The effect of double diffusion on the dynamics of turbulent plumes and jets
This thesis presents the experimental investigation of the effect of double diffusion on the dynamics of turbulent plumes and jets. Although it consists of seven distinct chapters, the work presented here can be broadly broken down into two parts. The first part addresses the effect of double diffusion on the rate of entrainment in turbulent plumes in the salt-fingering configuration. Plumes over a range of source buoyancy fluxes B0 and source density ratios Rρ are examined experimentally using two experimental methods. The first method is based on the filling-box technique introduced by Baines (1983), which allows the plume volume flux to be measured directly. The second method uses PIV and LIF to obtain simultaneous two-dimensional measurements of the velocity and scalar fields within the flow. The results obtained using both techniques reveal that when the plumes are double-diffusive (Rρ > 0) the rate of turbulent entrainment is not constant, with an up to 20% reduction in the value of the entrainment coefficient from the value found for single-diffusive plumes, i.e. plumes with Rρ = 0. The scale of reduction is found to be in direct relation to the source density ratio and is inversely related to the distance travelled by the plume, indicating that double-diffusive effects decrease as the plume evolves. Ultimately, double-diffusive plumes attain self-similarity and resemble the behaviour of single-diffusive plumes. We propose an explanation for the observed reduction in the entrainment coefficient on the basis of differential diffusion hindering large-scale engulfment at the edge of the plume. For the second part, we investigate the effect of double diffusion on the dynamics of initially neutrally-buoyant warm and salty turbulent jets discharged horizontally into stationary cooler freshwater ambient. Jets over a range of source Reynolds numbers and source temperature/salinity combinations are examined. In all cases, we observed sinking jet trajectories and the formation of salt fingers along the lower surface. Increasing the source concentration of both scalar properties led to more pronounced jet sinking trajectories and earlier formation of salt fingers, demonstrating the significance of the double-diffusive processes. We propose that is it the differential double-diffusive fluxes across the jet-ambient turbulent/non-turbulent interfaces that causes the build-up of negative buoyancy and hence the sinking motion. In addition, we make predictions on the onset point of the salt fingers based on the balance between diffusive processes and the jet entrainment, and compare them with the experimental observations.
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