Dissertations / Theses on the topic 'Open cavity flow'

The study of three-dimensional open cavity flow with and without cover plates at the edges of the cavity at freestream Mach number of 0.85 was performed. Such open cavities find their application in landing gear wells, bomb bay of an airplane and sunroof or window of an automobile. The aerodynamic noise and the flow field together in the above mentioned cavities lead to self-sustaining flow oscillations. These self-sustaining flow oscillations results in the structural failure in the surrounding of the cavity due to resonance phenomenon in the aircrafts and lead to loud and unbearable noise both in aircrafts and automobiles. Understanding this aeroacoustic phenomenon helps us in reducing the noise, increase the component stability and passenger comfort. Therefore, this problem has become an important topic of research for several decades in aerospace and automobile fields. In this thesis an effort was made to investigate the fully developed turbulent flow over an open cavity and to evaluate the effect of cover plates numerically using the detached eddy simulation (DES) formulation. For this purpose, computational fluid dynamics approach was utilized on the M219 cavity at Mach 0.85 using DES with the Spalart-Allmaras (S-A) turbulence model. Two cavity cases were modeled. The test case 1 is a three-dimensional open cavity without cover plates and the test case 2 is a three-dimensional open cavity with cover plates at the edges of the cavity. Furthermore, three variants of DES were considered for the test case 2 cavity namely Spalart- Allmaras, k-ω-SST and realizable k-ε model, whereas for the test case 1 cavity only the DES S-A model was employed.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering

Une écoulement de cavité ouverte tridimensionnel saturé non-linéairement est étudié par une approche spatio-temporelle utilisant des données expérimentales résolues à la fois en temps et en espace. Ces données ont été acquises dans deux plans longitudinaux, respectivement perpendiculaire et parallèle au fond de la cavité, dans le régime incompressible, en air ou en eau. À l'aide de multiples méthodes de décompositions globales en temps et en espace, les ondes et les structures cohérentes constituant la dynamique dans le régime permanent et pouvant être produites par des mécanismes d'instabilités différents sont identifiées et caractérisées.Tout d'abord, on approfondit la compréhension de l'effet des non-linéarités sur les oscillations auto-entretenues de la couche cisaillée impactante et leurs interactions avec l'écoulement intra-cavitaire. En particulier, l'analyse spectrale d'une portion de l'espace des paramètres permet de mettre en évidence un lien entre l'accrochage des modes d'oscillations auto-entretenues, la modulation d'amplitude au niveau du coin impactant et l'intermittence de ces modes. De plus, l'observation des basses fréquences intéragissant fortement avec les oscillations de la couche de mélange démontre l'existence d'une dynamique tridimensionnelle intrinsèque à l'intérieur de la cavité malgré les perturbations causées par la couche cisaillée instable.Les analyses de stabilité linéaire ont montré que des instabilités centrifuges peuvent résulter de la courbure induite par la recirculation. L'étude de la dynamique après saturation révèle de nombreuses structures cohérentes dont les propriétés sont quantifiées et classées en s'appuyant sur la forme des instabilités sous-jacentes: des ondes transverses progressives ou stationnaires. Enfin, certains comportements des structures saturées suggèrent que les mécanismes non-linéaires gouvernant le développement de l'écoulement une fois sorti du régime linéaire pourraient être étudiés dans le cadre des équations d'amplitude
A space-time study of a three-dimensional nonlinearly saturated open cavity flow is undertaken using time-resolved space-extended experimental data, acquired in both cross-stream and spanwise planes, in incompressible air and water flows. Through use of multiple modal decompositions in time and space, the waves and coherent structures composing the dynamics in the permanent regime are identified and characterised with respect to the instabilities arising in the flow.Effects of nonlinearities are thoroughly investigated in the impinging shear layer, regarding the self-sustained oscillations and their interactions with the inner-flow. In particular, the analysis conducted throughout the parameter space enlightens a global connection between the selection of locked-on modes and the amplitude modulation at the impingement and the mode switching phenomenon. Furthermore, observations of low frequencies interacting drastically with the shear layer flapping motion underline the existence of intrinsic coherent three-dimensional dynamics inside the cavity in spite of the shear layer disturbances.Linear stability analyses have demonstrated that centrifugal instabilities are at play along the main recirculation. The present investigation of the dynamics after onset of the saturation reveals numerous space-time coherent structures, whose properties are quantified and classified with respect to the underlying instabilities: travelling or standing spanwise waves. Finally, some patterns exhibited by the saturated structures suggest that the nonlinear mechanisms governing the mutations of the flow after the linear regime could gain more insight in the frame of amplitude equations

En aquest estudi s'analitzen els experiments i simulacions numèriques del flux de convecció mixta en una cavitat cúbica situada a la part inferior d'un canal quadrat. Els nombres de Reynolds en funció de la velocitat mitjana del flux i l'amplada del canal estan en el rang 100≤Re≤1500 i el nombre de Richardson varia entre 0.1≤Ri≤10. La tècnica PIV s'ha utilitzat per a les mesures en un canal d'aigua. Simulacions numèriques Tridimensionals s’han dut a terme amb un codi de volums finits de segon ordre considerant l'aproximació de Boussinesq ja que, per les condicions experimentals utilitzades, la variació de les propietats físiques amb la temperatura no té influència significativa en la topologia general de flux. Per 100≤Re≤1500 i Ri≤0.1 el flux és estacionari i consisteix en un remolí situat a l’interior de la cavitat cúbica que mostra velocitats majors a mesura que augmenta el nombre de Richardson. El flux és no estacionari a Re = 100 i Ri = 10. Prop de les parets laterals es produeixen, de forma alternada, ejeccions de flux, des de l’interior de la cavitat cap al canal, mentre que el flux entra a la cavitat des del canal a través de la part central de la cavitat. S'ha utilitzat una tècnica de mostreig condicional per elucidar l’estructura mitjana de l'evolució del flux turbulent a Ri = 10. S'ha trobat que les ejeccions de flux persisteixen durant tot el rang de Reynolds analitzat. Els números de Nusselt calculats estan d'acord amb els de les correlacions reportades a la literatura, vàlides per cavitats bidimensionals
En este estudio se analizan los experimentos y simulaciones numéricas del flujo de convección mixta en una cavidad cúbica situada en la parte inferior de un canal de sección cuadrada cuadrado. Los números de Reynolds en función de la velocidad media del flujo y la anchura del canal están en el rango 100≤Re≤1500 y el número de Richardson varía entre 0.1≤Ri≤10. La técnica PIV se ha utilizado para las medidas en un canal de agua. Se han llevado a cabo simulaciones numéricas tridimensionales con un código de volúmenes finitos de segundo orden, considerando la aproximación de Boussinesq ya que, en las condiciones experimentales utilizadas, la variación de las propiedades físicas con la temperatura no tiene influencia significativa en la topología general de flujo. En los rangos 100≤Re≤1500 y Ri≤0.1 el flujo es estacionario y consiste en un remolino situado en el interior de la cavidad cúbica que muestra velocidades mayores a medida que aumenta el número de Richardson. El flujo es no estacionario a Re = 100 y Ri = 10. Cerca de las paredes laterales se producen de forma alternada eyecciones de flujo, desde el interior de la cavidad hacia el canal, mientras que el flujo entra en la cavidad desde el canal a través de la parte central de la cavidad. Se ha utilizado una técnica de muestreo condicional para elucidar la estructura media de la evolución del flujo turbulento a Ri = 10. Se ha encontrado que las eyecciones de flujo persisten durante todo el rango de Reynolds analizado. Los números de Nusselt calculados están de acuerdo con los de las correlaciones reportadas en la literatura, válidas para cavidades bidimensionales.
In this study we analyze experiments and numerical simulations of steady and unsteady mixed convection flow in a cubical cavity located at the bottom of a square channel. The Reynolds numbers based on the mean flow velocity and the channel width are in the range 100≤Re≤1500 and the Richardson numbers vary within 0≤Ri≤10. Particle Image Velocimetry has been used for the measurements in a water channel. Three-dimensional direct numerical simulations have been carried out with a second order finite volume code considering the Boussinesq approximation since, for the experimental conditions considered, the variation of the physical properties with temperature has no significant influence on the overall flow topology. For 100≤Re≤1500 and Ri≤0.1 the flow is steady and it consists in a single roll that exhibits larger velocities as the Richardson number is increased. An unsteady periodic flow is found at Re=100 and Ri=10. Alternate flow ejections from the cavity to the channel occur near the lateral walls while the flow enters the cavity from the channel through the central part of the cavity. A conditional sampling technique has been used to elucidate the evolution of the mean unsteady turbulent flow at Ri=10. It has been found that the alternate flow ejections persist for all the Reynolds analyzed. The computed Nusselt numbers are in general agreement with a previously reported correlation, valid for two dimensional cavities of different aspects ratios.

Aeroacoustic whistles are a significant source of customer complaints to automotive manufacturers. Whistles can occur on many such components, but the relative position and configuration of rearview mirrors means they are a more problematic source of tonal noise on vehicles. The low subsonic complex turbulent flow, combined with small cavity scales, determines the possible whistle mechanisms. The one considered to be most problematic, fluid-dynamic cavity resonance, is the topic of this research thesis. The research scope is limited to the automotive environment of external rearview mirrors and the fluid-dynamic resonance mechanism: low subsonic Mach number, M = 0.05 - 0.13; laminar boundary layers; and two-dimensional, acoustically compact cavities. The low unit-cost of rearview mirrors and the desire to have simple identification and prediction schemes, that could be used by production engineers, determined an empirical approach. A search of the existing literature revealed that there were some data on cavities of the above scale in low Mach number flow, but quoted errors in empirical descriptions were large and there was very little research on the effects of flow yaw angle on the chosen resonance mechanism. The research therefore aims to determine whether existing empirical descriptions of fluid-dynamic cavity resonance are suitable for the prediction of the resonance characteristics, with sufficient accuracy to enable unambiguous identification of the presence of the resonance and its mechanism. A second aim is to investigate the effects of a feature of the automotive flow environment, flow yaw angle, on the resonance. Flow yaw angle is determined by those components of the flow in the same plane as the surface in which the cavity is situated. An experimental program was undertaken using a purpose-built aeroacoustic wind tunnel and a simple cavity model. Testing with two types of cavity configurations, as well as flow visualisation, investigated the main features of the resonance in time-averaged yawed flow. Within the scope of this thesis, it is shown that fluid-dynamic cavity resonance characteristics can be accurately identified by a simple empirical model, even in yawed flow. Various descriptors allow identification of the resonance threshold, stage, frequency and relative amplitude in non-yawed flow, while the frequency and stage can also be identified in yawed flow. The relative decrease in resonance amplitude in yawed flow, although identified for these experiments, would depend on the degree of spanwise variation in the boundary layer characteristics for a given cavity configuration. The results also identify significant issues with testing in a free jet tunnel, due to the nature of fluid-dynamic cavity resonance and the fluctuation energy content in free shear layers. Despite this, the thesis aims are achieved, and appropriate design guidelines are produced for automotive designers.

Ce travail porte sur la caractérisation de la dynamique intra-cavitaire en cavité ouverte, dont il existe peu d'études expérimentales, ainsi que sur le contrôle de cet écoulement. Nous avons réalisé une étude paramétrique des régimes primaire et secondaire de l'écoulement dans lesquels des structures de type Taylor-Görtler apparaissent. Nous avons identifié les seuils de bifurcation et montré leur nature systématiquement supercritique. Nous avons également montré que différentes familles de modes propagatifs ou stationnaires pouvaient être sélectionnées en fonction de la géométrie de la cavité. Cela a confirmé des prédictions réalisées dans des analyses de stabilité linéaire de l'écoulement de base. Nous avons montré que le régime secondaire résulte de la superposition d'ondes propagatives gauche et droite. Une tentative d'identification des coefficients des équations complexes couplées de Ginzburg-Landau décrivant cette dynamique a été conduite mais la sensibilité des coefficients à de multiples paramètres n'a pas permis d'obtenir des coefficients physiquement acceptables. Un forçage des oscillations de la couche cisaillée a été entrepris à l'aide d'un actionneur plasma froid à décharge à barrière diélectrique placé en amont de la cavité. L'analyse de la réponse de l'écoulement à un forçage périodique d'amplitude variable a permis d'identifier des plages d'accrochage en fréquence. Enfin, nous avons réalisé un contrôle en boucle fermée des oscillations de la couche cisaillée à l'aide d'une loi de contrôle à retard proposée par Pyragas dans le cadre des systèmes dynamiques chaotiques
This work is devoted to the characterization of the dynamic inside an open cavity flow, for which few experimental studies exist. A control of the flow has been also investigated. We have performed a parametric study of the first and second regime of the flow for which Taylor-Görtler vortices type appear. Bifurcation thresholds have been identified and their systematic supercritical nature has been highlighted. We have also showed that different family of propagating or stationary modes can be selected depending on the geometry of the cavity. This has confirmed predictions obtained by linear stability analysis of the base flow in the literature. We have showed that the second regime results from the superposition of left and right propagating waves. We attended to identify coefficients of the complex coupled Ginzburg-Landau equations that describe the dynamics but the values of the coefficients are sensitive to multiple parameters. A control of oscillations of the shear layer has been achieved by the mean of a plasma actuator with dielectric barrier discharge located upstream of the cavity. Locked regimes have been identified by the analysis of the flow response to a periodic perturbation with different amplitude. Finally, we have performed a closed loop control of the oscillations of the shear layer using a delay feedback control law proposed by Pyragas in the context of chaotic dynamical systems

Cavity flow induces strong flow oscillations, which increase noise, drag, vibration, and structural fatigue. This type of flow impacts a wide range of low speed applications, such as aircraft wheel wells, ground transportations, and pipelines. The objective of the current study is to examine the reverse flow interaction inside the cavity, which has a significant impact on the cavity flow oscillations. The study also investigates the impact of steady jets with different-configurations on the time-average field and the oscillations of the cavity separated shear layer. The purpose of the steady jets is suppressing the oscillations of the cavity separated shear layer. The experiments were performed for an open cavity with L/D = 4 at Reθ between 1.28×103 to 4.37×103. The steady jets were applied with different: momentum fluxes (J = 0.11 kg/m.s2,0.44 kg/m.s2 and 0.96 kg/m.s2), slot configurations (sharp edge and coanda), and blowing locations (blowing from the cavity leading and trailing edges). The data were acquired using qualitative (surface oil flow visualisation) and quantitative (hot-wire anemometry, laser Doppler anemometry, particle image velocimetry, and pressure measurements) flow diagnostics techniques. The study found that a low-frequency instability dominates the velocity spectra of the cavity separated shear layer. This instability decreases with increasing Reθ and is related to the reverse flow interaction. This interaction takes place when the reverse flow influences the sensitive separation point of the cavity separated shear layer. As a result, a large amplitude flapping wave is generated and propagates downstream of the cavity separated shear. It was also revealed that increasing J for the leading and trailing edges blowing enhances the reverse flow interaction and increases the broadband level of the unsteady wall pressure spectra. Thus, these types of jet blowing are not suitable for controlling the oscillations of the cavity separated shear layer.

Nous proposons un algorithme rapide et automatisé de contrôle par apprentissage automatique enrichi de méthodes de gradients (gMLC) pour l’optimisation de lois de contrôle en boucle fermée. Notre méthodologie alterne entre l’exploration de l’espace de recherche et l’exploitation des gradients locaux, et généralise la programmation génétique (GPC) et l’Explorative Gradient Method (EGM). L’algorithme gMLC est implémenté et testé numériquement, par la stabilisation d’un système multi-entrées multi-sorties, le pinball fluidique et expérimentalement, par le contrôle de la cavité ouverte. Dans les deux cas, gMLC a construit des lois de contrôle en boucle fermée permettant les meilleures performances répertoriées. Nous démontrons aussi que les mécanismes de contrôle pour la cavité reposent effectivement sur la rétroaction à partir de la mesure de l’état. La comparaison entre gMLC et GPC est toujours à l’avantage de gMLC aussi bien en termes de vitesse de convergence que de qualité de la solution finale. Le gain en vitesse d’apprentissage est d’au moins un facteur 10, permettant d’envisager le contrôle d’expériences complexes avec, par exemple, un grand nombre d’entrées et de sorties ou des tests multi-paramètres pour assurer la robustesse de l’apprentissage. Enfin, deux codes sont mis en ligne en libre accès: xMLC, basé sur le contrôle par programmation génétique et gMLC, basé sur notre nouvel algorithme
As main contribution we propose a fast and automated gradient-enriched machine learning control (gMLC) algorithm to learn feedback control laws. The framework alternates between explorative and exploitive gradient-based iterations, generalizing genetic programming control (GPC) and the Explorative Gradient Method (EGM). The gMLC algorithm has been demonstrated both numerically, with the stabilization of a MIMO system, the fluidic pinball and experimentally, with the control of the open cavity. In both cases, gMLC successfully built closed-loop control laws allowing the best performances so far. We prove, in particular, that the mechanisms behind the control of the cavity rely effectively on feedback. The benchmark of gMLC with GPC on both problems, shows that gMLC outperforms GPC both in terms of convergence speed and final solution efficiency. An acceleration of at least a factor 10 between the GPC and gMLC has been achieved, allowing the control of many experiments, e.g., with a large number of inputs and outputs or multiple parameters testing for robustness. The two developed codes are both freely available online: xMLC, based on GPC and gMLC, based on our new algorithm

碩士
國立臺灣大學
物理學研究所
107
Researches of Open Cavities are commonly used in gyrotrons and gyromonotron oscillators. Different from the enclosed cavities, it has well-defined geometrical boundary conditions. There are mainly two kinds in previous researches. One is under Time-Domain, analyzing and discussing the relation of different quality factor Q and resonant frequency with the basic structure or changing the structure or the material of the cavities. The other is under Frequency-Domain, injecting electromagnetic waves with different frequency. Then, observe and discuss the relation of coefficient and the injected frequency. The main result of this thesis is developed from the previous researches of Time-Domain. Basic from the knowledges of Electrodynamics, we denote the average value of the Poynting vector in the z-direction among a period as the net wave power flowing. Also, changing the structure, analyze the power flow of the wave with numerical simulations. Lastly, we give reasonable physical explanations in this thesis.

碩士
國立臺灣大學
物理學研究所
107
Open cavities are commonly used in research of gyrotrons. Because not closed geometrical boundary, the features that is basically different from close cavity. Hence, we use numerical simulation to understand its physical peoperty and phenomenon. Start from time domain models and declare the numerical algorithm and the boundary condition. We want to find the relation of quality factor and resonant frequency in time domain models in different structure. We also discuss the power flow in open cavities . By changing the structure of open cavities , we analyze the flow and decay of energies and forward power、backward power. We derive the formula of power flow by electrodynamics and check our guess by numerical simulation. Finally we try to give an appropriate physical explanation for the results.

碩士
國立臺灣海洋大學
輪機工程學系
107
In addition to the structural vibration of the fluid in the cavity, the flow noise generated by the fluid excitation and the structural vibration noise will affect the comfort of the outside world. Therefore, the unsteady oscillating flow field generated by the fluid flow through the cavity is of great importance in physics and engineering applications. In this thesis, the interaction between the flow field structure and the sound field generated by noise is studied for the open cavity flow issue in the three-dimensional rectangular cavity to explore the flow vortex structure and noise. One of the main parameters of the open cavity flow field is the ratio of the length of the depth (L/D), and the flow field with a length to depth ratio of less than 10 is generally called open cavity flow. There is a free shear layer at the opening of the hole. The free shear layer with different aspect ratio and Reynolds number will cross the opening or hit the cavity bottom and then flow out of the cavity, resulting in different flow fields structure and flow noise. This thesis mainly discusses the influence of three-dimensional vortex structure and noise interaction phenomenon in open cavity flow field at near critical Re value, focusing on the three-dimensional flow field and sound field evolution under different critical Re values and their differences. The vector field, topology and bifurcation theory are further used to analyze the instability of the flow field. The results of the study show that the mean static pressure, sound pressure level (Sound Pressure Level) and spectrum analysis are compared with experimental and numerical results. In the quantitative analysis, except that the harmonic frequency amplitude is higher than the experimental value, the rest is roughly consistent with the experiment.

碩士
國立臺灣大學
機械工程研究所
81
The purpose of the present study is to predict the flow field and heat transfer inside an open rectangular cavity by imposing impulsive start using a deterministic vortex method. The deterministic vortex method, proposed by Chang and his students (1991), is implemented to solve the viscous vorticity equation by interlacing a finite difference method for diffusion and a vortex-in-cell method for convection. Beside, this method has a good treatment on vorticity to update no-slip condition, it does not generate a large number of vortices as the random vortex method did. Therefore, a long time computation can be performed. Numerical results are presented to show the evolution of a primary eddy, secondary eddies, and the corresponding temperature fields for a Prandtl number of 2.1,cavity aspect ratios of 0.5, 0.8, 1.0, and 2.0, and for Reynolds number from 500 to 7000. The local and average Nusselt numbers on the walls for two different types of boundary coundition are also reported. When the flow reaches the steady state, numerical solutions are compared with both the predicted values of a close cavity, and experimental results[Wang(1993)] by using a naphthalene sublimation mass transfer method. Satisfactory agreements are found in the results of streamline patterns,temperature fields, and local Nusselt numbers.

碩士
國立臺灣海洋大學
輪機工程學系
104
This thesis mainly discusses the changes in the vortex structure of the three-dimensional open cavity flow by changing the aspect ratio of the open cavity which analyze the influence of changes in the structure of the vortex. Using the CFD (Computational Fluid Dynamics, CFD) open software, OpenFOAM®, to simulate different flow rates with geometric aspect ratio of the open cavity compare the influence and analysis of its pressure and vortex changes. The results clearly indicate the vortex generating with the open cavity by the constant direction of movement of the front edge of the downstream. In the bottom of open cavity, the inner pressure distribution is more uniform and holding a small value. The three-dimensional vortex generated by the open cavity and the shear layer in the lower wall has the same flow direction the interaction between vortex generated.

Thesis (Ph. D.)--University of Tennessee, Knoxville, 2002.
Title from title page screen (viewed on Sept. 5, 2002). Thesis advisor: Ahmad D. Vakili. Document formatted into pages (xv, 165 p. : ill. (some col.)). Vita. Includes bibliographical references (p. 158-161).