Direct numerical simulation of momentum and scalar internal boundary layers

Abstract

Direct numerical simulations (DNS) of turbulent open channel flow are performed to study and compare momentum and scalar internal boundary layers (IBL). An internal boundary layer (IBL) forms when a turbulent boundary layer is subjected to heterogeneous surface conditions. For example, a momentum IBL forms with a change in surface roughness and a scalar IBL forms due to a localised heat flux. Momentum IBLs are simulated using both a streamwise-varying roughness body forcing technique and a streamwise-varying wall shear stress. Passive scalar IBLs are simulated using both streamwise-varying scalar wall values and wall fluxes, for a homogeneous smooth wall. The roughness body forcing model is shown to reproduce the same IBL features as with the explicitly resolved roughness study of Rouhi et al. (2019), but at a reduced cost. Three common IBL detection methods are employed to estimate the power-law exponents in the growth rates of the IBL. The exponents for the momentum IBL cases are typically less than 0.7 for these methods, smaller than for the scalar-analogues of these methods. Furthermore, we analyse the streamwise variation of the root-mean-square wall-normal (vertical) velocity. We find that the ‘diffusion analogy’, or that momentum and scalar IBLs behave similarly, may not be an appropriate assumption, especially for rough-to-smooth transitions for momentum IBLs.