SPANWISE STRUCTURE IN THE NEAR-WALL REGION OF A TURBULENT BOUNDARY-LAYER

The behaviour of the streamwise velocity μ in the near-wall region of a turbulent boundary layer is obtained by analysing the data from an array of hot wires aligned in the spanwise direction. Conventional and conditional statistics are presented, relative to the occurrence of bursts and sweeps detected using a modified μ-level method. Sweeps have an average streamwise length which is twice as large as that of bursts while the average spanwise extent of sweeps is about 25% larger than that of bursts. Both instantaneous and conditionally averaged information is presented and discussed in the context of bursts and sweeps in the (x, z)-plane. Dependence on y+ is significant, and important differences are observed between instantaneous and conditionally averaged results. Conventional and conditional statistics of the velocity derivatives ∂μ/∂x and ∂μ/∂z provide some insight into the anisotropy of the mean-square velocity derivatives in the near-wall region. Conditionally averaged patterns of μ compare favourably with the numerical simulations of Kim (1985) in the near-wall region of a turbulent channel flow, at a comparable Reynolds number. © 1990, Cambridge University Press. All rights reserved.