Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session QD: Turbulence Simulations V |
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Chair: David Lee Cotrell, Lawrence Livermore National Laboratory Room: Long Beach Convention Center 102B |
Tuesday, November 23, 2010 12:50PM - 1:03PM |
QD.00001: Large-eddy simulation of flow past wind turbine rotors Iman Borazjani, Fotis Sotiropoulos Understanding the effects of atmospheric turbulence and terrain- specific flow phenomena on the aerodynamic performance of wind turbine rotors is critical perquisite for improving blade designs, developing effective flow control strategies and improving wind farm layouts. We develop a high resolution numerical method capable of carrying out large-eddy simulation of wind turbine flows in arbitrarily complex terrains. The method employs the curvilinear immersed boundary method coupled with overset grids and the governing equations can be solved both in the inertial and non- inertial frames. The method is validated by applying it to simulate the flow for the NREL phase VI wind turbine rotor for various operating points. In addition to LES, inviscid and unsteady RANS simulations are also carried out for all cases. The results from the different models are compared against each other and the experimental data and analyzed to provide turbulence statistics and the 3D flow structure in the rotor wake. [Preview Abstract] |
Tuesday, November 23, 2010 1:03PM - 1:16PM |
QD.00002: Conditional flow structures in a Large Eddy Simulation of the fully developed wind-turbine array boundary layer Claire Verhulst, Charles Meneveau Wind-turbines deployed in a large array experience a decrease in individual efficiency due to interactions among themselves and the atmospheric boundary layer (ABL). A fully developed flow regime can be established when this wind-turbine array is an order of magnitude longer than the height of the ABL. Under this condition, vertical entrainment of kinetic energy is essential for power extraction. In order to characterize this entrainment process, a Large Eddy Simulation of the fully developed wind-turbine atmospheric boundary layer (WTABL) is performed using a pseudo-spectral method with periodic boundary conditions in the horizontal directions. The wind-turbines are modeled as drag disks with a force proportional to the local disk-averaged velocity (Calaf et al. 2010, Phys. Fluids 22, 015110). Conditional averaging of the WTABL velocity field based on thresholds set on the instantaneous power extraction is performed to determine conditional coherent flow structures associated with large values of power extraction. Properties of the conditional structures are examined and their dependencies on WT loading factors are studied. [Preview Abstract] |
Tuesday, November 23, 2010 1:16PM - 1:29PM |
QD.00003: Large Eddy Simulation of stable and unstable atmospheric boundary layers over heterogeneous terrain Stimit Shah, Elie Bou-Zeid Large Eddy Simulations (LES) of the atmospheric boundary layer (ABL) are performed using a recently developed dynamic subgrid- scale (SGS) model. The model calculates both the Smagorinsky coefficient and the SGS Prandtl number dynamically based on the Lagrangian scale-dependent model in which required averages are accumulated in time, following fluid trajectories of the resolved velocity field. Simulations for both stable and unstable atmospheric boundary layers with heterogeneous surface fluxes are carried out to investigate the effect of surface variability on the turbulent kinetic energy budget and mixing in the ABL, with a special emphasis on the implications for turbulent transport similarity and turbulence closure in coarse atmospheric models. [Preview Abstract] |
Tuesday, November 23, 2010 1:29PM - 1:42PM |
QD.00004: Applying the v2f and the algebraic structure-based Reynolds stress closures to wind flow over complex terrain John O'Sullivan, Rene Pecnik, Gianluca Iaccarino Increasing worldwide wind energy production means wind farms are being constructed in areas where the terrain is more complex. Two important features of wind flow over complex terrain are flow separation and anisotropic turbulence. The most commonly used simulation approaches for wind flow use the Reynolds averaged Navier-Stokes (RANS) equations with a k-epsilon turbulence closure. This closure has difficulty in estimating separation accurately and cannot represent turbulent anisotropy. In other applications the v2f turbulence closure has shown a good ability to predict flow separation. Similarly the algebraic structure-based model (ASBM) has shown promise in capturing turbulent anisotropy. The flow over a representative hill which includes these features is calculated using the RANS equations with both the v2f and ASBM closures. A novel implementation of the ASBM closure is developed allowing a stable solution to be obtained. The results are compared with experimental data for the same flow and a good agreement is obtained for the separated region and the Reynolds stress components. Wall functions are developed for the v2f closure to enable the simulation of higher Reynolds number flows that include surface roughness. The results are compared with experimental data and accurately capture the separated region. [Preview Abstract] |
Tuesday, November 23, 2010 1:42PM - 1:55PM |
QD.00005: Optimization of turbine spacing in the fully developed wind turbine array boundary layer Charles Meneveau, Johan Meyers We consider the fully developed wind turbine array boundary layer, which is a regime of relevance when wind farms exceed the height of the atmospheric boundary layer by over an order of magnitude. Based on extensive LES studies of such boundary layers, a simple physics-based parameterization of the effective surface roughness was developed [see Calaf et al. Phys. Fluids {\bf 22}, 015110 (2010)]. The model depends upon wind turbine spacing, height, loading factors, ground roughness, etc. Using this model for induced surface roughness of large wind-farms, we proceed to establish optimal spacings between wind turbines, considering constant imposed geostrophic wind forcing. We examine the dependence of the optimal spacing on the ratio between cost of wind turbine and of land surface. We find that optimal average turbine spacing typically is between ten and twenty rotor diameters. This spacing is considerably higher than that used in conventional wind farms. [Preview Abstract] |
Tuesday, November 23, 2010 1:55PM - 2:08PM |
QD.00006: The LES of the channel flow in a non aligned system of coordinates Massimo Germano, Antonella Abb\`a The plane channel flow continues to be a very important test case for the verification and the validation of LES. In the channel flow test there is a privileged direction, usually one reference axis is oriented along the stream and the size of the computational box is increased in the streamwise direction in order to capture correctly the dominant turbulent structures and to produce a fully developed flow. All that is peculiar of this particular test, and in this paper we will investigate the sensitivity of the channel test to the particular alignment of the coordinate system with the mean flow. In a non aligned system of coordinates there is no privileged direction, there are two components of the forcing term, the mean pressure gradient, and the homogeneities of the Reynolds stresses are destroyed. In our paper we simulate the channel flow in a rotated system of coordinates, and we compare the results with the stream aligned data. We think that this test could evidence the flexibility of different LES codes and LES subgrid models to simulate the turbulent flow and to capture the correct statistical values in non aligned conditions. The first preliminary results are slightly contradictory: the resolved Reynolds stresses seem degraded while the mean flow is better predicted. The dynamic anisotropic subgrid model of Abb\`a, Cercignani and Valdettaro seems well fitted to represent correctly the large scales in non aligned conditions. [Preview Abstract] |
Tuesday, November 23, 2010 2:08PM - 2:21PM |
QD.00007: Isogeometric Variational Multiscale Large Eddy Simulation of Turbulent Flow through Annulus Channel Yousef Ghaffari Motlagh, Hyung Taek Ahn A large eddy simulation type variational multiscale method is performed for a turbulent annulus channel flow. The unsteady Navier-Stokes equations are solved numerically for two radius ratios. To model the annulus channel, Non-Uniformed Rational B-Spline (NURBS) basis functions are used. A key feature is to present geometry exactly by NURBS elements and the coarsest mesh encapsulates the exact geometry. In our calculation, we employ quadratic discretization that is $C^1$-continuous across element boundaries. The transverse curvature effect on near wall turbulent structures are analyzed for strong and weak curvature effects. To validate our results we compare them with available DNS data. [Preview Abstract] |
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