Plume Fluid Research Articles

Filling Box Flows in an Axisymmetric Porous Medium

We present an analytical solution for buoyancy-driven “filling box” flows in axisymmetric porous media having closed bottom and side boundaries. The flow consists first and foremost of a descending, point source plume. When plume fluid reaches the (horizontal) bottom boundary, it begins to flow radially outward in the form of an axisymmetric gravity current. The leading edge of the gravity current advances with time as $$t^{1/2}$$ until it reaches the vertical sidewalls. At this point, the flow is characterized by a vertically ascending “first front” that steadily advects towards the plume source. We assume the plume to be in a Darcy regime, i.e. $$\text{ Re }\mathop {\sim }\limits ^{<}\mathcal {O}(10)$$ , with $$\text{ Pe }>\mathcal {O}(1)$$ , where Re and Pe are, respectively, the Reynolds and Peclet numbers, and derive a similarity solution for the plume by applying a boundary layer approximation. Formulas are therefore obtained for the vertical variation of the plume volume flux and area-averaged concentration. The former result shows important qualitative differences with the analogue equation derived in the limit $$\text{ Pe }<\mathcal {O}(1)$$ . In particular, the plume volume flux is now predicted to explicitly depend on the reservoir permeability, plume buoyancy flux and fluid viscosity. The gravity current problem is likewise solved using a self-similar solution, this time adapted from the work of Lyle et al. (J Fluid Mech 543:293–302, 2005) but connected to the outflow conditions of the plume. Finally, in solving for the motion of the first front, we apply a volume flux balance equation and therefore estimate the time scale required for the first front to advect from the bottom of the control volume to the source elevation. By synthesizing the above results, we can estimate the total volume of source fluid and mass of solute that can be injected into an axisymmetric reservoir without overflow. Predictions can also be made for the time-variable mean concentration of this contaminated fluid layer, which must obviously be less than the source concentration.

Filling box flows in porous media

We report upon a theoretical and experimental investigation of a porous medium ‘filling box’ flow by specifically examining the details of the laminar descending plume and its outflow in a control volume having an impermeable bottom boundary and sidewalls. The plume outflow initially comprises a pair of oppositely directed gravity currents. The gravity currents propagate horizontally until they reach the lateral sidewalls at $y=\pm L$. The flow then becomes of filling box type, with a vertically ascending ‘first front’ separating discharged plume fluid below from ambient fluid above. The flow details are described analytically by first deriving a new similarity solution for Darcy plumes with $\mathit{Pe}&gt;O(1)$, where $\mathit{Pe}$ is the Péclet number. From the similarity solution so obtained, we then derive expressions for the plume volume flux and mean reduced gravity as functions of the vertical distance from the source. Regarding the plume outflow, a similarity solution adopted from Huppert &amp; Woods (J. Fluid Mech., vol. 292, 1995, pp. 55–69) describes the height and front speed of the gravity currents, whereas a semi-implicit finite difference scheme is used to predict the first front elevation versus time and horizontal distance. As with high-Reynolds-number filling box flows, that studied here is an example of a coupled problem: the gravity current source conditions are prescribed by the plume volume flux and mean reduced gravity. Conversely, discharged plume fluid may be re-entrained into the plume, be it soon or long after reaching the bottom impermeable boundary. To corroborate our model predictions, analogue laboratory experiments are performed with fresh water and salt water as the working fluids. Our experiments consider as independent variables the porous medium bead diameter and the plume source volume flux and reduced gravity. Predictions for the gravity current front position and height compare favourably against analogue measured data. Good agreement is likewise noted when considering either the mean elevation or the profile of the first front. Results from this study may be adopted in modelling geological plumes. For example, our equations can be used to predict the time required for discharged plume fluid to return to the point of injection in the case of aquifers closed on the sides and below by impermeable boundaries.

On the transport of heavy particles through a downward displacement-ventilated space

We investigate the transport of relatively heavy, small particles through a downward displacement-ventilated space. A flux of particles is supplied to the space from a localised source at a high level and forms a turbulent particle-laden plume which descends through the space. A constant flow of ambient fluid which does not contain particles is supplied to the space at a high level, while an equal amount of fluid is vented from the space at a low level. As a result of the entrainment of ambient fluid into the particle plume, a return flow is produced in the ambient fluid surrounding the plume in the lower part of the space. At steady state, particles are suspended by this return flow. An interface is formed which separates the ambient fluid in the lower part of the space, which contains particles, from the particle-free ambient fluid in the upper part of the space. New laboratory experiments show that the concentration of particles in the ambient fluid below the interface is larger than the average concentration of particles in the plume fluid at the level of the interface. Hence, as the plume fluid crosses the interface and descends through the particle-laden fluid underneath, it becomes relatively buoyant and forms a momentum-driven fountain. If the fountain fluid impinges on the floor, it then spreads radially over the surface until lifting off. We develop a quantitative model which can predict the height of the interface, the concentration of particles in the lower layer, and the partitioning of the particle flux between the fraction which sediments over the floor and that which is ventilated out of the space. We generalise the model to show that when particles and negatively buoyant fluid are supplied at the top of the space, a three-layer stratification develops in the space at steady state: the upper layer contains relatively low-density ambient fluid in which no particles are suspended; the central layer contains a mixture of ambient and plume fluid in which no particles are suspended; and the lower layer contains a suspension of particles in the same mixture of ambient and plume fluid. We quantify the heights of the two interfaces which separate the three layers in the space and the concentration of particles in suspension in the ambient fluid in the lower layer. We then discuss the relevance of the results for the control of airborne infections in buildings. Our experiments show that the three-layer stratification is subject to intermittent large-scale instabilities when the concentration of particles in the plume at the source is sufficiently small, or the rate of ventilation of the space is sufficiently large: we describe the transient concentration of particles in the space during one of these instabilities.

On the transport of heavy particles through an upward displacement-ventilated space

We explore the transport of heavy particles through an upward displacement-ventilated space. The space incorporates a localised source of buoyancy which generates a turbulent buoyant plume. The plume fluid is contaminated with a small concentration of particles, which are subject to gravitational settling. A constant flow of uncontaminated fluid is supplied at a low level into the space, while an equal amount of fluid is vented from the space at a high level. At steady state, a two-layer density stratification develops associated with the source of buoyancy. New laboratory experiments are conducted to explore how particles are transported by this flow. The experiments identify that the upper layer may either become well-mixed in particles or it may develop a vertical stratification in particle concentration, with the particle concentration decreasing with height. We develop a quantitative model which identifies that such stratification develops for larger particle setting speeds, or smaller ventilation rates. In accord with our experiments, the model predicts that the number of particles extracted from the space through the high-level vent is controlled by the magnitude of the particle stratification in the upper layer, and this in turn depends on the particle settling speed relative to the ventilation speed and also the cross-sectional area and height of the space. We compare the predictions of the model with measurements of the flux of particles vented from the space for a range of operating conditions. We explore the relevance of the model for the removal of airborne contaminants by displacement ventilation in hospital rooms, and we discuss how contamination is propagated in the room as a result of lateral mixing of pathogens in the upper layer.

Experimental investigation of the free convection velocity boundary layer and plume formation region for a heated horizontal cylinder

Particle image velocimetry measurements were obtained in the free convection water flow around a heated horizontal cylinder for a Rayleigh number of 1.33×106 and a Prandtl number of 5.98. Radial and circumferential velocity were investigated within and just outside the velocity boundary layer in a vertical plane perpendicular to the cylinder axis. Radial and axial velocity were observed in the early plume development just above the top of the cylinder in a vertical plane parallel to and passing through the cylinder axis. The submersion depth of the top of the cylinder below the free water surface normalized by cylinder diameter ranged from H/D=2 to 8, which includes depths for which the plume sways and meanders in periodic motion. The effects of plume swaying and meandering on the velocity boundary layer and plume formation were investigated. It was found that the approach flow outside the boundary layer was affected by penetrative convection caused by swaying and meandering of the plume even though there is no horizontal confinement. Measurements within the boundary layer as it nears the plume formation region and within the plume formation region itself reveal that penetrative convection in the approach flow altered the boundary layer and these effects propagated downstream into the plume formation region. The data therefore identified a feedback loop for which the plume fluid returns to the boundary layer sustaining the swaying motion. A direct cause of meandering was not determined.

Simulation of hydrothermal vents in the Izena Cauldron, Mid Okinawa trough, Japan and other Pacific locations

Simulation results of Jade hydrothermal vent in Okinawa trough, Japan and Dante hydrothermal vent in Endeavour ridge in the north east Pacific are presented using an improved version of MOHTV model. The results are presented along with some limited field comparisons. A limited study on the sensitivity of model results to key model parameters is also presented. The model used is capable of simulating the hydrodynamics, thermodynamics, and mineral formations in underwater hydrothermal vents. The transport and spread of the plume fluid and minerals formed are simulated in three stages: plume dynamics stage that is momentum and buoyancy driven; transition to far field conditions as a gravity current; and far field conditions where the mineral particles move according to advection diffusion governed by the ambient currents and mineral particle settling velocities that eventually lead to their bed deposition. Thermodynamics include the changes in plume temperature and its related properties such as the vent fluid density. Physico-chemical processes are the chemical reactions that occur in the plume due to the hot vent fluid mixing with cold ambient water. Chemical reactions form new compounds (mostly minerals), which change the plume properties and therefore its behavior. Model simulates the formation of minerals namely FeS, FeS2, ZnS, CuFeS2, CuS, PbS, CaSO4, BaSO4, and Particulate Manganese – PMn. This improved version of MOHTV considers a user defined particles size distribution, the bathymetry of the ocean bottom, and can be used to run long-term simulations. The model results compare reasonably well with the field data. The parametric analyses show, which input and/or ambient conditions most affect the distribution of particles.

Modelling of hydrothermal vent plumes to assess the mineral particle distribution

A new numerical model that integrates hydrodynamics, thermodynamics, and the chemistry of mineral formation in deepwater hydrothermal vents is developed. Transport and spread of plume fluid and the minerals formed are simulated in three stages: plume dynamics stage that is momentum and buoyancy driven; transition to far-field conditions as a gravity current; and far-field conditions where the mineral particles move according to advection–diffusion governed by ambient currents and settling velocities that eventually lead to bed deposition. Thermodynamics include the change in plume temperature and its related properties. Chemical reactions due to hot vent fluid mixing with cold entrained ambient water into the plume change its properties and the behaviour. The model considers the formation of several types of minerals. Model simulations and field measurements compare reasonably well for plumes in Atlantic and Pacific Oceans. Results for a simulation in East Pacific Rise 21°N are presented.

Laboratory experiments on two coalescing axisymmetric turbulent plumes in a rotating fluid

We investigate the early-time coalescence of two co-flowing axisymmetric turbulent plumes and the later-time flow of the induced vortices in a rotating, homogeneous fluid using laboratory experiments. The experiments demonstrate the critical importance of the rotation period Tf=2π/f, where f is the Coriolis parameter of the background rotation. We find that if the plumes’ sources are sufficiently “close” for the plumes to merge initially at an “early time” tm≲tr=3Tf/4, the experimentally observed merging height zme agrees well with the non-rotating theoretical relationship of zmt≈(0.44/α)x0&amp;lt;zr=5.5F01/4f-3/4, where α is the entrainment “constant” of the turbulent plumes, x0 is the separation distance between the two plume sources, F0 is the source buoyancy flux of each plume, and zr is the distance that the plume rises in the time tr before rotational effects become significant. Therefore, rotation does not affect the initial time to merger or the initial merger height of such “close” plumes. For “late” times t&amp;gt;tr, however, the flow dynamics are substantially more complicated, as the flow becomes significantly affected by rotation. The propagation and entrainment of the plumes becomes strongly affected by the vortices induced by the entrainment flow in a rotating environment. Also, the plume fluid itself starts to interact with these vortices. If the plumes have already initially merged by the time t=tr, a single vortex (initially located at the midpoint of the line connecting the two plume sources) develops, which both advects and modifies the geometry of the merging plumes. Coupled with the various suppressing effects of rotation on the radial plume entrainment, the “apparent” observed height of merger can vary substantially from its initial value. Conversely, for more widely separated “distant” plumes, where x0&amp;gt;xc=(25α/2)F01/4f-3/4, the plumes do not merge before the critical time tr when rotation becomes significant in the flow dynamics and two vortices are observed, each located over a plume source. The combined effect of these vortices with the associated suppression of entrainment by rotation thus significantly further delays the merger of the two plumes, which apparently becomes possible only through the merger of the induced vortices.

Time-dependent ventilation flows driven by opposing wind and buoyancy

We consider transient flow in a box containing an isolated buoyancy source, ventilated by a windward high-level opening and a leeward low-level opening, so that prevailing wind acts to oppose buoyancy-driven flow. Hunt &amp; Linden (J. Fluid Mech., vol. 527, 2005, p. 27) demonstrated that two stable steady states can exist above a critical wind strength: buoyancy-driven displacement ventilation with a two-layer stratification and wind-driven mixing ventilation with the whole interior contaminated by buoyant fluid. We present two time-dependent models for this system: a nonlinear ordinary differential equation (ODE) model following Kaye &amp; Hunt (J. Fluid Mech., vol. 520, 2004, p. 135), assuming ‘perfect’ vertical mixing of fluid within each layer, and a partial differential equation model assuming zero vertical mixing, following Germeles (J. Fluid Mech., vol. 71, 1975, p. 601).We apply these models to an initial-value problem – the filling box with constant opposing wind. The interface between the upper hot plume fluid and the lower cool ambient air can dramatically overshoot its final level before relaxing to equilibrium; in some cases, a fully contaminated transient can occur before the buoyancy-driven two-layer steady state is reached. However, we find that for an initially completely uncontaminated box, the system converges to a stable wind-driven steady state whenever it exists. By analysing phase diagrams of the ODE model for the flow, we establish a general method of determining which final state is attained and also explain the hysteresis observed by Hunt &amp; Linden (2005). We confirm these transient behaviours by conducting salt bath experiments in a recirculating flume tank and establish quantitative agreement between theory and experiment. Our ‘zero mixing’ model is more accurate than our ‘perfect mixing’ model for our experiments, as the upper layer remains stratified for a substantial time.

The formation and evolution of stratification during transient mixing ventilation

We investigate the buoyancy-driven ventilation of an enclosed volume of buoyant fluid, which is connected to the exterior through two openings at the top of the enclosure. An exchange flow becomes established, with outflow through one opening being matched by an equal and opposite inflow through the other vent. The inflowing flux of dense exterior fluid develops a turbulent buoyant plume, which mixes with the interior fluid as it cascades to the base of the fluid volume. An upward return flow gradually stratifies the confined volume of fluid, with a first front of relatively dense plume fluid advancing to the top of the space. Initially, the exchange flow is steady, but as the first front rises above the inflow opening, the flow rate wanes. The initial development of the exchange flow and interior stratification follows the classical filling box work of Baines &amp; Turner (J. Fluid Mech., vol. 37, 1969, p. 51), with a plume of constant buoyancy flux. Once the first front exits the space, the volume flux rapidly asymptotes to the form Q = Q0(τ/(τ+t)) from the initial value Q0, where τ is the buoyancy-driven draining time, based on the initial density contrast with the environment. The vertical structure of the interior density stratification asymptotes to a profile of constant shape whose amplitude decays in time as (τ/(τ+t))2, and we identify conditions under which the vertical variation in density is comparable to the difference between mean interior density and the exterior. This generalises the analysis presented by Linden, Lane-Serff &amp; Smeed (J. Fluid Mech., vol. 212, 1990, p. 309), who assumed that the interior fluid is well mixed. New laboratory experiments of the process are shown to be consistent with our predictions of the evolution of the flow, the interior stratification and the migration of contaminants. We also develop our analysis for situations where there are multiple stacks and show how this improves the mixing efficiency for a given ventilation flow. The model has relevance for the design of transient mixing ventilation in a building, especially when the effect of vertical stratification is important for ensuring thermal comfort.

Search for the proverbial mantle osmium sources to the oceans: Hydrothermal alteration of mid-ocean ridge basalt

We present Os and Sr isotopes and Mg, Os, and Sr concentrations for ridge-crest high-temperature and diffuse hydrothermal fluids, plume fluids and ridge-flank warm spring fluids from the Juan de Fuca Ridge. The data are used to evaluate the extent to which (1) the high- and low-temperature hydrothermal alteration of mid-ocean ridge basalts (MORBs) provides Os to the deep oceans, and (2) hydrothermal contributions of non-radiogenic Os and Sr to the oceans are coupled. The Os and Sr isotopic ratios of the high-temperature fluids (265–353 °C) are dominated by basalts ( 187Os/ 188Os = 0.2; 87Sr/ 86Sr = 0.704) but the concentrations of these elements are buffered approximately at their seawater values. The 187Os/ 188Os of the hydrothermal plume fluids collected ∼1 m above the orifice of Hulk vent is close to the seawater value (=1.05). The low-temperature diffuse fluids (10–40 °C) associated with ridge-crest high-temperature hydrothermal systems on average have [Os] = 31 fmol kg −1, 187Os/ 188Os = 0.9 and [Sr] = 86 μmol kg −1, 87Sr/ 86Sr = 0.709. They appear to result from mixing of a high-temperature fluid and a seawater component. The ridge-flank warm spring fluids (10–62 °C) on average yield [Os] = 22 fmol kg −1, 187Os/ 188Os = 0.8 and [Sr] = 115 μmol kg −1, 87Sr/ 86Sr = 0.708. The data are consistent with isotopic exchange of Os and Sr between basalt and circulating seawater during low-temperature hydrothermal alteration. The average Sr concentration in these fluids appears to be similar to seawater and consistent with previous studies. In comparison, the average Os concentration is less than seawater by more than a factor of two. If these data are representative they indicate that low-temperature alteration of MORB does not provide adequate non-radiogenic Os and that another source of mantle Os to the oceans must be investigated. At present, the magnitude of non-radiogenic Sr contribution via low-temperature seawater alteration is not well constrained. If non-radiogenic Sr to the oceans is predominantly from the alteration of MORB, our data suggest that there must be a different source of non-radiogenic Os and that the Os and Sr isotope systems in the oceans are decoupled.

Local implications for self-similar turbulent plume models

The local implications of the well-known flux conservation equations of Morton et al. (Proc. R. Soc. Lond. A, vol. 234, 1956, p.1) for plumes and jets are considered. Given the vertical velocity distributions of a model plume or jet, the divergence-free radial velocity distributions are calculated. It is shown that in general the velocity of the plume boundary is not described by the local total fluid velocity in this way. A two-fluid model tracking the evolution of both ‘plume’ and ‘ambient’ fluid is proposed which resolves this apparent inconsistency and also provides a way of explicitly describing the mixing process within a model plume. The plume boundary acts as a phase boundary across which ambient fluid is entrained, and the plume boundary moves at the velocity of the plume fluid. The difference between the plume-fluid radial velocity and the total fluid velocity quantifies in a natural way the purely horizontal entrainment flux of ambient fluid into the plume across the phase boundary at the plume edge.

Integral Model of a Multiphase Plume in Quiescent Stratification

The writers present a one-dimensional integral model to describe multiphase plumes discharged to quiescent stratified receiving waters. The model includes an empirical submodel for detrainment, and the capability to include dispersed phase dissolution. Model equations are formulated by conservation of mass, momentum, heat, dissolved species concentration, and salinity, and allow the tracking of dissolved material and changes in plume density due to solute density effects. The detrainment (or peeling) flux, Ep, is assumed to be a function of the dispersed phase slip velocity, ub, the integrated plume buoyancy, Bi, and the momentum of the entrained plume fluid, characterized by the fluid velocity, ui, given by the general relationship Ep=ε(ub∕ui)2(Bi∕ui2). The parameter ε is calibrated to laboratory experimental data. Because Ep is based on a force balance, this algorithm allows numerical models to reproduce a wide range of characteristic plume behavior. Such a predictive algorithm is important for applying models to field scale plumes, especially where chemical processes within the plume may alter plume buoyancy (and hence peeling behavior), as in the case of a CO2 droplet plume used for ocean sequestration of CO2.

Time-dependent plumes and jets with decreasing source strengths

The classical bulk model for isolated jets and plumes due to Morton, Taylor &amp; Turner (Proc. R. Soc. Lond. A, vol. 234, 1956, p. 1) is generalized to allow for time-dependence in the various fluxes driving the flow. This new system models the spatio-temporal evolution of jets in a homogeneous ambient fluid and Boussinesq and non-Boussinesq plumes in stratified and unstratified ambient fluids.Separable time-dependent similarity solutions for plumes and jets are found in an unstratified ambient fluid, and proved to be linearly stable to perturbations propagating at the velocity of the ascending plume fluid. These similarity solutions are characterized by having time-independent plume or jet radii, with appreciably smaller spreading angles ( is the conventional entrainment coefficient. These new similarity solutions are closely related to the similarity solutions identified by Batchelor (Q. J. R. Met. Soc., vol. 80, 1954, p. 339) in a statically unstable ambient, in particular those associated with a linear increase in ambient density with height.If the source buoyancy flux (for a rising plume) or source momentum flux (for a rising jet) is decreased generically from an initial to a final value, numerical solutions of the governing equations exhibit three qualitatively different regions of behaviour. The upper region, furthest from the source, remains largely unaffected by the change in buoyancy flux or momentum flux at the source. The lower region, closest to the source, is an effectively steady plume or jet based on the final (lower) buoyancy flux or momentum flux. The transitional region, in which the plume or jet adjusts between the states in the lower and upper regions, appears to converge very closely to the newly identified stable similarity solutions. Significantly, the predicted narrowing of the plume or jet is observed. The size of the narrowing region can be determined from the source conditions of the plume or jet. Minimum narrowing widths are considered with a view to predicting pinch-off into rising thermals or puffs.

Role of Slip Velocity in the Behavior of Stratified Multiphase Plumes

This paper presents direct measurements of multiphase plumes in stratification and correlates characteristic plume properties with the nondimensional slip velocity UN . UN is defined as the ratio of the bubble slip velocity us to a characteristic plume fluid rise velocity (BN)1∕4 ; B is the total kinematic buoyancy flux, and N is the buoyancy frequency. UN is derived by dimensional analysis and is compared to other nondimensional parameters for multiphase plumes in the literature. To investigate correlations of the nondimensional numbers with plume properties, laboratory experiments were conducted in linear stratification using dispersed phases of air, oil, and glass beads (creating an inverted plume). A new type of plume behavior is identified, called Type 1* , in which the horizontal motion in the first detrainment event disperses the relatively weak bubbles, creating a diffuse bubble plume that forms out of the intrusion. Measurements from these and previous experiments for type behavior, trap height, ...

Geochemical evolution of highly alkaline and saline tank waste plumes during seepage through vadose zone sediments

Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.

Water-column measurements of hydrothermal vent flow and particulate concentration using multibeam sonar

High-frequency multibeam systems enable water-column studies of fluid flow from deep ocean hydrothermal vent systems. Backscatter from high-temperature particulate-laden vent fluid (black smokers) is used to obtain three-dimensional images of the plume’s temporal and spatial structure. Backscattering intensity may provide a means to remotely estimate plume particulate concentration and turbulent micro-structure created by temperature and density fluctuations in the plume fluid, providing denser sampling in space and time than conventional point measurements. Acoustic Doppler measurements can provide estimates of the entrainment and transport of water, particulate matter, and larvae by hydrothermal systems, which is of interest in physical, chemical, and biological oceanography. Toward the goal of providing an acoustic means of inverting for such oceanographic parameters with a multibeam system, several mechanisms and models of volume scattering from the water column will be discussed. The results from field experiments with a calibrated 200-kHz multibeam system (Simrad SM2000) will be presented, as well as future developments and applications of mutlibeam systems to water-column scattering. [Work supported by NSF.]

Hydrothermal temperature changes at the southern Juan de Fuca Ridge associated with MW 6.2 Blanco Transform earthquake

The regional impact of transform seismicity on ridge-crest hydrothermal venting and ridge-transform dynamics is investigated using a June 1‐7, 2000, Blanco Transform earthquake sequence. The mainshock (Mw 6.2) and 170 foreshocks and aftershocks were located using T waves recorded on U.S. Navy hydrophones, and indicate that the active transform fault may be farther south than in previous tectonic models. During the earthquake sequence, two temperature probes were deployed in black smoker chimneys at the Vent1 and Plume hydrothermal fields along the southern Juan de Fuca Ridge. These two hydrothermal systems are ;39 km northwest of the mainshock’s acoustic location. Both probes show significant (.5 8C) temperature declines following the mainshock, the Vent1 temperature changes occurring over days to weeks while the Plume changes were coseismic. The Vent1 and Plume fluid temperature decreases are consistent with earthquakeinduced changes to permeability in the upper ocean crust. The evidence suggests that deep-ocean hydrothermal systems can be altered by large earthquakes even at a distance and across tectonic provinces, and ridge-crest seismic and magmatic activity are not the only causes of change to hydrothermal systems.

Vortex generation by line plumes in a rotating stratified fluid

We present the results of an experimental investigation of the generation of coherent vortical structures by buoyant line plumes in rotating fluids. Both uniform and stratified ambients are considered. By combining the scalings describing turbulent plumes and geostrophically balanced vortices, we develop a simple model which predicts the scale of the coherent vortical structures in excellent accord with laboratory experiments.We examine the motion induced by a constant buoyancy flux per unit length B, released for a finite time ts, from a source of length L into a fluid rotating with angular speed Ω = f/2. When the plume discharges into a uniformly stratified environment characterized by a constant Brunt–Väisälä frequency, N&gt;f, the fluid rises to its level of neutral buoyancy unaffected by the system rotation before intruding as a gravity current. Rotation has a strong impact on the subsequent dynamics: shear develops across the spreading neutral cloud which eventually goes unstable, breaking into a chain of anticyclonic lenticular vortices. The number of vortices n emerging from the instability of the neutral cloud, n = (0.65±0.1)Lf1/2/ (t1/2sB1/3), is independent of the ambient stratification, which serves only to prescribe the intrusion height and aspect ratio of the resulting vortex structures. The experiments indicate that the Prandtl ratio characterizing the geostrophic vortices is given by P = Nh/(fR) = 0.47±0.12; where h and R are, respectively, the half-height and radius of the vortices. The lenticular vortices may merge soon after formation, but are generally stable and persist until they are spun-down by viscous effects.When the fluid is homogeneous, the plume fluid rises until it impinges on a free surface. The nature of the flow depends critically on the relative magnitudes of the layer depth H and the rotational lengthscale Lf = B1/3/f. For H&gt;10Lf, the ascent phase of the plume is influenced by the system rotation and the line plume breaks into a series of unstable anticylonic columns of characteristic radius (5.3±1.0)B1/3/f which typically interact and lose their coherence before surfacing. When H&lt;10Lf, the system rotation does not influence the plume ascent, but does control the spreading of the gravity current at the free surface. In a manner analogous to that observed in the stratified ambient, shear develops across the surface current, which eventually becomes unstable and generates a series of anticyclonic surface eddies with characteristic radius (1.6±0.2)B1/3t1/3s /f2/3. These surface eddies are significantly more stable than their columnar counterparts, but less so than the lenticular eddies arising in the uniformly stratified ambient.The relevance of the study to the formation of coherent vortical structures by leads in the polar ocean and hydrothermal venting is discussed.

Experiments on a plume with off‐source heating: Implications for cloud fluid dynamics

We report here on a series of laboratory experiments on plumes, undertaken with the object of simulating the effect of the heat relase that occurs in clouds on condensation of water vapor. The experimental technique used for this purpose relies on ohmic heating generated in an electrically conducting plume fluid subjected to a suitable alternating voltage across specified axial stations in the plume flow [Bhat et al., 1989]. The present series of experiments achieves a value of the Richardson number that is toward the lower end of the range that characterizes cumulus clouds. It is found that the buoyancy enhancement due to heating disrupts the eddy structures in the flow and reduces the dilution owing to entrainment of ambient fluid that would otherwise have occurred in the central region of the plume. Heating also reduces the spread rate of the plume, but as it accelerates the flow as well, the overall specific mass flux in the plume does not show a very significant change at the heat input employed in the experiment. However, there is some indication that the entrainment rate (proportional to the streamwise derivative of the mass flux) is slightly higher immediately after heat injection and slightly lower farther downstream. The measurements support a previous proposal for a cloud scenario [Bhat and Narasimha, 1996] and demonstrate how fresh insights into certain aspects of the fluid dynamics of clouds may be derived from the experimental techniques employed here.