Thèses sur le sujet « Laminar layer »
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1
Bown, Nicholas William. « In-flight boundary layer studies on laminar flow nacelles ». Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299777.
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Choudhari, Meelan. « Boundary layer receptivity mechanisms relevant to laminar flow control ». Diss., The University of Arizona, 1990. http://hdl.handle.net/10150/184964.
Texte intégralRésumé :
Receptivity processes by which free-stream acoustic waves generate instability waves in boundary layers are investigated. Concentration is placed on mechanisms associated with local regions of short scale variation in wall suction or admittance distribution. These mechanisms are relevant to laminar flow control technology, in which suction is utilized to control the growth of boundary layer instabilities. The receptivity process requires a transfer of energy from the long wavelength of the free-stream disturbance to the short wavelength of the instability wave. In the case of wall suction, this occurs through the unsteady modulation, by the acoustic wave, of the short scale mean flow variation due to the steady wall suction. In the wall admittance mechanism, the boundary condition for the unsteady motion contains a short scale variation which directly scatters energy from the acoustic wave into the instability wave. The latter mechanism does not require a short scale adjustment in the mean boundary layer. Time harmonic, two and three-dimensional interactions are analyzed using the asymptotic, high Reynolds number, triple deck structure. The influence of subsonic compressibility is examined for the case of two-dimensional interactions, and a similarity transform is found which reduces the problem to an equivalent incompressible flow. For three-dimensional interactions, a similarity transform is possible only in the Fourier transform wavenumber space, and in the equivalent two-dimensional problem the frequency is complex. However, in many cases of practical interest, the imaginary component of this frequency is quite small and can be neglected. The acoustic wave orientation and the geometry of the wall suction or admittance distribution are found to significantly influence the amplitude of the generated instability wave. For an isolated, three-dimensional region of wall suction or admittance, instability wave growth is confined to a downstream, wedge shaped region. The saddle point method is utilized to calculate the characteristics of this instability wave pattern. In some ranges of parameter space, two saddle points are found to make comparable contributions. The instability wave pattern in these directions exhibits a beat phenomenon, due to constructive and destructive interference of the contributions from the two saddle points.
3
Mackerrell, O. S. « Some hydrodynamic instabilities of boundary layer flows ». Thesis, University of Exeter, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381355.
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Rogers, John B. « Numerical computations for laminar mixing layers between parallel supersonic streams ». Thesis, Georgia Institute of Technology, 1991. http://hdl.handle.net/1853/16441.
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5
Choudhari, Meelan 1963. « Boundary layer receptivity at a suction surface-hard wall junction ». Thesis, The University of Arizona, 1989. http://hdl.handle.net/10150/277030.
Texte intégralRésumé :
Receptivity refers to the generation of boundary layer instability waves by external disturbances. Recent work by M. E. Goldstein has set the general framework for the different types of boundary layer receptivity mechanisms. Therefore, it is now understood that receptivity occurs near the leading edge or at locations downstream where the boundary layer undergoes a rapid streamwise adjustment. The present work analyzes the receptivity due to a free-stream acoustic wave interacting with a suction surface--hard wall junction. In this case, receptivity occurs because of the rapid changes in wall suction distribution. Analytical expressions for the amplitude of the generated instability wave have been derived and numerical estimates provided for parameter values typical of hybrid laminar flow control applications. The importance of the junction receptivity as compared to other receptivity mechanisms has been assessed.
6
Fabbiane, Nicolò. « Adaptive and model-based control in laminar boundary-layer flows ». Licentiate thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-154052.
Texte intégralRésumé :
In boundary-layer flows it is possible to reduce the friction drag by breaking the path from laminar to turbulent state. In low turbulence environments, the laminar-to-turbulent transition is dominated by local flow instabilities – Tollmien-Schlichting (TS) waves – that exponentially grows while being con- vected by the flow and, eventually, lead to transition. Hence, by attenuating these disturbances via localised forcing in the flow it is possible to delay farther downstream the onset of turbulence and reduce the friction drag. Reactive control techniques are widely investigated to this end. The aim of this work is to compare model-based and adaptive control techniques and show how the adaptivity is crucial to control TS-waves in real applications. The control design consists in (i) choosing sensors and actuators and (ii) designing the system responsible to process on-line the measurement signals in order to compute an appropriate forcing by the actuators. This system, called compen- sator, can be static or adaptive, depending on the possibility of self-adjusting its response to unmodelled flow dynamics. A Linear Quadratic Gaussian (LQG) regulator is chosen as representative of static controllers. Direct numerical simulations of the flow are performed to provide a model for the compensator design and test its performance. An adaptive Filtered-X Least-Mean-Squares (FXLMS) compensator is also designed for the same flow case and its per- formance is compared to the model-based compensator via simulations and experiments. Although the LQG regulator behaves better at design conditions, it lacks robustness to small flow variations. On the other hand, the FXLMS compensator proved to be able to adapt its response to overcome the varied conditions and perform an adequate control action. It is thus found that an adaptive control technique is more suitable to delay the laminar-to-turbulent transition in situations where an accurate model of the flow is not available.I det tunna gränsskikt som uppstår en yta, kan friktionen minskas genom att förhindra omslag från ett laminärt till ett turbulent flöde. När turbulensnivån är låg i omgivningen, domineras till en början omslaget av lokala instabiliteter (Tollmien-Schlichting (TS) v ågor) som växer i en exponentiell takt samtidigt som de propagerar nedströms. Därför, kan man förskjuta omslaget genom att dämpa TS vågors tillväxt i ett gränsskikt och därmed minska friktionen.Med detta mål i sikte, tillämpas och jämförs två reglertekniska metoder, nämligen en adaptiv signalbaserad metod och en statiskt modellbaserad metod. Vi visar att adaptivitet är av avgörande betydelse för att kunna dämpa TS vågor i en verklig miljö. Den reglertekniska konstruktionen består av val av givare och aktuatorer samt att bestämma det system som behandlar mätsignaler (on- line) för beräkning av en lämplig signal till aktuatorer. Detta system, som kallas för en kompensator, kan vara antingen statisk eller adaptiv, beroende på om det har möjlighet till att anpassa sig till omgivningen. En så kallad linjär regulator (LQG), som representerar den statiska kompensator, har tagits fram med hjälp av numeriska simuleringar of strömningsfältet. Denna kompensator jämförs med en adaptiv regulator som kallas för Filtered-X Least-Mean-Squares (FXLMS) både experimentellt och numeriskt. Det visar sig att LQG regulatorn har en bättre prestanda än FXLMS för de parametrar som den var framtagen för, men brister i robusthet. FXLMS å andra sidan, anpassar sig till icke- modellerade störningar och variationer, och kan därmed hålla en god och jämn prestanda.Man kan därmed dra slutsaten att adaptiva regulatorer är mer lämpliga för att förhala omslaget fr ån laminär till turbulent strömning i situationer då en exakt modell av fysiken saknas.
QC 20141020
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Sattarzadeh, Shirvan Sohrab. « Boundary layer streaks as a novel laminar flow control method ». Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-181899.
Texte intégralRésumé :
A novel laminar flow control based on generation of spanwise mean velocity gradients (SVG) in a flat plate boundary layer is investigated where disturbances of different types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at; (i) generating stable and steady streamwise streaks in the boundary layer which set up spanwise gradients in the mean flow, and (ii) attenuating disturbance energy growth in the streaky boundary layers and hence delaying the onset of turbulence transition. The streamwise streaks generated by four different methods are investigated, which are spanwise arrays of triangular/rectangular miniature vortex generators (MVGs) and roughness elements, non-linear pair of oblique waves, and spanwise-periodic finite discrete suction. For all the investigated methods the boundary layer is modulated into regions of high- and low speed streaks through formation of pairs of counter-rotating streamwise vortices. For the streaky boundary layers generated by the MVGs a parameter study on a wide range of MVG configurations is performed in order to investigate the transient growth of the streaks. A general scaling of the streak amplitudes is found based on empiricism where an integral amplitude definition is proposed for the streaks. The disturbances are introduced as single- and broad band frequency twodimensional Tollmien–Schlichting (TS) waves, and three-dimensional single and a pair of oblique waves. In an attempt to obtain a more realistic configuration compared to previous investigations the disturbances are introduced upstream of the location were streaks are generated. It is shown that the SVG method is efficient in attenuating the growth of disturbance amplitudes in the linear regime for a wide range of frequencies although the disturbances have an initial amplitude response to the generation of the streaks. The attenuation rate of the disturbance amplitude is found to be optimized for an integral streak amplitude of 30% of the free-stream velocity which takes into account the periodic wavelength of the streaky base flow. The stabilizing effect of the streamwise streaks can be extended to the nonlinear regime of disturbances which in turn results in transition to turbulence delay. This results in significant drag reduction when comparing the skin friction coefficient of a laminar- to a turbulent boundary layer. It is also shown that consecutive turbulence transition delay can be obtained by reinforcing the streaky boundary layer in the streamwise direction. For the streaky boundary layer generated by pair of oblique waves their forcing frequency sets the upper limit for the frequency of disturbances beyond which the control fails.QC 20160208
8
Finnis, M. V. « Centrifugal instability of a laminar boundary layer on a concave surface ». Thesis, Cranfield University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.332090.
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Cruz, Erica Jeannette. « Interaction of a Dynamic Vortex Generator with a Laminar Boundary Layer ». Thesis, Rensselaer Polytechnic Institute, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10159646.
Texte intégralRésumé :
An experimental investigation was performed to study the fundamental interaction between a static and dynamic vortex generator with a laminar boundary layer. The effectiveness of static vortex generators (VGs) on delaying boundary layer separation is well established. However, as a passive flow control device, static VGs are associated with a drag penalty since they are always present in the flow. In the current study a piezoelectric-based dynamic vortex generator (DVG) was developed with the goal of mitigating the drag experienced when using a VG as a flow control device and exploring whether or not a DVG was more effective in flow mixing within the boundary layer. Experiments were conducted in a small wind tunnel, where the VG was flush mounted to the floor. The VG was rectangular in shape and erected into the flow with a mean height of the local boundary layer thickness, δ, or hm = 3 mm. The skew angle of the VG was &thetas; = 18° with respect to the incoming flow, oscillated at a driving frequency of f = 40 Hz with a peak to peak displacement (or amplitude) of 0.5·δ, or ha = 1.5 mm. During the experiments, the free stream velocity was held constant at U∞ = 10 m/s. This corresponded to a Reynolds number of Reδ ≈ 2000, which was based on the local boundary layer thickness at the center of the VG. Surface oil flow visualization experiments were performed to obtain qualitative information on the structures present in the flow, while Stereoscopic particle image velocimetry (SPIV) was used to provide quantitative measurements of the 3-D flow field at multiple spanwise planes downstream of the VG under both static and dynamic conditions. Several flow features were detected in the oil flow visualization experiments, including two vortical structures—the main vortex and primary horseshoe vortex—which were confirmed in the SPIV results. The time-averaged flow field showed similar results, though the strength of the vortices appeared less when the VG was actuated. However, phase-averaged data revealed the size, strength, and location of the vortices varied as a function of the actuation cycle, with peaks of vorticity magnitude being greater at certain phases as compared to the static case. The varying flow field associated with the dynamic motion of the DVG showed higher levels of turbulent kinetic energy, therefore confirming enhanced mixing in contrast to the static case.
10
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/.
Texte intégralRésumé :
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
11
Eustace, Richard. « Stabilisation of roughness particle induced turbulence using laminar flow control suction surfaces ». Thesis, University of Bristol, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299503.
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Ricco, Pierre Agostino. « Response of a compressible laminar boundary layer to free-stream turbulent disturbances ». Thesis, Imperial College London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.441995.
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Andrae, Johan. « Numerical Studies of Wall Effects of Laminar Flames ». Licentiate thesis, KTH, Chemical Engineering and Technology, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1258.
Texte intégralRésumé :
Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow.
The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition.
When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case.
For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects.
For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm).
As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures.
To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected.
Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow
QC 20100504
14
Collier, Fayette. « Curvature effects on the stability of three-dimensional laminar boundary layers ». Diss., Virginia Polytechnic Institute and State University, 1988. http://hdl.handle.net/10919/53679.
Texte intégralRésumé :
The linear stability equations which govern the growth of small periodic disturbances for compressible, three-dimensional laminar boundary layer flow are derived in an orthogonal curvilinear coordinate system. The parallel flow assumption is utilized in the derivation. The system of equations is solved using a finite difference scheme similar to that in a current state-of-the-art stability analysis code, COSAL. The LR method and the inverse Rayleigh iteration procedure are used to calculate the eigenvalues.
The stability of the three-dimensional compressible laminar boundary layer including the effects of streamline and surface curvature for flows past swept wings where crossflow type disturbances dominate is calculated. A parametric study is performed varying Reynolds number and sweep angle on an airfoil with a concave cutout in the leading edge region of the lower surface. It is known that convex curvature has a stabilizing effect on the laminar boundary layer. Conversely, concave curvature has a destabilizing effect. The magnitude of these effects for swept wing flows is determined. Non-stationary as well as stationary disturbances are calculated, and the most amplified frequencies are identified.
N-factor correlations at the measured location of transition are made utilizing flight test data. Results indicate that amplification rates and hence, N-factors, for swept wing flows over convex surfaces are reduced by about 30 to 50 percent when curvature effects are included in the linear stability analysis. In addition, comparisons are made with some experimental results on a swept concave-convex surface. Calculated velocity vector plots show good agreement with observed disturbances in the laminar boundary layer over the concave surface. The results of the calculations show that concave curvature destabilizes "crossflow” type disturbances with a 30 percent increase in amplification rate.Ph. D.
15
Tomorsky, Johanna. « Molecular Mechanisms of Laminar Circuit Formation in Visual Cortex ». Thesis, University of Oregon, 2019. http://hdl.handle.net/1794/24539.
Texte intégralRésumé :
The mammalian visual system develops to perform many complex tasks that allow us to perceive the natural world. These tasks rely on a dense network of synaptic connections transporting visual information both to and within visual cortex (V1). The laminar organization and functional properties of visual cortical neurons are largely conserved across mammals, and the mouse has been adopted as a model organism to study the development of this cortical circuit. Neurons in each cortical layer must find the correct synaptic partners for the optimal receipt, transfer, and processing of information. The molecular cues guiding the development of these connections, however, are largely unknown.
In this thesis, I identify and then examine the role of molecular factors important for synapse formation in layer 2/3 (L2/3) of visual cortex. L2/3 neurons are highly interconnected and fire selectively to a refined set of visual stimuli. The developmental refinement of these visual preferences has been shown to occur in the week following eye opening, corresponding with a period of intense synapse formation and dynamic gene expression in mouse V1. In Chapters II–IV, I use the TU-tagging technique to identify molecular factors enriched L2/3 neurons before and after eye opening and identify several candidate genes with potential functions in synapse formation.
In Chapter V, I examine the function of cell adhesion molecules nectin-1 and nectin-3, identified here as enriched in L2/3 visual cortex at eye opening, and previously shown to interact across synaptic junctions. I focus mainly on the effect of nectin-3 (having post-synaptic localization in hippocampus) on post-synaptic dendritic spine densities in developing L2/3 cortical neurons. I show that nectin-3 knockdown further increases spine densities after eye opening, while overexpressing a full length or truncated nectin-3 protein reduces spine densities. I conclude that nectin-3 may have a role in synapse formation following eye opening, and propose a mechanism describing the effects observed. Here, I describe a unique approach for understanding how cell-type specific connections are formed in visual cortex, beginning with the spatiotemporal examination gene expression and followed by the spatiotemporal manipulation of a single gene.
This dissertation includes previously published co-authored material.
16
Chuck, Chen. « Numerical simulation of oblique detonation and shock-deflagration waves with a laminar boundary-layer / ». Thesis, Connect to this title online ; UW restricted, 1990. http://hdl.handle.net/1773/9966.
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Sivasubramanian, Jayahar. « Numerical Investigation of Laminar-Turbulent Transition in a Cone Boundary Layer at Mach 6 ». Diss., The University of Arizona, 2012. http://hdl.handle.net/10150/228514.
Texte intégralRésumé :
Direct Numerical Simulations (DNS) are performed to investigate laminar-turbulent transition in a boundary layer on a sharp cone at Mach 6. The main objective of this dissertation research is to explore which nonlinear breakdown mechanisms may be dominant in a broad--band "natural" disturbance environment and then use this knowledge to perform controlled transition simulations to investigate these mechanisms in great detail. Towards this end, a "natural" transition scenario was modeled and investigated by generating wave packet disturbances. The evolution of a three-dimensional wave packet in a boundary layer has typically been used as an idealized model for "natural" transition to turbulence, since it represents the impulse response of the boundary layer and, thus, includes the interactions between all frequencies and wave numbers. These wave packet simulations provided strong evidence for a possible presence of fundamental and subharmonic resonance mechanisms in the nonlinear transition regime. However, the fundamental resonance was much stronger than the subharmonic. In addition to these two resonance mechanisms, the wave packet simulations also indicated the possible presence of oblique breakdown mechanism. To gain more insight into the nonlinear mechanisms, controlled transition simulations were performed of these mechanisms. Several small and medium scale simulations were performed to scan the parameter space for fundamental and subharmonic resonance. These simulations confirmed the findings of the wave packet simulations, namely that, fundamental resonance is much stronger compared to the subharmonic resonance. Subsequently a set of highly resolved fundamental and oblique breakdown simulations were performed. In these DNS, remarkable streamwise arranged "hot'' streaks were observed for both fundamental and oblique breakdown. The streaks were a consequence of the large amplitude steady longitudinal vortex modes in the nonlinear régime. These simulations demonstrated that both second--mode fundamental breakdown and oblique breakdown may indeed be viable paths to complete breakdown to turbulence in hypersonic boundary layers at Mach 6.
18
Combrinck, Madeleine Lelon. « Boundary layer response to arbitrary accelerating flow ». Thesis, University of Pretoria, 2016. http://hdl.handle.net/2263/61287.
Texte intégralRésumé :
This thesis was aimed developing a fundamental understanding of the boundary layer response
to arbitrary motion. In this context arbitrary motion was defined as the unsteady translation and
rotation of an object.
Research objectives were developed from the gaps in knowledge as defined during the literature survey.
The objectives were divided into three main activities; mathematical formulations for non-inertial
bulk flow and boundary layer equations, implementation of said formulations in a numerical solver and
simulations for various applications in arbitrary motion.
Mathematical formulations were developed for the bulk flow and boundary layer equations in arbitrary
motion. It was shown that the conservation of momentum and energy equations remains invariant
in the non-inertial forms. The conservations of momentum equation can at most have six fictitious terms
for unsteady arbitrary motion. The origin of the terms were found to be from transformation of the material
derivative to the non-inertial frame. All fictitious terms were found to be present in the boundary
layer equations, none could be eliminated during an order of magnitude analysis.
The vector form of the non-inertial equations were implemented in a novel OpenFOAM solver. The
non-inertial solver requires prescribed motion input and operate on a stationary mesh. Validation of the
solver was done using analytical solutions of a steady, laminar flat plate and rotating disk respectively.
Numerical simulation were done for laminar flow on a translating plate, rotating disk and rotating
cone in axial flow. A test matrix was executed to investigated various cases of acceleration and deceleration
over a range of 70 g to 700 000g. The boundary layer profiles, boundary layer parameters and
skin friction coefficients were reported.
Three types of boundary layer responses to arbitrary motion were defined. Response Type I is viscous
dominant and mimics the steady state velocity profile. In Response Type II certain regions of the
boundary layer are dominated by viscosity and others by momentum. Response Type III is dominated
by momentum. In acceleration the near-wall velocity gradient increases with increasing acceleration. In
deceleration separation occurs at a result of momentum changes in the flow.
The mechanism that causes these responses have been identified using the developed boundary layer
equations. In acceleration the relative frame fictitious terms become a momentum source which results
in an increase in velocity gradient at the wall. In deceleration the relative frame fictitious terms become
a momentum sink that induced an adverse pressure gradient and subsequently laminar separation.Hierdie tesis is gerig op die ontwikkeling van 'n fundamentele begrip aangaande die grenslaag reaksie op arbitrêre beweging. In hierdie konteks word arbitrêre beweging gedefinieer as die ongestadigde translasie en rotasie van 'n voorwerp. Navorsingsdoelwitte is ontwikkel uit die gapings soos omskryf in die literatuuroorsig. Die doelwitte is verdeel in drie hoof aktiwiteite; wiskundige formulerings vir ongestadigde vloei en grenslaag vergelykings, implementering van hierdie formulerings in 'n numeriese kode en simulasies vir verskeie gevalle van arbitrêre beweging. Wiskundige formulerings is ontwikkel vir die vloei en grenslaag vergelykings in arbitrêre beweging. Daar is bewys dat die behoud van massa en energie vergelykings onveranderd in die nie-inertiële vorms bly. Die behoud van momentum vergelyking kan hoogstens ses fiktiewe terme vir ongestadigde, arbitrêre beweging hê. Die oorsprong van die terme is vanuit die transformasie van die ongestadigde en adveksie terme (aan die linker kant van die momentum vergelyking) na die nie-inertiële raam. Alle fiktiewe terme is teenwoordig in die grenslaag vergelykings. Die vektor vorm van die nie-inertiële vergelykings is in 'n nuwe OpenFOAM oplosser geïmplementeer. Die nie-inertiële oplosser vereis voorgeskrewe beweging insette en werk op 'n stilstaande rooster. Die oplosser is getoets teen analitiese oplossings van 'n gestadigde, laminêre plaat plaat en 'n roterende skyf, onderskeidelik. Numeriese simulasies is gedoen vir laminêre vloei op 'n translerende plaat, roterende skyf en roterende konus in aksiale vloei. 'n Toets matriks is gebruik om ondersoek in te stel na gevalle van versnelling en vertraging oor 'n verskeidenheid van 70 g tot 700 000 g. Die grenslaag profiele, grenslaag parameters en oppervlak wrywingskoëffisiënte is aangemeld nie. Drie tipes grenslaag reaksies op arbitrêre beweging is gedefinieer. Reaksie Tipe I is viskeus dominant en boots die bestendige snelheidsprofiel na. In reaksie Tipe II sekere dele van die grenslaag is oorheers deur viskositeit en ander deur momentum. Reaksie Tipe III word in totaliteit oorheers deur momentum. In versnelling die snelheid helling teen die objek neem toe met toenemende versnelling. In vertraging is 'n negatiewe snelheidsprofiel waargeneem as gevolg van momentum veranderinge in die vloei. Die meganisme wat hierdie reaksies veroorsaak is geïdentifiseer deur die grenslaag vergelykings. In versnelling word die fiktiewe terme 'n bron van momentum. Dit lei tot 'n toename in snelheid helling op die objek. In vertraging word die fiktiewe terme 'n momentum gebruiker wat 'n negatiewe drukgradiënt veroorsaak en gevolglik laminêre vloei wegbreking veroorsaak.
Thesis (PhD)--University of Pretoria, 2016.
Mechanical and Aeronautical Engineering
PhD
Unrestricted
19
Erm, Lincoln. « Low-Reynolds-number turbulent boundary layers / ». Connect to thesis, 1988. http://eprints.unimelb.edu.au/archive/00000226.
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Hind, Michael D. « Quantification of flow structures generated by an oscillating fence actuator in a flat plate laminar boundary layer ». Laramie, Wyo. : University of Wyoming, 2008. http://proquest.umi.com/pqdweb?did=1798480911&sid=1&Fmt=2&clientId=18949&RQT=309&VName=PQD.
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Sinha, Roy Arijit. « Analysis and control of boundary layer transition on a NACA 0008 wing profile ». Thesis, KTH, Mekanik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-239931.
Texte intégralRésumé :
The main aim of this thesis was to understand the mechanism behind the classical transition scenario inside the boundary layer over an airfoil and eventually attempting to control this transition utilizing passive devices for transition delay. The initial objective of analyzing the transition phenomenon based on TS wave disturbance growth was conducted at 90 Hz using LDV and CTA measurement techniques at two different angles of attack. This was combined with the studies performed on two other frequencies of 100 and 110 Hz, in order to witness its impact on the neutral stability curve behavior. The challenges faced in the next phase of the thesis while trying to control the transition location, was to understand and encompass the effect of adverse pressure gradient before setting up the passive control devices, which in this case was miniature vortex generators. Consequently, several attempts were made to optimize the parameters of the miniature vortex generators depending upon the streak strength and stability. Finally, for 90 Hz a configuration of miniature vortex generators have been found to successfully stabilize the TS wave disturbances below a certain forcing amplitude, which also led to transition delay.
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Zincone, Eva. « Generation of laminar streaks in a Rankine-body boundary layer by free-stream vortical disturbances ». Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/19364/.
Texte intégral
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Voigts, Jakob. « The role of cortical layer six in the perception and laminar representation of sensory change ». Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/108887.
Texte intégralRésumé :
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, February 2017.Cataloged from PDF version of thesis. "September 2016."
Includes bibliographical references.
Neocortex learns predictive models of sensory input, allowing mammals to anticipate future events. A fundamental component of this process is the comparison between expected and actual sensory input, and the layered architecture of neocortex is presumably central to this computation. In this thesis, I examine the role of laminar differences, and specifically the role of layer 6 (L6) in the encoding and perception of stimuli that deviate from previous patterns. In awake mice, layer 4 neurons encode current stimulus deviations with a predominantly monotonic, faithful encoding, while neurons in layer 2/3 encode history dependent change signals with heterogeneous receptive fields. Corticothalamic (CT) cells in Layer 6 respond sparsely, but faithfully encode stimulus identity. Weak optogenetic drive of L6 CT cells disrupted this encoding in layer 6 without affecting overall firing rates. This manipulation also caused layer 2/3 to represent only current stimuli. In a head-fixed stimulus detection task, small stimulus deviations typically make stimuli more detectable, and the L6 manipulation removed this effect, without affecting detection of non-changing stimuli. Analogously, in free sensory decision making behavior, the manipulation selectively impaired perception of deviant stimuli, without affecting basic performance. In contrast, stronger L6 drive reduced sensory gain and impaired tactile sensitivity. These results show an explicit laminar encoding of stimulus changes, and that L6 can play a role in the perception of sensory changes by modulating responses depending on previous, or expected input. This finding provides a new perspective on how the layered cortical architecture can implement computations on hierarchical models of the world.
by Jakob Voigts.
Ph. D.
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Nchupang, Mojalefa Prince. « Stable and high order accurate finite difference method for the incompressible laminar boundary layer equations ». Master's thesis, Faculty of Engineering and the Built Environment, 2020. http://hdl.handle.net/11427/32732.
Texte intégralRésumé :
Numerical simulations of incompressible flows are unequivocally important due to their numerous industrial applications. These applications ranges from the large-scale fluid's flow modelling such as aerodynamics [1], atmospheric-ocean modelling [2] to a simple pipe flows in the petroleum industry [3]. This study is devoted to develop a provably stable and high order approximation for the incompressible laminar boundary layer equations. A new set of energystable boundary conditions are derived using the energy method. It is shown that both the weak and strong implementation of these boundary conditions yields an energy estimate. The semidiscrete problem is formulated by discretizing the continuous spatial derivatives using high order finite difference approximations on summation-by-parts form. The boundary conditions are implemented weakly using the simultaneous approximation terms methods. The discrete energy estimate is derived by mimicking the continuous analysis and hence, the numerical approximation is proved to be stable. The accuracy and linear stability of the developed scheme is also validated by solving the celebrated laminar flat plate flow problem. This is done by injecting the Blasius solution into the coefficient matrix as well as weak boundary conditions
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Ferro, Demetrio. « Effects of attention on visual processing between cortical layers and cortical areas V1 and V4 ». Doctoral thesis, Università degli studi di Trento, 2019. http://hdl.handle.net/11572/246290.
Texte intégralRésumé :
Visual attention improves sensory processing, as well as perceptual readout and behavior. Over the last decades, many proposals have been put forth to explain how attention affects visual neural processing. These include the modulation of neural firing rates and synchrony, neural tuning properties, and rhythmic, subthreshold activity. Despite the wealth of knowledge provided by previous studies, the way attention shapes interactions between cortical layers within and between visual sensory areas is only just emerging. To investigate this, we studied neural signals from macaque V1 and V4 visual areas, while monkeys performed a covert, feature-based spatial attention task. The data were simultaneously recorded from laminar electrodes disposed normal to cortical surface in both areas (16 contacts, 150 μm inter-contact spacing). Stimuli presentation was based on the overlap of the receptive fields (RFs) of V1 and V4. Channel depths alignment was referenced to laminar layer IV, based on spatial current source density and temporal latency analyses. Our analyses mainly focused on the study of Local Field Potential (LFP) signals, for which we applied local (bipolar) re-referencing offline. We investigated the effects of attention on LFP spectral power and laminar interactions between LFP signals at different depths, both at the local level within V1 and V4, and at the inter-areal level across V1 and V4. Inspired by current progress from literature, we were interested in the characterization of frequency-specific laminar interactions, which we investigated both in terms of rhythmic synchronization by computing spectral coherence, and in terms of directed causal influence, by computing Granger causalities (GCs). The spectral power of LFPs in different frequency bands showed relatively small differences along cortical depths both in V1 and in V4. However, we found attentional effects on LFP spectral power consistent with previous literature. For V1 LFPs, attention to stimuli in RF location mainly resulted in a shift of the low-gamma (∼30-50 Hz) spectral power peak towards (∼3-4 Hz) higher frequencies and increases in power for frequency bands above low-gamma peak frequencies, as well as decreases in power below these frequencies. For V4 LFPs, attention towards stimuli in RF locations caused a decrease in power for frequencies < 20 Hz and a broad band increase for frequencies > 20 Hz. Attention affected spectral coherence within V1 and within V4 layers in similar way as the spectral power modulation described above. Spectral coherence between V1 and V4 channel pairs was increased by attention mainly in the beta band (∼ 15-30 Hz) and the low-gamma range (∼ 30-50 Hz). Attention affected GC interactions in a layer and frequency dependent manner in complex ways, not always compliant with predictions made by the canonical models of laminar feed-forward and feed-back interactions. Within V1, attention increased feed-forward efficacy across almost all low-frequency bands (∼ 2-50 Hz). Within V4, attention mostly increased GCs in the low and high gamma frequency in a 'downwards' direction within the column, i.e. from supragranular to granular and to infragranular layers. Increases were also evident in an ‘upwards’ direction from granular to supragranular layers. For inter-areal GCs, the dominant changes were an increase in the gamma frequency range from V1 granular and infragranular layers to V4 supragranular and granular layers, as well as an increase from V4 supragranular layers to all V1 layers.
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Karasawa, Akane Sharon. « UNSTEADY TOTAL PRESSURE MEASUREMENT FOR LAMINAR-TO-TURBULENT TRANSITION DETECTION ». DigitalCommons@CalPoly, 2011. https://digitalcommons.calpoly.edu/theses/613.
Texte intégralRésumé :
This thesis presents the use of an unsteady total pressure measurement to detect laminar-to-turbulent transition. A miniature dynamic pressure transducer, Kulite model XCS-062-5D, was utilized to measure the total pressure fluctuations, and was integrated with an autonomous boundary layer measurement device that can withstand flight test conditions. Various sensor-probe configurations of the Kulite pressure transducer were first examined in a wind tunnel with a 0.610 m (2.0 ft) square test section with a maximum operational velocity of 49.2 m/s (110 mph), corresponding dynamic pressure of 1.44 kPa (30 psf). The Kulite sensor was placed on an elliptical nose flat plate where the flow was known to be turbulent. The Kulite sensor was then evaluated to measure total pressure fluctuations in laminar, turbulent, and transition of boundary layers developed on the flat plate in the same wind tunnel. The root-mean-square value of total pressure fluctuations was less than 1 % of the local free-stream dynamic pressure in the laminar boundary layer, but was about 2 % in the turbulent boundary layer. The value increased to 4 % in transition, indicating that the total pressure fluctuation measurements can be used not only to distinguish the laminar boundary layer from the turbulent boundary layer, but also to identify the transition region. The unsteady total pressure measurement was also conducted in a with a 2.13 m (7.0 ft) by 3.05 m (10.0 ft) section with similar operational velocity range as the previous wind tunnel. The Kulite sensor was placed on a wing model under laminar and transition conditions. The testing yielded similar results, demonstrating the usefulness of total pressure measurement for identifying the laminar-to-turbulent transition.
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Imayama, Shintaro. « Studies of the rotating-disk boundary-layer flow ». Doctoral thesis, KTH, Strömningsfysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-158973.
Texte intégralRésumé :
The rotating-disk boundary layer is not only a simpler model for the study of cross-flow instability than swept-wing boundary layers but also a useful simplification of many industrial-flow applications where rotating configurations are present. For the rotating disk, it has been suggested that a local absolute instability, leading to a global instability, is responsible for the small variation in the observed laminar-turbulent transition Reynolds number however the exact nature of the transition is still not fully understood. This thesis aims to clarify certain aspects of the transition process. Furthermore, the thesis considers the turbulent rotating-disk boundary layer, as an example of a class of three-dimensional turbulent boundary-layer flows. The rotating-disk boundary layer has been investigated in an experimental apparatus designed for low vibration levels and with a polished glass disk that gave a smooth surface. The apparatus provided a low-disturbance environment and velocity measurements of the azimuthal component were made with a single hot-wire probe. A new way to present data in the form of a probability density function (PDF) map of the azimuthal fluctuation velocity, which gives clear insights into the laminar-turbulent transition region, has been proposed. Measurements performed with various disk-edge conditions and edge Reynolds numbers showed that neither of these conditions a↵ect the transition process significantly, and the Reynolds number for the onset of transition was observed to be highly reproducible. Laminar-turbulent transition for a ‘clean’ disk was compared with that for a disk with roughness elements located upstream of the critical Reynolds number for absolute instability. This showed that, even with minute surface roughness elements, strong convectively unstable stationary disturbances were excited. In this case, breakdown of the flow occurred before reaching the absolutely unstable region, i.e. through a convectively unstable route. For the rough disk, the breakdown location was shown to depend on the amplitude of individual stationary vortices. In contrast, for the smooth (clean-disk) condition, the amplitude of the stationary vortices did not fix the breakdown location, which instead was fixed by a well-defined Reynolds number. Furthermore, for the clean-disk case, travelling disturbances have been observed at the onset of nonlinearity, and the associated disturbance profile is in good agreement with the eigenfunction of the critical absolute instability. Finally, the turbulent boundary layer on the rotating disk has been investigated. The azimuthal friction velocity was directly measured from the azimuthal velocity profile in the viscous sublayer and the velocity statistics, normalized by the inner scale, are presented. The characteristics of this three-dimensional turbulent boundary-layer flow have been compared with those for the two-dimensional flow over a flat plate and close to the wall they are found to be quite similar but with rather large differences in the outer region.QC 20150119
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Appelquist, Ellinor. « The rotating-disk boundary-layer flow studied through numerical simulations ». Doctoral thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-200827.
Texte intégralRésumé :
This thesis deals with the instabilities of the incompressible boundary-layer flow thatis induced by a disk rotating in otherwise still fluid. The results presented include bothwork in the linear and nonlinear regime and are derived from direct numerical sim-ulations (DNS). Comparisons are made both to theoretical and experimental resultsproviding new insights into the transition route to turbulence. The simulation codeNek5000 has been chosen for the DNS using a spectral-element method (SEM) witha high-order discretization, and the results were obtained through large-scale paral-lel simulations. The known similarity solution of the Navier–Stokes equations for therotating-disk flow, also called the von K ́arm ́an rotating-disk flow, is reproduced by theDNS. With the addition of modelled small simulated roughnesses on the disk surface,convective instabilities appear and data from the linear region in the DNS are anal-ysed and compared with experimental and theoretical data, all corresponding verywell. A theoretical analysis is also presented using a local linear-stability approach,where two stability solvers have been developed based on earlier work. Furthermore,the impulse response of the rotating-disk boundary layer is investigated using DNS.The local response is known to be absolutely unstable and the global response, onthe contrary, is stable if the edge of the disk is assumed to be at radius infinity. Herecomparisons with a finite domain using various boundary conditions give a globalbehaviour that can be both linearly stable and unstable, however always nonlinearlyunstable. The global frequency of the flow is found to be determined by the Rey-nolds number at the confinement of the domain, either by the edge (linear case) or bythe turbulence appearance (nonlinear case). Moreover, secondary instabilities on topof the convective instabilities induced by roughness elements were investigated andfound to be globally unstable. This behaviour agrees well with the experimental flowand acts at a smaller radial distance than the primary global instability. The sharpline corresponding to transition to turbulence seen in experiments of the rotating diskcan thus be explained by the secondary global instability. Finally, turbulence datawere compared with experiments and investigated thoroughly.QC 20170203
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Benyahia, Abdelkader. « Mise en oeuvre et évaluation d’un modèle de transition à équations de transport pour la simulation d’écoulements en turbomachines ». Thesis, Toulouse, ISAE, 2012. http://www.theses.fr/2012ESAE0034/document.
Texte intégralRésumé :
La prise en compte de la transition laminaire-turbulent dans les codes Navier-Stokes demeure problématique d’un point de vue numérique dès lors qu’on s’intéresse à des géométries complexes tridimensionnelles tel qu’un étage de turbomachine. Le modèle à équations de transport Re se propose de résoudre ces difficultés et d’ouvrir la voie à la modélisation automatique du phénomène pour des simulations RANS. Le travail exposé dans ce mémoire de thèse a consisté à développer dans le code elsA le modèle de transition Re et à évaluer l’aptitude de ce modèle à capturer avec précision le phénomène de la transition laminaire-turbulent dans le cadre de simulations RANS en turbomachine. Après une étude bibliographique, l’implémentation du modèle dans le code elsA est décrite ainsi que des premières simulations validant les développements réalisés. Par la suite, des cas documentés relatifs à des applications en aérothermique sont traités. Les prévisions par CFD des flux de chaleur à la paroi sont en bon accord avec l’expérience, démontrant ainsi la capacité du modèle à capturer avec précision la nature de la couche limite. Dans une dernière partie, le modèle est appliqué à la prévision de la transition par bulbe de décollement sur des aubages de turbine basse pression. Les résultats obtenus indiquent que le modèle doit être amélioré afin de modéliser correctement le phénomène de transition par bulbe de décollement. Ces travaux ont permis de valider l’utilisation du modèle de transition Re dans le code elsA pour une large gamme d’applications en turbomachine, et laissent entrevoir la perspective d’utiliser ce modèleThe modelisation of laminar to turbulent transition is still difficult in RANS computations while dealing with three dimensional computational domains like turbomachinery domains. The laminar to turbulent transition model Re has been proposed to solve these issues and permit the automatic modeling of laminar to turbulent transition in RANS codes. The Re model has been introduced into the code elsA and evaluated thanks to several computations relative to turbomachinery. After a bibliographical study, the implementation of the model into the code elsA and the inital results are described in this document. In a third part, aerothermal issues in turbomachinery are investigated. The estimations of the heat fluxes by CFD are in good agreement with the experimental data which demonstrate the capability of the model to capture laminar and turbulent regions. In a last part the phenomenon of separation induced transition occuring on low pressure turbine blade is studied. The results obtained while employing the Re model show that the model need to be refined for these applications. The studied performed have permited the validation the use of the laminar to turbulent transition model Re with the code elsA for very different applications in turbomachinery
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Yao, Hanxun. « A local correlation-based transition model for Spalart-Allmaras turbulence model ». Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/17382.
Texte intégralRésumé :
The Spalart Allmaras (S-A) Reynolds-Averaged Navier-Stokes (RANS) model has had considerable success in application to a wide range of turbulent flow problems. However, as turbulent kinetic energy k is not available in the S-A model, the application of sub-models of additional physical phenomena may not be possible. These phenomena could include turbulent transition models, which require a freestream turbulent intensity, or combustion closures which require a turbulent time scale, or broadband noise prediction based on k. This thesis composes of three main parts. In the first part, a comprehensive study of γ-〖Re〗_θ transition model is presented. In the second part, a local correlation-based γ-〖Re〗_θ transition model is coupled with the Spalart-Allmaras turbulence model (SA-γ-〖Re〗_θ model) in a structured parallelized Navier-Stokes solver, Flamenco and in the OpenFOAM package. The detailed and complete summary of the modified governing equations and a suite of validation and verification tests are given. The results obtained prove the validity of the SA-γ-〖Re〗_θ model. In the third part, a novel turbulence model, denoted the S-A-K model, is developed coupling the Spalart-Allmaras model with a transport equation for kinetic energy k, enabling coupling with the SA model with γ-〖Re〗_θ model (SAK-γ-〖Re〗_θ). The closure strategy of S-A-K model is proposed and validated against four standard benchmark cases. Good results are obtained using the S-A-K model compared to the results from the classical S-A turbulence model and the k-ω Shear Stress Transport Turbulence Model (k-ωSST) model, and experimental data.
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Marensi, Elena. « Nonlinear unsteady disturbances generated by the interaction of free-stream vorticity with a laminar boundary layer ». Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/15471/.
Texte intégralRésumé :
As a contribution towards understanding the impact of free-stream perturbations on laminar-to-turbulent boundary layer transition, we calculate the signature of unsteady disturbances engendered by the interaction of free-stream vortical fluctuations with a laminar boundary layer over a flat and a curved plate. We concentrate on low-frequency perturbations which, in the case of a flat plate, induce strong streamwise-elongated components of the boundary-layer signature, known as Klebanoff modes or streaks. In boundary layers over suitably curved concave walls, Klebanoff modes are expected to develop into Gortler vortices. The generation and nonlinear evolution of the induced perturbations, which acquire an O(1) magnitude, are described on a self-consistent and first-principle basis using the mathematical framework of the nonlinear unsteady boundary-region equations (NUBREs), subject to appropriate upstream and far-field boundary conditions. The nonlinear response of a compressible flat-plate boundary layer to free-stream vorticity is investigated first. The problem is governed by the compressible NUBREs, which are derived herein for the first time. The free-stream flow is studied by including the boundary-layer displacement effect and the solution is matched asymptotically with the boundary-layer flow. The nonlinear interactions inside the boundary layer drive an unsteady two-dimensional flow of acoustic nature in the outer inviscid region through the displacement effect. Analytical solutions are derived by exploiting the well-known analogy with the flow over a thin oscillating airfoil, which is used herein for the first time to study unsteady boundary layers. In the subsonic regime the perturbation is felt from the plate in all directions, while at supersonic speeds the disturbance only propagates within the dihedron defined by the Mach line. Numerical computations are performed for carefully chosen parameters that characterize three practical applications: turbomachinery systems, supersonic flight conditions and wind-tunnel experiments. The results show that nonlinearity plays a marked stabilizing role on the velocity and temperature streaks, and this is found to be the case for low-disturbance environment such as flight conditions. Increasing the free-stream Mach number inhibits the kinematic fluctuations but enhances the thermal streaks, relative to the free-stream velocity and temperature respectively, and the overall effect of nonlinearity becomes weaker. An abrupt deviation of the nonlinear solution from the linear one is observed in the case pertaining to a supersonic wind tunnel. Large-amplitude thermal streaks and the strong abrupt stabilizing effect of nonlinearity are two new features of supersonic flows. In the second part of the thesis, the generation and nonlinear development of unsteady Gortler vortices in an incompressible boundary layer over a concave plate is studied. The centrifugal force caused by the concavity of the wall is included in the incompressible NUBREs. The results show that the stabilizing effect on nonlinearity is significantly intensified in the presence of centrifugal forces. Sufficiently downstream the nonlinear vortices generated at different free-stream turbulence levels Tu are stabilized to the same amplitude, suggesting that the initial intensity of the forcing becomes unimportant. At low Tu the perturbation undergoes a quasi-exponential growth with the growth rate being enhanced for lower frequencies and more curved plates. At higher Tu, in the typical range of turbomachinery applications, the Gortler vortices do not exhibit an exponential growth as nonlinearity saturates rapidly, and the wall curvature does not influence the boundary-layer response. Good quantitative agreement with direct numerical simulations and experimental data is obtained.
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Fuentes, Andres. « Interactions between the reaction zone and soot field in a laminar boundary layer type diffusion flame ». Thesis, University of Edinburgh, 2006. http://hdl.handle.net/1842/1765.
Texte intégralRésumé :
The concurrent spreading of a boundary layer type diffusion flame is studied. The impossibility of obtaining a low velocity laminar flow without any perturbation induced by buoyancy has lead to the development of an experimental apparatus for use in micro-gravity facilities. Based on previous experimental observations, an original numerical approach has been developed showing, first the dominating role of the radiative heat transfer on the structure of the flame and second the major role of the soot on the extinction phenomenon at the flame trailing edge. The influence of the forced flow velocity, the fuel injection velocity and oxygen concentration on the geometry of the flame has been examined by imaging of CH* and OH* radicals spontaneous emission. Laser-Induced Incandescence (LII) is used to determine the soot field concentration in the flame. The soot formation has been studied by Laser Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAHs). The interaction between the reaction zone and the field of soot formation/oxidation is taken into account to analyze the flame length. These results can be used as the experimental input data for a future complete validation of numerical model simulating the soot formation and oxidation in this kind of flame.
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da, Rocha Pinto João M. « Wall-based feedback control of a compressible laminar boundary layer subjected to free-stream vortical disturbances ». Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/11998/.
Texte intégralRésumé :
This thesis presents theoretical and numerical results on the penetration of small amplitude free-stream vortical disturbances into a compressible laminar boundary layer, the formation and evolution of streamwise-elongated, low-frequency fluctuations inside the boundary layer and the wall-based feedback control of such disturbances. The theoretical formulation of the low-frequency disturbances, also called laminar streaks or Klebanoff modes, builds upon the works of Leib, Wundrow & Goldstein [43], Ricco & Wu [58] and Ricco [56], and it is based on the compressible linearised unsteady boundary region equations. For the first time, the incompressible framework by Ricco [56] is extended to the compressible case. The initial and outer boundary conditions for the outer layer compressible disturbances are therefore derived and put into context of the compressible Klebanoff modes analysis by Ricco & Wu [58]. Numerical results on the boundary region equations for the compressible and incompressible cases are presented. The general adjoint theory is presented and applied to the compressible linear unsteady boundary region equations for the first time. The theoretical formulation considers blowing and suction and wall thermal actuation to attenuate the Klebanoff modes. This further develops the works of Cathalifaud & Luchini [13] on spatial control for the incompressible linear boundary region equations and of Zuccher, Luchini & Bottaro [72] for the incompressible nonlinear boundary region equations. However, the previous studies were limited to the incompressible cases and neglected the free-stream turbulent forcing. Numerical solutions of the attenuated Klebanoff modes via an iterative feedback algorithm are presented, focusing on optimal wall-normal blowing suction.
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Brandt, Luca. « Numerical studies of bypass transition in the Blasius boundary layer ». Doctoral thesis, KTH, Mechanics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3517.
Texte intégralRésumé :
Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak.
Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed.
Keywords:Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.
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Li, Hon Yee. « Constant Voltage Hot-Wire Anemometry for the Boundary Layer Data System ». DigitalCommons@CalPoly, 2013. https://digitalcommons.calpoly.edu/theses/1105.
Texte intégralRésumé :
To continue the development of the Boundary Layer Data System (BLDS), a constant voltage hot-wire anemometer (CVA) is implemented into the BLDS for flight-testing. The hot-wire anemometer was chosen as an alternative to the traditional pressure probe because of the ability to measure both average velocity and fluctuating velocity within the boundary layer. Previous work done on the benchtop has led to the design of miniaturization, flight-capable hardware for the BLDS. The next step in the development of the BLDS – CVA calls for quantifying the accuracy of the boundary layer measurements measured by the CVA system. To do this, numerous turbulent boundary layer velocity and fluctuating velocity profiles were taken on a flat-plate at various speeds within the Cal Poly 2x2 wind tunnel with both the traditional pressure probe and the CVA. These test resulted showed agreement between the hot-wire and pressure probe data. Once this was completed the new CVA hardware was tested along with the new software that was written for the BLDS – CVA. In addition, due to the limited memory space onboard the BLDS – CVA, an approximation had to be developed to convert the average voltage data from the BLDS – CVA to the average velocity data due to the non-linear calibration function. The approximation developed was able to match the exact values from a traditional calibration. Lastly, due to the inability to perform a laboratory calibration of the hot-wire at altitude, where the conditions differ significantly from the ground conditions, a new procedure for hot-wire calibration was developed. The method developed was validated through wind tunnel testing and a computer thermal/electric model. With the completion of this work, the BLDS – CVA is ready for flight-testing.
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Heinrich, Roland Adolf Eberhard. « Flat-plate leading edge receptivity to various free-stream disturbance structures ». Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184860.
Texte intégralRésumé :
The receptivity process by which two-dimensional, time-harmonic freestream disturbances generate instability waves in the incompressible Blasius boundary layer is investigated analytically. The importance of the leading edge region and the linear nature of the receptivity process are discussed, and Goldstein's (1983a, 1983b) theoretical framework for the leading edge receptivity problem is reviewed. His approach utilizes asymptotic matching of a region close to the leading edge, which is governed by the linearized unsteady boundary layer equation, with a region further downstream, which is described by an Orr-Sommerfeld type equation. The linearized unsteady boundary layer equation is solved numerically, using the slip velocity and pressure gradient obtained from the inviscid interaction of the freestream disturbance with the semi-infinite plate. A new method is developed to extract the receptivity coefficient from this numerical solution. The receptivity coefficient determines the amplitude of the instability wave--a quantity not available from classical stability theory. The freestream disturbances investigated are oblique plane acoustic waves, vortical gusts of various orientations convected downstream with freestream speed U(∞), and a Karman vortex street passing above the plate surface with speed U(p). In addition, the case of a semi-infinite plate in a channel of finite width subject to an upstream traveling acoustic wave on the upper plate surface is considered. For oblique acoustic waves, the dominant receptivity mechanism is related to scattering of the waves by the leading edge. In contrast, for vortical gusts the receptivity produced by leading edge scattering is very small. The boundary layer receptivity to a Karman vortex street is found to be a strong function of the speed ratio U(p)/U(∞). A pronounced influence of channel walls, which is related to the alternate cut-on of higher modes in the upstream and downstream channel halves, is found. A comparison of the present results with available experiments shows good qualitative and quantitative agreement.
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Crouch, Jeffrey D. « The nonlinear evolution of secondary instabilities in boundary layers ». Diss., Virginia Polytechnic Institute and State University, 1988. http://hdl.handle.net/10919/53684.
Texte intégralRésumé :
Following the concepts of stability analysis, a study is made of the pre-breakdown stage of transition to turbulence in boundary layers. The first step consists of a ’decoupling’ of the primary and secondary instabilities. A perturbation method is used to solve for the primary wave, in the absence of any secondary disturbances. Once the wave is calculated, it is decomposed into a basic flow portion and an interaction portion. The basic flow portion acts as a parametric excitation for the secondary wave. The interaction portion then captures the resonance effects of the secondary back onto the primary. A perturbation method is also used for the secondary and interaction components. The results obtained are in three principal forms: Landau constants, amplitude growth curves, and velocity functions. While in good agreement with experiments and simulations, these results offer new explanations to the observed processes. In addition, a physically-based transition criteria is established.Ph. D.
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Shin, Yong-su [Verfasser], et Ulrich [Akademischer Betreuer] Rist. « Stability of a laminar streaky boundary-layer behind a roughness element / Yong-su Shin. Betreuer : Ulrich Rist ». Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2015. http://d-nb.info/107984225X/34.
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Vasudevan, K. P. « Transition Zone In Constant Pressure Boundary Layer With Converging Streamlines ». Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/272.
Texte intégralRésumé :
The laminar-turbulent transition in viscous fluid flows is one of the most intriguing problems in fluid dynamics today. In view of the enormous applications it has in a variety of fields such as aircraft design, turbomachinery, etc., scientists have now realized the importance of tackling this problem effectively. Three-dimensional flows are usually associated with pressure gradient, streamline curvature, streamline convergence / divergence etc., all acting simultaneously. Towards a better understanding of the transition process and modeling the transition zone, it is important to study the effect of each of these parameters on the transitional flow. The present work aims at studying experimentally the effect of lateral streamline convergence alone on the laminar-turbulent transition zone under constant stream-wise pressure.
The experimental setup consists of a low turbulence wind tunnel with its test section modified to cause lateral streamline convergence under constant pressure. This is achieved by converging the side-walls and appropriately diverging the roof, thus maintaining a constant stream-wise pressure. The half angle of convergence is chosen as 100 , which is approximately the same as the half of the turbulent spot envelope in constant pressure two-dimensional flows.
Experiments are carried out to analyze the development of the laminar and transitional boundary layers, intermittency distribution in the transition zone and the overall characteristics of an artificially induced turbulent spot.
The laminar velocity profiles are found to be of the Blasius type for two-dimensional constant pressure flows. However, the converging streamlines are found to contribute to an increased thickness of the boundary layer as compared to the corresponding two-dimensional flow.
The intermittency distribution in the transition zone is found to follow the universal intermittency distribution for two-dimensional constant pressure flow. A simple linear-combination model for two-dimensional flows is found to perform very well in predicting the measured velocity profiles in the transition zone.
An artificially introduced turbulent spot is found to propagate along a conical envelope with an apex cone angle of 220 which is very nearly the value for a corresponding constant pressure two-dimensional flow. The spot shapes and celerities are also comparable to those in two-dimensional flow.
In summary, the present study brings out many similarities between a constant pressure laterally converging flow and a constant pressure two-dimensional flow.
40
Valenzuela, Calva Fernando. « Numerical investigation of the interaction of synthetic jets with a laminar boundary layer and the effect of jet orientation ». Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/numerical-investigation-of-the-interaction-of-synthetic-jets-with-a-laminar-boundary-layer-and-the-effect-of-jet-orientation(5cc80987-a9e1-48ac-9e0f-18067c5f48fe).html.
Texte intégralRésumé :
In 2009, based on its commitment to take action on the climate change, the aviation industry accorded a group of objectives to reduce carbon dioxide emissions. Although only 2% of all human-induced carbon dioxide (CO2) emissions are produced by the global aviation industry, the aviation industry is set to grow in the next 30 years. In order to maintain this growth without increasing its negative environmental impact, the future aircraft have to be cleaner and greener. In order to reduce carbon emissions and increase the operative efficiency, novel technologies have been developed and applied on aircraft. One of the recently introduced technologies is the flow control over the wing by employing active flow control methods. Amongst the active flow control methods, synthetic jets have emerged as a developing and promising technology. The latter have been extensively investigated since 1990 in laboratory based investigations. In spite of the fact that many experimental studies have been performed to design synthetic jet actuators for optimal flow control, due to the the vast number of operating parameters involved, and the lack of current measurement technologies, they can be impractical and highly expensive. Hence, there is a need for a systematic analysis to establish the optimal operating conditions with the highest effectiveness at the cost of minimum energy input, and the most suitable orientation of synthetic jet orifices. This would require enhanced comprehension of the inherent features of synthetic jets and their corresponding near wall effects. By using numerical simulations with a commercial CFD software (Star-CCM+), this thesis investigates some features associated with synthetic jet performance that are not fully understood, such as: • The optimal working configuration of a synthetic jet array embedded into a laminar detached boundary layer for flow separation control. • The effect of orifice orientation (inclined and skewed synthetic jets) over normal synthetic jets and their optimal working configuration in an attached laminar boundary layer.
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Imayama, Shintaro. « Experimental study of the rotating-disk boundary-layer flow ». Licentiate thesis, KTH, Mekanik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-95147.
Texte intégralRésumé :
Rotating-disk flow has been investigated not only as a simple model of cross flow instability to compare with swept-wing flow but also for industrial flow applications with rotating configurations. However the exact nature of laminar-turbulent transi- tion on the rotating-disk flow is still major problem and further research is required for it to be fully understood, in particular, the laminar-turbulent transition process with absolute instability. In addition the studies of the rotating-disk turbulent boundary- layer flow are inadequate to understand the physics of three-dimensional turbulent boundary-layer flow. In present thesis, a rotating-rotating disk boundary-layer flow has been inves- tigated experimentally using hot-wire anemometry. A glass disk with a flat surface has been prepared to archieve low disturbance rotating-disk environment. Azimuthal velocity measurements using a hot-wire probe have been taken for various conditions. To get a better insight into the laminar-turbulent transition region, a new way to describe the process is proposed using the probability density function (PDF) map of azimuthal fluctuation velocity. The effect of the edge of the disk on the laminar-turbulent transition process has been investigated. The disturbance growth of azimuthal fluctuation velocity as a function of Reynolds number has a similar trend irrespective of the various edge conditions. The behaviour of secondary instability and turbulent breakdown has been in- vestigated. It has been found that the kinked azimuthal velocity associated with secondary instability just before turbulent breakdown became less apparent at a cer- tain wall normal heights. Furthermore the turbulent breakdown of the stationary mode seems not to be triggered by its amplitude, however, depend on the appearance of the travelling secondary instability. Finally, the turbulent boundary layer on a rotating disk has been investigated. An azimuthal friction velocity has been directly measured from the azimuthal velocity profile in the viscous sub-layer. The turbulent statistics normalized by the inner and outer sclaes are presented.QC 20120529
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Högberg, Markus. « Optimal Control of Boundary Layer Transition ». Doctoral thesis, KTH, Mechanics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3245.
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Khapko, Taras. « Transition to turbulence in the asymptotic suction boundary layer ». Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-141344.
Texte intégralRésumé :
The focus of this thesis is on the numerical study of subcritical transition to turbulence in the asymptotic suction boundary layer (ASBL). Applying constant homogeneous suction prevents the spatial growth of the boundary layer, granting access to the asymptotic dynamics. This enables research approaches which are not feasible in the spatially growing case. In a first part, the laminar–turbulent separatrix of the ASBL is investigated numerically by means of an edge-tracking algorithm. The consideration of spanwise-extended domains allows for the robust localisation of the attracting flow structures on this separatrix. The active part of the identified edge states consists of a pair of low- and high-speed streaks, which experience calm phases followed by high energy bursts. During these bursts the structure is destroyed and re-created with a shift in the spanwise direction. Depending on the streamwise extent of the domain, these shifts are either regular in direction and distance, and periodic in time, or irregular in space and erratic in time. In all cases, the same clear regeneration mechanism of streaks and vor- tices is identified, bearing strong similarities with the classical self-sustaining cycle in near-wall turbulence. Bifurcations from periodic to chaotic regimes are studied by varying the streamwise length of the (periodic) domain. The resulting bifurcation diagram contains a number of phenomena, e.g. multistability, intermittency and period doubling, usually investigated in the context of low-dimensional systems. The second part is concerned with spatio–temporal aspects of turbulent ASBL in large domains near the onset of sustained turbulence. Adiabatically decreasing the Reynolds number, starting from a fully turbulent state, we study low-Re turbulence and events leading to laminarisation. Furthermore, a robust quantitative estimate for the lowest Reynolds number at which turbulence is sustained is obtained at Re QC 20140213
44
Atalayer, Senem Hayriye. « Analysis Of Stability And Transition In Flat Plate Compressible Boundary Layers Using Linear Stability Theory ». Master's thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/2/12605366/index.pdf.
Texte intégralRésumé :
In this study, numerical investigations of stability and transition problems were performed for 2D compressible boundary layers over a flat plate in adiabatic wall condition. Emphasis was placed on linear stability theory. The mathematical formulation for 3D boundary layers with oblique waves including detailed theoretical information was followed by use of the numerical techniques for the solution of resulting differential system of the instability problem, consequently an eigenvalue problem.
First, two-dimensional sinusoidal disturbances were analyzed at various Mach numbers including the subsonic, transonic, supersonic and even hypersonic flow speeds. In this case, the second mode (acoustic mode), namely the Mack mode, and its behavior with the increasing Mach number were visualized. The results were then compared with the available data in literature concluding with good agreements.
Secondly, similar analysis was carried out for oblique waves. Here, not only the effect of flow speed but also the effect of wave orientation was demonstrated. For this purpose, instability problem was solved for several wave angles at each Mach number in the range of M=0 and M=5. In this respect, the angle at which the waves were most unstable was also obtained at each investigated flow speed. The resultant stability diagrams corresponding to M=4 and higher Mach numbers for which both first and the second modes appear revealed that plane waves were more stable than oblique waves for the Tollmien-Schlichting mode, however, this was the opposite for the acoustic mode where oblique waves were observed to be more stable.
As a final step, estimation of the transition location was handled for the most unstable wave condition. Smith-Van Ingen transition method was applied as the prediction device. The results representing the influence of Mach number on transition Reynolds number were then compared with the experimental data as well as the numerical ones in literature ending up with very good agreements.
45
Dagenhart, J. Ray. « Crossflow stability and transition experiments in a swept-wing flow ». Diss., Virginia Tech, 1992. http://hdl.handle.net/10919/39084.
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Tadjfar, Mehran. « Receptivity of a laminar boundary layer to the interaction of a three-dimensional roughness element with time-harmonic free-stream disturbances / ». The Ohio State University, 1991. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487686243820977.
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Samad, Abdul. « The convective instability of the boundary-layer flow over families of rotating spheroids ». Thesis, University of Leicester, 2011. http://hdl.handle.net/2381/27576.
Texte intégralRésumé :
The majority of this work is concerned with the local-linear convective instability analysis of the incompressible boundary-layer flows over prolate spheroids and oblate spheroids rotating in otherwise still fluid. The laminar boundary layer and the perturbation equations have been formulated by introducing two distinct orthogonal coordinate systems. A cross-sectional eccentricity parameter e is introduced to identify each spheroid within its family. Both systems of equations reduce exactly to those already established for the rotating sphere boundary layer. The effects of viscosity and streamline-curvature are included in each analysis. We predict that for prolate spheroids at low to moderate latitudes, increasing eccentricity has a strong stabilizing effect. However, at high latitudes of ϴ ≥ 60, increasing eccentricity is seen to have a destabilizing effect. For oblate spheroids, increasing eccentricity has a stabilizing effect at all latitudes. Near the pole of both types of spheroids, the critical Reynolds numbers approach that for the rotating disk boundary layer. However, in prolate spheroid case near the pole for very large values of e, the critical Reynolds numbers exceed that for the rotating disk. We show that high curvature near the pole of prolate spheroids is responsible for the increase in critical Reynolds number with increasing eccentricity. For both types of spheroids at moderate eccentricity, we predict that the most amplified modes travel at approximately 76% of the surface speed at all latitudes. This is consistent with the existing studies of boundary-layer flows over the related rotating-disk, -sphere and -cone geometries. However, for large values of eccentricity, the traveling speed of the most amplified modes increases up to approximately 90% of the surface speed of oblate spheroids and up to 100% in the prolate spheroid case.
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Appelquist, Ellinor. « Direct numerical simulations of the rotating-disk boundary-layer flow ». Licentiate thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-146087.
Texte intégralRésumé :
This thesis deals with the instabilities of the incompressible boundary-layer flow that is induced by a disk rotating in otherwise still fluid. The results presented are mostly limited to linear instabilities derived from direct numerical simulations (DNS) but with the objective that further work will focus on the nonlinear regime, providing greater insights into the transition route to turbulence. The numerical code Nek5000 has been chosen for the DNS using a spectral-element method in an effort to reduce spurious effects from low-order discretizations. Large-scale parallel simulations have been used to obtain the present results. The known similarity solution of the Navier–Stokes equation for the rotating-disk flow, also called the von Karman flow, is investigated and can be reproduced with good accuracy by the DNS. With the addition of small roughnesses on the disk surface, convective instabilities appear and data from the DNS are analysed and compared with experimental and theoretical data. A theoretical analysis is also presented using a local linear-stability approach, where two stability solvers have been developedbased on earlier work. A good correspondence between DNS and theory is found and the DNS results are found to explain well the behaviour of the experimental boundary layer within the range of Reynolds numbers for small amplitude (linear) disturbances. The comparison between the DNS and experimental results, presented for the first time here, shows that the DNS allows (for large azimuthal domains) a range of unstable azimuthal wavenumbers β to exist simultaneously with the dominantβ varying, which is not accounted for in local theory, where β is usually fixed for each Reynolds number at which the stability analysis is applied. Furthermore, the linear impulse response of the rotating-disk boundary layer is investigated using DNS. The local response is known to be absolutely unstable. The global response is found to be stable if the edge of the disk is assumed to be at infinity, and unstable if the domain is finite and the edge of the domain is placed such that there is a large enough pocket region for the absolute instability to develop. The global frequency of the flow is found to be determined by the edge Reynolds number.QC 20140708
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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.
Texte intégralRésumé :
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.
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Knörnschild, Ulrich. « Untersuchungen zum laminar-turbulenten Transitionsprozess bei Anregung und Dämpfung schräglaufender Tollmien-Schlichting-Wellen ». Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2002. http://nbn-resolving.de/urn:nbn:de:swb:14-1018531593187-75100.
Texte intégralRésumé :
Als Teilprojekt des Themenkreises III "Transitionskontrolle" des Schwerpunkt-Forschungsprogramms "Transition" der Deutschen Forschungsgemeinschaft, konzentriert sich diese Arbeit auf experimentelle Grundlagenuntersuchungen zum laminar-turbulenten Grenzsichtumschlag. Die Experimente wurden in der Grenzschicht einer ebenen, parallel angeblasenen, hydraulisch glatten Platte durchgeführt. Einen besonderen Schwerpunkt bildet die Abhängigkeit der Entwicklung der Instabilitäten, der sogenannten Tollmien-Schlichting Wellen, von deren Schräglaufwinkel zur Plattenvorderkante. Weiterhin wird der Einfluss zahlreicher Parameter wie z.B. des Schalldruckpegels und der Anregungsfrequenz diskutiert. Die Anregung der Tollmien-Schlichting Wellen erfolgte über periodisches Ausblasen / Ansaugen von Luft durch oberflächenbündige Schlitze quer zur Strömungsrichtung. Mit einem zeitlich hochauflösenden, restlichverstärkendem Kamerasystem konnten Aufnahmen der Strömungsvisualisierung erzielt werden, die unter anderem die zeitliche Entwicklung von Wirbelstrukturen (Lambda- Wirbel) zeigen. Zur Analyse der experimentell gewonnen Daten werden vergleichend Berechnungen nach der "Linearen Stabilitätstheorie" diskutiert. Einen weiteren Schwerpunkt bilden Untersuchungen zur aktiven Transitionskontrolle. Dabei wird der künstlich angeregten Tollmien Schlichting Welle eine gegenphasige Störwelle stromab überlagert. Es konnte nachgewiesen werden, das mit diesem Verfahren entsprechend des Superpositionsprinzips, die anfängliche Störamplitude der Tollmien Schlichting Welle deutlich reduziert wird. Es kommt zu einer fast vollständigen Störauslöschung. Untersuchungen im Nahfeld der Störeinkopplung, sowohl im Bereich der Anregnung als auch der gegenphasigen Dämpfungseinkopplung, zeigen deren Auswirkung auf die Entwicklung der GrenzschichtA sub-project of the working group III, "Transition Control" of the German Research Community's project "Transition", this paper is focused on experimental fundamental investigations in the field of laminar-turbulent transition. The experiments were carried out in the boundary layer of a flat plat with tangential blowing. The main topic is the development of instabilities, or so-called Tollmien-Schlichting Waves (TSWs), based on the oblique angle between the TSWs and the leading edge. In addition the influence of other parameters, including the sound-pressure level and the frequency of the TSWs are discussed. The instabilities are initialised by suction and blowing through flush, oblique slots in the surface of a flat plate. Pictures of the flow visualisation, recorded with a high-speed camera system, show the time-resolved development of structures (Lamda-Vortices) within the boundary layer. In order to analyse the experimental data, a comparison is made between it and numerical calculations corresponding to the Linear Instability Theory. Another main topic is the investigation of the active transition control. According to the Super Position Principle a second wave with opposite phase is superimposed on the TSW. It can be demonstrated that this technique works with oblique waves too. The initialised instabilities can almost completely be cancelled out. Investigations very close to the initialising slots of the TSW with a high special resolution show their influence on the development of the boundary layer