Buoyancy-driven Flow Research Articles

Bidensity particle-laden exchange flows in a vertical duct

Buoyancy-driven exchange flow in a vertical duct is studied theoretically for a light pure fluid and a heavy fluid. The latter is a suspension composed of a Newtonian fluid and two populations of negatively buoyant particles of the same size but different densities (bidensity). In a previous study (Mirzaeian & Alba, J. Fluid Mech., vol. 847, 2018, pp. 134–160), the authors developed a lubrication model for monodensity suspension of particles of uniform size. The main observation of the monodensity study was the discovery of particle-enriched zones near heavy and light fluid fronts due to the relative motion of particles and the fluid. Distinct from the previous work, here, mismatched densities instigate a relative motion of lighter and heavier particles in addition to the movement of fluids. Other than the previously observed enrichment case, the bidensity case gives rise to a novel flow regime where there is enrichment of heavy particles but depletion of light particles near the interface. The transition to this regime is governed by a balance between the densities of heavy and light particles as well as those of light and carrying fluids for a given choice of initial volume fractions of particles. Such density balance is characterized by two dimensionless parameters comprising light and heavy particle-to-carrying-fluid density ratios. The transition mechanism is studied through additional simulations, revealing that the former increases with initial volume fraction of particles of either type, while the latter contrarily decreases. The effect of other parameters on the flow are discussed within the context of the paper.

Visualization of Mass Transfer Between Source and Seeping Water in a Variable Aperture Fracture—Impact of Tracer Density

An experiment with a vertical slot with horizontally seeping water with a dye diffusing from below was performed to help validate and visualize the Q-equivalent model, which describes the mass transfer rate from a source into flowing water, such as that in a repository for nuclear waste. The Q-equivalent model is used for quantifying mass transport in geological repositories. However, the tracer propagated much slower and to a lesser extent than predicted by the model. It was found that the tracer gave rise to a small density gradient that induced buoyancy-driven flow, overwhelming that driven by the horizontal hydraulic gradient. This dramatically changed the mass transfer from the dye source into the water in the slot. For the release of contaminants, this can have detrimental as well as beneficial effects, depending on whether positive or negative buoyancy is induced. These observations led to an analysis of when and how density differences in a repository can influence the release and further fate of escaping radionuclides in waste repositories. This and other experiments also showed that laboratory experiments aimed at visualizing flow and mass transfer processes in fractures could be very sensitive to the heating of the dye tracers by the lighting in the laboratory.

Turbulent mixing of a passive scalar in the ocean mixed layer

We study the 2D turbulent mixing of a passive scalar in the ocean mixed layer. As an example, we examine a steady-state convective mixed layer in which the boundary conditions are chosen so that the system reaches a dynamical equilibrium. In this idealized case, we parameterize the horizontally and temporally averaged fluxes as a functional of the horizontally and temporally averaged property gradients. Here, 〈w′c′〉=−∫dz′K(z|z′)∂〈c〉∕∂z′, where K(z|z′) is the eddy diffusivity kernel which describes the vertical transport by eddies at any vertical location z. The full kernel K(z|z′) is computed by adding passive scalars to a buoyancy-driven flow field in a 2D DNS of the ocean surface layer. This functional form of the eddy diffusivity highlights both local and non-local effects of the mixing of a passive scalar, and is based on an unapproximated representation of the idealized physics. This type of formulation can be further extended to other problems in turbulence concerning the mixing of a passive scalar to determine a parameterization based on an accurate representation of ocean physics.

Experimental investigation of the effect of perforated fins on thermal performance enhancement of vertical shell and tube latent heat energy storage systems

A key challenge in the deployment of practical latent heat energy storage systems employing phase change materials is the inherent low thermal conductivity of these materials. The present research is motivated by the need to intensify the buoyancy-driven convection flow in the phase change material to enhance the thermal performance of the system. In this paper, for the first time, the effect of applying perforated fins on the thermal performance enhancement of a vertical shell and tube latent heat energy storage heat exchanger is experimentally investigated and the results are compared with those of the unfinned and solid finned heat exchangers as the base cases. Lauric acid as the phase change material is placed in the shell side and the water is passed through the inner tube. The shells of the heat exchangers were made of transparent Plexiglas tubes to enable the visual comparison of the melting processes. The fins and tubes were made of copper. The melting process of phase change material is studied under different inlet water flow rates (0.5 and 1 l/min) and temperatures (55 and 65 °C). The experimental results showed that the time-averaged Nusselt number of the perforated finned heat exchanger is about 30% higher than that of the solid finned heat exchanger due to the minor hindering effect of the perforated fins on the development of the convection flows. Moreover, the total melting time of the perforated finned heat exchanger is about 7% lower than that of the solid finned heat exchanger.

Characterizing Density Flow Regimes of Three Rivers with Different Physicochemical Properties in a Run-Of-The-River Reservoir

Inflow mixing affects the spatiotemporal heterogeneity of water quality in reservoirs. Reservoir water quality management requires accurate prediction of density flow regimes to understand the spatiotemporal distribution of dissolved and particulate nutrients and organics. This study aims to characterize the mixing and circulation of three rivers with different physicochemical properties in a run-of-the-river (ROR) reservoir, using high-frequency monitoring and three-dimensional (3D) hydrodynamic modeling. The Aquatic Ecosystem Model (AEM3D) was constructed for the reservoir and calibrated with high-frequency data obtained from May–June 2016, accurately reproducing the observed spatiotemporal variations of flow velocity, water temperature, and electrical conductivity (EC) in the reservoir. High-frequency data and 3D model results showed that mixing of the rivers in the ROR reservoir is governed by density flow regimes formed by influent water temperature differences. At the confluence, colder and warmer river influents formed underflows and surface buoyant overflows, respectively. The spatial arrangement of flow direction, water residence time, and EC concentration were largely controlled by the buoyancy-driven flow. Stagnant areas with long residence times corresponded with areas of observed algal blooms and hypoxia. High-frequency sensor technology, combined with 3D hydrodynamic modeling, is effective for understanding the complex flow regimes and associated water quality characteristics in ROR-type reservoirs.

Buoyancy-driven instabilities and particle deposition in a Taylor-Couette apparatus

Abstract The present paper aims at studying the particle trajectories and sedimentation inside Taylor-Couette buoyancy-driven flows. The dynamical and thermal features of Taylor–Couette-flows inside a three-dimensional differentially heated cavity are investigated for Reynolds numbers Re ranging from 67.3 to 392.7 and Grashof numbers Gr between 764.4 ≤ Gr ≤ 3822.1. The results indicate a strong interaction between natural convection and the base Taylor-Couette flow due to rotation for a weak radial temperature gradient. A spectral analysis allows to identify different flow regimes. For discrete particle simulations, the Lagrangian Particle Tracking method is used to follow the particle trajectories inside the Taylor-Couette apparatus. Water droplets are considered as solid spherical particles with different diameters (10 ≤ Dp ≤ 35 μm). The time analysis of suspended and deposited particles along different walls shows that the rotation of the inner-cylinder coupled to the natural convection influences significantly the time and location of the particle deposition.

The effect of wind on the convective heat transfer from the floor of single-sided naturally ventilated cubical enclosures

In mild climatic conditions, residential houses are often ventilated by windows on a single exterior wall. The resulting airflow in single-sided naturally ventilated spaces is complex due to the involvement of buoyancy and wind driven forces that can reinforce, or restrict, the buoyancy-driven flow. The consequence of these effects on indoor flow-fields, and particularly convective heat transfer on the floor, needs examination. To understand the influence of wind conditions, this work utilized computational fluid dynamics to examine the flow in, and heat transfer from the floor of, a single-sided partly open air-filled cubical enclosure. The results indicate that both the indoor flow-field and convective heat transfer at the floor are strongly influenced by the outside wind conditions. As such, there is a significant scope to improve the relationships used to estimate the convective heat transfer from the floor of naturally ventilated buildings by including the effect of outdoor wind conditions.

Transport mechanisms during water droplet evaporation on heated substrates of different wettability

The evaporation of water droplets placed on heated hydrophilic, hydrophobic, and superhydrophobic substrates is numerically investigated. Simplified analytical models for droplet evaporation only include vapor diffusion transport in the surrounding gas domain and assume an isothermal droplet interface at the substrate temperature. The comprehensive model developed in this study accounts for all of the pertinent transport mechanisms. The interface is cooled via absorption of latent heat during evaporation, and the saturated vapor concentration is coupled to local temperature at the droplet interface. Conjugate heat and mass transfer are solved throughout the system using temperature-dependent physical properties. Buoyancy-driven convective flows (induced by both species concentration and temperature gradients) in the droplet and gas domains are also simulated. The evaporation rates predicted as a function of the substrate wettability (contact angle from 10° to 160°) and substrate temperature (40 °C to 65.4 °C) are validated against experiments from the literature. The modeling approach yields quantitative insights into the influence of these transport mechanisms on the evaporation characteristics. As substrate temperature is increased, the buoyancy-induced convection significantly increases the evaporation rate by up to ~60% on the hottest substrate compared to the diffusion-based model, by enhancing vapor transport in the gas domain. Simultaneously, the liquid-gas interface is increasingly cooled by evaporation, leading to a large temperature drop across the droplet height, ~18 °C for a 3 μL droplet evaporating on the superhydrophobic substrate at 60 °C. This significantly alters the distribution of the vapor fraction and evaporation flux along the interface and suppresses the evaporation rate (by ~53%). When both factors are considered together, the net effect (namely, enhancement or suppression) on the evaporation rate is dependent on the competition between the buoyancy-induced convection and evaporative cooling. On hydrophilic substrates, the evaporative cooling effect is weak because the flat droplet shape results in a relatively small temperature difference between the interface and the heated substrate; upward gas-phase natural convection is dominant and enhances evaporation. On the hydrophobic substrate, these respective suppression and enhancement effects counterbalance each other. On the superhydrophobic substrate, the effect of evaporative cooling is further amplified by the large thermal resistance between the substrate and interface, dominating the transport process and entirely suppressing the influence of upward natural convection in the gas phase.

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.

Cookoff modeling of a melt cast explosive (Comp-B)

A universal cookoff model (UCM) is applied to the melt cast explosive Comp-B composed of RDX and TNT. The UCM uses a simple kinetic mechanism with rates and thermophysical properties determined specifically for Comp-B. The success of the UCM is primarily attributed to the flexible form of the rate expression as well as accurate thermophysical properties obtained from small-scale experiments. The rate expressions use distributed activation energies in conjunction with rate multipliers to account for accelerations caused by 1) dissolved RDX, 2) liquid RDX, and 3) pressure. Our finite element model addresses Comp-B cookoff from the pristine state, through melting of the TNT binder, partial dissolution of RDX in the hot TNT, and melting of the remaining RDX as the Comp-B thermally ignites. Typically, the UCM is used for explosives that do not flow. However, we have included a buoyancy-driven flow model to account for multiphase fluid movement. Predicted temperature fields were sensitive to flow, which caused the hotter material to rise. Our predictions of ignition times were also sensitive to RDX dissolving in hot TNT causing an acceleration of the RDX decomposition.

Heat transfer and boundary layer analyses of laminar and turbulent natural convection in a cubical cavity with differently heated opposed walls

The buoyancy-driven flow in a cubical cavity with differently heated opposed walls has been chosen to investigate the laminar and turbulent heat transfer in enclosed domains. This type of configuration contains the essential physics existing in flows where transport between a fluid and an adjacent solid surface is controlled by the existence of boundary layers. The dependence of the wall heat transfer on the buoyancy force over the range Ra=105 to Ra=5.4×108 has been investigated, for air (Pr=0.7), using Direct Numerical Simulations. Under the assumption of mixed convection flow within the boundary layer produced by the combined effect of buoyancy and forced convection due to the large-scale flow, we assessed the validity of classical 2D boundary layer solutions in this flow configuration. The resulting characterization of the near-wall flow allows to derive new semi-analytical models for wall transfer applications in enclosed cavities.

Natural convection from cross blade inside a nanofluid-filled cavity using ISPH method

Purpose The purpose of this study is to apply the incompressible smoothed particle hydrodynamics method for simulating the natural convection flow inside a cavity including cross blades or circular cylinder cylinder. Design/methodology/approach The base fluid is water and copper-water nanofluid is treated as a working fluid. The left and rights walls are maintained at a cool temperature, the horizontal cavity walls are isolated and the inner shape was heated. The physical parameters are the length of the blades L_Blade, the number of cross blades, circular cylinder radius L_R, Rayleigh number Ra and the nanoparticles volume fraction. Findings The results reveal that the lengths of the cross blade, number of the blades and radius of the circular cylinder is working as an enhancement factor for heat transfer and fluid flows inside a cavity. Adding nanoparticles augments heat transfer and reduces the fluid flow intensity inside a cavity. The best case for buoyancy-driven flow was obtained when the inner shape is the circular cylinder at a higher Rayleigh number. Originality/value This work uses a distinctive numerical method to study the natural convection heat from cross blades inside a cavity filled with nanofluid. It provides a new analysis of this issue and presented good results.

Experimental Validation of ANSYS CFX for Transient Flows With Heat Transfer in a Tubular Heat Exchanger

Abstract Within the European SESAME project, extensive experimental work was performed and complemented by the development, application, and validation activities of thermal-hydraulic simulation tools for different scales. The TALL-3D experimental facility, operated by KTH Royal Institute of Technology in Stockholm, is designed for thermal-hydraulic experiments with lead-bismuth eutectic (LBE) coolant at natural and forced circulation conditions. The heat generated in the primary TALL-3D circuit is transferred via a double-tube counter-flow heat exchanger (HX) to the secondary side, which is cooled by glycerol oil. Validation calculations with the coupled simulation code ATHLET-ANSYS CFX within the European THINS project showed that one of the major challenges for this one-dimensional–three-dimensional (1D–3D) simulation of the experimental facility lies in the calculation of the HX with ATHLET. This is due to the fact that glycerol oil properties are not yet available for ATHLET. In order to better understand the flow and heat transfer phenomena inside the HX, 3D stand-alone calculations with ANSYS CFX were carried out for the SESAME experiment TG03.S301.03. The data generated in this experimental run is challenging for validation of computational fluid dynamics (CFD) codes, because the test combines heat transfer between buoyancy-driven LBE flow in the primary circuit and a turbulent oil flow in the secondary TALL-3D side. Moreover, the validation gets even more difficult for the CFD approach due to the fact that both coolants are nonunity Prandtl fluids, and this requires a careful modeling of the turbulent heat flux. In this paper, the TALL-3D HX behavior during test TG03.S301.03 is analyzed, and the results of the ANSYS CFX calculations are compared with measurements.

Hydrogeological control on carbon dioxide input into the atmosphere of the Chauvet-Pont d'Arc cave.

Carbon dioxide (CO2) concentration (CDC) is an essential parameter of underground atmospheres for safety and cave heritage preservation. In the Chauvet cave (South France), a world heritage site hosting unique paintings dated 36,000years BP, a high-sensitivity monitoring, ongoing since 1997, revealed: 1) two compartments with a spatially uniform CDC, a large volume (A) (40,000 to 80,000m3) with a mean value of 2.20±0.01% vol. in 2016, and a smaller remote room (B) (2000m3), with a higher mean value of 3.42±0.01%; 2) large CDC annual variations with peak-to-peak amplitude of 2% and 1.6% in A and B, respectively; 3) long-term changes, with an increase of CDC and of its annual amplitude since 1997, then faster since 2013, reaching a maximum of 4.4% in B in 2017, decreasing afterwards. While a large effect of seasonal ventilation is ruled out, monitoring of seepage at two dripping points indicated that the main control of CDC seasonal reduction was transient infiltration. During periods of water deficit, calculated from surface temperature and rainfall, CDC systematically increased. The carbon isotopic composition of CO2, correlated with water excess, is consistent with a time-varying component of CO2 seeping from above. The CO2 flux, which is the primary driver of CDC in A and B, inferred using box modelling, was found to confirm the relationship between water excess and reduced CO2 flux into A, compatible with a more constant flux into B. A buoyancy-driven horizontal CO2 flow model in the vadose zone, hindered by water infiltration, is proposed. Similarly, pluri-annual and long-term CDC changes can likely be attributed to variations of water excess, but also to increasing vegetation density above the cave. As CDC controls the carbonate geochemistry, an increased variability of CDC raises concern for the preservation of the Chauvet cave paintings.

Non-axis-Symmetric Transport Characteristics of an Evaporating Water Droplet Sessile on Heated Horizontal Superhydrophobic Substrates

Abstract Evaporation of sessile droplets is an important fundamental problem. The present three-dimensional numerical model considers various effects including vapor diffusion, buoyancy-driven flow, and evaporative cooling, with conjugate heat and mass transfer solved throughout the computational domain. Evaporation of a sessile water droplet with an initial volume of 3 μL is investigated on heated horizontal superhydrophobic substrates. A non-axis-symmetric rolling flow is presented inside the droplet rather than the axis-symmetric recirculation flow predicted by a two-dimensional axisymmetric model. This is because the axis-symmetric flow is not stable on heated superhydrophobic substrate and sensitive to any perturbations. The evaporative cooling along the droplet interface is observed, but the coolest point appears on the one side of the droplet instead of the droplet top owing to the rolling flow inside the droplet. Influence of relative humidity is also discussed and it indicates a stronger impact on substrates with relatively lower temperature. The present numerical study reveals the important underlying transport characteristics, which provides new insights into evaporation of water droplets resting on heated horizontal superhydrophobic substrates.

A minimal model for predicting ventilation rates of subterranean caves

The ventilation of carbon dioxide within subterranean caves regulates the growth of speleothems --- mineral deposits found in caves that provide important clues about past climate. While previous studies have used internal temperature measurements to predict ventilation rates, such data would not be available for the task of climate reconstruction. Here, we develop a parsimonious model to predict ventilation rates from knowledge of outside temperatures and the cave's physical dimensions only. In the model, ventilation arises from buoyancy-driven flows created in passageways that connect to the outside. A few key simplifications leads to a system amenable to perturbation analysis, resulting in explicit expressions for ventilation rates. We compare these predictions to time-resolved, in-situ measurements of transported cave gases (carbon dioxide and radon). The theory accurately accounts for the timing and magnitude of seasonal and synoptic variations of these gases, and is therefore diagnostic of seasonal bias in speleothem growth.

Liquid velocity fluctuations and energy spectra in three-dimensional buoyancy-driven bubbly flows

We present a direct numerical simulation (DNS) study of pseudo-turbulence in buoyancy-driven bubbly flows for a range of Reynolds ( ) and Atwood ( ) numbers. We study the probability distribution function of the horizontal and vertical liquid velocity fluctuations and find them to be in quantitative agreement with the experiments. The energy spectrum shows a scaling at high and becomes steeper on reducing . To investigate spectral transfers in the flow, we derive the scale-by-scale energy budget equation. Our analysis shows that, for scales smaller than the bubble diameter, the net transfer because of the surface tension and the kinetic energy flux balances viscous dissipation to give scaling of the energy spectrum for both low and high .

Topological equivalence between two classes of three-dimensional steady cavity flows: A numerical-experimental analysis

The present study concerns Lagrangian transport and (chaotic) advection in three-dimensional (3D) flows in cavities under steady and laminar conditions. The main goal is to investigate topological equivalences between flow classes driven by different forcing; streamline patterns and their response to nonlinear effects are examined. To this end, we consider two prototypical systems that are important in both natural and industrial applications: a buoyancy-driven flow (differentially heated configuration with two vertical isothermal walls) and a lid-driven flow governed by the Grashof (Gr) and the Reynolds (Re) numbers, respectively. Symmetries imply fundamental similarities between the streamline topologies of these flows. Moreover, nonlinearities induced by fluid inertia and buoyancy (increasing Gr) in the buoyancy-driven flow vs fluid inertia (increasing Re) and single- or double-wall motion in the lid-driven flow cause similar bifurcations of the Lagrangian flow topology. These analogies imply that Lagrangian transport is governed by universal mechanisms, and differences are restricted to the manner in which these phenomena are triggered. Experimental validation of key aspects of the Lagrangian dynamics is carried out by particle image velocimetry and 3D particle-tracking velocimetry.

An efficient parallel algorithm for DNS of buoyancy-driven turbulent flows

In this paper, an explicit low-storage simplified M — stage Runge-Kutta (SRK) scheme for high Reynolds-number incompressible flows is presented. In the SRK scheme, the Poisson equation is solved only once in the final substage of each time step. By taking advantage of the SRK scheme and the advanced hybrid MPI+MPI model, we have developed an efficient parallel solver for buoyancy-driven turbulent flow. The spatial and temporal accuracies of the solver are validated with Taylor-Green vortex flow. Both the RK and SRK schemes are implemented for the simulation of turbulent Rayleigh-Benard convection as well as Rayleigh-Taylor flow. The results show that the SRK scheme can save approximately 20% of the computation time.

Efficient high order accurate staggered semi-implicit discontinuous Galerkin methods for natural convection problems

In this article we propose a new family of high order staggered semi-implicit discontinuous Galerkin (DG) methods for the simulation of natural convection problems. Assuming small temperature fluctuations, the Boussinesq approximation is valid and in this case the flow can simply be modeled by the incompressible Navier-Stokes equations coupled with a transport equation for the temperature and a buoyancy source term in the momentum equation. Our numerical scheme is developed starting from the work presented in [1, 2, 3], in which the spatial domain is discretized using a face-based staggered unstructured mesh. The pressure and temperature variables are defined on the primal simplex elements, while the velocity is assigned to the dual grid. For the computation of the advection and diffusion terms, two different algorithms are presented: i) a purely Eulerian upwind-type scheme and ii) an Eulerian-Lagrangian approach. The first methodology leads to a conservative scheme whose major drawback is the time step restriction imposed by the CFL stability condition due to the explicit discretization of the convective terms. On the contrary, computational efficiency can be notably improved relying on an Eulerian-Lagrangian approach in which the Lagrangian trajectories of the flow are tracked back. This method leads to an unconditionally stable scheme if the diffusive terms are discretized implicitly. Once the advection and diffusion contributions have been computed, the pressure Poisson equation is solved and the velocity is updated. As a second model for the computation of buoyancy-driven flows, in this paper we also consider the full compressible Navier-Stokes equations. The staggered semi-implicit DG method first proposed in [4] for all Mach number flows is properly extended to account for the gravity source terms arising in the momentum and energy conservation laws. In order to assess the validity and the robustness of our novel class of staggered semi-implicit DG schemes, several classical benchmark problems are considered, showing in all cases a good agreement with available numerical reference data. Furthermore, a detailed comparison between the incompressible and the compressible solver is presented. Finally, advantages and disadvantages of the Eulerian and the Eulerian-Lagrangian methods for the discretization of the nonlinear convective terms are carefully studied.