Jet Currents Research Articles

Effect of Background Gas on the Current Emitted from Taylor Cones

The current-flow rate characteristics I( Q) of electrified liquid cone jets of highly conductive liquids ( K> 10 -2 S/m) are shown to be insensitive to the nature (air, CO 2, SF 6) and pressure (130 Torr < p < 1500 Torr) of the surrounding gas in the domain of values in which the cone is stable. Yet the mobility of the droplets produced by the breakup of the emitted jet depends considerably on gas pressure. Furthermore, the width of the spray they form increases with gas pressure. We conclude that the I( Q) curve is unaffected by space charge phenomena. This is consistent with earlier evidence indicating that the jet current is fixed entirely inside the meniscus. A cone jet of given flow rate Q is thus an ideal current generator, practically insensitive to the distribution of free charge in the gas or the surrounding electrodes. The minimum flow rate for stability of the cone, Q min , is also found to be independent of gas pressure.

Numerical analysis of the distribution of currents in a plasma jet, with the Hall effect taken into account

Numerical analysis of the distribution of electric currents in a plasma medium, with the Hall effect taken into account, has been conducted in many works dedicated to studying intrachannel flows in various MHD devices. The aim of the present paper is to study the peculiarities of the current distribution in stationary underexpanded plasma jets flowing into a neutral medium, with the magnetic field applied in such a manner as to be directed along the jet axis. This topic has been virtually neglected in the literature although its study would be of interest in designing and using certain types of MHD engines and accelerators. Although the problem seems to be somewhat simpler than that presented because of the absence of electrodes in the computational domain, it nevertheless has its own special features which are due to the fact that it has a free boundary and that the jet is not limited down the flow.

Baroclinic flows, transports, and kinematic properties in a cyclonic‐anticyclonic‐cyclonic ring triad in the Gulf of Mexico

During October‐November 1986 the baroclinic circulation of the central and western Gulf of Mexico was dominated by an anticyclonic ring that was being bisected by two north and south flanking cyclonic rings. The baroclinic circulation revealed a well‐defined cyclonic‐anticyclonic‐cyclonic triad system. The anticyclone's collision against the western gulf continental slope at 22.5°N, 97°W originated the north and south flanking cyclonic rings. The weakening of the anticyclone's relative vorticity, during the collision, was compensated by along‐shelf north (26 cm s−1) and south (58 cm s−1) jet currents and by the anticyclone's flanking water mass's gain of cyclonic vorticity from lateral shear contributed by east (56 cm s−1) and west (42 cm s−1) current jets with individual mass transports of ∼18 Sv. Within the 0–1000 and 0–500 dbar layers and across 96°W the magnitudes of the colliding westward transports were 17.80 and 8.59 Sv, respectively. These corresponding transports were 85 and 94% balanced by along‐shelf jet currents north and south of the anticyclone's collision zone. This indicates that only minor amounts (&lt;15%) of the anticyclone's colliding westward transports might have flowed into the western gulf's continental shelf water mass or else they sank into deeper water along the continental slope during the anticyclone's collision event. The resultant effect of the coupled interaction between the anticyclone and the cyclonic pair was the surging of the water mass in the cyclones and its sinking in the anticyclone. This mechanism controlled the magnitude, direction, location of vertical advection, and transfer of kinetic energy from the upper to the deeper water layers. Our vertical transport estimates through the 1000‐m‐depth surface revealed a net vertical descending transport of 0.4 Sv for the ring triad system. This mass flux occurred primordially within the south central gulf region and most likely constituted a principal mechanism that propelled the gulf's deep horizontal circulation. The volume renewal time is ∼5 years for the ring triad system within 0–1000 dbar. The volume renewal time for the gulf's deep water layer (2000–3000 dbar), estimated as a function of its horizontal outflowing mass flux (1.96 Sv), is of the same order of magnitude and reveals that the deeper layer of the Gulf of Mexico is as well ventilated as its upper layer (0–1000 dbar). The ring triad's surface kinematic properties were derived from the sea surface baroclinic circulation field referenced to 500 dbar. Within this layer, individual ring geometries were conserved. Maximum tangential ring velocities were 60 and 58 cm s−1, for the north and south cyclones respectively, and 30 cm s−1 for the anticyclone. The corresponding periods of revolution were 16, 19, and 26 days, and vertical velocities calculated at the rings' peripheries, where maximum horizontal divergence was encountered, were 1.5, 1.0, and −1.0 m d−1.

The effect of boundary conditions and turbulent exchange intensity on the adaptation calculations of the tropical Atlantic hydrothermodynamic fields

Reduction of turbulent exchange coefficients leads to an intensification of jet currents and allows a more correct description of the temperature field's spatial structure. These fields virtually do not depend on the type of fluid boundary conditions of the considered area, which, however, essentially affect meridional heat transport (MHT). It is indicated that a two-month-long adaptation period does not suffice for the equatorial zone MHT to become stable.

Seasonal cooling and blooming in tropical oceans

The relative importance of tropical pelagic algal blooms in not yet fully appreciated and the way they are induced not well understood. The tropical Atlantic supports pelagic blooms together equivalent to the North Atlantic spring bloom. These blooms are driven by thermocline tilting, curl of wind stress and eddy upwelling as the ocean responds to intensified basin-scale winds in boreal summer. The dimensions of the Pacific Ocean are such that seasonal thermocline tilting does not occur, and nutrient conditions are such that tilting might not induce bloom, in any case. Divergence at the equator is a separate process that strengthens the Atlantic bloom, is more prominent in the eastern Pacific, and in the Indian Ocean induces a bloom only in the western part of the ocean. Where western jet currents are retroflected from the coast off Somalia and Brazil, eddy upwelling induces prominent blooms. In the eastward flow of the northern equatorial countercurrents, positive wind curl stress induces Ekman pumping and the induction of algal blooms aligned with the currents. Some apparent algal bloom, such as that seen frequently in CZCS images westwards from Senegal, must be due to interference from airborne dust.

Air‐sea boundary layer dynamics in the presence of mesoscale surface currents

In the presence of surface currents, a shear stress at the air‐sea interface is induced by the surface currents. In the case of a unidirectional current, a quadratic stress law leads to a stress curl proportional to and opposing the surface current vorticity even with a uniform wind. This causes a spindown effect on the surface vorticity field at a rate proportional to the wind speed. In the steady state, or in slowly varying processes which can be treated as parametrically developing quasi‐steady states, the surface‐layer potential vorticity modulation causes upwelling and downwelling patterns associated with the surface‐current vorticity. These effects are analyzed for an idealized jet current, and for a physical situation characteristic of a Gulf Stream boundary ring along the Florida Keys, where the induced transport patterns may be important for onshore transport of fish and spiny lobster larvae, as well as for onshore transport to the Florida Keys of general flotsam transported past them by the Gulf Stream. The spindown time scale (t*) for a 1.5‐layer system is H/( ρ′cdVa) for a surface jet on the deformation radius scale (where H is the thickness of the surface layer, Va the surface wind speed, ρ′ the air to water density ratio and cd the surface drag coefficient) and increases for large horizontal scales in proportion to the current width squared. For a typical wind speed of 5 m/s and a density normalized drag coefficient ρ′cd= 2 × 10−6, t* is on the order of 1 month for a 30‐m surface layer. In the more general case of a stratified interior water column, the vorticity spindown directly affects only the potential vorticity of the surface layer and generally leads to subsurface velocity and vorticity maxima for mesoscale eddies and jets.

Jet behavior in the flute

Time snapshot profiles of an acoustically excited flute jet were measured with a Pitot tube and with a hot wire anemometer. Both methods showed the jet broadened substantially under excitation, and developed a subsidiary peak on the side opposite the main moving peak. Variation of phase, jet swing, and inferred current signal amplitude with drive intensity and frequency, and response to a complex wave were consistent with pressure gradient as the stimulus for the jet motion, rather than acoustic velocity or negative displacement. Observation of signals generated by the jet when blowing a nonresonant flute tube and embouchure while excited by either simple or complex waves in the same tube, gave overall transfer functions that were consistent with the assumption of jet response to acoustic velocity in conjunction with tube drive by injected jet current.

Spatial features of the near-surface and midwater circulation patterns off western and southern South Africa and their role in the life histories of various commercially fished species

Currents on the South African shelf have been measured on 13 cruises since an Acoustic Doppler Current Profiler (ADCP) was installed on F.R.S. Africana in 1989. Most previous descriptions of the West Coast (Benguela) system, including the baroclinic jet currents off the Cape Peninsula and Cape Columbine, the latter clearly diverging into a dominant offshore arm and a weaker inshore arm, have been confirmed. This baroclinic flow originates along the south-western margin of the Agulhas Bank. Measurements of the inshore features of the Agulhas Current have shown strong WSW – SSW flow aligned with the bathymetry, but with significant shelf-edge north-eastward return flows. The velocity differences between the surface and depth reveal barotropic dominance in all inshore areas and over the central Agulhas Bank. The West Coast outer shelf displays variable baroclinic effects, reflecting the importance of frontal features and cross-shelf flow. SSW flow on the outer eastern Agulhas Bank appears responsible for an offshore loss of eggs from this region. The funnelling of anchovy eggs from the western Agulhas Bank to the West Coast was clear, but this "funnel" was found to have holes through which substantial loss of eggs offshore is also probable. The currents on the Agulhas Bank and the West Coast are discussed with regard to the life histories of various commercially fished species, in addition to anchovy. The study concludes that hypotheses relating environmental factors and recruitment need to consider the variety of potential options in terms of life history contained within the prevailing current system.

Time domain simulation and jet behavior in the flute.

A time domain simulation using parameters closely imitating a Boehm flute gave mode oscillation frequencies varying properly with blowing velocity, terminating tangent to an edge-tone characteristic obtained from the same model. Second harmonic generation was much enhanced by a reflection function representing stretched modes. Stagnation pressure and hot-wire speed measurements yielded profiles for a real jet that broadened greatly under acoustic excitation, with development of a subsidiary peak. Jet swing amplitudes and phases for different frequencies agreed with pressure gradient as the stimulating force, not velocity or negative displacement. A jet blowing a flute embouchure coupled to a nonreflecting, separately excited air column, produced signals that implied an overall jet transfer function consistent with the assumptions of velocity stimulation and injected jet current drive made in the computer simulation.

Spatial heterogeneity and dynamics of vernal phytoplankton species in the Baltic Sea in April–May 1986

The mesoscale phytoplankton distribution was studied as part of an international joint Baltic Sea Patchiness Experiment (PEX'86) during April–May 1986 in the open Baltic Proper. The study period covered the peak phase of the phytoplankton spring bloom. The spatio-temporal dynamics of four dominating phytoplankton species, Achnanthes taeniata, Chaetoceros spp., Skeletonema costatum and Thalassiosira levanderi were studied within an area of 20×40 nmi with grids of 2 and 4 nmi spatial resolution. The results showed highly varying spatial distributions for all species, and the variability was accentuated on the synoptic space scale before the development of the seasonal thermocline. The maxima of Chaetoceros spp. and Thalassiosira levanderi coincided with the anticyclonic and cyclonic eddies prevailing in the area. Skeletonema costatum was found in high abundances only within a warmer water mass of higher salinity advecting into the area. The results pointed out that different successional stages can simultaneously be found even in adjacent water masses and both the phytoplankton growth and composition during the bloom peak phase in the Central Baltic depend on complex factors, mainly those connected with mesoscale hydrodynamic features (eddies, frontal zones, jet currents).

Export production and the distribution of fish larvae and their prey in a coastal jet frontal region

Fine-scale spatial sampling series and vertical cross-sections were used to obtain quasisynoptic images of the distribution of fish larvae and their prey in the permanent frontal region formed by the Gaspe coastal jet current and the adjacent Anticosti gyre (NW Gulf of St. Lawrence, Canada). The accumulation of large diatoms triggered the reproduction of copepods in the Gasp6 current where eggs and nauplii (the main prey of first-feeding fish larvae) were 10 to 20 times more abundant than in the gyre. Estuarine circulation resulted in the coincidence of the small and abundant larvae of capelin Mallotus villosus and sand lance Ammodytes sp. with this intense production of their food in the jet current. The large and less abundant larvae of redfish Sebastes sp. and Arctic shanny Stichaeuspunctatus exploited the scarcer food resources of the Anticosti gyre. We conclude that opportunistic species producing large numbers of small offspring with limited foraging skills depend on massive export production at hydrographic singularities (i.e. salient hydrographic features) for reproduction. Species producing fewer but larger and more competent larvae can colonize less productive areas of the ocean. Plankton dynamics in the dispersal area of the early larval stages appear to be a pnmary constraint defining the life strategy of a fish species.

The exchange of fluid mass between quasi-geostrophic and ageostrophic motions during the reflection of Rossby waves from a coast. I. The case of an infinite rectilinear coast

When the problem of the reflection of spatially localized Rossby waves from a coast is treated using the quasigeostrophic (QG) approximation, the total fluid mass and the along-shore circulation calculated from the geostrophic height field are not conserved. To understand the correct mass balance and the degree to which the QG equations and boundary conditions may be in error, we analyze an initial-value problem for the Laplace tidal equations on a β-plane in the asymptotic limit ϵ ⪡ 1, where ϵ is the ratio of the spatial scale of the motion to the Earth's radius. It is shown that there is a coupling between QG and O(ϵ) fields. Physically, the coupling occurs by a peculiar adjustment process in the O(ϵ) approximation in which fast gravity waves are permanently generated to build up a quasi-stationary edge Kelvin wave. Different temporal scales (large for O(1) Rossby waves and small for the O(ϵ) gravity waves make comparable the contributions of the waves to the mass and circulation balance equations. However, QG analysis itself describes the reflection of Rossby waves correctly, but is incomplete, and for satisfactory balances one has to take into account the fields of both orders of the approximation. Applications of the results to closed basins, baroclinicity, and variable bottom topography are discussed. It is conjectured that an interaction of strong oceanic eddies with a coast (continental slope) may give rise to noticeable along-shore jet currents.

Phytoplankton dynamics in a coastal jet frontal region

We studied the physiological status and turbulent transport of phytoplankton across the quasi-permanent 150 km long estuarine front separating the Gaspe current from the Anticosti gyre (NW Gulf of St. Lawrence, Canada). In early June, chl a concentrations reached 35 pg 1-' in the current, but remained < 1 pg 1-' in the gyre. At that time of maximum river runoff and horizontal shear stress, the phytoplankton biomass developing in the current was rapidly transported across the front by turbulent mixing. The strong correlation between chl a and salinity (r = -0.96) lndcated that the rate of crossfrontal dilution far exceeded the rate of phytoplankton production. The N : C and C .ch l a ratios increased and decreased, respectively, with cross-frontal mixlng as nutrient-deficient cells from the Gaspe current experienced higher nutrient concentrations and a deepening of the mixed layer in the front. As freshwater runoff and frontal mixing decreased in summer, phytoplankton biomass accumulated in the density front and at the base of the jet current, where low advection and sufficient light and nutrient conditions prevailed. Sporadic upwelling along the front contributed an estimated 5 and 9 O/O of the total seaward flux of nitrate and silicate, respectively. Thus, nutrients from the drainage basin of the St. Lawrence, rather than the frontal upwelling of deep water, would b e responsible for the bulk of the inorganic nutrient flux from the NW Gulf of St. Lawrence to the southern Gulf and the Scotian Shelf.

Nonlinear Rossby adjustment in a channel: beyond Kelvin waves

Nonlinear advective adjustment of a discontinuity in free-surface height under gravity and rotation is considered, using the method of contour dynamics. After linear wave-adjustment has set up an interior jet and boundary currents in a wide ([dbl greater-than sign] one Rossby radius) channel, fluid surges down-channel on both walls, rather than only that wall supporting a down-channel Kelvin wave. A wedgelike intrusion of low potential vorticity fluid on this wall, and a noselike intrusion of such fluid on the opposite wall, serve to reverse the sign of relative vorticity in the pre-existing currents. For narrower channels, a coherent boundary-trapped structure of low potential vorticity fluid is ejected at one wall, and shoots ahead of its parent fluid. The initial tendency for the current to concentrate on the ‘right-hand’ wall (the one supporting a down-channel Kelvin wave in the northern hemisphere) is defeated as vorticity advection shifts the maximum to the left-hand side. Ultimately fluid washes downstream everywhere across even wide channels, leaving the linearly adjusted upstream condition as the final state. The time necessary for this to occur grows exponentially with channel width. The width of small-amplitude boundary currents in linear theory is equal to Rossby's deformation radius, yet here we find that the width of the variation in velocity and potential vorticity fields deviates from this scale across a large region of space and time. Comparisons of the contour dynamics solutions, valid for small amplitude, and integration of the shallow-water equations at large amplitude, show great similarity. Boundary friction strongly modifies these results, producing fields more closely resembling the linear wave-adjusted state. Observed features include those suggestive of coastally trapped gravity currents. Analytical results for the evolution of vorticity fronts near boundaries are given in support of the numerical experiments.

Non-inductively driven currents in JET

Neutral beam heating data from JET have been analysed in detail to determine what proportion of the current is driven non-inductively. It is found that in low density limiter discharges, currents of the order of 0.5 MA are driven, while in H-mode plasmas currents of the order of 0.7 MA are measured. These measured currents are found to be in reasonable agreement with theoretical predictions based on neoclassical models. In low density plasmas the beam driven current is large while the neoclassical bootstrap current dominates H-mode plasmas.

Physical and biological features across an upwelling front in the southern Benguela

The upwelling front of the Cape Columbine upwelling centre was intensively studied, physically and biologically, along a repeated transect during December 1984 following a quiescent phase in the upwelling cycle. Three distinct zones were evident, an inshore zone influenced by upwelling, an offshore warm oligotrophic zone and a transitional frontal zone separating the two. Salinity proved to be a useful indicator of recent water movements. There was evidence of intrusions and mixing of water types within the frontal zone, possibly accounting for the elevated phytoplankton biomass recorded there. Floral and faunal changes occurred between the frontal and offshore zones, corresponding to the thermal front. The predominant flow was alongshore, with strong equatorward jet currents, making the interpretation of cross-shelf gradients difficult in this dynamic area. Aspects of the distributions of organisms and their productivity across the upwelling front are described with respect to the hydrographic parameters...

Water wave propagation across a shearing current

An analogue of the mild-slope equation is obtained for small-amplitude water waves propagating across a shearing current. This is shown to conserve wave action flux. The results of numerical solutions of this equation are presented for both jet and shear-layer currents. These support two conjectures which have appeared in the literature.

Investigations into the mechanism of electrohydrodynamic spraying of liquids: II. Mechanism of stable jet formation and electrical forces acting on a liquid cone

The mechanism of stable jet formation was investigated by observing the motion of the liquid in a jet close to the capillary tip from which it emerges under the influence of an electric field. Tracer particles of lycopodium were inserted in the liquid and streak photographs were taken of a stable jet formed under the application of 10.5 kV and a flow rate of 30 ml/h. These photographs show an axisymmetric circulation of the liquid in the conical base of the jet, invalidating previous theories that have assumed a uniform velocity profile in the liquid cone. This axisymmetric motion of the liquid in the jet was explained in terms of interfacial electrical shear stresses. Due to the semi-insulating nature of the liquid, there will exist a potential difference between the base of the capillary and the tip of the cone. This potential drop ensures that the interface is subjected to a tangential electric field in the direction of flow and hence an electric shear stress on the surface of the cone. Both tangential and normal fields on the cone were calculated from a knowledge of the jet profile and current. The tangential field on the surface of the cone was calculated by considering it to be a section of a sphere and dividing it into five sections. Each section was assumed to form an equipotential surface normal to the direction of flow and to have a constant current flowing through it. The normal field was calculated numerically using a computer program that estimates the potential distribution within a region subject to given boundary conditions using the finite element method. The results of calculation showed that the shear force acting on the cone was about 5 to 10 times smaller than the normal force. However, the normal force was found to be fairly constant and independent of applied voltage, within the range studied. In contrast, the shear force showed a tendency to decrease with an increase in applied voltage. It was also observed that as the voltage was increased, the length of the jet decreased. This could be explained in terms of the reduction of tangential shear force with an increase in applied voltage, thus reducing jet stability.

Numerical simulation and analysis of the mean coastal circulation off California

A two-dimensional numerical model is applied to a coastal ocean wherein alongshore elevation and density gradients, normally calculated by a three-dimensional model, are instead supplied by climatologically averaged data for the California Current System between 25 and 40°N. Surface wind stress is also obtained from climatological data. Both surface and bottom boundary layers are resolved in the model calculations; a second moment turbulence closure submodel supplies vertical diffusivities. Near steady state solutions are possible when surface buoyancy flux is imposed at the surface. Model results are as follows: Southward wind stress produces a broad equatorward current with an embedded coastal jet in accordance with previous studies. Positive wind stress curl reduces the jet current and produces a poleward undercurrent which then surfaces as the curl is increased. The jet currents are reduced and poleward flow increases as bottom steepness increases; to a lesser extent, inclusion of the beta effect has a similar effect. The existence of near bottom, poleward or equatorward flow is explained rather simply in terms of the bottom stress resulting from the alongshore balance of surface wind stress and vertically integrated pressure gradient, the latter involving the alongshore surface elevation and density gradient. A further finding is that the upwelling circulation associated with wind stress is confined to the top 200 to 300 m of the ocean along the California coast.

Transport of anchovy, Engraulis capensis Gilchrist, eggs and early larvae by a frontal jet current

Transport of anchovy, Engraulis capensis Gilchrist, eggs and early larvae by a frontal jet current