Near-Wall Turbulence Structure over Misaligned Riblets

Abstract

In the early 1980’s NASA Langley Research Center conducted a parametric study to show that up to 8\% in turbulent skin-friction reduction can be obtained from riblets on a flat plate with zero pressure gradient, at moderate Reynolds numbers [1]. This extensive work was followed by the studies of turbulent boundary layers with riblets under different flow conditions; on 3-D bodies [2,3,4], with pressure gradients [2,4,5], at high Mach numbers [3,6] and high Reynolds numbers [3,4,6]. Detailed studies on structural changes in the near-wall turbulence were also carried out [7,8,9]. These results indicate, basically, that the drag-reduction performance of riblets under such flow conditions does not differ much from the early NASA results on a flat plate. The recent flight test carried out by Airbus Industries using an A320 aircraft and the field trial by BMT and ARE using a high speed buoyancy propelled vehicle [4] have convincingly demonstrated this point. There are, however, still a few unresolved problems in the practical applications of riblets. The alignment of riblets in a three-dimensional turbulent boundary layer, such as over a swept wing or near the stern of a ship hull, is one of these unsolved problems.