THE PROPAGATION OF INTRUSION FRONTS OF HIGH-DENSITY RATIOS

The propagation of gravity fronts of high density ratios has been stud ied experimentally (exchange flow) and by computer simulation. Non-Bou ssinesq fronts are known to occur in certain safety problems (chemical spills and fires), and we have investigated seven gas combinations gi ving density ratios from near unity to well over 20. The results are p resented in terms of a density parameter rho which remains finite bot h in the weak (rho = 0) and the strong (rho* = 1) limit. The front ve locities, measured by means of hot wires, are found to fall on two dis tinct curves, one for the slower light-gas fronts and one for the fast er heavy-gas fronts. Two fractional depths, PHI = 1/2 (lock exchange) and PHI = 1/6, have been investigated in detail and results for the in teresting case PHI --> 0 have been obtained by extrapolation. To aid i n the extrapolation and for comparison, all experimental (and some int ermediate) cases have been simulated by means of a general purpose CFD -code (PHOENICS). Good agreement is found for cases without convergenc e problems, i.e. for heavy-gas fronts of density ratio less than 5. Fu rther information on frontal shape etc. has been obtained from visuali zation. The extrapolations to infinite depth indicate a limiting speed for both the heavy- and light-gas fronts close to the values predicte d from shallow-layer theory for the analogous dam-break problem.