Experimental Investigation of Oscillating Riblets for Turbulent Drag Reduction

Direct shear stress measurements were carried out on a riblet test plate with rectangular riblet grooves in a fully turbulent oil channel flow. The band-shaped riblets were tilted in spanwise direction by means of an electric drive up to an amplitude of 30 degrees and a frequency of 4 Hz. Those oscillating riblets were intended to combine the drag reducing effect of conventional, stationary riblets with that of a laterally oscillating wall. Shear stress measurements on oscillating riblets with a cross-section optimal for stationary riblets yielded improvements in the drag-reducing capability of order of the measurement accuracy. For elongated riblets, a measurable increase in the drag reduction was obtained at certain values of the dimensionless oscillation period. Those results are in qualitative congruence to results from DNS simulations previously published. However, the maximum drag reduction of stationary riblets could not be increased by oscillating riblets. By 3D-PIV measurements, the successful generation of an additional spanwise velocity component in the near-wall region was proven.