Roughness effects in turbulent forced convection

Abstract

We conducted direct numerical simulations of turbulent flow over three-dimensional sinusoidal roughness in a channel. A passive scalar is present in the flow with Prandtl number Pr=0.7 , to study heat transfer by forced convection over this rough surface. The minimal-span channel is used to circumvent the high cost of simulating high-Reynolds-number flows, which enables a range of rough surfaces to be efficiently simulated. The near-wall temperature profile in the minimal-span channel agrees well with that of the conventional full-span channel, indicating that it can be readily used for heat-transfer studies at a much reduced cost compared to conventional direct numerical simulation. As the roughness Reynolds number, k+ , is increased, the Hama roughness function, ΔU+ , increases in the transitionally rough regime before tending towards the fully rough asymptote of κ−1mlog(k+)+C , where C is a constant that depends on the particular roughness geometry and κm≈0.4 is the von Kármán constant. In this fully rough regime, the skin-friction coefficient is constant with bulk Reynolds number, Reb . Meanwhile, the temperature difference between smooth- and rough-wall flows, ΔΘ+ , appears to tend towards a constant value, ΔΘ+FR . This corresponds to the Stanton number (the temperature analogue of the skin-friction coefficient) monotonically decreasing with Reb in the fully rough regime. Using shifted logarithmic velocity and temperature profiles, the heat-transfer law as described by the Stanton number in the fully rough regime can be derived once both the equivalent sand-grain roughness ks/k and the temperature difference ΔΘ+FR are known. In meteorology, this corresponds to the ratio of momentum and heat-transfer roughness lengths, z0m/z0h , being linearly proportional to the inner-normalised momentum roughness length, z+0m , where the constant of proportionality is related to ΔΘ+FR . While Reynolds analogy, or similarity between momentum and heat transfer, breaks down for the bulk skin-friction and heat-transfer coefficients, similar distribution patterns between the heat flux and viscous component of the wall shear stress are observed. Instantaneous visualisations of the temperature field show a thin thermal diffusive sublayer following the roughness geometry in the fully rough regime, resembling the viscous sublayer of a contorted smooth wall.