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

In order to examine the Reynolds number effect on the turbulent structure of a d-type rough wall boundary later, the bursting phenomenon which had major contribution to the turbulent production was investigated by means of the VITA analysis. The mean bursting frequency and conditional averaged velocity patterns of u, v and uv components were obtained in the low Reynolds number range of R_θ=800-5000. The scaling of the mean bursting frequency with the conventional scales in boundary layers, inner, outer and mixed time scales fails in the low Reynolds number range of the present d-type rough wall flow. The peak value and time duration of the conditional averaged signals both in the ejection and sweep processes depend on the Reynolds number in the transitional rough regime of R_θ<2000. It is expected that the behavior of the non-linear interaction between the boundary layer and low momentum fluid particle in the transverse grooves, which leads enhancement of the momentum exchange and large turbulent production rate is essential to describe the Reynolds number effect over the d-type roughness.