Transient evolution and backlayering of buoyancy-driven contaminants in a narrow inclined space

The transport of buoyancy-driven contaminants in narrow inclined spaces, for example underground tunnels, is frequently seen in engineering. We carry out a series of experiments to investigate the transient evolution of buoyancy-driven flow in a narrow inclined tank. The negative buoyancy is introduced by releasing brine through a localized source located at the ceiling of the tank which is immersed in fresh water. We adopt a light attenuation technique to measure the transient distribution of the reduced gravity in the tank. In each of the experiments, the reduced gravity of the downstream current is stratified in the transverse direction of the tank but almost keeps constant in the longitudinal direction. A model is established to calculate the average reduced gravity at the downstream side of the buoyant source after the flow reaches steady state. The reduced gravity of the backlayering flow is significantly larger than that of the downward current. A simple model is proposed to estimate the maximum transient backlayering length. Moreover, the non-dimensional time tb,max/H/g0'sinθ for the transient backlayering flow to reach its maximum length is determined. The propagation velocity of the front of the current depends on the buoyancy flux of the front. The buoyancy-driven contaminates propagate downstream with constant velocity in the steady state.