Boundary Layer Structure over a Two-Dimensional Rough Wall

Abstract

Turbulence measurements for a zero pressure gradient boundary layer over a two-dimensional (2-D) roughness are presented and compared to previous results for a smooth wall and a three-dimensional (3-D) roughness [5]. The present experiments were made on transverse square bars in the fully-rough flow regime. The 2-D bars lead to significant changes in the turbulence in the outer flow. Reynolds stresses, particularly ${\overline{{v^{\prime}}^{2}}}^{+}$and $-{\overline{u^{\prime} v^{\prime}}}\,^{+}$, increase, although the mean flow is not as significantly affected. Large scale turbulent motions originating at the wall lead to increased spatial scales in the outer flow. The dominant feature of the outer flow, however, remains hairpin vortex packets which have similar inclination angles for all wall conditions. The differences between boundary layers over 2-D and 3-D roughness are attributable to the scales of the motion induced by each type of roughness. It is also noted that there are fundamental differences in the response of internal and external flows to strong wall perturbations, with internal flows being less sensitive to roughness effects due to their boundary conditions.