Effect of Roughness on Surface Boundary Conditions for Large-Eddy Simulation |
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Authors: | Rob Stoll Fernando Porté-agel |
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Institution: | (1) St. Anthony Falls Laboratory, Department of Civil Engineering, University of Minnesota, Minneapolis, MN, 55414, U.S.A;(2) National Center for Earth-Surface Dynamics, U.S.A |
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Abstract: | An important parameterization in large-eddy simulations (LESs) of high- Reynolds-number boundary layers, such as the atmospheric
boundary layer, is the specification of the surface boundary condition. Typical boundary conditions compute the fluctuating
surface shear stress as a function of the resolved (filtered) velocity at the lowest grid points based on similarity theory.
However, these approaches are questionable because they use instantaneous (filtered) variables, while similarity theory is
only valid for mean quantities. Three of these formulations are implemented in simulations of a neutral atmospheric boundary
layer with different aerodynamic surface roughness. Our results show unrealistic influence of surface roughness on the mean
profile, variance and spectra of the resolved velocity near the ground, in contradiction of similarity theory. In addition
to similarity-based surface boundary conditions, a recent model developed from an a priori experimental study is tested and it is shown to yield more realistic independence of the results to changes in surface roughness.
The optimum value of the model parameter found in our simulations matches well the value reported in the a priori wind-tunnel study. |
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Keywords: | Aerodynamic roughness length Atmospheric boundary layer Large-eddy simulation Surface boundary conditions |
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