Session ES: Geophysical: Oceanographic I

Fate and Transport of Fluid Mixed at the Boundary of a Lake

Several studies have shown that fluxes in stratified water bodies are controlled by turbulence and mixing at sloping boundaries, but fewer have investigated the fate of the mixed fluid. We conducted field experiments in two Iowa lakes to track fluid mixed on the sloping sides as it moved into the interior of the lake. Measurements from a meteorological station characterize the forcing, while measurements from thermistor chains and acoustic profilers provide information on the resulting internal waves. Tracer measurements combined with microstructure profiling illustrate the boundary mixing and transport of mixed fluid. In one case in the smaller of the two lakes, turbulence created by currents from long internal waves mixed fluid that propagated from the shore as an intrusion governed by a balance between inertia and buoyancy. In another case, the transport is more likely due to a jet corresponding to the second vertical mode of the internal waves. Recent results from a larger lake will be used to determine the effect of lake size and geometry on the transport.   

Direct Numerical Simulation of a Model Estuary

We investigate mixing and sedimentation processes in laboratory-scale estuaries by means of high-resolution Navier-Stokes simulations. A positively buoyant, sediment-laden freshwater river is considered that enters a notional ocean, upon which particles sediment out at the lower boundary of the current. The computational setup, while employing a simplified geometry, accounts for the most important features of a typical estuary. The flow is studied in a spatially developing framework that allows us to obtain statistically stationary solutions for freshwater/saltwater mixing rates and particulate settling profiles. Details of the particle settling process are investigated both during the initial transient phase, as well as for statistically stationary conditions. We observe qualitatively good agreement of the settling mechanisms with corresponding laboratory experiments. The properties of the particulate plume in the freshwater current are analyzed as a function of the particle Stokes settling velocity.   

The distribution of age in a coastal river plume

Fluid age and residence time are arguably the most important variables for predicting changes in water quality in aquatic systems; however, no existing experimental technique can directly resolve them, and our understanding of the relationship between the distribution of age and fluid mechanical principals is incomplete. We present a novel experimental technique for determining the spatial distribution of the age of fluid parcels in laboratory flows. We use the technique to investigate the retention of fluid in a coastal river plume using a series of experiments on a 2m diameter rotating table. In this system, an anticyclonic eddy, or bulge, sets up near the river mouth and accumulates approximately half of the water discharged from the river. Initially, fluid collects continually in the core of the bulge such that the maximum age in the eddy after 10 rotation periods is approximately 6 rotation periods. Subsequently, the bulge becomes baroclinically unstable and the aged core is distorted by smaller coherent eddies. The instability breaks down the retention process and transports fluid from the aged core into the coastal current. The instability also appears to periodically modulate the partitioning of new fluid between the bulge and the coastal current, resulting an coherent pulses of fluid in discreet age classes in the age distribution.   

Internal-Tide Scattering by 2D Topography: Experimental Study

Scattering of internal tides is an important mechanism to understand mixing and energy transfer in the ocean. Numerical and oceanographic studies have shown that topographies can be responsible of conversion from low modes to higher modes, leading to transfer from large to smaller scale inducing local mixing and higher damping rate along propagation. To understand and quantify more precisely this mechanism, we generate a mode-1 internal tide using a new configuration for the wavemaker recently developed by Gostiaux~{\it et al.}. Experiments have been realized at the Coriolis Turntable in Grenoble (France), a cylindrical rotating tank of $13$~m diameter. The velocity fields observed using PIV technique are analyzed in terms of modal decomposition. Knowing that 97$\%$ of the generated internal tide energy flux is associated to a mode-1 internal tide, we analyze its interaction with a 2D gaussian topography. Subcritical and supercritical bathymetry are considered according to the frequency of the incoming internal tide. Estimations of the amount of reflected energy by the topography, such as the scattering into higher modes of the transmitted wave field, are in good agreement with numerical predictions.   

Rippled Bed Evolution over Wave Groups: Implications to Bottom Roughness Calculations

Outside of the surf zone, seabed ripples are the source of significant dissipation of free surface gravity energy. The dissipation is a function of both the ripple geometry and the hydrodynamic forcing. Observations of natural irregularly rippled beds and the oscillatory, two-dimensional, time-varying velocity field were collected using a submersible Particle Image Velocimetry (PIV) system in both a full-scale and smaller-scale environments. The full-scale observations were obtained over natural sand beds, whereas the smaller-scale observations were obtained over artificial low specific gravity beds. Bedform evolution regimes characterized by the ripple radius of curvature were examined relative to measures of the non-dimensional bed stress (the grain roughness Shields parameter), the non-dimensional pressure gradient (the Sleath parameter), and the water column coherent structure formation from the ripples (the swirling strength). Anorbital bedforms were found to respond to individual waves by modulating amplitude as wave groups passed. These observations suggest that the bottom dissipation due to movable sediment beds may be more dynamic than previously assumed.   

LES of an oscillating current over an inclined slope

Large-Eddy Simulations (LES) are performed to investigate the dynamics of a stratified bottom boundary layer on a continental slope under a tidal current. Turbulent mixing is observed to be different between the upslope and downslope phases of the flow. The observed difference is found to be related to the phase-dependent modulation of the stratification.Flow instabilities and turbulence in the bottom boundary layer excite internal gravity waves that propagate far from the wall region. The wave field during the upslope and downslope flow exhibits significant differences. The slope angle is varied and found to play an important role in determining properties of the bottom boundary layer.   

Settling of porous particles through a pycnocline

Downward carbon flux in the Ocean is largely governed by particle settling. Most marine particles are highly porous and settle at low Reynolds numbers. We present results of an experimental investigation of porous spheres settling through a thin density interface at $O(0.1) [Preview Abstract]

3D experimental investigation of vortex dipoles in shallow water

Vortex dipoles are often associated to sediments transport. Shallow dipoles have often been expected to exhibit Quasi-2D dynamics. However, recent lab experiments have shown the existence of a horizontal spanwise vortex at the front of the dipole. It is of great importance to describe and quantify their dynamics. We investigate dipole evolution and spanwise vortex formation. Reproducible laminar dipoles are created with flap apparatus. For fixed flaps gap, shallowness of the dipole and dipole propagation velocity are the parameters controlling the flow. PIV measurements were performed in two horizontal planes and the vertical symmetry one for a wide range of control parameters. These measurements led us to define criteria on the spanwise vortex generation and dynamics. Complete 3D geometry and dynamics of the dipole are obtained using 3D-3C scanning PIV. Transition from a Q-2D dipole to a 3D dipole considering the spanwise vortex has been enhanced thanks to this powerful technique.   

Turbidity Currents in Complex Topographies

We consider particle-laden gravity currents interacting with complex seafloor topographies, such as mini-basins, ridges or meandering channels. Both two- and three-dimensional Navier-Stokes simulations are employed in order to investigate their dynamics, entrainment and depositional behavior for a range of flow and geometrical parameters. We observe that coherent vortical structures generated by topographical effects can lead to the formation of strong nonuniformities in the sediment deposit. Results from a parametrical study are discussed, based on two-dimensional simulations of depositing currents produced by a lock-exchange configuration flowing through a minibasin, in order to quantify the effects of the geometrical parameters and particle settling speed on the sediment deposit fields. The over-spill and lateral deposit profile is studied for flows passing through meandering channels with the continuous inflow and outflow from the system.   

The inhibiting effect of stratified mixing on surface-stress-driven flow in a cylinder

We extend previous work of Boyer, Davies \& Guo (1997: Fluid Dyn. Res. Vol. 21 pg 381-401) to consider the evolution of an initially two-layer stratified fluid in a cylindrical tank which is driven by a horizontal rotating disc. The flow induced by the disc drives entrainment at the interface, and the layer nearer to the disc deepens. Through high-frequency conductivity probe measurements, we establish that the deepening layer is well-mixed, and the interface depth between the two evolving layers appears to be largely constant. Under certain circumstances, we find that the rate of increase in depth of the deepening layer decreases with time, at variance with the results of Boyer et al, and implying that the characteristic velocities in the deepening layer decrease as the upper layer deepens. Such imperfect, decaying spin-up is a natural consequence of the total energy budget of the flow, as the combined power requirements of entrainment and layer homogenization can inhibit the characteristic velocities of the deepening layer approaching the (constant) velocities of the driving disc, as assumed by Boyer et al. We investigate the dependence of both the entrainment and the efficiency of the mixing on the external parameters of the flow, in particular the bulk Richardson number defined in terms of the initial layer depth, interfacial reduced gravity, and disc azimuthal velocity.