A wind tunnel study of dense gas dispersion in a neutral 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 neutral boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion paper treats the stable boundary layer case. Entrainment velocities, W, were 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*. Results for Richardson numbers in the range Ri*(15 were found to be well "tted by the empirical expression: W/u*"0.65/(1#0.2Ri*). Flow visualisation studies showed layered plume structures with a sharp upper interface at higher Richardson numbers and in this regime turbulent motion below the interface became progressively more intermittent as Ri* increased. Measured turbulence levels collapsed within such high Richardson number plumes and #ow and dispersion were signi"cantly a!ected by molecular processes. Up-welling above the source was observed when the emission speed exceeded the approach #ow friction velocity, though there was no clear evidence that this a!ected plume behaviour away from the immediate vicinity of the source.