Pressure Gradient Effect on the Turbulence Structure over Riblet Surface

During the last ten years or so, a large number of turbulence research work has been carried out on techniques for reducing turbulent skin friction drag. The work has led to the development of riblet technology and progress has been such that riblets are becoming an accepted passive means of drag reduction. To explain the mechanisms of drag reduction a number of models have been proposed (Refs. 1–4) and there have been a few experimental studies which support these models. Among them is a conceptual model proposed by Choi (Refs. 2 and 4). He argued with experimental data that the near-wall burst takes place between a pair of counter rotating vortices developed from the concentrated vortices in a near-wall region of turbulent boundary layer (see Figure 1). The high shear stress of turbulence boundary layers is produced during the near-wall burst when the high-momentum fluid is brought down toward the wall between the pair of vortices. These studies have yielded interesting results but were limited to zero pressure gradient conditions.