1,720,967 research outputs found
Book review: Small-scale Processes in Geophysical Fluid Flows, by L.H. Kantha and C.A. Clayson, Academic Press, 2000, 888pp
No full textMulti-layer hydraulic exchange flows
No full textFlows between ocean basins are often controlled by narrow channels and shallow sills. A multi-layer hydraulic control theory is developed for exchange flow through such constrictions. The theory is based on the inviscid shallow-water equations and extends the functional approach introduced by Gill (1977) and developed by Dalziel (1991). The flows considered are those in rectangular–cross-section channels connecting two large reservoirs, with a single constriction (sill and/or narrows). The exchange flow depends on the stratification in the two reservoirs, represented as a finite number of immiscible layers of (different) uniform density. For most cases the flow is ‘controlled’ at the constriction and often at other points along the channel (virtual controls) too. As with one- and two-layer hydraulics, controls are locations at which the flow passes from one solution branch to another, and at which (at least) one internal wave mode is stationary. The theory is applied to three-layer flows, which have two internal wave modes, predicting interface heights and layer fluxes from the given reservoir conditions. The theoretical results for three-layer flows are compared to a comprehensive set of laboratory experiments and found to give good agreement. The laboratory experiments also show other features of the flow, such as the formation of waves on the interfaces. The implications of the results for oceanographic flows and ocean modelling are discussed
Dynamics of the flow within a rift valley segment of the Mid-Atlantic Ridge (abstract of paper to be presented at AGU/ASLO 2000 Ocean Sciences Meeting, San Antonio, TX, 24-28 January 2000)
No full textRotating gravity currents. Part 1. Energy loss theory
No full textA comprehensive energy loss theory for gravity currents in rotating rectangular channels is presented. The model is an extension of the non-rotating energy loss theory of Benjamin (J. Fluid Mech. vol. 31, 1968, p. 209) and the steady-state dissipationless theory of rotating gravity currents of Hacker (PhD thesis, 1996). The theory assumes the fluid is inviscid, there is no shear within the current, and the Boussinesq approximation is made. Dissipation is introduced using a simple method. A head loss term is introduced into the Bernoulli equation and it is assumed that the energy loss is uniform across the stream. Conservation of momentum, volume flux and potential vorticity between upstream and downstream locations is then considered. By allowing for energy dissipation, results are obtained for channels of arbitrary depth and width (relative to the current). The results match those from earlier workers in the two limits of (i) zero rotation (but including dissipation) and (ii) zero dissipation (but including rotation). Three types of flow are identified as the effect of rotation increases, characterized in terms of the location of the outcropping interface between the gravity current and the ambient fluid on the channel boundaries. The parameters for transitions between these cases are quantified, as is the detailed behaviour of the flow in all cases. In particular, the speed of the current can be predicted for any given channel depth and width. As the channel depth increases, the predicted Froude number tends to , as for non-rotating flows
Observations of mixed layer deepening during an Antarctic gale
No full textObservations of mixed layer deepening made during a gale in February 2005 near an ice shelf, Fimbulisen, Antarctica, are reported. The observations were made from the RRS James Clark Ross in the lee of the ice shelf, using repeated downcasts (“yo-yo”) of a conductivity-temperature-depth package, together with shipboard meteorological and other measurements. The mixed layer deepened from less than 40?m to over 120?m over the course of 27?h, with a very rapid deepening from 80?m to 120?m over a period of under 11?h. The mixed layer became both colder and fresher, with the change in salinity and heat content likely to be caused by melting ice. Oxygen isotope results suggest the source of the fresh water was melting sea ice rather than precipitation or ice shelf melt. The input of melt water at the surface stabilizes the mixed layer, so extra energy is required to deepen the mixed layer. The observations suggest that approximately 1.8% of the available “wind-work” energy was used to mix the upper water column, while the stabilizing surface buoyancy flux inhibits the turbulence in the mixed layer, limiting the mixing length to 1.6?m. The eventual depth of the mixed layer is in line with estimates based on the planetary length scale u*/f. The rate of mixed layer deepening is given by Ue/u*?=?0.035. The apparent peak ice melting rate was approximately 60?mm?hr?1, although this is likely to be exaggerated by convergence and downwelling
Internal bores in two-layer exchange flows over sills
No full textInternal bores are a common feature of tidally modulated two-layer exchange flows through straits and over sills. Even where the forcing changes smoothly, the flow may adjust with sudden jumps in the position of the interface between the two layers. The resulting flow configuration, with a hydraulically controlled exchange flow (at the sill) coupled with a propagating internal hydraulic jump (known as a bore), is investigated with mathematical models and laboratory experiments. The study concentrates on two-dimensional flow in a rectangular channel with a sill. The parameters considered are the depth of the channel compared to the depth over the sill, the depth of the interface before the passage of the bore and the strength of the net flux through the channel.The theory is based on shallow water equations and hydraulic control theory and includes the effects of a steady net flow through the channel (driven, for example, by the tide). Once the depth of the channel is twice the depth over the sill, further changes in geometry have relatively little effect on the flow. The bore velocity and fluxes are strongly affected by the strength of any net flow. The laboratory experiments model pure exchange flows (with no net flow) and give detailed information about the bores themselves. In many cases an undular bore is produced, with a well-defined wave train on the interface behind the front of the bore. The wavelengths and amplitudes of these internal waves are quantified and a brief comparison with similar internal waves observed in the Strait of Gibraltar is presented
Rotating gravity currents. Part 2: Potential vorticity theory
No full textAn extension to the energy-conserving theory of gravity currents in rectangular rotating channels is presented, in which an upstream potential vorticity boundary condition in the current is applied. It is assumed that the fluid is inviscid; that the Boussinesq approximation applies; that the fundamental properties of momentum, energy, volume flux and potential vorticity are conserved between upstream and downstream locations; and that the flow is dissipationless. The upstream potential vorticity in the current is set through the introduction of a new parameter , that defines the ratio of the reference depth of the current to the ambient fluid. Flow types are established as a function and the rotation rate, and a fourth flow geometry is identified in addition to the three previously identified for rotating gravity currents. Detailed solutions are obtained for three cases 0.5, 1.0 and 1.5, where \delta\,{<}\,1 is relevant to currents originating from a shallow source and \delta\,{>}\,1 to currents where the source region is deeper than the downstream depth, for example where a deep ocean flow encounters a plateau. The governing equations and solutions for each case are derived, quantifying the flow in terms of the depth, width and front speed. Cross-stream velocity profiles are provided for both the ambient fluid and the current. These predict the evolution of a complex circulation within the current as the rotation rate is varied. The ambient fluid exhibits similar trends to those predicted by the energy-conserving theory, with the Froude number tending to at the right-hand wall at high rotation rates. The introduction of the potential vorticity boundary condition into the energy-conserving theory does not appear to have a substantial effect on the main flow parameters (such as current speed and width); however it does provide an insight into the complex dynamics of the flow within the current
Experimental studies of rotating exchange flow
No full textOcean basins are connected by straits and passages, geometrically limiting important heat and salt exchanges which in turn influence the global thermohaline circulation and climate. Such exchange can be modeled in an idealized way by taking into consideration the density-driven two-layer flow along a strait under the influence of rotation. We use a laboratory model of a lock exchange between two reservoirs of different density through a flat-bottom channel with a horizontal narrows, set up on two different platforms: a 1 m diameter turntable, where density interface position was measured by dye attenuation, and the 14 m diameter turntable at Coriolis/LEGI (Grenoble, France), where correlation imaging velocimetry, a particle imaging technique, allowed us to obtain for the first time detailed measurements of the velocity fields in these flows. The influence of rotation is studied by varying a parameter, Bu, a type of Burger number given by the ratio of the Rossby radius to the channel width at the narrows. In addition, a two-layer version of the Miami Isopycnic Coordinate Model (MICOM) is used, to study the cases with low Burger number. Results from experiments by Dalziel [1988. Two-layer hydraulics: maximal exchange flows. Ph.D. Thesis, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, see also http://www.damtp.cam.ac.uk/lab/people/sd103/papers/1988/Thesis_Dalziel.pdf] are also included for comparison. Time-mean exchange fluxes for any Bu are in close agreement with the inviscid zero-potential vorticity theory of Dalziel [1990. Rotating two-layer sill flows. In: Pratt, L.J. (Ed.), The Physical Oceanography of Sea Straits. Kluwer Academic, Dordrecht, pp. 343–371] and Whitehead et al. [1974. Rotating hydraulics of strait and sill flows. Geophysical Fluid Dynamics 6, 101–125], who found that fluxes for Bu>1 mainly vary with channel width, similar to non-rotating flow, but for Bu<1 are only limited by the Rossby radius. We also show theoretically that non-zero-potential vorticity results in only a small increase in the predicted exchange flux around Bu1. The flow characteristics are found to be very different for small and large Burger numbers: for Bu>1 a steady, two-layer flow was observed that persisted across the channel at the narrows with only some across-channel variation. The distribution of the Froude number is found to give some evidence for hydraulic control in a manner similar to that of non-rotating flows under the influence of bottom drag. Flow for Bu<0.5 does not appear to reach a steady state but instead is characterized by an unsteady, meandering current and several eddies in the strait. Similar instabilities also occur in wide oceanic straits, where several mechanisms, such as barotropic and baroclinic instability, have been proposed and could also be one cause of time variability in our experiments. Both the laboratory experiments and the MICOM results suggest that in the presence of bottom drag or side wall friction some features of the flow, such as the location of the channel crossing, become sensitive to the initial conditions. These effects differ in flows with Bu>1 and Bu<1
Modeling hydrography and marine sedimentation in the Cariaco Basin since the Last Glacial Maximum
No full textThe Cariaco Basin has shallow connections with the Caribbean Sea, and these are further reduced at times of lower sea level, such as at the Last Glacial Maximum (LGM). A numerical model was developed to describe the oceanography and biogenic sedimentation in the Cariaco Basin and nearby Caribbean. The model is run with different sea levels in order to simulate the changing oceanography and the development of deep water anoxia in the Cariaco Basin since the LGM. In the main sequence of numerical experiments, the surface forcing is kept fixed at present?day values while the sea level is changed in order to separate the effects of sea level from the effects of climate. As the sea level rises, the main sedimentation zone moves first to the shallow broad northern sill and NE part of the Cariaco Basin and then, once sea level reaches approximately 60 m below present, moves south to the northern coast of mainland Venezuela. The model shows that there would be an overall increase in sedimentation in the basin as the sea level rises, even if there was no change in the surface forcing. However, the model also shows that sedimentation at particular points in the basin exhibits more complicated behavior, which needs to be taken into account when interpreting individual records. Preliminary numerical experiments examine the effects of changing surface forcing while keeping the sea level at LGM values, and the applicability of a mathematical hydraulic control model in this case is also considered
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