An experimental study of the spread of buoyant water into a rotating environment

Abstract

This thesis examines previously unresolved issues regarding the fluid dynamics of the spread of buoyant water into a rotating environment. We focus in particular on the role that finite potential vorticity and background turbulence play in determining the flow properties.

When water of an anomalous density enters into an oceanic basin, gravity-driven surface flows can be established as a result of the density difference. These flows are often of a sufficiently large scale that the dynamics are affected by the Coriolis force arising from the rotation of the earth. This causes the formation of a large outflow gyre near to the source which feeds into a propagating gravity current that is confined to the coast. Previous experimental work in this field has sought to simplify the problem through the use of a point source and a quiescent ambient. We extend this work to provide a better representation of the real-world flow by introducing a source of finite depth and background turbulence to the rotating ambient.

This study seeks to answer three key questions that are critical to the understanding of the flow behaviour in this scenario. First, what is the effect of the finite potential vorticity of the outflow on the properties of the outflow vortex and the boundary current? Second, what role does the presence of the the outflow vortex play in determining the behaviour of the current? Third, what is the effect of background turbulence on the flow properties? To carry out the investigation, experiments were conducted in the laboratory and compared with a theoretical description of the flow. The currents are generated inside a rotating tank filled with saltwater by the continuous release of buoyant freshwater from a source structure located at the fluid surface. A horizontal source of finite depth is used to introduce finite potential vorticity into the outflow. The impact of background turbulence is examined by introducing an oscillating grid into the rotating tank.

We find that the finite potential vorticity of the outflow plays an important role in determining the flow properties for sufficiently low Rossby and Froude number. As the value of these parameters is increased a zero potential vorticity model is able to capture the key elements of the flow behaviour. The outflow vortex is found to act as a time-varying source to the boundary current, with the current velocity fixed by the vortex velocity field. The vortex vorticity is seen to decrease with time, while the vortex radius continues to increase at late times despite the vortex having reached a limiting depth, which enables potential vorticity to be conserved and the current to be supplied with a non-zero velocity. Finally, the structure of the background turbulence is found to be key in determining the effect that it has on the flow properties, with different behaviours observed for three-dimensional and quasi- two-dimensional turbulence.