Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G12: Vortex Dynamics and Vortex Flows IV |
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Chair: Melissa Green, Syracuse University Room: 336 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G12.00001: Investigating three-dimensional wake topology of a low aspect ratio dual step cylinder with 2D PIV measurements Chris Morton, Serhiy Yarusevych A dual step cylinder is composed of a large diameter cylinder (D) of small aspect ratio (L/D) attached to the mid-span of a small diameter cylinder (d). The present work investigates the flow past dual step cylinders for Re$_{\mathrm{D}} \quad =$ 2100, 0.2 $\le $ L/D $\le $ 3, and 1.33 $\le $ D/d $\le $ 2.67. Experiments are completed in a water flume facility employing Laser Doppler Velocimetry (LDV) and planar Particle Image Velocimetry (PIV), as well as hydrogen bubble flow visualization. Turbulent vortex shedding occurs in the wake of the dual step cylinder for all the cases investigated. However, wake topology and vortex dynamics are influenced significantly by the geometrical parameters of the model, namely, L/D and D/d. A novel method is introduced for reconstructing salient features of the three-dimensional wake topology using phase-averaged 2D PIV measurements. The results show that flow development in the small cylinder wake away from the large cylinder is similar to that expected for a uniform cylinder of the same diameter. However, complex three-dimensional vortex deformations and splitting occur downstream of the large diameter cylinder. Four distinct flow regimes are identified based on changes in large cylinder wake development: (i) vortex shedding at a frequency lower than that expected for a uniform cylinder, (ii) irregular shedding, (iii) vortex shedding at a frequency higher than that for a uniform cylinder, and (iv) suppression of large cylinder vortex shedding. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G12.00002: Numerical investigation of vortex shedding behind a square cylinder oscillating in a closed channel John Pitre, Joseph Bull We investigate the vortex shedding behavior of a square cylinder oscillating longitudinally in a closed channel at a high blockage ratio (\textgreater 80{\%}). The square cylinder translates parallel to the long axis of the channel with a sinusoidal velocity of a given oscillation frequency. The incompressible Navier-Stokes equations in the Arbitrary Lagrangian-Eulerian formulation are solved using the finite element method. We vary both the Reynolds number and the Womersley number in order to quantify their effects on the vortex shedding. From the computational results, we calculate the Strouhal number and use this as a metric for the shedding behavior. At high Reynolds numbers, the flow is characterized by the presence of distinct vortex roll cells with long persistence times. At high Womersley numbers, these roll cells are increasingly likely to interact with the cylinder as it reverses direction and translates back across the channel. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G12.00003: A Lagrangian Coherent Structures Analysis of the Unsteady Wake Behind a Circular Cylinder Matthew Rockwood, Jacob Morrida, Melissa Green The experimentally measured unsteady wake behind a circular cylinder was studied and compared with numerical results. The location and evolution of coherent structures, or vortices, in the flow were analyzed to facilitate the understanding of the vortex shedding physics in the near wake region. This understanding is critical to the control of vortex shedding from bluff bodies. The two-component velocity data was collected using a DPIV measurement system, and Eulerian vortex criteria were applied along with a Lagrangian coherent structures (LCS) analysis to determine the properties of the wake. The LCS analysis utilizes the Finite Time Lyapunov Exponent (FTLE) method to objectively determine the locations of vortex boundaries in the flow. This technique offers new insight on the development of the unsteady wake, and shows an objective change in curvature of the LCS in the region where the new vortex will form before traditional Eulerian techniques show any changes. This information can be used to highlight regions to be targeted by flow control techniques. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G12.00004: Investigating wake topology of a single step cylinder with tomographic PIV Serhiy Yarusevych, Sina Rafati, Fulvio Scarano Wake vortex shedding from a single step cylinder is investigated experimentally using Tomographic Particle Image Velocimetry (TOMO PIV). The model geometry is comprised of two circular cylinders of different diameters joined concentrically. Experiments are conducted in a low-speed wind tunnel for a range of cylinder diameter ratios 1.14 $\le $ D/d $\le $ 2.67 and Reynolds numbers 2000 $\le $ Re$_{D\, }\le $ 5000. The employed TOMO PIV system consists of six CCD cameras subtending an arc and an Nd:YAG laser. LaVision DaVis 8 is used for image acquisition and processing. For the range of parameters investigated, turbulent vortex shedding occurs in the single-step cylinder wake. The difference in diameters leads to a variation in vortex shedding frequency, producing complex three-dimensional vortex interactions in the wake region downstream of the step. The use of TOMO PIV enables quantitative visualization and analysis of the attendant intricate vortex dynamics. Vortex filaments are visualized by the Q-criterion, and the topology of recurring vortex patterns is investigated. Reduced order modeling is used to identify dominant vortex interactions, providing added insight into the wake development. The results are used to reconstruct salient topological features of the near wake region and to investigate the effect of diameter ratio and Reynolds number on the wake topology. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G12.00005: The turbulent wake of a submarine model at varying pitch and yaw angle Anand Ashok, Tyler Van Buren, Alexander Smits Experiments are reported to examine the effects of pitch and yaw angle on the mean flow and turbulence in the wake of an axisymmetric DARPA SUBOFF submarine model. Measurements in the wake were performed at a Reynolds number based on the length of 2.4 $\times$ 10$^{6}$. Three component velocity measurements were taken at eight cross-stream planes, downstream of the trailing edge of the model (2 \textless\ x/D \textless\ 26), using Stereoscopic Particle Image Velocimetry. The pitch and yaw angles were in the range 0 to $\pm$ 10$^{\circ}$. Two-point, crossed wire measurements in the wake of the same submarine model in the axisymmetric configuration over a wide range ofnumbers based on the length between 1 $\times$ 10$^{6}$ and 67 $\times$ 10$^{6}$ are also presented. Work supported by ONR Grant N00014-13-1-0174. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G12.00006: Shedding characteristics along the span in the wake of a low-aspect-ratio pyramid Zahra Hosseini, Mouhammad El Hassan, Robert Martinuzzi The aim of the present work is to extract the 3D vortical structures in the wake of a low-aspect-ratio pyramid to study the cellular shedding patterns and interactions between structures with different scales along the span. The velocity fields measured, using Time Resolved PIV, at 2D planes in the wake shows the formation of anti-symmetric Karman shedding and instances of symmetrically positioned structures that interrupt the shedding. In such instances the Reynolds stresses drop significantly. The coherent vortical structures were extracted using the first three POD modes, the first two composing a harmonic pair and the third a symmetric mode capturing the low frequency modulations. The energy of the third mode is almost constant along the span which can be related to the spanwise coupling of the structures. The energy of the harmonic pair is significantly larger at the base but drops quickly at higher spans and becomes comparable to that of the third mode. In the symmetric configurations, the harmonic pair amplitude drops sharply and the symmetric mode amplitude is rather high. To understand the mechanisms that result in such interruptions, the 3D structures will be reconstructed from isolated planar measurements using a low order model based on the most dominant POD modes and their correlation with the surface pressure. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G12.00007: Vortices Behind Asymmetric Bodies Forming Closed Wakes Alan Elcrat, Ken Miller, Luca Zannetti We describe flows past bodies in 2D inviscid flow which are uniform at infinity, and in which there are two point vortices standing in equilibrium with the flow. The bodies are not symmetric, and in general there is a circulation around the body plus vortices. This model then has three degrees of freedom which can be reduced by specifying separation points on the body. In particular for bodies with sharp corners the Kutta condition can be imposed at these corners. There results a model for the wake in which, in general, there is flow from upstream infinity to downstream infinity through the recirculation region ie the separating streamlines do not rejoin. The wake is not closed. We give examples in which the remaining free parameters can be used to force closure of the wake ie to have a closed recirculation region. These include arcs for which the Kutta condition is imposed at each end and Joukouski airfoils with the Kutta condition imposed at the trailing edge. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G12.00008: Circulation shedding in viscous starting flow past a flat plate Monika Nitsche, Ling Xu Numerical simulations of viscous flow past a flat plate moving in direction normal to itself reveal details of the vortical structure of the flow. At early times, most of the vorticity is attached to the plate. We introduce a definition of the shed circulation at all times and show that it indeed represents vorticity that separates and remains separated from the plate. Scaling laws for the shed circulation, and contributions to the shedding rate across various boundary components are presented, as well as their dependence on Reynolds number. The simulations provide benchmark results to evaluate simpler separation models such as point vortex and vortex sheet models. A comparison with vortex sheet results is included. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G12.00009: Pressure-gradient Mechanism for Vortex Shedding in External Flows Michael Boghosian, Kevin Cassel In our previous research, a pressure-gradient mechanism is identified as a likely cause of vortex splitting and shedding in constricted two-dimensional channel flows. We now find this mechanism present in the canonical two-dimensional external flows of (a) the cylinder in crossflow and (b) the flow over a step via numerical simulations of the unsteady, two-dimensional, incompressible Navier--Stokes equations. The details of the pressure-gradient mechanism are presented for Reynolds numbers typically found in the literature. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G12.00010: The Interplay of Acceleration and Vorticity Fields in the Tip Region of Massively-Separated Flows David Rival, Jochen Kriegseis The influence of seemingly analogous plate kinematics (plunge vs. tow) on instantaneous forces has been investigated. Simultaneous measurements by means of three-dimensional particle tracking velocimetry (3D-PTV) and a six-component force/moment sensor have been performed. Despite identical effective shear-layer velocities and effective angles of attack, the force histories vary between the two cases (plunge and tow). To uncover this discrepancy, a combined analysis of vorticity, Lagrangian (total) fluid acceleration and vortex-force contribution (Lamb vector) has been performed. It is found that leading-edge vortex (LEV) and tip vortex (TV) formation are nearly identical during the acceleration phase for both cases. However, at the end of acceleration the tow LEV rolls off the plate. As such, the development of vortex force also ceases once this roll-off process begins. Also TV strength as well as its relative positioning to the plate surface influences the instantaneous force. Based on a Lamb-vector analysis of the TV, the present work provides insight into the underlying cause-effect relation. Particularly, it is demonstrated that the sensitivity of the resulting vortex-force formation is dependent on the interplay between streamwise vorticity and spanwise (inboard) velocity. [Preview Abstract] |
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