Influence of Wave-Like Riblets on Turbulent Friction Drag

This article reports on a numerical and experimental study of the turbulent drag on riblet surfaces, where the trapezoidal riblet grooves were formed in a wave-like sinusoidal or zigzag pattern. The aim was to enhance the drag-reducing capabilities of conventional, straight riblet grooves by an additional contribution that originates from the induced oscillating lateral flow component. By means of a comprehensive parameter study in an oil channel at Re between 10,000 and 30,000 and DNS simulations at Re

τ

=180, suitable waveform parameters are sought, with which wave-like riblets produce a drag reduction larger than that of their straight counterparts. For a riblet cross-section shape that is known to be optimal for straight grooves, no such beneficial drag modification could be demonstrated. With a riblet groove cross-section different from the optimum shape, an augmented attainable drag reduction in comparison to straight riblet grooves was found within a certain range of the waveform amplitude. The improvement amounts up to 1.3%-points in terms of drag reduction. Wave-like riblets with reduced riblet height never outperformed the drag reduction of straight riblet grooves of optimal cross-section form, but exhibit a similar drag reduction in the best cases investigated. It is shown that this favourable influence on the riblet-modified turbulent drag persists under a mild misalignment of the riblets to the main flow direction.