Boundary Layers of Air Adjacent to Cylinders

Abstract

Using existing heat transfer data, a relatively simple expres- sion was developed for estimating the effective thickness of the boundary layer of air surrounding cylinders. For wind velocities from 10 to 1000 cm/second, the calculated boundary- layer thickness agreed with that determined for water vapor diffusion from a moistened cylindrical surface 2 cm in diam- eter. It correctly predicted the resistance for water vapor move- ment across the boundary layers adjacent to the (cylindrical) inflorescence stems of Xanthorrhoea australis R. Br. and Scirpus validus Vahl and the leaves of Allium cepa L. The boundary-layer thickness decreased as the turbulence intensity increased. For a turbulence intensity representative of field conditions (0.5) and for vwindd between 200 and 30,000 cm2/ second (where V,ind is the mean wind velocity and d is the cylinder diameter), the effective boundary-layer thickness in centimeters was equal to 0.58 N/(/,/ wnd