The effect of an adverse pressure gradient on the drag reductionperformance of manipulators

Abstract

This paper presents skin friction data obtained with sublayer-scale fence, razor blade, and log-layer-sized Preston tube for both natural and manipulated boundary layers subjected to a strong adverse pressure gradient. Comparative measurements were made, under nominally zero pressure gradient conditions, with a floating element drag balance. The results provide support for established formulas and recently proposed corrections for dp/dx and d 2 p/dx 2. They also reveal that the adverse pressure gradient had remarkably little effect on the magnitude of the C f reduction obtained by introducing an optimized tandem plate manipulator into the outer part of the boundary layer (to the extent that separation occurred when the manipulator was present). This finding is assessed in the light of other recent results and earlier floating element measurements. In addition, the skin friction distributions measured in the experiment are shown to be predicted remarkably accurately with an algebraic stress model. However, the computational results indicate an associated increase in the shape factor which probably limits any net benefit even prior to the onset of separation.