Effect of roughness texture on transient, accelerating~channel flows

The effect of surface~roughness texture on non-equilibrium, accelerating wall turbulence is studied~using direct numerical~simulation of transient periodic channels. The~smooth-wall base case is compared with two irregular rough~surfaces: i) a small-wavelength sand-grain roughness and ii) a~multiscale turbine-blade roughness. The flow is~accelerated from a bulk~Reynolds number of 3000 to 12000 in a short time interval, rendering the~rough-wall flows~transitionally-rough to fully-rough. The smooth wall undergoes~reverse transition towards a laminar-like state with quasi-1D~turbulence, before~re-transition into a new equilibrium state. In contrast, near a rough wall a more~isotropic Reynolds stress~tensor and a higher friction coefficient are observed.~This is mainly due to the fast responses of the form-induced~Reynolds-stress~production and pressure work to the increased shear, both contributing significantly~to higher Reynolds-stress isotropy. As the characteristics of the form-induced~fluctuations are important for these mechanisms, the roughness~texture determines~the initial rate of turbulence response. Results show that the roughness~geometry is important in a non-equilibrium turbulence over a rough wall.