Flows Induced by the Impingement of a Two-Dimensional Thermal on a Density Interface

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  • 1 Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona
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Abstract

Laboratory experiments were carried out to investigate the fluid motions resulting from the impingement of a descending thermal on a sharp density interface with the hope of gaining insight into lead-induced convection in polar oceans. Before the impingement, the thermal behaves as in an unstratified fluid and descends as a vortex pair. The evolution after the impingement, however, showed a strong dependence on the Richardson number of the density interface RiRi = lDΔb/wD2, where lD and wD, are the length and velocity scales of the thermal just prior to the impingement and Δb is the buoyancy jump at the interface. When Ri > 10, upon impingement, the thermal splits into two separate vortices without deforming the density interface significantly. Subsequently, a sharp-nosed front propagating on the density interface emerges from each vortex as a consequence of the gravitational collapse. When Ri < 5, the thermal penetrates deep into the density interface and bounces back owing to the baroclinic force. During this rebound, the thermal loses the characteristics of a vortex pair and collapses along the density interface in the form of an intrusive gravity current. The measurements included the depth of penetration of the thermal into the density interface, the propagation velocity of the gravity current, the velocity of the secondary flow induced in the upper layer, and the properties of interfacial waves generated at the density interface.

Abstract

Laboratory experiments were carried out to investigate the fluid motions resulting from the impingement of a descending thermal on a sharp density interface with the hope of gaining insight into lead-induced convection in polar oceans. Before the impingement, the thermal behaves as in an unstratified fluid and descends as a vortex pair. The evolution after the impingement, however, showed a strong dependence on the Richardson number of the density interface RiRi = lDΔb/wD2, where lD and wD, are the length and velocity scales of the thermal just prior to the impingement and Δb is the buoyancy jump at the interface. When Ri > 10, upon impingement, the thermal splits into two separate vortices without deforming the density interface significantly. Subsequently, a sharp-nosed front propagating on the density interface emerges from each vortex as a consequence of the gravitational collapse. When Ri < 5, the thermal penetrates deep into the density interface and bounces back owing to the baroclinic force. During this rebound, the thermal loses the characteristics of a vortex pair and collapses along the density interface in the form of an intrusive gravity current. The measurements included the depth of penetration of the thermal into the density interface, the propagation velocity of the gravity current, the velocity of the secondary flow induced in the upper layer, and the properties of interfacial waves generated at the density interface.

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