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 HN: Vortex Flows: Vortex-Induced Vibrations and Unsteady Flows |
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Chair: Doug Bohl, Clarkson University Room: Long Beach Convention Center 202C |
Monday, November 22, 2010 10:30AM - 10:43AM |
HN.00001: Long flexible cylinders subject to vortex-induced vibrations in shear flow exhibit preferentially counter-clockwise figure-eight trajectories at lock-in Remi Bourguet, Yahya Modarres-Sadeghi, Michael Triantafyllou Long flexible cylindrical structures placed in cross-flow current exhibit large amplitude, self-excited, vortex-induced vibrations. The flow excites structure when the vortex shedding frequency is synchronized with the frequency of cross-flow vibration, a condition referred to as lock-in. When the in-line and cross-flow vibrations occur with a frequency ratio of two, as is generally observed in this context, the cylinder exhibits figure-eight trajectories. We investigate the existence of a link between the occurrence of the lock-in condition and the orientation of these figure-eight trajectories, in shear flow, by means of a joint analysis of the structure response and wake pattern, based on detailed numerical simulation and experimental results. We show that trajectories in which the cylinder moves upstream at the extremes of the cross-flow motion (counter-clockwise trajectories) are preferred by the system to establish the lock-in condition. Also, we emphasize the impact of this orientation on fluid-structure energy exchanges. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HN.00002: Influence of flexibility and corner shape on three-dimensional vortex structures in translating plates Daegyoum Kim, Morteza Gharib In order to understand the complicated vortex formation process of the flexible propulsors in nature, three-dimensional vortex structures generated by impulsively translating low aspect-ratio plates with a 90\r{ } angle of attack were studied experimentally. Rigid and flexible thin plastic plates were used to find the effect of plate flexibility on the development of vortex structure. The tip vortex motion is one of the obvious differences between the rigid and flexible plates. While the tip vortex moves upward in the flat-rigid plate case, it stays near the tip in the flexible plate case, which results in significantly different three-dimensional vortex morphology near the tip region. In addition, the dynamics of the vortex near a corner region was compared among three different corner angles for impulsively translating plates. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HN.00003: A study of vortex-induced vibration using hybrid cyber-physical supports A. Mackowski, C.H.K. Williamson In this work, we study the vortex dynamics of fluid-structure interaction using a novel technique, which we call Cyber- Physical Fluid Dynamics. Typically, when studying flow-induced vibration, one needs to select essential parameters for the body, such as mass, spring stiffness, and damping. Normally, these parameters are set physically by selecting mechanical elements. However, in our approach, which utilizes a water channel, a computer-controlled XY positioning system, and a force-feedback control system, we are able to impose mass- spring-damping forces in real time and with significant precision. A similar concept was pioneered by a group at MIT (Miller 1996; Hover, Techet, Triantfyllou 1997), in studies of vortex-induced vibration of cables. The present research expands on this technique, both with a new control system and the first ever implementation of Cyber-Physical Fluid Dynamics in a continuously flowing facility. Although the use of a cyber- physical system has clear advantages over using a fixed, physical experiment, there are serious challenges to overcome in the design of the governing control system. We will explore aspects of the control system and strategies for reducing the limitations of this approach. We shall present experimental results from selected problems of fluid-structure interaction using this new technique. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HN.00004: Physics behind vortex-induced vibration reduction using an oblique trailing edge hydrofoil Amirreza Zobeiri, Francois Avellan, Mohamed Farhat The issue of vortex-induced vibration based on the phenomenon of vortex shedding behind a bluff body is a major problem in hydraulic machinery. Resulting fluctuating forces may lead to excessive vibrations and premature cracks. It is well known that a hydrofoil with an oblique trailing edge reduces vibration as compared to that with a blunt trailing edge. However physics behind this is not fully understood. The purpose of the present work is to conduct an experimental investigation of vortex shedding dynamics in the wake of an oblique trailing edge hydrofoil to understand the phenomena and the reasons for vibration reduction. This could help optimize the trailing edge shape and diminish the induced vibration. A velocity survey in the hydrofoil wake is performed via Laser-Doppler and Particle Image velocimetry using the Proper-Orthogonal-Decomposition technique for post-processing. In addition, flow induced vibration measurements and high speed visualization are performed. The high-speed videos clearly demonstrate alternate shedding of the vortices transforming into nearly simultaneous shedding at the hydrofoil trailing edge. As a result, partial cancellation is observed for upper and lower vortices, accompanied by the thickening of the lower vortex core that is believed to be the primary reason of the vibration reduction. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HN.00005: Self-Excited Energy Harvesting in Uniform Fluid Flows Using Piezoelectric Generators Dogus Akaydin, Niell Elvin, Yiannis Andreopoulos A novel energy harvesting configuration consisting of a circular cylinder attached to a piezoelectric beam is investigated experimentally in this study. The cylinder is attached to the free end of a cantilever beam and undergoes bending deformation due to vortex shedding under uniform fluid flow. The periodic change of strain along the piezoelectric beam generates an alternating voltage that can be used to power an electrical circuit such as a wireless sensor. Two major governing parameters were studied: First, the ratio of the length of the cylinder to its diameter and second, the ratio of the length of the beam to the diameter of the cylinder. Both parameters alter the forcing frequency and magnitude of the flow, natural frequency of the structure and ultimately determine the magnitude of the resultant vibrations. The configuration is different from those previously studies since it is a combination of an oscillating cylinder with a flexible splitter plate and the vibrations are induced within a steady, uniform flow. Three dimensionality of the flow complicates its structure and nonlinear oscillations and lock-in phenomena were observed in experiments with beams of low stiffness. It was also observed that aerodynamic interference with several components of the harvester can significantly alter the harvested power. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HN.00006: The dynamics of vortex shedding of flow past a vertical flat plate; LES studies Carlos Velez, Marcel Ilie Although, apparently flow-induced vibrations represent a canonical problem, a complete understanding of the fluid-structure mechanism of interaction has not yet been achieved. One of the issues associated with flow induced vibrations stems from the vortex shedding phenomenon. It is well known that vortex streets are formed in the wake of bluff bodies over a wide range of Reynolds numbers. The periodic shedding of vortices may result in significant fluctuating loading on the body. When the shedding frequency is close to one of the characteristic frequencies of the body, the resonant oscillations of the body can be excited, causing damaging instabilities. Various studies regarding the vortex shedding resonance or ``lock-on'' phenomenon have been conducted. However, the effect of Reynolds number on the ``lock-on'' regime is yet to be fully understood. The ``lock-on'' phenomenon is of critical importance for the analysis of flow-induced vibration, when the aeroelastic response of the structure is considered. In the present research the influence of sweeping angle on the plate tip vortex formation is studied numerically using large eddy simulation (LES). The results show an increase in magnitude and size of vertical structures developed in the wake of the plate. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HN.00007: Oscillatory vortex formation behind a movable plat Marija Vukicevic, Gianni Pedrizzetti INTRODUCTION: A wide spectra of application, from industrial to environmental and biological, involve fluid-structure interaction (FSI) at a fundamental level. We investigate a 2D FSI problem for a rigid structure hinged on a wall, freely rotating by the action of an oscillatory fluid flow. METHODS: The Navier-Stokes equations are solved simultaneously with the body dynamics. An accurate numerical solution is developed on the conformal map of the time-varying physical domain. RESULTS: The FSI is primarily influenced by the vortex formation process and by the interaction between vortices generated during the sequential flow oscillations. The emerging bodies can be arranged into a three main groups. The first, made of heavy bodies, terminates the motion during the first few oscillations with the impact of the body on the wall. On the other extreme, the third group made of relatively light bodies presents a flow-driven motion that oscillates periodically in time. In a wide intermediate range, the body oscillates in time presenting non periodic features. CONCLUSIONS: The process of oscillatory vortex formation in presence of fluid-structure interaction shows the emergence of various phenomena that were analyzed in details. In this specific application the results demonstrate that the FSI range from linear to chaotic interaction and finite-time collapse. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HN.00008: Computational Analysis of Vortex Formation Over a Plunge Oscillating Flat Plate with Various Slip Conditions John Palmore, Muhammad Sharif, Amy Lang A thorough understanding of small scale aerodynamics is important for the design of micro air vehicles. Since they fly in the same Re regime as that of insects, these animals can provide biologically inspired designs. This study looks at how an alteration to the surface slip condition affects the aerodynamic flow over a wing at low Re. Butterflies have small scales (on the order of 100 microns in length) that line the surface of their wings, and it is hypothesized that these scales can affect the slip condition over their wings altering vortex formation and possibly leading to improved flight characteristics. As an initial test to this hypothesis, the flow over an infinitely thin, two-dimensional flat plate was studied using the CFD software FLUENT. The no-slip condition was modified by directly altering the shear stress distribution over the plate. In addition, the action of flapping was simulated by varying the angle of attack as a function of time between -60 and 60 degrees. Multiple shear stress distributions, varying from shear free to no-slip, and multiple flapping frequencies were tested to discern the effects on vortex formation; lift and drag were also analyzed. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HN.00009: Fluid flow over an elliptical cylinder undergoing a rotationally-oscillating motion Esam Alawadhi The near-wake behind an elliptical cylinder undergoing rotationally-oscillating motion will be simulated using the finite element method at a low Reynolds number, Re=200. The simulations will be carried out by varying the angle of attack between $\pm $10$^{\circ}$, $\pm $20$^{\circ}$, and $\pm $30$^{\circ}$, while the considered range of dimensionless oscillation frequency is St$_{o}$/2$<$St$<$4$\times $St$_{o}$, where St$_{o}$ is the natural Strouhal frequency of a stationary elliptical cylinder. The solver is coupled with a mesh movement scheme using the Arbitrary Lagrangian-Eulerian kinematics to simulate the flow-structural interaction. Fluid mechanics results will be presented in terms of instantaneous and time-average lift and drag coefficients, flow streamline, and vortices contours. [Preview Abstract] |
Monday, November 22, 2010 12:27PM - 12:40PM |
HN.00010: ABSTRACT WITHDRAWN |
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