Дисертації з теми "Rough boundary"
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1
Alexander, William Nathan. "Sound from Rough Wall Boundary Layers." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/29246.
Повний текст джерелаАнотація:
Turbulent flow over a rough surface produces sound that radiates outside the near wall region. This noise source is often at a lower level than the noise created by edges and bluff body flows, but for applications with large surface area to perimeter ratios at low Mach number, this noise source can have considerable levels. In the first part of this dissertation, a detailed study is made of the ability of the Glegg & Devenport (2009) scattering theory to predict roughness noise. To this end, comparisons are made with measurements from cuboidal and hemispherical roughness with roughness Reynolds numbers, hu_Ï /ν, ranging from 24 to 197 and roughness height to boundary layer thickness ratios of 5 to 18. Their theory is shown to work very accurately to predict the noise from surfaces with large roughness Reynolds numbers, but for cases with highly inhomogeneous wall pressure fields, differences grow between estimation and measurement. For these surfaces, the absolute levels were underpredicted but the spectral shape of the measurement was correctly determined indicating that the relationship of the radiated noise with the wavenumber wall pressure spectrum and roughness geometry appears to remain relatively unchanged. In the second part of this dissertation, delay and sum beamforming and least-squares analyses were used to examine roughness noise recorded by a 36-sensor linear microphone array. These methods were employed to estimate the variation of source strengths through short fetches of large hemispherical and cuboidal element roughness. The analyses show that the lead rows of the fetches produced the greatest streamwise and spanwise noise radiation. The least-squares analysis confirmed the presence of streamwise and spanwise aligned dipoles emanating from each roughness element as suggested by the LES of Yang & Wang (2011). The least-squares calculated source strengths show that the streamwise aligned dipole is always stronger than that of the spanwise dipole, but the relative magnitude of the difference varies with frequency.Ph. D.
2
Kutlar, Ahmet Ihsan. "Turbulant boundary layers on rough painted surfaces." Thesis, University of Liverpool, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.359178.
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3
Dyer, Luke Oliver. "Parabolic boundary value problems with rough coefficients." Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/33276.
Повний текст джерелаАнотація:
This thesis is motivated by some of the recent results of the solvability of elliptic PDE in Lipschitz domains and the relationships between the solvability of different boundary value problems. The parabolic setting has received less attention, in part due to the time irreversibility of the equation and difficulties in defining the appropriate analogous time-varying domain. Here we study the solvability of boundary value problems for second order linear parabolic PDE in time-varying domains, prove two main results and clarify the literature on time-varying domains. The first result shows a relationship between the regularity and Dirichlet boundary value problems for parabolic equations of the form Lu = div(A∇u)−ut = 0 in Lip(1, 1/2) time-varying cylinders, where the coefficient matrix A = [aij(X, t)] is uniformly elliptic and bounded. We show that if the Regularity problem (R)p for the equation Lu = 0 is solvable for some 1 < p < then the Dirichlet problem (D*) 1 p, for the adjoint equation L*v = 0 is also solvable, where p' = p/(p − 1). This result is analogous to the one established in the elliptic case. In the second result we prove the solvability of the parabolic Lp Dirichlet boundary value problem for 1 < p ≤ ∞ for a PDE of the form ut = div(A∇u)+B ·∇u on time-varying domains where the coefficients A = [aij(X, t)] and B = [bi(X, t)] satisfy a small Carleson condition. This result brings the state of affairs in the parabolic setting up to the current elliptic standard. Furthermore, we establish that if the coefficients of the operator A and B satisfy a vanishing Carleson condition, and the time-varying domain is of VMO-type then the parabolic Lp Dirichlet boundary value problem is solvable for all 1 < p ≤ ∞. This is related to elliptic results where the normal of the boundary of the domain is in VMO or near VMO implies the invertibility of certain boundary operators in Lp for all 1 < p < ∞. This then (using the method of layer potentials) implies solvability of the Lp boundary value problem in the same range for certain elliptic PDE. We do not use the method of layer potentials, since the coefficients we consider are too rough to use this technique but remarkably we recover Lp solvability in the full range of p's as the elliptic case. Moreover, to achieve this result we give new equivalent and localisable definitions of the appropriate time-varying domains.
4
Tarada, F. H. A. "Heat transfer to rough turbine blading." Thesis, University of Sussex, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379448.
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5
Rapetto, Marco. "Rough surfaces in contact : artificial intelligence and boundary lubrication." Licentiate thesis, Luleå tekniska universitet, Maskinelement, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-25874.
Повний текст джерелаАнотація:
Interacting surfaces are found in mechanical systems and components. Since engineered surfaces are not perfectly smooth, only a fraction of the nominal surface area is actually in contact. This fraction is denoted as the real area of contact, Ar, and is formed by the sum of the contact spots between the two touching surfaces. If these contacting surfaces are sliding, then friction and wear occur in these actual contacts. Friction and wear may be controlled by lubrication: depending on the operating conditions different types of lubrication regime exist. When the surfaces are completely separated by the fluid film and load is carried by hydrodynamic action, contacts operate in hydrodynamic regime. When the load is carried by the lubricating fluid and asperity contact, the regime becomes mixed lubrication. In boundary lubrication, surfaces are in contact and the load is carried by surface asperities. In many cases this is the critical lubrication regime that governs the life of the components. Due to the complexity of thin film boundary lubrication, design of lubricated interfaces is still a trial-and-error process. The mechanism of formation and rupture of oxide layers and boundary layers is not completely known and a reliable model for rough surfaces in boundary lubrication is currently lacking. This study focuses on boundary lubrication regime: the effect of surface roughness on the real area of contact is investigated and a numerical model for the sliding interaction between two asperities in sliding contact is developed. Numerical simulations of normal, dry, friction free, linear elastic contact of rough surfaces are performed. A variational approach is followed and the FFT-technique is used to speed up the numerical solution process. Five different steel surfaces are measured using a Wyko optical profilometer and several 2-D profiles are taken. The real area of contact and the pressure distribution over the contact length are calculated for all the 2-D profiles. A new slope parameter is defined. An artificial neural network is applied to determine the relationship between the roughness parameters and the real area of contact. Boundary lubrication mechanism is usually controlled by the additives present in the oil that form low friction, protective layers on the wearing surfaces. Chemical reactions between the lubricant molecules and the asperity surface may take place. These reactions are activated by certain values of pressure and temperature. Fundamental research on the influence of surface roughness on contact conditions is hence required and is a key factor in understanding the wear mechanism in boundary lubrication condition since pressure distribution, shear stresses, frictional heating, mechanical wear highly depends on surface topography. Modelling boundary lubrication requires knowledge in many fields: contact mechanics, thermodynamics, surface chemistry etc, thus different sub-models interacting each other must be created. It is complicated and may be not feasible within a foreseeable time period to take into account all the different parameters and evaluate them. Artificial intelligence is a way to overcome the problem and determine the relationship between input parameters and desired outputs. An elasto-plastic analytical model is used to determine the variation of pressure distribution and shear stress during the collision process of two asperities in sliding contact. The outputs of the elasto-plastic model are inputs of the thermal model that calculates the temperature rise during the collision process. The desorption of the adsorbed layer is determined by using existing adsorption theories and finally the probability of wear is computed at each time step of the collision process. Different results obtained using different adsorption theories and different input parameters are compared.Godkänd; 2008; 20080512 (ysko)
6
Rapetto, Marco Pietro. "Rough surfaces in contact : artificial intelligence and boundary lubrication /." Luleå : Luleå University of Technology, 2008. http://epubl.ltu.se/1402-1757/2008/16/.
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7
Ross, Christopher Roger. "Direct and inverse scattering by rough surfaces." Thesis, Brunel University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318675.
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8
Chang, Chung-Yie. "Boundary value problems for differential equations driven by rough signals." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299835.
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9
Smith, Benjamin Scott. "Wall Jet Boundary Layer Flows Over Smooth and Rough Surfaces." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/27597.
Повний текст джерелаАнотація:
The aerodynamic flow and fluctuating surface pressure of a plane, turbulent, two-dimensional wall jet flow into still air over smooth and rough surfaces has been investigated in a recently constructed wall jet wind tunnel testing facility. The facility has been shown to produce a wall jet flow with Reynolds numbers based on the momentum thickness, Re&delta = &deltaUm/&nu, of between 395 and 1100 and nozzle exit Reynolds numbers, Rej = Umb/&nu, of between 16000 and 45000. The wall jet flow properties (&delta, &delta*, &theta, y1/2, Um, u*, etc.) were measured and characterized over a wide range of initial flow conditions and measurement locations relative to the wall jet source. These flow properties were measured for flow over a smooth flow surface and for flow over roughness patches of finite extent. The patches used in the current study varied in length from 305 mm to 914 mm (between 24 and 72 times the nozzle height, b) and were placed so that the leading edge of the patch was fixed at 1257 mm (x/b = 99) downstream of the wall jet source. These roughness patches were of a random sand grain roughness type and the roughness grain size was varied throughout this experiment. The tests covered roughness Reynolds numbers (k+) ranging from less than 2 to over 158 (covering the entire range of rough wall flow regimes from hydrodynamically smooth to fully rough). For the wall jet flows over 305 mm long patches of roughness, the displacement and momentum thicknesses were found to vary noticeably with the roughness grain size, but the maximum velocity, mixing layer length scale, y1/2, and the boundary layer thickness were not seen to vary in a consistent, determinable way. Velocity spectra taken at a range of initial flow conditions and at several distinct heights above the flow surface showed a limited scaling dependency on the skin friction velocity near the flow surface.
The spectral density of the surface pressure of the wall jet flow, which is not believed to have been previously investigated for smooth or rough surfaces, showed distinct differences with that seen in a conventional boundary layer flow, especially at low frequencies. This difference is believed to be due to the presence of a mixing layer in the wall jet flow. Both the spectral shape and level were heavily affected by the variation in roughness grain size. This effect was most notable in overlap region of the spectrum. Attempts to scale the wall jet surface pressure spectra using outer and inner variables were successful for the smooth wall flows. The scaling of the rough wall jet flow surface pressure proved to be much more difficult, and conventional scaling techniques used for ordinary turbulent boundary layer surface pressure spectra were not able to account for the changes in roughness present during the current study. An empirical scaling scheme was proposed, but was only marginally effective at scaling the rough wall surface pressure.Ph. D.
10
Arens, Tilo. "The scattering of elastic waves by rough surfaces." Thesis, Brunel University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.311560.
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11
Gagné, Jean-François. "An improved method for modelling fully rough turbulent boundary layer flows." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/mq36888.pdf.
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12
Tachie, Mark Francis. "Open channel turbulent boundary layers and wall jets on rough surfaces." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/NQ63927.pdf.
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13
Kreuter, Marcel [Verfasser]. "Vector-valued elliptic boundary value problems on rough domains / Marcel Kreuter." Ulm : Universität Ulm, 2019. http://d-nb.info/1177882434/34.
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14
Stewart, Douglas John. "Finite element schemes for elliptic boundary value problems with rough coefficients." Thesis, Brunel University, 1998. http://bura.brunel.ac.uk/handle/2438/5529.
Повний текст джерелаАнотація:
We consider the task of computing reliable numerical approximations of the solutions of elliptic equations and systems where the coefficients vary discontinuously, rapidly, and by large orders of magnitude. Such problems, which occur in diffusion and in linear elastic deformation of composite materials, have solutions with low regularity with the result that reliable numerical approximations can be found only in approximating spaces, invariably with high dimension, that can accurately represent the large and rapid changes occurring in the solution. The use of the Galerkin approach with such high dimensional approximating spaces often leads to very large scale discrete problems which at best can only be solved using efficient solvers. However, even then, their scale is sometimes so large that the Galerkin approach becomes impractical and alternative methods of approximation must be sought. In this thesis we adopt two approaches. We propose a new asymptotic method of approximation for problems of diffusion in materials with periodic structure. This approach uses Fourier series expansions and enables one to perform all computations on a periodic cell; this overcomes the difficulty caused by the rapid variation of the coefficients. In the one dimensional case we have constructed problems with discontinuous coefficients and computed the analytical expressions for their solutions and the proposed asymptotic approximations. The rates at which the given asymptotic approximations converge, as the period of the material decreases, are obtained through extensive computational tests which show that these rates are fundamentally dependent on the level of regularity of the right hand sides of the equations. In the two dimensional case we show how one can use the Galerkin method to approximate the solutions of the problems associated with the periodic cell. We construct problems with discontinuous coefficients and perform extensive computational tests which show that the asymptotic properties of the approximations are identical to those observed in the one dimensional case. However, the computational results show that the application of the Galerkin method of approximation introduces a discretization error which can obscure the precise asymptotic rate of convergence for low regularity right hand sides. For problems of two dimensional linear elasticity we are forced to consider an alternative approach. We use domain decomposition techniques that interface the subdomains with conjugate gradient methods and obtain algorithms which can be efficiently implemented on computers with parallel architectures. We construct the balancing preconditioner, M,, and show that it has the optimal conditioning property k(Mh(^-1)Sh) =< C(1 + log(H/h))^2 where Sh is the discretized Steklov—Poincaré operator, C> 0 is a constant which is independent of the magnitude of the material discontinuities, H is the maximum subdomain diameter, and h is the maximum finite element diameter. These properties of the preconditioning operator Mh allow one to use the computational power of a parallel computer to overcome the difficulties caused by the changing form of the solution of the problem. We have implemented this approach for a variety of problems of planar linear elasticity and, using different domain decompositions, approximating spaces, and materials, find that the algorithm is robust and scales with the dimension of the approximating space and the number of subdomains according to the condition number bound above and is unaffected by material discontinuities. In this we have proposed and implemented new inner product expressions which we use to modify the bilinear forms associated with problems over subdomains that have pure traction boundary conditions.
15
Sharma, Sanjib. "Uniform Flow Development Length in a Rough Laboratory Flume." OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1620.
Повний текст джерелаАнотація:
Test sections in laboratory studies should be fully developed and uniform if they are to be generalized. The objective of this study is to develop a model for predicting the uniform flow development length (Lunif) in a rough laboratory flume as a function of hydraulic parameters and bed particle roughness height (ks). Using an ADV time-averaged point velocity was measured in developing and fully developed turbulent subcritical rough open-channel flows. A series of laboratory tests were carried out in a 6.1-m-long and 0.46-m-wide rectangular channel. Tests were conducted with fine gravel (d50 = 5.8 mm, ks = 3.1d90 = 0.026 m), medium gravel (d50 = 14 mm, ks = 0.068 m), and with fine /medium gravel (d50 = 11 mm, ks = 0.04 m). For each test, longitudinal point velocity measurements were made along the center of the channel at five elevations, and at thirteen longitudinal stations. The study concluded that for flow to be uniform, the flow depth and mean cross-sectional velocity must be constant. In addition, root mean square of the fluctuating component of the velocity, RMS(u'), which is the measure of the turbulence intensity, should be uniform in order for a flow to consider uniform. Thus, RMS(u') is one of the indicative measures for determining the location where the developing flow is fully developed and uniform. The results showed that increasing the bed roughness height decreases the uniform flow development length. Using the dimensional and statistical analyses Lunif was estimated as a linear function of Reynold's particle number and Froude number.
16
Watt, Robert McFarlane. "Effects of surface roughness on the boundary-layer characteristics of turbine aerofoils." Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.330065.
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17
Sen, Mehmet Ali. "Proper Orthogonal Decomposition Methodology to Understand Underlying Physics of Rough-Wall Turbulent Boundary Layer." Fogler Library, University of Maine, 2007. http://www.library.umaine.edu/theses/pdf/SenMA2007.pdf.
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18
Kerevanian, G. K. "Experimental investigation of turbulent boundary layers on uniform, well-defined rough walls." Thesis, Queen's University Belfast, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269051.
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19
Meyers, Timothy Wade. "The Rough Wall High Reynolds Number Turbulent Boundary Layer Surface Pressure Spectrum." Thesis, Virginia Tech, 2014. http://hdl.handle.net/10919/25880.
Повний текст джерелаАнотація:
There have been very few studies investigating the rough wall pressure spectra under fully rough flows, which are relevant to many common engineering applications operating within this regime. This investigation uses the Virginia Tech Stability Wind Tunnel to perform experiments on a series of high Reynolds number zero pressure gradient turbulent boundary layers formed over rough walls in an effort to better understand and characterize the behavior of the rough wall pressure spectrum. The boundary layers were fully rough, and the boundary layer height remained sufficiently larger than the height of the roughness elements. Two rough surfaces were tested. One consisted of an array of 1-mm ordered hemispherical elements spaced 5.5-mm apart, and the other contained 3-mm hemispherical elements randomly spaced, but with the same element density as 1/3 of the 1-mm ordered roughness. The wall pressure spectrum and its scaling were then studied in detail, and it was found that the rough wall turbulent pressure spectrum at vehicle relevant conditions is defined by three scaling regions. One of which is a newly discovered high frequency scaling defined by viscosity, but controlled by the friction velocity adjusted to exclude the pressure drag on the roughness elements. Based on these three scaling regions an empirical model describing the wall pressure spectra for hydraulically smooth, traditionally rough, and fully rough flows was explored. Two point wall pressure fluctuations were also analyzed for each surface condition, and it was found that the roughness inhibits the convective velocities within the inner portions of the boundary layer.Master of Science
20
Forest, Jonathan Bradley. "The Wall Pressure Spectrum of High Reynolds Number Rough-Wall Turbulent Boundary Layers." Thesis, Virginia Tech, 2012. http://hdl.handle.net/10919/31114.
Повний текст джерелаАнотація:
The presence of roughness on a surface subject to high Reynolds number flows promotes the formation of a turbulent boundary layer and the generation of a fluctuating pressure field imposed on the surface. While numerous studies have investigated the wall pressure fluctuations over zero-pressure gradient smooth walls, few studies have examined the effects of surface roughness on the wall pressure field. Additionally, due to the difficulties in obtaining high Reynolds number flows over fully rough surfaces in laboratory settings, an even fewer number of studies have investigated this phenomenon under flow conditions predicted to be fully free of transitional effects that would ensure similarity laws could be observed. This study presents the efforts to scale and describe the wall pressure spectrum of a rough wall, high Reynolds number turbulent boundary layer free of transitional effects. Measurements were taken in the Virginia Tech Stability Wind Tunnel for both smooth and rough walls. A deterministic roughness fetch composed of 3-mm hemispheres arranged in a 16.5-mm square array was used for the rough surface. Smooth and rough wall flows were examined achieving Reynolds numbers up to Reθ = 68700 and Reθ = 80200 respectively, with the rough wall flows reaching roughness based Reynolds numbers up to kg+ = 507 with a simultaneous blockage ratio of δ/kg = 76. A new roughness based inner variable scaling is proposed that provides a much more complete collapse of the rough wall pressure spectra than previous scales had provided over a large range of Reynolds numbers and roughness configurations. This scaling implies the presence of two separate time scales associated with the near wall turbulence structure generation. A clearly defined overlap region was observed for the rough wall surface pressure spectra displaying a frequency dependence of Ï -1.33, believed to be a function of the surface roughness configuration and its associated transport of turbulent energy. The rough wall pressure spectra were shown to decay more rapidly, but based on the same function as what defined the smooth wall decay.Master of Science
21
Konidaris, Spyridon G. "Electromagnetic scattering from rough surfaces using the On-Surface Radiation Boundary Condition (OSRC) method." Thesis, Monterey, California. Naval Postgraduate School, 1990. http://hdl.handle.net/10945/30624.
Повний текст джерелаАнотація:
Approved for public release, distribution unlimitedElectromagnetic scattering from rough surfaces is of prime importance in the engineering field since it affects communications, radar, remote sensing, acoustics, etc. The actual problem of scattering from rough surfaces is complicated and involves three dimensional scattering from either lossy or dielectric, electrically large surface. Integral equations are widely utilized to solve this kind of problem but this solution to the problem is generally computationally intensive. In the On-Surface Radiation Boundary Condition (OSRC) method, a higher order radiation condition is imposed directly on the surface of the scatterer. This reduces the integral equation for the scattered field to a line integral which can be easily evaluated numerically. In this thesis, the OSRC method is used to formulate the problem of scattering from periodic rough, two dimensional surfaces illuminated by a transverse magnetic, plane electromagnetic wave. Three geometric surfaces are considered. A comparison is made between the present formulation, the exact solution, and the physical optics approximation.
22
Sapkota, Deependra. "TRIPPING OF THE BOUNDARY LAYER DEVELOPMENT LENGTH OVER ROUGH AND FULLY TURBULENT SUBCRITICAL FLUME." OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1801.
Повний текст джерелаАнотація:
The distance required for flow entering a laboratory channel to become fully-developed and uniform can be substantial. Given the need to establish fully-developed uniform flow, if the length of a laboratory channel is not substantial then it likely that the flume cannot be used to conduct open-channel flow research. In laboratory studies where the channel bed is hydraulically rough, the noted problem can be lessened by minimizing the length over which the flow becomes fully-developed and uniform (Lunif). For this study it is hypothesized that if bed material with a roughness height (ks, ∆) is placed at the channel entrance and ks, ∆ is greater has the roughness height of bed material placed throughout the channel (ks, bed) then Lunif can be reduced. The length over which the larger bed material is referred to as the tripping zone length (∆). A second hypothesis for this study is that if ∆ is longer, then Lunif will be shorter. The primary objective of this study is to test the above mentioned hypothesis and to develop a relationship for predicting Lunif as a function of Δ. For this study, physical tests were performed in a rectangular Plexiglas flume with a variable slope. The flume was 6.1 m long, 45.7 cm wide, and 45.7 cm deep. The channel has smooth walls and the bed was lined with gravel (median particle size, d50 = 8.5 mm or 22 mm). Similarly tripping zone was lined with gravel of larger size (median particle size, d50 = 13 mm or 58 mm).Twelve tests were conducted for the study. For each test, longitudinal point velocity measurements (u) were made along the channel center, at five elevations (z), and at twelve longitudinal stations (x). An Acoustic Doppler Velocimeter was used to measure u. Lunif was determined by considering four indications of flow uniformity. Results indicate that having a tripping zone decreases Lunif and the magnitude of the decrease in Lunif was dependent on ∆. A function is presented for predicting Lunif /H = f (Rep, Fr, and Δ/H) where Rep is the Reynold's particle number, Fr is the Froude number and H is the flow depth.
23
McClain, Stephen Taylor. "A discrete-element model for turbulent flow over randomly-rough surfaces." Diss., Mississippi State : Mississippi State University, 2002. http://library.msstate.edu/etd/show.asp?etd=etd-04032002-140007.
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George, Jacob. "Structure of 2-D and 3-D Turbulent Boundary Layers with Sparsely Distributed Roughness Elements." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/27935.
Повний текст джерелаАнотація:
The present study deals with the effects of sparsely distributed three-dimensional elements on two-dimensional (2-D) and three-dimensional (3-D) turbulent boundary layers (TBL) such as those that occur on submarines, ship hulls, etc. This study was achieved in three parts: Part 1 dealt with the cylinders when placed individually in the turbulent boundary layers, thereby considering the effect of a single perturbation on the TBL; Part 2 considered the effects when the same individual elements were placed in a sparse and regular distribution, thus studying the response of the flow to a sequence of perturbations; and in Part 3, the distributions were subjected to 3-D turbulent boundary layers, thus examining the effects of streamwise and spanwise pressure gradients on the same perturbed flows as considered in Part 2. The 3-D turbulent boundary layers were generated by an idealized wing-body junction flow. Detailed 3-velocity-component Laser-Doppler Velocimetry (LDV) and other measurements were carried out to understand and describe the rough-wall flow structure. The measurements include mean velocities, turbulence quantities (Reynolds stresses and triple products), skin friction, surface pressure and oil flow visualizations in 2-D and 3-D rough-wall flows for Reynolds numbers, based on momentum thickness, greater than 7000. Very uniform circular cylindrical roughness elements of 0.38mm, 0.76mm and 1.52mm height (k) were used in square and diagonal patterns, yielding six different roughness geometries of rough-wall surface. For the 2-D rough-wall flows, the roughness Reynolds numbers, based on the element height (k) and the friction velocity, range from 26 to 131. Results for the 2-D rough-wall flows reveal that the velocity-defect law is similar for both smooth and rough surfaces, and the semi-logarithmic velocity-distribution curve is shifted by an amount depending on the height of the roughness element, showing that this amount is a function of roughness Reynolds number and the wall geometry. For the 3-D flows, the data show that the surface pressure gradient is not strongly influenced by the roughness elements. In general, for both 2-D and 3-D rough-wall TBL, the differences between the two roughness patterns (straight and diagonal), as regards the mean velocities and the Reynolds stresses, are limited to about 3 roughness element heights from the wall.
The study on single elements revealed that the separated shear layers emanating from the top of the elements form a pair of counter rotating vortices that dominate the downstream flow structure. These vortices, termed as the roughness top vortex structure (RTVS), in conjunction with mean flow, forced over and around the elements, are responsible for the production of large Reynolds stresses in the neighborhood of the element height aft of the elements. When these elements are placed in a distribution, the effects of RTVS are not apparent. The roughness elements create a large region of back flow behind them which is continuously replenished by faster moving fluid flowing through the gaps in the rough-wall. The fluid in the back flow region moves upward as low speed ejections where it collides with the inrushing high speed flow, thus, leading to a strong mixing of shear layers. This is responsible for the generation of large levels of turbulent kinetic energy (TKE) in the vicinity of the element height which is transported, primarily, by turbulent diffusion. As regards the 3-D rough-wall TBL, the effect of flow three-dimensionality is seen in the large skewing of the distributions of mean velocities, Reynolds stresses and TKE, aft of the elements. In general, the regions of large TKE production-rates seem to propagate in the direction of the local velocity vector at the element height. The data-sets also enable the extraction of the turbulent flow structure to better describe the flow physics of these rough-wall turbulent boundary layers.Ph. D.
25
Bomminayuni, Sandeep Kumar. "Large eddy simulation of turbulent flow over a rough bed using the immersed boundary method." Thesis, Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/34821.
Повний текст джерелаАнотація:
Study of turbulent flow over a rough bed is highly important due to its numerous applications in the areas of sediment transport and pollutant discharge in streams, rivers and channels. Over the past few decades, many experimental studies have been conducted in this respect to understand the underlying phenomenon. However, there is a scarcity in the number of computational studies conducted on this topic. Therefore, a Large Eddy Simulation (LES) of turbulent flow over a rough channel bed was conducted to contribute further understanding of the influence of bed roughness on turbulent flow properties. For this purpose, an efficient, second order accurate 'immersed boundary method' was implemented into the LES code Hydro3d-GT, and validated for flow past bluff bodies.
LES results from the present study showed excellent agreement with previous experimental studies on flow over rough beds. An in-depth analysis of time varying turbulent quantities (like the velocity fluctuations) revealed the presence of coherent structures in the flow. Also, a three dimensional visualization of the turbulent structures provided a good picture of the flow, especially in the near bed region, which is quite difficult to accomplish using experimental studies.
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Wang, Lu Ph D. Massachusetts Institute of Technology. "Self-shrinkers of mean curvature flow and harmonic map heat flow with rough boundary data." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/67817.
Повний текст джерелаАнотація:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 59-63).
In this thesis, first, joint with Longzhi Lin, we establish estimates for the harmonic map heat flow from the unit circle into a closed manifold, and use it to construct sweepouts with the following good property: each curve in the tightened sweepout, whose energy is close to the maximal energy of curves in the sweepout, is itself close to a closed geodesic. Second, we prove the uniqueness for energy decreasing weak solutions of the harmonic map heat flow from the unit open disk into a closed manifold, given any H¹ initial data and boundary data, which is the restriction of the initial data on the boundary of the disk. Previously, under an additional assumption on boundary regularity, this uniqueness result was obtained by Rivière (when the target manifold is the round sphere and the energy of initial data is small) and Freire (for general target manifolds). The point of our uniqueness result is that no boundary regularity assumption is needed. Also, we prove the exponential convergence of the harmonic map heat flow, assuming that the energy is small at all times. Third, we prove that smooth self-shrinkers in the Euclidean space, that are entire graphs, are hyperplanes. This generalizes an earlier result by Ecker and Huisken: no polynomial growth assumption at infinity is needed.
by Lu Wang.
Ph.D.
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Alveroğlu, Burhan. "The convective instability of the BEK system of rotating boundary-layer flows over rough disks." Thesis, University of Leicester, 2016. http://hdl.handle.net/2381/37977.
Повний текст джерелаАнотація:
A numerical study investigating the effects of surface roughness on the stability properties of the BEK system of flows is introduced. The BEK system of flows occur in many engineering applications such as turbo-machinery and rotor-stator devices, therefore they have great practical importance. Recent studies have been concerned with the effects of surface roughness on the von Kármán flow. The aim of this thesis is to investigate whether distributed surface roughness could be used as a passive drag reduction technique for the broader BEK system of flows. If it can, what is “the right sort of roughness?" To answer these questions, a linear stability analysis is performed using the Chebyshev collocation method to investigate the effect of particular types of distributed surface roughness, both anisotropic and isotropic, on the convective instability characteristics of the inviscid Type I (cross-flow) instability and the viscous Type II instability. The results reveal that all roughness types lead to a stabilisation of the Type I mode in all flows within the BEK family, with the exception of azimuthally-anisotropic roughness (radial grooves) within the Bődewadt flow which causes a mildly destabilising effect. In the case of the Type II mode, the results reveal the destabilising effect of radially-anisotropic roughness (concentric grooves) on all the boundary layers, whereas both azimuthally-anisotropic and isotropic roughness have a stabilising effect on the mode for Ekman and von Kármán flows. Moreover, an energy analysis is performed to investigate the underlying physical mechanisms behind the effects of rough surfaces on the BEK system. The conclusion is that isotropic surface roughness is the most effective type of the distributed surface roughness and can be recommended as a passive-drag reduction mechanism for the entire BEK system of flows.
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Walshe, John D. "CFD modelling of wind flow over complex and rough terrain." Thesis, Loughborough University, 2003. https://dspace.lboro.ac.uk/2134/7827.
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A model has been developed using the general-purpose Navier-Stokes solver CFX4 to simulate Atmospheric Boundary Layer flow over complex terrain. This model has been validated against the measured data from the Askervein Hill experiment, and has been shown to perform well. The CFD model is also compared to the WAsP linear model of wind flow over topography, and a significant improvement is noted for flow over complex topography. Boundary conditions, gridding issues and sensitivity to other solver parameters have all been investigated. An advanced roughness model has been developed to simulate flow over forest canopies, using a resistive body force within the canopy volume. The model is validated against measured data for simple 2D cases, and for a complex 3D case over real topography. The model is shown to give a more physically realistic profile for the wind speed in and just above forest canopies than the standard roughness length model used in most CFD simulations. An automated methodology for setting up CFD simulations using the models described has been developed. A custom pre-processing package to implement this has been written, to enable the use of the CFD methodology in a commercial environment.
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Rasnick, Matthew Byron. "The Noise of a Boundary Layer Flowing Over Discrete Roughness Elements." Thesis, Virginia Tech, 2010. http://hdl.handle.net/10919/33202.
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This study focuses on measuring and normalizing the roughness noise of multiple roughness types across numerous layouts and flow speeds. Using the Virginia Tech Anechoic Wall Jet Facility, far field noise was recording for the flow of a turbulent wall jet boundary layer over cubes, hemispheres, and gravel, with element heights in the range of 14.3 - 55.2% of the boundary layer thickness. The sound radiated from the various layouts showed that the elements acted as independent sources when separated by three element diameters center-to-center or more. When the elements were placed shoulder to shoulder, interaction between the elements and shielding of the higher velocity flow lowered the noise per element produced. The far field roughness noise was then normalized using the theory of Glegg et al. (2007), which assumes a dipole efficiency factor. Comparisons were made between the theoretical drag spectrum model proposed by Glegg et al. (1987) and a modified version of this model made using the empirical data gathered. Overall, the theory of Glegg et al. (2007) succeeds greatly in collapsing the data into its non-dimensional drag spectra, but the original model spectrum did not fit well. The modified spectrum showed much greater fit with the data at all layouts and speeds. The collapse of the data using the theory of Glegg et al. (2007) confirms that roughness noise is dipole in nature.Master of Science
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Mansour-Tehrani, Mehrdad. "Spacial distribution and scaling of bursting events in boundary layer turbulence over smooth and rough surfaces." Thesis, University College London (University of London), 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261297.
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Joseph, Liselle AnnMarie. "Pressure Fluctuations in a High-Reynolds-Number Turbulent Boundary Layer over Rough Surfaces of Different Configurations." Diss., Virginia Tech, 2017. http://hdl.handle.net/10919/79630.
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The pressure fluctuations under a high Reynolds Number, rough-wall, turbulent, boundary layer have been studied in the Virginia Tech Stability Wind Tunnel. Rough surfaces of varying element height (1-mm, 3-mm), shape (hemispheres, cylinders) and spacing (5.5-mm, 10.4-mm, 16.5-mm) were investigated in order to ascertain how the turbulent pressure fluctuations change with changes in roughness geometry. Rough surfaces which contain two types of elements are investigated and relationships between the combination surface and the individual surfaces have been uncovered. Measurements of the wall pressure fluctuations were made using pinhole microphones and hotwire measurements were made to obtain the velocity and turbulence field.
Among the principal findings is the development of two scaling laws for the low frequency pressure fluctuations. Both of these are based on the idea that the defect between the edge velocity and some local boundary layer velocity sustains the pressure fluctuations in the outer regions of the flow. The first scaling uses the broadband convection velocity as the local velocity of the large scale pressure fluctuations. The second scaling uses the mean boundary layer velocity. Both these scalings appear more robust than the previously proposed scalings for the low frequency region and are able to scale the pressure spectra of all the data to within 3.5-dB.
In addition, it was proven that the high frequency shear friction velocity scaling of Meyers et al. (2015) is universal to rough surfaces of different element shape and density. Physical insights into the shear friction velocity, on which this scaling is based, have been revealed. This includes an empirical formula which estimates the element pressure drag coefficient from the roughness density and the Reynolds number.
The slopes in the mid-frequency region were found to vary with element density and microphone location such that a useful scaling could not be determined for this region. The possibility of an overlap region is explored and the expectation of a -1 slope is disproved. It is hypothesised that an evanescent decay of the mid-frequency pressure fluctuations occurs between their actual location and the wall where they are measured. A method for accounting for this decay is presented in order to scale the pressure fluctuations in this region.
Lastly, a piecewise interpolation function for the pressure spectrum of rough wall turbulent boundary layers was proposed. This analytical function is based on the low frequency scaling on mean velocity and the high frequency scaling of Meyers et al. (2015) The mid-frequency is estimated by a spline interpolation between these two regions.Ph. D.
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Repasky, Russell James. "Turbulent Boundary Layers over Rough Surfaces: Large Structure Velocity Scaling and Driver Implications for Acoustic Metamaterials." Thesis, Virginia Tech, 2019. http://hdl.handle.net/10919/90796.
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Turbulent boundary layer and metamaterial properties were explored to initiate the viability of controlling acoustic waves driven by pressure fluctuations from flow. A turbulent boundary layer scaling analysis was performed on zero-pressure-gradient turbulent boundary layers over rough surfaces, for 30,000≤〖Re〗_θ≤100,000. Relationships between fluctuating pressures and velocities were explored through the pressure Poisson equation. Certain scaling laws were implemented in attempts to collapse velocity spectra and turbulence profiles. Such analyses were performed to justify a proper scaling of the low-frequency region of the wall-pressure spectrum. Such frequencies are commonly associated with eddies containing the largest length scales. This study compared three scaling methods proposed in literature: The low-frequency classical scaling (velocity scale U_τ, length scale δ), the convection velocity scaling (U_e-U ̅_c, δ), and the Zagarola-Smits scaling (U_e-U ̅, δ). A default scaling (U_e, δ) was also selected as a baseline case for comparison. At some level, the classical scaling best collapsed rough and smooth wall Reynolds stress profiles. Low-pass filtering of the scaled turbulence profiles improved the rough-wall scaling of the Zagarola-Smits and convection velocity laws. However, inconsistent scaled results between the pressure and velocity requires a more rigorous pressure Poisson analysis. The selection of a proper scaling law gives insight into turbulent boundary layers as possible sources for acoustic metamaterials. A quiescent (no flow) experiment was conducted to measure the capabilities of a metamaterial in retaining acoustic surface waves. A point source speaker provided an acoustic input while the resulting sound waves were measured with a probe microphone. Acoustic surface waves were found via Fourier analysis in time and space. Standing acoustic surface waves were identified. Membrane response properties were measured to obtain source condition characteristics for turbulent boundary layers once the metamaterial is exposed to flow.Master of Science
Aerodynamicists are often concerned with interactions between fluids and solids, such as an aircraft wing gliding through air. Due to frictional effects, the relative velocity of the air on the solid-surface is negligible. This results in a layer of slower moving fluid near the surface referred to as a boundary layer. Boundary layers regularly occur in the fluid-solid interface, and account for a sufficient amount of noise and drag on aircraft. To compensate for increases in drag, engines are required to produce increased amounts of power. This leads to higher fuel consumption and increased costs. Additionally, most boundary layers in nature are turbulent, or chaotic. Therefore, it is difficult to predict the exact paths of air molecules as they travel within a boundary layer. Because of its intriguing physics and impacts on economic costs, turbulent boundary layers have been a popular research topic. This study analyzed air pressure and velocity measurements of turbulent boundary layers. Relationships between the two were drawn, which fostered a discussion of future works in the field. Mainly, the simultaneous measurements of pressure on the surface and boundary layer velocity can be performed with understanding of the Pressure Poisson equation. This equation is a mathematical representation of the boundary layer pressure on the surface. This study also explored the possibility of turbulent-boundary-layer-driven-acoustic-metamaterials. Acoustic metamaterials contain hundreds of cavities which can collectively manipulate passing sound waves. A facility was developed at Virginia Tech to measure this effect, with aid from a similar laboratory at Exeter University. Microphone measurements showed the reduction of sound wave speed across the metamaterial, showing promise in acoustic manipulation. Applications in metamaterials in the altering of sound caused by turbulent boundary layers were also explored and discussed.
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Zhang, Libiao. "Modelling uncertain decision boundary for text classification." Thesis, Queensland University of Technology, 2016. https://eprints.qut.edu.au/102042/1/Libiao_Zhang_Thesis.pdf.
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Text classification is to classify documents into predefined categories by learned classifiers. Classic text classifiers cannot unambiguously describe decision boundary between relevant and irrelevant documents because of uncertainties caused by feature selection and knowledge learning. This research proposes a three-way decision model for dealing with uncertain decision boundary based on rough sets and centroid solution to improve classification performance. It partitions training samples into three regions by two main boundary vectors, and resolves the boundary region by two derived boundary vectors to generate decision rules for making 'two-way' decisions.
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Grissom, Dustin Leonard. "A Study of Sound Generated by a Turbulent Wall Jet Flow Over Rough Surfaces." Diss., Virginia Tech, 2007. http://hdl.handle.net/10919/28336.
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The far field acoustics generated by turbulent flow over rough surfaces has been experimentally investigated in an acoustically treated wall jet facility. The facility allows direct measurement of the far field sound from small patches of surface roughness, without contamination from edge or other aerodynamic noise sources. The facility is capable of generating turbulent boundary layer flows with momentum thickness Reynolds numbers between 450 and 1160. The variation of surface conditions tested cover the range from hydrodynamically smooth surfaces through most of the transitional range, with h+ variations from 3 to 85. Single microphone narrow band acoustic spectra, measured in the far field, show sound levels as much as 15 dB above the background from 0.186 m2 roughness patches. The measurements revealed the spectral shape and level variations with flow velocity, boundary layer thickness, and roughness size; providing the first data set large enough to assess the affects of many aerodynamic properties on the acoustic spectra. Increases in the size of grit type roughness produced significant increases in acoustic levels. Patches of hydrodynamically smooth roughness generated measurable acoustic levels, confirming that acoustic scattering is at least one of the physical mechanisms responsible for roughness noise. The shapes of the measured spectra show a strong dependence on the form of the surface roughness. The acoustic spectra generated by periodic two-dimensional surfaces have a much narrower louder peak than that generated by three-dimensional grit type roughness. Measurements also show the orientation of the two-dimensional surface significantly affects the acoustic levels and directivity.
The variation of sound levels with flow velocity and roughness size suggests the acoustic field is significantly affected by changes in the near wall flow due to the presence of the roughness. Current models of noise generated by rough surfaces predict the general trends seen in measurements for flows over grit and two-dimensional roughness in the range of 20
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Ren, Huiying. "Experimental Studies of Turbulent Boundary Layers Over a Rough Forward-facing Step and its Coarse Scale Resolution Approximations." Wright State University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=wright1292449621.
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Moon, Jiyoung. "Rheological Behavior of Complex Fluid with Deformable and Rigid Particles." Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/17106.
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Predicting the rheological properties of particles in matrix is one of the most challenging and complicated problems in material and fluid sciences. The complication is arisen by the particles collision and interactions with the surrounding fluid. A full description of the rheology of particles requires a complete understanding of the interactions between particles, interaction between the particles and the matrix fluid, and interactions between channel and particles. Thus consideration of above factors can lead to a better understanding of the rheological behavior of suspensions with particles flow. In this thesis, fluid with deformable particle and fluid with rigid particles are considered. A combination model of the three dimensional lattice Boltzmann method (LBM) and the immersed boundary method (IBM) are used to simulate these suspension systems. For the single particle deformation in the flow, the boundary thickness and value on transit time in a microchannel was analyzed. To see the physics behind the single particle in a micro channel, the path selection (navigation) of a single moving particle in a microfluidic channel network was analyzed. To see the interaction between wall property and suspension flow, deformable particles in hydrophobic and hydrophilic surface microfluidic channels was analyzed. To see the effect of particle roughness on rheology, the results of measuring the viscometric flow of concentrated rigid-sphere suspensions with constant-viscosity matrices, both Newtonian (silicone oil) and non-Newtonian were presented. Finally, the rough particle was analyzed by lattice Boltzmann method to find the physics behind the experimental results.
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Leaman, Nye Abigail. "Scattering of internal gravity waves." Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/238679.
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Internal gravity waves play a fundamental role in the dynamics of stably stratified regions of the atmosphere and ocean. In addition to the radiation of momentum and energy remote from generation sites, internal waves drive vertical transport of heat and mass through the ocean by wave breaking and the mixing subsequently produced. Identifying regions where internal gravity waves contribute to ocean mixing and quantifying this mixing are therefore important for accurate climate and weather predictions. Field studies report significantly enhanced measurements of turbulence near 'rough' ocean topography compared with those recorded in the ocean interior or near more gradually varying topography (e.g. Toole et al. 1997, J. Geophys. Res. 102). Such observations suggest that interaction of waves with rough topography may act to skew wave energy spectra to high wavenumbers and hence promote wave breaking and fluid mixing. This thesis examines the high wavenumber scatter and spatial partitioning of wave energy at 'rough' topography containing features that are of similar scales to those characterising incident waves. The research presented here includes laboratory experiments using synthetic schlieren and PIV to visualise two-dimensional wavefields produced by small amplitude oscillations of cylinders within linear salt-water stratifications. Interactions of wavefields with planar slopes and smoothly varying sinusoidal topography are compared with those with square-wave, sawtooth and pseudo knife-edge profiles, which have discontinuous slopes. Far-field structures of scattered wavefields are compared with linear analytical models. Scatter to high wavenumbers is found to be controlled predominantly by the relative slopes and characterising length scales of the incident wavefield and topography, as well as the shape and aspect ratio of the topographic profile. Wave energy becomes highly focused and the spectra skewed to higher wavenumbers by 'critical' regions, where the topographic slope is comparable with the slope of the incident wave energy vector, and at sharp corners, where topographic slope is not defined. Contrary to linear geometric ray tracing predictions (Longuet-Higgins 1969, J. Fluid Mech. 37), a significant back-scattered field can be achieved in near-critical conditions as well as a forward scattered wavefield in supercritical conditions, where the slope of the boundary is steeper than that of the incident wave. Results suggest that interaction with rough benthic topography could efficiently convert wave energy to higher wavenumbers and promote fluid mixing in such ocean regions.
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Tournier, Simon. "Contribution à la modélisation de la diffusion électromagnétique par des surfaces rugueuses à partir de méthodes rigoureuses." Thesis, Toulouse, ISAE, 2012. http://www.theses.fr/2012ESAE0008/document.
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Cette thèse traite de la diffusion par des surfaces rugueuses monodimensionnelles. Les surfaces présentant des petites échelles de variations nécessitent une discrétisation fine pour représenter les effets de diffusion sur le champ diffracté, ce qui augmente les coûts numériques. Deux aspects sont considérés : la réduction de la taille du problème en construisant une condition aux limiteséquivalente traduisant les effets des variations rapides et la réduction du nombre d’itérations nécessaires pour résoudre le système linéaire issu de la méthode des moments par une méthode basée sur les sous-espaces de Krylov. En ce qui concerne la réduction de la taille du problème, une technique d’homogénéisation est utilisée pour transformer la condition aux limites posée sur lasurface rugueuse par des paramètres effectifs. Ces paramètres sont déterminés par des problèmes auxiliaires qui tiennent compte des échelles fines de la surface. Dans le cas de surfaces parfaitement métalliques, la procédure est appliquée en polarisation Transverse Magnétique (TM) et Transverse Électrique (TE). Une impédance équivalente de Léontovich d’ordre 1 est déduite.Le procédure est automatique et les ordres supérieurs sont dérivés pour la polarisation TM. La procédure d’homogénéisation est aussi appliquée pour des interfaces rugueuses séparant deux milieux diélectriques. En ce qui concerne la réduction du nombre d’itérations, un préconditionneur, basé sur des considérations physiques, est construit à partir des modes de Floquet. Bien que le préconditionneur soit initialement élaboré pour des surfaces périodiques, nous montrons qu’il est aussi efficace pour des surfaces tronquées éclairées par une onde plane. L’efficacité des deux aspects présentés dans cette thèse est numériquement illustrée pour des configurations d’intérêtThis work is about the scattering by monodimensional rough surfaces. Surfaces presenting small scales of variations need a very refined mesh to finally capture the scattering field behaviour what increases the computational cost. Two aspects are considered : the reduction of the problemsize through an effective boundary condition incorporating the effect of rapid variations and the reduction of the number of iterations to solve the linear system arising from method of moments by a method based on Krylov subspace. Firstly, an homogenization process is used to convert the boundary condition on the rough interface into effective parameters. These parameters are determined by the solutions of auxiliary problems which involve the detailed profile of the interface. In the case of perfectly metallic surfaces, the process is applied to the E- and H-polarization and an Leontovich impedance of order 1 is deduced. The process is automatic and higher orders are derived for E-polarization. The homogenization process is also applied to dielectric rough interfaces. Secondly, a physically-based preconditioner is built with Floquet’s modes. Although the preconditioner has been designed for periodical surfaces, it was shown to be efficient in the case of truncated surfaces illuminated by a plane wave. The efficiency of both aspects is numerically illustrated for some configurations of interest
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Kubwimana, Thierry. "Simulation de l'écoulement atmosphérique au voisinage d'une tête de tunnel." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEC023.
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La conception d’un système de ventilation mécanique dans un tunnel nécessite de recenser tous les phénomènes physiques mis en jeu dans le mouvement de l’air dans le tunnel. Et ceci afin d’établir les capacités de ventilation nécessaires au regard d’objectifs règlementaires. On peut compter parmi ces phénomènes les effets atmosphériques, et notamment l’effet du vent, susceptible de générer des surpressions ou dépressions à proximité des ouvertures d’un tunnel et par conséquent d’induire ou de modifier un courant d’air établi à l’intérieur de celui-ci. Le présent travail entend contribuer à une meilleure compréhension ainsi qu’à une meilleure prise en compte de l’écoulement atmosphérique extérieur dans les études de ventilation de tunnel.Modélisations expérimentale et numérique ont été mises en œuvre pour cela. Des essais en soufflerie ont été menés dans la soufflerie atmosphérique de l’École Centrale de Lyon et ont fait appel à différentes techniques (PIV, anémométrie à fils chauds, micromanomètre) pour mesurer les caractéristiques moyennes et turbulentes de l’écoulement atmosphérique au voisinage d’un tunnel. Et nous avons également employé les approches numériques moyenne (RANS) et filtrée (LES) pour simuler l’écoulement atmosphérique autour d’un tunnel.La représentation de l’écoulement atmosphérique, instationnaire et turbulent, en entrée d’un domaine de calcul LES pose des difficultés. Nous avons, au cours de ce travail, implémenté un générateur synthétique de conditions amont dans le code de calcul FLUENT et, à l’appui des résultats expérimentaux, établi le paramétrage optimal d’une simulation LES de couche limite atmosphérique pleinement rugueuse.Deux configurations de tunnel ont ensuite été étudiées par voies numérique et expérimentale. Dans une première série d’essais, le champ de pression sur la tête d’un tunnel assimilée à la section frontale d’une cavité parallélépipédique a été étudié. Les comparaisons entre les différentes approches ont mis en évidence l’influence de la géométrie du tunnel et du bâti environnant, ainsi que la meilleure performance de l’approche LES dans la caractérisation de l’écoulement turbulent. Et dans une deuxième série d’essais, nous nous sommes rapprochés d’une configuration réelle et avons instrumenté une maquette de tête de tunnel ouverte dans lequel nous pouvions créer un courant d’air dirigé vers l’intérieur ou l’extérieur de l’ouvrage. Les résultats ont montré une interaction importante entre la couche limite atmosphérique et le jet pariétal tridimensionnel issu du tunnelThe design of a mechanical ventilation system in a tunnel requires to identify all the physical phenomena involved in the movement of the air in the tunnel. That is in order to establish the necessary ventilation capacities with regard to regulatory objectives. Atmospheric effects feature among the mechanisms likely to generate overpressures or depressions near the openings of a tunnel and consequently to induce or to modify the airflow established inside. This research work intends to contribute to a better understanding as well as a better consideration of the external atmospheric effects in tunnel ventilation studies.Experimental and numerical modeling have been completed. Wind tunnel tests were carried out in the atmospheric wind tunnel of the École Centrale de Lyon and used different techniques (PIV, hot wire anemometry, micromanometer) to measure the mean and turbulent statistics of the atmospheric flow in the vicinity of a tunnel. Time averaged (RANS) and filtered (LES) turbulence models were also used to simulate the atmospheric flow around a tunnel.The suitable representation of the unsteady turbulent atmospheric flow at the inlet of an LES computational domain remains an issue. During this work, we implemented a synthetic turbulence generator in the CFD code Fluent and, through comparison with experimental data, derived the optimal setup for the simulation of a fully rough atmospheric boundary layer.Thereafter, two tunnel configurations were studied by numerical and experimental means. In a first series of tests, the pressure field at the front section of a rectangular cavity was studied. The comparisons between the different approaches highlighted the influence of the geometry of the tunnel and the arrangement of the surrounding urban-like environment, as well as a better performance of the LES model in the description the turbulent flow. And in a second series of tests, we got closer to a realistic configuration and instrumented an open tunnel in which we could create an airflow directed towards the outside or the inside of the structure. The results showed a significant interaction between the atmospheric boundary layer and the three-dimensional wall jet from the tunnel
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Rouzès, Maxime. "Etude expérimentale de l'hydrodynamique d'un écoulement turbulent à surface libre sur fond rugueux à faible submersion." Phd thesis, Toulouse, INPT, 2015. http://oatao.univ-toulouse.fr/14164/1/rouzes.pdf.
Повний текст джерелаАнотація:
L'étude concerne les couches limites turbulentes dans le cas d'écoulements à surface libre sur fond rugueux homogène. Afin de réaliser cette étude, deux dispositifs de mesure PIV par stéréoscopie (PIV 2D-3C) ont été mis en place avec comme double objectif de fournir les lignes directrices au design d'un système stéréoscopique PIV in situ et d'étudier l'influence de la faible submersion des éléments rugueux sur la structure universelle de la couche limite sur fond rugueux, i.e. pour des submersions h/D comprises entre 0,33 et 0,66 (avec h la hauteur des rugosités et D la hauteur d'eau). Pour le premier objectif, le dispositif de mesure a permis un accès optique facilité par un point de vue incliné des caméras à la zone proche des rugosités dans des conditions naturelles d'écoulement (turbidité et éclairement naturels). Les mesures de vitesse ont été faites dans un canal hydraulique de petite dimension (12 m x 0,5 m x 0,25 m) rempli d'hémisphères positionnées en quinconce. Une étude paramétrique de l'influence de l'inclinaison des caméras ainsi que de la turbidité de l'eau sur la qualité des mesures de vitesse a été entreprise suivie par une nouvelle méthodologie basée sur l'analyse de l'intensité lumineuse dans le système. Il a été montré que l'écoulement est correctement résolu jusqu'à une turbidité d'environ 25 NTU avec un angle d'inclinaison par rapport au plan vertical de mesure de 25°. Pour le second objectif, les investigations expérimentales ont été réalisées dans une veine hydraulique de plus grande dimension (26 m x 1,10 m x 0,50 m), dont le fond rugueux est constitué par des cubes en PVC de 2 cm de côté comme dans l'étude de Florens et al. (2013). Les résultats mettent en évidence que l'étendue de la sous-couche rugueuse augmente avec la submersion pour finalement occuper toute la colonne d'eau dans le cas de la plus faible submersion (h/D=0,66). Malgré cela, une loi logarithmique est tout de même observée, et ce, quelle que soit la submersion étudiée.
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Toussaint, Damien. "Couche limite turbulente sur paroi rugueuse : étude expérimentale et modélisation." Thesis, Toulouse, INPT, 2020. http://www.theses.fr/2020INPT0122.
Повний текст джерелаАнотація:
Les écoulements sur parois rugueuses apparaissent dans de nombreuses situations environnementales ou industrielles, notamment dans l'aéronautique. Par rapport au cas idéal d'une paroi lisse, une augmentation de traînée et du flux de chaleur pariétal, susceptible de dégrader les performances d'un système (aile, machine tournante...), est induite par les rugosités. La prise en compte des effets des rugosités est ainsi souhaitable dans tout processus de conception. Plusieurs approches permettent de modéliser les effets de ces rugosités. Dans le cadre de cette étude, on considère la Méthode par Eléments Discrets Rugueux (DERM). Celle-ci, dont l'objectif est représenter de manière moyennée l'influence d'une paroi rugueuse sur un écoulement, est obtenue en appliquant une moyenne de volume et d'ensemble aux équations de Navier-Stokes. De nouveaux termes, notamment de traînée des rugosités et de tensions turbulentes et dispersives, apparaissent alors dans les équations. Cette étude consiste donc à proposer des pistes de fermeture pour ces termes. Pour ce faire, une campagne de mesures par LDV est réalisée afin de caractériser un écoulement de couche limite turbulente sur paroi rugueuse. Un accent particulier est accordé à l'obtention de mesures au plus près des rugosités, notamment dans leurs sillages, là où la littérature est pauvre en données. Une base de données numérique d'écoulements de canal turbulents sur paroi rugueuse est également constituée à l'aide de simulations RANS réalisées sur des rugosités résolues. Les résultats numériques sont confrontés aux mesures afin de s'assurer qu'ils sont bien représentatifs de l'écoulement réel. Ils sont enfin analysés afin de mettre en lumière le comportement des termes de l'approche DERM et de construire des modèles de fermetureTurbulent flows over rough walls can be found in various environmental and engineering situations. Focusing on aeronautical applications, unwanted rough surfaces are likely to decrease the overall efficiency of a system, skin friction, and heat transfer coefficients being generally increased compared to ideal smooth-wall situations. A careful account of the aerodynamic effects induced by surface roughness is therefore important in a design process, and several prediction strategies may be considered. The Discrete Element Roughness Method (DERM) is considered in the present study. This approach aims at resolving the averaged roughness effects and is derived by ensemble- and volume-averaging the Navier–Stokes equations, yielding three unknown terms in the momentum equation: the Reynolds stress and dispersive stress tensors and the average drag force acting on the roughness elements. The present work aims at proposing guidance for their modelling. First, a LDV measurement campaign was conducted to investigate turbulent boundary layer flows over rough surfaces. Measurements were especially performed within the roughness wake regions, that are not extensively investigated in the literature. Second, RANS simulations of turbulent channel flows over resolved rough surfaces were performed. The numerical results were then compared to the experimental data to assess the validity of the RANS simulations. Finally, these numerical results were analysed to highlight the behavior of the specific DERM terms and propose guidance for their modeling
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Salizzoni, Pietro. "Mass and momentum transfer in the urban boundary layer." Ecully, Ecole centrale de Lyon, 2006. http://www.theses.fr/2006ECDL0015.
Повний текст джерелаАнотація:
The dispersion of pollutants in the atmosphere in urban areas is a complex process which depends on several physical phenomena. The present work analyzes the mechanisms of the mass and momentum exchange in the urban atmospheric boundary layer. In particular we focused our attention on the lower part of the atmospheric boundary layer, where the °ow dynamics are typically determined by the size and the density of the buildings and by the street geometry. In order to analyze these processes we have performed a wind tunnel investigation of the °ow dynamics and scalar dispersion in the near-ground region of a neutral atmospheric boundary layer. An idealized street geometry was simulated by an array of 2D parallel canyons, made of a set of square section bars placed normal to the wind; the spacing of the bars (i. E. The ratio H=W between building height and canyon width) could be varied. The velocity measurements have been performed by means of hot-wire anemometry and Particle Image velocimetry (PIV) whilst passive scalar concentration measurements have been performed with a Flame Ionization Detector (FID). In the ¯rst part of the work we studied the in°uence of small scale roughness (roof shape, chimney. . . . ) at the top of the buildings on the °ow and the dispersion in the turbulent stream above buildings roofs. The in°uence of the roof roughness was studied by adding small scale 2D roughness elements to the tops of the bars. In order to evaluate the mass and momentum exchange in the boundary layer above the obstacles, di®erent experiments were conducted for each geometrical con¯guration: the pro¯les of mean and °uctuating velocities were measured above the obstacle roof; a passive scalar was released from a an elevated line source and from a ground level source, and concentration pro¯les were measured downstream of the source. We veri¯ed that the presence of a smaller scale roughness is felt by the overlying °ow only if the larger scale obstacles are su±ciently packed together. The smaller scale structures produced by the small scale roughness in°uence the °ow dynamics if their size is the same order of that of the eddies shed by the shear layer developing at the canopy top: that happens if the canyon width is not too large (i. E. For street aspect ratio H=W » 1). In the second part of the work we focused on the processes that determine the mass exchange between the recirculating region within the street canyons and the external °ow. The goal of the study was to evaluate how di®erent conditions within and outside the cavity determine the velocity and concentration ¯elds within the cavity itself; the aim was to ¯nd the appropriate reference velocity and length scales that characterize the mass exchange between the recirculating region and the external °ow. We veri¯ed that the exchange processes are dependent on the canyon geometry as well as on the intensity of the external turbulence, but are not sensitive to the external integral length scale. As a general conclusion we may say that the mass and momentum exchange between a recirculating region and the external °ow is a process which is driven by the °ow instabilities, arising within the shear layer which develops at the interface between the two region, and it is in°uenced even by the turbulent kinetic energy °uxes from the external °ow toward the cavity.
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Dietz, Otto. "Linear and non-linear properties of light." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://dx.doi.org/10.18452/17474.
Повний текст джерелаАнотація:
Alle optischen Systeme haben den gleichen Zweck: Sie manipulieren Eigenschaften des Lichts, durch Interaktion mit Materie. In dieser Arbeit werden zwei wichtige Teilaspekte aus diesem Kontext untersucht, im linearen und im nicht-linearen Bereich. In Teil I werden die bekannten Bragg-Reflexionen in neuem Licht betrachtet. Bragg Reflexion findet statt, wenn Licht mit einem periodischen Medium interagiert. Die Bragg-Bedingung verknüpft den Gitterabstand in einem Kristall mit der Wellenlänge, die von ihm reflektiert wird. In dieser Arbeit werden die Bragg Reflexionen in gewellten Wellenleitern untersucht. Es wird gezeigt, dass die Bragg-Bedingung nicht ausreicht, um die Streuung in diesen Wellenleitern zu verstehen. Es wird numerisch und analytisch demonstriert, dass unebene Ränder eine neue Reflexionsbedingung schaffen, die über das einfache Bragg-Bild hinausgeht. Dieser Streueffekt, der Square Gradient Bragg-Mechanismus ist aus statistischen Streuansätzen bekannt. Er hängt mit der Krüummung des Randes zusammen und hat einen starken Einfluss auf die Wellenleitung in diesen Systemen. In dieser Arbeit wird die erste allgemeine Theorie für den Square Gradient Bragg Streumechanismus vorgestellt, die es ermöglicht, Voraussagen für einzelne Wellenleiter mit beliebig deformierten Rändern zu treffen. Eine weitere wichtige Eigenschaft des Lichts wird in Teil II dieser Arbeit untersucht: Die Verschränkung zwischen zwei Photonen. Verschränkung ist ein intuitiv nicht verständliches Phänomen, weil es in der uns umgebenden klassischen Welt kein Analogon hat. Insbesondere verletzt es unsere implizite Annahme eines lokalen Realismus, weil voneinander entfernte Teilchen scheinbar instantan miteinander wechselwirken können. In dieser Arbeit wird eine neue und verstimmbare Quelle für verschränkte Photonen entworfen. Die Photonenpaare werden in nicht-linearen Kristallen erzeugt, aber ihre Verschränkung wird rein geometrisch erzwungen. Dieser geometrische Ansatz erlaubt es, die Frequenz der Photonen einzustellen. Hier übertrifft diese neue Quelle ihre Vorgänger, die ausführlich besprochen werden. Die Verschränkung der erzeugten Photonen wird experimentell nachgewiesen.Any optical experiment, any optical technology is only about one thing: Manipulating the properties of light through interaction with matter. This thesis will address two important issues in this broad context, in the linear and in the non-linear regime. In Part I, the well-known Bragg reflection is revised. Bragg reflection takes place whenever light interacts with a periodic structure. The famous Bragg condition relates the lattice spacing in a crystal to the wavelength which is effectively reflected by that lattice. In this thesis the Bragg reflection in dielectric waveguides is investigated. It is shown that the Bragg condition is not sufficient to describe the scattering situation in waveguides with corrugated boundaries. It is demonstrated, analytically and numerically, that corrugated boundaries cause a new type of reflection condition, which goes beyond the Bragg picture. This scattering mechanism, the Square Gradient Bragg Scattering, is known from statistical scattering approaches. It is connected to the curvature of the boundary and has a strong influence on the wave propagation in these systems. Here the first general theory for Square Gradient Bragg Scattering is presented, which allows for making predictions for single corrugated waveguides with arbitrary boundaries. Another important property of light is investigated in Part II of this thesis: The entanglement of two photons. Entanglement is a counter-intuitive phenomenon, because it has no classical analogy. It especially violates our assumption of local realism, because distant particles seemingly act on each other instantaneously. In this thesis a new tunable and portable source of photon pairs is designed. The photon pairs are created in non-linear crystals, but their entanglement is enforced in a purely geometrical manner. This geometrical approach makes the setup tunable. This is where the new design supersedes its predecessor, which will be discussed in detail. The entanglement of the generated photons is demonstrated experimentally.
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Introïni, Clément. "Interaction entre un fluide à haute température et un béton : contribution à la modélisation des échanges de masse et de chaleur." Thesis, Toulouse, INPT, 2010. http://www.theses.fr/2010INPT0074/document.
Повний текст джерелаАнотація:
Lors d'un hypothétique accident grave de réacteur à eau sous pression, un mélange de matériaux fondus, appelé corium, issu de la fusion du cœur peut se relocaliser dans le puits de cuve constitué par un radier en béton. Les codes d'évaluation réacteur pour simuler la phénoménologie de l'interaction corium-béton sont basés sur une description à grande échelle des échanges qui soulève de nombreuses questions, tant sur la prise en compte des phénomènes multi-échelles mis en jeu que sur la structure adoptée de la couche limite au voisinage du front d'ablation. Dans ce contexte, l'objectif principal de ce travail consiste à aborder le problème de la structure de la couche limite par simulation numérique directe. Ce travail s'inscrit dans le cadre plus général d'une description et d'une modélisation multi-échelle des échanges, c'est-à-dire de l'échelle locale associée au voisinage du front d'ablation jusqu'à l'échelle du code d'évaluation réacteur. Une telle description multi-échelle des échanges soulève le problème de la description locale de l'écoulement multiphasique multiconstituant mais aussi le problème du changement d'échelle et en particulier le passage de l'échelle locale à l'échelle de description supérieure dite macroscopique associée aux mouvements convectifs dans le bain de corium. Parmi les difficultés associées au changement d'échelle, nous nous intéressons à la problématique de la construction de conditions aux limites effectives ou lois de parois pour les modèles macroscopiques. Devant la complexité du problème multiphasique multiconstituant posé au voisinage du front, cette contribution a été abordée sur un problème modèle. Des conditions aux limites dites effectives ont été construites dans le cadre d'une méthode de décomposition de domaine puis testées pour un problème d'écoulement laminaire de convection naturelle sur parois rugueuses. Mˆeme si le problème traité reste encore éloigné des applications visées, cette contribution offre de nombreuses perspectives et constitue une première étape d'une modélisation multiéchelle des échanges pour la problématique de l'interaction corium-béton. Dans le cas plus complexe des écoulements multiphasiques multiconstituants et devant les difficultés expérimentales associées, le développement de lois de parois pour les outils existants aux échelles de description supérieures nécessite, au préalable, de disposer d'un outil de simulation numérique directe de l'écoulement au voisinage du front d'ablation. L'outil développé dans ce travail correspond à un modèle de Cahn-Hilliard/Navier-Stokes pour un mélange diphasique (liquide-gaz) compositionnel (corium-béton fondu) s'appuyant sur une description du système selon trois paramètres d'ordre associés respectivement aux fractions volumiques du gaz et aux deux espèces miscibles de la phase liquide ainsi que sur une décomposition de l'énergie libre selon une contribution diphasique et compositionnelle. Les équations de transport sont dérivées dans le cadre de la thermodynamique des processus irréversibles et résolues sur la base d'une application éléments finis de la plate-forme PELICANS. Plusieurs expériences numériques illustrent la validité et les potentialités d'application de cet outil sur des problèmes diphasiques et/ou compositionnels. Enfin, à partir de l'outil développé, nous abordons par simulation numérique directe une étude de la structure de la couche limite au voisinage du front d'ablation pour des bétons siliceux et silico-calcaireIn the late phases of some scenario of hypothetical severe accident in Pressurized Water Reactors, a molten mixture of core and vessel structures, called corium, comes to interact with the concrete basemat. The safety numerical tools are lumped parameter codes. They are based on a large averaged description of heat and mass transfers which raises some uncertainties about the multi-scale description of the exchanges but also about the adopted boundary layer structure in the vicinity of the ablation front. In this context, the aim of this work is to tackle the problem of the boundary layer structure by means of direct numerical simulation. This work joins within the more general framework of a multi-scale description and a multi-scale modeling, namely from the local scale associated with the vicinity of the ablation front to the scale associated with the lumped parameter codes. Such a multi-scale description raises not only the problem of the local description of the multiphase multicomponent flow but also the problem of the upscaling between the local- and the macro-scale which is associated with the convective structures within the pool of corium. Here, we are particularly interested in the building of effective boundary conditions or wall laws for macro-scale models. The difficulty of the multiphase multicomponent problem at the local scale leads us to consider a relatively simplified problem. Effective boundary conditions are built in the frame of a domain decomposition method and numerical experiments are performed for a natural convection problem in a stamp shaped cavity to assess the validity of the proposed wall laws. Even if the treated problem is still far from the target applications, this contribution can be viewed as a first step of a multi-scale modeling of the exchanges for the molten core concrete issue. In the more complicated case of multiphase multicomponent flows, it is necessary to have a direct numerical simulation tool of the flow at the local scale to build wall laws for macro-scale models. Here, the developed tool corresponds to a Cahn-Hilliard/Navier-Stokes model for a two-phase compositional system. It relies on a description of the system by three volume fractions and on a free energy composed by a two-phase part and a compositional part. The governing equations are derived in the frame of the thermodynamic of irreversible processes. They are solved on the basis of a finite element application of the object-oriented software component library PELICANS. Several numerical experiments illustrate the validity and the potentialities of application of this tool on two-phase compositional problems. Finally, using the developed tool, we tackle by means of direct numerical simulation the problem boundary layer structure in the vicinity of the ablation front for limestone-sand and siliceous concretes
45
Bey, Mohamed Amine. "Modélisation mathématique et simulations numériques des écoulements sanguins dans des artères avec ou sans stents." Thesis, Sorbonne Paris Cité, 2015. http://www.theses.fr/2015USPCD027/document.
Повний текст джерелаАнотація:
Cette thèse est consacrée à la modélisation mathématique et simulations numériques des écoulements sanguins dans des artères en présence d’une endoprothèse vasculaire de type stent. La présence de stent peut être considérée comme une perturbation locale d’un bord lisse d’écoulement, plus précisément les parois de l’artère sont assimilées à une surface fortement rugueuse. Nous nous sommes principalement intéressés au contrôle de la régularité H² sur un modèle simplifié permettant de prendre en compte l’effet de ces stents lorsque le flux sanguin est gouverné par une équation de Laplace (en lien avec la composante axiale de la vitesse d’écoulement) avec une condition aux limites de type Dirichlet, dans un domaine à bord rugueux (en fonction d’un petit paramètre ε). Dans une première partie, nous soulevons la question d’existence et d’unicité de la solution de ce modèle d’écoulement sanguin et nous traitons la régularité H² par des techniques d’analyse variationnelle. Une étude minutieuse permet de contrôler la régularité H² en O(ε−1). Le deuxième axe est dédié à l’étude de la régularité H² par des analyse asymptotiques multiéchelles. Nous montrons que la norme H² de la solution de ce modèle d’écoulement sanguin est singulière en O(ε−½ ). D’autre part, nous améliorons les ordres de convergence des résultats existants concernant la construction des approximations multiéchelles. Dans un troisième temps, nous présentons des estimations d’erreur et des résultats numériques. Ces résultats illustrent le bien fondé des estimations d’erreur sur le plan pratique. Nous montrons bien l’importance des méthodes asymptotiques qui se révèlent plus efficaces qu’un calcul directThis thesis is devoted to mathematical modeling and numerical simulations of the blood-flows in arteries in the presence of a vascular prosthesis of type stent. The presence of stent can be considered as a local perturbation of a smooth edge of flow, more precisely the walls artery can be seen as a strongly rough surface.Weare mainly interested in controlling the H² regularity of a simplified model which takes into account the impact of these stents when the blood flow is controlled by a Laplace equation (in link with the axial component rateof flow) with a Dirichlet boundary condition, in a domain with a rough board (according to a small parameter ε). First, we raise the question of existence and unicity of the solution of this model of blood-flow and we study the H² regularity using variational analysis methods. By a detailed study, we control the H² regularity of order O(ε−1). The second part is devoted to the study of the regularity H² regularity using multi-scale analysis.We prove that the H² norm of the solution of this model is singular of order O(ε−½). Moreover, we improve the convergence rate of the existing results on the construction of the multi-scale approximation. Finally, we present an error estimation and numerical results. These numerical results illustrate the well-founded of the error estimates on a practical level. We show the importance of the asymptotic methods that seem to be more effective than a direct computation
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SU, ZI-YI, and 蘇子義. "Rough turbulent boundary layer under short-crested waves." Thesis, 1991. http://ndltd.ncl.edu.tw/handle/70815213839789348475.
Повний текст джерела
47
Shah, Mohammad Khalid. "Skin friction characteristics in smooth-wall, transitionally rough and fully rough turbulent boundary layer." 2004. http://hdl.handle.net/1993/15762.
Повний текст джерела
48
YUAN, JUNLIN. "Large-Eddy Simulations of Accelerating Boundary Layer Flows Over Rough Surfaces." Thesis, 2011. http://hdl.handle.net/1974/6846.
Повний текст джерелаАнотація:
Large-eddy simulations are carried out to study the combined
effects of roughness and favourable pressure gradient in boundary layer flows,
where the high acceleration (on smooth walls) may cause flow reversion to the
quasi-laminar state. A sand-grain roughness model is used, with the
no-slip boundary condition modeled by an immersed boundary method.
The properties and accuracies of the scheme are studied,
the roughness model is validated, and the spatial-resolution requirements are
determined.
The roughness model is applied to boundary layers subject to mild or strong
acceleration, with simulations carried out underlining the effects of
three parameters: the acceleration parameter, the roughness height,
and the inlet Reynolds number.
The roughness effects are limited to the roughness sublayer; the outer layer is
affected indirectly only, through the changes that roughness causes in the
relaminarization and retransition processes.
The roughness significantly affects the inner-layer
quantities like the friction velocity and the friction coefficient, while
the local Reynolds number, the outer-layer mean velocity, as well as the Reynolds
stresses beyond the roughness sublayer, are not sensitive to the roughness.
The acceleration decreases the Reynolds stresses in the
overlap region and promotes a laminar-like velocity profile.
The acceleration leads to stabilization of near-wall structures and causes
one-dimensional turbulence. The roughness generates small-scale structures
at the bottom wall, which disturb the larger structures originally
stabilized by the pressure gradient, leading to a decrease in the Reynolds-stress
anisotropy. Roughness increases the Reynolds stresses in the roughness sublayer
and tends to restore the fully turbulence flow early. The inlet Reynolds number
affects the flow stability by determining the viscous length scale
compared to the roughness length scales, and by determining how far
the roughness effect extents into the boundary layer.Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2011-10-17 11:19:08.063
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Morgan, Jonathan Philip. "Linear and Non-linear Interactions in a Rough-Wall Turbulent Boundary Layer." Thesis, 2019. https://thesis.library.caltech.edu/11453/17/Jonathan-Morgan-final-thesis-submission.pdf.
Повний текст джерелаАнотація:
This thesis explores the linear and non-linear interactions which take place in a rough-wall turbulent boundary through experiments and modeling. In order to derive physics-based models for the relation between roughness geometry and flow physics, two very simple periodic roughnesses were 3D printed and placed in a boundary layer wind tunnel for separate experiments. Hot-wire measurements were taken at a grid of points within a single period of the roughness in order to map the spatial variation of important flow statistics in way that allows correlation back to the roughness geometry. Time averaged streamwise velocity and the power spectrum of instantaneous streamwise velocity were both found to vary coherently with the roughness. The spatial variation of the time averaged velocity was identified as the linear result of the roughness, as it has identical wavenumber and frequency to the static roughness geometry. Modeling the time-averaged velocity field as a response mode of the linear resolvent operator was found to be reasonable for certain wavenumbers. The spatial distribution of the power spectrum was shown to be a non-linear effect of the roughness; the power spectrum only measures the energy of convecting modes, which necessarily have non-zero frequency and cannot correlate linearly to the static roughness. The spatial modulation of the power spectrum was found to be indicative of non-linear triadic interactions between the static velocity Fourier modes and pairs of convecting modes, as allowed by the Navier-Stokes equations. A low-order model for these interactions, and their effect on the power spectrum, was constructed using resolvent response modes to represent all velocity Fourier modes. The model was found to qualitatively predict the modulation of the power spectrum for several sets of wavenumbers. The success of such a simple model suggests that it presents a useful low-order understanding of non-linear forcing between scales in rough-wall boundary layers.
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Joshi, Sumedh Mohan. "Quantifying three dimensional effects in acoustic rough surface scattering." Thesis, 2011. http://hdl.handle.net/2152/ETD-UT-2011-05-3377.
Повний текст джерелаАнотація:
Interface roughness can have a significant effect on the scattering of sound energy, and therefore an understanding of the effects of roughness is essential
to making predictions of sound propagation and transmission underwater. Many models of roughness scattering currently in use are two dimensional
(2D) in nature; three dimensional (3D) modeling requires significantly more
time and computational resources. In this work, an effort is made to quantify the effects of 3D scattering in order to assess whether or under what conditions
3D modeling is necessary. To that end, an exact 3D roughness scattering model is developed based on a commercially available finite element package. The finite element results are compared with two approximate scattering models (the Kirchhoff approximation and first order perturbation theory) to establish the validity and regimes of applicability of each. The rough surfaces are realizations generated from power spectra measured from the sea floor. However, the surfaces are assumed to be pressure release (as on an air-water interface). Such a formulation is nonphysical, but allows the assessment of the validity of
the various modeling techniques which is the focus of this work. The comparison between the models is made by calculating the ensemble average of the scattering from realizations of randomly rough surfaces. It is shown that a
combination of the Kirchhoff approximation and perturbation theory models recovers the 3D finite element solution.text