A Family of Vortices to Study Axisymmetric Vortex Breakdown and Reconnection

A new analytic model describing a family of vortices has been developed to study some of the axisymmetric vortex breakdown and reconnection fluid dynamic processes underlying body-vortex interactions that are frequently manifested in rotorcraft and propeller-driven fixed-wing aircraft wakes. The family of vortices incorporates a wide range of prescribed initial vorticity distributions -- including single or dual-core vorticity distributions. The result is analytical solutions for the vorticity and velocities for each member of the family of vortices. This model is of sufficient generality to further illustrate the dependence of vortex reconnection and breakdown on initial vorticity distribution as was suggested by earlier analytical work. This family of vortices, though laminar in nature, is anticipated to provide valuable insight into the vortical evolution of large-scale rotor and propeller wakes

Illustrative Flow Visualization of 4D PC-MRI Blood Flow and CFD Data

Das zentrale Thema dieser Dissertation ist die Anwendung illustrativer Methoden auf zwei bisher ungelöste Probleme der Strömungsvisualisierung. Das Ziel der Strömungsvisualisierung ist die Bereitstellung von Software, die Experten beim Auswerten ihrer Strömungsdaten und damit beim Erkenntnisgewinn unterstützt. Bei der illustrativen Visualisierung handelt es sich um einen Zweig der Visualisierung, der sich an der künstlerischen Arbeit von Illustratoren orientiert. Letztere sind darauf spezialisiert komplizierte Zusammenhänge verständlich und ansprechend zu vermitteln. Die angewendeten Techniken werden in der illustrativen Visualisierung auf reale Daten übertragen, um die Effektivität der Darstellung zu erhöhen. Das erste Problem, das im Rahmen dieser Dissertation bearbeitet wurde, ist die eingeschränkte Verständlichkeit von komplexen Stromflächen. Selbstverdeckungen oder Aufrollungen behindern die Form- und Strömungswahrnehmung und machen diese Flächen gerade in interessanten Strömungssituationen wenig nützlich. Auf Basis von handgezeichneten Strömungsdarstellungen haben wir ein Flächenrendering entwickelt, das Silhouetten, nicht-photorealistische Beleuchtung und illustrative Stromlinien verwendet. Interaktive Flächenschnitte erlauben die Exploration der Flächen und der Strömungen, die sie repräsentieren. Angewendet auf verschiedene Stromflächen ließ sich zeigen, dass die Methoden die Verständlichkeit erhöhen, v.a. in Bereichen komplexer Strömung mit Aufwicklungen oder Singularitäten. Das zweite Problem ist die Strömungsanalyse des Blutes aus 4D PC-MRI-Daten. An diese relativ neue Datenmodalität werden hohe Erwartungen für die Erforschung und Behandlung kardiovaskulärer Krankheiten geknüpft, da sie erstmals ein dreidimensionales, zeitlich aufgelöstes Abbild der Hämodynamik liefert. Bisher werden 4D PC-MRI-Daten meist mit Werkzeugen der klassischen Strömungsvisualisierung verarbeitet. Diese werden den besonderen Ansprüchen der medizinischen Anwender jedoch nicht gerecht, die in kurzer Zeit eine übersichtliche Darstellung der relevanten Strömungsaspekte erhalten möchten. Wir haben ein Werkzeug zur visuellen Analyse der Blutströmung entwickelt, welches eine einfache Detektion von markanten Strömungsmustern erlaubt, wie z.B. Jets, Wirbel oder Bereiche mit hoher Blutverweildauer. Die Grundidee ist hierbei aus vorberechneten Integrallinien mit Hilfe speziell definierter Linienprädikate die relevanten, d.h. am gefragten Strömungsmuster, beteiligten Linien ausgewählt werden. Um eine intuitive Darstellung der Resultate zu erreichen, haben wir uns von Blutflußillustrationen inspirieren lassen und präsentieren eine abstrakte Linienbündel- und Wirbeldarstellung. Die Linienprädikatmethode sowie die abstrakte Darstellung der Strömungsmuster wurden an 4D PC-MRI-Daten von gesunden und pathologischen Aorten- und Herzdaten erfolgreich getestet. Auch die Evaluierung durch Experten zeigt die Nützlichkeit der Methode und ihr Potential für den Einsatz in der Forschung und der Klinik.This thesis’ central theme is the use of illustrative methods to solve flow visualization problems. The goal of flow visualization is to provide users with software tools supporting them analyzing and extracting knowledge from their fluid dynamics data. This fluid dynamics data is produced in large amounts by simulations or measurements to answer diverse questions in application fields like engineering or medicine. This thesis deals with two unsolved problems in flow visualization and tackles them with methods of illustrative visualization. The latter is a subbranch of visualization whose methods are inspired by the art work of professional illustrators. They are specialized in the comprehensible and esthetic representation of complex knowledge. With illustrative visualization, their techniques are applied to real data to enhance their representation. The first problem dealt with in this thesis is the limited shape and flow perception of complex stream surfaces. Self-occlusion and wrap-ups hinder their effective use in the most interesting flow situations. On the basis of hand-drawn flow illustrations, a surface rendering method was designed that uses silhouettes, non-photorealistic shading, and illustrative surface stream lines. Additionally, geometrical and flow-based surface cuts allow the user an interactive exploration of the surface and the flow it represents. By applying this illustrative technique to various stream surfaces and collecting expert feedback, we could show that the comprehensibility of the stream surfaces was enhanced – especially in complex areas with surface wrap-ups and singularities. The second problem tackled in this thesis is the analysis of blood flow from 4D PC-MRI data. From this rather young data modality, medical experts expect many advances in the research of cardiovascular diseases because it delivers a three-dimensional and time-resolved image of the hemodynamics. However, 4D PC-MRI data are mainly processed with standard flow visualizaton tools, which do not fulfill the requirements of medical users. They need a quick and easy-to-understand display of the relevant blood flow aspects. We developed a tool for the visual analysis of blood flow that allows a fast detection of distinctive flow patterns, such as high-velocity jets, vortices, or areas with high residence times. The basic idea is to precalculate integral lines and use specifically designed line predicates to select and display only lines involved in the pattern of interest. Traditional blood flow illustrations inspired us to an abstract and comprehensible depiction of the resulting line bundles and vortices. The line predicate method and the illustrative flow pattern representation were successfully tested with 4D PC-MRI data of healthy and pathological aortae and hearts. Also, the feedback of several medical experts confirmed the usefulness of our methods and their capabilities for a future application in the clinical research and routine

Response of a swirl-stabilized flame to transverse acoustic excitation

This work addresses the issue of transverse combustion instabilities in annular gas turbine combustor geometries. While modern low-emissions combustion strategies have made great strides in reducing the production of toxic emissions in aircraft engines and power generation gas turbines, combustion instability remains one of the foremost technical challenges in the development of next generation combustor technology. To that end, this work investigates the response of a swirling flow and swirl-stabilized flame to a transverse acoustic field is using a variety of high-speed laser techniques, especially high-speed particle image velocimetry (PIV) for detailed velocity measurements of this highly unsteady flow phenomenon. A description of the velocity-coupled transverse instability mechanism is explained with companion measurements describing each of the velocity disturbance pathways. Dependence on acoustic frequency, amplitude, and field symmetry is discussed. Significant emphasis is placed on the response of a swirling flow field to a transverse acoustic field. Details of the dynamics of the vortex breakdown bubble and the shear layers are explained using a wide variety of measurements for both non-reacting and reacting flow cases. This thesis concludes with an overview of the impact of this work and suggestions for future research in this area.PhDCommittee Chair: Tim Lieuwen; Committee Member: Ari Glezer; Committee Member: Jerry Seitzman; Committee Member: Lakshmi Sankar; Committee Member: Suresh Meno

Prandtl-Batchelor theorem for three-dimensional flows slowly varying in one direction and its application to vortex breakdown

In this work, the Prandtl-Batchelor theorem is extended to three-dimensional flows slowly varying in one direction by using asymptotic techniques, and thus overcoming the problem of having non-closed streamlines for recirculating three-dimensional flows. The derived equations turned out to be an analogue of the quasi-cylindrical equations used for describing behavior of streamwise vortices, rotating jets, vortex breakdown phenomenon and some other problems. Hence, the derived equations may be used for studying similar phenomena in non-axisymmetric cases. In order to apply such a system of equations to particular problems, a computational code was developed and validated by reproducing numerical results available in the literature. This code was constructed in two parts, one part considered the parabolic system of partial differential equations as decoupled from the Poisson equation and the second part solved the nonlinear Poisson equation by using an iterative method. Finally, these two algorithms were joined in order to solve the entire system. Once the code was available, it was used to investigate possible non-axisymmetric effects on the position of vortex breakdown phenomenon. The results of this study suggest that non-axisymmetric effects precipitate the onset of vortex breakdown. From all this work, two articles were written, one article was published in the Journal of Fluid Mechanics (see Appendix D) and the second article will be submitted

Vortical flow. Part 2. Flow past a sphere in a constant-diameter pipe

This paper describes an experimental and numerical investigation of concentrated vortex flow past a sphere in a constant-diameter pipe. As the swirl was increased at a fixed sphere Reynolds number of approximately 1100, the length of the mean downstream separation bubble decreased. For a small range of swirl intensity, an almost stagnant separation bubble formed on the upstream hemisphere. A further increase in swirl caused the bubble to become unstable and develop into an unsteady spiral disturbance. At very high swirl the downstream separation bubble was eliminated and an unsteady separation zone extended far upstream. Calculations of the vorticity field from surface fits to azimuthal and axial velocity data suggest that upstream separation is caused by the distortion of vortex filaments in the diverging flow approaching the sphere. Numerical solutions of steady inviscid axisymmetric flow past a sphere exhibit a fold in the vicinity of upstream separation. It is suggested that this accounts for the extreme sensitivity encountered in the experiments.T. W. Mattner, P. N. Joubert and M. S. Chon

An experimental analysis of critical factors involved in the breakdown process of leading edge vortex flows

Experimental crosswire measurements of the flowfield above a 70 and 75 degree flat plate delta wing were performed at a Reynolds number of 250,000. Survey grids were taken normal to the platform at a series of chordwise locations for angles of attack of 20 and 30 degrees. Axial and azimuthal vorticity distributions were derived from the velocity fields. The dependence of circulation on distance from the vortex core as well as on chordwise location was examined. The effects of nondimensionalization in comparison with other experimental data was made. The circulation distribution scales with the local semispan and grows approximately linearly in the chordwise direction. For regions of the flow outside of the vortex subcore, the circulation at any chordwise station was observed to vary logarithmically with distance from the vortex axis. The circulation was also found to increase linearly with angle of incidence at a given chordwise station. A reduction in the local circulation about the vortex axis occurred at breakdown. The spanwise distribution of axial vorticity was severely altered through the breakdown region and the spanwise distribution of axial vorticity present appeared to reach a maximum immediately preceding breakdown. The local concentration of axial vorticity about the vortex axis was reduced while the magnitude of the azimuthal vorticity decreased throughout the breakdown zone. The axial vorticity components with a negative sense, found in the secondary vortex, remained unaffected by changes in wing sweep or angle of attack, in direct contrast to the positive components. The inclusion of the local wing geometry into a previously derived correlation parameter indicated that the circulation of growing leading edge vortex flows were similar at corresponding radii from the vortex axis. It was concluded that the flow over a delta wing, upstream of the breakdown regions and away from the apex and trailing edge regions, is conical. In addition, the dominating factors leading to the onset of breakdown are felt to be the local circulation of the vortex and the accompanying pressure field

Interaction of thrust vectoring jets with wing vortical flows

It has been widely anticipated that thrust vectoring could be an effective method of providing sufficient levels of stability and control for highly manoeuvrable and flexible Unmanned Combat Air Vehicles (UCAVs). The present project aims to understand the interactions of delta wing vortical flows and thrust vectoring, with an emphasis on unsteady aspects. Food-colouring dye flow visualization, Laser-induced fluorescent flow visualization, Particle Image Velocimetry (PIV) and force measurements were conducted in the water and wind tunnels over a range of dimensionless frequencies and jet momentum coefficients. Both slender and nonslender wings were tested with the purpose of understanding the effect of sweep angle on the aerodynamics-propulsion interaction. The interaction of statically pitched trailing-edge jets with leading-edge vortices over stationary delta wings was studied. It was found that under-vortex blowing with rectangular nozzle at stall and post-stall regimes could yield the maximum effectiveness of trailing-edge blowing, due to the promotion of earlier reattachment and delay of vortex breakdown. The effect of nozzle geometry can be important, because the entrainment effect of the jet depends on it. Studies of the flow field reveal strong jet-vortex interactions, distortion of jet vortices, and merging of wing and jet vortices. The dynamic responses of wing vortical flows to dynamic trailing-edge blowing exhibit hysteresis and phase lag, which increases with the increasing dimensionless frequency of jet momentum. Time delay for the decelerating jet is significantly larger than that for the accelerating jet. Sweep angle has no significant influence on the effect of unsteady trailing-edge blowing. From a design aspect, hysteresis and time delay need to be considered for the flight control systems.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Formation and evolution of vortex rings with weak to moderate swirl and their implications for enhancing vortex ring circulation

The formation of swirling vortex rings and their early time evolution, resulting from the controlled discharge of an incompressible fluid into a stationary equivalent fluid bulk, is explored both numerically and experimentally for swirl number S ∈ [0, 1]. For the numerical work, two practically realisable inlet conditions are investigated with swirl simultaneously superposed onto a linear momentum discharge; the corresponding circulation based Reynolds number is 7500. The results reveal that, for S > 1/2, the addition of swirl promotes the breakdown of the leading primary vortex ring structure, giving rise to the striking feature of significant negative vorticity, or opposite sign vorticity (OSV), generation in the region surrounding the primary vortex ring core, whose strength scales with S2. Through a non-linear interaction with the vortex breakdown, the radius of the primary toroidal vortex core is rapidly increased; consequently, the self-induced propagation velocity of the leading ring decreases with S and vortex stretching along the circular primary vortex core increases counteracting viscous diffusion effects. The latter governs the evolution of the peak vorticity intensity and the swirl velocity magnitude in the primary ring core, the circulation growth rate of this ring, as well as the vorticity intensity of the trailing jet and hence its stability. This combination of effects leads to an increased dimensionless kinetic energy for the primary ring with increasing S and results in an almost linearly decreasing circulation based formation number, F. In a rigorous complementary experimental investigation, OSV is observed by introducing swirl using a rotating pipe, varying the time period before the piston stroke to achieve the desired swirl strength at a Reynolds number of 1000. Rotating pipe is found to generate a secondary flow altering the inlet condition. Nevertheless, it is observed, using short periods of pipe rotation and higher angular speed, that it is possible to generate a swirling vortex ring with less OSV production and all the related effects discussed above. The relation between F and the radius of the vortex ring is investigated through manipulation of ring radius growth, achieved through its interaction with a preceding vortex ring. Reducing radius growth, facilitates an increase of the circulation of the vortex ring, which in turn affects its F value

Flow Control Applications

Flow control has a long history with many successes across a plethora of applications. This report addresses the characteristics of the approaches that are actually used, why they are used, the many approaches that are not used, and why. Analysis indicates ways forward to increase applicability/usefulness, and efficiency of flow control research. Overall, greater and more effective progress in flow control requires utilization of far more detailed information early in the research process regarding application details and requirements