COMMON VORTICAL STRUCTURE OF TURBULENT FLOWS OVER SMOOTH AND ROUGH BOUNDARIES

The bursting phenomenon is a common feature of turbulent boundary laye rs irrespective of the wall roughness condition. Consistent with Theod orsen's conjecture, recent direct numerical simulation studies have sh own that the dynamics of this bursting process over smooth walls is di rectly linked to the presence of Powerful vortical structures with a g eneral horseshoe-type configuration. The present paper describes the r esults of physical experiments that demonstrate that these vortex stru ctures are also present in turbulent boundary layers over rough walls. They appear to form the central element in a recurring, highly nonlin ear cycle of turbulence and burst generating instability. Novel veloci ty measurement techniques were used in the investigation that, for the first time in physical experimental fluid mechanics research, allowed quasi-instantaneous vortex lines to be traced through a three-dimensi onal block of flow space to reveal the vortical structures embedded in the shear flowfield. Preliminary test results are also presented that indicate that, as in the smooth wall case, the instability structures over rough boundaries have a preferred spanwise wavelength that scale s with the roughness dimension.