Effective Thickness and Resistance of the Air Boundary Layer Adjacent to Spherical Plant Parts

Abstract

Using existing heat transfer data a rather simple expression is developed for the effective thickness of the boundary layer of air adjacent to spheres. The calculated boundary layer thickness agreed fairly well with that determined for water vapour diffusion from a wet spherical surface 5.90 cm in diameter for wind velocities from 10 to 1000 cm s⁻¹. At a turbulence intensity representative of field conditions (0.5), the measured boundary layer thickness averaged 11 per cent less than for a relatively low turbulence intensity of 0.01. For wind velocities up to 100 cm s⁻¹, the resistance of the boundary layer for water vapour diffusion was actually greater than the resistance of the basidiocarps of Lycoperdon perlatum, L. polymorphum, and Scleroderma australe. The tissue resistance for water vapour movement from the fruiting bodies of other members of the Basidiomycetes was also low (below 10 s cm⁻¹), while the resistance of 20 different spherical fruits ranged from 30 to over 5000 s cm⁻¹. The thickness of the air boundary layer for spheres can depend on the square root of a characteristic dimension divided by the wind velocity, as has already been shown for cylindrical and flat plant material.