Representation of Surface Roughness in Hybrid Turbulence Simulations

The influence of surface roughness on overlying turbulence is represented by a drag layer, with a quadratic drag law. The drag coefficient is considered to be a function of effective sandgrain roughness. The challenge to hybrid modeling is posed as being to calibrate that drag law, such that the calibration be nearly the same for either RANS or LES. To that end the coefficient is first calibrated for LES. With roughness represented by the drag formula, the RANS model and boundary condition are adjusted to provide consistency in the log layer. Then, it is found that RANS and LES calibrations become nearly the same. The model is validated by simulations of a rough wall boundary layer, of rough to smooth junction flow, of a rough ramp, and of streamwise roughness strips.