Coherent structures and subfilter-scale dissipation in LES: A field study

The success of large-eddy simulation (LES) of atmospheric boundary layer turbulence relies on the ability of the subfilter-scale (SFS) models to reproduce the effect of the unresolved (subfilter) scales on the resolved scales accurately. Intermittent behavior of the momentum and scalar fluxes due to local features of the turbulence has been especially problematic to modeling. This work reports results from a field study designed and carried out to study the role of coherent structures on the local transfer of energy and temperature variance between resolved and subfilter scales. Data used in this study were taken from high-resolution measurements of velocity and temperature made at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility located on the salt flats of Western Utah. In particular, 16 tri-axial sonic anemometers were used in an arrangement with a 6m-high vertical array (10 anemometers) that intersected a 3m-wide horizontal array (7 anemometers) 2m above the ground. The data collected are used to calculate subfilter-scale stresses and fluxes, and subfilter-scale dissipation rates (i.e., the transfer rates between resolved and subfilter scales) of energy and temperature variance. With these quantities, conditional averaging is used to study the relation of strong positive (forward-scatter) and negative (backscatter) SFS dissipation events to local features of the flow. The conditionally averaged vertical and horizontal flow fields reveal vortical structures, inclined downwind at angles close to 16 degrees during near-neutral stability and as large as 34 degrees during convective conditions. These inclined vortical structures match the conceptual model of a hairpin vortex (with head and trailing legs) around which sweep and ejection events are found. Localized regions of large forward-scatter are found on the upper trailing edge of these structures whereas localized regions of large backscatter are found on the lower leading edge of the same type of structures.