科研之友
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摘要
The influence of surface roughness on a high Reynolds number turbulent boundary layer is investigated in the light of Townsend%26apos;s wall similarity hypothesis. There is conflicting information in the literature about the effect of surface roughness on the flow structure in the outer layer. Flows over roughness consisting of spanwise rods appear to show unusually strong interaction between the wall and the wake region. It has been suggested that flows which show strong modifications in the outer layer suffer from lack of scale separation between the roughness scale and the largest scale given by the boundary layer thickness. One of the flows which have documented significant roughness effects across the entire layer is the boundary layer developing over roughness made up of spanwise aligned square bars. Here we compare one such flow taken at relatively low Reynolds number with measurements for a boundary layer which develops at much higher Reynolds number. The high Reynolds number (Re-theta = 32 600) assures that the range of scale is wide, while at the same time the size of the roughness elements is sufficiently large for the flow to be fully rough. Both sets of data are compared to a smooth wall reference boundary layer and from probability distributions, the quadrant contributions to the shear stress and triple correlation profiles, we show that the outer layer changes are significantly reduced when increasing the Reynolds number and thereby the range of scales. In this case the wall perturbation of the flow is shown to be much smaller than reported in other experiments using the same geometry. Here we find that the modifications in the turbulent structure are limited to about five times the roughness scale. This is in accordance with the wall similarity hypothesis.
- 出版日期2012-7
