A study of the spanwise structure of coherent eddies in the viscous wall region

Publisher Website
Google Scholar
Abstract
Simultaneous measurements of the longitudinal and spanwise components sx and sz of the fluctuating velocity gradient at the wall of a pipe and the fluctuating velocity u and w at various distances from the wall have been analysed in order to provide information on the origin of flow-oriented wall eddies. Special conditional-averaging techniques, which use multiprobe measurements, are developed to capture the wall structures as they evolve in both space and time. It is found that wall eddies of wavelength λ+ ≈ 100, detected from measured patterns of sz(z), are closely related to spanwise variations of the velocity at y+ = 20 and 40. Analysis of measurements from probes arranged in the direction of the mean flow supports the notion that the wall structures result from the propagation of spanwise disturbances of the velocity at some distance from the wall, both downstream and towards the wall. A flow model, different from vortex models hypothesized by previous investigators, is proposed to account for the relation of flow phenomena in the viscous wall region to the low-speed wall streaks.