Turbulent characteristics of Internal Boundary Layers

The nature of turbulence within an internal boundary layer (IBL) is examined using a large-eddy simulation (LES) model, which explicitly resolves the three-dimensional turbulent eddies rather than parameterizing their effect, using a nested grid technique to better resolve eddies very close to the surface. Two idealized IBLs are simulated by changing surface roughnesses within a neutral atmosphere, to produce a rough-to-smooth transition and a smooth-to-rough transition. IBL generation has been traditionally conceived as resulting from differing natures of turbulence produced by differing surfaces. The present investigation supports this by exploiting the capability of LES modelling to obtain very long time averages under stationary conditions, so as to investigate turbulence quality in addition to turbulence magnitude. Turbulence quality, identified by skewness and kurtosis values, is shown to vary over the differing surfaces and the IBL interface can be identified through differences in those qualities. However, it is also demonstrated that the IBL growth is not necessarily controlled by the local underlying surface but rather by the rougher of the two surfaces, which may be the upwind surface. Turbulent mixing coefficients (Ks) will be presented and compared to those values expected over a horizontally homogeneous surface. The importance of pressure gradient forces is also examined: while immediately downwind of the surface interface turbulent stress divergence is balanced by advection, as is usually assumed in simplified IBL models, further downwind the stress divergence is instead balanced by pressure induced accelerations.