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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 v m , the shear rate γ v and the total boundary layer thickness d+δ v ; 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 v 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 v 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.
Acknowledgements
We gratefully acknowledge the Research Grants Council of Hong Kong SAR for support of this work under grant no. CUHK 403003. Tong is supported by the Research Grants Council of Hong Kong SAR under grant no. HKUST 603003.
