  Direct Numerical Simulations of two superposed fluids in a channel with a
textured surface on the lower wall have been carried out. A parametric study
varying the viscosity ratio between the two fluids has been performed to mimic
both {\bf idealised} super hydrophobic and liquid infused surfaces and assess
its effect on the frictional, form and total drag for three different textured
geometries: longitudinal square bars, transversal square bars and staggered
cubes. The interface between the two fluids is assumed to be slippery in the
streamwise and spanwise directions and not deformable in the vertical
direction, corresponding to the ideal case of infinite surface tension. To
identify the role of the fluid-fluid interface, an extra set of simulations
with a single fluid has been carried out and compared to the results obtained
with two fluids of same viscosity separated by the interface. The drag and the
maximum wall-normal velocity fluctuations were found to be highly correlated
for all the surface configurations, whether they reduce or increase the drag.
This implies that the structure of the near-wall turbulence is dominated by the
total shear and not by the local boundary condition of super-hydrophobic,
liquid--infused or rough surfaces.
