Reynolds-number dependence of the structure of a turbulent boundary layer

Conditional averages of longitudinal, normal velocity and temperature fluctuations and of their products have been obtained in a slightly heated boundary layer with zero pressure gradient over a momentum-thickness Reynolds-number range 990 ≤ R m ≤ 7100. These averages are based on the identification of coherent temperature fronts that extend across most of the layer. The average period between fronts is approximately independent of R m when R m is greater than about 1500. The streamwise length scale of the fronts and the magnitude of velocity and temperature derivatives associated with the fronts scale on the thickness of the layer except for R m , less than about 3000. This scaling is consistent with the Reynolds-number independence, for R m greater than about 3000, of both mean and turbulent velocity and temperature fields. Conditional averages are discussed in the context of Head & Bandyopadhyay's (1978) suggestion, based on smoke-flow visualization, that the boundary layer consists almost exclusively of hairpin eddies.