Characterization of Flow Structures at the Front of Cylindrical Gravity Current Fronts

CANTERO, M.I.; GARCÍA, C.M.; GARCÍA M.H.; BALACHANDAR, S.

Santa Fe. Argentina

Congreso; ENIEF 2006. XV Congreso sobre Métodos Numéricos y sus Aplicaciones; 2006

Asociación Argentina de Mecánica Computacional

Three dimensional direct numerical simulations are presented for cylindrical density currentsusing 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 10^5, 1:5 10^6 and 10^7, respectively) in order to identify differences in the flow 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 fluids rolls up forming Kelvin-Helmholtz vortices. The formation of the first 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 smallscale 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 flow 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 flow spreads, while the local Reynolds number of the flow decreases. Due to the high resolution of the simulations, we have been able to link the lobe and cleft structure to local flow patterns and vortical structures. In the near front region and body of the current several hairpin vortices populate the flow. Laboratory experiments have been performed at the higher Reynolds numbers and the results have been compared to the simulation results. The agreementhas been documented to be very good.