Effects caused by small discrete two-dimensional roughness elements immersed in turbulent boundary layers

Abstract

In order to determine the downstream consequences of the presence of small discrete surface discontinuities situated on otherwise smooth surfaces and subjected to six equilibrium two-dimensional adverse-pressure-gradient turbulent boundary-layer flows, these conditions were first established in a special-purpose wind tunnel. A surface discontinuity is small if it lies within the logarithmic region of the undisturbed boundary layer. Immediately downstream of such discontinuities flow separation ensues. After the subsequent reattachment, measurements were made of the downstream boundary-layer development. Even in strong adverse pressure gradients the local increments of momentum thickness caused by these roughness elements were well predicted by Gaudet & Johnson's zero-pressure-gradient correlation. With highly adverse pressure gradients it was found that these small surface discontinuities have little influence on the flow downstream. The essential outcome of this work is that the incremental drag of small roughness elements depends solely on local wall variables. Thus, when the pressure gradient is strongly adverse and the local skin friction is correspondingly small, the incremental drag of the roughness element becomes similarly small. After reattachment, it has an insignificant effect on the flow downstream of it.