Radial intrusions from turbulent plumes in uniform stratification

Laboratory experiments investigate the radial spread of an intrusion created by a turbulent forced plume in uniformly stratified ambient fluid. The flow evolution is determined as it depends upon the ambient buoyancy frequency, N, and the source momentum and buoyancy fluxes, M-0 and F-0, respectively. The plume reaches its maximum vertical extent, Z(m), collapses back upon itself as a fountain and then spreads radially outwards at its neutral buoyancy depth, Z(s), where the intrusion has the same density as the ambient. Through theory and experiments we determine that Z(s) = f(sigma) H-p, in which H-p = (M0F0-1/2)-F-3/4, sigma = (M0N/F-0)(2), and f(sigma) alpha sigma(-3/8) for sigma less than or similar to 50 and f(sigma) alpha sigma(-1/4) for sigma greater than or similar to 50. In the inertia-buoyancy regime the intrusion front advances in time approximately as R-f alpha t(3/4), consistent with models assuming a constant buoyancy flux into the intrusion. Where the intrusion first forms, at radius R-1, its thickness h(1) is approximately constant in time. The thickness of the intrusion as a whole, h(r, t), adopts a self-similar shape of the form h/h(1) similar or equal to [(R-f -r)/(R-f -R-1)](p), with p similar or equal to 0.55 +/- 0.03. The comparison of these results to large volcanic plumes penetrating into and spreading in the stratosphere is discussed. (C) 2014 AIP Publishing LLC.