Transport of gases to and from surfaces with bluff and wave‐like roughness elements

Wind tunnel experiments have been done on the transport of a radio‐active vapour of thorium B, and water vapour, to and from surfaces with spherical, cylindrical and wave‐like roughness elements. The results are analysed in terms of the dimensionless number B of Owen and Thomson.For values of the roughness parameter u_* z₀ v⁻¹ between 10 and 1,000, the results were found to conform approximately to Owen and Thomson's empirical formula \documentclass{article}\pagestyle{empty}$ \[B^{ - 1} = \alpha \left({\frac{{u_* h}}{\nu }} \right)^m \left({\frac{\nu }{D}} \right)^n \] $ Where h (= 30 z₀) is the equivalent sand roughness, D the diffusivity of the vapour, and α, m and n are constants with values 0·5, 0·45 and 0·8 respectively. Experiments with aerodynamically smooth surfaces give results in accordance with von Kármán's equation.Wind profiles over large bodies of water imply rather small values of the roughness length and u_* z₀ v⁻¹ generally lies in the range 0·1 to 10. In this region, the bluff‐body effect on momentum transport is small, and is counteracted by the fact that D for water vapour is greater than v. As a result, B⁻¹ is small, and the resistances to transport of momentum and water vapour are nearly the same. Experiments on the rate of evaporation of water in a tank in the wind tunnel give values of the bulk evaporation coefficient, referred to a height 5 cm above the surface, in good agreement with theory.