Vortical motion contributions to stress transport in turbulent boundary layers

Abstract

Four-wire hot-wire probe measurements are used to examine the relationships between the motions bearing spanwise vorticity, ωz, and the mechanisms responsible for turbulent stress transport. The measurements were carried out in thick zero pressure gradient turbulent boundary layers over the Reynolds number range, 1010<Rθ<4850. Because of the scale of the flow, the spatial resolution of the probe was, in general, very good. The capabilities of the probe allowed all of the terms in the balance equation for 〈u2〉, except the pressure strain correlation, to be directly measured. Regarding wall region vorticity dynamics, velocity–vorticity correlation measurements are used to investigate ωz motion contributions to the gradients of the turbulent stresses. In particular, the measured positive correlation between v and ωz for y+<15 is shown to be consistent with the large ∂〈−uv〉/∂y gradient in this region, and physically, with the outward motion of sublayer streaks. Relationships between the motions bearing ωz and the sweep and ejection bursting events are examined, and the results are discussed in connection with the turbulent diffusion term in the budget for 〈u2〉. The present results indicate that in the near-wall region (but outside the sublayer) approximately two-thirds of the contributions to the turbulent diffusion term come from the correlation between the uv shear product and ωz fluctuations. The overall results are discussed relative to coherent motion interactions and the known properties of the inner region vorticity field.