The possibility of drag reduction by outer layer manipulators in turbulent boundary layers

Abstract

Experiments were carried out with the aim of investigating the possibility of obtaining a net drag reduction on a finite body by manipulating the outer layer structure of the turbulent boundary layer. The experiments were carried out in a 260 m long towing tank, where large eddy breakup devices (LEBU’s) were used in single and tandem configurations on a large flat plate. The total drag was measured directly by a force gauge, and the geometrical configuration parameters, as well as the chord Reynolds number, were varied over wide ranges. The highest chord Reynolds number tested was 260 000, which is within the range of interest for practical applications. Also, the device drag was measured directly in flight, which enabled evaluation of the performance of the manipulators, as well as determination of the total skin friction reduction. Despite a substantial skin friction reduction for certain configurations (7% averaged over the part downstream from the manipulators) as well as a low device drag, no drag reduction was found. The present results, and a critical evaluation of previously reported data, make any substantial net drag reduction by use of LEBU’s at high Reynolds numbers seem implausible.