A wind tunnel study of dense gas dispersion in a stable boundary layer over a rough surface

Measurements of the vertical entrainment velocity into two-dimensional dense gas plumes over fully rough surfaces
were carried out as part of a co-operative research programme with wind tunnel facilities in the USA. This paper presents
results obtained for stable boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion
paper treats the neutral boundary layer case. Mean velocity and temperature, turbulent normal and shear tresses,
temperature #uctuations and heat #uxes were measured and used to demonstrate that a moderately stable atmospheric
boundary layer had been successfully simulated in the tunnel. Entrainment velocities,=, were then deduced from the
streamwise development of the concentration "eld, non-dimensionalised with respect to the friction velocity in the
undisturbed #ow, u*, and correlated with the plume Richardson number, Ri*. Higher non-dimensional entrainment
speeds,=/u*, were observed for Ri*'5 in the stable boundary layer than in the neutral boundary layer, the di!erence
growing with increasing Richardson number. Emission velocity ratios, =/u*, were however larger in the stable
experiments, and exceeded one at about Ri*"18. Entrainment in the stable boundary layer appeared therefore to be
more sensitive to emission velocity ratio than in the neutral case. Entrainment behaviour for Ri*)5 followed that found
in the neutral boundary layer. In this regime, use of the neutral boundary layer entrainment speed correlation is unlikely
to lead to the over-prediction of plume dilution rates in moderately stable boundary layers.