The propagation of gravity fronts of high density ratios has been studied experimentally (exchange flow) and by computer simulation. Non-Boussinesq fronts are known to occur in certain safety problems (chemical spills and fires), and we have investigated seven gas combinations giving density ratios from near unity to well over 20. The results are presented in terms of a density parameter ρ* which remains finite both in the weak (ρ* = 0) and the strong (ρ* = 1) limit. The front velocities, measured by means of hot wires, are found to fall on two distinct curves, one for the slower lightgas fronts and one for the faster heavy-gas fronts. Two fractional depths, Φ = ½ (lock exchange) and Φ = ⅙, have been investigated in detail and results for the interesting case Φ → 0 have been obtained by extrapolation. To aid in the extrapolation and for comparison, all experimental (and some intermediate) cases have been simulated by means of a general purpose CFD-code (PHOENICS). Good agreement is found for cases without convergence problems, i.e. for heavy-gas fronts of density ratio less than 5. Further information on frontal shape etc. has been obtained from visualization. The extrapolations to infinite depth indicate a limiting speed for both the heavy- and light-gas fronts close to the values predicted from shallow-layer theory for the analogous dam-break problem.

Low-momentum releases of buoyant material from area sources are investigated in the context of the model of Morton, Taylor & Turner (1956) (MTT). The general solution of the model equations is shown to include the case of a converging–diverging flow which we have used to model plumes from the area sources. These solutions have as asymptotes certain power solutions in terms of height, i.e. the MTT solution at large heights and a non-entraining solution near the source. The new solutions exhibit distinct and interesting flow features such as a neck (point of minimum cross-section) and a velocity peak somewhat above. The new results have been compared with the few known experimental data sources and reasonable agreement is demonstrated. In the process we have also examined the importance of the Boussinesq approximation and find results recently published not to be valid except in one special case.

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The flow structure associated with a line bubble plume in shallow water is investigated. G. I. Taylor has proposed the use of such plumes as wavebreakers. To be effective the surface current generated should be stable for a distance of the order of the wavelength, which in turn could be several times the depth. It appears that the formation of recirculating cells can affect the wavebreaking potential of line bubble plumes. The paper presents observations and measurements of the cell structure associated with line bubble plumes as obtained in a model towing basin of dimensions 1 × 1 × 40 m. A recirculating region was found on both sides of the plume, but the secondary (and higher-order) cell structures proposed by other investigators were not observed. The primary cells were found to be appreciably longer than those reported in the literature for vertical plane jets. The difference can in part be attributed to different definitions of cell length. The definitions used herein are based on observables, both on the water surface and in the interior of the flow, and they lead to consistent measures of length. Bubble-plume parameters (such as entrainment coefficient) are known to depend on gas flow rate, and it was found that the length of the primary cell is a weak function of this variable as well. Additional experiments with a vertical plane jet were conducted for comparison. Longer cells than those previously reported were again observed. The paper contains a complete theory for line bubble plumes, including the effects of compressibility, bubble slip and finite release volume, as well as a simplified similarity analysis useful in estimating plume properties and horizontal-current depth and velocity.