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1
Manca, Eleonora. "Effects of Posidonia oceanica seagrass on nearshore waves and wave-induced flows." Thesis, University of Southampton, 2010. https://eprints.soton.ac.uk/195257/.
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This work focuses on the effects of the large Mediterranean seagrass Posidonia oceanica on coastal waves and wave-induced flows, which has significant implications for coastal protection. Investigations were made on both a natural shallow Posidonia oceanica bed and, in controlled conditions of full-scale Posidonia mimics under regular and irregular waves. In the field, waves and currents were monitored during low energy conditions and a Mistral wind event. Data were collected on the distribution of Posidonia patches, density and canopy height, as well as bed sediment type and bathymetry. In the flume, measurements were made of water surface elevation along the flume and oscillatory flows at 3 locations and 4 elevations, under several wave conditions, water depths and for 2 canopy densities. The mimics were designed carefully to recreate the hydraulic behaviour of Posidonia plants under waves. Field results indicate that shallow Posidonia meadows are effective at reducing wave energy under low wave energy conditions and small wave amplitudes. The flume experiments confirm this trend. Under both regular and irregular waves, drag coefficients decrease with increasing Reynolds vegetation numbers; wave dissipation factors decrease with wave orbital amplitude. Under spectral waves, most wave energy dissipation occurs at the peak spectral frequency and it is largest for the least energetic wave spectra. At high wave Reynolds numbers, the canopyinduced hydraulic roughness (r) appears to be a function of the canopy element density only, and the empirical formula of Nielsen (1992) is successfully applied. However, more work is required in low energy conditions to examine the range of validity of the formula. In natural conditions under small amplitude waves, attenuation of wave-induced flows is negligible in the upper canopy; flume experiments confirm this trend. The typical flow intensification at the canopy top, measured for other seagrasses, occurs only for tests with the largest wave amplitudes, whilst, under smaller waves, flow intensification is located within the upper part of the canopy. In the lower canopy, flows are always reduced and flows decelerate exponentially with increasing orbital amplitude. This is a novel observation in flexible canopies. The artificial canopy, like the natural Posidonia bed, enhances flow asymmetries at the canopy top, especially under waves with large wave orbital amplitudes. This is thought to be a mechanism to enhance shoreward drift. Turbulence in the artificial canopy, under regular waves, peaks at the canopy top, as occurs under unidirectional flows and for other seagrass beds exposed to waves. Vertical turbulent exchanges are enhanced at the edge of the seagrass patch and are larger for lower submergence ratios (the ratio of canopy height to water depth). A reduction in submergence ratio in the flume, also causes increased shear stresses at the top of the canopy, lower wave height decay and reduced oscillatory flow attenuation in the lower part of the canopy. The denser canopy, in the conditions tested, increases relative roughness (r/A), wave attenuation, in canopy oscillatory flow reduction and turbulent kinetic energy at the top of the canopy. Oscillatory flows characterised by small orbital amplitudes can penetrate further into the canopy than larger orbital velocities, inducing a larger drag, thus increasing wave dissipation, as proposed for rigid canopies (corals). This is manifested as a thinner canopy boundary layer under small orbital amplitude waves than the large amplitude waves. A conceptual model is proposed to summarise these findings. Under storm conditions Posidonia is believed to be less efficient at reducing wave energy, however it remains effective at reducing sediment transport locally and, by inducing a preferential shoreward drift, at preventing sand dispersal offshore
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Aldridge, Christopher John. "Density-wave oscillations in two-phase flows." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260741.
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Moroney, Gerard. "Internal wave wakes in stratified shear flows." Thesis, University of Liverpool, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399177.
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Wikramanayake, Palitha Nalin. "Turbulent wave-current bottom boundary layer flows." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/14353.
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Stephen, Adam Vercingetorix. "POD methods in baroclinic flows." Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.302401.
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Dominy, Robert Gerald. "Rarefied hypersonic shock wave and blunt body flows." Thesis, Imperial College London, 1988. http://hdl.handle.net/10044/1/47034.
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Ingram, David M. "Numerical prediction of blast wave flows around rigid structures." Thesis, Manchester Metropolitan University, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.332898.
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Forster, Graham Keith. "Instability and wave-growth within some oscillatory fluid flows." Thesis, University of St Andrews, 1996. http://hdl.handle.net/10023/14087.
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Oscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.
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Mavromoustaki, Aliki. "Long-wave dynamics of single- and two-layer flows." Thesis, Imperial College London, 2011. http://hdl.handle.net/10044/1/6452.
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Thin-film flows are central to a number of industrial, biomedical and daily-life applications, which include coating flow technology, enhanced oil recovery, microfluidics, and surfactant replacement therapy. Though these systems have received a lot of attention in a variety of settings, the understanding of the dominant physics governing the flows is not completely thorough; this is especially true in cases where the free surface of the film or, in two-layer flows, the fluid-fluid interface is susceptible to instabilities leading to the break-up of the film and the formation of fingering patterns. The elucidation of the underlying mechanisms behind the onset of these instabilities is of utmost importance to several industrial processes. The work in this thesis focusses on modelling the dynamics of thin-film flows in the presence of complexities; the latter arise from the presence of surface-active chemicals and spatial confinement. The lubrication approximation, which is valid in the limit of small film aspect ratios, is used to simplify the governing equations; this facilitates the derivation of an evolution equation for the interfacial position. This methodology is employed extensively in the present thesis to examine co- and counter-current two-layer flows in a closed, rectangular channel and the dynamics of a thin film laden with surfactant, driven to climb up an inclined substrate. In the two-fluid case, the dynamics of the flow are described by a single, two-dimensional, fourth-order nonlinear partial differential equation. Analysis of the one-dimensional flow demonstrate the existence of travelling-wave solutions which take the form of Lax shocks, undercompressive shocks, and rarefaction waves. In unstably-stratified cases, a Rayleigh-Taylor mechanism spawns the formation of large-amplitude capillary waves. A wide range of parameters is studied, which include the density and viscosity ratios of the two fluids, the flow configuration (whether co- or counter-current), the heights of the films at the channel ends and the channel inclination. The stability of these structures to perturbations in the spanwise direction, is also examined through a linear stability analysis and transient, two-dimensional numerical simulations. These analyses demonstrate successfully that some of the structures observed in the one-dimensional flow are unstable to fingering phenomena. In the case of the climbing film, two configurations are examined, namely, constant flux and constant volume whereby the evolution equation for the interface is coupled to convective-diffusive equations for the concentration of surfactant, present in the form of monomers and micelles. The former are allowed to exist at the gas-liquid and liquid-solid interfaces, and in the bulk; the latter can only be present in the bulk. For the constant flux case, the flow is simulated by a continuously-fed uncontaminated fluid and surfactant at the flow origin allowed to spread on a solid substrate which has been prewetted by a thin, surfactant-free precursor layer. The constant volume configuration simulates the deposition of a finite drop, laden with surfactant, spreading on a thin, uncontaminated film. In the absence of spanwise disturbances, the one-dimensional solutions demonstrate how the climbing rate and the structural deformation of the film are influenced by gravity, and physico-chemical parameters such as surfactant concentration (whether above or below the critical micelle concentration), and rates of adsorption of monomers at the two interfaces. The stability of the flow is examined through linear theory and transient solutions of the full, nonlinear, two-dimensional system of equations revealing the growth of spanwise perturbations into full-length fingers. A brief introduction to the experimental design of an apparatus, aimed at validating channel flow results, is also described. The objective of the experiment was to investigate the physical features associated with the counter-current, pressure-driven flow of a gas-liquid system. Preliminary experimental results revealed that upon perturbing the flow, an initially uniform liquid film becomes unstable, resulting in the formation of fingers which elongated downstream as time progressed. Finally, we conclude with recommendations for future work, representing natural extensions to the theoretical work described in the present thesis.
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Hartmann, Axel [Verfasser]. "Experimental Analysis of Wave Propagation at Buffet Flows / Axel Hartmann." Aachen : Shaker, 2012. http://d-nb.info/106904623X/34.
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Jiang, Feng. "The development of linear wave packets in unbounded shear flows." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.385425.
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Maestri, Joseph. "Vortex-wave interactions and exact coherent structures in shear flows." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/51109.
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Recent studies of transition to turbulence in linearly stable shear flows have been dominated by finding, and characterising, so-called exact coherent structures which exist as either equilibrium or travelling wave solutions of the governing Navier-Stokes equations. These structures have been shown to act as either edge states between laminar and turbulent flow or become associated with attractors for turbulent flows, making the quest to understand these structures equally as mathematically interesting as it is industrially relevant. In this thesis, vortex-wave interaction theory, an asymptotic approach based on the assumption of large Reynolds number, is used as a means of finding such structures in plane Couette flow. A new, iterative, numerical method will be developed to solve the governing interaction equations that will allow us to circumvent many of the difficulties in calculating such structures (in particular at high Reynolds numbers) and perform analyses into the state-space of solutions, as well as explore their stability properties. A number of previously found exact coherent structures will be shown to be vortex-wave interaction states, including the so-called upper branch sinuous mode and the class of solutions called mirror-symmetric modes. A number of receptivity type problems will also be considered. Namely, the use of periodic blowing/suction on the channel walls and the introduction of variations in the wall shape. The effect of these two techniques on the skin friction drag and the wave amplitude will be discussed in the context of whether they are beneficial in terms of laminar flow control. Further, it will be shown that the use of periodic blowing/suction leads to a new class of synchronous mirror-symmetric modes and variations in the wall shape lead to a new class of inhomogeneous vortex-wave interaction states.
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Guarino, Maria Vittoria. "Mountain wave breaking in atmospheric flows with directional wind shear." Thesis, University of Reading, 2017. http://centaur.reading.ac.uk/75850/.
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In this thesis, mountain wave breaking triggered by directional wind shear is investigated using numerical simulations of idealized and semi-idealized orographic flows. Idealized simulations are used to produce a regime diagram to diagnose conditions for wave breaking in Richardson number-dimensionless mountain height parameter space. It is found that, in the presence of directional shear, wave breaking can occur over lower mountains than in a constant-wind case. Furthermore, the extent of regions within the simulation domain where Clear-Air Turbulence (CAT) is expected increases with terrain elevation and background wind shear intensity. Analysis of the model output, supported by theoretical arguments, suggest the existence of a link between wave breaking and the relative orientations of the incoming wind vector and the horizontal velocity perturbation vector. This condition provides a possible diagnostic for CAT forecast in directional shear flows. The stability of the flow to wave breaking in the transition from hydrostatic to nonhydrostatic mountain waves is also investigated. Wave breaking seems to be inhibited by non-hydrostatic effects for weak wind shear, but enhanced for stronger wind shear. In the second part of the thesis, a turbulence encounter observed over the Rocky Mountains (in Colorado, USA) is studied. The role of directional shear in causing wave breaking is isolated from other possible wave breaking mechanisms through various sensitivity tests. The existence of an asymptotic wake, as predicted by Shutts for directional shear flows, is hypothesized to be responsible for a significant downwind transport of unstable air detected in cross-sections of the flow. Finally, critical levels induced by directional shear are studied by spectral analysis of the horizontal velocity wave perturbations. This is done for a fully idealized flow and for the more realistic flow corresponding to the investigated turbulence encounter. In these 2D power spectra, a rotation of the most energetic wave modes with the background wind and their selective absorption can be found. Such behaviour is consistent with the mechanism leading to wave breaking in directional shear flows.
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Wikramanayake, Palitha Nalin. "Velocity profiles and suspended sediment transport in wave-current flows." Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/12542.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1993.Includes bibliographical references (leaves 225-230).
by Palitha Nalin Wikramanayake.
Ph.D.
15
Vieira, Rafael Fontes. "A numerical study on shock wave - boundary layer interaction flows." Instituto Tecnológico de Aeronáutica, 2013. http://www.bd.bibl.ita.br/tde_busca/arquivo.php?codArquivo=2883.
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This thesis addresses the important problem of shock wave--boundary layer interaction (SBLI) flows for aerospace engineering applications. Moreover, the work emphasizes the need for high fidelity simulations for the appropriate treatment of such flows. In this context, RANS solvers appear as a cost effective CFD approach. Therefore, the present work conducts studies in such a way to identify and to understand limitations, strengths and capabilities of RANS simulations for SBLI flows. Since turbulence modeling is an important issue on the accuracy of such simulations, the efforts here are concentrated on assessing the capabilities of several models that range from linear eddy-viscosity models (EVM) to Reynolds-stress closures (RSM). It would be expected that a RSM-type model could provide better solutions for a 3-D turbulent boundary layer under the action of high adverse pressure gradients, once such models allow for anisotropy between the Reynolds stress components. In order to achieve such goals, the configurations presented at the 2010 AIAA SBLI Workshop are chosen as the current test cases. Such test cases deal with high speed flows and very complex phenomena, including boundary layer separation. Meshes, composed of hexahedral and wedge elements, have been built. Mesh refinement and grid convergence studies are performed in order to identify a grid with a good compromise between accuracy and computational cost. In any event, even using the baseline grids, the present work has found that the computations are considerably expensive. Several simulations are presented for the test cases. Although no turbulence model has remarkably shown an outstanding performance over the others, the present work indicates that the SST and SA closures are the ones providing the best results for the test cases of interest here. Nonetheless, the two closures still present shortcomings in the simulation of SBLI flows. The overall simulation results using the RSM closure for the present SBLI test cases are not better than the SA and SST results. One must observe that the latter are much simpler turbulence models. Additional studies shall be focused on providing more robustness to the simulations with the 7-equation RSM turbulence model.
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Ford, Rupert. "Gravity wave generation by vortical flows in a rotating frame." Thesis, University of Cambridge, 1993. https://www.repository.cam.ac.uk/handle/1810/251624.
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Pinilla, Camilo Ernesto. "Numerical simulation of shear instability in shallow shear flows." Thesis, McGill University, 2008. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=115697.
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The instabilities of shallow shear flows are analyzed to study exchanges processes across shear flows in inland and coastal waters, coastal and ocean currents, and winds across the thermal-and-moisture fronts. These shear flows observed in nature are driven by gravity and governed by the shallow water equations (SWE). A highly accurate, and robust, computational scheme has been developed to solve these SWE. Time integration of the SWE was carried out using the fourth-order Runge-Kutta scheme. A third-order upwind bias finite difference approximation known as QUICK (Quadratic Upstream Interpolation of Convective Kinematics) was employed for the spatial discretization. The numerical oscillations were controlled using flux limiters for Total Variation Diminishing (TVD). Direct numerical simulations (DNS) were conducted for the base flow with the TANH velocity profile, and the base flow in the form of a jet with the SECH velocity profile. The depth across the base flows was selected for the' balance of the driving forces. In the rotating flow simulation, the Coriolis force in the lateral direction was perfectly in balance with the pressure gradient across the shear flow during the simulation. The development of instabilities in the shear flows was considered for a range of convective Froude number, friction number, and Rossby number. The DNS of the SWE has produced linear results that are consistent with classical stability analyses based on the normal mode approach, and new results that had not been determined by the classical method. The formation of eddies, and the generation of shocklets subsequent to the linear instabilities were computed as part of the DNS. Without modelling the small scales, the simulation was able to produce the correct turbulent spreading rate in agreement with the experimental observations. The simulations have identified radiation damping, in addition to friction damping, as a primary factor of influence on the instability of the shear flows admissible to waves. A convective Froude number correlated the energy lost due to radiation damping. The friction number determined the energy lost due to friction. A significant fraction of available energy produced by the shear flow is lost due the radiation of waves at high convective Froude number. This radiation of gravity waves in shallow gravity-stratified shear flow, and its dependence on the convective Froude number, is shown to be analogous to the Mach-number effect in compressible flow. Furthermore, and most significantly, is the discovery from the simulation the crucial role of the radiation damping in the development of shear flows in the rotating earth. Rings and eddies were produced by the rotating-flow simulations in a range of Rossby numbers, as they were observed in the Gulf Stream of the Atlantic, Jet Stream in the atmosphere, and various fronts across currents in coastal waters.
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Bura, Romie Oktovianus. "Laminar/transitional shock-wave/boundary-layer interactions (SWBLIs) in hypersonic flows." Thesis, University of Southampton, 2004. https://eprints.soton.ac.uk/47605/.
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Numerical investigations of laminar shock-wave/boundary-layer interactions (SWBLIs) in hypersonic flow have been carried out at M∞ = 6.85 and M∞ ≈ 8, with unit Reynolds numbers ranging from 2.0 x 106 m- l to 7.60 x 106 m- l. This thesis deals with a simplified 2-D geometric configuration to simulate SWBLIs on vehicle surfaces or engine intakes, i.e. the interaction of an oblique shock (produced by a wedge) impinging on an incoming laminar boundary-layer on an isothermal flat plate. The numerical simulations were performed with weak/moderate to strong shock. The results were compared with available theoretical and experimental results. Limited experimental work at M∞ = 6.85 for obtaining qualitative data were performed to provide the location of separation and re-attachment points using surface oil flow. Schlieren photographs were taken to provide the general flow features. A comprehensive analysis was performed on the 2-D numerical results with various Mach numbers, Reynolds numbers and shock strengths, to verify whether numerical solutions were able to confirm the established trends for the laminar free-interaction concept. An analysis was also performed using a well-established power-law relationship of pressure and heat flux in the region of interactions. An unstable first oblique mode disturbance was imposed with the strongest wedge angle, 9°, at M∞ = 6.85 and unit Reynolds number 2.45 x 106 m- l to determine the boundary-layer stability and its propensity to undergo transition in the linear regime. Several unsteady 3-D simulations were performed with varied parameters. Streamwise vortices were generated in all cases especially downstream of maximum separation bubble height. However, as the amplifications of the disturbance were quite small, transition was found to be unlikely at these conditions
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Murray, Neil Paul. "Three-dimensional turbulent shock-wave : boundary-layer interactions in hypersonic flows." Thesis, Imperial College London, 2007. http://hdl.handle.net/10044/1/7963.
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Weng, Chenyang. "Theoretical and numerical studies of sound propagation in low-Mach-number duct flows." Doctoral thesis, KTH, MWL Marcus Wallenberg Laboratoriet, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-168031.
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When sound waves propagate in a duct in the presence of turbulent flow, turbulent mixing can cause attenuation of the sound waves extra to that caused by the viscothermal effects. Experiments show that compared to the viscothermal effects, this turbulent absorption becomes the dominant contribution to the sound attenuation at sufficiently low frequencies. The mechanism of this turbulent absorption is attributed to the turbulent stress and the turbulent heat transfer acting on the coherent perturbations (including the sound waves) near the duct wall, i.e. sound-turbulence interaction. The purpose of the current investigation is to understand the mechanism of the sound-turbulence interaction in low-Mach-number internal flows by theoretical modeling and numerical simulations. The turbulence absorption can be modeled through perturbation turbulent Reynolds stresses and perturbation turbulent heat flux in the linearized perturbation equations. In this thesis, the linearized perturbation equations are reviewed, and different models for the turbulent absorption of the sound waves are investigated. A new non–equilibrium model for the perturbation turbulent Reynolds stress is also proposed. The proposed model is validated by comparing with experimental data from the literature, and with the data from Direct Numerical Simulations (DNS) of pulsating turbulent channel flow. Good agreement is observed.QC 20150526
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Thomas, Michael Dominic. "Nonlinear stability of flows over rigid and flexible boundaries." Thesis, University of St Andrews, 1990. http://hdl.handle.net/10023/14273.
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This work assesses the importance of nonlinearity in the stability of flows over compliant and rigid walls, and comprises three main parts. The first part considers inviscid flow with a free surface over a flexible boundary. The dispersion relation is obtained, and the conditions for linear instability investigated. The linear dispersion relation is then used to show that the conditions for nonlinear three-wave resonance are often met. In some circumstances, the resonance may be of 'explosive' sort, involving waves of opposite energy sign; but non-explosive resonant configurations are most common. Next, the wave- amplitude evolution equations for three-wave resonance are derived, firstly by a 'direct' approach, and then via a variational (averaged Lagrangian) method. Results agree with those of Case & Chiu (1977) for capillary-gravity waves, and Craik & Adam (1979), for three-layer fluid flow, on taking the appropriate limits. We also consider a nonlinear model for the flexible boundary. In the second part, stability of Blasius flow over a compliant surface is studied. This extension of rigid-wall work of Craik (1971) and Hendriks (appendix to Usher & Craik 1975) determines the quadratic interaction coefficients of three-wave resonance, and complements the linear analysis of Carpenter & Garrad (1985, 1986) and others. First, the linear eigenvalue spectrum is investigated for various values of the wall parameters. Then, resonant triads are located and the quadratic interaction coefficients determined numerically. By way of introduction some rigid-wall results are also presented, extending those of Hendriks.
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Campbell, Lucy J. "Nonlinear critical layer development of forced wave packets in geophysical shear flows." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ64529.pdf.
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Toomer, Christine Ann. "Weakley nonlinear baroclinic wave theory and its relevance to laboratory annulus flows." Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46583.
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Foutter, Richard R. (Richard Robert). "A numerical investigation of weak shock wave effects in reacting nozzle flows." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/50293.
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Gambino, John Piero 1974. "A comparison of sediment transport models for combined current and wave flows." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/47420.
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Jian, Wei. "Smoothed particle hydrodynamics modelling of dam-break flows and wave structure interactions." Thesis, University of Cambridge, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648472.
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Pelanti, Marica. "Wave propagation algorithms for multicomponent compressible flows with applications to volcanic jets /." Thesis, Connect to this title online; UW restricted, 2005. http://hdl.handle.net/1773/6784.
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Asproulias, Ioannis. "RANS modelling for compressible turbulent flows involving shock wave boundary layer interactions." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/rans-modelling-for-compressible-turbulent-flows-involving-shock-wave-boundary-layer-interactions(e2293c9d-de93-4e97-b8b8-967ec0b682d8).html.
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The main objective of the thesis is to provide a detailed assessment of the performance of four types of Low Reynolds Number (LRN) Eddy Viscosity Models (EVM), widely used for industrial purposes, on flows featuring SWBLI, using experimental and direct numerical simulation data. Within this framework the two-equation linear k-ε of Launder and Sharma (1974) (LS), the two-equation linear k-ω SST, the four-equation linear φ-f of Laurence et al. (2004) (PHIF) and the non-linear k-ε scheme of Craft et al. (1996b,1999) (CLSa,b) have been selected for testing. As initial test cases supersonic 2D compression ramps and impinging shocks of different angles and Reynolds numbers of the incoming boundary layer have been selected. Additional test cases are then considered, including normal shock/isotropic turbulence interaction and an axisymmetric transonic bump, in order to examine the predictions of the selected models on a range of Mach numbers and shock structures. For the purposes of this study the PHIF and CLSa,b models have been implemented in the open source CFD package OpenFOAM. Some results from validation studies of these models are presented, and some explorations are reported of certain modelled source terms in the ε-equation of the PHIF and CLSb models in compressible flows. Finally, before considering the main applications of the study, an examination is made of the performance of different solvers and numerical methods available in OpenFOAM for handling compressible flows with shocks. The performance of the above models, is analysed with comparisons of wall-quantities (skin-friction and wall-pressure), velocity profiles and profiles of turbulent quantities (turbulent kinetic energy and Reynolds stresses) in locations throughout the SWBLI zones. All the selected models demonstrate a broadly consistent performance over the considered flow configurations, with the CLSb scheme generally giving some improvements in predictions over the other models. The role of Reynolds stress anisotropy in giving a better representation of the evolution of the boundary layer in these flows is discussed through the performance of the CLSb model. It is concluded that some of the main deficiencies of the selected models is the overestimation of the dissipation rate levels in the non-equilibrium regions of the flow and the underestimation of the amplification of Reynolds stress anisotropy, especially within the recirculation bubble of the flows. Additionally, the analysis of the performance of the considered EVM's in a normal shock/isotropic turbulence interaction illustrates some drawbacks of the EVM formulation similar to the ones observed in normally-strained incompressible flows. Finally, a hybrid Detached Eddy Simulation (DES) approach is incorporated for the prediction of the transonic buffet around a wing.
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Isoni, Andrea. "Vortex wave interaction theory to understand self sustaining processes in transitional flows." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/24950.
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In this work the self-sustaining processes are investigated within a Couette flow de- veloping a method able to apply directly the stress jumps predicted by the vortex wave interaction theory. The challenge of the approach is to implement a technique able to directly implement the stress jumps and to implement a procedure able to deform the mesh to the flow variations. The derivation of the vortex wave interaction theory is also discussed and the numerical formulations of the governing equations are discretized through a spectral/hp element method. The method turns out to agree with the other approaches already utilised in literature and the results repro- duce a constraint of the mathematically inviscid flow suggesting that the flow is weakly dependent on the viscosity. The characteristics of the obtained flow are then discussed. These Navier-Stokes solutions are then perturbed by a sinusoidal wall forcing to study the robustness of the self-sustained mechanism by varying the amplitude of the forcing. The results show the possibility to control the behaviour of the flow and the effectiveness of the considered forcing to induce a drag reduction. Overcoming a certain amplitude threshold, a breakdown of the flow occurs in which the vortex core splits into multiple cores. Also after the breakdown the vortex wave interaction theory has been able to generate a self-sustained multiple core flow.
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Leszczyszyn, Antin M. "Resonant generation and refraction of dispersive shock waves in one-dimensional nonlinear Schrödinger flows." Thesis, Loughborough University, 2011. https://dspace.lboro.ac.uk/2134/9233.
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In the Thesis, two important theoretical problems arising in the theory of one-dimensional defocusing nonlinear Schrödinger (NLS) flows are investigated analytically and numerically: (i) the resonant generation of dispersive shock waves (DSWs) in one-dimensional NLS flow past a broad repulsive penetrable barrier; and (ii) the interaction of counter-propagating DSW and a simple rarefaction wave (RW), which is referred to as the DSW refraction problem. The first problem is motivated by the recent experimental observations of dark soliton radiation in a cigar-shaped BEC by sweeping through it a localised repulsive potential; the second problem represents a dispersive-hydrodynamic counterpart of the classical gas-dynamics problem of the shock wave refraction on a RW, and, apart from its theoretical significance could also find applications in superfluid dynamics. Both problems also naturally arise in nonlinear optics, where the NLS equation is a standard mathematical model and the `superfluid dynamics of light' can be used for an all-optical modelling of BEC flows. The main results of the Thesis are as follows: (i) In the problem of the transcritical flow of a BEC through a wide repulsive penetrable barrier an asymptotic analytical description of the arising wave pattern is developed using the combination of the localised ``hydraulic'' solution of the 1D Gross-Pitaevskii (GP) equation with repulsion (the defocusing NLS equation with an added external potential) and the appropriate exact solutions of the Whitham-NLS modulation equations describing the resolution of the upstream and downstream discontinuities through DSWs. We show that the downstream DSW effectively represents the train of dark solitons, which can be associated with the excitations observed experimentally by Engels and Atherton (2008). (ii) The refraction of a DSW due to its head-on collision with the centred RW is considered in the frameworks of two one-dimensional defocusing NLS models: the standard cubic NLS equation and the NLS equation with saturable nonlinearity, the latter being a standard model for the light propagation through photorefractive optical crystals. For the cubic nonlinearity case we present a full asymptotic description of the DSW refraction by constructing appropriate exact solutions of the Whitham modulation equations in Riemann invariants. For the NLS equation with saturable nonlinearity, whose modulation system does not possess Riemann invariants, we take advantage of the recently developed method for the DSW description in non-integrable dispersive systems to obtain key parameters of the DSW refraction. In both problems, we undertake a detailed analysis of the flow structure for different parametric regimes and calculate physical quantities characterising the output flows in terms of relevant input parameters. Our modulation theory analytical results are supported by direct numerical simulations of the corresponding full dispersive initial value problems (IVP).
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Wan, Bangjun. "A numerical study of conjugate flows and flat-centred internal solitary waves in a continuously stratified fluid." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/mq25896.pdf.
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Waywell, M. N. "Predictions of wave and tidally induced oscillatory flows with Reynolds stress turbulence models." Thesis, University of Salford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308264.
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Yang, Tian-Shiang. "Nonlinear interaction of long-wave distrubances with short-scale oscillations in stratified flows." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/37555.
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Buckland, Hannah. "Combined current, wave and turbulent flows and their effects on tidal energy devices." Thesis, Swansea University, 2014. https://cronfa.swan.ac.uk/Record/cronfa42509.
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This thesis considers the effect of disruptive waves and turbulence on a horizontal axis tidal stream turbine (TST), in terms of performance and survivability. The TST reaction to transient fluid flow is modelled analytically using Blade Element Momentum Theory (BEMT). Standard BEMT corrections are evaluated for the TST application and an alternative optimisation method is proposed for the steady state BEMT, improving compatibility with transient and depth dependent inflow, as well as the non-dimensionalisation constant needed to calculate the performance coefficients. Also, an alternative BEMT tip and hub loss implementation has led to a significant improvement of the turbine axial force prediction and in the high induction region. Validation studies are presented for BEMT coupled with regular, nonlinear wave theory and good agreement is found with published experimental data. A novel method to simulate irregular sea states is developed to couple with BEMT and a combined reactive coupling of waves and current is implemented. The TST performance in an irregular sea state is considered against turbine performance with real ADCP data and a good agreement is found. This work evaluates the BEMT implementation for the specific application of modelling TST's and significantly improves the fundamental theory, applicability and quality of results in this case.
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Sansica, Andrea. "Stability and unsteadiness of transitional shock-wave/boundary-layer interactions in supersonic flows." Thesis, University of Southampton, 2015. https://eprints.soton.ac.uk/385891/.
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The aim of this research is to study the effect of transition location on the interaction between an oblique shock-wave and a boundary-layer. A large set of direct and large eddy simulations are performed with an in-house high-order fully-parallelised finite difference compressible Navier-Stokes solver to study the inherent instability and unsteady behaviour of laminar, transitional and turbulent interactions. The numerical simulations are compared with the experiments conducted at the Novosibirsk State University as part of the EU-FP7 TFAST project, providing a better understanding of the fundamental mechanisms of the shock-wave/boundary-layer interaction (SWBLI). As well as the characteristics of the interactions, interest is also focused on methods to control the transition location. Three distinct forcing techniques are used to obtain different transition scenarios for a laminar SWBLI at free-stream Mach number of 1.5. An oblique mode breakdown caused by forcing the most unstable eigenmodes, predicted by the local linear stability theory, is compared with a bypass-like transition due to high-amplitude free-stream acoustic disturbances. A non-thermal plasma flow actuation device is also used, however showing a low applicability to supersonic flows due to the high electric power required to trigger transition. Attention is also focused on the response of a laminar shock-induced separation bubble. For both 2D and 3D configurations, a low-frequency response is found for the first time in a laminar SWBLI, even when the separation bubble is only forced internally, therefore supporting the idea that the separated region is influenced by internal mechanisms. The SWBLI is further analysed via linear and nonlinear stability approaches, including local stability theory or parabolised stability equations based tools. The response of the separated region for increasing shock intensities is studied and the stability based tools provide satisfactory results even for largely separated boundary-layers.
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Patel, Rupa Ashyinkumar. "Theory and computation on nonlinear vortex/wave interactions in internal and external flows." Thesis, Imperial College London, 1997. http://hdl.handle.net/10044/1/8597.
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Buttgereit, Ruediger Volker. "The fluid dynamic study of ducted and unducted Wells turbines in undirectional and oscillatory flows." Thesis, Imperial College London, 1998. http://hdl.handle.net/10044/1/8885.
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Neuenhahn, Thomas [Verfasser]. "Investigation of the shock wave, boundary layer interaction of scramjet intake flows / Thomas Neuenhahn." Aachen : Shaker, 2010. http://d-nb.info/1007974192/34.
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Dorcheh, Saied Ahmad Mortazavi. "Effect of rigid vegetation on the velocity, turbulence, and wave structure in open channel flows." Thesis, Cardiff University, 2007. http://orca.cf.ac.uk/56179/.
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Vegetation plays an important role in changing the flow characteristics and flooding of natural channels. In this study a series of laboratory programmes were undertaken to investigate the effects of emergent and submerged vegetation on the flow structure in compound, simple and wide channels. The vegetation consisted of rigid rods replicating tree vegetation. In a laboratory flume, wooden rods were used to replicate this with three density states and one non-vegetated state being used to compare their effects on the flow characteristics in the flume. Velocities were measured using 3-D Acoustic Doppler Velocimeters (ADV), with one downward facing and one upward facing probe. The results showed that the velocities decreased within and near the vegetation zones, and the flow accelerated and transferred towards the upper part of the rods, or to the non-vegetated zone, along the flume. The vegetation density was shown to control the magnitude of these effects. Also, in the transition zones between the floodplain and the main channel in the compound channel, or in transition zones between the vegetated and non-vegetation zones, or near the top of the vegetation in the simple and wide channels, the velocity fluctuations and Reynolds stresses were observed to have relatively high values. The magnitude of the velocity fluctuations and the Reynolds stresses were shown to be highly depended upon the vegetation density. Vegetation reduces the energy and increases the flow depth, with a reduction in the velocities. It also protects the channel bed against erosion due to resistance of the flow and enhances settlement of sediment. Behind the rods the longitudinal velocity was observed to be very low, but other velocities (transverse and vertical) were high. Also velocity fluctuations and Reynolds stress components were higher behind the rods. However, as the distance from the rods increased, then the longitudinal velocity increased and the transverse and vertical velocities and Reynolds stresses all decreased. For many conditions in vegetated open channel flows, rods may produce transverse waves due to vortex shedding. Some experiments in the wide channel, of 1200 mm width, 10 m length, and 24 mm rods diameter were undertaken using different rod arrangements and densities to investigate the frequency and amplitude of these waves. From these experiments, and a subsequent analysis of the results, a new equation was developed for calculating the Strouhal number for water and two new equations were established for calculating the wave amplitude for different rod arrangements and densities. These equations were experimentally proved to better match the data variations than the conventional equation.
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Febrer, Alles Gemma. "A hybrid approach for inclusion of acoustic wave effects in incompressible LES of reacting flows." Thesis, Loughborough University, 2012. https://dspace.lboro.ac.uk/2134/11979.
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LLean premixed combustion systems, attractive for low NOx performance, are inherently susceptible to thermo-acoustic instabilities - the interaction between unsteady heat release and excited acoustic wave effects. In the present work, a hybrid, coupled Large Eddy Simulation (LES) CFD approach is described, combining the computational efficiency of incompressible reacting LES with acoustic wave effects captured via an acoustic network model. A flamelet approach with an algebraic Flame Surface Density (FSD) combustion model was used. The ORACLES experiments - a perfectly premixed flame stabilised in a 3D sudden expansion - are used for validation. Simulations of the inert flow agree very well with experimental data, reproducing the measured amplitude and distribution of turbulent fluctuations as well as capturing the asymmetric mean flow. With reaction the measured data exhibit a plane wave acoustic mode at 50Hz. The influence of this plane wave must be incorporated into the LES calculation. Thus, a new approach to sensitise the incompressible LES CFD to acoustic waves is adopted. First an acoustic network model of the experimental geometry is analysed to predict the amplitude of the 50Hz mode just before the flame zone. This is then used to introduce a coherent plane wave at the LES inlet plane at the appropriate amplitude, unlike previous LES studies, which have adopted a "guess and adjust" approach. Incompressible LES predictions of this forced flow then show good agreement with measurements of mean and turbulent velocity, as well as for flame shape, with a considerable improvement relative to unforced simulations. To capitalise on the unsteady flame dynamics provided by LES, simulations with varying forcing amplitude were conducted and analysed. Amplitude dependent Flame Transfer Functions (FTFs) were extracted and fed into an acoustic network model. This allowed prediction of the stable/unstable nature of the flame at each forcing amplitude. An amplitude at which the flame changed from unstable to stable would be an indication that this coupled approach was capable of predicting a limit cycle behaviour. With the current simple FSD combustion model almost all cases studied showed a stable flame. Predictions showed considerable sensitivity to the value chosen for the combustion model parameter but specially to the acoustic geometric configuration and boundary conditions assumed showing evidence of limit cycle behaviour for some combinations. Nevertheless, further work is required to improve both combustion model and the accuracy of acoustic configuration and boundary condition specification.
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Khyyer, Abbas. "Improved Particle Methods by Refined Differential Operator Models for Free-Surface Fluid Flows." 京都大学 (Kyoto University), 2008. http://hdl.handle.net/2433/66215.
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Kyoto University (京都大学)0048
新制・課程博士
博士(工学)
甲第14147号
工博第2981号
新制||工||1442(附属図書館)
26453
UT51-2008-N464
京都大学大学院工学研究科都市環境工学専攻
(主査)教授 後藤 仁志, 教授 細田 尚, 准教授 牛島 省
学位規則第4条第1項該当
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Jones, Samuel Edward. "Symmetries in the kinematic dynamos and hydrodynamic instabilities of the ABC flows." Thesis, University of Exeter, 2013. http://hdl.handle.net/10871/14689.
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This thesis primarily concerns kinematic dynamo action by the 1:1:1 ABC flow, in the highly conducting limit of large magnetic Reynolds number Rm. The flow possesses 24 symmetries, with a symmetry group isomorphic to the group O24 of orientation-preserving transformations of a cube. These symmetries are exploited to break up the linear eigenvalue problem into five distinct symmetry classes, which we label I-V. The thesis discusses how to reduce the scale of the numerical problem to a subset of Fourier modes for a magnetic field in each class, which then may be solved independently to obtain distinct branches of eigenvalues and magnetic field eigenfunctions. Two numerical methods are employed: the first is to time step a magnetic field in a given symmetry class and obtain the growth rate and frequency by measuring the magnetic energy as a function of time. The second method involves a more direct determination of the eigenvalue using the eigenvalue solver ARPACK for sparse matrix systems, which employs an implicitly restarted Arnoldi method. The two methods are checked against each other, and compared for efficiency and reliability. Eigenvalue branches for each symmetry class are obtained for magnetic Reynolds numbers Rm up to 10^4 together with spectra and magnetic field visualisations. A sequence of branches emerges as Rm increases and the magnetic field structures in the different branches are discussed and compared. All symmetry classes are found to contain a dynamo, though dynamo effectiveness varies greatly between classes, suggesting that the symmetries play an important role in the field amplification mechanisms. A closely related problem, that of linear hydrodynamic stability, is also explored in the limit of large Reynolds number Re. As the same symmetry considerations apply, the five symmetry classes of the linear instability can be resolved independently, reducing the size of the problem and allowing exploration of the effects of the symmetries on instability growth rate. Results and visualisations are obtained for all five classes for Re up to 10^3, with comparisons drawn between the structures seen in each class and with those found in the analogous magnetic problem. For increasing Re, multiple mode crossings are observed within each class, with remarkably similar growth rates seen in all classes at Re=10^3, highlighting a lack of dependence on the symmetries of the instability, in contrast with the magnetic problem. This thesis also investigates the problem of large-scale magnetic fields in the 1:1:1 ABC flow through the introduction of Bloch waves that modify the periodicity of the magnetic field relative to the flow. Results are found for a field with increased periodicity in a single direction for Rm up to 10^3; it is established that the optimal scale for dynamo action varies as Rm increases, settling on a consistent scale for large Rm. The emerging field structures are studied and linked with those of the original dynamo problem. On contrasting this method with a previous study in which the flow is instead rescaled, it is shown that the use of Bloch waves drastically increases the range of possible scales, whilst cutting required computing time. Through a multiple-scale analysis, the contribution from the alpha-effect is calculated for the 1:1:1 ABC flow and is seen in growth rates for Rm << 1.
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Silvestri, Lorenzo. "Numerical study of wind-wave interfacial phenomena." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/14643/.
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A DNS simulation has been performed in order to study the wind-wave interaction, especially focusing the attention on the interfacial phenomena. The numerical model is founded to reproduce gravity waves generation under the action of a turbulent wind. The VOF (volume of fraction) method is used in order to capture and model the free surface, while the wind is generated by imposing an external pressure gradient. This last choice, in contrast with the usual ways of simulating wind by a moving wall, turned out to be very effective from both the computational and the physical point of view.
The simulation can be described by two fundamental parameters: the friction Reynolds number is about 330 and the wave age is 35. The wave age has been calculated by a spectral analysis of the free surface: through this analysis the wavelength of the main ripple is found to be about 5 cm, while its phase speed results to be 0.7 m/s. This wavelength and phase speed seems reasonable by comparing it with experiments and observations.
The turbulent boundary layer is significantly modified by waves in a way that resembles rough turbulence. The spatially and time averaged mean velocity profile is affected by the roughness of waves as much as the fluctuating field. In this study a sea surface roughness of 0.017 is founded by observing the behaviour of the wind close to the free surface. This similarity between a rough wall and waves, can be relevant for modelling flows over wavy walls or predicting wind and wave currents, especially in the Geophysical fluid dynamic field.
Finally, the Phillips mechanism (turbulent pressure fluctuations along the free surface) and the sheltering mechanism (positive and negative pressure fluctuations respectively in front and on the leeside of the wave due to the boundary layer separation) have been clearly observed.
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Holmberg, Andreas. "Experimental Determination of Aeracoustic Sources in Low Mach Number Internal Flows." Licentiate thesis, KTH, MWL Strömningsakustik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-26133.
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In this thesis, the in-duct experimental methods for determining aeroacoustic N-ports of in-duct elements are discussed and improved. The scattering matrix determination methods and the related wave decomposition methods are evaluated from measurements in an empty duct carrying a mean flow. The improvements of a new over-determination method for the source part of the N-port is studied using simulations and measurements; in quiescent air as well as measurements of the flow associated noise of a mixer plate, here a triangular plate inserted at an angle in a duct. The new method is shown to improve suppression of random errors while no improvement is achieved for bias errors. Further, the methods are applied in the study of two different aeroacoustic phenomena; one is the effect on the flow associated noise of the triangular plate achieved by varying the bending stiffness. For the most resilient plate tested, it is observed that when the Strouhal number of the flow noise coalesce with the Helmholtz number of a specific eigen-mode of the plate, the noise is drastically dampened. There is also a weaker broad band effect. The other phenomena studied is the amplification and attenuation obtained for sound waves propagating in a T-junction of rectangular ducts. It is found that by adding only 10% of inflow in the side branch relative to that in the main branch, the amplification is heavily increased. By adding another 10% the amplification is again similar to that of no side branch flow. Adding further flow lessens the effects still.QC 20101118
Experimental characterization of aero-acoustic sources
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Harlander, Uwe. "Some simple solutions of trapped Rossby waves in zonal barotropic multiple-jet flows." Universitätsbibliothek Leipzig, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-215362.
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This study concerns on trapped Rossby waves and local Rossby wave packets in zonal basic flows with two or more prominent extreme values (so called multiple-jet flows). For simplicity, most of the calculations are performed on the f-plane, but an extension to the ß-plane is also discussed. Under the assumption that the basic fiow is a solution to a special type of second order ordinary differential equation, we show that then the amplitudes of stationary trapped Rossby waves are solutions to an equation of the same type. We investigate the occurrence of trapped modal waves as well as the rays of wave action radiation in a particular multiple-jet flow. Further we consider the development of the local wavenumbers of Rossby wave packets in such a flow, with and without a zonally oriented reflective boundary. lt is found that wave action can propagate in the zonal direction only when the boundary is present. Otherwise the rays of wave action radiation form a closed curveDiese Studie beschäftigt sich mit gefangenen Rossby Wellen und lokalen gefangenen Rossbywellenpaketen in einer zonalen Strömung mit zwei oder mehreren ausgeprägten Extrema (sog. Strömungen mit mehrfachen Jets). Der Einfachheit halber werden die meisten Berechnungen auf der f-Ebene angestellt, eine Erweiterung auf die ß-Ebene wird allerdings auch diskutiert. Wenn man annimmt, dass der Grundstrom die Lösung einer bestimmten gewöhnlichen Differentialgleichung zweiter Ordnung ist, kann man zeigen, dass die Amplituden gefangener Wellen Lösungen des gleichen Gleichungstypes sind. Wir betrachten die Bedingungen für das Auftreten gefangener modaler Wellen, als auch die Pfade der Wellenenergie-Abstrahlung in einer bestimmten Mehrfach-Jet-Strömung. Ferner untersuchen wir die Entwicklung der lokalen Wellenzahlen von Wellenpaketen in einer solchen Strömung, mit und ohne eines zonal orientierten reflektierenden Randes. Wir finden, dass sich Wellenenergie auf der f-Ebene nur dann in zonaler Richtung ausbreiten kann, falls eine solcher Rand vorhanden ist. Anderenfalls ergeben die Strahlen der Wellenenergie Ausbreitung eine geschlossene Kurve
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Sigfrids, Timmy. "Hot wire and PIV studies of transonic turbulent wall-bounded flows." Licentiate thesis, KTH, Mechanics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1577.
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The compressible turbulent boundary layer developing over atwo-dimensional bump which leads to a supersonic pocket with aterminating shock wave has been studied. The measurements havebeen made with hot-wire anemometry and Particle ImageVelocimetry (PIV).
A method to calibrate hot-wire probes in compressible ow hasbeen developed which take into account not only the ow velocitybut also the inuence of the Mach number, stagnation temperatureand uid density. The calibration unit consists of a small jetow facility, where the temperature can be varied. The hot wiresare calibrated in the potential core of the free jet. The jetemanates in a container where the static pressure can becontrolled, and thereby the gas density. The calibration methodwas verfied in the at plate zero pressure gradient turbulentboundary layer in front of the bump at three different Machnumbers, namely 0.3, 0.5 and 0.7. The profiles were alsomeasured at different static pressures in order to see theinuence of varying density. Good agreement between the profilesmeasured at different pressures, as well as with the standardlogarithmic profile was obtained.
The PIV measurements of the boundary layer ow in front ofthe 2D bump showed good agreement with the velocity profilesmeasured with hotwire anemometry. The shock wave boundary layerinteraction was investigated for an inlet Mach number of 0.69.A lambda shock wave was seen on the downstream side of thebump. The velocity on both sides of the shock wave as measuredwith the PIV was in good agreement with theory. The shock wavewas found to cause boundary layer separation, which was seen asa rapid growth of the boundary layer thickness downstream theshock. However, no back ow was seen in the PIV-data, probablybecause the seeding did not give enough particles in theseparated region. The PIV data also showed that the shock wavewas oscillating, i.e. it was moving approximately 5 mm back andforth. This distance corresponds to about five boundary layerthicknesses in terms of the boundary layer upstream theshock.
Descriptors:Fluid mechanics, compressible ow,turbulence, boundary layer, hot-wire anemometry, PIV, shockwave boundary layer interaction, shape factor.
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Ben, Hassan Saïdi Ismaïl. "Numerical simulations of the shock wave-boundary layer interactions." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS390/document.
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Les situations dans lesquelles une onde de choc interagit avec une couche limite sont nombreuses dans les industries aéronautiques et spatiales. Sous certaines conditions (nombre de Mach élevé, grand angle de choc…), ces interactions entrainent un décollement de la couche limite. Des études antérieures ont montré que la zone de recirculation et le choc réfléchi sont tous deux soumis à un mouvement d'oscillation longitudinale à basse fréquence connu sous le nom d’instabilité de l’interaction onde de choc / couche limite (IOCCL). Ce phénomène appelé soumet les structures à des chargement oscillants à basse fréquence qui peuvent endommager les structures.L’objectif du travail de thèse est de réaliser des simulations instationnaires de l’IOCCL afin de contribuer à une meilleure compréhension de l’instabilité de l’IOCCL et des mécanismes physiques sous-jacents.Pour effectuer cette étude, une approche numérique originale est utilisée. Un schéma « One step » volume fini qui couple l’espace et le temps, repose sur une discrétisation des flux convectifs par le schéma OSMP développé jusqu’à l’ordre 7 en temps et en espace. Les flux visqueux sont discrétisés en utilisant un schéma aux différences finies centré standard. Une contrainte de préservation de la monotonie (MP) est utilisée pour la capture de choc. La validation de cette approche démontre sa capacité à calculer les écoulements turbulents et la grande efficacité de la procédure MP pour capturer les ondes de choc sans dégrader la solution pour un surcoût négligeable. Il est également montré que l’ordre le plus élevé du schéma OSMP testé représente le meilleur compromis précision / temps de calcul. De plus un ordre de discrétisation des flux visqueux supérieur à 2 semble avoir une influence négligeable sur la solution pour les nombres de Reynolds relativement élevés considérés.En simulant un cas d’IOCCL 3D avec une couche limite incidente laminaire, l’influence des structures turbulentes de la couche limite sur l’instabilité de l’IOCCL est supprimée. Dans ce cas, l’unique cause d’IOCCL suspectée est liée à la dynamique de la zone de recirculation. Les résultats montrent que seul le choc de rattachement oscille aux fréquences caractéristiques de la respiration basse fréquence du bulbe de recirculation. Le point de séparation ainsi que le choc réfléchi ont une position fixe. Cela montre que dans cette configuration, l’instabilité de l’IOCCL n’a pas été reproduite.Afin de reproduire l’instabilité de l’IOCCL, la simulation de l’interaction entre une onde de choc et une couche limite turbulente est réalisée. Une méthode de turbulence synthétique (Synthetic Eddy Method - SEM) est développée et utilisée à l’entrée du domaine de calcul pour initier une couche limite turbulente à moindre coût. L’analyse des résultats est effectuée en utilisant notamment la méthode snapshot-POD (Proper Orthogonal Decomposition). Pour cette simulation, l’instabilité de l’IOCCL a été reproduite. Les résultats suggèrent que la dynamique du bulbe de recirculation est dominée par une respiration à moyenne fréquence. Ces cycles successifs de remplissage / vidange de la zone séparée sont irréguliers dans le temps avec une taille maximale du bulbe de recirculation variant d’un cycle à l’autre. Ce comportement du bulbe de recirculation traduit une modulation basse fréquence des amplitudes des oscillations des points de séparation et de recollement et donc une respiration basse fréquence de la zone séparée. Ces résultats suggèrent que l’instabilité de l’IOCCL est liée à cette dynamique basse fréquence du bulbe de recirculation, les oscillations du pied du choc réfléchi étant en phase avec le point de séparationSituations where an incident shock wave impinges upon a boundary layer are common in the aeronautical and spatial industries. Under certain circumstances (High Mach number, large shock angle...), the interaction between an incident shock wave and a boundary layer may create an unsteady separation bubble. This bubble, as well as the subsequent reflected shock wave, are known to oscillate in a low-frequency streamwise motion. This phenomenon, called the unsteadiness of the shock wave boundary layer interaction (SWBLI), subjects structures to oscillating loads that can lead to damages for the solid structure integrity.The aim of the present work is the unsteady numerical simulation of (SWBLI) in order to contribute to a better understanding of the SWBLI unsteadiness and the physical mechanism causing these low frequency oscillations of the interaction zone.To perform this study, an original numerical approach is used. The one step Finite Volume approach relies on the discretization of the convective fluxes of the Navier Stokes equations using the OSMP scheme developed up to the 7-th order both in space and time, the viscous fluxes being discretized using a standard centered Finite-Difference scheme. A Monotonicity-Preserving (MP) constraint is employed as a shock capturing procedure. The validation of this approach demonstrates the correct accuracy of the OSMP scheme to predict turbulent features and the great efficiency of the MP procedure to capture discontinuities without spoiling the solution and with an almost negligible additional cost. It is also shown that the use of the highest order tested of the OSMP scheme is relevant in term of simulation time and accuracy compromise. Moreover, an order of accuracy higher than 2-nd order for approximating the diffusive fluxes seems to have a negligible influence on the solution for such relatively high Reynolds numbers.By simulating the 3D unsteady interaction between a laminar boundary layer and an incident shock wave, we suppress the suspected influence of the large turbulent structures of the boundary layer on the SWBLI unsteadiness, the only remaining suspected cause of unsteadiness being the dynamics of the separation bubble. Results show that only the reattachment point oscillates at low frequencies characteristic of the breathing of the separation bubble. The separation point of the recirculation bubble and the foot of the reflected shock wave have a fixed location along the flat plate with respect to time. It shows that, in this configuration, the SWBLI unsteadiness is not observed.In order to reproduce and analyse the SWBLI unsteadiness, the simulation of a shock wave turbulent boundary layer interaction (SWTBLI) is performed. A Synthetic Eddy Method (SEM), adapted to compressible flows, has been developed and used at the inlet of the simulation domain for initiating the turbulent boundary layer without prohibitive additional computational costs. Analyses of the results are performed using, among others, the snapshot Proper Orthogonal Decomposition (POD) technique. For this simulation, the SWBLI unsteadiness has been observed. Results suggest that the dominant flapping mode of the recirculation bubble occurs at medium frequency. These cycles of successive enlargement and shrinkage of the separated zone are shown to be irregular in time, the maximum size of the recirculation bubble being submitted to discrepancies between successive cycles. This behaviour of the separation bubble is responsible for a low frequency temporal modulation of the amplitude of the separation and reattachment point motions and thus for the low frequency breathing of the separation bubble. These results tend to suggest that the SWBLI unsteadiness is related to this low frequency dynamics of the recirculation bubble; the oscillations of the reflected shocks foot being in phase with the motion of the separation point
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Harlander, Uwe. "Some simple solutions of trapped Rossby waves in zonal barotropic multiple-jet flows." Wissenschaftliche Mitteilungen des Leipziger Instituts für Meteorologie ; 17 = Meteorologische Arbeiten aus Leipzig ; 5 (2000), S. 44-60, 2000. https://ul.qucosa.de/id/qucosa%3A15148.
Full textAbstract:
This study concerns on trapped Rossby waves and local Rossby wave packets in zonal basic flows with two or more prominent extreme values (so called multiple-jet flows). For simplicity, most of the calculations are performed on the f-plane, but an extension to the ß-plane is also discussed. Under the assumption that the basic fiow is a solution to a special type of second order ordinary differential equation, we show that then the amplitudes of stationary trapped Rossby waves are solutions to an equation of the same type. We investigate the occurrence of trapped modal waves as well as the rays of wave action radiation in a particular multiple-jet flow. Further we consider the development of the local wavenumbers of Rossby wave packets in such a flow, with and without a zonally oriented reflective boundary. lt is found that wave action can propagate in the zonal direction only when the boundary is present. Otherwise the rays of wave action radiation form a closed curve.Diese Studie beschäftigt sich mit gefangenen Rossby Wellen und lokalen gefangenen Rossbywellenpaketen in einer zonalen Strömung mit zwei oder mehreren ausgeprägten Extrema (sog. Strömungen mit mehrfachen Jets). Der Einfachheit halber werden die meisten Berechnungen auf der f-Ebene angestellt, eine Erweiterung auf die ß-Ebene wird allerdings auch diskutiert. Wenn man annimmt, dass der Grundstrom die Lösung einer bestimmten gewöhnlichen Differentialgleichung zweiter Ordnung ist, kann man zeigen, dass die Amplituden gefangener Wellen Lösungen des gleichen Gleichungstypes sind. Wir betrachten die Bedingungen für das Auftreten gefangener modaler Wellen, als auch die Pfade der Wellenenergie-Abstrahlung in einer bestimmten Mehrfach-Jet-Strömung. Ferner untersuchen wir die Entwicklung der lokalen Wellenzahlen von Wellenpaketen in einer solchen Strömung, mit und ohne eines zonal orientierten reflektierenden Randes. Wir finden, dass sich Wellenenergie auf der f-Ebene nur dann in zonaler Richtung ausbreiten kann, falls eine solcher Rand vorhanden ist. Anderenfalls ergeben die Strahlen der Wellenenergie Ausbreitung eine geschlossene Kurve.
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Brahmi, Nassim. "Etude numérique de la propagation et l’atténuation des ondes de choc en milieux confinés Turbulent structures of shock-wave diffraction over 90° convex corner Analysis of shock-wave diffraction over double concave cylindrical wedges. Part I: Shock dynamics Analysis of shock-wave diffraction over double cylindrical wedges. Part II: Vorticity generation Achieving an optimal shock-wave mitigation inside open channels with cavities for weak shock waves: a computational study." Thesis, Normandie, 2020. http://www.theses.fr/2020NORMIR09.
Full textAbstract:
La propagation des ondes de choc implique des interactions complexes entre ondes et milieux environnants, ce qui engendre plusieurs phénomènes tels que la réflexion, la diffraction, etc. Pour clarifier davantage la physique associée à ces phénomènes, des simulations numériques hautes résolutions ont été réalisées. En particulier l'analyse de la diffraction des ondes de choc sur deux surfaces concaves cylindriques a révélé que les angles de transition, d'une réflexion régulière à une réflexion de Mach, augmentent avec le nombre de Mach, alors qu'ils sont presque égaux sur les deux surfaces concaves pour les régimes de Mach transsoniques et relativement plus important sur la deuxième surface pour les nombres de Mach plus élevés. Ceci prouve que l'écoulement est en mesure de conserver l'historique des événements passés sur l'ensemble du processus pour des nombres de Mach élevés. L'analyse de l'équation de transport de vorticité a montré, pour la première fois, que la diffusion de la vorticité due aux effets visqueux est assez importante par rapport au terme baroclinic pour les faibles nombres de Mach, alors que cette tendance est inversée pour les nombres de Mach les plus élevés. L'étude a également montré que le stretching de la vorticité dû aux effets de compressibilité joue un rôle important dans la production de vorticité. A la base de ces simulations numériques, une relation universelle a été proposée, permettant de prédire la trajectoire et la vitesse de l'onde incidente en fonction du nombre de Mach incident, du rayon de courbure de la géométrie et des propriétés du gaz. Par la suite, l'étude de la propagation des ondes de choc et leur atténuation dans des conduites de différentes hauteurs et présentant des cavités circulaires creuses de différentes profondeurs a été effectuée. Les résultats ont montré l'importance de la réduction de la hauteur du canal et le changement de la position de la section réduite en plus de l'angle de diffraction et de la profondeur de la cavité pour une meilleure atténuation des ondes. Un arrangement optimal de la position/hauteur du canal et de l'emplacement / profondeur de la cavité a été trouvéThe propagation of shock waves involves complex interactions between waves and surrounding media, which gives rise to several phenomena such as reflection, diffraction, etc. To shed more light into the fundamental physics associated with these phenomena, high resolution numerical simulations were carried out. In particular, analysis of shock diffraction over double concave cylindrical wedges revealed that the transition angles, from regular to Mach reflection, increase with the Mach number, whereas they are found to be almost the same over the two concave surfaces for the transonic Mach regimes and relatively larger on the second surface for high ones showing that the flow is capable of retaining the memory of the past events over the entire process for the high Mach numbers. The analysis of the vorticity equation balance showed, for the first time, that the diffusion of the vorticity due to the viscous effects is quite important compared to the baroclinic term for low Mach numbers, while this trend is inverted for higher Mach numbers. The study also showed that the stretching of the vorticity due to the compressibility effects plays an important role in the vorticity production. On the basis of these numerical simulations, an approximate universal relation is proposed, allowing to predict the incident-shock trajectory and velocity as a function of the incident-shock Mach number, the radius of curvature of the geometry, and the gas properties. Afterward, the study of shock-waves propagation and their attenuation in channel flow having different heights and exhibiting a hollow circular cavities with different depths has been clone. The results also showed the importance of reducing the height of the channel and changing the position of the reduced section in addition to the diffraction angle and the cavity depth for better shock-waves attenuation. A subtle arrangement of channel position/height and a cavity location/depht was found
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Aziz, Saduman. "Perfect Gas Navier-stokes Solutions Of Hypersonic Boundary Layer And Compression Corner Flows." Phd thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12606661/index.pdf.
Full textAbstract:
The purpose of this thesis is to perform numerical solutions of hypersonic, high temperature, perfect gas flows over various geometries. Three dimensional, thin layer, compressible, Navier-Stokes equations are solved. An upwind finite difference approach with Lower Upper-Alternating Direction Implicit (LU-ADI) decomposition is used.
Solutions of laminar, hypersonic, high temperature, perfect gas flows over flat plate and compression corners (qw=5°, 10°
, 14°
, 15°
, 16°
, 18°
and 24°
) with eight different free-stream and wall conditions are presented and discussed. During the analysis, air viscosity is calculated from the Sutherland formula up to 1000°
K, for the temperature range between 1000 º
K and 5000 º
K a curve fit to the estimations of Svehla is applied. The effects of Tw/T0 on heat transfer rates, surface pressure distributions and boundary layer characteristics are studied. The effects of corner angle (&
#952
w) on strong shock wave/boundary layer interactions with extended separated regions are investigated. The obtained results are compared with the available experimental data, computational results, and theory.