Numerical study of drag reduction mechanism on surfaces with transverse triangular microstructures

One of the important passive techniques to reduce the drag force is the use of microstructured surfaces. The structures of these surfaces, which are from the order of nanometers to several hundred micrometers, can be created randomly or in a regular and controlled manner, with different geometries and configurations on the surface, and by affecting the fluid flow, they can change the amount of drag. With the aim of studying the physics governing microstructures, this article will investigate the parameters resulting from air flow passing them, which include drag components, velocity profiles and shear stress. For this purpose, triangular microstructures with the same base and height of 50, 100, 200, 400 and 800 µm have been used, which are transversely exposed to air flow with velocity of 5 m/s and 25 m/s. Due to the emphasis of some articles on the flow slipping over the microstructures, the velocity profiles on these surfaces have been investigated, but finally, the change in the amount and direction of the shear stress has been described as the main mechanism of viscous drag reduction. Then, the effect of the size of the structures and the velocity of the flow has been investigated. The obtained results show that the trapped vortices among the transverse structures can reduce the viscous drag by reducing the amount of shear stress around the peaks and reversing its direction in the valleys. On the other hand, creating a pressure gradient inside and around the structures will lead to creating pressure drag. The sum of these two drag components, which depend on the size of the microstructures and the flow velocity, will finally determine the increase or decrease of the total drag.