Windtunnel modelling of a release of a heavy gas near a building

A heavy gas cloud from a hemispherical continuous release is investigated experimentally. The cloud generated is a bifurcated one with strong concentration gradients towards the centre line as well as the edges of the cloud. The shape of the cloud is documented by measurements of velocities and concentrations. Models of buildings are introduced into the cloud and the cloud is shown to be strongly modified by interacting with the model. The horseshoe vortex system found at the base of an obstacle in a shear flow is shown to reduce the concentration level on the walls to a very low level, below 2% by volume for most of the tests. It is demonstrated that this effect depends on the ratio of cloud to model height.