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.