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 dens e gas plumes over fully rough surfaces were carried out as part of a co-ope rative research programme with wind tunnel facilities in the USA. This pape r presents results obtained for neutral boundary layer conditions in the En Flo wind tunnel at the University of Surrey; a companion paper treats the s table boundary layer case. Entrainment velocities, W-E, were deduced from t he streamwise development of the concentration field, non-dimensionalised w ith respect to the friction velocity in the undisturbed flow, u*, and corre lated with the plume Richardson number, Ri*. Results for Richardson numbers in the range Ri* < 15 were found to be well fitted by the empirical expres sion: W-E/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 progressive ly more intermittent as Ri* increased. Measured turbulence levels collapsed within such high Richardson number plumes and flow and dispersion were sig nificantly affected by molecular processes. Up-welling above the source was observed when the emission speed exceeded the approach flow friction veloc ity, though there was no clear evidence that this affected plume behaviour away from the immediate vicinity of the source. (C) 2001 Published by Elsev ier Science Ltd.