Direct numerical simulation of turbulent channel flow with wall velocity disturbances

This paper considers the effect of applying nonzero velocity fluctuations along a flat smooth wall, mainly with the aim of understanding how conditions at the wall interact with the outer turbulent flow. Such a study is expected to be of use in formulating effective strategies for wall turbulence control. Three direct numerical simulations of a turbulent plane channel flow are carried out, starting with a flow field with no-slip conditions. Each simulation evolves by imposing (on one wall) only one nonzero velocity component. When a nonzero longitudinal velocity fluctuation $u_1$ is applied, drag reduction occurs. With a nonzero spanwise velocity fluctuation $u_3,$ the flow is very similar to that in an unperturbed channel. However, the use of a nonzero wall-normal velocity fluctuation $u_2$ results in structural changes similar to those observed in a direct numerical simulation of a turbulent flow over a rough surface. From the present simplified simulations, the inference is that the salient characteristics of rough wall flows reflect mainly the presence of a nonzero wall-normal normal velocity distribution at the interface between the roughness cavities and the external flow.