Effects of trigonometric ripples on flow separation control in rectangular S-ducts

Abstract This paper proposes a parametric design method of the sinusoidal rippled surfaces on rectangular S-duct to restrain the flow separation and reduce total pressure loss, which works as the passive flow control method inspired by the morphology of intertidal seabed dunes and the pectoral fins of humpback whales. Multiple geometric models with various rippled surface configurations were automatically generated by three key variables: ripple depth ( d ), ripple angle ( β ), and the location of the ripple (upper surface, lower surface, upper and lower surfaces). Subsequently, numerical simulations were conducted to study flow separation. The results indicate that an increase in both ripple angle and ripple depth leads to a gradual decrease in the total pressure recovery coefficient ( Cpt ). The configuration with ripples applied solely on the lower surface achieves the best performance in improving the Cpt . Specifically, an S-duct configuration with a ripple depth of 1.5 mm and a ripple angle of 0° applied to the lower surface achieves a maximum 8.7 % reduction in TPLC relative to the flat model without ripples. The findings demonstrate that introducing rippled surfaces within S-ducts is an effective passive flow control method without requiring external energy input.