Spanwise structure in the near-wall region of a turbulent boundary layer

Abstract

The behaviour of the stream wise velocity u 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 span wise direction. Conventional and conditional statistics are presented, relative to the occurrence of bursts and sweeps detected using a modified u-level method. Sweeps have an average stream wise length which is twice as large as that of bursts while the average span wise 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 ∂u/∂x and ∂u/∂z provide some insight into the anisotropy of the mean-square velocity derivatives in the near-wall region. Conditionally averaged patterns of u compare favourably with the numerical simulations of Kim (1985) in the near-wall region of a turbulent channel flow, at a comparable Reynolds number.