Axisymmetric gravity currents in a rotating system: experimental and numerical investigations

The propagation at high Reynolds number of a heavy, axisymmetric gravity cu rrent of given initial volume over a horizontal boundary is considered in b oth rotating and non-rotating situations. The investigation combines experi ments with theoretical predictions by both shallow-water approximations and numerical solutions of the full axisymmetric equations. Attention is focus ed on cases when the initial ratio of Coriolis to inertia forces is small. The experiments were performed by quickly releasing a known cylindrical vol ume of dense salt water of 2 m diameter at the centre of a circular tank of diameter 13 m containing fresh ambient water of typical depth 80 cm. The p ropagation of the current was recorded for different initial values of the salt concentration, the volume of released fluid, the ratio of the initial height of the current to the ambient depth, and the rate of rotation. A maj or feature of the rotating currents was the attainment of a maximum radius of propagation. Thereafter a contraction-relaxation motion of the body of f luid and a regular series of outwardly propagating pulses was observed. The frequency of these pulses is slightly higher than inertial, and the amplit ude is of the order of magnitude of half the maximum radius. Theoretical pr edictions of the corresponding gravity currents were also obtained by (i) p reviously developed shallow-water approximations (Ungarish & Huppert 1998) and (ii) a specially developed finite-difference code based on the full axi symmetric Navier-Stokes equations. The 'numerical experiments' provided by this code are needed to capture details of the flow field (such as the non- smooth shape of the interface, the vertical dependence of the velocity fiel d) which are not reproduced by the shallow-water model and are very difficu lt for, or outside the range of, accurate experimental measurement. The com parisons and discussion provide insight into the flow field and indicate th e advantages and limitations of the verified simulation tools.