VORTICAL STRUCTURES AND COHERENT MOTION IN TURBULENT-FLOW OVER SMOOTH AND ROUGH BOUNDARIES

It has been recognized for some years that the bursting phenomenon is a common feature of turbulent boundary layers irrespective of wall roughness condition. In confirmation of Theodorsen's original conjecture, there is now a convincing body of evidence, particularly deriving from recent direct numerical simulation studies, that the dynamics of this bursting process over smooth walls, is directly linked to the presence of powerful vortical structures with a general horseshoe-type configuration, embedded in the wall flow field. The present paper describes the results of physical experiments which demonstrate that these vortex structures are also present in turbulent boundary layers over rough walls and that they are similarly linked to the bursting events. Novel velocity measurement techniques were used in the investigation which, for the first time in physical fluid mechanics research, allowed quasi-instantaneous vortex lines to be traced through a three-dimensional block of flow space to reveal the vortical structures. The preliminary results from a second investigation are also presented which demonstrate that, just as in the smooth wall case, the spanwise flow structure adjacent to a rough boundary exhibits a well defined cross-flow wavelength. The measurements indicate that this wavelength, which reflects the spanwise scale of the near-wall vortex structures, is directly proportional to the size of the boundary roughness elements under fully rough conditions. Consistent with Townsend's 'attached-eddy' hypothesis, the vortical structures are observed to increase in scale with increasing wall distance while remaining attached back to the vorticity generation zone at the boundary. The paper commences with a review of the status of current knowledge relating to coherent vortical structures in turbulent boundary layers and its interpretation in the context of large scale geophysical flows.