On the Manner in Which Outer Layer Disturbances Affect Turbulent Boundary Layer Skin Friction

Abstract

This paper presents results, from both experimental studies and turbulence modelling, of the manipulated flow behind flat plate devices introduced into the outer region of turbulent boundary layers. An analysis of mean velocity profiles, in conjunction with drag balance and other measurements for uτ, reveals only small changes in the log law and little influence of plate thickness on skin friction, except in the immediate vicinity of the manipulator where variations in the wall static pressure are recorded. However the drag reducing efficiency of the prefered tandem plate arrangement is found to depend simultaneously on plate length and gap. These observations are discussed together with recent turbulence measurements and previous flow visualisation. Implications emerge regarding the relative importance of the various proposed drag reduction mechanisms, and, in the light of these, alternative ‘triplet’ and ‘quadruplet’ configurations are investigated. It is concluded that larger nett drag reductions may be attainable, while the skin friction distribution downstream of a single plate is shown to be predicted quite accurately by an Algebraic Stress Model.