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 G24: Aerodynamics II |
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Chair: Fulvia Scarano, TU Delft Room: 319 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G24.00001: Dynamic instability of small-scale wind turbine blades Pariya Pourazarm, Yahya Modarres-Sadeghi, Matthew Lackner Future wind turbine blades will become larger, and therefore more flexible. For more flexible blades, the ratio of the estimated critical speed for dynamic instability to the operating speed decreases, and the blades are more susceptible to such instabilities. In the current work, the dynamic instability of a rotating wind turbine blade is studied using a numerical stability analysis and supported by experimental results. For the experimental component of the work, a series of tests were conducted in a wind tunnel. The blades were designed using relatively thin, low Reynolds number airfoils and built using rapid-prototyping methods with a flexible material. As the oncoming wind speed was increased, the beam natural frequencies varied, up to a critical wind speed at which two structural modes coalesced and resulted in a coupled-mode flutter. A theoretical model based on coupled flexural-torsional beam equations subjected to aerodynamic loadings is derived to study the flow-induced instability for the designed blade. The model also predicts the onset of instability at a critical wind speed at which one of the structural modes experiences a negative damping. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G24.00002: Studies of Mini-Turbines Stacey Chan, Masaki Endo, Michael Romanko, C.H.K. Williamson Urban environments are inaccessible to large wind turbines of the classical ``windmill'' design. By exploring small-scale vertical-axis wind turbines (VAWTs), wind energy can possibly be harvested from the constrained spaces within cities. We present a comprehensive study of blade offset pitch angle and relative blade size (ratio of blade chordlength/turbine diameter, c/D). We find that the optimal pitch angle for a symmetric blade is the angle at which the midpoint chordline is tangent to the turbine circumference. Also, a turbine with conventional blades of small c/D ratio (c/D $=$ 0.12) -- typical of large scale turbines -- do not operate well at low Reynolds numbers. On the other hand, the maximum coefficient of power for turbines with larger c/D ratio (c/D $=$ 0.36) is much higher than for the conventional small-blades. As blade size increases, the operating range of TSR (Tip Speed Ratio) also increases, making large-chord turbines more robust to the prevailing wind conditions. Surprisingly, the regime of TSR for maximum power extracted, at these low Reynolds numbers, corresponds with small or even negative power predictions, based on streamtube theory. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G24.00003: Performance Optimization and Analysis of Variable-Pitch Vertical-Axis Wind Turbines Dietmar Rempfer, Peter Kozak The blades of conventional vertical-axis wind turbines (VAWT) operate in a complex unsteady environment, characterized by periodically changing relative flow velocity and angle of attack, accentuated by passage through the wake of preceding blades. For many operating regimes, in particular for operation at low tip-speed ratio which is of interest in order to reduce mechanical loads, the blades experience dynamic stall, reducing overall efficiency and leading to significant torque fluctuations. Periodic pitch variation of the turbine blades may therefore be considered in order to avoid stall and increase efficiency. In this presentation we will discuss gains in operating characteristics and efficiency that can be obtained by such a strategy. We will describe a full optimization of turbine efficiency based on double-multiple streamtube models. In addition, we will compare these results, and discuss the physics of the associated flows using data obtained from two-dimensional Navier-Stokes simulations of such turbines. It will be shown that, while peak efficiency of a variable-pitch VAWT is only moderately higher than the one of a conventional fixed-pitch VAWT, we can achieve a much broader maximum, leading to significantly improved performance in practical use. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G24.00004: Fluid-Structure Interaction Simulations of a Parked Wind Turbine Rotor Blade under Steady and Unsteady Inflow Conditions Robert Campbell, Balaji Jayaraman, Adam Lavely, Javier Motta-Mena, Ganesh Vijayakumar Tightly coupled fluid-structure interaction (FSI) simulations are performed for an NREL 5 MW rotor blade in the parked configuration for steady and unsteady inflow conditions. The FSI solver employs a partitioned approach that couples OpenFOAM as the flow solver and an author-developed structural finite element solver. Sub-iterations are employed to ensure convergence of the flow and structural response every solution time step. The simulations are performed for the NREL 5 MW blade, with the structural response represented by a modal summation solution. A custom fluid mesh motion solver allows the fluid mesh motion to occur primarily in a region local to the blade, while maintaining the mesh quality near the blade surface. The time-accurate blade response allows the approximation of a linear structural model to be assessed for the NREL 5 MW blade. Details of the FSI solver, including the mesh motion scheme and solution times are presented. Comparisons of blade loadings for steady and unsteady inflow conditions demonstrate the importance of blade flexibility for these simulations. Supported by the US Department of Energy. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G24.00005: Proper Orthogonal Decomposition analysis of Large Eddy Simulation data of a single wind turbine wake with uniform inflow Claire VerHulst, Robert Mikkelsen, Jens Norkaer Sorensen, Charles Meneveau Large Eddy Simulations have been performed using the EllipSys3D code to model the NREL 5 MW reference wind turbine with uniform incoming flow at 10 m/s. Instantaneous snapshots of the velocity field are decomposed using a fully three-dimensional Proper Orthogonal Decomposition (POD) analysis. This method unambiguously identifies the bases of velocity fields that most efficiently represent the turbulent kinetic energy of the snapshots on average. The structure of the resulting POD modes and the evolution of their magnitude in time provide insight into the dynamics of breakdown of tip vortices and recovery of the wake velocity deficit. In this presentation, we will discuss the structure of the first few POD modes and how the observed structure relates to the dynamics of the wind turbine wake. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G24.00006: Evaluation of drag forcing models for vertical axis wind turbine farms Brian Pierce, Parviz Moin, John Dabiri Vertical axis wind turbines (VAWTs) have the potential to produce more power per unit area than horizontal axis wind turbines (HAWTs) in a wind farm setting (Kinzel et al. J. Turb. [2012]), but further understanding of the flow physics is required to design such farms. In this study we will model a large wind farm of VAWTs as an array of 100 circular cylinders which will allow a comparison with a laboratory experiment (Craig et al. DFD 2013). The geometric complexity and high Reynolds numbers necessitate phenomenological modeling of the interaction of the turbine with the fluid, which is done through point drag models similar to those found in canopy flow simulations (e.g. Dupont et al. J. Fluid Mech. [2010]). We will present a detailed study of the point drag model performance for flow over one cylinder, providing an evaluation of the model's fidelity as it relates to quantities of interest for the VAWT farm. Next we will present results for flow through the cylinder array, emphasizing validation of the model and insight into VAWT wind farm dynamics. We will also discuss the effect of wall modeling on the calculations, as the Reynolds number of the problem requires the application of wall modeling of the turbulent boundary layer above the ground to keep the cost manageable. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G24.00007: Large eddy simulations of vertical axis wind turbines to optimize farm design Seyed Hossein Hezaveh, Elie Bou-Zeid Wind energy production, and research have expanded considerably in the past decade. These efforts aim to reduce dependence on fossil fuels and the greenhouse gas emissions associated with current modes of energy production. However, with expanding wind farms, the land areas occupied by such farms become a limitation. Recently, interest in vertical axis wind turbines (VAWTs) has increased due to key advantages of this technology: compared to horizontal axis turbines, VAWTs can be built with larger scales, their performance is not sensitive to wind direction, and the ability to place their generators at the bottom of the mast can make them more stable offshore. In this study, we focus on how the Atmospheric Boundary Layer (ABL) will react to the presence of large VAWT farms. We present a state-of-art representation of VAWTs using an actuator line model in a Large Eddy Simulations code for the ABL. Validations are made against several experimental datasets, which include flow details and power coefficient curves, the wake of an individual turbine is visualized and analyzed, and the interaction of adjacent turbines is investigated in view of optimizing their interactions and the configuration of VAWT farms. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G24.00008: Numerical study of ocean wave effect on offshore wind farm Lian Shen, Di Yang, Charles Meneveau Wind power at sea has become increasingly important in renewable energy study. For energy harvesting, winds over oceans have many advantages over winds on land, for example, larger and open surface area, faster wind speed, and more wind resource close to high population regions. On the other hand, the presence of ocean waves introduces complexities to wind turbines. There is a critical need to study the dynamical interactions among marine atmospheric boundary layer, ocean wave field, and floating turbines. In this research, we study offshore wind farm by performing large-eddy simulations for winds coupled with potential-flow-theory based simulations for broadband irregular waves, with the wind turbines represented by an actuator disk model. Our results show that windseas at different development stages result in different sea-surface roughness and have an appreciable effect on wind profile and the energy extraction rate of the turbines. If swells are present, swell-to-wind momentum and energy transfer further changes the wind field to introduce oscillations in as well as modify the mean of the wind power. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G24.00009: Power Optimization of Wind Farms in Large Eddy Simulations Johan Meyers, Jay Prakash Goit As the understanding of wind-farm aerodynamics broadens, our interest is shifting towards exploring the possibilities of optimising and improving the power-extraction of a wind farm. In the present work we couple flow simulations performed using Large Eddy Simulations (LES) with gradient based optimization to control individual turbine in a farm, so as to achive an increase in the total power. The controls in our optimization problem are the thrust coefficients $C_{\rm {T,n}}'\left(t\right)$ of individual turbines as function of time. We use a gradient-based algorithm for the optimization and the gradients are computed using the adjoint method. In the first step we verify the adjoint calculated gradient by comparing it to the forward simulation based gradient obtained from finite difference of the cost function and find that errors remain below 5$\% $. We further elaborate the optimization techniques, and present results for a number of cases of wind-farm boundary layer cases. We also discuss how the thrust coefficient $C_{\rm {T,n}}'(t)$ evolves with time for different turbine locations. We also present and interpret results of the adjoint fields. [Preview Abstract] |
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