Experimental and Computational Study of the Effect of Two-Dimensional Roughness on the Mean Velocity of a Laminar Boundary Layer.

Abstract

In this report an experimental and computiational study of the flow behind a two-dimensional square roughness element placed in a laminar boundary layer on a flat plate is described Numerical solutioins of the two-dimensional N-S equations were obtained by the MAC(Marker-and-Cell)method. The experiment was carried out in a low-turbulence wind tunnel. The flow behind the roughness element consisted of a separated region and an external layer. There was no mass transfer between these regions. The computational and the experimental results for these regions were compared. The extent of the separated region depends on κ / δ*κ, where κ is the height of the roughness element and δ*κ is the displacement thickness at the roughness element. When κ / δ*κ is less than a critical value, the computational results agree with the experimental results. The boundary-layer flow separates at the roughness element and returns at 70-κ to the laminar boundary layer. In this case the length of the separated region increases with κ / δ*κ. When κ / δ*κ is larger than the critical value, the length of the separated region decreases with increasing κ / δ*κ, but the computational results do not agree with the experimental ones.