The propagation of a gravity current into a linearly stratified fluid

The constant initial speed of propagation (V) of heavy gravity currents, of density PC, released from behind a lock and along the bottom boundary of a tank containing a linearly stratified fluid has been measured experimentally and calculated numerically. The density difference, bottom to top, of the stratification is (rho(b) - rho(0)) and its intrinsic frequency is N. For a given ratio of the depth of released fluid (h) to total depth (H) it has been found that the dimensionless internal Froude number, Fr = V/NH, is independent of the length of the lock and is a logarithmic function of a parameter R = (rho(C) - rho(0))/(rho(b) - rho(0)), except at small values of h/H and R close to unity. This parameter, R, is one possible measure of the relative strength of the current (rho(C) - rho(0)) and stratification (rho(b)-rho(0)). The distance propagated by the current before this constant velocity regime ended (X-tr), scaled by h, has been found to be a unique function of Fr for all states tested. After this phase of the motion, for subcritical values of Fr, i.e. less than 1/pi, internal wave interactions with the current resulted in an oscillation of the velocity of its leading edge. For supercritical values, velocity decay was monotonic for the geometries tested. A two-dimensional numerical model incorporating a no-slip bottom boundary condition has been found to agree with the experimental velocity magnitudes to within +/-1.5%.