Turbulent Drag Reduction using Sinusoidal Riblets with Triangular Cross-Section

It is known that longitudinal ribs manufactured in a flat surface act to reduce turbulent skin-friction drag, providing a moderate drag reduction of 4 to 8%. It is shown in this paper that this value can be increased by at least 50% if sinusoidal-like rods are used instead of conventional straight riblets. Large Eddy Simulation of a turbulent flow over a riblet-covered surface is performed for three cases: straight riblets and sinusoidal riblets with two different values of wavelength. All riblets have triangular cross-section. It is found that drag reduction with sinusoidal riblets depend strongly on the wavelength, showing a benefit over straight riblets for a larger value of the wavelength, and an opposite trend for a smaller value. Different nature of the flow over straight and sinusoidal riblet surfaces is revealed by looking at crossflow motion in transverse planes, mean and instantaneous streamwise vorticity, and organized coherent structures. Turbulent statistics is compared between all three cases, crossflow turbulence intensity is reduced for sinusoidal riblets as opposed to straight riblets.

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