Boundary layer turbulence in transitional and developed states

Using the recent DNS by Wu and Moin (2010) of a flat-plate boundary layer with a passively heated wall, statistics of the turbulence in transition at Reθ = 500 where spots merge (distributions of the mean velocity, rms velocity and vorticity component fluctuations, kinetic energy production and dissipation rates and enstrophy) have been compared to these statistics for the developed boundary layer turbulence at Reθ = 1850. When the distributions in the transitional region, determined in narrow planes 0.03Reθ wide, exclude regions and times when the flow is not turbulent, they closely resemble the distributions in the developed turbulent state at the higher Reynolds number, especially in the buffer and sublayers. The skin friction coefficient, determined in this conditional manner in the transitional flow is, of course, much larger than that obtained by including both turbulent and non-turbulent information there, and is consistent with a value obtained by extrapolating from the developed turbulent region. Individual hairpin vortices have been identified using vorticity lines in the transitional and developed turbulence, and they have quite similar characteristics in both cases. Some of these vortices appear to emerge out of sheets of unorganized vorticity in the viscous sublayer. Hairpin vortices are closely associated with the processes that transport momentum and heat within the boundary layer. An octant analysis based on the combinations of signs of the velocity and temperature fluctuations, u, v and θ, shows that this transport is predominantly of the mean gradient type over much of both the transitional and developed flows. The results add strong evidence to the view that there is little difference between the structure and transport processes of a developed turbulent boundary layer and of turbulent spots that appear in transition.