Influence of an anisotropic slip-length boundary condition on turbulent channel flow

The effects of an anisotropic Navier slip-length boundary condition on turbulent channel flow are investigated parametrically by direct numerical simulations. The slip-length boundary condition is made direction dependent by specifying the value of the slip length independently for the streamwise and spanwise direction. The change in drag is mapped versus a wide range of streamwise and spanwise slip-length combinations at two different friction Reynolds numbers, Re-tau 0 = 180 and Re-tau 0 = 360. For moderate slip lengths both drag-reducing and drag-increasing slip-length combinations are found. The percentage drag increase saturates at approximately 60% for high spanwise slip. Once a threshold value for the streamwise slip length is exceeded, drag is reduced in all cases irrespective of the value of the spanwise slip length. The Reynolds number appears to have only little influence on the change in drag for the moderate Reynolds numbers studied here. A detailed comparison with the implicit theoretical formula of Fukagata [Phys. Fluids 18, 051703 (2006)], which relates the change in drag with the streamwise and spanwise slip length, has been made. In general, this formula gives a fair representation of the change in drag; a modified version of this relation is presented, which improves the prediction for the change in drag for small slip length values and reduces the number of free parameters contained in the model. The effects of the slip-length boundary condition on the flow are further investigated using mean flow and turbulence statistics. For drag-neutral slip-length combinations the level of turbulent fluctuations is approximately unchanged. The presence of a slip-length boundary condition affects both the level of wall-shear stress fluctuations and the degree of intermittency of the wall-shear stress probability density function. The correlation statistics of the velocity field show that a high spanwise slip length causes a disruption of the near-wall streaks, while high streamwise slip favours an increasing streak regularity. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4719780]