Alignment trends of velocity gradients and subgrid-scale fluxes in the turbulent atmospheric boundary layer

Field experimental data in the atmospheric surface layer are analyzed using tools from statistical geometry. The data consist of velocity measurements from sonic anemometer arrays. In the context of large eddy simulations (LES), these arrays permit the spatial filtering needed to separate large from small scales. Time series of various quantities relevant to LES are evaluated from the data. Results show that the preferred filtered fluid deformation is axisymmetric extension and the preferred subgrid stress state is axisymmetric contraction. The filtered fluctuating vorticity shows preferred alignments with the mean vorticity, with the streamwise direction, and with the intermediate strain-rate eigenvector. The alignment between eigenvectors of the subgrid-scale stress and filtered strain rate is used to test eddy viscosity and mixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model.