B. Loubet et al., An evaluation of the wind-tunnel technique for estimating ammonia volatilization from land: Part 2. Influence of the tunnel on transfer processes, J AGR ENG R, 72(1), 1999, pp. 83-92
The wind-tunnel technique is widely used to measure ammonia emissions from
extensive surfaces. However, few studies have been made, within the tunnel,
of the turbulent flow which drives the transfer processes above the experi
mental surface. Experiments have been carried out to determine the transfer
characteristics within the tunnel, and the influence of the external wind
on these characteristics. Two source types have been used, one with constan
t surface emission of CO2, and one with constant surface concentration of w
ater vapour.
Both water vapour emission rate and wind speed measurements show the buildi
ng up of a boundary layer above the experimental surface, which was dramati
cally disturbed by the external wind in the first metre. Further downwind,
the boundary layer stabilizes to a height of about 10 cm, below which the c
haracteristics were equivalent to those of the atmospheric boundary layer,
as shown by CO2 and wind speed measurements. However, above the boundary la
yer, the wind speed in the tunnel was consistently lower than the external
wind speed, showing that if the wind speed at 25 cm is used to drive the tu
nnel flow rate, the transfer rate in the end part of the tunnel would be co
nsistently higher than outside. (C) 1999 Silsoe Research Institute.