The Boundary Layer over aPopulusLeaf

Abstract

Air flow over a Populus leaf was investigated using a hot-wire anemometer. When the air flow in the wind tunnel was laminar, the boundary layer was often turbulent at a wind speed of only 1.5 m s⁻¹, particularly when encouraged by uneven topography and roughness of the surface, as on the lower side of the leaf. The smoother upper surface behaved in a similar way to a flat plate when at low wind speeds, and the profiles of wind speed could be shown to be equivalent to those expected from laminar boundary layer theory. Nevertheless, the boundary layer became turbulent at Reynolds numbers much lower than those required to cause the transition to turbulence in a flat plate. Turbulent air flow in the wind tunnel greatly increased boundary layer turbulence but had only a small effect on evaporation from a model of the leaf. The evaporation rates observed were 2.5 times higher than expected from theory, irrespective of the turbulence regime.