DOI: 10.1615/TSFP6
TURBULENCE STRUCTURE FOR TWO- AND THREE-DIMENSIONAL ROUGHNESS
ABSTRAKT
Turbulence measurements for a zero pressure gradient boundary layer over a two-dimensional roughness are presented and compared to previous results for a smooth wall and a three-dimensional roughness. The present experiments were made on transverse square bars in the fully-rough flow regime. The turbulence structure was documented through the fluctuating velocity components and two-point correlations of the fluctuating velocities. The 2-D bars lead to significant changes in the turbulence in the outer flow. Reynolds stresses, particularly ν'2+ and − u'ν'+ , 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. Three-dimensional roughness produces turbulence scales of the order of the roughness height, k, while the motions generated by 2-D roughness may be much larger due to the width of the roughness elements.