Direct Numerical Simulations and Experiments of Turbulence Over Regular Roughness From the Transitionally Rough to the Fully Rough Regime

Abstract

We present results of experiments and fully resolved, direct numerical simulations (DNSs) on turbulent flows over regular arrays of cylindrical roughness elements. The experimental campaign was conducted in the Virginia Tech Stability Wind Tunnel for K+ ≈ 225 − 400 at ReT ≈ 7110 − 16700, where the ‘+’ superscript represents wall-unit scaling with kinematic viscosity and the friction velocity. DNSs were conducted for K+ = 5, 10, 15, and 20 at ReT ≈ 190 and for K+ = 20 at ReT ≈ 380. Data from experiments and DNSs are presented and discussed for the roughness function, equivalent sand-gain roughness, mean flow velocity profiles, turbulence statistics, and spectral energy densities. Experimental results suggest the method used to estimate the equivalent sand-grain roughness from mean velocity profiles is sensitive to the selection of the logarithmic region considered in the regression fit, highlighting the need for an improved analytical method to estimate roughness parameters from the mean velocity profiles. DNS results suggest there is a progressive departure from smooth-wall-like turbulence for all cases except the smallest roughness size investigated. We hypothesise the differences are related to the nonlinear interaction of the texture-coherent flow with the background turbulence and plan to assess the importance of this mechanism in future work.