Thèses : « Turbulent shear layers »

1

Abu-Hijleh, Bassam Abdel-Kareem A.-R. « Structure of supersonic turbulent reattaching shear layers / ». The Ohio State University, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487676261012304.

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2

Luo, Jian Yang. « Calculation of turbulent shear layers over highly curved surfaces ». Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/11500.

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3

Sreedhar, Madhu K. « Large eddy simulation of turbulent vortices and mixing layers ». Diss., This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-06062008-163324/.

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4

Wang, Kan. « Computational investigation of aero-optical distortions by turbulent boundary layers and separated shear layers ». Thesis, University of Notre Dame, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3578995.

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Aero-optical distortions are detrimental to airborne optical systems. To study distortion mechanisms, compressible large-eddy simulations are performed for a Mach 0.5 turbulent boundary layer and a separated shear layer over a cylindrical turret with and without passive control in the upstream boundary layer. Optical analysis is carried out using ray tracing based on the computed density field and Gladstone-Dale relation.

In the flat-plate boundary layer, the effects of aperture size, Reynolds number, small-scale turbulence, different flow regions and beam elevation angle are examined, and the underlying flow physics is analyzed. Three momemtum-thickness Reynolds numbers, Re_&thetas; = 875, 1770 and 3550, are considered. It is found that the level of optical distortions decreases with increasing Reynolds number within the Reynolds number range considered. The contributions from the viscous sublayer and buffer layer are small, while the wake region plays a dominant role followed by the logarithmic layer. By low-pass filtering the fluctuating density field, it is shown that small-scale turbulence is optically inactive. Consistent with previous experimental findings, the distortion magnitude is dependent on the propagation direction due to anisotropy of the boundary-layer vortical structures. Density correlations and length scales are analyzed to understand the elevation-angle dependence and its relation to turbulence structures. The applicability of Sutton's linking equation to boundary-layer flows is examined, and excellent agreement between linking equation predictions and directly integrated distortions is obtained when the density length scale is appropriately defined.

The second case studied involves a separated shear layer over a cylindrical turret with a flat window, with inflow from a flat-plate boundary layer with and without passive control devices. The flow and optical results show reasonable agreement with experimental data for the baseline case without control. Aperture size effect, frequency spectra of OPD and two-point spatial correlations of OPD are investigated. The similarities and differences of distortion characteristics compared to those induced by turbulent boundary layers are discussed. The distortions by a separated shear layer are much larger in magnitude and spatially less homogeneous than those induced by an attached boundary layer. It is found that pressure fluctuations are significant and play a dominant role in inducing density fluctuations and associated optical distortions in a separated shear layer, in contrast to the dominant role of temperature fluctuations in a turbulent boundary layer. When passive control is applied using a row of thin and tall pins in the upstream boundary layer, the numerical results confirm key experimental findings. The flow above the optical window is characterized by two distinct shear layers, whose combined effect leads to a significant reduction of density fluctuation magnitude in the main shear layer and associated optical distortions compared to the uncontrolled flow with a single strong shear layer.

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Hipp, Hans Christoph 1959. « Numerical investigation of mode interaction in free shear layers ». Thesis, The University of Arizona, 1988. http://hdl.handle.net/10150/276871.

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Numerical simulations of incompressible, two-dimensional, monochromatically and bichromatically forced laminar free shear layers are performed on the basis of a vorticity-velocity formulation of the complete Navier-Stokes equations employing central finite differences. Spatially periodic shear layers developing in time (temporal model) are compared with shear layers developing in the stream-wise direction (spatial model). The regimes of linear growth and saturation of the fundamental are quantitatively scrutinized, the saturation of the subharmonic and vortex merging are investigated, and the effects of a forcing phase-shift between fundamental and subharmonic. For the spatial model the appearance of an unforced subharmonic was also examined. It was found that contrary to temporal shear layers a significant control of vortex merging by means of a forcing phase-shift and vortex shredding are not possible in spatial shear layers due to strong dispersion.

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6

Schmidt, Martin Arnold. « Microsensors for the measurement of shear forces in turbulent boundary layers ». Thesis, Massachusetts Institute of Technology, 1988. http://hdl.handle.net/1721.1/14781.

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Ciochetto, David S. « Analysis of Three Dimensional Turbulent Shear Flow Experiments with Respect to Algebraic Modeling Parameters ». Thesis, Virginia Tech, 1997. http://hdl.handle.net/10919/36808.

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The extension of the theory for two dimensional turbulent boundary layers into three dimensional flows has met with limited success. The failure of the extended models is attributed to the anisotropy of the turbulence. This is seen by the turbulent shear stress angle lagging the flow gradient angle and by the behavior of the Reynolds shear stresses lagging that of the mean flow. Transport equations for the turbulent shear stresses were proposed to be included in a modeling effort capable of accounting for the lags seen in the flow. This study is aimed at developing algebraic relationships between the various Reynolds-averaged terms in these modeling equations. Particular emphasis was placed on the triple products that appear in the transport equations. Eleven existing experimental data sets were acquired from the original authors and re-examined with respect to developed and existing parameters. A variety of flow geometries were collected for comparison. Emphasis was placed on experiments that included all six components of the Reynolds stress tensor and triple products. Parameters involving the triple products are presented that appear to maintain a relatively constant value across regions of the boundary layer. The variation of these parameters from station to station and from flow to flow is discussed. Part of this study was dedicated to parameters that were previously introduced, but never examined with respect to the data that was collected. Results of these parameters are presented and discussed with respect to agreement or disagreement with the previous results. The parameters presented will aid in the modeling of three dimensional turbulent boundary layers especially with models that employ the transport equations for the Reynolds stresses.
Master of Science

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McGinnis, David C. « Aero Optic Characterization of Highly Turbulent Free Shear Layers Over a Backward Facing Step ». University of Cincinnati / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1367928372.

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9

Martin, Martin Laura. « Numerical study of sound scattering by isolated elliptic vortices and turbulent jet shear layers ». Electronic Thesis or Diss., Ecully, Ecole centrale de Lyon, 2024. http://www.theses.fr/2024ECDL0025.

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Cette étude est consacrée à la diffusion d'ondes acoustiques par des tourbillons isolés et couches de cisaillement de jet turbulentes. Lorsque les ondes acoustiques traversent un volume de turbulence, les fluctuations de la turbulence modifient la direction de propagation des ondes. En outre, si la turbulence évolue dans le temps, le contenu spectral du son change également, ce qui entraîne un élargissement spectral. Afin de mieux comprendre ces phénomènes, une série d'analyses numériques a été réalisée. Pour ce faire, un code fourni par Siemens a été utilisé, dans lequel les Equations d'Euler Linéarisées sont résolues par la Méthode de Galerkin Discontinue. Il simule la propagation des ondes acoustiques sur un écoulement de base défini par l'utilisateur. Pour prendre en compte l'élargissement spectral, le code a été modifié pour pouvoir interpoler en temps et en espace des données externes dépendant du temps dans l’écoulement de base. L'interpolation a été testée par des différentes études de convergence du champ de pression diffusé par une couche de mélange bidimensionnelle. D'autres caractéristiques ont également été mises en œuvre pour faire face aux instabilités numériques causées par l'inhomogénéité de l’écoulement de base. Dans un premier temps, la diffusion des ondes acoustiques causée par un tourbillon elliptique de Kirchhoff isolé est étudiée. Lorsque le tourbillon est fixé dans l'espace, l'étude se concentre sur les effets de l'ellipticité, de l'orientation du tourbillon par rapport à la direction de propagation de l'onde acoustique incidente, de la vitesse tangentielle du tourbillon et de sa taille par rapport aux ondes acoustiques. La diffusion a été également étudiée lorsque le tourbillon est convecté. Une attention particulière a été accordée à son ellipticité et à la vitesse de convection. Les résultats montrent que l'ellipticité et surtout l'orientation du tourbillon jouent un rôle clé dans la diffusion. Enfin, l'étude de la diffusion du son par les couches de cisaillement des jets turbulents est menée, où la source acoustique est située à l'axe du jet. Pour cela, les données interpolées dans l'écoulement de base du code DGM appartiennent à une base de données externe de jets ronds simulés par LES. Ces jets ont des nombres de Mach variant entre 0,3 et 1,3, et leur température est 1, 1,5 ou 2,25 fois la température ambiante. Ces paramètres modifient les propriétés des fluctuations turbulentes. Le contenu spectral de ces fluctuations est donc comparé entre les jets. Ensuite, les champs de pression obtenus avec des écoulements de base moyens et des écoulements de base turbulents, ainsi que la différence entre eux, sont présentés. Leurs directivités sont également discutées, ainsi que les spectres du champ acoustique. Les spectres sont caractérisés par une tonalité centrale à la fréquence de la source et deux lobes latéraux. Ils sont symétriques pour des nombres de Mach élevés. La position des lobes latéraux se rapproche du ton central et leurs niveaux augmentent avec la température du jet pour des jets à nombre de Mach constant, ce qui peut s'expliquer par les changements subis par les fluctuations de la turbulence
This study is consecrated to the scattering of acoustic waves by isolated vortices and turbulent jet shear layers. When the acoustic waves pass through a volume of turbulence, the fluctuations in the turbulence change the propagation direction of the waves. In addition, if the turbulence evolves in time, there is also a change in the sound spectral content, causing spectral broadening. In order to better understand these phenomena, a series of numerical analyses have been carried out. For this purpose, a code provided by Siemens has been used where the Linearised Euler Equations are solved by the Discontinuous Galerkin method. It simulates the acoustic wave propagation over a base flow defined by the user. To take into account the spectral broadening, the code has been modified to be able to interpolate time-dependent external data in time and space onto the base flow. The interpolation has been tested by different convergence studies of the pressure field scattered by a 2-dimensional mixing layer. Other features have been also implemented to cope with the numerical instability waves caused by the inhomogeneity of the base flow. Initially, the scattering of acoustic waves caused by an isolated Kirchhoff elliptic vortex is investigated. When the vortex is fixed in space, the study focuses on the effects of the ellipticity, the orientation of the vortex regarding the direction of propagation of the incident acoustic wave, the tangential velocity of the vortex and its size regarding the acoustic waves. The scattering has been investigated also when the vortex is convected. Special attention has been devoted to its ellipticity and the velocity convection. The results show that the ellipticity and especially the orientation of the vortex play a key role in the scattering. Finally, the study of the scattering of sound by turbulent jet shear layers is conducted, where the acoustic source is located at the jet axis. For that, the data interpolated in the base flow of the DGM code belong to an external database of round jets simulated by LES. These jets have Mach numbers varying between 0.3 and 1.3, and their temperature is 1, 1.5 or 2.25 times the ambience temperature. These parameters modify the properties of the turbulent fluctuations. Therefore, the spectral content of these fluctuations is compared between the jets. After that, the pressure fields obtained with mean base flows and turbulent base flows, and the difference between them are presented. Their directivities are also discussed, as well as the spectra of the acoustic field. The spectra are characterized by a central tone at the source frequency and two lateral lobes. They are symmetric for high Mach numbers. The position of the lateral lobes shifts closer to the central tone and their levels increase with the jet temperature for jets with constant Mach number, which can be explained by the changes undergone by the turbulence fluctuations

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Miller, Ronald J. « A Study of Passive Scalar Mixing in Turbulent Boundary Layers using Multipoint Correlators ». Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7574.

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This study analyzes a turbulent passive scalar field using two-point and three-point correlations of the fluctuating scalar field. Multipoint correlation functions are investigated because they retain scaling property information and simultaneously probe the concentration field for the spatial structure of the scalar filaments. Thus, multipoint correlation functions provide unique information about the spatial properties of the concentration filaments. The concentration field is created by the iso-kinetic release of a high Schmidt number dye into a fully developed turbulent boundary layer of an open channel flow. The concentration fields were previously measured using the planar laser-induced fluorescence technique. The two-point correlations of the fluctuating scalar field indicate that as the scalar field evolves downstream, the anisotropic influence of the tracer injection method diminishes, and the scalar field becomes dominated by the mean velocity shear. As the scalar filaments align with the mean velocity gradient, the elliptical shape associated with the contours of the correlation function tilts in the direction of the mean velocity gradient. As a result, the two-point correlation contours of the concentration fluctuations indicate that anisotropic conditions (i.e. the tilted, asymmetric, elliptical shape) develop as a consequence of the mean velocity shear. Three-point correlations of the fluctuating scalar field are calculated based on configuration geometries defined by previous researchers. The first configuration follows Mydlarski and Warhaft (1998), which employs two cold-wire measurements and Taylor's frozen turbulence hypothesis. The three-point correlation contours of the concentration fluctuations associated with the cold-wire measurements exhibit a symmetric characteristic V-shape. Similar symmetric properties are observed in the current study. The second set of configurations follows on recent theoretical predictions, which indicate that the three-point correlation of the fluctuating scalar field is dependent on the size, shape, and orientation of the triangle created by the three points. The current study analyzes two geometric configurations (isosceles and collinear). The geometric configurations are defined to ensure that the influence of the shape remains constant as the configuration is rotated, translated, and dilated. Additionally, the scaling exponent in the inertial-convective regime is calculated to determine the dependence of the correlation function on the size of the triangle pattern.

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Freeman, Jeffrey L. « On the Growth Rate of Turbulent Mixing Layers : A New Parametric Model ». DigitalCommons@CalPoly, 2014. https://digitalcommons.calpoly.edu/theses/1160.

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A new parametric model for the growth rate of turbulent mixing layers is proposed. A database of experimental and numerical mixing layer studies was extracted from the literature to support this effort. The domain of the model was limited to planar, spatial, nonreacting, free shear layers that were not affected by artificial mixing enhancement techniques. The model is split into two parts which were each tuned to optimally fit the database; equations for an incompressible growth rate were derived from the error function velocity profile, and a function for a compressibility factor was generalized from existing theory on the convective Mach number. The compressible model is supported by a detailed evaluation of the currently accepted models and practices, including error analysis of the convective Mach number derivation and a critical analysis of Slessor’s re-normalization technique which affected his 1998 compressibility parameter. Analysis of the database suggested that a distinction should be made between thickness definitions that are based on the velocity profile and those based on the density profile. Additionally, the accumulation of different normalization approaches throughout the literature was shown to have introduced non-physical variance in the trends. Resolution of this issue through a consistent normalization process has greatly improved the normality and scatter of the data and the goodness-of-fit of the models, resulting in R2 = 0.9856 for the incompressible model and R2 = 0.9004 for the compressible model.

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12

Guskey, Christopher R. « NEAR WALL SHEAR STRESS MODIFICATION USING AN ACTIVE PIEZOELECTRIC NANOWIRE SURFACE ». UKnowledge, 2013. http://uknowledge.uky.edu/me_etds/27.

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An experimental study was conducted to explore the possible application of dynamically actuated nanowires to effectively disturb the wall layer in fully developed, turbulent channel flow. Actuated nanowires have the potential to be used for the mixing and filtering of chemicals, enhancing convective heat transfer and reducing drag. The first experimental evidence is presented suggesting it is possible to manipulate and subsequently control turbulent flow structures with active nanowires. An array of rigid, ultra-long (40 μm) TiO2 nanowires was fabricated and installed in the bounding wall of turbulent channel flow then oscillated using an attached piezoelectric actuator. Flow velocity and variance measurements were taken using a single sensor hot-wire with results indicating the nanowire array significantly influenced the flow by increasing the turbulent kinetic energy through the entire wall layer.

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13

COHEN, JACOB. « INSTABILITIES IN TURBULENT FREE SHEAR FLOWS ». Diss., The University of Arizona, 1986. http://hdl.handle.net/10150/188143.

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The evolution of the large scale structures and the mean field were investigated in axisymmetric and plane mixing layers. Some aspects of the linear instability of an axisymmetric jet have been demonstrated. The axisymmetric geometry admits two additional length scales with relation to the two-dimensional shear layer: the radius of the jet column and the azimuthal wavelength. The importance of these two length scales in governing the instability of an axisymmetric jet was explored. The special case of a thin axisymmetric shear layer was analyzed and the results stressing the evolution of different azimuthal modes were compared with some phase-locked data which was produced by subjecting the jet to axisymmetric and helical excitation. The importance of the initial spectral distribution in a natural jet was demonstrated when it is used as an input to the amplification curve obtained from linear stability theory to predict a measured spectral distribution at a further downstream location. The inclusion of the nonlinear terms in the stability analysis reveals two main interactions: mean flow-wave interaction and wave-wave interaction. The modification of the mean flow of an axisymmetric jet was examined by exciting two azimuthal modes simultaneously. The interaction resulted in an azimuthal modulation of the mean velocity profile having a cosine shape. Effectively, the geometry of the jet was modified without changing the geometry of the nozzle. The coupling between an excited periodic disturbance and the mean flow was analyzed and the spatial evolution of both were compared with experimental results obtained in a plane mixing layer. The behavior of the concommittant Reynolds stresses is discussed in detail. The conditions under which one disturbance will transfer energy to another were derived and demonstrated in an axisymmetric jet. The interaction between a large amplitude plane wave with a weak subharmonic component was shown to enhance the amplification rate of the subharmonic. It was further shown that the nonlinear interaction between two azimuthal modes can produce a third azimuthal mode which was not initially present in the flow. The coupling between a fundamental wave and its subharmonic in a parallel plane mixing layer was demonstrated numerically.

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Naaseri, Masud. « Studies of complex three-dimensional turbulent flows ». Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/7379.

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Slessor, Michael David Dimotakis Paul E. « Aspects of turbulent-shear-layer dynamics and mixing / ». Diss., Pasadena, Calif. : California Institute of Technology, 1998. http://resolver.caltech.edu/CaltechETD:etd-03292005-085835.

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Soteriou, Marios C. « Numerical study of turbulent combustion in a shear layer ». Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/12333.

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Johnstone, Henry Webb 1956. « CONFINED JET-INDUCED MIXING AT A DENSITY INTERFACE (TURBULENT, SHEAR FLOW) ». Thesis, The University of Arizona, 1987. http://hdl.handle.net/10150/292003.

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Ailinger, Kevin Gerard. « Measurements of surface shear stresses under a three-dimensional turbulent boundary layer using oil-film laser interferometry ». Thesis, This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-11012008-063040/.

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Oljaca, Miodrag. « Optical phase distortions in a plane shear layer ». Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/18178.

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Baum, Bryan Alan. « The extension of rapid distortion theory to stratified shear flows ». Diss., Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/25971.

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21

Lindgren, Björn. « Flow facility design and experimental studies of wall-bounded turbulent shear-flows ». Doctoral thesis, KTH, Mechanics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3454.

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The presen present thesis spans a range of topics within thearea of turbulent flows, ranging from design of flow facilitiesto evaluation aluation of scaling laws and turbulence modelingdeling aspects through use of experimental data. A newwind-tunnel has been designed, constructed and evaluated at theDept. of Mechanics, KTH. Special attention was directed to thedesign of turning vanes that not only turn the flow but alsoallow for a large expansion without separation in the corners.The investigation of the flow quality confirmed that theconcept of expanding corners is feasible and may besuccessfully incorporated into low turbulence wind-tunnels. Theflow quality in the MTL wind-tunnel at the Dept. of Mechanics,KTH, was as also in investigated confirming that it still isvery good. The results are in general comparable to thosemeasured when the tunnel was as new, with the exception of thetemperature variation ariation that has decreased by a factorof 4 due to an improved cooling system.

Experimental data from high Reynolds number zeropressure-gradient turbulent layers have been investigated.These studies have primarily focused on scaling laws withe.g.confirmation of an exponential velocity defect lawin a region, about half the size of the boundary layerthickness, located outside the logarithmic overlap region. Thestreamwise velocity probability density functions in theoverlap region was found to be self-similar when scaled withthe local rms value. Flow structures in the near-wall andbuffer regions were studied ande.g. the near-wall streak spacing was confirmed to beabout 100 viscous length units although the relative influenceof the near-wall streaks on the flow was as found to decreasewith increasing Reynolds number.

The separated flow in an asymmetric plane diffuser wasdetermined using PIV and LDV. All three velocity componentswere measured in a plane along the centerline of the diffuser.Results for mean velocities, turbulence intensities andturbulence kinetic energy are presented, as well as forstreamlines and backflow coefficientcien describing theseparated region. Instantaneous velocity fields are alsopresented demonstrating the highly fluctuating flow. Resultsfor the above mentioned velocity quantities, together with theproduction of turbulence kinetic energy and the secondanisotropy inariant are also compared to data from simulationsbased on the k -wformulation with an EARSM model. The simulation datawere found to severely underestimate the size of the separationbubble.

Keywords:Fluid mechanics, wind-tunnels, asymmetricdiffuser, turbulent boundary layer, flow structures, PDFs,modeling, symmetry methods.

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Šálený, Vratislav. « Numerická simulace hluku generovaného nestabilitami ve smykové vrstvě ». Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2018. http://www.nusl.cz/ntk/nusl-383530.

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Predicting and inhibiting aerodynamically generated noise for fast moving vehicles such as cars, aircraft and trains is increasingly important. The tonal noise generated by the shear-layer instability of air flowing around the cavity opening is especially one of the most significant and most intense sources of aerodynamically generated noise. Computational aeroacoustics (CAA) based on the CFD simulations of compressible Navier-Stokes equations offers the most general approach to predicting those aerodynamically induced sounds. Aeroacoustics is practically always associated with turbulent flow and turbulence is the major challenge for CFD simulations. Four different turbulence modelling approaches are examined in this work. Three of them belong to the LES method category and one uses the URANS approach. Appropriate numerical discretization and iteration schemes have been identified for each of these approaches and implemented in the OpenFOAM open source CFD platform. The accuracy, computational performance and convergence reliability of those schemes have been subsequently studied during three-dimensional CFD simulations on a model of a suitable real object. The CFD simulation results are validated by a measurement. An organ pipe has been chosen as the object of this CAA research because it uses self-sustained oscillations, commonly referred as shear-layer (Rossiter) modes, as the source of its tone generation. The numerical simulation of the shear layer modes, respectively the noise generated by instability in the shear layer, is the subject of this work.

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23

Hellum, Aren. « Intermittency and the viscous superlayer in a single stream shear layer ». Diss., Connect to online resource - MSU authorized users, 2006.

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24

Kandil, Sherif M. « A computational study of mixing in a liquid jet impinging on an immiscible liquid layer ». Morgantown, W. Va. : [West Virginia University Libraries], 2001. http://etd.wvu.edu/templates/showETD.cfm?recnum=1941.

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Thesis (M.S.)--West Virginia University, 2001.
Title from document title page. Document formatted into pages; contains xii, 113 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 95-98).

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Semeraro, Onofrio. « Feedback control and modal structures in transitional shear flows ». Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-29754.

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Two types of shear flows are investigated in this thesis; numerical simulations are performed for the analysis and control of the perturbation arising in a boundary layer over a flat plate, whereas PIV measurements are analysed for the investigation of a confined turbulent jet. Modal structures of the flows are identified: the aim is to understand the flow phenomena and to identify reduced-order models for the feedback control design. The attenuation of three-dimensional wavepackets of streaks and Tollmien-Schlichting (TS) waves in the boundary layer is obtained using feedback control based on arrays of spatially localized sensors and actuators distributed near the rigid wall. In order to tackle the difficulties arising due to the dimension of the discretized Navier-Stokes operator, a reduced-order model is identified, preserving the dynamics between the inputs and the outputs; to this end, approximate balanced truncation is used. Thus, control theory tools can be easily handled using the low-order model. We demonstrate that the energy growth of both TS wavepackets and streak-packets is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is investigated by varying the number of actuators and ensors, the Reynolds number and the pressure gradient. The configuration can be possibly reproduced in experiments, due to the localization of sensing and actuation devices. A complete analysis of a confined turbulent jet is carried out using timeresolved PIV measurements. Proper orthogonal decomposition (POD) modes and Koopman modes are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.
QC 20110214

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Semeraro, Onofrio. « Active Control and Modal Structures in Transitional Shear Flows ». Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-117916.

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Flow control of transitional shear flows is investigated by means of numerical simulations. The attenuation of three-dimensional wavepackets of Tollmien-Schlichting (TS) and streaks in the boundary layer is obtained using active control in combination with localised sensors and actuators distributed near the rigid wall. Due to the dimensions of the discretized Navier-Stokes operator, reduced-order models are identified, preserving the dynamics between the inputs and the outputs of the system. Balanced realizations of the system are computed using balanced truncation and system identification. We demonstrate that the energy growth of the perturbations is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is analysed by varying the number of actuators and sensors, the Reynolds number, the pressure gradient and by investigating the nonlinear, transitional case. We show that delay of the transition from laminar to turbulent flow can be achieved despite the fully linear approach. This configuration can be reproduced in experiments, due to the localisation of sensing and actuation devices. The closed-loop system has been investigated for the corresponding twodimensional case by using full-dimensional optimal controllers computed by solving an iterative optimisation based on the Lagrangian approach. This strategy allows to compare the results achieved using open-loop model reduction with model-free controllers. Finally, a parametric analysis of the actuators/ sensors placement is carried-out to deepen the understanding of the inherent dynamics of the closed-loop. The distinction among two different classes of controllers – feedforward and feedback controllers - is highlighted. A second shear flow, a confined turbulent jet, is investigated using particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) modes and Koopman modes via dynamic mode decomposition (DMD) are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.

QC 20130207

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27

Ecker, Tobias. « Turbulence Statistics and Eddy Convection in Heated Supersonic Jets ». Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/51687.

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Supersonic hot jet noise causes significant hearing impairment to the military workforce and results in substantial cost for medical care and treatment. Detailed insight into the turbulence structure of high-speed jets is central to understanding and controlling jet noise. For this purpose a new instrument based on the Doppler global velocimetry technique has been developed. This instrument is capable of measuring three-component velocity vectors over ex-tended periods of time at mean data-rates of 100 kHz. As a demonstration of the applicability of the time-resolved Doppler global velocimetry (TR-DGV) measurement technique, statistics of three-component velocity measurements, full Reynolds stress tensors and spectra along the stream-wise direction in a cold, supersonic jet at exit Mach number Mj = 1.4 (design Mach number Md = 1.65) are presented. In pursuance of extending the instrument to planar op- eration, a rapid response photomultiplier tube, 64-channel camera is developed. Integrating field programmable gate array-based data acquisition with two-stage amplifiers enables high-speed flow velocimetry at up to 10 MHz. Incor- porating this camera technology into the TR-DGV instrument, an investigation of the perfectly expanded supersonic jet at two total temperature ratios (TTR = 1.6 and TTR = 2.0) was conducted. Fourth-order correlations which have direct impact on the intensity of the acoustic far-field noise as well as convective velocities on the lip line at several stream-wise locations were obtained. Comprehensive maps of the convective velocity and the acoustic Mach number were determined. The spatial and frequency scaling of the eddy convective velocities within the developing shear layer were also investigated. It was found that differences in the radial diffusion of the mean velocity field and the integral eddy convective velocity creates regions of locally high convective Mach numbers after the potential core. This, according to acoustic analogies, leads to high noise radiation efficiency. The spectral scaling of the eddy convec- tive velocity indicates intermittent presence of large-scale turbulence structures, which, coupled with the emergence of Mach wave radiation, may be one of the main driving factors of noise emission observed in heated supersonic jets.
Ph. D.

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Mayo, Jr David Earl. « The Turbulence Structure of Heated Supersonic Jets with Offset Total Temperature Non-Uniformities ». Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/93532.

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Noise induced hearing loss is a large concern for the Department of Defense. Personnel on aircraft carriers are exposed to dangerous noise levels of noise from tactical aircraft, causing hearing damage which results in significant costs for medical care and treatment. Additionally, NASA and the FAA have begun to investigate the viability of reintroducing supersonic commercial transport in the United States and one of the largest problems to address is reducing the noise impact of these aircraft on communities. The overarching goal of jet noise research is to optimize noise reduction techniques for supersonic jets. In order to achieve this, a more complete theoretical framework which links the jet boundary conditions to the turbulence production in the jet plume and the far-field radiated noise must be established. The research presented herein was conducted on the hypothesis that introducing thermal non-uniformities into a heated supersonic jet flow can favorably alter the turbulence structure in the jet shear layer, leading to reductions in radiated noise. To investigate the impact of temperature on the turbulence development in the jet, spatially resolved three-component velocity vectors were acquired using particle image velocimetry (PIV) performed on two small-scale perfectly expanded Mach 1.5 jet flows, one with a uniform temperature profile and another containing a geometrically offset temperature non-uniformity. Using the PIV data, the mean velocities, Reynolds stresses, and correlation coefficients were obtained from both jet flows and compared to analyze changes in the mean turbulence field. Small but significant reductions in the shear layer turbulence were observed in the near nozzle region of the thermally offset jet when compared to the uniform jet case. The changes result in a thickening of the shear layer nearest the location of the cold plume which alters the integral length scales of the coherent turbulent structures in the offset jet in a manner consistent with other techniques presented in the literature that reduce jet noise. Applying quadrant analysis, a conditional averaging technique, to the jet turbulence plume revealed changes in the statistical flow field of Reynolds shear stress structures. The changes provide strong evidence of the presence of intermittent stream-wise vortical structures which serve to reduce the spatial correlation levels of turbulence in the thermally offset jet flow when compared to the uniform baseline jet.
Doctor of Philosophy
Increasingly large and powerful engines are required as the mission requirements for tactical aircraft become more advanced. These demands come at the cost of an increased production of noise which is particularly hazardous to crewpersons operating on Navy aircraft carriers during take-off and landing. Noise-induced hearing loss from extended exposure to high noise levels has become a major medical expenditure for the Navy. To address this issue in tactical aircraft engines, the sources of jet plume noise must be reduced, but doing so requires improved understanding of the connections between nozzle boundary conditions, the jet turbulence plume, and the radiated noise while keeping in consideration system constraints and performance requirements. The current study introduces a novel method for controlling supersonic jet noise induced by turbulence through the introduction of an offset non-uniform temperature perturbation at the nozzle mouth. Non-invasive flow measurements were conducted using stereoscopic particle image velocimetry to obtain high-resolution velocity and turbulence data. Analysis of the flow data indicate that an offset reduced temperature plume introduced at the nozzle exit has a first-order effect on the turbulence evolution which result in small, but significant reductions in jet noise levels. The reductions observed are attributed to a disruption in the coherence of the primary noise generating turbulence structures in the jet plume which are associated with the formation of stream-wise vortical structures induced by the cold plume.

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Stella, Francesco. « Caractérisation d’un décollement turbulent sur une rampe : entraînement et lois d’échelle ». Thesis, Orléans, 2017. http://www.theses.fr/2017ORLE2043/document.

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Les décollements turbulents massifs sont des phénomènes communs qui peuvent causer des pertes et de nuisances aérodynamiques importantes dans les écoulements industriels, par exemple à l’arrière d’une aile d’avion. Ce travail contribue à leur compréhension par l’analyse phénoménologique d’un décollement turbulent, représentatif d’un grand nombre d’écoulements réels. Le premier objectif est d’identifier les lois d’échelle des décollements turbulents, notamment en rapport avec les caractéristiques de l’écoulement à l’amont de la rampe. Un deuxième objectif est l’analyse, à grande et à petite échelle, des mécanismes de transport de fluide qui pilotent le fonctionnement des décollements. A cet effet, une approche originale est proposée, basée sur une description expérimentale et analytique de la couche cisaillée décollée et des interfaces turbulentes qui la délimitent. Nos résultats suggèrent que les lois d’échelle du décollement varient de façon complexe selon l’interaction de la couche limite à l’amont, de la couche cisaillée et de l’écoulement potentiel extérieur. La taille du décollement est liée à l’intensité de l’entraînement turbulent de masse dans la couche cisaillée, qui à son tour dépend de la turbulence dans la couche limite, bien à l’amont du point de décollement. Cette dépendance pourrait s’appliquer à toute la gamme d’échelles turbulentes responsables du transport de masse. Ces observations montrent clairement le rôle de la couche cisaillée dans le fonctionnement des décollements massifs et suggèrent la faisabilité de stratégies de contrôle nouvelles, de type retro-action ou prédictif, basée sur l’entrainement turbulent
Massive turbulent separations are common phenomena that can cause sizeable aerodynamical losses and detrimental effects in industrial flows, for example on airplane wings. This work contributes to their understanding with a phenomenological analysis of a canonical turbulent separation, representative of a large number of real flows. The first objective is to identify the scaling laws of turbulent separations, in particular with respect to their dependencies on the characteristics of the flow upstream of the ramp. A second objective is the analysis, both at large and small scale, of the transfert mechanisms that drive the functioning of separated flows. To this end, a new approach is proposed, centered on the experimental and analytical description of the separated shear layer and of the turbulent interfaces that bound it. Our results suggest that the scaling laws of the separated flow vary in a complex way, in function of the interaction of the incoming boundary layer, the separated shear layer and the free-stream. The size of the separation is related to the intensity of turbulent mass entrainment within the shear layer, which in turn depends on the turbulence in the incoming boundary layer, well upstream of the separation point. This dependency might apply over the entire range of turbulent length scales that are responsible for mass transfer. These observations clearly show the role of the shear layer in the functioning of massive separation. They also suggest the feasibility of new control strategies, both of feedback and feed-forward type, based on turbulent entrainment

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Gunasekaran, Sidaard. « Relationship Between the Free Shear Layer, the Wingtip Vortex and Aerodynamic Efficiency ». University of Dayton / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1470231642.

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Maji, S., P. R. Hanmaiahgari, R. Balachandar, Jaan H. Pu, A. M. Ricardo et R. M. L. Ferreira. « A review on hydrodynamics of free surface flows in emergent vegetated channels ». MDPI, 2020. http://hdl.handle.net/10454/17820.

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Yes
This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.

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Akins, David J. « Control of the Turbulent Shear Layer Downstream of a Backward Facing Step using Nanosecond Pulse Driven Surface Plasma Discharges : Effects of Pulse Energy ». Thesis, The University of Arizona, 2016. http://hdl.handle.net/10150/613578.

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The influence of pulse energy on nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuation applied to the turbulent shear layer downstream of a backward facing step (BFS) is examined experimentally. The ns-DBD control mechanism, which is believed to be primarily thermal in contrast to most other flow control actuators, has been demonstrated in various high speed shear flows yet questions on fundamental physics and scaling remain unanswered. In this work, ns-DBD pulse amplitude is varied between 0.13mJ/cm-0.88mJ/cm per pulse in order to understand its effects on the excitation of a turbulent shear layer. This work is carried out at a fixed actuation frequency of 1000Hz which corresponds to St(θ) ≈ 0.005 based on initial momentum thickness. Both mean and phase-averaged data indicate a preference for the 0.33mJ/cm and 0.56mJ/cm pulse amplitudes. However, further analysis concludes that 0.33mJ/cm is the most favorable as seen from momentum thickness growth and TKE distribution. Further analysis through the use of schlieren imaging suggests that depreciating control authority for the highest pulse amplitude of 0.88mJ/cm may be a result of either increased plasma three dimensionality resulting in non-uniform heating, or a thermal saturation mechanism (overheating). Additional theoretical studies are required to substantiate these claims and to decipher between the two.

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Mathey, Fabrice. « Ecoulements cisaillés réactifs : étude par modélisation sous-maille du mélange et simulation numérique des grandes échelles ». Université Joseph Fourier (Grenoble), 1997. http://www.theses.fr/1997GRE10215.

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Le formalisme de la simulation des grandes echelles (les) pour l'etude des ecoulements turbulents est presente. Des tests, en configuration de turbulence homogene isotrope et de couche de melange temporelle, mettent en evidence le comportement des differents modeles de viscosite turbulente vis-a-vis de la dissipation generee par les correcteurs de flux du schema ppm. Une solution est proposee permettant d'eviter une dissipation numerique trop importante pour les ecoulements en turbulence developpee et d'eviter le developpement d'instabilites numeriques dues aux conditions initiales. Les modeles de melange sous-maille specifiques aux ecoulements reactifs sont ensuite presentes. Deux approches sont en particulier retenues. Une approche stochastique, permettant de reconstruire jusqu'aux plus petites echelles l'evolution d'un champ de scalaire : le lem (linear eddy model). On montre que cette approche permet en particulier de reproduire des lois d'echelles observees dans les experiences et s'ecartant des theories classiques du melange. Puis une approche probabiliste ou la structure du champ a petite echelle est presumee a partir d'une fonction de densite de probabilite. Cette derniere approche necessite la modelisation de la variance sous-maille de la fraction de melange. Pour cela, une nouvelle modelisation est presentee, permettant des simulations les sans parametres ajustable. Ce modele est valide par des tests a priori a partir de simulations lem a grands nombres de reynolds, puis par des simulations de couche de melange temporelle reactive. Dans ce dernier cas, les resultats obtenus lors des phases de transition et de turbulence developpees sont compares aux experiences de la litterature. On montre que les quantites de produits sont correctement representees par le modele sous-maille de melange. Ces simulations permettent de mettre en evidence les mecanismes de la transition de melange et le role des tourbillons longitudinaux.

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Bagheri, Shervin. « Analysis and control of transitional shear flows using global modes ». Doctoral thesis, Stockholm : Department of Mechanics, Royal Institute of Technology, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11894.

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Huret, Thomas. « Génération en soufflerie d'écoulements cisaillés représentatifs des écoulements environnementaux de couches limites atmosphériques par des dispositifs passifs ». Electronic Thesis or Diss., Centrale Lille Institut, 2024. http://www.theses.fr/2024CLIL0011.

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Dans le domaine de l'ingénierie du vent, les dispositifs les plus couramment utilisés à la fois en milieux académiques et industriels pour générer des écoulements de couches limites atmosphériques en soufflerie consistent à associer des rugosités de paroi à un alignement de «spires» (flèches) en amont. Si des modèles permettent effectivement de concevoir a priori ce type de dispositifs passifs pour généré un champs moyen donné, le contrôle du profil d’intensité turbulente nécessite un processus d’ajustements par essai-erreur long et coûteux à réitérer chaque nouvelle configuration souhaitée.En parallèle, des études récentes sur les écoulements turbulents en aval de grilles régulières et fractales ont établis qu’un modèle d’interaction de sillages permet de prédire l’intensité turbulente générée, dans des configurations sans cisaillement moyen. L’extension de ces lois d’échelles à des écoulements cisaillés non-uniformes rendrait possible un contrôle indépendant des profils de vitesse moyenne et d’intensité turbulente dès la phase de conception, contrairement aux dispositifs de « spires » actuels. Pour étudier expérimentalement cette possibilité, notre travail s’intéresse à des grilles passives d'un type particulier –des Grilles Multi-échelles Inhomogènes (MIG) – qui permettent l’ajustement local des paramètres du modèle d’interaction de sillages en fonction de l’altitude, tout en maitrisant le champs moyen généré. Ils se trouvent que les dispositifs de « spires » peuvent conceptuellement être assimilés à un cas particulier de grilles MIG « continues ».Dans un premier temps, un algorithme de conception de grilles MIG est développé pour générer un champs moyen donné, première étape avant une tentative de contrôle de la turbulence. Cet algorithme est validé expérimentalement par la conception de grilles MIG et de « spires » dédiés à la génération d’écoulements moyens représentatifs de tout ou partie d’une couche limite atmosphérique neutre. En particulier, ces études préliminaires révèlent l’existence d’un régime d’écoulement défavorable à l’établissement du champs moyen souhaité pour certaines configurations, ce qui amène à suggérer un ensemble de critères sur les paramètres géométriques locaux de grille pour les éviter.En outre, les résultats montrent que le modèle d’interaction de sillages établi initialement en écoulement uniforme permet effectivement un bon « collapse » des mesures d’intensité turbulente en aval de chaque niveau horizontal de grille MIG discrètes, même en présence d’un cisaillement moyen. Pour autant, une variabilité non-négligeable subsiste en fonction de l’intensité de ce cisaillement. Par ailleurs, même si un « collapse » des mesures en aval de « spires » est également observé sur une gamme étendue d’altitudes, la courbe résultant de cette mise à l’échelle ne coïncide pas avec celle obtenue en aval de grilles MIG discrètes.Afin de tenter d’expliquer à la fois l’influence du cisaillement moyen sur le déclin de turbulence et la différence de régime observée entre grilles MIG discrètes et spires, un modèle est développé à partir d’hypothèses simplificatrices sur l’équation sur l’énergie cinétique turbulente. Ce modèle est complété par des modèles empiriques décrivant l’évolution des contraintes de Reynolds et celle des longueurs intégrales. Un modèle similaire pour le taux de dissipation turbulente reste à établir.L’analyse des différentes hypothèses utilisées pour établir notre modèle de turbulence révèlent des variations de cisaillements et de vorticité longitudinale qui sont inattendues à la fois en termes de complexité et de persistance en aval des dispositifs générateurs. Les structures d’écoulement mises au jour sont considérées comme la cause principale des différences observées entre spires et grilles MIG discrètes. Notre travail indique ainsi la nécessité d’orienter l’effort de modélisation vers les écoulements de « sillage proche », directement autour des dispositifs passifs étudiés
In the field of wind engineering, a common practice for both research and industrial applications involving the generation of a neutral atmospheric boundary layer (ABL) in a wind tunnel consists in associating a roughness fetch with an upstream array of “spires”. This passive method enables to tailor the mean velocity profile on the basis of quantitative guidelines. At the expense of a further time-consuming trial-and-error design process, these devices can be adjusted to generate representative turbulent intensity profiles.In parallel, the downstream evolution of regular and fractal grid-generated turbulence has been recently shown to scale with a wake-interaction model which can be used to predict basic properties of downstream turbulence profiles in case of zero mean shear. If this scaling law were to apply to grid-generated shear flow, it would enable the independent tailoring of turbulent intensity and mean velocity profiles without trial-and-error, contrary to previous spires devices. In order to investigate these ideas experimentally, our work makes use of Multiscale Inhomogeneous Grids (MIG), a new type of passive device defined to vary the turbulence scaling parameters with altitude, while enabling a tailoring of the mean velocity profile. In particular, it is shown that "spires" represent a specific case of "continuous" MIG grids.A general MIG design algorithm is developed for the generation of a prescribed mean flow profile. It is experimentally validated for the design of both discrete MIG grids and spires aiming to generate full-depth and part-depth neutral atmospheric boundary layer configurations. The devices are studied over both smooth and rough walls in the SCL-PIV wind tunnel of ONERA Lille using both Hot-Wire Anemometry (HWA) and stereo Particle Image Velocimetry (S-PIV). This validation process reveals the existence of a defective grid regime for specific local geometric configurations of MIG grids. A set of criteria on local geometric parameters (i.e. local obstruction and local mesh aspect ratio) is suggested to avoid this defective grid regime.The turbulence intensity decay downstream of the designed MIG grids is then studied by scaling its streamwise profile measured downstream of each horizontal grid level by the corresponding wake-interaction parameters for zero shear grid flows. A very good collapse of the turbulence decay is observed for all discrete MIG grids, with however a slight remnant effect of the local mean shear. Moreover, a collapse of spires-generated curves is also observed over a specific range of altitudes, but on a different collapse curve than for discrete MIG grids.An attempt to take into account both the remaining effect of mean shear on the turbulence decay and the observed collapse discrepancy between discrete and continuous MIG grids leads to the development of a simplified Turbulent Kinetic Energy (TKE) model. Several terms of this model remain to be closed in order to provide an a priori prediction. Successful closing empirical models are developed for both Reynolds stress and integral length scales, but not for the turbulent dissipation rate due to a lack of experimental data.The experimental investigation of the different hypotheses of the TKE model surprisingly revealed complex mean shear and streamwise vorticity patterns persisting far downstream of both spires and discrete MIG grids. These diverse and complex structures, originating from the near-wake flows around the upstream devices, are acknowledged as the main cause of discrepancy between spires and MIG grids. Tailoring turbulence intensity for ABL generation in wind tunnel appears to require an understanding and a modeling of this near-wake complexity

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Malla, Bhupatindra. « Study of High-speed Subsonic Jets using Proper Orthogonal Decomposition ». University of Cincinnati / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1352397174.

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Millican, Anthony J. « Bio-Inspired Trailing Edge Noise Control : Acoustic and Flow Measurements ». Thesis, Virginia Tech, 2017. http://hdl.handle.net/10919/78376.

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Trailing edge noise control is an important problem associated mainly with wind turbines. As turbulence in the air flows over a wind turbine blade, it impacts the trailing edge and scatters, producing noise. Traditional methods of noise control involve modifying the physical trailing edge, or the scattering efficiency. Recently, inspired by the downy covering of owl feathers, researchers developed treatments that can be applied to the trailing edge to significantly reduce trailing edge noise. It was hypothesized that the noise reduction was due to manipulating the incoming turbulence, rather than the physical trailing edge itself, representing a new method of noise control. However, only acoustic measurements were reported, meaning the associated flow physics were still unknown. This thesis describes a comprehensive wall jet experiment to measure the flow effects near the bio-inspired treatments, termed “finlets” and “rails,” and relate those flow effects to the noise reduction. This was done using far-field microphones, a single hot-wire probe, and surface pressure fluctuation microphones. The far-field noise results showed that each treatment successfully reduced the noise, by up to 7 dB in some cases. The surface pressure measurements showed that the spanwise coherence was slightly reduced when the treatments were applied to the trailing edge. The velocity measurements clearly established the presence of a shear layer near the top of the treatments. As a whole, the dataset led to the shear-sheltering hypothesis: the bio-inspired treatments are effective based on reducing the spanwise pressure correlation and by sheltering the trailing edge from turbulent structures with the shear layer they create.
Master of Science
This thesis describes a project aimed at developing a technology inspired by the silent flight of owls, with the end goal of using this technology to reduce the noise generated by wind turbines. Specifically, the phenomenon known as "trailing edge noise" is the primary source of wind turbine noise, and is the noise source of interest here. It occurs when air turbulence (which can be thought of as unsteady air fluctuations) crashes into the rear (trailing) edge of wind turbine blades, scattering and producing noise. Typically, methods of reducing this noise source involve changing the shape of the trailing edge; this may not always be practical for existing wind turbines. Recently, inspired by the downy covering of owl feathers, researchers developed treatments that can be applied directly to the trailing edge, significantly reducing trailing edge noise. This bio-inspired concept was verified with numerous acoustic measurements. Based on those measurements, researchers hypothesized that the noise reduction was achieved by manipulating the incoming turbulence before it scattered off the trailing edge, rather than by changing the existing wind turbine blade, representing a new method of trailing edge noise control. However, as only acoustic measurements (not flow measurements) were reported, the changes in turbulence could not be examined. With the above motivation in mind, this thesis describes a comprehensive wind tunnel experiment to measure the changes in the aerodynamics and turbulence near the bio-inspired treatments, and relate those changes to the reduction in trailing edge noise. This was done using a hot-wire probe to measure the aerodynamics, as well as microphones to measure the radiated noise and surface pressure fluctuations. As a whole, the experimental results led to the shear-sheltering hypothesis: the bio-inspired treatments are effective based on the creation of a shear layer (a thin region between areas with different air speeds) which shelters the trailing edge from some turbulence, as well as by de-correlating surface pressure fluctuations along the trailing edge.

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Rehab, Hichem. « Structure de l'écoulement et mélange dans le champ proche des jets coaxiaux ». Université Joseph Fourier (Grenoble), 1997. http://www.theses.fr/1997GRE10074.

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Le present travail est une etude experimentale de l'ecoulement dans le champ proche des jets coaxiaux a fort rapport des vitesses initiales entre le courant annulaire et central. Les objectifs de l'etude sont de clarifier les mecanismes physiques qui gouvernent la dynamique de l'ecoulement et les regimes d'ecoulement. Deux regimes d'ecoulement sont identifies selon les valeurs du rapport des vitesses initiales comparee a une certaine valeur critique. Le premier regime est caracterise par un cone potentiel central dont la longueur est inversement proportionnelle au rapport des vitesses. Un modele d'entrainement base sur la conservation de la masse et le taux d'entrainement turbulent a travers les couches de melange permet d'expliquer et de predire la longueur du cone potentiel central. Le rapport des vitesses initiales represente le parametre cle de l'ecoulement. Au dela d'une valeur critique du rapport des vitesses, le cone potentiel central se brise et un ecoulement de recirculation s'installe. La bulle de recirculation qui se forme a un mouvement de precession a basse frequence et de large amplitude comparee au mode de jet. Le regime de recirculation favorise un melange precoce a la sortie des injecteurs par rapport au regime non recirculant. Les etapes de l'etablissement de la structure fine du melange dans ces jets sont analysees a l'aide d'un scalaire passif a gran nombre de schmidt. On distingue une etape de declenchement des instabilites de cisaillement, puis un processus de reduction des longueurs a etirement constant jusqu'aux echelles diffusionnelles. Le temps necessaire pour atteindre ces echelles depend du nombre de schmidt, et sa valeur elevee induit une pente anormale (-1) sur le spectre des fluctuations de concentration dans la gamme des echelles inertielles.

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Si-Ameur, Mohamed. « Simulations numériques de mélanges turbulents dans les écoulements cisaillés supersoniques ». Université Joseph Fourier (Grenoble ; 1971-2015), 1994. http://www.theses.fr/1994GRE10214.

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L'etude du melange turbulent dans des ecoulements cisailles supersoniques, couche de melange confinee entre parois et jet faiblement sous detendu, est abordee a l'aide de simulations numeriques tridimensionnelles instationnaires. Un code aux volumes finis est utilise dans cette optique. La technique de simulation des grandes echelles a ete utilisee, les effets sous-maille etant pris en compte par les proprietes de l'algorithme numerique ppm qui filtre les concentrations de vorticite a petite echelle. Cette technique nous a permis de considerer certains caracteres des ecoulements reels: prise en compte des parois, simulation explicite du developpement spatial, resolution a des echelles de l'ordre de la taille des dispositifs reels. Les effets de parois sont etudies en approximation bidimensionnelle et en situation reelle tridimensionnelle. Les resultats montrent que les calculs bidimensionnels sont peu realistes pour representer le caractere tridimensionnel de l'ecoulement. Les zones de melange sont analysees qualitativement et quantitativement. La formation de sous-couches au niveau de la zone de melange est evidente pour m#c 0. 6. Une schematisation d'un modele de melange dans une couche de melange compressible est proposee

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Khelif, Djamal. « Contribution a l'etude de la couche de melange plane : conditions de formation et evolution de la structure tourbillonnaire ». Poitiers, 1987. http://www.theses.fr/1987POIT2256.

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Etude experimentale de la structure tourbillonnaire dans une zone de melange confinee formee par la rencontre de deux ecoulements plans paralleles debouchant a vitesses differentes dans un canal rectangulaire. Visualisation des deux ecoulements incidents permettant de determiner l'influence de ces derniers sur les grosses structures coherentes. Traitement numerique de sequences d'images

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Bennaceur, Iannis. « Etude numérique de la diffusion d'une onde acoustique par une couche de cisaillement turbulente à l'aide d'une simulation aux grandes échelles ». Thesis, Aix-Marseille, 2017. http://www.theses.fr/2017AIXM0187/document.

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Lors des mesures acoustiques dans les souffleries à veine ouverte, les ondes acoustiques émises par une maquette ou une source située dans la veine se propagent dans la couche de cisaillement turbulente qui se forme aux abords du jet avant d’être reçues par les microphones localisés en dehors. L’onde acoustique interagit avec le champ de vitesse turbulent de la couche de mélange ce qui a pour effet de modifier son contenu spectral, de redistribuer spatialement son énergie et de moduler sa phase et son amplitude, on parle alors de diffusion acoustique. Cette thèse a consisté à l’étude de la diffusion d’une onde acoustique par une couche de cisaillement turbulente à l’aide d’une simulation numérique aux grandes échelles. Pour cela, il a d’abord été nécessaire de réaliser la simulation numérique aux grandes échelles d’une couche de cisaillement turbulente plane dans son régime auto-similaire. Dans un second temps, nous avons simulé l’interaction entre une onde acoustique et l’écoulement turbulent afin d’étudier les caractéristiques du champ de pression diffusé qui en résulte. Nous avons notamment vérifié que la simulation était capable de prédire précisément les fréquences sur lesquelles est répartie la plupart de l’énergie acoustique ainsi que la forme du spectre de pression diffusé. Finalement, le champ de vitesse du milieu turbulent qui est corrélé avec l’enveloppe du champ de pression diffusé a été reconstruit à l’aide de la méthode de l’estimation stochastique linéaire. Cette méthode nous a notamment permis de visualiser les larges structures turbulentes qui interviennent principalement dans le mécanisme de diffusion acoustique
During open jet wind tunnel measurements, the acoustic waves emitted by a device or an acoustic source located inside the flow propagate inside the turbulent shear layer that develops at the periphery of the jet before being received by microphones located outside the flow. The acoustic wave interacts with the turbulent velocity field leading to a change of directivity, a phase and amplitude modulation as well as a spectral re-distribution of the acoustic energy over a band of frequencies. This phenomenon is known as acoustic scattering. This work has consisted in the study of the scattering of an acoustic wave by a turbulent shear layer using large-eddy simulation. The first step of the study has consisted in the large-eddy simulation of a turbulent shear layer in its self-similar state. In a second second step, the direct computation of the interaction between the acoustic wave and the turbulent flow has been performed in order to study the characteristics of the resulting scattered pressure field. It has been shown that the numerical simulation is able to accurately predict the frequencies on which the main part of the scattered energy is redistributed, as well as the shape of the scattered pressure spectrum. Finally, the turbulent velocity field which is correlated with the envelope of the scattered pressure field is reconstructed using the linear stochastic estimation method. This method has enabled the visualization of the large turbulent structures that mainly take part in the acoustic scattering mechanism

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Tarbouriech, Laurent. « Développement d'une méthode de vélocimétrie par images de particules pour les grandes dimensions : application à l'étude expérimentale d'un sillage turbulent soumis à la rotation ». Grenoble INPG, 1996. http://www.theses.fr/1996INPG0212.

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Les objectifs de cette these etaient, dans un premier temps, de developper une methode de mesure globale et non intrusive adaptee aux grandes dimensions de la plaque tournante coriolis du legi (13 m de diametre). Dans un second temps, il fallait appliquer cette methode au sillage plan turbulent soumis a la rotation solide. La methode developpee permet de realiser des mesures de champs de vitesse a deux composantes par images de particules (piv). Grace a la tres haute definition offerte par l'acquisition sur film des images, nos mesures s'etendent sur plusieurs metres carres avec une erreur theorique maximum de 6%. De plus, nous acquerons 250 champs de vitesse consecutifs grace auxquels nous accedons, sous certaines conditions, aux donnees statistiques des vitesses, de la vorticite et de la divergence. Dans le sillage a nombre de reynolds eleve (re=5. 10#3 et re=10#4), les mesures par piv montrent qu'au moins deux mecanismes sont responsables de la destabilisation de la couche cisaillee anticyclonique. Le premier se manifeste par l'augmentation du melange turbulent et, en consequence, par l'elargissement du cisaillement considere. Le second est accompagne par une forte structuration tridimensionnelle. Nous proposons de decrire les effets de la rotation a l'aide d'un nombre de rossby totalement local (ro#1=<>/2) qui est le rapport de la vorticite verticale moyenne sur le parametre de coriolis. Il semble que la destabilisation se manifeste lorsque ro#1<-0,5 avec un maximum d'effet quand ce parametre est proche de -1. Les mesures par piv sont completees avec des mesures par profileurs doppler acoustiques et avec des visualisations. Ces dernieres montrent, notamment, certains comportements originaux des structures longitudinales (tridimensionnelles) et nous permettent de proposer une interpretation des termes des equations de la vorticite

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Kaya, Serpil. « Reynolds-averaged Navier-stokes Computations Of Jet Flows Emanating From Turbofan Exhausts ». Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/2/12610078/index.pdf.

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This thesis presents the results of steady, Reynolds-averaged Navier-Stokes (RANS) computations for jet flow emanating from a generic turbofan engine exhaust. All computations were performed with commercial solver FLUENT v6.2.16. Different turbulence models were evaluated. In addition to turbulence modeling issues, a parametric study was considered. Different modeling approaches for turbulent jet flows were explained in brief, with specific attention given to the Reynolds-averaged Navier-Stokes (RANS) method used for the calculations. First, a 2D ejector problem was solved to find out the most appropriate turbulence model and solver settings for the jet flow problem under consideration. Results of one equation Spalart-Allmaras, two-equation standart k-&
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, realizable k-&
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and SST k-&
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turbulence models were compared with the experimental data provided and also with the results of Yoder [21]. The results of SST k-&
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and Spalart-Allmaras turbulence models show the best agreement with the experimental data. Discrepancy with the experimental data was observed at the initial growth region of the jet, but further downstream calculated results were closer to the measurements. Comparing the flow fields for these different turbulence models, it is seen that close to the onset of mixing section, turbulence dissipation was high for models other than SST k-&
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and Spalart-Allmaras turbulence models. Higher levels of turbulent kinetic energy were present in the SST k-&
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and Spalart-Allmaras turbulence models which yield better results compared to other turbulence models. The results of 2D ejector problem showed that turbulence model plays an important role to define the real physics of the problem. In the second study, analyses for a generic, subsonic, axisymmetric turbofan engine exhaust were performed. A grid sensitivity study with three different grid levels was done to determine grid dimensions of which solution does not change for the parametric study. Another turbulence model sensitivity study was performed for turbofan engine exhaust analysis to have a better understanding. In order to evaluate the results of different turbulence models, both turbulent and mean flow variables were compared. Even though turbulence models produced much different results for turbulent quantities, their effects on the mean flow field were not that much significant. For the parametric study, SST k-&
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turbulence model was used. It is seen that boundary layer thickness effect becomes important in the jet flow close to the lips of the nozzles. At far downstream regions, it does not affect the flow field. For different turbulent intensities, no significant change occurred in both mean and turbulent flow fields.

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Scheller, Johannes. « Electroactive morphing for the aerodynamic performance improvement of next generation airvehicles ». Phd thesis, Toulouse, INPT, 2015. http://oatao.univ-toulouse.fr/14479/1/scheller_partie_1_sur_2_2.pdf.

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The need to improve the aerodynamic performance of air vehicles is the origin of intense research on the real-time optimization of the airfoil shape. This real-time optimization can only be achieved by morphing the airfoil using adequate materials and actuators. The object of this thesis is to study smart-material actuators for aerodynamic performance optimization on different time scales (low-frequent and high-frequent actuation). First, the effects of the distinct actuation types, low-frequency large-displacement shape-memory alloy (SMA) and high-frequency low-displacement piezoelectric, on the surrounding flow are analyzed separately using dedicated time-resolved particle image velocimetry (TR-PIV) measurements. The experiments showed the deformation capacity of the SMA technology under realistic aerodynamic loads. Furthermore, it was highlighted that despite the limited actuation frequency the “quasi-static” hypothesis has to be carefully adapted for the Reynolds number range of 200.000. The PIV measurements conducted behind the piezoelectrically actuated trailing edge showed the capacity of the actuator to reduce the shear-layer instability modes. An open-loop optimum actuation frequency of 60 Hz has been identified. Secondly, a hybridization of the two previously studied technologies has been proposed. The implied actuators, SMAs and macro fiber composites (MFCs), have been modelled and the combined actuation capacity has been demonstrated. The designed prototype NACA4412 airfoil has been tested in the S4 wind-tunnel of IMFT and it was shown that the combination of the two technologies allows acting on the shear-layer vortices as well as control the lift.

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Brun, Christophe. « Étude expérimentale et numérique de l'interaction forte entre sillages d'obstacles cylindriques ». Université Joseph Fourier (Grenoble ; 1971-2015), 1998. http://www.theses.fr/1998GRE10049.

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Cette etude s'interesse aux phenomenes caracteristiques de la formation et de l'interaction des sillages turbulents en aval d'obstacles cylindriques en configuration complexe. Pour le bas regime sous-critique de nombre de reynolds nous effectuons une investigation de la zone de proche sillage de facon a la fois experimentale via des mesures de vitesse par anemometrie laser doppler et numerique via des simulations 2d avec raffinement du maillage en paroi. Dans un premier temps nous etudions le cas de l'interaction forte entre deux cylindres en parallele dont l'ecartement bord a bord est inferieur au diametre. L'effet de blocage se traduit par l'apparition d'un jet inter-tubes. Deux regimes sont identifies par rapport a une valeur critique re#c du nombre de reynolds. En deca de re#c le jet se plaque de facon stable du cote de l'un des cylindres provoquant une disymetrie du double sillage forme en aval. Des nombres de strouhal differents sont retrouves dans la zone arriere de chacun des tubes. Au-dela de re#c le jet adopte un mouvement de battement aleatoire de part et d'autres de l'axe median des tubes. Nous relions l'apparition de ce phenomene d'intermittence a grande echelle de l'ecoulement a la naissance d'instabilites 2d de kelvin-helmholtz au sein de la couche cisaillee decollee. Ces deux effets d'asymetrie et de battement de l'ecoulement (selon le regime) s'estompent au-dela de cinq diametres pour laisser place a un sillage unique qui tend vers une etat de turbulence quasiment developpee. Les phenomenes de melange a grande echelle et de transfert d'energie et dissipation a petite echelle sont favorises par cette configuration d'interaction forte. Enfin, nous etudions experimentalement le cas plus complexe de l'ecoulement au sein et en aval d'un faisceau de tubes en quinconce a des regimes de reynolds equivalents. Les proprietes de melange sont accentuees par rapport au cas geometriquement et hydrodynamiquement equivalent de l'interaction forte entre deux tubes.

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Novozámský, Adam. « Střih větru jako nebezpečný jev v letectví ». Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2014. http://www.nusl.cz/ntk/nusl-231313.

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This thesis is about wind shear and its influence on aviation. There is wide theoretical description of wind shear and it’s implemented into aviation practice. There are also methods of observation, forecasting and reporting, for both pilots and meteorologist described. In this thesis, pilots can find useful recommendations for dealing with wind shear during the flight. Every reader can also read about famous aviation accidents and look at models of weather that caused those accidents. At the end of thesis there are analyses of a rawiosonde measurements related to wind shear reports. This whole thesis is focused on increasing awareness of wind shear in aviation from angle of view of both pilots and aviation meteorologist.

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Lang, Daniel Bernard. « Laser Doppler Velocity and Vorticity Measurements in Turbulent Shear Layers ». Thesis, 1985. https://thesis.library.caltech.edu/3291/10/Lang.1985.thesis_opt.pdf.

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A Laser Doppler Velocimeter (LDV) system was developed to measure the instantaneous spanwise vorticity, -w_z, in a turbulent shear layer. It was necessary to design and fabricate the LDV optics and processing electronics, as no commercially available LDV systems met the specifications of measuring the velocity at four closely spaced points to the requisite accuracy. Measurements were also made of the instantaneous u, v, u', v', and -u'v'. The instantaneous vorticity was processed to obtain an estimate of its probability density function, from which the mean and rms values were estimated. It was also possible to separate the irrotational fraction of the flow (-w_z ≈ 0) from the rotational (intermittent) fraction of the flow (-w_z ≠ 0). The development of the intermittency profiles, based on vorticity, as a function of the downstream distance from the splitter plate was studied. A notable feature is that the vorticity is found to have values opposite the mean sense of rotation, i.e., -w_z(t) < 0, a significant fraction of the time. Additionally, a detailed study was performed to evaluate the approximation of -∂v/∂x, in terms of various local temporal derivatives ∂v/u(y)∂t. The optimum choice for u(y) can be found and is influenced by the relative local convection velocities of the small and large scale structures.

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Hall, Jeffery Lawrence. « An experimental investigation of structure, mixing and combustion in compressible turbulent shear layers ». Thesis, 1991. https://thesis.library.caltech.edu/3727/1/Hall_jl_1991.pdf.

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Two-dimensional, compressible, turbulent shear layers are studied in a new wind tunnel facility. Both reacting and non-reacting flows are investigated, with one free stream velocity supersonic and the other subsonic. The combustion experiments are based on the use of low concentrations of hydrogen, nitric oxide and fluorine gases. Side-view Schlieren photographs of these reacting and non-reacting flows appear devoid of the two-dimensional, large-scale structures seen in incompressible flows. Comparison with all-subsonic flows produced in the same facility suggests that this lack of two-dimensional structure is due to the presence of the supersonic high-speed free stream velocity. Travelling shock and expansion waves are observed in the high compressibility flows, evidently created by turbulent structures convecting at supersonic velocities. Such waves are seen only in the low-speed fluid, with apparent convection velocities much higher than those predicted on the basis of isentropic pressure-matching arguments. The measured shear layer growth rates agree with previous results by other experimenters, except for a few cases at low compressibility and low density ratio. The fast chemistry regime is attained in some of the high compressibility flows tested. "Flip" experiments conducted in this regime indicate that the volume fraction of mixed fluid in the layer is substantially reduced as compared to previous incompressible results. These same flip experiments also reveal that compressibility significantly alters the entrainment ratio. Finally, it is observed that the shear layer growth rate is relatively insensitive to incident shock /expansion waves and significant heat release inside the shear layer.

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Mahle, Inga [Verfasser]. « Direct and large eddy simulation of inert and reacting compressible turbulent shear layers / Inga Mahle ». 2007. http://d-nb.info/985359358/34.

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Roberts, Fredrick Allen. « Effects of a Periodic Disturbance on Structure and Mixing in Turbulent Shear Layers and Wakes ». Thesis, 1985. https://thesis.library.caltech.edu/1123/1/Roberts_fa_1985.pdf.

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Large scale structure and mixing processes are investigated in chemically reacting wakes and shear layers to which a periodic disturbance is applied. The experiments employ a diffusion-limited acid-base reaction to directly measure the extent of mixing. Optical diagnostics used include laser absorption and laser induced fluorescence. Absorption of laser light by reacted product provides a measure of cross-stream average product. Fluorescence was measured by a self-scanning linear photodiode array using high speed computer data acquisition to obtain the product distribution across the layer.

Previous results showing that forcing alters the structure and growth rate of shear layers are confirmed. Forcing artificially extends the lifetime of vortices whose size is consistent with the disturbance wavelength. Amalgamation of smaller vortices is enhanced over that in the natural layer until the frequency locked scale is achieved. At high Reynolds number product measurements show reduction of product with forcing. At moderate Reynolds numbers, on the other hand, there is an increase in product when forced. In one case a five fold increase in product was observed. The differences are related to the different effects of forcing on entrainment, composition ratio and secondary structure.

A dramatic, order of magnitude increase in mixing was discovered for certain forced wake flows. This effect is strongly associated with an interaction between the spanwise organized wake vortices and the test-section side walls.

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