Spatial distribution and scaling of bursting events in boundary layer turbulence over smooth and rough surfaces

This study sets out to further investigate the nature of coherent structures in turbulent boundary layers, especially over rough surfaces. A comprehensive review of the existing knowledge of the turbulence generation mechanics including the bursting phenomenon was carried out. The bursting phenomenon involves the lifting and violent ejection of low-speed fluid from the near wall region. This process has been shown to produce the majority of the turbulence kinetic energy present in the flow, and also to dominate mass, momentum and vorticity transport across the shear layer. An experimental investigation was undertaken to evaluate the effect of roughness on the spanwise distribution and scaling of bursting events in boundary layer turbulence. The experiments were carried out in an open channel, where a combination of four flow rates (Reynolds number range of 5000-11000) and five bed conditions, one smooth and four rough (ranging from transitional to fully rough), were used. The experimental method involved the quantitative analysis of instantaneous spanwise velocity distributions obtained using the hydrogen bubble flow visualisation technique. Spatial spectral analysis and visual counting techniques were used to investigate the spatial distribution of bursting events as a function of wall distance and wall roughness. The results 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 vortical structures driving the bursting process, is directly proportional to the roughness length scale, under fully rough conditions. The spanwise wavelength was also observed to increase in scale with increasing wall distance. These results are discussed in comparison with the smooth wall case, and an eddy viscosity scaling parameter is considered with a view to providing a more effective basis for the numerical modelling of rough boundary flow, and assisting in the development of closure models, linking the mean velocity field and the Reynolds stress shear field based on coherent structure concepts