Measurement of Spanwise motions and secondary flows in a turbulent boundary layer formed over drag-increasing riblets*

We experimentally investigate the effect of dispersive stresses and near-wall large-scale motions on the drag increase by non-optimal trapezoidal riblets. This investigation requires measurement in the vicinity of the riblet tips and within the valleys. Performing particle image velocimetry is ideal, however, the strong reflection from the surface prevents accessing any information close to the surface. For this reason, we use a novel technique to minimize the light reflection from the riblet surface and permit measurement in the vicinity of the wall. This technique works by coating the riblets with Rhodamine paint that emits light at a different wavelength when illuminated with a green light laser. We use two stereoscopic particle image velocimetry systems to visualize the flow above the riblet tips and within the valleys.

This work was supported by the Air Force Office of Scientific Research (AFOSR, program manager: Gregg Abate) under award number FA2386-21-1-4018 (program managers: David Newell and Ryan Carr, AOARD) and the Australian Research Council under Discovery Project DP170102595.