On the omnidirectionality of a system with eight spinning rods for wake control of a circular cylinder in laminar regime

Abstract

We investigate the omnidirectionality of a system comprised of a main circular cylinder (of diameter D) fitted with eight rotating rods (of diameter d=D/20) acting as wake-control devices over the incoming flow of velocity U∞ and viscosity ν. The gap between rods and main body is G/D=1/100. Finite-volume simulations were conducted at a Reynolds number Re=U∞D/ν=100 in two configurations: One in which all rods spun with the same angular velocity (case 0); and another, in which the rotation speeds were inspired by potential-flow theory (case 1). We show through analysis of hydrodynamic coefficients, vorticity contours and power consumption that both configurations reduced mean drag, eliminated the vortex wake, and were negligibly affected in terms of power loss by the angle of attack. Nevertheless, the two cases showed different behaviours regarding omnidirectionality: While case 1 became ever “more omnidirectional” (that is, less susceptible to the angle of attack to render null mean lift and suppress vortex shedding) with increasing angular speeds, case 0 produced an imbalanced, yet steady, wake that led to larger absolute values of mean lift at higher speeds. The relative importance of each rod of the system is explained, and it becomes clear that merely introducing actuating power into the rods’ motion does not translate directly into better wake control. Rather, it is essential that the rotation speeds are appropriately weighted according to the position of the rods relative to regions of boundary and shear layer, and wake of the main body.