Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session L2: Energy: Wind Power |
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Chair: Mahesh Bandi, OIST Graduate University Room: A106 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L2.00001: The Spectrum of Wind Power Fluctuations Mahesh Bandi Wind is a variable energy source whose fluctuations threaten electrical grid stability and complicate dynamical load balancing. The power generated by a wind turbine fluctuates due to the variable wind speed that blows past the turbine. Indeed, the spectrum of wind power fluctuations is widely believed to reflect the Kolmogorov spectrum; both vary with frequency $f$ as $f^{-5/3}$. This variability decreases when aggregate power fluctuations from geographically distributed wind farms are averaged at the grid via a mechanism known as geographic smoothing. Neither the $f^{-5/3}$ wind power fluctuation spectrum nor the mechanism of geographic smoothing are understood. In this work, we explain the wind power fluctuation spectrum from the turbine through grid scales. The $f^{-5/3}$ wind power fluctuation spectrum results from the largest length scales of atmospheric turbulence of order 200 km influencing the small scales where individual turbines operate. This long-range influence spatially couples geographically distributed wind farms and synchronizes farm outputs over a range of frequencies and decreases with increasing inter-farm distance. Consequently, aggregate grid-scale power fluctuations remain correlated, and are smoothed until they reach a limiting $f^{-7/3}$ spectrum. [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L2.00002: Coordinated Control of Cross-Flow Turbines Benjamin Strom, Steven Brunton, Brian Polagye Cross-flow turbines, also known as vertical-axis turbines, have several advantages over axial-flow turbines for a number of applications including urban wind power, high-density arrays, and marine or fluvial currents. By controlling the angular velocity applied to the turbine as a function of angular blade position, we have demonstrated a 79 percent increase in cross-flow turbine efficiency over constant-velocity control. This strategy uses the downhill simplex method to optimize control parameter profiles during operation of a model turbine in a recirculating water flume. This optimization method is extended to a set of two turbines, where the blade motions and position of the downstream turbine are optimized to beneficially interact with the coherent structures in the wake of the upstream turbine. This control scheme has the potential to enable high-density arrays of cross-flow turbines to operate at cost-effective efficiency. Turbine wake and force measurements are analyzed for insight into the effect of a coordinated control strategy. [Preview Abstract] |
Monday, November 21, 2016 4:56PM - 5:09PM |
L2.00003: Multiscale simulations of a horizontal axis tidal turbine in a tidal site Arturo Fernandez, Andres Tejada-Martinez, Mario Juha We present multiscale simulations of a three-bladed horizontal axis tidal turbine (HATT) in a tidal site. The power and wake generated by the HATT are computed using an Overset Large Eddy Simulation (OLES) technique, where the velocity field is split into a background and a perturbation field. The background flow is a tidal boundary layer with a depth of 10 m, which exhibits a logarithmic distribution for the averaged streamwise velocity. This flow is computed via Large Eddy Simulation for an open channel flow with a time varying pressure gradient. The interaction between this background flow and the HATT is captured using a line actuator model, which generates the perturbation flow representing the turbulent wake. These simulations assume the turbine to have a diameter of 2.5 m and the hub to be located 7.5 m from the bottom floor. The results show the power coefficient to exhibit a cyclical behavior and the power spectral density to have peaks at multiples of one third of the rotational frequency. The turbulent wake exhibits the classical behavior with the near wake dominated by helicoidal vortical structures, and the far wake characterized by diffusion phenomena. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L2.00004: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 5:22PM - 5:35PM |
L2.00005: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 5:35PM - 5:48PM |
L2.00006: Leading Edge Vortex Formation of an Oscillating Foil in Energy Harvest Modes James Liburdy, Firas Siala, Alexander Totpal The vortex formation number, represents a non-dimensional time scale to achieve its maximum circulation, defined as $\Gamma / c U_{SL}, $where \quad $\Gamma $ is circulation, $c $is foil chord length, and $U_{SL\thinspace }$is the feeding shear-layer velocity. It has been observed that the formation number may be universal, with an optimum value of 4. In this study, phase-locked particle image velocimetry measurements are used to investigate the effect of foil flexibility on the vortex formation number of the leading edge vortex (LEV) of an oscillatory foil in the energy harvesting mode. The goal is to provide insights as to how surface flexibility can enhance energy harvesting efficiencies. Experiments are conducted using three airfoil configurations: fully rigid, flexible trailing edge and flexibility leading edge. The objective is to identify the ability of surface flexibility to enhance the LEV circulation and its associated shear-layer. Results are presented in terms of the vortex formation number for a range of operating reduced frequencies, $k =$ 0.05 -- 0.2, which reportedly yield the most efficient energy harvesting conditions. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L2.00007: The effects of passive foil flexibility on the energy extraction performance of an oscillating foil operating at low reduced frequencies Alexander D. Totpal, Firas F. Siala, James A. Liburdy With a goal to improve energy extraction efficiency from an oscillating foil, direct aerodynamic force measurements are used to study the effect of surface flexibility of an oscillating foil operating in the energy harvesting regime. The experiments are conducted in a closed-loop wind tunnel at a low reduced frequencies range of 0.04 -0.06. The pitching amplitude was varied from 45 to 90 degrees and the phase shift between pitching and heaving motions was varied from 30 to 120 degrees. Three different airfoil configurations were tested: fully rigid, flexible leading edge and flexible trailing edge. In addition, phase-locked particle image velocimetry (PIV) measurements were taken at the higher efficiency cases, and are used to help interpret trends seen in the force measurement data. The timing and position of the leading edge vortex along the foil, which has been shown to be crucial to energy extraction, is investigated in order to help explain why certain operating conditions yield larger efficiencies. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L2.00008: Optimal control of wind turbines in a turbulent boundary layer Ali Emre Yilmaz, Johan Meyers In recent years, optimal control theory was combined with large-eddy simulations to study the optimal control of wind farms and their interaction with the atmospheric boundary layer [1,2]. The individual turbine's induction factors were dynamically controlled in time with the aim of increasing overall power extraction. In these studies, wind turbines were represented using an actuator disk method. In the current work, we focus on optimal control on a much finer mesh (and a smaller computational domain), representing turbines with an actuator line method. Similar to Refs.~[1,2], optimization is performed using a gradient-based method, and gradients are obtained employing an adjoint formulation. Different cases are investigated, that include a single and a double turbine case both with uniform inflow, and with turbulent-boundary-layer inflow. [1] Goit Jay, Meyers Johan (2015). Optimal control of energy extraction in wind-farm boundary layers. Journal of Fluid Mechanics 768, 5-50. [2] Goit Jay, Munters Wim, Meyers Johan (2016). Optimal coordinated control of power extraction in LES of a wind farm with entrance effects. Energies 9 (1), art.nr. 29 [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L2.00009: Optimal control of wind-farm boundary layers: effect of turbine response time Wim Munters, Johan Meyers Complex turbine wake interactions play an important role in overall energy extraction in large wind farms. Current control strategies optimize individual turbine power, and lead to significant energy losses in wind farms compared to lone-standing turbines. In recent work, an optimal control framework for dynamic induction control of wind farms and their interaction with the atmospheric boundary layer (ABL) was introduced, with the aim of mitigating such losses. The framework applies a receding horizon methodology, in which the ABL state is modeled through large-eddy simulations. Previously, the framework was applied to both fully-developed (Goit and Meyers 2015, J Fluid Mech, 768, 5--50) and spatially developing wind farms (Goit et al. 2016, Energies, 9, 29), for which respective energy gains of 16\% and 7\% were obtained, albeit at the cost of additional turbine loading variability. Here, we quantify the trade-off between increased power extraction and smoothed turbine dynamics by varying the turbine response time in the control framework. We consider simulation cases restricted to underinduction compared to Betz-optimal induction, as well as cases that also allow overinduction. In addition, efforts on replicating optimized power gains with practical controllers are presented. [Preview Abstract] |
Monday, November 21, 2016 6:27PM - 6:40PM |
L2.00010: Resonance wave pumping: wave mass transport pumping Remi Carmigniani, Damien Violeau, Morteza Gharib It has been previously reported that pinching at intrinsic resonance frequencies a valveless pump (or Liebau pump) results in a strong pulsating flow. A free-surface version of the Liebau pump is presented. The experiment consists of a closed tank with a submerged plate separating the water into a free-surface and a recirculation section connected through two openings at each end of the tank. A paddle is placed at an off-centre position at the free-surface and controlled in a heaving motion with different frequencies and amplitudes. Near certain frequencies identified as resonance frequencies through a linear potential theory analysis, the system behaves like a pump. Particle Image Velocimetry (PIV) is performed in the near free surface region and compared with simulations using Volume of Fluid (VOF) method. The mean eulerian mass flux field ($\rho \boldsymbol{u}$) is extracted. It is observed that the flow is located in the vicinity of the surface layer suggesting Stokes Drift (or Wave Mass Transport) is the source of the pumping. A model is developped to extend the linear potential theory to the second order to take into account these observations. [Preview Abstract] |
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