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 E12: Vortex Dynamics and Vortex Flows III |
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Chair: Stavros Tavoularis, University of Ottawa Room: 336 |
Sunday, November 24, 2013 4:45PM - 4:58PM |
E12.00001: Jet Interactions in a Feedback-Free Fluidic Oscillator in the Transition Region Mehmet Tomac, James Gregory The details of the jet interactions and oscillation mechanism of a feedback-free type fluidic oscillator are studied in this work. Flow rate-frequency measurements indicate the existence of three distinct operating regimes: low flow rate, transition, and high flow rate regions. This study presents results from the transition regime, extracted by using refractive index-matched particle image velocimetry (PIV). A newly-developed sensor configuration for frequency measurements in the refractive index-matched fluid and a phase-averaging method that minimizes jitter will be discussed. Experimental results indicate that the interactions of the two jets create three main vortices in the mixing chamber. One vortex vanishes and forms depending on the oscillation phase and plays a key role in the oscillation mechanism. The other two vortices sustain their existence throughout the oscillation cycle; however, both continuously change their size and strength. The resulting complex flow field with self-sustained oscillations is a result of the combination of many interesting phenomena such as jet interactions and bifurcations, viscous effects, vortex-shear layer interactions, vortex-wall interactions, instabilities, and saddle point creations. [Preview Abstract] |
Sunday, November 24, 2013 4:58PM - 5:11PM |
E12.00002: Characterization of self-excited fluidic energy harvesters in uniform flows Vahid Azadeh Ranjbar, Coralie Cler, Niell Elvin, Yiannis Andreopoulos Energy harvesters consisting of a low aspect-ratio hollow circular cylinder attached to the free end of a cantilevered beam which is partially covered by piezoelectric patches near its clamped end to produce electrical power output have been investigated experimentally and analytically. The unsteady nature of vortex shedding is described by the van der Pol equation, a non-conservative oscillator with non-linear damping, which models the near wake dynamics that is coupled with the harvester's equation of motion. This model helps to describe and predict the vortex induced vibration phenomena such as lock-in range, maximum amplitude of oscillations and extension of structural oscillations far away lock-in range with a better physical insight. Both free vibration and wind tunnel tests were carried out to characterize the harvester. Based on the wind tunnel tests data, there is a remarkable difference in magnitude and frequency of the lift force between stationary and oscillating cylinders subjected to stationary uniform flow. Moreover, maximum electrical power output occurs at a forcing frequency somewhat higher than the structural resonance frequency. These experimental results are in good agreement with the results of the mathematical model. [Preview Abstract] |
Sunday, November 24, 2013 5:11PM - 5:24PM |
E12.00003: Fluid-membrane dynamics of energy harvesting eel immersed in a square cylinder wake Ying Zheng Liu, Yue Long Yu A comprehensive study was performed of fluid-membrane dynamics of energy harvesting eel placed behind a square cylinder. A low aspect-ratio of PVDF membrane was employed for the experiments in a low-speed wind tunnel; simultaneous measurements of the closed loop voltage and the flapping motion were made by using a digital oscilloscope and a high-speed camera, respectively. The experimental Reynolds number based on the width of the cylinder was Re$=$ 1,000-10,000. Influence of electric resistances on the power output was determined by varying the electric resistances in the closed loop. A state-of-the-art algorithm of image analysis, which was proposed for accurately identifying arbitrarily moving and deforming boundary, was performed on the consecutively recorded membrane, resulting in the digitized data of the flapping membrane. The pattern of the flapping membrane under different Reynolds numbers and electric resistances was compared. Time history of the membrane tip deflection, power spectral determined at different locations along the length of the membrane, spectral feature of the voltage were demonstrated for correlation analysis. [Preview Abstract] |
Sunday, November 24, 2013 5:24PM - 5:37PM |
E12.00004: Estimating the pressure forcing on a flexible piezoelectric beams exerted by a passing vortex using time-resolved PIV data Oleg Goushcha, Niell Elvin, Yiannis Andreopoulos A cantilever flexible beam instrumented with a piezoelectric patch and immersed in a flow can be used to harvest fluidic energy. Pressure distribution induced by naturally present vortices in a turbulent fluid flow can force the beam to oscillate producing electrical current. Maximizing the power output of such an electromechanical fluidic system is a challenge. In order to understand the pressure force exerted on the beam in a fluid flow where vortices of different scales are present, an experimental facility was set up to observe the interaction of individual vortices with the beam and record the time-resolved PIV data around the beam. Using the time-resolved PIV data, the pressure Poisson equation is solved by using a Green function's approach to obtain the pressure distribution over the beam. The beam is instrumented at the base with a piezoelectric patch, a strain gage and a force sensor whose output data are compared to the results from the pressure Poisson equation solution. A large negative pressure peak is observed as the vortex core travels over the beam responsible for the net lift force deflecting the beam towards the center of the vortex core. [Preview Abstract] |
Sunday, November 24, 2013 5:37PM - 5:50PM |
E12.00005: Energy Harvesting from an Oscillating Flat Plate in a Uniform Flow Benjamin Strom, Jennifer Franck, Kenneth S. Breuer A flat plate of aspect ratio 4.12 was pitched sinusoidally about the center chord in a uniform flow over a range of frequencies, amplitudes and mean angles of attack with the objective of studying the system's energy harvesting capabilities. Energy transfer from the fluid to plate, calculated from the torque and angular position, was found to be positive over a wide range of pitching parameters with an optimal mean angle of attack of approximately 15 degrees. Energy transfer per cycle was found to correlate linearly with a proposed measure of the total circulation in the leading edge vortex. The characteristic length determining the leading edge vortex strength appeared to differ for angles of attack less than and greater than the optimum. Comparisons with unsteady Large Eddy Simulations give insight into the vortex dynamics and mechanisms of energy transfer. [Preview Abstract] |
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