Flow in a narrow-gap annulus: drag reduction by inner cylinder azimuthal oscillation

The axial flow through a thin gap between two concentric cylinders is investigated using direct numerical simulation (DNS) of the Navier–Stokes equations. It is shown that by imposing a stationary, harmonic, azimuthal velocity at the inner cylinder wall, drag reduction is always attained while power saving is limited to a portion of the explored amplitude–axial wavelength space. The largest values of drag reduction and power saving are 44 percent sign 44 % $44\,%$ and 11 percent sign 11 % $11\,%$ , respectively, obtained for different amplitude–axial wavelength pairs. It has been verified that the effect of such a control on a convex surface aligns well with the known concept in plane channel flows of depressing in turn the pressure–strain, the radial Reynolds stress, the turbulent shear stress and ultimately the viscous drag. A direct connection between the wall motion, through the Stokes layer, and the pressure–strain reduction has been identified as the root cause of the passive flow control effectiveness.