NEAR-WALL BEHAVIOR OF SEPARATED AND REATTACHING FLOWS

Two separated and reattaching flows produced by a sudden expansion in a pipe have been studied experimentally. Velocity measurements were made close to the reattachment surface using a new type of pulsed-wire probe. These data show the near-wall flow to be very different from a normal turbulent boundary layer. Mean-velocity profiles do not obey the law of the wall and cannot be correlated outside the linear sublayer using the friction velocity. However, they do contain semilogarithmic regions that appear to form tangents to the linear sublayer profile. A satisfactory description of the backflow mean velocity profile is obtained by incorporating this observation into Simpson's model. The distribution of streamwise turbulence intensity in the near-wall region appears independent of the mean skin friction. It is, however, related to the root-mean-square of skin friction fluctuations expressed as a friction velocity. A simple one-dimensional model of the near-wall flow suggests that the form of the turbulence intensity profile is dependent on the frequency of large-scale velocity fluctuations.