Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A14: Wind Energy: Modeling and Simulation |
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Chair: Charles Meneveau, Johns Hopkins University Room: Georgia World Congress Center B301 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A14.00001: Toward validation of an overset CFD flow solver for wind energy applications. Enrico Fabiano, Shreyas Ananthan, Jayanarayanan Sitaraman, Michael Alan Sprague The analysis and design of the next generation of wind farms relies on the accurate numerical prediction of the turbulent airflow around multiple wind turbines. This computational aerodynamic problem is characterized by an atmospheric turbulent inflow, multiple bodies in relative motion, i.e. rotors and towers, and complex wake interactions. Furthermore, the size of the computational domain requires the numerical resolution of length scales that vary from several kilometers to the viscous length scales of the boundary layer of the blades. Overset-grids Computational Fluid Dynamics methods (CFD) offer an elegant and efficient solution for the simulation of blade-resolved wind farm aerodynamic problems: multiple body-conforming meshes can be embedded in several wake-capturing grids of different resolution. In this work, we present the initial validation of Nalu-Wind, an overset unstructured CFD flow solver for wind energy applications. The accuracy of the newly implemented overset methodology is investigated by simulating the complex unsteady airflow around a modern wind turbine airfoil and the turbulent flow around a NACA 0015 wing. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A14.00002: Effect of interpolation on convergence of overset grids using implicit hole cutting Ashesh Sharma, Shreyas Ananthan, Jayanarayanan Sitaraman, Michael Alan Sprague Accurately predicting the aerodynamic behavior of a wind farm requires high fidelity modeling of the individual components and their interaction with each other. Components in a wind farm include blades, nacelles, hubs, and towers, which possess complex geometries. The need to resolve flow structures around complex geometries has driven the research on overlapping grids commonly referred to as the overset grid approach. Exchange of information between overlapping grids is at the core of an overset framework and usually involves interpolation of data between grids. An important question to ask when using overlapping grids is how this interpolation influences the accuracy and convergence of the system. The answer depends on the behavior of the region of overlap as the overlapping meshes are refined or coarsened. This work investigates this question in detail from an implicit hole-cutting standpoint wherein a cell selection process determines computationally inactive regions based on how the meshes overlap with each other. We quantify our study using 3D problems simulating heat conduction and Low Mach fluid flow discretized using the CVFEM. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A14.00003: Turbulence generation in a large eddy simulation of a wind farm coupling meso- and micro scale Christian Santoni, Edgardo J García-Cartagena, Umberto Ciri, Giacomo Valerio Iungo, Stefano Leonardi Mesoscale circulation is of key importance for the study of wind farms in real conditions. Vertical mixing in Numerical Weather Prediction models is limited to resolutions on the order of 103 meters and, consequently, is unable to capture the intermediate scale turbulent structures impinging the wind turbine rotor and their wake interaction. To reconstruct these intermediate scales variability, we performed Large Eddy Simulations of a wind farm in north Texas, resolving both the mesoscale wind variations and the microscale by coupling WRF with our in-house LES code. Five one-way nested domains are solved with WRF to model the wind mesoscale wind conditions, retaining a resolution of 50 meters at the wind farm. An additional high-resolution nested domain is performed with our in-house LES code in the innermost domain in WRF from where we obtained the inlet boundary conditions. Numerical results are compared against SCADA, LiDAR and meteorological measurements. The variability of the inflow in the innermost domain is such to generate turbulence intensities in good agreement with SCADA. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A14.00004: Filtered lifting line theory and its application to the actuator line model Luis Martinez, Charles Vivant Meneveau In this work, a new formulation for a filtered version of Prandtl's lifting line theory is presented. In this formulation, a Gaussian filter is applied to the infinitesimally thin line of vorticity source, resulting in a volumetric distribution of vorticity along the blade. This formulation is then used to predict the induced velocity along the blade from the actuator line model. Using the new formulation, a correction is developed which allows the actuator line model to provide results independent of the smearing length scale ε. The correction is then tested on large eddy simulations of flow past a fixed wing with constant and elliptic chord distributions. Excellent agreement is found in all cases when using the correction. Finally, the correction is tested on a simulation of flow past a wind turbine. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A14.00005: A lifting line theory for yawed wind turbine actuator disks Carl Shapiro, Dennice F Gayme, Charles Vivant Meneveau Deflecting wind turbine wakes by yawing, which can increase wind farm power production, is receiving increased interest. Achieving this benefit requires improved predictions of the transverse velocity induced by the transverse component of the thrust force and the circulation of the counter-rotating vortex pair shed from the rotor. We focus on the inviscid region directly behind the turbine, which we treat as a porous lifting surface (airfoil). Prandtl's lifting line theory is applied by distributing the total transverse force along a lifting line through the center of the rotor. The resulting circulation distribution is elliptic and the theory is used to predict the induced constant transverse velocity in the wake and the counter-rotating vortex pair's circulation. The average velocity through the rotor and streamwise velocity in the wake are then found using mass and momentum conservation. The lifting line model is compared to numerical simulations of an actuator disk under uniform inflow and found to more accurately predict the transverse velocity (wake skewness) angle than other models. Using it to generate an initial condition also improves the accuracy of wake models for yawed turbines. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A14.00006: Effect of order of accuracy of fluid-structure-interaction algorithms on actuator-line simulations of wind turbines Ganesh Vijayakumar, Michael Alan Sprague Actuator-line simulations of wind turbines often use loose coupling for the fluid-structure interaction (FSI) algorithm that reduces the numerical order of accuracy of fluid and structural solver. Demonstrating expected convergence rates with spatial- and temporal-grid refinement is the ``gold standard'' of code and algorithm verification. With increasing size and flexibility of modern wind turbines, the order of accuracy of the FSI algorithm could also play a major role in the prediction of coupled unsteady aerodynamic effects. In this work, we demonstrate that a new FSI algorithm is second-order accurate using the method of manufactured solutions for actuator-line CFD simulations with a simplified structural model. We then implement the same FSI algorithm for actuator-line simulations coupled to the multi-physics OpenFAST wind turbine simulation tool. We demonstrate the effects of the order of accuracy of the FSI algorithm on simulations of wind turbines in a wind farm. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A14.00007: Wake Dynamics of Wind Turbines for Various Operating Conditions. Søren Andersen Wind turbines traditionally operate as "greedy individuals", but more and more research now explore possibilities of controlling wind turbines to improve performance of entire wind farms. Two main approaches have usually been investigated, namely induction based control and wake steering control. The former aims to decrease the wake deficit, whereas the latter aims to improve the performance of subsequent turbines by deflecting the wake away from the next turbine. The wake dynamics are investigated for different control scenarios to elucidate on how and why the performance of the next turbines could potentially improve. The wind turbine and its wake is modelled using EllipSys3D(Michelsen, 1992, and Sørensen, 1995), where large eddy simulation is fully coupled to the aero-elastic tool, Flex5(Øye, 1996) through the actuator line method(Sørensen and Shen, 2002). Various statistical measures as well as proper orthogonal decomposition(POD) is used to analyse the dynamics and large coherent structures of the turbulent wakes. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A14.00008: An improved BEM-type model for Vertical-Axis Wind Turbines by incorporation of base-suction effects Anis A. Ayati, Konstantinos Steiros, Mark Miller, Subrahmanyam Duvvuri, Claudia E Brunner, Marcus Hultmark We present a modified double-multiple streamtube (DMST) model for vertical-axis wind turbines (VAWT). This approach builds on blade element momentum theory (BEMT) and accounts for the azimuthal dependence of blade aerodynamics by discretizing the flow into a set of adjacent streamtubes, each featuring two actuator disks in tandem. BEMT equations are solved twice in each streamtube, once at the upstream and once at the downstream turbine half cycle, respectively. The Rankine-Froude actuator disk theory, which traditionally constitutes the momentum aspect of BEMT is replaced by a recently developed model for the drag of two-dimensional flat plate of arbitrary porosity. This model takes into account the effect of base suction in the wake and yields realistic wake velocities. This is critical for DMST models as wake velocities from upstream actuators are used as input to downstream streamtubes. Global performance predictions are compared to the unique Princeton HRTF wind tunnel measurements conducted at full dynamic similarity with respect to full-scale turbines operating at high Re-numbers ($0.5\times10^6<Re_D<5\times10^6$), with varying tip-speed ratios ($0.75<TSR<2.5$) and solidities ($0.45<St<1.12$). The proposed model performs significantly better than conventional DMST models. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A14.00009: A nonlinear model for wind turbine blade flutter Pieter Boersma, Xavier Amandolese, Yahya Modarres-Sadeghi Larger wind turbine blades are more susceptible to flow-induced instabilities such as coupled-mode flutter. Flutter research in wind turbine blades has been primarily focused on predicting the critical flutter point and less so on post-critical flutter behavior. However, the goal of establishing control mechanisms for wind turbine blade flutter and the possibility of subcritical instabilities have made it essential to derive nonlinear models for these instabilities. We present a nonlinear model for wind turbine blade instability in which the structure is represented by a set of coupled nonlinear partial differential equations, which retain up to third order nonlinearities, and the flow is represented using an ONERA dynamic stall model. The nonlinear model is obtained by coupling these equations, and is solved using the Galerkin technique. The results indicate post-critical limit cycle oscillation, whose amplitude increases with increasing wind speed. |
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