3A.6 Investigation of Subgrid-Scale Turbulence in the Atmospheric Surface Layer using AHATS Field Data

Tuesday, 3 August 2010: 4:45 PM
Torrey's Peak I&II (Keystone Resort)
Khuong X. Nguyen, Clemson University, Clemson, SC; and S. P. Oncley, T. W. Horst, P. P. Sullivan, and C. Tong

Data obtained in the atmospheric surface layer during the recent Advection Horizontal Array Turbulence Study (AHATS) field program are used to study issues met in large-eddy simulation (LES) of atmospheric boundary layer. The array technique, which has been successfully employed in several previous programs, is extended to include a second array to measure the advection of the subgrid-scale (SGS) stress. Pressure sensors are also deployed to measure the fluctuating pressure, enabling separation of the resolvable- and subgrid-scale pressure. We analyze the subgrid-scale terms in the joint probability density function (JPDF) of the resolvable-scale velocity, which must be reproduced by the SGS model in order for LES to predict correctly the resolvable-scale velocity JPDF. These terms include the conditional SGS stress (on the resolvable-scale velocity), the conditional SGS stress production rate, the conditional resolvable-scale pressure, and the conditional resolvable-scale pressure-strain rate correlation. The results show that the conditional resolvable-scale pressure (containing contributions from the SGS stress) has a strong dependence on the resolvable-scale velocity, being higher for negative vertical velocity and positive streamwise velocity fluctuations. The dependence of the conditional resolvable-scale pressure-strain rate correlation on the vertical resolvable-scale velocity generally has the opposite trends for positive and negative streamwise velocity fluctuations. Therefore, this term can enhance or counter the conditional SGS stress production term. The results can be used in statistical a posteriori SGS model tests to examine model performance in predicting the pressure effects. We also analyze the advection and pressure terms in the SGS stress budget, which are important for understanding the dynamics of the SGS stress and for modeling the SGS stress using the transport equation.
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