A study of the closure problem for pressure-scalar covariances

Perhaps the most commonly used closure for the pressure-correlation terms in second-order closure models is Rotta's return-to-isotropy parameterization, which was originally developed for shear flows. It is not clear that it alone is adequate for application to convective turbulence, however, because of the pervasive effects of buoyancy on turbulence structure. This closure problem is studied for the pressure-scalar term in the scalar flux equation with a data set generated through large-eddy simulation (LES) of a convective boundary layer. The pressure field is resolved into turbulence-turbulence, mean-shear, buoyancy, Coriolis force, and subgrid-scale components, and it is found that the buoyancy and turbulence-turbulence components dominate. The buoyancy contribution to the pressure-gradient/scalar covariance is one-half of the buoyancy production term in the flux equation, to a good approximation, while the turbulence-turbulence contribution can be parameterized adequately with Rotta's return-to-isotropy assumption.