Revisiting Nieuwstadt's collapsed boundary layer

This work investigates the collapse of turbulence in stably stratified boundary layers. It partly revisits the work of Nieuwstadt (2005), who studied a stably stratified, smooth channel flow by means of direct numerical simulation (DNS) of the Navier-Stokes equations. Besides DNS, the present work also studies the potential of a local-similarity model with respect to simulation of the collapse phenomenon. The flow has a typical Reynolds number in the order ~7000

It is shown that, with respect to the mean variables and the fluxes, the analogy between the local-similarity model and the DNS-model is close up to the point of collapse. This is rather surprising in view of the fact that a low Renolds number flow is simulated with a local similarity model that originates from high-Re number atmospheric observations (see abstract by Moene et al.).

However, after the collapse both models behave differently, even in a qualitative sense:

The local similarity model shows a clear recovery of turbulence, whereas the DNS remains in collapsed stated. The difference is partly explained by the fact that Reynolds similarity does not hold any longer after the collapse (viscous effects start to dominate) and this discrepancy will be studied in the future.

The ultimate goal is to understand and predict collapse of turbulence in the atmosphere in terms of large-scale forcing parameters. The potential of this philosophy will be illustrated with the help of observations from the KNMI-CABAUW site that give a new perspective on this issue.