Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A18: Vortex Dynamics: Energy Harvesting and Atmospheric Flows |
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Chair: Dennice Gayme, Johns Hopkins University Room: 2004 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A18.00001: Optimization of energy harvesting efficiency of an oscillating hydrofoil: Sinusoidal and Non-sinusoidal trajectories Michael Miller, Ben Strom, Kenneth Breuer, Shreyas Mandre We determine the feasibility of applying optimization algorithms to an oscillating hydrofoil's motion trajectory to determine maximum efficiency of energy capture. Optimization is performed using the Nelder-Meade downhill simplex method. The objective function is the energy captured measured experimentally in run-time with an oscillating hydrofoil capable of measuring mechanical energy capture in a laboratory flume. For sinusoidal trajectories, optimization is performed over pitch and heave amplitudes as well as frequency; this system is shown to be capable of optimization in run-time. The optimum efficiency of 30{\%} is found for a pitch amplitude of 70$^{\circ}$, a heave amplitude of 0.8*chord and a dimensionless frequency of 0.13. To treat non-sinusoidal trajectories, we expand them in a truncated Fourier series and consider the coefficients of this series as variables for optimization. The sinusoidal case is simply an extreme case of such a truncated Fourier series, with only one term in the series retained. We present a systematic method for optimization over general non-sinusoidal trajectories by including more and more terms in the Fourier series. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A18.00002: Vortex shedding from vertical axis wind turbine blades under linear motion Reeve Dunne, Beverley McKeon A NACA 0018 airfoil was pitched and surged sinusoidally in in a mean free stream flow at Re\(_\textrm{c}=100,000\) to simulate the flow over vertical axis wind turbine (VAWT) blades. Angle of attack variations between \(\alpha = \pm 30^\circ\) and velocity variation of \(\frac{\textrm{U}_{\textrm{max}}-\textrm{U}_{\textrm{min}}}{\textrm{U}_{\textrm{mean}}}=.80\) at a reduced frequency \(\textrm{k}=\frac{\Omega \textrm{c}}{2 \textrm{U}_\infty}=.12\) result in strong dynamic stall on the blade. Multiple flow regimes occur during the airfoil motion resulting in vortex shedding over a large range of frequencies. A model of the phase averaged (based on airfoil angle of attack and velocity) flow developed using dynamic mode decomposition highlights the evolution of the leading edge or dynamic stall vortex at the airfoil frequency. Instantaneous results show vortex shedding at frequencies up to 100 times higher than the frequency of the pitch/surge motion and smeared out by the phase averaging process. The implications for forcing on the blade (and associated wind turbine) are described. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A18.00003: Vortex Identification in the Wake of a Wind Turbine Array Aleksandr Aseyev, Raul Cal A 4 x 3 wind turbine array boundary layer is analyzed through Particle Image Velocimetry data gathered directly forward and aft of the first and last row turbines at the centerline in a wind tunnel. Vortex identification techniques are able to capture vortical structures. Q-criterion, {\$}Delta{\$}-criterion, and {\$}lambda-2{\$} criterion are evaluated and compared for this flow. Q-criterion and {\$}lambda-2{\$} criterion provided a clear indication of regions where vortical activity exists while the {\$}Delta{\$}-criterion is not able to capture these regions. Galilean decomposition, Reynolds decomposition, vorticity, and swirling strength were used to further understand the location and behavior of the vortices. The various criterion displayed the high magnitude vortices, resulting from the blade tips and located immediately in areas of high shear. Using Galilean and Reynolds decomposition, swirling motions are shown hugging vortex regions in agreement with the identification criterion. The percentages used in the Galilean decomposition were 20 and 50 percent of a convective velocity of 7 m/s. As the vortices convect downstream, these vortices weaken in magnitude to approximately 25 percent of those present in the near wake. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A18.00004: Constructive interference in arrays of energy harvesters in fluid flows Vahid Azadeh Ranjbar, Oleg Goushcha, Niell Elvin, Yiannis Andreopoulos In the present work we demonstrate some unique opportunities which exist to increase the power harvested with fluidic piezoelectric generators by almost two orders of magnitude higher than existing methods by exploiting dynamic non-linearities and deploying multi-element arrays in carefully selected positions in a fluid flow field. These ac-coupled generators convert fluid kinetic energy, which otherwise would be wasted, into electrical energy. The available power in a flowing fluid is proportional to the cube of its velocity and if it is properly harvested can be used for continuously powering very small electronic devices or can be rectified and stored for intermittent use. Additional experimental work has shown that non-linear arrays of such energy harvesters can produce high output voltages in a very broadband range of frequencies. In our work, we investigate the effect of geometric parameters such as spatial arrangement and the mutual interference between the elements of a non-linear array on their overall performance and efficiency characteristics. Analytical tools based on the non-linear van der Pol oscillator have been also developed and verified with experimental data. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A18.00005: Energy Harvesting of a Flapping Airfoil in a Vortical Wake Z. Charlie Zheng, Zhenglun Wei We study the response of a two-dimensional flapping airfoil in the wake downstream of an oscillating D-shape cylinder. The airfoil has either heaving or pitching motions. The leading edge vortex (LEV) and trailing edge vortex (TEV) of the airfoil play important roles in energy harvesting. Two major interaction modes between the airfoil and incoming vortices, the suppressing mode and the reinforcing mode, are identified. However, distinctions exist between the heaving and pitching motion in terms of their contributions to the interaction modes and the efficiency of the energy extraction. A potential theory and the related fluid dynamics analysis are developed to analytically demonstrate that the topology of the incoming vortices corresponding to the airfoil is the primary factor that determines the interaction modes. Finally, the trade-off between the input and the output is discussed. It is found that appropriate operational parameters for the heaving motion are preferable in order to preserve acceptable input power for energy harvesters, while appropriate parameters for the pitching motion are essential to achieve decent output power. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A18.00006: Buoyancy-Induced Columnar Vortices Mark Simpson, Ari Glezer Naturally-occurring, buoyancy-driven columnar vortices (``dust devils'') that are driven by an instability of the thermally stratified, ground-heated air layer and are sustained by entrainment of the ground-heated air, occur spontaneously in the natural environment with core diameters of 1-50 m and heights up to one km. These vortices convert low-grade waste heat in the air layer overlying the warm surface into a flow with significant kinetic energy that may be exploited for power generation by coupling the vortex to a vertical-axis turbine. The considerable kinetic energy of the vortex column cannot be explained by buoyancy alone, and the fundamental mechanisms associated with the formation, evolution, and dynamics of an anchored, buoyancy-driven columnar vortex are investigated in a laboratory facility using a heated ground plane and an azimuthal array of flow vanes. The present investigation focuses on the vortex formation, structure, and the dependence of its scaling and strength on the thermal resources and the characteristic scales of the anchoring flow vanes using stereo-PIV with specific emphasis on the production, advection, and tilting of vorticity within the entrained boundary layer. Approaches for the manipulation of these mechanisms for increasing the available kinetic energy and therefore the generated power are also investigated. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A18.00007: Numerical Investigation of Buoyancy-Induced Columnar Vortices Nicholas Malaya, Roy Stogner, Robert Moser Buoyancy driven columnar vortices arise naturally in the atmosphere. A new energy harvesting approach makes use of this phenomenon by creating and anchoring the vortices artificially and extracting energy from them. In this talk, we explore the characteristics of these ``solar vortices'' through numerical simulation. Computational models of the turning vane system used to generate the solar vortex and the turbine used to extract energy have been developed. The formulation of these models and their validation against available experimental measurements will be discussed, as will the details of the columnar vortex structure and its interaction with the turbine. In addition, the computational models are being used to optimize the turning vane configuration and the turbine characteristics to maximize the power extraction, and to characterize the effects of environmental conditions such as cross winds and topography. Preliminary results from these studies will also be presented. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A18.00008: On a possible mechanism for the generation of cyclonic vortices regime in a precessing cylindrical container Waleed Mouhali, Thierry Lehner We report experimental observations obtained by particle image velocimetry of the behavior of a flow driven by rotation and precession in a cylindrical container. This study is motivated by dynamo effect and geophysics applications. Precessional motion forces inertial waves whose amplitude are predicted by a linear inviscid theory. But, various flow regimes are identified experimentally according to the value of the control parameter $\varepsilon $ the precession rate : the ratio of the precession frequency $\Omega_{\mathrm{P}}$ to the rotation frequency $\Omega_{\mathrm{R}}$ ($\varepsilon =\frac{\Omega_{P} }{\Omega_{R} })$. When $\varepsilon $ is increased from small values, after a linear regime, we have observed a differential rotation followed by the apparition of four permanent cyclonic vortices as a consequence of instability (eruption of jets from the lateral edges of the cylinder). We propose a mechanism for this instability based on a precedent study : we have proved that the nonlinear mode coupling of two inertial waves of azimuthal wave number $m =$ 0 and $m =$ 1 (mode forced by the precession) in the inviscid regime creates differential rotation also observed experimentally at small $\varepsilon $. The profile of the azimuthal mean velocity and the corresponding axial mean vorticity both show an inflexion point in their radial profile. We show that when the control parameter $\varepsilon $ is increased from low values, the forced mode $m =$ 1 can become instable in this induced differential rotation. It could be responsible for the observed instability and for the cyclones formation within the volume after a subsequent Kelvin-Helmholtz type instability. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A18.00009: Piezoelectric Energy Harvesters in Isotropic Turbulence Amir Danesh-Yazdi, Oleg Goushcha, Niell Elvin, Yiannis Andreopoulos In the present work, we will report experimental and analytical results related to the extraction of fluidic energy in decaying homogeneous, isotropic turbulence using cantilever beams with attached piezoelectric patches of various materials. Turbulence carries mechanical energy distributed over a range of temporal and spatial scales and the resulting interaction of these scales with the immersed piezoelectric beams creates a strain field in the beam which generates electric charge. Experiments are carried out in large scale wind tunnels in which passive, semi-passive and active turbulence-generating grids are used to excite the piezoelectric cantilever beams at various distances from the grids. We observe that the average power generated in the piezoelectric layer obeys an exponential decay law with respect to the dimensionless distance parameter, as predicted from our theoretical hypothesis. The pertinent parameters that influence the power output of the beams are identified as (1) the dimensionless distance of the beam from the grid with respect to the grid size and (2) the dimensionless length of the beam with respect to the turbulence integral length scale. Furthermore, the efficiencies associated with each step of the energy conversion process in the beams are discussed. [Preview Abstract] |
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