IMPACTS OF LATERAL SPREADING AND UPSTREAM CONDITIONS ON BUOYANT RIVER PLUMES: MIXING, STRUCTURE AND PLUME DYNAMICS
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This dissertation investigates the nature of physical processes associated with buoyant river plumes under the impacts of rotation, lateral spreading and upstream conditions. Two sets of laboratory experiments, one focused on the whole rotating buoyant plume, and the other zoomed into the near field plume, are studied. When lighter fluid is released into denser water, buoyancy causes the lighter intruding flow to run on top of the ambient water. This baroclinic flow propagates both in the offshore direction due to strong momentum from the inflow source and laterally in the alongshore direction because of the horizontal pressure gradient. This energetic region is the so-called near-field region where the plume behaves like a buoyant jet and is characterized by high momentum and strong stratification. Under the influence of the Coriolis force, the density driven flow is then guided along the coast, forming an anti-cyclonic bulge (in northern hemisphere). In this large scale region, called far-field region, the flow is more geophysical and less energetic. The transition between the two regions is the mid-field region in which the fluid transfers from an energetic flow into a geophysical current. The first laboratory experiment is designed to simulate the dynamics of two ad- jacent coastal river plumes in a rotating reference frame. The plumes are generated on a rotating table using two identical fresh water inlets, with blue and red dye indicating upstream and downstream river flows, respectively. We successfully calculate the depth field for the combined two-plume system and differentiate between the two plumes using a two-dimensional calibration map. With the upstream coastal current acting as the ambient condition, the downstream plume bulge does not reach a steady condition. The downstream bulge is pulled into the upstream bulge, forming a larger re-circulating bulge which then becomes unstable. The coastal current transport can be calculated by assuming a geostrophic cross-balance balance with an empirical coefficient &alpha = 0.6. The impact of lateral spreading on mixing was investigated for laboratory scale non-rotating stratified-shear plumes. The experiment is begun with the release of a vertical wall of freshwater and the simultaneous activation of a pump which supplies freshwater to the filling basin. Velocity and density fields are obtained using the combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) method, while the lateral spreading rate is determined using the optical thickness method (OTM). The vertical mixing is parameterized by the turbulent buoyancy flux, which is calculated using a control volume approach. Both the lateral spreading rate and mixing are related to inflow Fr, where the lateral spreading rate decreases with Fr<sub>i</sub> while mixing increases with Fr<sub>i</sub>. By comparing the mixing in the laterally confined and unconfined cases, we observe that although the lateral spreading significantly modifies the plume vertical structure, it does not change the local turbulent buoyancy flux. On the other hand, the lateral spreading increases the horizontal area over which mixing occurs and as a result it increases the net dilution of river water at a fixed distance from the river mouth. Unlike the local mixing process, we observe that the plume structure is signif- icantly different in the laterally confined and unconfined plumes. We hypothesize that the laterally spreading river plume might be a source of non-linear internal solitary waves with trapped cores. Such waves are commonly observed in fjords, straits and coastal ocean, where a strong shear-stratified flow meets dramatical topography changes. A series of small scale Kelvin-Helmholtz instability billows are generated along the edge of the large-scale waves, propagating downstream and finally breaking at the wave trough. This phenomenon highly increases the mixing and entrainment at the edge and trailing edges of the wave while it inhibits the mixing at the frontal side of the wave.
- Civil engineering