  The interaction between a fluid and a wall is described with a certain
boundary condition for the fluid velocity at the wall. To understand how fluids
behave near a rough wall, the fluid velocity at every point of the rough
surface may be provided. This approach requires detailed knowledge of, and
likely depends strongly on the roughness. Another approach of modeling the
boundary conditions of a rough wall is to coarse grain and extract a
penetration depth over which on average the fluid penetrates into the
roughness. In this work we show that for a broad range of periodic roughness
patterns and relative flow velocities, a universal penetration depth function
can be obtained. We obtain these results with experiments and complementary
numerical simulations. Our results show that wall roughness boundary conditions
can be captured with an average ``slip length'' and so indicate that surface
patterning yields extensive control over wall slip.
