Hydrodynamics of flat plates in cross-flow near the free surface

Aquesta tesi presenta els resultats d'un estudi realitzat per comprendre l'efecte de la superfície lliure sobre les forces generades per una placa plana que es mou en un fluid estacionari, de manera que la superfície de la placa és perpendicular a la direcció d’empenta. El treball es basa en un estudi paramètric que inclou la relació d'aspecte, el nombre de Reynolds i la profunditat d'immersió mesurada com la distància entre la superfície lliure i la vora superior de la placa. S'han utilitzat mesures de força i tècniques de visualització quantitativa de flux. S'ha demostrat que les forces d'arrossegament augmenten bruscament just sota de la superfície lliure, essent l'augment més destacat en les plaques de baixa relació d'aspecte. L'augment abrupte en el coeficient d'arrossegament es deu a l'existència del flux que apareix entre la placa i la superfície lliure i que resulta en la formació d'una zona de recirculació prop de la base de la placa. En general, les tendències són independents del número de Reynolds, excepte quan les relacions d'aspecte estan en el rang de 0,75 a 1,33, amb la placa a prop de la superfície lliure. Sukruth Satheesh A més, s'han investigat dues configuracions de plaques diferents. En primer lloc, plaques rígides amb porositat en diferents ubicacions i, en segon lloc, flexibilitat estructural. Ambdós conceptes han resultat en una reducció significativa del coeficient d'arrossegament, especialment a prop de la superfície lliure. S'ha demostrat que el mecanisme de reducció d'arrossegament en models porosos es deu a la interacció entre els dolls formats en els orificis, les capes de cisalla i el flux que apareix entre la placa i la superfície lliure. Amb flexibilitat estructural, la reconfiguració de la plaça implica la reducció de l'arrossegament.Esta tesis presenta los resultados de un estudio realizado para comprender el efecto de la superficie libre sobre las fuerzas generadas por una placa plana que se mueve en el seno de un fluido estacionario, de manera que la superficie de la placa es perpendicular a la dirección de empuje. El trabajo se basa en un estudio paramétrico que incluye la relación de aspecto, el número de Reynolds y la profundidad de inmersión medida como la distancia entre la superficie libre y el borde superior de la placa. Se han usado medidas de fuerza y técnicas de visualización cuantitativa de flujo. Se ha demostrado que las fuerzas de arrastre aumentan bruscamente justo debajo de la superficie libre, siendo el aumento más prominente en las placas de baja relación de aspecto. El aumento abrupto en el coeficiente de arrastre se debe a la existencia de un flujo encauzado entre la placa y la superficie libre que resulta en la formación de una zona de recirculación cerca de la base de la placa. En general, las tendencias son independientes del número de Reynolds, excepto cuando las relaciones de aspecto están en el rango de 0.75 a 1.33, con la placa cerca de la superficie libre. Además, se han investigado dos configuraciones de placa diferentes. Primero, placas rígidas con porosidad en distintas ubicaciones y, en segundo lugar, flexibilidad estructural. Ambos conceptos han resultado en una reducción significativa del coeficiente de arrastre, especialmente cerca de la superficie libre. Se ha demostrado que el mecanismo de reducción de arrastre en modelos porosos se debe a la interacción entre los chorros formados en los orificios, las capas de cizalladura y el flujo que aparece entre la placa y la superficie libre. Con flexibilidad estructural, la reconfiguración de la placa implica la reducción del arrastre.This thesis presents the results of a study conducted to understand the effect of free surface on forces generated by a flat plate being towed in a quiescent fluid, with the plate surface being normal to the towing direction. The work is based on a parametric study involving aspect ratio, Reynolds number and submergence depth, with the depth always being measured as the distance between free surface and the upper edge of the plate. Force and quantitative flow visualization techniques have been employed to understand the flow physics. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this jump in drag being more prominent in low aspect ratio plates. The abrupt rise in the drag is due to the existence of a gap-flow at the free surface resulting in the formation of a recirculating flow in close proximity to the base region of plate. Overall, the trends are Reynolds number independent, except when the aspect ratios are in the range from 0.75 to 1.33, and the plate was near the free surface. Furthermore, two different plate configurations have been investigated. First, rigid plates with porosity at distinct locations and secondly structural flexibility. Both concepts have resulted in significant drag reduction, especially near the free surface. The mechanism of drag reduction for porous models has been shown to be due to the interaction between the jets formed at the holes, the shear layers and the gap-flow. With structural flexibility, reconfiguration leads to drag reduction

Large Eddy Simulation of Riverflows

Keynote Lecture

Large Eddy Simulation and Analysis of Shear Flows in Complex Geometries

In the present work, large eddy simulation is used to numerically investigate two types of shear flows in complex geometries, (i) a novel momentum driven countercurrent shear flow in dump geometry and (ii) a film cooling flow (inclined jet in crossflow). Verification of subgrid scale model is done through comparisons with measurements for a turbulent flow over back step, present cases of counter current shear and film cooling flow. In the first part, a three dimensional stability analysis is conducted for countercurrent shear flow using Dynamic mode decomposition and spectral analysis. Kelvin-Helmholtz is identified as primary instability mechanism and observed as global mode at a specific parameter. Mechanism of global mode synchronization over distinct spatial location is studied. In the second part, the flow physics of film cooling flows is analysed. The origin, evolution of various coherent flow structures and their role in film cooling heat transfer is studied based on detailed flow visualization. Further, the contribution of coherent structures in film cooling heat transfer and mixing is studied through modal analysis. Low frequency modes are found to have large contribution in cooling surface adiabatic temperature fluctuation while high frequency modes play larger role in bulk mixing. Finally, a new contoured crater shape is developed and shown to have improved performance at shallow depth compared to earlier designs

Turbulent Flow Visualization over Surface-mounted Finite-height Cylinders and Square Prisms

The study of turbulent flows over surface-mounted, finite-height bluff bodies like cylinders and square prisms have various engineering and industrial applications. The flow field around and in the wake of these bodies is sufficiently complex due to the interactions of the ground plane boundary layer with the separating shear layers from the free end and the sides of these bodies. As such straightforward geometries produce large, complex wakes, there has been increased interest in the literature in the past few decades on examining this flow field experimentally. Recent advancements in computational power have also facilitated the numerical calculation of these flows at higher spatial and temporal resolutions, which were otherwise inestimable by experiments alone. However, the instantaneous flow field in the wake, and specifically above the free end, which is a major contributor to the three-dimensionality of this flow field, has not been well understood. Beyond numerical simulations or experiments, a modern challenge in turbulence research has been to identify and classify the instantaneous energetic structures ensconced in a turbulent flow field. Velocity gradient methods like the swirling strength criterion and the Q-criterion have successfully tendered a mathematical definition to isolate vortex structures embedded in turbulent flow. Another promising approach called proper orthogonal decomposition (POD) has also become popular in the past couple of decades, as it provides for a low-dimensional approximation of a high-dimensional turbulent flow field. Thus, the aim of this thesis is to pursue fundamental studies of the flow topologies above the free end of a surface-mounted cylinder and square prism, as well as in the wake of a surface-mounted square prism, to obtain insightful representations of these flow fields using enhanced post-processing methodologies like the swirling strength criterion, Q-criterion, and POD. The first manuscript (presented as Chapter 2) investigated the flow field obtained from Particle Image Velocimetry (PIV) above the free end of a surface-mounted finite square prism in the vertical symmetry plane at a Reynolds number of Re = 4.2x10^4 for four different aspect ratios AR = 9, 7, 5 and 3. The POD methodology was able to capture the energetic flow features within a small number of energy modes. A qualitative analysis of the energy modes revealed a pair of shear sub-layers from the free end, as well as several vortex structures within these layers, likely evidence of Kelvin-Helmholtz (KH) instabilities. The swirling strength criterion was also used to find other structural features in the near-wake above and behind the free end, and the changes in the flow topologies with aspect ratio were demonstrated. The second manuscript (presented in Chapter 3) investigated the flow field obtained from PIV above a surface-mounted finite cylinder in several horizontal planes close to the free end and parallel to it, at a Reynolds number of Re = 4.2x10^4 for four different aspect ratios AR = 9, 7, 5 and 3. The POD methodology was able to capture the energetic flow features within a small number of energy modes. A qualitative analysis of the energy modes revealed pairs of symmetric, side-tip counter-rotating vortices, a pair of counter-rotating vortices on each side of the midline, as well as evidence of alternate undulation of flow on either side of the midline. Flow topologies were shown to vary with aspect ratio, with a general trend of a vertical compression of the wake at higher aspect ratios due to a stronger entrainment of flow from the outer freestream velocity and a weakening influence of the ground plane. The third manuscript (presented in Chapter 4) investigated the time-averaged three-dimensional flow field obtained from Large Eddy Simulation (LES) around and in the wake of a surface-mounted finite square prism at a Reynolds number of Re = 500 for aspect ratio AR = 3. The dominant flow features include the horseshoe vortex enclosing the base junction, and a pair of counter-rotating tip vortices emerging from the sides of the prism close to the free end. The tip vortices were shown to descend towards the ground plane in the wake owing to downwash effects. Other flow features using streamlines and vorticity were also demonstrated, including the mean recirculation zone (Bt vortex) and the base junction vortex (Nw vortex) in the vertical symmetry plane, a pair of symmetric vortices in the horizontal planes, as well as several separating shear layers and foci around the square prism surfaces. The fourth manuscript (presented in Chapter 5) performed a 3D POD analysis on the instantaneous flow field obtained from Large Eddy Simulation (LES) in the wake of a surface-mounted finite-height square prism at a Reynolds number of Re = 500 for aspect ratio AR = 3. The energetic POD energy modes revealed several dominant streamwise vortex tubes. A bifurcation of a streamwise vortex strand across the vertical symmetry plane was also observed, manifesting evidence of an alternating, upward-inclined streamwise connector strand traversing downstream and culminating in a vortex core. The POD temporal coefficients revealed strong periodicity and were used to obtain the phase information for this flow field. Accordingly, two lower order reconstructions opposite in phase were corroborated with the instantaneous flow field, further substantiating the alternating half-loop structure traversing downstream

Large eddy simulation of river flows

Keynote Lecture

Vortex detection and tracking in massively separated and turbulent flows

The vortex produced at the leading edge of the wing, known as the leading edge vortex (LEV), plays an important role in enhancing or destroying aerodynamic force, especially lift, upon its formation or shedding during the flapping flight of birds and insects. In this thesis, we integrate multiple new and traditional vortex identification approaches to visualize and track the LEV dynamics during its shedding process. The study is carried out using a 2D simulation of a flat plate undergoing a 45 degree pitch-up maneuver. The Eulerian 1 function and criterion are used along with the Lagrangian coherent structures (LCS) analyses including the finite-time Lyapunov exponent (FTLE), the geodesic LCS, and the Lagrangian-Averaged Vorticity Deviation (LAVD). Each of \h{these} Lagrangian methods \h{is} applied at the centers and boundaries of the vortices to detect the vortex dynamics. The techniques enable the tracking of identifiable features in the flow organization using the FTLE-saddles and -saddles. The FTLE-saddle traces have shown potential to identify the timing and location of vortex shedding, more precisely than by only studying the vortex cores as identified by Eulerian techniques. The traces and the shedding times of the FTLE-saddles on the LEV boundary matches well with the plate lift fluctuation, and indicates a consistent timing of LEV formation, growth, shedding. The formation number and vortex shedding mechanisms are compared in the thesis with the shedding time and location by the FTLE-saddle, which validates the result of the FTLE-saddles and provide explanations of vortex shedding in different aspects (vortex strength and flow dynamics). The techniques are applied to more cases involving vortex dominated flows to explore and expand their application in providing insight of flow physics. For a set of experimental two-component PIV data in the wake of a purely pitching trapezoidal panel, the Lagrangian analysis of FTLE-saddle tracking identifies and tracks the vortex breakdown location with relatively less user interaction and provide a more direct and consistent analysis. For a simulation of wall-bounded turbulence in a channel flow, tracking FTLE-saddles shows that the average structure convection speed exhibits a similar trend as a previously published result based on velocity and pressure correlations, giving validity to the method. When these Lagrangian techniques are applied in a study of the evolution of an isolated hairpin vortex, it shows the connection between primary and secondary hairpin heads of their circulation and position, and the contribution to the generation of the secondary hairpin by the flow characteristics at the channel wall. The current method of tracking vortices yields insight into the behavior of the vortices in all of the diverse flows presented, highlighting the breadth of its potential application