Laterally unconfined neutral or negative buoyancy inflow into a quiescent ambient over sloping bottom

This work studies the nearshore hydrodynamics of a shallow turbulent flow entering a laterally unconfined quiescent ambient with a sloping bottom boundary. Examples of such flow are neutrally buoyant ebb tidal jets and hyperpycnal river plumes entering open waters.Laboratory experiments were applied in a shallow open channel connected to a sloping bottom tank. A three-dimensional (3D) CFD model based on large eddy simulation is established and validated using the experimental data. The validated model not only provides detailed 3D flow field information but also extends the experimentally studied range of initial and boundary conditions. With neutral buoyancy, turbulent inflow is classified as a tidal jet, whose transverse velocity profile gradually transforms from a top-hat profile into a Gaussian distribution. Bounded by a sloping boundary, the jet extends vertically, resulting in an abrupt decrease in centerline velocity near the channel mouth. Under the combined effects of the vertical extension and lateral entrainment, the jet undergoes lateral contraction before spreading laterally. Shear layers are generated at both sides of the jet, converging towards the centerline with increasing offshore distance. The momentum thickness of the shear layers increases longitudinally until the shear layers from both sides meet and merge at the centerline. Kelvin Helmholtz-type coherent structures (KHCS) develop inside the shear layers, contributing remarkably to turbulent kinetic energy production and momentum transfer (70%-80%). The KHCS enlarge with the growth of the shear layers and extend vertically as the jet spreads in the vertical direction. Nevertheless, the Strouhal number of KHCS remains almost constant (~0.079). After the shear layers from both sides meet at the centerline, the jet undergoes a "flag-like" flapping motion.With negative buoyancy, the turbulent inflow is classified as a hyperpycnal plume. Examples are cool and sediment-laden river flowing into open water. Under the influence of negative buoyancy, the plume converges and forms a triangular shape on the water surface but spreads laterally near the bottom boundary. At a certain distance (xp), the plume is totally submerged and plunges towards the bottom boundary. The discharge of the plume increases during plunging as a result of ambient water entrainment. In a fixed geometry, the mean flow field is controlled by the initial densimetric Froude number (Frd-0). The xp value and entrainment coefficient (E, quantifying the entrained ambient water) increase with the Frd-0 value. The experimental and numerical results are comparable to observations at the Rhône River mouth (Lake Geneva) and field measurements at other similar river mouths in terms of xp and E values. Shear layers are observed at the two sides of the plume, whose lateral growth is suppressed significantly by negative buoyancy. In a 3D view, the plume-ambient interface is a hooked face under the water surface, and thus the direction of shear-induced vorticity changes with depth, resulting in the unique 3D shapes of KHCS. The Strouhal number of KHCS decreases with increasing local Richardson number, and a coherent region containing KHCS forms and considerably contributes to the total mixing (approximately 40%) between the plume and ambient water in the nearshore region.