On the origin of streamwise vortices in a turbulent boundary layer

Abstract

Several experiments have suggested that streamwise vortices, with their accompany-ing low-momentum streaks in a turbulent boundary layer have a characteristic spanwise wavelength of approximately λ z + = 100. Here a mechanism is proposed which selects a comparable spanwise wavelength and produces counter-rotating streamwise vortices in a turbulent boundary layer. Examining the equations which describe the deviation of the velocity field from its time-average, it is found that a resonance (Benney & Gustavsson 1981), is associated with the mean-velocity profile. As an integral part of this resonance, there is a mean secondary flow which has a spanwise wavelength λ z + = 90 and whose velocities exhibit a streamwise vortex structure similar to those observed.