Large structure in a turbulent boundary layer

Abstract

Using an array of hot wires and wall shear stress probes results are presented which support the hypothesis that an organized structure exists in the boundary layer. From the correlation between wall shear stress and velocity it is shown that this structure is at an oblique angle to the free stream. The inferred angle from this correlation is found to agree remarkably with recent flow visualization photographs. Evidence is found that the large scale motion in the organized structure produces a slowly varying component in the wall shear stress and, importantly, a high frequency large amplitude fluctuation occurring near the maximum in the slowly varying wall shear. It is suggested that this latter fluctuation is associated with the bursting phenomenon. A model which accounts for these results is described. It leads to a description of the bursting process in terms of a rotational instability and while too approximate to give more than good order of magnitude estimates, it gives values of the wall streak spacing which are not inconsistent with previous measurements.