Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session LN: Vortex Flows: Bluff Bodies |
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Chair: Bhimsen Shivamoggi, University of Central Florida Room: Long Beach Convention Center 202C |
Monday, November 22, 2010 3:35PM - 3:48PM |
LN.00001: On the spanwise variation of vortex shedding in the wake of slender cones Chetan Jagadeesh, Michael Gaster The present work aims to study the spanwise variation of vortex shedding in the wake of slender cones at low Reynolds numbers. The occurrence of modulated velocity fluctuations is associated with cellular vortex shedding, with the discontinuities responsible for these modulations occurring at discrete locations along the span. Hot-wire data showed that the increase in amount of taper resulted in the increase in the number of vortex shedding cells in the wake, with the velocity fluctuations being modulated at all locations along the span of highly tapered cones. Comparing flow visualization images, hot-wire signals and corresponding streamwise velocity contours, one can confirm that the activity seen in the junction of parallel and tilted vortex lines is indeed the discontinuity and the discontinuity appears to move along the span of the cone in time. The tilting of vortices is periodic and continues along the span of the cone as time progresses and is surmised that as the vortex lines reach a certain large angle the process gets disrupted, resulting in the so-called discontinuity. This process then occurs at the next spanwise location, until it reaches a local diameter where the Reynolds number is too low for periodic vortex shedding. Thus the movement of the discontinuity in the wake of the highly tapered cone could lead to a scenario where the so-called cells could be considered to be non-stationary, or in other words moving. [Preview Abstract] |
Monday, November 22, 2010 3:48PM - 4:01PM |
LN.00002: The Effect of Relative Submergence and Shape on the Wake of a Low-Aspect-Ratio Wall-Mounted body Seyed Mohammad Hajimirzaie, Craig Wojcik, James Buchholz Wall-mounted bodies in boundary layer flows are ubiquitous in nature and engineering applications and significantly enhance momentum and scalar transport in their vicinity. In this experimental study we evaluate the role of relative submergence (the ratio of flow depth to obstacle height) and shape on the wakes around four different wall-mounted obstacles. We consider four obstacle geometries: semi-ellipsoids with the major and minor axes of the base ellipses aligned in the streamwise and transverse directions, and two cylinders with matching aspect ratios D/H (where D is the maximum transverse dimension and H is the obstacle height). The aspect ratios considered are 0.67 and 0.89. DPIV was used to interrogate the flow. Streamwise structures observed in the mean wake include tip, base, and horseshoe vortex pairs as well as additional structures apparently not previously observed. The presence of a base vortex for such low-aspect-ratio obstacles is unexpected, and its strength increases with decreasing relative submergence. We will discuss hypotheses on the mechanisms of generation of the base and tertiary structures and their interconnection with the rest of the vortex skeleton. [Preview Abstract] |
Monday, November 22, 2010 4:01PM - 4:14PM |
LN.00003: Experimental Investigations of the Flow past Circular Cylinders with Stepwise Discontinuities Chris Morton, Serhiy Yarusevych Circular cylinders with step discontinuities in diameter are often encountered in engineering applications, e.g., finned tube heat exchangers and civil structures. This investigation is focused on wake vortex shedding from cylinders with one or two step discontinuities in diameter along the span. For uniform flow past a single step cylinder, the flow development is dependent on the Reynolds number (Re) and the ratio of the large cylinder diameter (D) to the small cylinder diameter (d), known as the diameter ratio (D/d). For a dual step cylinder, in addition to these two parameters, the aspect ratio of the large cylinder (L/D) is expected to influence vortex dynamics. This study has been performed for Re = 1050, D/d = 2, and a range of aspect ratios from 0.2 to 17. Experimental measurements have been acquired in a water flume facility using laser Doppler velocimetry and flow visualization. The results show that vortex shedding occurs in spanwise vortex cells of constant frequency. Vortex connections form between the spanwise vortices in these cells downstream of the step discontinuities, and vortex dislocations occur at cell boundaries. For a dual step cylinder, the aspect ratio is found to have a profound effect on vortex shedding, resulting in several distinct flow regimes. [Preview Abstract] |
Monday, November 22, 2010 4:14PM - 4:27PM |
LN.00004: Horseshoe Vortex Dynamics in Cylinder-Wall and Cylinder-Endplate Junctions Adam Blackmore, Tayfun Aydin, Alis Ekmekci Unsteady horseshoe vortex systems forming near circular cylinder-wall and circular cylinder-end plate junctures are studied experimentally using Particle Image Velocimetry (PIV). Wall mounted cylinder is exposed to a flow with boundary layer thickness of 0.24$D$ at the juncture, and the endplate mounted cylinder is exposed to flow that undergoes separation at the leading edge of the endplate and reattaches before the junction. PIV measurements on the cylinder-wall junction show presence of two primary horseshoe vortices. The vortex closest to the body (the first vortex) is periodically fed by the second vortex which develops further upstream. The trajectories of the horseshoe vortices on the plane of symmetry show that the second vortex shows several oscillations in the approach flow direction before merging closer to the first vortex, which also oscillates at the same frequency. The horseshoe vortex system at the cylinder-endplate junction shows quasi-periodicity over a certain leading edge distance, but has a more complex unsteady behaviour due to the separation/reattachment of the flow prior to the juncture. [Preview Abstract] |
Monday, November 22, 2010 4:27PM - 4:40PM |
LN.00005: Secondary Vortex Structures in Vortex Generator Induced Flow Clara Velte, Valery Okulov, Martin Hansen Passive rectangular vane actuators can induce a longitudinal vortex that redistributes the momentum in the boundary layer to control the flow. Recent experiments [1] as well as previous studies [2] have shown that a secondary vortex of opposite sign is generated along with the primary one, supposedly from local separation of the boundary layer due to the primary vortex. 2D flow visualizations of a vortex in the vicinity of a boundary support this hypothesis [3]. These secondary vortices are studied for various configurations -- single generator, counter- and co-rotating cascades. The objective is to study their removal through cancelation in cascades using Stereoscopic Particle Image Velocimetry and flow visualization.\\[4pt] [1] Velte, Hansen and Okulov, {\emph{J. Fluid Mech.}} {\bf{619}}, 2009.\\[0pt] [2] Zhang, {\emph{Int. J. Heat Fluid Flow}} {\bf{21}} 2000.\\[0pt] [3] Harris, Miller and Williamson, {\emph APS abstract} 2009. [Preview Abstract] |
Monday, November 22, 2010 4:40PM - 4:53PM |
LN.00006: Leading Edge Vortex Detection Using On-Body Pressure Sensing Jeff Dusek, Jason Dahl, Michael Triantafyllou Ongoing experiments within the Center for Environmental Sensing and Modeling (CENSAM) have shown that the low pressure region characteristic of a vortex allows for their detection and tracking using pressure sensors alone. While early experiments were conducted with wall mounted pressure sensors and externally generated vortices, a new series of experiments has succeeded in detecting separated flow generated by the sensing body. A combined pressure sensing and particle image velocimetry (PIV) approach was used to detect the leading edge vortex shed from a hydrofoil accelerated at a fixed angle of attack. A NACA 0018 foil was instrumented with four pressure sensors at discrete locations along the foil in the chord-wise direction. When accelerated from rest, the traces from each of the four pressure sensors displayed a distinctive, transient drop, consistent with results observed in previous experiments. From the pressure sensor results, it was theorized that a leading edge vortex was being created, and subsequently shed and convected along the foil chord. Two-dimensional PIV techniques were used to image the flow near the foil surface, allowing the anticipated vortex formation and shedding to be verified. [Preview Abstract] |
Monday, November 22, 2010 4:53PM - 5:06PM |
LN.00007: Experimental evidence of 3D flows around corners at low Reynolds number Josue Sznitman, David Clifton, Dexter Scobee, Howard Stone, Alexander Smits Recently, Rusconi et al. (\textit{J R Soc Interface}, 2010) have observed the formation of suspended filamentous biofilms in the middle plane of curved microchannels under laminar flow conditions. Motivated by such findings, we investigate experimentally the structure of 3D bounded viscous flows in the proximity of corners, at low Reynolds numbers ($Re < 0.01$). Beyond the location of the corner, shear driven flows are geometrically confined within rectangular-like channels of varying aspect ratios. Characteristic flows are experimentally generated using dynamic similarity in a tow-tank filled with a highly viscous silicon oil. Quantitative flow measurements are obtained using PIV; regions of interest are interrogated by scanning a series of 2D planes in the vicinity of the corner. Past the corner, flows are predominantly uniaxial along the ductal length. However, near the corner, there exists a weak yet well-defined vortical flow structure; this secondary motion is experimentally observed via the deflection of PIV velocity vectors in the spanwise flow direction. Such flow structure yields a net steady-state flow focusing effect in the middle of the duct plane; the structure and strength of such focusing effect are discussed in the present talk. [Preview Abstract] |
Monday, November 22, 2010 5:06PM - 5:19PM |
LN.00008: Dynamic Wind Loads and Vortex Structures in the Wake of a Wind Turbine Hui Hu, Zifeng Yang, Partha Sarkar We report an experimental study to characterize the dynamic wind loads and evolution of wake vortex flow structures downstream of a horizontal axis wind turbine (HAWT). ~The experiments were conducted in a wind tunnel with a wind turbine model placed in a boundary layer flow developed over rough and smooth surfaces in order to study the effects of roughness and the resulting velocity and turbulence fields on the wake characteristics and fatigue loads acting on the wind turbine. ~~In addition to measuring dynamic wind loads (both aerodynamic forces and moments) acting on the wind turbine model using a six-component load cell, a high-resolution Particle Image Velocimetry (PIV) system was used to make phase-locked flow field measurements to quantify the time-evolution of the wake vortex and turbulence flow structures shedding from wind turbine blades. ~The detailed flow field measurements were correlated with the wind load measurements to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on wind turbines. [Preview Abstract] |
Monday, November 22, 2010 5:19PM - 5:32PM |
LN.00009: Flow over Superhydrophobic Hydrofoils Robert Daniello, Jonathan Sullivan, Jonathan Rothstein For several years, superhydrophobic surfaces which are chemically hydrophobic with micron or nanometer scale surface features have been considered for their ability to reduce drag in microfluidic devices. More recently it has been demonstrated that superhydrophobic surfaces reduce friction coefficient in turbulent flows as well. In this talk, we will consider the effect of superhydrophobic surfaces on drag, lift and fluid-structure interactions of hydrofoils and the effect of superhydrophobicity on separation point and vortex structure at high angles of attack. Drag reductions and significant changes to the fluid structure interactions are observed with the presence of superhydrophobic coatings. Hydrofoils are coated with patterned microridge geometries from 5$\mu $m to 30$\mu $m. Selectively coated symmetrical hydrofoils are also examined to characterize the effect of superhydrophobicity on lift behavior. Particle image velocimetry, streak images and direct force measurements will be presented. Experiments were conducted over the range of Reynolds numbers 100$<$\textit{Re}$<$10,000 demonstrating the effect of the coating on laminar, transitioning and turbulent flow regimes. [Preview Abstract] |
Monday, November 22, 2010 5:32PM - 5:45PM |
LN.00010: Influence of Slip on the Flow Past Superhydrophobic Circular Cylinders Jonathan Rothstein, Robert Daniello, Nangelie Ferrer, Pranesh Muralidhar Superhydrophobic surfaces have been shown to produce significant drag reduction for both laminar and turbulent flows of water through large and small-scale channels. In this presentation a series of experiments will be presented which investigate the effect of superhydrophobic-induced slip on the flow past a circular cylinder. In these experiments, circular cylinders are coated with a series of superhydrophobic surfaces fabricated from PDMS with well-defined micron-sized patterns of surface roughness. We will show that the presence of the superhydrophobic surface has a significant effect on both the drag coefficient of the cylinder and vortex shedding dynamics in its wake. When compared to a smooth, no-slip cylinder, we will show that the drag coefficient of the superhydrophobic cylinder decreases, the separation moves towards the rear of the cylinder and the vortex shedding frequency increases. In addition, we will show that the vortex shedding dynamics are very sensitive to changes of feature spacing, size and orientation. [Preview Abstract] |
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