Large-Eddy Structure of d-Type Rough Wall Boundary Layer Manipulated with Longitudinal Thin Ribs. Two-Point Triple Velocity Correlation Function and Diffusion Terms.

Abstract

Large-eddy structure of a d-type rough wall boundary layer manipulated with longitudinal thin ribs placed within transverse grooves has been investigated experimentally in terms of a dynamical equation for a two-point velocity correlation function, UAUB. Two-point triple velocity correlation functions involved in diffusion terms were measured in order to obtain spatial structure and the scales of three-dimensional vortical motions. It is seen that a pair of streamwise vortices with low-speed common-up flow is the primary structure. The spanwise stretching motion and interaction with high-speed sweeps from the outer layer make a dominant contribution to energy transfer from the large-scale eddy to smaller-scale one. The longitudinal thin ribs restrict the spanwise stretching motion and low-speed common-up flow and then reduce the strength of streamwise vortices which also make a dominant contribution to the turbulent production. The effect of wall modification with the longitudinal thin ribs can be found in the large-scale structure in the outer layer.