Single-Point Closures in a Neutrally Stratified Boundary Layer

Abstract

Closure assumptions often employed in single-point closure models for boundary-layer applications are evaluated against a neutrally stratified planetary boundary-layer flow generated by large-eddy simulation. The contributions from slow and rapid terms to fluctuating pressure are calculated directly from simulated fields. The slow pressure terms are compared with Rotta-type return to isotropy assumptions, both for the components of the Reynolds tensor and for a passive scalar. A simple proportionality between the time scales for dissipation of turbulent kinetic energy and for return to isotropy is found to be a good approximation in the upper two- thirds of the boundary layer. In the lower one-third of the layer, however, this ratio is found to increase by a factor of 2. Closure constants depending on anisotropy are examined and their usefulness determined. Significant contributions of the rapid terms are found for all second moments except vertical velocity variance and vertical scalar flux. Two sets of often-used closure assumptions for the rapid terms are compared with the explicitly calculated data. Also, the rapid terms show that the use of constant closure coefficients are for the tested parameterizations to be viewed at best as a first approximation. Length scales for dissipation of turbulent kinetic energy and scalar variance are extracted and compared with commonly used forms.