Large-Eddy Simulations of Plane and Radial Wall-Jets over Smooth and Rough Surfaces

Abstract

Large-eddy simulations were carried out to investigate the flow dynamics of wall jets over smooth and rough surfaces. Results were validated against data in the literature. A sand-grain roughness model is used, based on an immersed boundary method. To understand the extent to which the outer/inner layer modifies the inner/outer layer and the extent to which the effect of roughness spreads away from the wall, instantaneous and mean flow fields were investigated. For the Reynolds numbers and roughness heights considered in this study, the effect of roughness is mostly confined to the near-wall region in both plane and radial configurations. There is no structural difference between the outer layer over smooth and rough surfaces. Roughness does not affect either the size of the outer-layer structures or the scaling of the profiles of Reynolds stresses in the outer layer. However, in the inner layer, roughness redistributes stresses from streamwise to wall-normal and spanwise directions. Contours of joint probability-density function of the streamwise and wall-normal velocity fluctuations at the bottom of the logarithmic region match those of the turbulent boundary layer at the same height; traces of the outer-layer structures were detected at the top of the logarithmic region, indicating that they do not affect the flow very close to the wall, but still modify a major portion of the inner layer. Simulations of plane and radial wall-jets at several Re numbers were then investigated to, first, compare the plane and radial wall-jets and, second, to quantify the interaction of inner and outer layers. In both cases, the local Reynolds number is an important determining factor in characterization of the flow. The joint probability density function analysis shows that the local Reynolds number determines the level of intrusion of the outer layer into the inner layer. As the local Reynolds number increases, the thickness of the overlap layer becomes smaller, and the inner layer of the wall jet becomes more similar to the conventional turbulent boundary layer, i.e., the extent of the logarithmic region of the wall jets increases and its slope gets closer to the universal law of the wall.