Coherent structure of a d-type rough wall boundary layer by means of the four-quadrant analysis : low reynolds number effect

In order to investigate the low Reynolds number effect on the turbulent structure of a d-type rough wall boundary layer, fractional contribution of the stress generating motions to the Reynolds shear stress was qualitatively examined with the four-quadrant analysis. The second and forth quadrant events (ejections and sweeps) play major role in fractional contribution and also in intermittency through the layer. In the low Reynolds number flow fields, the second quadrant has dominant contribution to the Reynolds shear stress, while the contribution from the other three quadrants are reduced. Comparison of the ratio of fractional contribution from the second quadrant to that of the fourth quadrant among three types of surface, d-type rough, k-type rough and smooth, a certain difference of the near-wall turbnlent structure is found. It is expected that the difference can be explained with the dominant vortical structure. The violent ejections in the outer layer was investigated in detail. The mean frequency and conditional velocity patterns show that the violent ejections is associated with the large-scale spanwise vortical structure in the outerlayer and also the near-wall bursting phenomenon.