The smooth-wall-like behaviour of turbulence over drag-altering surfaces: a unifying virtual-origin framework

We conduct direct simulations of turbulent channels imposing different virtual origins for all three velocities using Robin, slip-like boundary conditions to study the effect displacing perceived by flow components, a mechanism common small-textured surfaces. also explore this opposition control. For riblets, Luchini et al. (1991) proposed that their could be reduced offset between streamwise- and spanwise-velocity origins, latter being origin turbulence. Later results on superhydrophobic surfaces suggest apparent wall-normal-velocity play role. Our support relevant parameter is mean The former determined streamwise slip length, but depends both wall-normal spanwise lengths. velocity fluctuations, in turn, has negligible turbulence, at least drag-reduction regime. This suggests quasi-streamwise vortices, which induce cross surface, key determining while near-wall streaks, are associated with plays secondary In framework, changes quantities typically reported flow-control literature shown only result choice absent when referenced experienced Other than shift origin, we show turbulence remains essentially smooth-wall-like, simple expression can predict drag reproduced priori introducing into smooth-wall eddy-viscosity framework.