A CRITICAL ASSESSMENT OF THE APPLICATION OF WALL FUNCTIONS TO THE PREDICTION OF TURBULENT FLOW OVER ROUGH SURFACES USING k―ε MODELS

摘要

In this paper, the application of wall functions to the prediction of turbulent flow over rough surfaces with k―εmodels has been critically assessed. Wall functions, as a near-wall treatment, were used in the simulation of fully developed turbulent pipe flow and a turbulent boundary layer for hydraulically smooth and fully rough flow with sandgrain roughness. In addition, experimental studies were undertaken for a (Re_θ = 4990) zero pressure gradient turbulent boundary layer over a smooth surface to validate some of the numerical results. On a semi-log plot using inner coordinates, roughness shifts the mean velocity profile downward and to the right. The wall function formulation for rough surface predicts a significantly lower value for the friction factor in pipe flow, and a lower skin friction coefficient in zero pressure gradient turbulent boundary layer. In addition, the formulation fails to accurately predict the effect of roughness on the turbulence kinetic energy in the outer region of the flow. The results presented in this paper suggest that use of a wall function formulation is feasible for rough surfaces. However, it is time for a more careful calibration for both transitional and fully rough flows by the CFD community.