Heat and moisture transfer from flat surfaces in intermittent flow: a laboratory study
1980
Schuepp, P.H. (McGill Univ., Ste Anne-de-Bellevue, Quebec (Canada). MacDonald Campus, Dept. of Agricultural Chemistry and Physics)
Air flow near the ground is often intermittent (gusty), particularly in and around vegetation, but steady state models are frequently used to described heat transfer and evaporation, with time-averaged horizontal wind substituted for steady state flow velocity. Measurements of heat and moisture transfer in intermittent flow were carried out in the laboratory, with rectangular velocity pulses of variable pulse height, duration and spacing, and the results compared with the transfer obtained in steady state flow. The intermittent heat transfer recorded was up to about 30 per cent higher than steady state transfer with the same mean velocity but it appears that a quantitative description of intermittent moisture transfer by the mean horizontal wind would be completely inadequate with underestimation of up to 100 per cent or more at very low mean winds. Mean horizontal velocity ceases to be a meaningful parameter for the quantitative description of evaporation, particularly in the presence of turbulence-producing barriers, and some evidence is presented that barriers, while reducing mean wind, do not necessarily reduce evaporation.
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