Pablo Huq

The characteristics of the recirculating bulge region in coastal buoyant outflows

A relatively large recirculating gyre, or bulge (anticyclonic buoyant lens) region attached to the source has been observed to occur in some numerical simulations and in some oceanic outflows. The purpose of this paper is to determine the dynamics of such recirculating bulge regions. Laboratory experiments were conducted for the purpose of examining the characteristics, evolution, and impact of a recirculating bulge in a coastal buoyant outflow. All experiments were performed such that the buoyant layer was thin compared to the total water depth, and the width of the bay exit was approximately equal to the internal Rossby deformation radius of the buoyant outflow. The geometry of the bay-exit (radius of curvature and exit angle) was varied in order to produce experiments with recirculating bulge formation, and experiments without. Measurements of the density and velocity fields were undertaken. The dimensions of the recirculating bulge were found to evolve identically (when appropriately scaled) for all experiments in which such a bulge formed, regardless of bay-exit geometry. When recirculating bulges formed, it was determined that the freshwater storage within the bulge was approximately 60-70% of the source freshwater flux. The impact on the attached coastal current downshelf of the recirculating bulge was found to be significant: the width, length, and velocities of such attached coastal currents were much reduced in comparison to coastal currents that evolved in the absence of a recirculating bulge. Growth rates of the bulge were measured with time: the recirculating bulge was found to grow radially as ∼t 2/5 and vertically as ∼t 1/5 . Baroclinic instabilities were observed in the recirculating bulge. The instabilities evolved as multiple rotating cores within the larger anticyclonic gyre. The presence of the instabilities in the recirculating bulge may account for its relatively large radial growth but weak vertical growth.

Scaling Analysis for the Interaction between a Buoyant Coastal Current and the Continental Shelf: Experiments and Observations

Abstract This paper addresses how the geometrical parameters (ambient ocean depth, H; bottom slope, α) of coastal bathymetry affect the evolution of buoyant coastal currents flowing over a sloping continental shelf. Scaling arguments are presented that show the coastal current dynamics can be classified by a two-variable nondimensional parameter space: the ambient depth parameter, h/H, and the bottom slope parameter, R/yb. The ratio h/H is the fraction of the available depth occupied by the buoyant layer; the bottom slope parameter is the ratio of two horizontal length scales, the internal Rossby radius R to the bottom-trapped width yb. The scale depth h is derived from geostrophic dynamics and is representative of the depth of the buoyant layer of the coastal current. The resulting parameter space is delineated by surface-advected currents that do not depend upon the bottom slope parameter and bottom-trapped currents that do. For bottom-trapped coastal currents, the across-shore width and downstream velo...

The role of outflow geometry in the formation of the recirculating bulge region in coastal buoyant outflows

Density-driven coastal currents are a common feature in the worlds coastal oceans. These currents may separate from the coastline due to variations in geometry. Past studies have shown that this separation may produce two distinctly different flow states: a continuation of the coastal current, or a recirculating gyre downshelf of, and attached to, the separation point. Laboratory experiments of coastal buoyant outflows (rotationally dominated, buoyancy driven) were undertaken to examine the role of bay geometry on the evolution of the outflow. Experiments were conducted on a rotating turntable in relatively deep water (such that the buoyant layer depth was much less than the total ocean depth). The geometry of the bay-exit was varied, both in exit angle (θ) and in radius of curvature (r c ). The width of the bay was varied such that the bay exit Kelvin number (a ratio between the width of the bay exit and the internal Rossby deformation radius) was order 1 for all experiments. A recirculating bulge (a large, anticyclonic gyre joining the coastal current to the buoyant source) was occasionally observed to form. Results are compared to the Bormans and Garrett (1989) hypothesis: this hypothesis is found to explain a portion of the results only. Geometrical arguments are presented that build upon the Bormans and Garrett hypothesis that parameterizes the magnitude of the flow separation between the buoyant fluid and the exit. A separation ratio, Γ, is defined as a ratio between the inertial turning radius of the flow and the maximum offshore distance between the separated flow and the coast. A recirculating bulge was observed to form for flows with values of Γ > 0.5. The separation ratio, r, is shown to be equivalent to the impact angle, Φ, of the buoyant fluid re-encountering the wall. The impact angle governs the upshelf and downshelf volume flux of the impacting fluid: recirculating bulge formation is found to occur when at least 50% of the source volume flux returns to the source region. This is equivalent to an impact angle greater than or equal to 60-degrees.

Mixing due to grid-generated turbulence of a two-layer scalar profile

In this experimental study the mixing of passive scalars that arises from shear-free decaying grid-generated turbulence is examined. The flow configuration consists of two homogeneous layers of equal density separated by a sharp interface between different mean concentrations of passive scalar. Both layers flow through a turbulence-generating grid. To determine the effect of diffusivity the scalar was heat in one experiment and salt in a second. The Schmidt number – the ratio of momentum to species diffusivity – was 7 and 700 for heat and salt respectively. Velocity, scalar and flux fields were mapped and flow visualization was undertaken to study the flows. Integral lengthscales and (scalar) flux were determined to be independent of diffusivity, whereas the scalar Taylor microscales varied with Schmidt number, S c , thus illustrating the disparate effects of diffusivity on large and small scales. The time series of signals showed a correspondence between locations of w θ extrema and uw extrema. Flux w θ arose from intermittent events; and the magnitude of the time-averaged flux

Turbulence evolution and mixing in a two-layer stably stratified fluid

\overline{w\theta}

A laboratory study of buoyant plumes in laminar and turbulent crossflows

was found to depend on the frequency, rather than on variations in the amplitude of w θ events.

Role of micro-convection due to non-affine motion of particles in a mono-disperse suspension

The results of an experimental study of shear-free decaying grid-generated turbulence on both sides of a sharp interface between two homogeneous layers of different densities are presented. The evolution of turbulence and mixing were examined by simultaneously mapping the velocity (u, w) and density fields @) and the vertical mass flux F(= pW/p’w’) together with flow visualization in a low-noise water tunnel. Buoyancy was induced by salinity differences so the value of the Schmidt number S, = 700. Density stratification altered the inertial-buoyancy force balance (most simply expressed by Nt, the product of the buoyancy frequency N and turbulent timescale t) so as to attenuate turbulent velocity fluctuations, vertical motions and interfacial convolutions, normalized density fluctuations, vertical flux mass, and mean interfacial thickness. Vertical velocity fluctuations w’ were found to increase with distance from the interface, whereas the u’-distribution can be non-monotonic. The maximum value of the mass flux, F, was found to be about 0.5 which was less than the typical value of 0.7 for thermally stratified wind tunnel experiments for which S, = 0.7. The vertical mass flux can be a combination of down-gradient and counter-gradient transport with the ratio varying with Nt (e.g. at Nt z 5, the flux is counter-gradient). The flux Richardson number Rf was found to increase monotonically to values of approximately 0.05.

On the Transport of Buoyant Coastal Plumes

Abstract The results of laboratory experiments of buoyant plumes in crossflows are presented. Specific comparisons are made of plume evolution for laminar and weakly turbulent crossflows for otherwise identical flow parameters to establish that plume dilution rates differ significantly even for weakly turbulent crossflows. Decaying, grid-generated turbulence constituted the flow field of the weakly turbulent crossflow. Measurements comprised analyses of flow visualization, hot-film anemometry and conductivity probe data. Plume thicknesses and widths are found to be greater for turbulent crossflows, although they both grow as χ 2/3 for laminar and turbulent crossflows. The effect of turbulence in the crossflow is to increase entrainment and dilution with the consequence of lower plume trajectories. It is found, relative to the values of dilution for buoyant plumes in a laminar crossflow, that dilutions of plumes in a weakly turbulent crossflow (with kinetic energy dissipation rates two orders of magnitude smaller than internal plume dissipation rates) were greater by 33%. Trajectories can be predicted by an integral model with a modified β, the entrainment coefficient, which relates the ratio of dissipation rates within and outside the plume.

Measurements of Turbulence and Dispersion in Three Idealized Urban Canopies with Different Aspect Ratios and Comparisons with a Gaussian Plume Model

Non-affine translation of suspended particles in a flowing suspension causes micro-convection of the fluid in the vicinity of the particle which affects the local transport mechanisms in the suspension. To investigate the importance of micro-convection on heat transport, a unit cell approach is adopted. A parametric study investigates the influence of particle fall velocity in a quiescent fluid and particle volume fraction on micro-convection in a sedimenting suspension. The thermal flux across the unit cell is shown to be a function of modified Peclet number that includes the influence of volume fraction for the range of volume fractions (0 < e < 0.3) and for Pe ≈ O(1). This is used to investigate the influence of hindered settling on transport processes such as heat transfer occurring in a sedimenting suspension.

Observations of jets in density stratified crossflows

Abstract The role of discharge conditions and shelf geometry on the transport of coastal plumes is studied with a fully nonlinear, primitive equation hydrodynamic model. The physical setting is an estuarine channel with a small discharge Rossby number. By simulating different discharge magnitudes, buoyant plumes are shown to be succinctly described by a simple coastal front model. Three results emerge from the model analysis. First, the plume transport is given by T = γ0(g′ph2/2f ), where γ0 is a parameter dependent on the ratio of the front and the plume widths, g′p is the plume reduced gravity, h is the plume maximum depth, and f is the Coriolis parameter. Second, this model links the plume transport directly to upstream river conditions with T = γQr, where Qr is the river outflow and γ is a parameter that relates to entrainment, the geometry of the plume front and shelf slope, and the fraction of freshwater carried downshelf. Third, these equations reduce to analytic results previously established for ...