Effect of roughness texture on a transient accelerating channel flow*

The topographical effects of wall roughness on the response of a turbulent channel to a very rapid acceleration is studied using direct numerical simulations. Flows over two surfaces with same roughness height, but with different roughness topographies are compared: a sand-grain roughness and a multiscale turbine-blade roughness. The flow is accelerated from a bulk Reynolds Number of 3000 to 12000 over a short time interval. 

During this time, the rough-wall flows change from transitionally rough to fully rough.

It has been shown that, unlike a smooth-wall flow, the rough-wall flows recover to the new equilibrium state quickly without reverse transition, and that roughness geometry affects the rate of such recovery.

In this talk, we show that such roughness effects are attributed to the form-induced turbulent fluctuations, which are intensified by the acceleration. 

First, the form-induced velocities and their strain rates are characterized. Then we show that they significantly affect near-wall pressure fluctuations and Reynolds stress anisotropy, by analyzing various source terms of the pressure Poisson equation.

This work demonstrates that the transient response of rough-wall turbulence to acceleration depends strongly on the texture of the roughness.