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

Abstract

The large-eddy structure of a d-type rongh wall boundary layer manipulated with longitudinal thin ribs for turbulent drag reduction has been investigated experimentally in terms of a dynamical equation for a two-point velocity correlation uA uB. Two-point double velocity correlation functions involved in the production terms were measured in order to focus on the role of the large eddies on the turbulent production process. Some noticeable features of the two-point velocity correlation functions show that the well-known hairpin eddy or streamwise rotating vortices predominantly affect the production terms. In the boundary layer manipulated with the longitudinal thin ribs, the magnitude of the two-point velocity correlation functions is reduced only in a relatively small spatial distance of 0.1δ, which is comparable to Taylor's micro length scale in the present turbulent field. This reduction of the two-point velocity correlation is seen throghout the layer.