Comparison between turbulent boundary layers and channels from direct numerical simulations
Seminar Room 1, Newton Institute
Preliminary results are presented from a new simulation of the zero-pressure-gradient turbulent boundary layer at Re_\theta=1000-2100, which are compared to simulations of turbulent channels at similar Reynolds numbers. Even the low order statistic differ considerably between the two flows, including for some quantities the values within the buffer layer. In general the pressure and the transverse velocity fluctuations are stronger in boundary layers than in channels, even if the wall-parallel scales that can be derived from the spectra and the two-point correlations are simular in both cases. On the other hand, the streamwise fluctuation intensities are roughly similar in both flows, but their scales are shorter and narrower in boundary layers than in channels.
The difference between the two flows is traced to an excess of production of the streamwise turbulent energy in the outer part of the boundary layers compared to channels (by a factor of order two), which is associated with the presence of a stronger wake component of the mean velocity profile. Most of this excess production is compensated by stronger pressure fluctuations and by the pressure-strain term, whicht redistribute the energy to the transverse components. The difference in the statistic can be traced in experimental results at higher Reynolds numbers, although qunatities such as the pressure fluctuations and the energy budgets are not available for them. These results suggest that caution should be used in mixing different flows when document, for example, Reynolds number effects in shear turbulence.