Дисертації з теми "Instability and transition"
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1
Zhao, Yongling. "Instability and Transition of Natural Convection Boundary Layers." Thesis, The University of Sydney, 2014. http://hdl.handle.net/2123/13126.
Повний текст джерелаАнотація:
The present work is concerned with the instability and transition of vertical natural convection boundary layers, which are investigated by numerical simulations and particle image velocimetry (PIV) experiments. Two-dimensional direct stability analyses are firstly implemented to study the instability characteristics of the boundary layers, and three-dimensional direct numerical simulations are then conducted to investigate the transitions of the boundary layers. Two broad categories of transitions, that is, the natural transition and the controlled transition, are investigated in this thesis. For the investigation of the controlled transitions, the K-type and H-type transitions are examined respectively. Furthermore, a PIV measurement of the flow characteristics of the boundary layers under natural transition and a preliminary PIV experiment of the K-type transition are conducted to provide qualitative and quantitative evidences for validating the numerical results.
2
Picella, Francesco. "Retarder la transition vers la turbulence en imitant les feuilles de lotus." Thesis, Paris, ENSAM, 2019. http://www.theses.fr/2019ENAM0014/document.
Повний текст джерелаАнотація:
Nombreuses stratégies de contrôle ont été récemment proposées par la communauté scientifique afin depouvoir réduire la traînée dans les écoulements pariétaux. Entre autres, les Surfaces Superhydrophobes (SHS) ontmontré leurs capacités de pouvoir réduire considérablement le frottement pariétal d’un écoulement liquide grâce à laprésence de microbulles de gaz piégées dans les nano-rugosités de la surface. Dans des conditions géométrique etthermodynamique données pour lesquelles la transition de mouillage est évitée (condition pour laquelle normalementla taille des rugosités qui caractérise la SHS est de plusieurs ordres de grandeur plus petite que l'échellecaractéristique de l'écoulement principal), on peut atteindre ce qu’on appelle ‘l'effet Lotus’, pour lequel l'écoulementglisse à la paroi, avec une vitesse différente de zéro.. Dans ce cadre, nous nous sommes proposés d’étudier, à l’aidede simulations numériques l’influence des SHS sur la transition laminaire-turbulent dans un écoulement de canal.Pour cela, nous avons réalisé une série de simulations numériques directes (DNS), allant de l'état laminaire au casturbulent pleinement développé, en traitant la plupart de scénarios de transition connu en littérature. Des analyses destabilité locale et globale ont aussi été réalisées afin de déterminer l’influence de ces surfaces sur la première phasedu processus de transition. Bien que la procédure de déclenchement de la transition contrôlée (type K, H, C,...) soitbien décrite dans la littérature, cela n’est pas le cas pour les transitions naturelles. À cette fin, une nouvelle méthode aété développée pour déclencher puis étudier la transition naturelle dans des écoulements de type canal. Cette méthodeest basée sur des mécanismes de réceptivité de l'écoulement (resolvent global) permettant de construire un forçagevolumique spécifique. Plusieurs approches pour modéliser les SHS ont été utilisées, de complexités croissantes, touten tenant en compte des caractéristiques physiques de ces surfaces. Dans un premier temps, une condition deglissement homogène a été utilisée et son influence analysée. Chaque rugosité a été ensuite discrétisée spatialement,d’abord avec une alternance de condition limite sur une surface plate, ensuite en tenant compte de la dynamique del’interface gaz-liquide par une méthode Lagrangienne-Eulerienne Arbitraire (ALE). Nous avons montré que les SHSpermettent d’efficacement retarder les transitions contrôlées mais qu’en revanche elles ont peu d’influence sur lestransitions naturelles (développant des stries de vitesse). En effet, ce comportement dérive de l'équilibre entre deuxeffets contradictoires. D’un côté, le glissement pariétal nuit au développement des structures cohérentes de typehairpin , en altérant le processus de vortex stretching-tilting . D’autre part, le mouvement de l’interface gaz-liquideinteragit avec les structures cohérentes de l'écoulement, en produisant des vitesses normales à la paroi favorisantdavantage le processus de sweep-ejection et entraînant le développement de structures en forme d’arche. Nous avonsmontré que les interfaces gaz-liquide statiques retardent la transition de façon analogue à une condition aux limiteshomogène (si l’hétérogénéité pariétale est petite). En revanche la prise en compte de leur dynamique limite le retardde la transition, montrant l’importance du modèle de SHS dans les écoulements transitionnelsMany passive control strategies have been recently proposed for reducing drag in wall-bounded shearflows. Among them, underwater SuperHydrophobic Surfaces (SHS) have proven to be capable of dramaticallyreducing the skin friction of a liquid flowing on top of them, due to the presence of gas bubbles trapped within thesurface nano-sculptures. In specific geometrical and thermodynamical conditions for which wetting transition isavoided (in particular, when the roughness elements characterizing the SHS are several orders of magnitude smallerthan the overlying flow), the so-called ’Lotus effect’ is achieved, for which the flow appears to slip on the surfacewith a non zero velocity. In this framework, we propose to study, by means of numerical simulations, the influence ofSHS on laminar-turbulent transition in a channel flow. To do so we have performed a series of direct numericalsimulations (DNS), from the laminar to the fully turbulent state, covering the majority of transition scenarios knownin the literature, as well as local and global stability analysis so to determine the influence of SHS onto the initialstages of the process. While the conditions for observing controlled K-type transition in a temporal channel flow arewell defined, this is not the case for uncontrolled ones. To this end, a novel theoretical numerical framework has beendeveloped so to enable the observation of natural transition in wall-bounded flows. This method, similarly to theFree-Stream-Turbulence framework available for the boundary layer flow, is capable of triggering uncontrolledtransition t hrough flow receptivity to a purpose-built forcing. Different surface modellings for the superhydrophobicsurfaces are tested. First, homogeneous slip conditions are used. Then, the spatial heterogeneity of the SHS has beenconsidered by modelling it as a flat surface with alternating slip no-slip boundary conditions. Finally, the dynamics ofeach microscopic liquid-gas free-surface has been taken into account by means of a fully coupled fluid-structuresolver, using an Arbitrary Lagrangian Eulerian formulation. We show that while SHS are ineffective in controllingtransition in noisy environment , they can strongly delay transition to turbulence for the K-type scenario . Thisbehaviour results from the balance of two opposing effects. On one hand slippery surfaces inhibit the development ofcharacteristic hairpin vortices by altering the vortex stretching-tilting process. On the other hand, the movement ofthe gas-liquid free-surfaces interacts with the overlying coherent structures, producing wall-normal velocities thatenhance the sweep-ejection process, leading to a rapid formation of hairpin-like head vortices. Thus, whenconsidering flat interfaces transition time is strongly increased, while taking into account the interface dynamicsinduces smaller changes with respect to the no-slip case, indicating the need for an appropriate modelling of SHS fortransition delay purposes
3
Patel, Sanjay. "Computational modelling of instability and transition using high-resolution methods." Thesis, Cranfield University, 2007. http://hdl.handle.net/1826/3235.
Повний текст джерелаАнотація:
This thesis concerns the numerical investigation of suddenly expanded flows featuring
separation, instabilities and transition, in the context of Implicit Large Eddy Simulation
(ILES). The study of separated flows through suddenly expanded geometries is a classic
yet complex area of research. These types of flows feature instabilities which may
lead to bifurcation. Non-linear bifurcation is of great importance when considering
hydrodynamic stability and the mechanism of laminar to turbulent flow transition.
A detailed numerical investigation of various high-resolution methods and their ability
to correctly predict the flow through a suddenly expanded and contracted geometry
demonstrates that the choice of the particular numerical method employed can lead
to an incorrect solution of the flow. The key di erence between the various highresolution
methods employed is in the calculation of the nonlinear wave-speed dependent
term. It is shown that the nonlinearity of this term provides an asymmetric
dissipation to the flow which triggers symmetry-breaking bifurcation in a fully symmetric
computational set-up. High-resolution simulations of three-dimensional flow
through a plane suddenly expanded channel at low Reynolds numbers show that this
type of flow is characterised by a symmetric separation of the fluid which is nominally
two-dimensional in the spanwise direction. Increasing the Reynolds number reveals
a symmetry-breaking bifurcation of the fluid flow which becomes three-dimensional
as Reynolds number is further increased. Simulations confirm that it is this threedimensional
disturbance which leads to the onset of time-dependent flow characterised
by the periodic shedding of vortices from the upstream recirculation zones.
Preconditioning techniques which aim to alleviate sti ness in the calculation of the
advective fluxes for low Reynolds number flows are shown to be unsuitable for flows
featuring instabilities. The added dissipation to the flow causes the prediction of an
incorrect stable solution or to an improper estimation of the size of the separation
bubbles.
Simulations of a synthetic jet issuing into quiescent air using various slope limiters
manage to capture the flow physics relatively well. Limiters are used to avoid a scheme
from being oscillatory and provide non-linear dissipation in the region of excessively
large gradients. The various limiters di er with regards to the amount of dissipation
they provide to the flow, hence the solution obtained is dependent on the limiter used.
4
Yoshimura, Kazuyuki. "Mode instability and chaoticity transition in one-dimensional anharmonic lattices." Kyoto University, 1997. http://hdl.handle.net/2433/202314.
Повний текст джерела
5
Robey, H. F. Liepmann H. W. Liepmann H. W. "The nature of oblique instability waves in boundary layer transition /." Diss., Pasadena, Calif. : California Institute of Technology, 1986. http://resolver.caltech.edu/CaltechETD:etd-05242007-150746.
Повний текст джерела
6
Savin, Deborah Jane. "Linear and nonlinear aspects of interactive boundary layer transition." Thesis, University College London (University of London), 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.243306.
Повний текст джерела
7
Hagan, J. "Nonlinear instabilities and transition to turbulence in magnetohydrodynamic channel flow." Thesis, Coventry University, 2013. http://curve.coventry.ac.uk/open/items/cc5976b0-419c-4944-a2ff-3af446a03d05/1.
Повний текст джерелаАнотація:
The present study is concerned with the stability of a flow of viscous conducting liquid driven by a pressure gradient between two parallel walls in the presence of a transverse magnetic field, which is investigated using a Chebyshev collocation method. This magnetohydrodynamic counterpart of the classic plane Poiseuille flow is generally known as Hartmann flow. Although the magnetic field has a strong stabilizing effect, the turbulence is known to set in this flow similarly to its hydrodynamic counterpart well below the threshold predicted by the linear stability theory. Such a nonlinear transition to turbulence is thought to be mediated by unstable equilibrium flow states which may exist in addition to the base flow. Firstly, the weakly nonlinear stability analysis carried out in this study shows that Hartmann flow is subcritically unstable to small finite-amplitude disturbances regardless of the magnetic field strength. Secondly, two-dimensional nonlinear travelling wave states are found to exist in Hartmann flow at substantially subcritical Reynolds numbers starting from Ren = 2939 without the magnetic field and from Ren ∼ 6.50 × 103Ha in a sufficiently strong magnetic field defined by the Hartmann number Ha. Although the latter value is by a factor of seven lower than the linear stability threshold Rel ∼ 4.83×104Ha and by almost a factor of two lower than the value predicted by the mean-field (monoharmonic) approximation, it is still more than an order of magnitude higher than the experimentally observed value for the onset of turbulence in this flow. Three-dimensional disturbances are expected to bifurcate from these two-dimensional travelling waves or infinity and to extend to significantly lower Reynolds numbers. The by-product of this study are two developments of numerical techniques for linear and weakly nonlinear stability analysis. Firstly, a simple technique for avoiding spurious eigenvalues is developed for the solution of the Orr-Sommerfeld equation. Secondly, an efficient numerical method for evaluating Landau coefficients which describe small amplitude states in the vicinity of the linear stability threshold is introduced. The method differs from the standard approach by applying the solvability condition to the discretised rather than the continuous problem.
8
Hosseini, Seyed Mohammad. "Stability and transition of three-dimensional boundary layers." Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-123175.
Повний текст джерелаАнотація:
A focus has been put on the stability characteristics of different flow types existing on air vehicles. Flow passing over wings and different junctions on an aircraft face numerous local features, ranging from different pressure gradients, to interacting boundary layers. Primarily, stability characteristics of flow over a wing subject to negative pressure gradient is studied. The current numerical study conforms to an experimental study conducted by Saric and coworkers, in their Arizona State University wind tunnel experiments. Within that framework, a passive control mechanism has been tested to delay transition of flow from laminar to turbulence. The same control approach has been studied here, in addition to underling mechanisms playing major roles in flow transition, such as nonlinear effects and secondary instabilities. Another common three-dimensional flow feature arises as a result of streamlines passing through a junction, the so called corner-flow. For instance, this flow can be formed in the junction between the wing and fuselage on a plane. A series of direct numerical simulations using linear Navier-Stokes equations have been performed to determine the optimal initial perturbation. Optimal refers to a perturbation which can gain the maximum energy from the flow over a period of time. Power iterations between direct and adjoint Navier- Stokes equations determine the optimal initial perturbation. In other words this method seeks to determine the worst case scenario in terms of perturbation growth. Determining the optimal initial condition can help improve the design of such surfaces in addition to possible control mechanisms.QC 20130604
RECEPT
9
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.
Повний текст джерелаАнотація:
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
10
Schmidt, Oliver [Verfasser]. "Numerical investigations of instability and transition in streamwise corner-flows / Oliver Schmidt." München : Verlag Dr. Hut, 2014. http://d-nb.info/1052375626/34.
Повний текст джерела
11
Wang, Yaxing. "Instability and transition of boundary layer flows disturbed by steps and bumps." Thesis, Queen Mary, University of London, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.404889.
Повний текст джерелаАнотація:
An investigation of the influence of various wall perturbations on the development of instability waves has been carried out through a series of experiments to study different aspects of the stability/transition problem. The first experiment was devoted to an investigation of the effects of an isolated step on the transition of the boundary layer over a flat plate. Steps of various heights, both positive and negative, produced an upstream movement of the transition point, which was interpreted as arising from an increase in the amplification of the instability waves. The transition Reynolds number and the resulting change in the so-called 'N-factor', used for transition prediction, has been correlated with the step height to provide a useful empirical engineering design tool. The second experiment concerned the effect of a two-dimensional (2-D) sharpedged short bump on instability waves (Tollmien-Schlichting (TS) waves). The experiment was designed to study the mechanisms by which a sharp-edged surface roughness element, in the form of a 2-D bump, modified the evolution of TS wave and hence affected the process of transition. This investigation demonstrated that the interaction of a 2-D TS wave with a 2-D bump was a many-sided complex problem. The bump played multiple roles as a disturbance generator, an amplifier and a promoter of three-dimensionality. The crucial parameter was the height. Nonlinearity was the main factor that caused the enhancement of boundary layer transition and manifested itself in the appearance of higher harmonics in the spectra. In the last experiment the bump was three-dimensional (3-D) and in the form of a cylinder. The experimental results revealed that the effect of a 3-D bump was confined to a wedge-shaped region downstream of the bump. The mean velocity distortion had a distinct spanwise distribution from which a pair of streamwise vortex structures could be inferred. The interaction of a 2-D TS wave with a 3-D bump generated oblique waves with a dominant band of spanwise wavenumbers as predicted by theoretical analysis. Three thresholds were found that defined bump height ranges where there was no-effect, a linear behaviour and a nonlinear one.
12
Jouravleva, Karina. "Telomere-driven chromosome instability impacts the genetic program through genome-wide epigenetic reprogramming." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066286.
Повний текст джерелаАнотація:
Le raccourcissement télomérique est la source majeure de l'instabilité chromosomique (CIN) au cours de la progression tumorale. Nous avons montré que les cellules humaines embryonnaires de rein (cellules HEK) ayant traversé une période de CIN subissent des vastes changements dans l'expression des microARNs, ce qui induit une transition épithélio-mésenchymateuse (TEM), un processus permettant aux cellules cancéreuses épithéliales migrer et envahir de nouveaux tissus et former des métastases. Notre travail a aussi suggéré que les cellules ayant subi une TEM étaient capables de former des tumeurs dans un microenvironnement sénescent. De surcroît, cette évolution dans la capacité tumorale était associée à une dérégulation supplémentaire des microARNs et à l'acquisition des propriétés des cellules souches. Afin d'étudier comment ce potentiel est mis en place au cours de l'instabilité chromosomique et au contact avec le microenvironnement sénescent, nous avons modulé les niveaux d'expression de miR-145 et avons démontré que la répression de miR-145 était nécessaire pour le développement des caractéristiques des cellules souches. Afin de mieux comprendre l'impact de CIN sur le programme génétique des cellules épithéliales, nous avons utilisé des approches de haut débit et avons caractérisé les changements des paysages chromatiniens et leur mise en place dans les cellules ayant traversé une période de CIN. Nos résultats révèlent pour la première fois que l'instabilité télomérique modifie profondément la distribution des marques d'histones en conduisant aux changements d'expression des gènes et au processus de transformation des cellules épithéliales pré-tumoralesTelomere shortening is a major source of chromosome instability (CIN) at early stages during carcinogenesis. However, the mechanisms through which telomere-driven CIN (T-CIN) contributes to the acquisition of tumor phenotypes remain uncharacterized. We have shown that human epithelial kidney (HEK) cells undergo massive microRNA deregulation upon CIN, in particular a miR-200-dependent epithelial-mesenchymal transition (EMT), which is thought to enable epithelial cancer cells to migrate and invade other tissues to form metastases. Our work also indicated that CIN+ cells that underwent EMT were able to form tumors in a senescent microenvironment. Notably, this progression in tumor capacity was associated with further microRNA deregulation and the manifestation of enhanced stem-like properties. To investigate how stem-like properties are acquired in CIN+ cells in the contact with senescent microenvironment we adapted knockdown and overexpression approaches to modulate miR-145 expression, and demonstrated that enhanced stem-like properties depended on miR-145 repression. To fully apprehend the impact of CIN on the genetic program of epithelial cells, we used an unbiased approach to characterize the chromatin state of HEK CIN+ cells and uncover genome wide redistributions that were in direct correlation with gene expression changes. Our results reveal for the first time that T-CIN profoundly modifies the chromatin landscape genome-wide thereby fueling the transformation process of pre-tumor epithelial cells
13
Nicholls, Ursula Joy. "An investigation of a structural phase transition : the R-point instability in KMnF₃." Thesis, University of Edinburgh, 1987. http://hdl.handle.net/1842/15519.
Повний текст джерела
14
Valiev, Damir. "Flame Dynamics and Deflagration-to-Detonation Transition." Doctoral thesis, Stockholm : Industriell teknik och management, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4875.
Повний текст джерела
15
Liu, Chonghui. "A numerical investigation of instability and transition in adverse pressure gradient boundary layers." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape16/PQDD_0010/NQ36999.pdf.
Повний текст джерела
16
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.
Повний текст джерелаАнотація:
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
17
Hosseini, Seyed Mohammd. "On stability, transition and turbulence in three-dimensional boundary-layer flows." Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-177617.
Повний текст джерелаАнотація:
A lot has changed since that day on December 17, 1903 when the Wright brothers made the first powered manned flight. Even though the concepts behind flying are unaltered, appearance of stat-of-the-art modern aircrafts has undergone a massive evolution. This is mainly owed to our deeper understanding of how to harness and optimize the interaction between fluid flows and aircraft bodies. Flow passing over wings and different junctions on an aircraft faces numerous local features, for instance, acceleration or deceleration, laminar or turbulent state, and interacting boundary layers. In our study we aim to characterize some of these flow features and their physical roles. Primarily, stability characteristics of flow over a wing subject to a negative pressure gradient are studied. This is a common condition for flows over swept wings. Part of the current numerical study conforms to existing experimental studies where a passive control mechanism has been tested to delay laminarturbulent transition. The same flow type has also been considered to study the receptivity of three-dimensional boundary layers to freestream turbulence. The work entails investigation of effects of low-level freestream turbulence on crossflow instability, as well as interaction with micron-sized surface roughness elements. Another common three-dimensional flow feature arises as a resultof stream-lines passing through a junction, the so-calledcorner-flow. For instance, thisflow can be formed in the junction between the wing and fuselage on aplane.A series of direct numerical simulations using linear Navier-Stokes equationshave been performed to determine the optimal initial perturbation. Optimalrefers to perturbations which can gain the maximum energy from the flow overa period of time. In other words this method seeks to determine theworst-casescenario in terms of perturbation growth. Here, power-iterationtechnique hasbeen applied to the Navier-Stokes equations and their adjoint to determine theoptimal initial perturbation. Recent advances in super-computers have enabled advance computational methods to increasingly contribute to design of aircrafts, in particular for turbulent flows with regions of separation. In this work we investigate theturbulentflow on an infinite wing at a moderate chord Reynolds number of Re= 400,000 using a well resolved direct numerical simulation. A conventional NACA4412 has been chosen for this work. The turbulent flow is characterizedusing statistical analysis and following time history data in regions with interesting flow features. In the later part of this work, direct numerical simulation has been chosen as a tool to mainly investigate the effect of freestream turbulence on the transition mechanism of flow from laminar to turbulent around a turbine blade.QC 20151125
18
Valiev, Damir. "The role of Landau-Darrieus instability in flame dynamics and deflagration-to-detonation transition." Licentiate thesis, Stockholm : Materialvetenskap, Kungliga Tekniska högskolan, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4334.
Повний текст джерела
19
Moradi, Nour Zeinab. "Simulation and modal analysis of instability and transition in a steady eccentric stenotic flow." Thesis, KTH, Mekanik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-118945.
Повний текст джерелаАнотація:
Direct numerical simulation (DNS) of steady flow, with Re = 750 at inlet, through stenosed pipe has been done to study transition and turbulence of the flow in the post-stenosis area. The pipe has 75% constriction by area reduction and 5% eccentricity of the main pipe diameter at the throat. A sinusoidal Gaussian force is implemented to keep turbulent in the domain. The result shows acceptable agreement with previous study has been done by Fischer et al. [4]. We simulated the case by NEK5000 which benefits from the spectral element method (SEM) as a higher order accurate method. To have a better understanding of the turbulent flow, we have done the modal decomposition to obtain coherent structures. Among several methods for modal decomposition, we considered Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) for current study. The methods have been implemented in Fortran, accelerated using OpenMP and is potentially settled for computation of large data sets. DMD implementation shows 2.5 speed up. The stuctures correspond to the implementation force are extracted by POD however they have not been recognised by dynamic decomposition.
20
Trajkova, Natasha. "Instability and volatility of economic growth under transition : an application of exogenous growth theory." Thesis, Staffordshire University, 2013. http://eprints.staffs.ac.uk/2031/.
Повний текст джерелаАнотація:
The aim of this thesis is to explore growth processes in transition economies (TEs) by analysing differences between growth patterns in the course of transition and the smooth growth paths characteristic of developed market economies. Accordingly, the thesis builds upon the neoclassical growth theory 10 the transition context to develop a modified theoretical model that conceptualizes transition as a non~linear process consisting of three distinct stages or regimes of growth: the crash or adjustment stage; the recovery stage; and the take~off stage. Namely, instead of describing transition as a movement along one steady state linear growth path, this new approach depicts transition as a process of radical adjournments or shifts between growth paths caused by big structural changes in the economy. This theoretical model is tested not only informally against the observed growth patterns under transition, but also through a series of econometric investigations: (1) Perron's procedure for testing for structural breaks in the presence of a unit root in the data series; (2) a univariate Markov Switching Model (MSM) for assessing (a) whether or not the different hypothesized regimes exist in the data an~ (b) if different growth regimes do exist, both the instability between and volatility within growth regimes; and (3), a multivariate MS VAR model estimated as a small vector autoregression that repeats the univariate MSM investigation into growth regimes but conditional on both physical and human capital variables. The empirical evidence supports the concept of non~linear growth characterised by structural changes and regime shifts. In particular, the univariate MS analysis suggests that most TEs (19 from 26) have passed through all three regimes or stages of transition, with variations across groups in terms of the recorded mean GDP growth rates and the volatility in each regime. Conversely, the multivariate analysis brings forward a somewhat different depiction. Namely, although generally confirming the idea of instability and volatility, the MS VAR analysis suggests that only an elite "few", the five most developed TEs, now EU members have managed to pass through all three stages of transition, as identified in our theoretical model. They can be regarded as having completed their journey by becoming developed market economies In contrast; all the others recorded only two distinct regimes. This result is consistent with our theoretical model in identifying three main stages or growth regimes in the transition process. Finally, the thesis appraises a new notion of transition as a process of dramatic non ~ linear changes that require correspondingly bold policies, particularly if the third regime leading to the developed market economy status is to be attained. Although this thesis does not prescribe specific policy recommendations, it does provide a particular perspective for policymaking, namely one oriented to long-run supply-side reforms.
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Plogmann, Benjamin [Verfasser]. "Experimental investigations on roughness initiated instability and transition in airfoil boundary layers / Benjamin Plogmann." München : Verlag Dr. Hut, 2015. http://d-nb.info/1079768254/34.
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Rowley, Craig Emlyn. "Transient behaviour in boundary layers close to the transition from convective to absolute instability." Thesis, Keele University, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.398909.
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Lefort, Eric. "Caractérisation des bifurcations et de la dynamique d'une lentille thermique par analyse spectrale." Rouen, 1987. http://www.theses.fr/1987ROUES019.
Повний текст джерелаАнотація:
Un faisceau laser est focalisé sur la face d'entrée d'une cuve transparente remplie d'un liquide absorbant. Une lentille thermique est formée; les oscillations de la lentille sont la manifestation visible d'oscillations hydrodynamiques du liquide dans la cuve. Le système présente un comportement de transition laminaire turbulente, un certain nombre de bifurcations ayant été observées. Analyse spectrale du signal temporel
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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.
Повний текст джерелаАнотація:
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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Marais, Anthony. "Influence du vieillissement statique sur la transition ductile-fragile des aciers au C-Mn." Phd thesis, École Nationale Supérieure des Mines de Paris, 2012. http://pastel.archives-ouvertes.fr/pastel-00820120.
Повний текст джерелаАнотація:
Les aciers ferritiques pour structures industrielles présentent une transition fragile-ductile de ténacité et de résilience avec leur température. Leur résistance à la rupture fragile joue un rôle essentiel dans la certification de la sécurité des structures industrielles importantes. De nos jours, le souci de performance et de longévité sont des points clés pour des acteurs majeurs comme EDF.Dans ces études de transition ductile-fragile, de ténacité et de résilience, la ténacité est prédite à partir de la résilience. Plusieurs travaux antérieurs ont déjà montré que la probabilité de rupture par clivage peut être correctement décrite dans le palier fragile par une approche locale de la rupture. Mais ces études supposent que le matériau ne subit pas de vieillissement sous déformation, ce qui est en fait rarement pertinent pour les aciers bas carbone et peu calmés. Le travail a consisté d'une part à caractériser le comportement et d'autre part à en proposer une modélisation robuste et explicite des phénomènes observés. La caractérisation a consisté en la réalisation d'essais de traction entre -150°C et 20°C à plusieurs vitesses de déformation. Un modèle capable de simuler le vieillissement statique est identifié en mettant en place une stratégie adéquate et systématique. Des essais de résilience permettent de construire la courbe de transition ductile-fragile du matériau pour différentes conditions afin de comprendre et d'observer l'influence du vieillissement statique sur la rupture. Enfin, la modélisation de la rupture fragile a été décrite pour toutes les conditions expérimentales testées en utilisant le modèle de comportement développé et identifié dans la partie précédente afin de prédire numériquement la transition pour les différentes conditions du matériau.
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Micchi, Gianluca. "Mechanical signatures of the current-blockade instability in suspended carbon nanotubes." Thesis, Bordeaux, 2016. http://www.theses.fr/2016BORD0337/document.
Повний текст джерелаАнотація:
Le couplage fort entre le transport électronique dans une boîte quantique à un seul niveau et un oscillateur nano-mécanique couplé capacitivement peut conduire à une transition vers un état mécaniquement bistable et bloqué en courant. Son observation est à portée de main dans les expériences de pointe menées sur les nanotubes de carbone. Nous étudions donc la réponse mécanique du système et plus précisément la fonction spectrale de déplacement, la réponse linéaire à une solicitation externe et le comportement pendant le retour à l'équilibre. Nous montrons qu'il existe une relation étroite entre les grandeurs électriques (telles le courant électrique et la fonction spectrale des fluctuations du courant) et mécaniques. Nous constatons qu'en augmentant le couplage électromécanique, les deux fonctions spectrales présentent un pic qui s'élargit et se déplace vers les basses fréquences alors que le temps de déphasage de l'oscillateur se raccourcit. Ces effets sont maximaux à la transition où les non-linéarités dominent la dynamique et sont robustes vis-à-vis de l'effet des fluctuations extérieures et de la dissipation. Ces caractéristiques fortes ouvrent la voie à la détection de la transition vers l'état de blocage du courant dans des dispositifs actuellement étudiées par plusieurs groupesThe strong coupling between electronic transport in a single-level quantum dot and a capacitively coupled nano-mechanical oscillator may lead to a transition towards a mechanically-bistable and blocked-current state. Its observation is at reach in carbonnanotube state-of-art experiments. Therefore, we investigate the mechanical response of the system, namely the displacement spectral function, the linear response to a driving, and the ring-down behavior, and the electric response, namely the electric current and current spectral function. We show that a close relation between electric and mechanical quantities exists. We find that, by increasing the lectromechanical coupling, the peak in both spectral functions broadens and shifts at low frequencies while the oscillator dephasing time shortens. These effects are maximum at the transition where nonlinearities dominate the dynamics, and are robust towards the effect of external uctuations and dissipation. These strong signatures open the way to detect the blockade transition in devices currently studied by several groups
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Lu, Meichen. "Transition between flow regimes in porous media using magnetic resonance velocimetry : from laminar to turbulent." Thesis, University of Cambridge, 2019. https://www.repository.cam.ac.uk/handle/1810/288744.
Повний текст джерелаАнотація:
The primary aim of this thesis is to investigate the transition between different flow regimes in porous media. The complete transition spectrum of single-phase flow, from creeping flow to inertial, unsteady laminar, and turbulent flow regimes, was examined in sphere packings. Further understanding of the fundamental fluid dynamics was derived based on the pore-scale flow visualisation using magnetic resonance velocimetry (MRV). Spiral imaging was selected as the ultrafast imaging protocol to probe the transient phenomena, and the acquisition was further accelerated by combining subsampling and compressed sensing reconstruction. In a random sphere packing column with column-to-diameter ratio of 3.44, the inertial effect and the onset of unsteady regime were examined with respect to the principal flow characteristics: the inertial core/channeling, backflow, and helical vortices. Helical vortices have been observed experimentally in a random packing for the first time, and the analogy between the swirling flow and helical vortices provides insight into the design and operation of packed bed reactors. Another new observation is that the transition to the unsteady regime is a highly heterogeneous process, where the evolution of the flow instability depends on the pore geometry. Moreover, pixelwise validation was achieved between the experimental and simulation results on three-dimensional velocity fields in the inertial regime; this is enabled by an image-based meshing pipeline, which reproduces the geometry of the random packing in MRV for the numerical simulation. The unsteady regimes were further investigated using a regular sphere packing, the simple cubic packing (SCP). The spectral analysis, in both the random and regular packing, revealed a route to chaos from the steady to periodic, quasi-periodic, and chaotic dynamics, which was only predicted numerically before. During the transition to turbulence, the coherent structures were extracted using proper orthogonal decomposition (POD), which yields a coherent picture regarding the turbulent dynamics, when combined with the skewness, flatness, and quadrant analysis. Furthermore, it was found that the macroscopic properties converged at lower Reynolds number than the microscopic features. In conclusion, the opportunity to measure flow fields at high spatial and temporal resolution will play an increasingly significant role in the advancement of fundamental fluid dynamics. In this thesis, MRV is used, which is particularly advantageous for non-invasive measurements in opaque systems. This thesis provides the experimental and analysis toolkits for such studies and has demonstrated the contribution to characterising and understanding different flow regimes in porous media.
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Weirauch, Daniel R. "A high-resolution record of climate instability spanning ~1.0 million years across the mid-Pleistocene transition." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 131 p, 2007. http://proquest.umi.com/pqdweb?did=1472642111&sid=21&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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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.
Повний текст джерелаАнотація:
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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Chiu, Hsin-Yao. "Family Structure and Family Instability: Evaluating Their Influence on Adult Outcomes." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3398.
Повний текст джерелаАнотація:
Family structure is a widely used independent variable. However, in recent years researchers have questioned its predicting power. They also strive to improve its use. This study drew on Holman's (2001) theoretical model and examined the association between the family structure independent variables (Family Structure, Family Instability, and Parental Romantic Partners) and the outcome variables (Coming to Terms, Self-esteem, Maturity, and Depression). The analysis was conducted with a sample of 3,705 individuals (2,316 females and 1,389 males) randomly selected from the entire population that completed the Relationship Evaluation Questionnaire (RELATE). Results showed that Family Instability was a stronger predictor than Family Structure and Parental Romantic Partners. Among all of the outcome variables, Coming to Terms had the strongest association with each of the independent variables. Implications of the results are discussed.
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Gazanion, Bertrand. "Transition laminaire-turbulent dans un conduit à paroi débitante." Thesis, Toulouse, ISAE, 2014. http://www.theses.fr/2014ESAE0053/document.
Повний текст джерелаАнотація:
Ce travail s’inscrit dans le cadre de la prévision des oscillations de pression interne des moteurs à propergol solide. Il consiste à étudier la transition laminaire-turbulent de l’écoulement interne,modélisé par celui dans un conduit cylindrique à paroi débitante, et son lien avec l’instabilité naturelle de cet écoulement, le Vortex Shedding Pariétal (VSP). La démarche s’est organisée en trois temps. Des mesures antérieures sur un montage gaz froid, reproduisant l’écoulement modèle,sont analysées afin de mettre en évidence la transition laminaire-turbulent. Cette transition est ensuite imposée dans des simulations URANS afin de permettre l’étude de son influence sur les modes VSP. Enfin, une approche LES est mise en place pour simuler le développement de la transition dans les conditions de l’expérience ; dans ce but, une stratégie de perturbation spatiale de l’écoulement est utilisée. Cette étude met en avant quatre résultats principaux. La transition laminaire-turbulent découle de l’amplification spatiale des modes VSP. La simulation de ce processus met en évidence une forte influence de la perturbation numérique ajoutée à l’écoulement. D’autre part, les simulations URANS montrent que la transition réduit l’amplification des modes VSP et les oscillations de pression interne résultantes. Le rôle de la transition dans l’absence d’oscillations de pression lorsque le domaine a un grand rapport d’aspect, jusqu’alors supposé dans la littérature,est ainsi confirmé. Une particularité importante de cette transition est qu’elle dépend de la position radiale, l’écoulement étant turbulent près de la paroi débitante et laminaire au cœurThe present work is related to the prediction of oscillations in solid rocket motors inner flow. It consists in a study of the laminar-turbulent transition of the motor’s inner flow, which is represented by a cylindrical injection-driven flow, and the relation between this phenomenon and the natural instability named Parietal Vortex Shedding (PVS). Three aspects have been analyzed.First of all, previous cold-gas experiments – reproducing the injection driven flow – are analyzed in order to highlight the transition laminar-turbulent transition. This transition is then imposedin URANS simulations to enable a study of its influence on the PVS modes. Finally, Large Eddy Simulations are performed to simulate the laminar-turbulent process. A strategy based on spatial steady disturbances is used to ease this process. The mains conclusions of this work are the following ones. The laminar-turbulent transition is a consequence of the spatial amplification of PVS modes. Simulations of this process highlight a strong influence of the injected numerical disturbances. The URANS simulations show that this transition reduces the amplification of PVSmodes, and the resulting pressure oscillations levels. These results confirm the role of the transitionin the absence of pressure oscillations when the motor cavity is long. A distinctive feature ofthis transition is its dependence on the radial position, which leads to the coexistence of a laminar region in the channel core and a turbulent region near the injecting wall at a given axial position
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Farano, Mirko. "Using nonlinear optimization to understand coherent structures in turbulence and transition." Thesis, Paris, ENSAM, 2017. http://www.theses.fr/2017ENAM0047/document.
Повний текст джерелаАнотація:
Cette thèse vise à démêler les principaux mécanismes impliqués dans les écoulements transitoires et turbulents. L’idée centrale est d'utiliser une technique d’optimisation non linéaire pour étudier l’origine et le rôle des structures cohérentes habituellement observées dans ces écoulements. Cette méthode a été utilisée dans trois contextes différents. Tout d’abord, un écoulement laminaire linéairement stable a été considéré et l'optimisation a été utilisée pour calculer les perturbations les plus amplifiées parmi toutes les perturbations capables de déclencher une transition vers la turbulence. Une fois que la turbulence est bien établie, une optimisation non linéaire entièrement 3D maximisant l'énergie cinétique turbulente est utilisée pour étudier les structures cohérentes qui peuplent l’écoulement turbulent et les mécanismes responsables de la croissance et de l’échange d’énergie (optimale) sont étudiés. Ensuite, une approche de type système dynamique est appliquée aux équations du mouvement. La géométrie de l’espace des phases est étudiée en utilisant la théorie de la croissance transitoire pour évaluer l’importance des variétés stable et instable dans la dynamique. Dans le même cadre, un algorithme de minimisation non linéaire est utilisé pour calculer les connexions hétérocliniques parmi les solutions invariantes des équations de Navier-StokesThis thesis aims at unraveling the main mechanisms involved in transitional and turbulent flows. The central idea is that of using a nonlinear optimization technique to investigate the origin and role of coherent structures usually observed in these flows. This method has been used in three different contexts. First, a linearly stable laminar flow has been considered and the optimization has been used to compute the most amplified perturbations among all disturbances able to trigger transition to turbulence. Once turbulence is well established, a fully 3D nonlinear optimization maximizing the turbulent kinetic energy is used to study coherent structures populating turbulent shear flow as well as investigate the mechanisms responsible for the energy (optimally) growth and exchange. Then, a dynamical system approach is applied to fluid flow equations. The geometry of the state space is investigated by using transient growth theory to reveal the importance of the stable and unstable manifold. In the same framework, a nonlinear minimization algorithm is used to compute heteroclinic connections among invariant solutions of the Navier-Stokes equations
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Puckert, Dominik [Verfasser]. "Experimental Investigation on Global Instability and Critical Reynolds Number in Roughness-Induced Laminar-to-Turbulent Transition / Dominik Puckert." München : Verlag Dr. Hut, 2019. http://d-nb.info/1181515130/34.
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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.
Повний текст джерелаАнотація:
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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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.
Повний текст джерелаАнотація:
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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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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Tyagi, P. K. "Linear Instability Of Laterally Strained Constant Pressure Boundary Layer Flows." Thesis, Indian Institute of Science, 2001. https://etd.iisc.ac.in/handle/2005/265.
Повний текст джерелаАнотація:
The linear instability of laterally diverging/converging flows is an important aspect towards understanding the laminar-transition process in many viscous flows. In this work the linear instability of constant pressure laterally diverging/converging flow has been investigated.
The laminar velocity field for laterally diverging/converging flows, under the source/sink approximation, has been reduced to two-dimensional flows. This reduction is alternative to the Mangier transformation used earlier. For a constant pressure laterally strained flow, the laminar velocity is found to be governed by the Blasius equation for flow over a flat plate.
The non-parallel linear instability of constant pressure laterally strained flows has been examined. The instability equation is found to be same as that for the Blasius flow. This implies that the stability is same as that for the Blasius flow. A lateral divergence/convergence is shown to alter the Reynolds number from that in a two-dimensional flow. The instability of a laterally converging/diverging flow thus can be obtained from the available results for the Blasius flow by scaling the Reynolds numbers. This leads to the result that while a diverging flow is more unstable than the Blasius flow, a converging flow is more stable. Some additional relevant results are also presented.
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Tyagi, P. K. "Linear Instability Of Laterally Strained Constant Pressure Boundary Layer Flows." Thesis, Indian Institute of Science, 2001. http://hdl.handle.net/2005/265.
Повний текст джерелаАнотація:
The linear instability of laterally diverging/converging flows is an important aspect towards understanding the laminar-transition process in many viscous flows. In this work the linear instability of constant pressure laterally diverging/converging flow has been investigated.
The laminar velocity field for laterally diverging/converging flows, under the source/sink approximation, has been reduced to two-dimensional flows. This reduction is alternative to the Mangier transformation used earlier. For a constant pressure laterally strained flow, the laminar velocity is found to be governed by the Blasius equation for flow over a flat plate.
The non-parallel linear instability of constant pressure laterally strained flows has been examined. The instability equation is found to be same as that for the Blasius flow. This implies that the stability is same as that for the Blasius flow. A lateral divergence/convergence is shown to alter the Reynolds number from that in a two-dimensional flow. The instability of a laterally converging/diverging flow thus can be obtained from the available results for the Blasius flow by scaling the Reynolds numbers. This leads to the result that while a diverging flow is more unstable than the Blasius flow, a converging flow is more stable. Some additional relevant results are also presented.
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Chiffaudel, Arnaud. "Dynamique spatio-temporelle de la convection a bas nombre de prandtl." Paris 7, 1988. http://www.theses.fr/1988PA077037.
Повний текст джерелаАнотація:
Etude experimentale de la convection de rayleigh-benard. Description et caracterisation des regimes dynamiques successifs de l'etat stationnaire a l'etat turbulent dans le cadre de theories faiblement non-lineaires
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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.
Повний текст джерелаАнотація:
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
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Chrust, Marcin. "Etude numérique de la chute libre d'objets axisymétriques dans un fluide newtonien." Phd thesis, Université de Strasbourg, 2012. http://tel.archives-ouvertes.fr/tel-00749659.
Повний текст джерелаАнотація:
La mémoire présente une étude numérique des trajectoires non-verticales d'objets en chute ou en ascension libre dans un fluide newtonien initialement au repos. Une méthode numérique originale combinant une discrétisation spatiale spectrale et la décomposition du domaine a été implémentée à cet effet. Le code obtenu a été exploité pour apporter de nouvelles connaissances sur des objets fixes et objets libres. Pour les objet fixes, ellipsoïdes et cylindres, l'étude très complète des divers états de la transition a permis d'établir un lien entre le scénario de transition de la sphère et du disque infiniment mince. La simulation numérique d'objets libres a apporté des résultats très complets sur la chute de disques minces et de cylindres de faible épaisseur. Plusieurs questions soulevées dans les travaux précédents ont trouvés des réponses claires. Une étude paramétrique exhaustive, jamais abordée précédemment, portant sur des ellipsoïdes est décrite dans le dernier chapitre du mémoire.
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Stewart, I. J. "A model for transition by attachment line contamination and an examination of cross-flow instability in three-dimensional boundary layers." Thesis, Cranfield University, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380625.
Повний текст джерела
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Gurun, Akif Murat. "Interactions of Tollmien-Schlichting Waves and Stationary Transient Disturbances." Case Western Reserve University School of Graduate Studies / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=case1133374397.
Повний текст джерела
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Martínez, Germán Andrés Gaviria. "Towards natural transition in compressible boundary layers." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/18/18148/tde-24052017-114027/.
Повний текст джерелаАнотація:
In this work, a DNS code was developed to investigate problems on transition in subsonic compressible boundary layer on a flat plate. Code validation tests were performed for linear and nonlinear stages of transition, on incompressible and compressible regimes. The focus of the present work is to investigate natural transition in subsonic boundary layers modeled by wave packets; and perform a preliminary study of transition induced by white noise. Three main problems were considered, namely, a DNS simulation and analysis of the ex- periment (MEDEIROS; GASTER, 1999b) of wave packet evolution on incompressible boundary layer, the influence of compressibility on wave packet evolution at subsonic Mach numbers and finally, a preliminary study of the evolution of a white noise perturbation in the boundary layer at Mach 0.2 and Mach 0.9. Comparisons between numerical and experimental results show remarkably good agreement in the linear and nonlinear stages, in both, spatial and Fourier spaces. A numerical simulation of this experiment and the analysis carried out is not available in the literature for wave packets in the incompressible boundary layer. The nonlinear modal analysis performed established the existence of tuned fundamental and subharmonic resonance of H-type and K-type in the packet. Influence of compressibility in the wave packet evolution was here investigated in boundary layers at Mach 0.7 and Mach 0.9. There are no works reported in the literature on wave packets in compressible subsonic boundary layer. In the linear regime, the oblique modes were the most unstable for Mach > 0.7, as expected by the results of the literature. In the nonlinear regime, strong streaks were observed, associated with low frequency modes that eventually decay downstream. An isolated wave packet at Mach 0.9 showed nonlinear amplification only in the subharmonic band, which may be associated to H-type or detuned resonance. However this packet has a relatively stable character. On the other hand, at Mach 0.9 spanwise interaction of wave packet pairs were more unstable than the isolated case, because stable modes for the isolated packet evolution becomes unstable in the wave packet interaction. This scenario evidenced the presence of oblique transition. Finally, the nonlinear evolution of the same white noise disturbance at Mach 0.2 and Mach 0.9 were observed to be completely different. In the incompressible boundary layer localized lambda vortex structures were observed, that could be associated to the local presence of H-type and/or K-type resonance. In the compressible regime, longitudinal vortex structures distributed across the entire domain seemed to be linked to oblique transition. In the white noise evolution, compressibility seems to have a stronger effect than in the wave packet evolution. In the conditions considered, the wave packet interaction appear to be a better representation of white noise compressible transition scenario.No presente trabalho, um código DNS (Direct Numerical Simulation) foi desenvolvido para abordar problemas de transição para turbulência em camada limite subsônica compressível em uma placa plana. Foram realizados testes de validação de código , nos regimes linear e não linear do processo de transição, nos regimes incompressível e compressível. O foco do presente trabalho é estudar transição natural modelada por meio de pacotes de onda em camada limite compressível subsônica, e realizar uma análise preliminar da transição induzida por ruído branco. Três assuntos principais foram considerados: uma simulação DNS e uma análise comparativa com o experimento (MEDEIROS; GASTER, 1999b) sobre a evolução de um pacote de ondas em camada limite incompressível, a influência da compressibilidade na evolução de pacotes de ondas no regime subsônico, e por último, um estudo preliminar da transição induzida por ruído branco em Mach 0.2 e Mach 0.9. As comparações realizadas entre a solução numérica e os dados experimentais mostram uma boa concordância, nos regimes linear e não linear, tanto no espaço físico quanto no espaço de Fourier. A simulação numérica deste experimento e a análise realizada neste trabalho, não são encontradas na literatura para o regime incompressível. A análise modal não linear aplicada aos resultados, permitiu identificar a presença das ressonâncias tipo H e tipo K no pacote de ondas. A influência da compressibilidade na evolução dos pacotes de onda foi estudada em Mach 0.7 e Mach 0.9. Na literatura não há trabalhos sobre pacotes de ondas no regime sub- sônico. No regime linear da transição, os modos oblíquos resultam ser os mais instáveis para Mach > 0.7, como era de esperar, de acordo com os resultados da literatura. No regime não linear, foram observadas estrias de moderada amplitude, associadas com modos de baixa frequência que acabam decaindo. O pacote de ondas em Mach 0.9 apresentou amplificação não linear somente na banda subharmônica, que pode ser associada com transição tipo H ou ressonância dessintonizada. No entanto, o comportamento geral neste regime é estabilizante. Por sua vez, a interação entre pacotes de ondas em Mach 0.9 mostrou um comportamento desestabilizante, pois a interação acaba gerando amplificação não linear em modos que decaem no pacote isolado. Os modos amplificados sugerem a presença do mecanismo de transição oblíqua. Finalmente, a evolução da mesma perturbação constituída por ruído branco em Mach 0.2 e Mach 0.9, resultaram ser completamente diferentes. Na camada limite incompressível foram observados vórtices tipo lambda, que poderiam ser gerados pela presença localizada das ressonâncias tipo H e/ou tipo K. No regime compressível foram observados vórtices distribuidos em todo o domínio, o que sugere a presença da transição oblíqua. Na transição gerada por ruído branco a compressibilidade teve uma influência maior que no pacote de ondas. Nas condições estudadas, a interação entre pacotes de ondas parece ser uma melhor representação do ruído branco no regime compressível.
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She, Zhen-Su. "Instabilites et dynamique a grande echelle en turbulence." Paris 7, 1987. http://www.theses.fr/1987PA077241.
Повний текст джерелаАнотація:
Travaux analytiques et numeriques en dynamique des fluides incompressibles regis par l'equation de navier-stokes. On relie la visco-elasticite de la turbulence a l'invariance de galilee. Equilibre thermique des protuberances solaires dans les bouches magnetiques coronales
46
Brandt, Luca. "Study of generation, growth and breakdown of streamwise streaks in a Blasius boundary layer." Licentiate thesis, KTH, Mechanics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1256.
Повний текст джерелаАнотація:
Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern.
Descriptors:Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.
QC 20100518
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Boulfelfel, Salah Eddine. "Atomic Scale Investigation of Pressure Induced Phase Transitions in the solid State." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2009. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-25283.
Повний текст джерелаАнотація:
In this work, atomic scale investigation of pressure-induced transformations in the solid state have been carried out. A series of compounds including GaN, ZnO, CaF2, and AgI, in addition
to elemental phosphorus have been studied. The corresponding transition mechanisms have been
elucidated with a clear description of atomic displacements and intermediate structures involved
therein.
In the first group of compounds, the long standing debate on the transition path of the
wurtzite(WZ)-to-rocksalt(RS) transition in semiconductors, GaN and ZnO was resolved using geometrical
modeling combined with molecular dynamics (MD) simulations conducted in the frame
of transition path sampling (TPS) method. In GaN, a two-step mechanism through a metastable
intermediate phase with a tetragonal structure iT has been revealed from simulations. In ZnO,
the tetragonal intermediate structure was kinetically less stable, although still part of the real
transition mechanism. It appeared at the interface between WZ and RS as consequence of a layers
shearing. The transition regime in ZnO was characterized by a competition between iT structure
and another hexagonal intermediate with hexagonal symmetry iH. Although possible, the latter
is not functional for the transition.
In both cases, GaN and ZnO, two points of agreement with experiments have been revealed.
The tilting of structures after transition, and the phonon mode softening associated with atomic
displacements leading to the tetragonal structure iT
In the second group of compounds, the investigation of transitions in superionic conductors,
CaF2 and AgI, demonstrated a different and particular behavior of atomic motion under pressure.
The solid-solid reconstruction of CaF2 structure was shown to be initiated and precedented by high
disorder of the anionic sublattice. The percolation of fluoride ions through voids in the fluorite
structure created a thin interface of liquid like state. The sparce regions caused by the departure
of anions facilitates the cation sublattice reconstruction.
In AgI, ion diffusion during the wurtzite/zincnlende(ZB)$rocksalt transition was more pronounced
due to the extended stacking disorder WZ/ZB. The Ag+ ions profited not only from the
structure of the interface but used the combination of interstitial voids offered by both phases,
WZ and ZB, to achieve long diffusion paths and cause the cation sublattice to melt. Clearly, a
proper account for such phenomena cannot be provided by geometry-designed mechanisms based
on symmetry arguments.
In phosphorus, the question of how the stereochemically active lone pairs are reorganized during
the orthorhombic (PI) to trigonal (PV) structural transition was answered by means of simulations.
Computation was performed at different levels theory.
First, the mechanism of the transition was obtained from TPS MD simulations. MD runs
were performed within density functional tight binding method (DFTB). The analysis of atomic
displacements along the real transformation path indicated a fast bond switching mechanism.
In a second step, the nature of the interplay between orbitals of phosphorus during the bond
switching was investigated. A simultaneous deformation of lone pair and P−P bond showed
a mutual switching of roles during the transformation. This interplay caused a low dimensional
polymerization of phosphorus under pressure. The corresponding structure formed as zigzag linear
chain of fourfold coordinated phosphorus atoms (· · ·(P(P2))n · · ·) at the interface between PI and
PV phases.
A further result of this work was the development of a simulation strategy to incorporate
defects and chemical doping to structural transformations. On top of the transition path sampling
iterations, a Monte Carlo like procedure is added to stepwise substitute atoms in the transforming
system. Introducing a chemically different dopant to a pure system represents a perturbation to
the energy landscape where the walk between different phases is performed. Therefore, any change
in the transition regime reflects the kinetic preference of a given structural motif at times of phase
formation.
This method was applied to the elucidation of WZ-RS transition mechanism in the series of
semiconducting compounds AlN, GaN, and InN. Simulations showed that In atoms adopt the
same transformation mechanism as in GaN and favor it, while Al atoms demonstrated a significant
reluctance to the path going through tetragonal intermediate iT. The difference between transition
regime in mixed systems InxGa1−xN and AlxGa1−xN is in agreement with experiments on high pressure
behavior of AlN, GaN, and InN. While transitions in GaN and InN are reversible down
to ambient conditions, AlN is stable.
The work presented in this thesis constitutes the seed of new perspectives in the understanding
of pressure-induced phase transformations in the solid state, where the physics and the chemistry
are brought together by means of computer simulations.
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Le, Quéré Patrick. "Etude de la transition à l'instationnarité des écoulements de convection naturelle en cavité verticale différentiellement chauffée par méthodes spectrales chebyshev." Poitiers, 1987. http://www.theses.fr/1987POIT2003.
Повний текст джерелаАнотація:
On developpe un algorithme d'integration des equations bidimensionnelles instationnaires de navier-stokes d'un fluide de bomussinesq qui couple une discretisation spatiale des variables dependantes par approximation spectrale de chebyshev a une discretisation temporelle de type differences-finies du second ordre. Les cavites etudiees ont des rapports de forme de 1 a 10. Suivant le rapport de forme et les conditions aux limites thermiques, il existe trois mecanismes differents de transition a l'instationnarite qui correspondent a des bifurcations de hopf supercritiques
49
Zhou, Wei. "Instabilités de trajectoires de sphères, ellipsoïdes et bulles." Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAD038/document.
Повний текст джерелаАнотація:
La thèse présente une étude numérique des instabilités de trajectoires de sphères, d'ellipsoïdes aplatis et de bulles en mouvement libre sous l'action de la gravité, de la poussée d'Archimède et des forces hydrodynamiques. Le chapitre sur les sphères reprend, complète et étend l'étude numérique de Jenny et al. (2004) en se concentrant sur la transition au chaos et sur les trajectoires chaotiques. Les résultats montrent la différence entre le scénario de transition au chaos de sphères de faible et de grand rapport de densité. Plusieurs grandeurs statistiques sont proposées afin de fournir une caractérisation quantitative des états chaotiques. Elle permettent de mettre en relation les états ordonnées et chaotiques et offrent une possibilité de comparaison objective de données aléatoires d'origine numérique ou expérimentale. L'étude, très extensive, du comportement d'ellipsoïdes aplatis établit le lien entre les disques et les sphères en faisant varier l'aplatissement des objets depuis infiniment plat jusqu'à presque sphérique. Les huit diagrammes d'état présentés permettent de comprendre l'effet de la forme des ellipsoïdes sur le scénario de transition. Le cas d'ellipsoïdes presque sphériques montre que de faibles imperfections de la forme peuvent avoir in impact significatif sur les trajectoires de sphères de très faible rapport de densité. Pour les bulles considérées dans la limite de rapport de densité et viscosité az/liquide nul, l'étude se concentre sur l'analyse de stabilité linéaire et aboutit à la courbe de stabilité marginale dans le plan des paramètres nombre de Bond – nombre de Galilée en tenant compte de la déformation des bulles au moment de la perte de leur axisymétrie. Plus deux décades de nombres de Bond, entre 0,1 et 20, sont couvertes. Les résultats montrent clairement l'effet de la déformation de la bulle sur le seuil de l'instabilitéThe thesis presents a numerical study of path instabilities for spheres, oblate spheroids and bubbles moving freely under the effect of the gravity, buoyancy and hydrodynamic forces. For spheres, the parametric study of Jenny et al. (2004) is revisited, improved end extended with a special focus on the chaotic states. The results reveal that the effect of density ratio responsible for different oblique oscillating states of low and high frequencies has a significant impact both on the onset of chaos and on the behavior of fully chaotic states. Several quantitative statistical quantities are proposed and shown to be relevant for establishing the relation between chaotic and ordered states and for an objective comparison of random data of numerical or experimental origin. The extensive study on freely moving spheroids establishes the link between disks and spheres by varying the aspect ratio of spheroids from infinitely flat to almost spherical. The state diagrams provided for eight different aspect ratios of spheroid show in detail how the transition scenario varies depending of the body shape. The investigation of almost spherical spheroids reveals the specificities of the dynamics of light imperfect spheres.For the deformable gas bubble in the limit of zero gas/liquid density and viscosity ratio, a marginal stability curve is given in the two-parameter plane of the Galileo and the Bond number indicating the critical Galileo numbers for the loss of stability of vertical trajectories. The numerical investigation covers more than two decades of Bond number going from 0.1 to 20. The results clearly show the crucial role of the surface deformation
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Graumer, Pierre. "Etude du mélange gazeux produit par instabilité de Richtmyer-Meshkov en régime initial périodique faiblement diffus." Thesis, Toulouse, ISAE, 2019. http://www.theses.fr/2019ESAE0015/document.
Повний текст джерелаАнотація:
Le travail de thèse présenté dans ce manuscrit propose une analyse expérimentale du dé-veloppement spatio-temporel d’une zone de mélange (air/hélium) initiée par instabilité deRichtmyer-Meshkov (IRM). Cette étude s’appuie sur la mise en oeuvre d’un tube à chocspositionné verticalement et sur le développement d’un nouveau protocole expérimental associéà un système innovant de génération de l’interface initiale entre les deux espèces gazeuses enprésence. Ce système est basé sur un dispositif d’obturation/ouverture composé d’un rideau rigiderétractable et d’une série volets mobiles. La caractérisation de l’interface initiale et de l’évolutionspatio-temporelle de la zone de mélange ainsi obtenue est effectuée en exploitant les résultats dedifférentes techniques de mesures telles que la visualisation strioscopique (Schlieren) résolue entemps, la tomoscopie plan laser (TPL) et la Vélocimétrie par Imagerie de Particules (PIV). Enpremier lieu, différentes campagnes de mesures visant à caractériser l’interface initiale ont permisde quantifier la répétabilité du système et de démontrer ses capacités à générer une interfacepériodique faiblement diffuse. Dans un second temps, une étude du mélange gazeux obtenu pourun jeu de paramètres expérimentaux donné, est proposée. L’analyse s’intéresse en particulieraux mécanismes d’initiation et de transition a la turbulence de la zone de mélange produite parl’IRM. L’interaction entre cette zone de mélange en cours de développement et le choc réfléchisur l’extrémité supérieure du tube (phénomène de rechoc) est également étudiée dans l’optique deconfirmer la transition turbulente de la zone de mélangeThis work proposes an experimental analysis of the spatio-temporal development of an air/heliummixing zone promoted by the Richtmyer-Meshkov instability (RMI). This study relies on the useof a vertical shock tube and on the development of a new experimental protocol associated with aninnovative device for the generation of an initial interface between two gazeous species. This deviceconsists a rigid retractable curtain and of a series of rotating shutters. The characterization ofthis initial interface and the spatio-temporal evolution of the RMI-induced mixing zone is carriedout by exploiting the results of various experimental methods such as time resolved Schlierenvisualizations, planar laser mie scattering and Particle Image Velocimetry (PIV). In a first step,various measurement campaigns have made it possible to quantify the repeatability of the newdevice and to demonstrate its ability to generate a periodic, weakly diffused interface. In a secondstep, a study of the gaseous mixing for a given set of experimental parameters is proposed. Theanalysis focuses on the understanding of the underlying mechanisms driving the gaseous interfaceformation and the transition to turbulence of the RMI-induced mixing. The interaction betweenthis mixing zone and the reflected shock from the upper end of the tube (re-shock phenomenon)is also studied in order to confirm the turbulent transition of the mixing zone