Buoyancy-driven Exchange Flows Research Articles

Contaminant transport by human passage through an air curtain separating two sections of a corridor: Part II – Two zones at different temperatures

Air curtains are installed in open doorways of a building to reduce buoyancy-driven exchange flows across the doorway. Although an air curtain allows an unhampered passage of humans and vehicles, the interaction of this traffic with an air curtain is not well understood. In this study, we investigate the problem of the simultaneous interaction between the air curtain, the wake of a moving person and the buoyancy-driven flow arising due to the density difference across the doorway. To this end, we conduct small-scale waterbath experiments with fresh water and salt water solutions to achieve different fluid densities. As a model of human passage, a vertical cylinder is pulled through a planar jet representing an air curtain and separating two zones at different densities. For a fixed travel distance of the cylinder before and after the air curtain, the average infiltration flux of dense fluid into the light fluid side increases with increasing cylinder velocity. Remarkably, we find that the infiltration flux is independent of the density difference across the doorway and is mainly due to the interaction between the air curtain and the cylinder wake with negligible effects from the buoyancy-driven flow. Furthermore, the infiltration flux is also independent of the travel direction of the cylinder. As a consequence, the sealing effectiveness of an air curtain reduces with an increasing cylinder speed and this reduction is independent of the direction of the buoyancy-driven flow. We provide a theoretical explanation for the observed changes in the effectiveness curve of the air curtain as the function of the deflection modulus. Dye visualisations of the air curtain and the cylinder wake are used to examine the re-establishment process of the air curtain after its disruption by the cylinder. We observe that the re-establishment time of the air curtain and the infiltration in the cylinder wake increases with an increasing cylinder speed.

Convective tipping point initiates localization of basaltic fissure eruptions

Basaltic fissure eruptions may evolve rapidly and unpredictably complicating hazard management. Localization of an elongate fissure to one or more focused vents may take days to months, and depends on fluid dynamic processes, such as thermally-driven viscous fingering, in the sub-volcanic plumbing system. However, fluid dynamics in a dyke geometry are poorly understood. We perform scaled analogue experiments to investigate convective magma exchange flow within a dyke-like conduit, and discover flow regimes ranging from chaotic mingling to stable, well-organized exchange, over the parameter space relevant for natural eruptions. Experiments are scaled via the Grashof number Gr, which is a Reynolds number for buoyancy-driven exchange flows. We propose that chaotic exchange at high Gr hinders thermally-driven localization by suppressing viscous fingering, whereas flow organization at low Gr enhances localization. Consequently, progressive decrease in Gr through increasing magma viscosity or decreasing dyke width pushes a fissure eruption towards a tipping point that results in rapid localization. Our findings indicate that current conceptual models for magma flow in a dyke require revision to account for this convective tipping point, and provide a quantitative framework for understanding the evolution of fissure eruptions.

Buoyancy-driven exchange flows in inclined ducts

Buoyancy-driven exchange flows arise in the natural and built environment wherever bodies of fluids at different densities are connected by a narrow constriction. In this paper we study these flows in the laboratory using the canonical stratified inclined duct experiment, which sustains an exchange flow in an inclined duct of rectangular cross-section over long time periods (Meyer & Linden, J. Fluid Mech., vol. 753, 2014). We study the behaviour of these sustained stratified shear flows by focusing on three dependent variables of particular interest: the qualitative flow regime (laminar, wavy, intermittently turbulent, or fully turbulent), the mass flux (net transport of buoyancy between reservoirs), and the interfacial thickness (thickness of the layer of intermediate density between the two counter-flowing layers). Dimensional analysis reveals five non-dimensional independent input parameters: the duct aspect ratios in the longitudinal direction $A$ and spanwise direction $B$, the tilt angle $\theta$, the Reynolds number $Re$ (based on the initial buoyancy difference driving the flow), and the Prandtl number $Pr$ (we consider both salt and temperature stratifications). After reviewing the literature and open questions on the scaling of regimes, mass flux, and interfacial thickness with $A,B,\theta,Re,Pr$, we present the first extensive, unified set of experimental data where we varied systematically all five input parameters and measured all three output variables with the same methodology. Our results in the $(\theta,Re)$ plane for five sets of $(A,B,Pr)$ reveal a variety of scaling laws, and a non-trivial dependence of all three variables on all five parameters, in addition to a sixth elusive parameter. We further develop three classes of candidate models to explain the observed scaling laws: ...

Buoyancy-driven unbalanced exchange flow through a horizontal opening

Buoyancy-driven exchange flows occur in a variety of natural and industrial situations, including nuclear and hydraulic engineering, oceanography and building ventilation. Balanced exchange flows, whereby there is simultaneously an equal volume flux transferred vertically upwards and downwards through a horizontal opening, have previously been described theoretically. However, until now there has been no theoretical description of unbalanced exchange flows, whereby the volume flux in one direction through an opening exceeds that in the other. The model developed herein examines the growth of perturbations on the density interface at an opening made in a horizontal plane that connects buoyant fluid below with denser fluid above. By considering the interface as it is advected away from the plane of the opening by a bulk flow imposed in the vertical, we quantify the exchange for the unbalanced case. The model successfully predicts the Froude number criterion, which corresponds directly to the minimum dimensionless flow rate of the imposed flow, for the onset of unbalanced exchange across circular openings found experimentally. Additionally, comparisons made between the exchanges predicted and measured show excellent agreement across the entire range of possible flows, from unidirectional flow, through unbalanced exchange to balanced exchange. Consideration is given to applications of the model to ocean outfall design and to the prediction of building ventilation flows. For natural ventilation, the theoretical model we derive for unbalanced exchange bridges the gap in the prediction of air flow rates between displacement flows, where the flow is unidirectional, and balanced exchange flows.

Bistability of buoyancy-driven exchange flows in vertical tubes

Buoyancy-driven exchange flows are common to a variety of natural and engineering systems, ranging from persistently active volcanoes to counterflows in oceanic straits. Laboratory experiments of exchange flows have been used as surrogates to elucidate the basic features of such flows. The resulting data have been analysed and interpreted mostly through core–annular flow solutions, the most common flow configuration at finite viscosity contrasts. These models have been successful in fitting experimental data, but less effective at explaining the variability observed in natural systems. In this paper, we demonstrate that some of the variability observed in laboratory experiments and natural systems is a consequence of the inherent bistability of core–annular flow. Using a core–annular solution to the classical problem of buoyancy-driven exchange flows in vertical tubes, we identify two mathematically valid solutions at steady state: a solution with fast flow in a thin core and a solution with relatively slow flow in a thick core. The theoretical existence of two solutions, however, does not necessarily imply that the system is bistable in the sense that flow switching may occur. Through direct numerical simulations, we confirm the hypothesis that core–annular flow in vertical tubes is inherently bistable. Our simulations suggest that the bistability of core–annular flow is linked to the boundary conditions of the domain, which implies that is not possible to predict the realized flow field from the material parameters of the fluids and the tube geometry alone. Our finding that buoyancy-driven exchange flows are inherently bistable systems is consistent with previous experimental data, but is in contrast to the underlying hypothesis of previous analytical models that the solution is unique and can be identified by maximizing the flux or extremizing the dissipation in the system. Our results have important implications for data interpretation by analytical models and may also have interesting ramifications for understanding volcanic degassing.

Non-isothermal buoyancy-driven exchange flows in inclined pipes

We study non-isothermal buoyancy-driven exchange flow of two miscible Newtonian fluids in an inclined pipe experimentally. The heavy cold fluid is released into the light hot one in an adiabatic small-aspect-ratio pipe under the Boussinesq limit (small Atwood number). At a fixed temperature, the two fluids involved have the same viscosity. Excellent qualitative and quantitative agreement is first found against rather recent studies in literature on isothermal flows where the driving force of the flow comes from salinity as opposed to temperature difference. The degree of flow instability and mixing enhances as the pipe is progressively inclined towards vertical. Similar to the isothermal limit, maximal rate of the fluids interpenetration in the non-isothermal case occurs at an intermediate angle, β. The interpenetration rate increases with the temperature difference. The degree of fluids mixing and diffusivity is found to increase in the non-isothermal case compared to the isothermal one. There has also been observed a novel asymmetric behavior in the flow, never reported before in the isothermal limit. The cold finger appears to advance faster than the hot one. Backed by meticulously designed supplementary experiments, this asymmetric behavior is hypothetically associated with the wall contact and the formation of a warm less-viscous film of the fluid lubricating the cold more-viscous finger along the pipe. On the other side of the pipe, a cool more-viscous film forms decelerating the hot less-viscous finger. Double diffusive effects associated with the diffusion of heat and mass (salinity) are further investigated. In this case and for the same range of inclination angles and density differences, the level of flow asymmetry is found to decrease. The asymmetric behaviour of the flow is quantified over the full range of experiments. Similar to the study of Salort et al. [“Turbulent velocity profiles in a tilted heat pipe,” Phys. Fluids 25(10), 105110-1–105110-16 (2013)] for tilted heat pipes, a small Richardson number of Ri≈0.05 is found, above which flow laminarization occurs. In terms of the dimensionless numbers of the problem, it is found that the interpenetrative speeds of the heavy and light fluid layers in non-isothermal and double-diffusive cases increase with the dimensionless temperature difference, rT, Atwood number, At, Grashof number, Gr, Reynolds number, Re, Nahme number, Na, and Péclet number, Pe but decreases with Prandtl number, Pr, and Brinkman number, Br.

Study on the Experiments and the Numerical Analysis of Exchange Flow Behavior in the Unstably Stratified Field

In the flow mechanism of unstably stratified field, occurs after Rayleigh-Taylor instability. Buoyancy-driven exchange flows were investigated the helium-air flow in the vertical narrow pathway between upper air chamber and lower helium chamber. Exchange flows may occur following the opening of a window for ventilation, when fire breaks out in a room, as well as when a pipe ruptures in a high temperature gas-cooled nuclear reactor. The numerical analysis and experiment in this paper was carried out in a test chamber filled with helium and the flow was visualized using the smoke wire method. The flow behavior was recorded by a high-speed camera combined with a computer system. The image of the flow was transferred to digital data, and the flow velocity was measured by PTV and PIV software. The mass fraction in the test chamber was measured using electronic balance. The detected data was arranged by the densimetric Froude number of the exchange flow rate derived from the dimensional analysis. A method of mass increment was developed and applied to measure the exchange flow rate. As the result, it is revealed that three dimensional structure of counter current exchange flow in the narrow flow path such as rotation and circulation flows by the optical system, mass inclement method and numerical analysis of moving particle method as well as HSMAC method.

Mixing efficiency in controlled exchange flows

Turbulence and mixing are generated by the shear between two counter-flowing layers in hydraulically controlled buoyancy-driven exchange flows through a constriction. From direct measurements of the density distribution and the amount of turbulent mixing in steady laboratory exchange flows we determine the overall efficiency of the mixing. For sufficiently large Reynolds numbers the mixing efficiency is 0.11(±0.01), independent of the aspect ratio and other details of constriction geometry, in good agreement with a scaling analysis. We conclude that the mixing in shear flows of this type has an overall efficiency significantly less than the maximum value widely proposed for stratified turbulence.

C204 高速度カメラ撮影による対向置換流挙動の可視化

Buoyancy-driven exchange flows of helium-air were investigated through inclined a small opening. Exchange flows may occur following a window opening as ventilation, fire in the room as well as a pipe rupture accident in a high temperature gas-cooled nuclear reactor. The experiment has carried out by a test chamber filled with helium and flow was visualized by the smoke wire method. By removing the cover plate placed on the top of the opening, the exchange flow initiated. Air enters the test chamber and the mass of the gas mixture in the test chamber increased. The flow behavior has recorded by a high-speed camera with a computer system. The image of the flow was transferred to the digital data, thus the flow velocity was measured by PTV software. The volumetric exchange flow rate was evaluated from the PTV method and mass increment method. In the case of inclination openings, the results of both methods were compared. The detected data was arranged by the densimetric Floude number of the exchange flow rate that derived from the dimensional analysis.

Sulfate distribution in a multi-basin, saline lake

Devils Lake in northeastern North Dakota (USA) is a terminal lake of glacial origin with a substantial sulfate content. Since 1940, the lake water level has risen over 13.5m and salinity has decreased. Devils Lake consists of a series of interconnected basins with sulfate concentration increasing in an eastward direction from 450mg/l in West Bay to 3000mg/l in East Devils Lake. Using sulfate as a tracer and linking measurements of specific conductance with observed time series of water surface elevations, wind, and water velocity, this paper illustrates the magnitude and dynamics of mass transport and exchange processes between the multiple basins of Devils Lake. Measurements show small horizontal salinity gradients within the lake basins, but large salinity gradients in some of the exchange zones connecting basins. Vertical salinity gradients were observed in small basins and near exchange zones with strong horizontal salinity differentials. These instances of vertical stratification were associated with buoyancy driven exchange flows and wind sheltering. Exchange flow velocities between selected basins were correlated with wind direction and speed. Interbasin exchange flows were found to reverse direction frequently in response to the prevailing wind direction. Flow direction frequencies in one specific exchange zone were 31% west to east, 48% east to west, and 21% bidirectional (stratified). At the lake level observed in 2000, time averaged, measured interbasin exchange flow rates were 30--40m3/s compared to tributary inflow rates to the lake of 3--9m3/s.

Three-dimensional numerical simulations of dust mobilization and air ingress characteristics in a fusion reactor during a LOVA event

Dust mobilization and air ingress characteristics in a fusion reactor during a loss-of-vacuum accident (LOVA) event were analyzed numerically using a newly developed thermal-hydraulic analysis code under the conditions of a compressible flow and three-dimensional cylindrical coordinates. Some physical models on the motion of dust were considered to resolve the dust mobilization characteristics. Air ingress behavior through breaches was calculated by using the compressible Navier–Stokes equations. It was clarified quantitatively from the present numerical results that the predicted average pressures in a vacuum vessel (VV) agree well with the experimental results within an error of 10% for different breach diameters and the dust mobilization depends on the flow configurations of the buoyancy-driven exchange flows through breaches and the dust density, and the rate of dust released from the VV through the breaches to the outside depends on the breach positions on the VV, and furthermore, the present numerical approach is very effective to estimate the thermal-hydraulic phenomena inside the VV after the LOVA event occurs.

Temperature distributions in a Tokamak vacuum vessel of fusion reactor after the loss-of-vacuum events occurred

If a loss-of-vacuum event (LOVA) occurred in a fusion reactor, buoyancy-driven exchange flows would take place at breaches of a vacuum vessel (VV) due to the temperature difference between the inside and outside of the VV. The exchange flows may bring mixtures of activated materials and tritium in the VV to the outside through the breaches, and remove decay heat from the plasma-facing components of the VV. Therefore, the LOVA experiments were carried out under the conditions that one or two breaches were opened and that the VV was heated to a maximum 200°C, using a small-scaled LOVA experimental apparatus. Air and helium gas were provided as working fluids. Fluid and wall temperature distributions in the VV were measured and the flow patterns in the VV were estimated from these temperature distributions. It was found that: (1) the exchange mass in the VV depended on the breach positions; (2) the exchange flow at the single breach case became a counter-current flow when the breach was at the roof of the VV and a stratified flow when it was at the side wall; (3) and that at the double breach case, a one-way flow between two breaches was formed.

Experimental Study on Buoyancy-Driven Exchange Flows through Breaches of a Tokamak Vacuum Vessel in a Fusion Reactor under the Loss-of-Vacuum-Event Condition

As one of thermofluid safety studies in the International Thermonuclear Experimental Reactor, buoyancy-driven exchange flow behavior through breaches of a vacuum vessel (VV) has been investigated quantitatively by using a preliminary loss-of-vacuum-event (LOVA) apparatus that simulated the tokamak VV of a fusion reactor with a small-scaled model. To carry out the present experiments under the atmospheric pressure condition, helium gas and air were provided as the working fluids. The inside of the VV was initially filled with helium gas and the outside was atmosphere. The breaches on the VV under the LOVA condition were simulated by opening six simulated breaches to which were set the different positions on the VV. When the buoyancy-driven exchange flow through the breach occurred, helium gas went out from the inside of the VV through the breach to the outside and air flowed into the inside of the VV through the breach from the outside. The exchange rate in the VV between helium gas and air was calculated from the measured weight change of the VV with time since the experiment has started. Experimental parameters were breach position, breach number, breach length, breach size, and breach combination. The present study clarifies that the relation between the exchange rate and the breach position of the VV depended on the magnitude of the potential energy from the ground level to the breach position, and then, the exchange rate decreased as the breach length increased and as the breach size decreased.

Exchange Flow Characteristics in a Tokamak Vacuum Vessel of Fusion Reactor Under the Loss-of-Vacuum Conditions

When a Tokamak vacuum vessel of fusion reactor is broken, buoyancy-driven exchange flows will take place through breaches after the inside pressure of the vacuum vessel (VV) becomes equal to the outside pressure. The exchange flow may bring a mixture of activated dusts and tritium from the inside of the VV to the outside through the breaches. Moreover, the exchange flow may remove decay heat from the plasma-facing components. A preliminary LOVA (Loss Of VAcuum event) apparatus was constructed to investigate quantitative heat transfer characteristics of the exchange flows through the breaches under the LOVA conditions. The results of this study, the relationship between Froude numbers and breach locations in the VV was determined and empirical correlations for the average Froude numbers were derived.

特集 復元と可視化 ヘリウム‐空気置換流の可視化解析

A buoyancy-driven exchange flow through an opening in a partition can be induced by density difference between the gases of each side. The exchange flow rate and flow behavior depend on the density difference and the geometric configuration of the opening. The geometry of the opening is characterized by its diameter, length and inclination angle.This paper describes an experimental study on the buoyancy-driven exchange flow between helium filled in a chamber and the outside air through a small opening by means of visualization. The opening with large length-to-diameter ratio appears at the top of the chamber. Buoyancy-driven exchange flows through the opening are visualized by making use of laser-light-sheet technique. The results of visualization show exchange flow behavior by turbulent diffusion and its transition to stratified flow caused by increasing of inclination angle of the opening.Key words : Buoyancy-driven Exchange Flow, Visualization, Laser-light-sheet Technique, Turbulent Diffusion, Stratified Flow

69 ヘリウム―空気置換流の流速分布計測および可視化

This paper presents the experimental results of visualization into the buoyancy-driven exchange flows between helium and air through a vertical pipe. This type flow might be caused by a pipe rupture accident in a high temperature gas cooled reactor. The exchange flows induced by density difference are complicated by Kelvin-Helm-holtz instability arising on internal contact surface.The exchange flows formed in the vertical pipe connecting between the helium vessel and ambient air are visualized by means of the laaer-light-sheet illumination technique. The results obtained show the transient process of flow development in the pipe, and unsteady, three dimensional structure of the flows.

小口径開口部を通るヘリウム・空気置換流の流量測定と流れの可視化

This paper deals with an experimental investigation on buoyancy-driven exchange flows through horizontal and inclined openings. The method of the mass increment was developed to measure the flow rate in helium-air system and a displacement fringe technique was adopted in Mach-Zehnder interferometer to visualize the flow. As the result, the followings were obtained : Flow visualization results indicate that the upward and downward plumes of helium and air break through the opening intermittently, and they swing in the lateral direction through the horizontal opening. It is clearly visualized that the exchange flows through the inclined openings take place smoothly and stably in the separated passages. The inclination angle for the maximum Froude number decreases with increasing length-to-diameter ratio in the helium-air system, on the contrary to Mercer's experimental results in the water-brine system indicating that the angle remains almost constant.

ヘリウム-空気置換流の流速分布計測および可視化

In this paper, experimental results of visualization and velocity measurements of the buoyancy-driven exchange flows through the small opening are described. This type flow might be caused by a pipe rupture accident in a high temperature gas cooled reactor.After helium gas had been filled in the test cylinder, the cap covered the top of the opening was abruptly removed. Then an ascending helium current issuing from the opening and an entering air current into the test cylinder were occurred. The ascending flows seeded with fine particles were illuminated by sweeping beam scanned by a polygon mirror in vertical and horizontal planes. And multi-exposure specklegrams of the flows were recorded on the films. The velocity vector diagrams of the helium current were obtained from the specklegrams by means of Young's fringe method.Experimental results show that maximum velocity of helium current depends on the shapes of the opening with/without throat.

Combined Natural Convection and Forced Flow Through Small Openings in a Horizontal Partition, With Special Reference to Flows in Multicompartment Enclosures

Estimates of the magnitude of buoyancy-driven exchange flows through openings in partitions that separate compartments are needed to assess the movement of toxic gases and smoke through buildings. An experiment using water and brine as a substitute for a light gas moving in a dense gas was designed to measure combined forced and buoyancy-driven exchange flow through a single opening in a horizontal partition. No theoretical treatment exists for this configuration. The same apparatus was used to determine the magnitude of the forced flow required to purge the opening of the oppositely directed buoyant component (i.e., the “flooding” limit). Finally, combined forced and buoyancy-driven flows in a multicompartment enclosure were measured. It has been demonstrated that the combined forced and buoyancy-generated flows in the multicompartment structure can be predicted by a direct application of the results of the study of exchange flow through a single opening.