Atmospheric stability effect on subgrid scale physics for large-eddy simulation

F. Porté-Agel1,3,M. Pahlow, 1,3, C. Meneveau2,3 and M. Parlange1,3,
1Department of Geography and Environmental Engineering, 2Department of Mechanical Engineering, 3Center for Environmental and Applied Fluid Mechanics, The Johns Hopkins University, Baltimore MD 21218

ABSTRACT: Field measurements in the atmospheric boundary layer were carried out to identify the effect of atmospheric stability on subgrid scale physics for large-eddy simulation. The basic instrumentation setup consisted of 12 three-dimensional sonic anemometers arranged in two parallel horizontal arrays (7 sensors in the lower array and 5 sensors in the upper array). Data from this setup are used to compute the subgrid-scale (SGS) heat flux and SGS dissipation of the temperature variance under stable und unstable stability conditions. The relative contribution of the SGS vertical flux to the total turbulent flux increases when going from unstable to stable conditions. The relative importance of negative SGS dissipation = (backscatter) events becomes larger under stable conditions. The model coefficients for two well-known SGS models (eddy-viscosity and non linear) are computed. Model coefficients are found to depend strongly on stability. Under both stable and unstable conditions, large negative SGS dissipation is associated with the onset of ejection evetns while large positive SGS dissipation tends to occur during the onset of sweep events. These findings are also supported by conditionally sampled two-dimensional velocity and temperature fields obtained using the twelve anemometers placed in a vertical array.