其他摘要：Three dimensional direct numerical simulations are presented for cylindrical density currents using the Boussinesq approximation for small density difference. Three Reynolds numbers (Re) are investigated (895, 3450 and 8950, this particular choice corresponds to values of Grashof number of 105, 1:5 _ 106 and 107, respectively) in order to identify differences in the _ow structure and dynamics, and to compare with planar density currents. The simulations are performed using a fully de-aliased pseudospectral method. The simulated _ows present the main features observed in experiments for the large Re. As the current develops, it transitions through different phases of spreading, namely acceleration, slumping, inertial and viscous. Soon after release the interface between light and heavy _uids rolls up forming Kelvin-Helmholtz vortices. The formation of the _rst vortex sets the transition between acceleration and slumping phases. Vortex formation continues only during the slumping phase. The coherent Kelvin-Helmholtz vortices undergo azimuthal instabilities and eventually breakdown into small scale turbulence. In the case of planar currents this turbulent region extends over the entire body of the current, while in the cylindrical case it only extends to the near-front region. The _ow develops threedimensionality right from the initial acceleration phase. During this phase, incipient lobes and clefts start to form at the lower frontal region. These instabilities grow in size and extend to the upper part of the front. Lobes and clefts continuously merge and split and, thus result in a complex pattern that dynamically evolves. The wavelength of the lobes grows as the _ow spreads, while the local Reynolds number of the _ow decreases. Due to the high resolution of the simulations, we have been able to link the lobe and cleft structure to local _ow patterns and vortical structures. In the near front region and body of the current several hairpin vortices populate the _ow. Laboratory experiments have been performed at the higher Reynolds numbers and the results have been compared to the simulation results. The agreement has been documented to be very good.
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