Experimental study of velocity boundary layer near a rough conducting surface in turbulent natural convection

Abstract

Dual-beam intensity cross-correlation spectroscopy is used to measure the viscous boundary layer over a rough conducting surface in turbulent convection in water. It is found that the velocity boundary layer near the rough surface can be characterized by three quantities: the maximum velocity ν_m, the shear rate γ_ν and the total boundary layer thickness d + δ_ν; all of them obey power laws of Rayleigh number Ra. The maximum velocity v_m is located outside the rough surface and its numerical value remains the same as that in the smooth cell. The Ra-dependence of d + δ_ν is found to be the same as that in the smooth cell, but the power-law amplitude is increased by a factor close to 2. Because of the formation of small eddies inside the roughening grooves, the measured γ_ν is reduced considerably in the groove region. It is also found that the Nusselt number Nu ∼ Ra^0.35, which is larger in both magnitude and scaling exponent as compared to the smooth case.