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• Critical lift force coefficient verifiable in polydisperse, turbulent flows. • Findings from single bubble experiments valid in complex flows. • Separation of different bubble sizes due to the lift force. • Separation efficiency of bubble columns defined. • New evaluation method developed based on the two-fluid model. The lift force acting on particles, drops, and bubbles in a shear field is a well know effect that was extensively investigated since the 1980s. Experiments with single bubbles in a linear shear field reveal that the lift force coefficient has at a specific bubble size a zero where the coefficient switches its sign from positive to negative. Solving the lateral force balance for a polydisperse bubbly flow with the liquid flow field usually found in a bubble column, the lift force causes a spatial separation of small, which tend to the wall, and big bubbles, which tend to the center. Bubbles with a zero lift force coefficient on the other hand are equally distributed over the cross section. In order to investigate the influence of the flow field on the force balance, simulations with the Euler-Euler two-fluid model were conducted. The simulations are in good agreement with experiments and showed that the liquid flow field found in the used bubble column has a minor influence on the steady state distribution of the bubbles. From evaluating six different bubble column experiments conducted in air/water, the critical diameter at which the lift force coefficient is zero is determined between 5.1 and 5.2mm. This result is very close to the critical diameter that we were able to determine in previous, single-bubble experiments. Therefore, the conclusion is possible that the lift force coefficients that are determined with single bubble experiments are applicable to the complex flow field found in polydisperse bubbly flows at low void fractions and low turbulence levels. Moreover, a very simple experimental setup is described to determine the critical diameter in complex flow situations, which is the most important point when it comes to modelling the lift force. Such a setup is therefore beneficial when complex substance mixtures used in a specific industrial or environmental application needs to be investigated since lift force correlations are only available for clean, ideal substances. [ABSTRACT FROM AUTHOR]

Many large calving events at Greenland's marine‐terminating glaciers generate globally detectable glacial earthquakes. We perform a cross‐correlation analysis using regional seismic data to identify events below the teleseismic detection threshold, focusing on the 24 hr surrounding known glacial earthquakes at Greenland's three largest glaciers. We detect additional seismic events in the minutes prior to more than half of the glacial earthquakes we study and following one third of them. Waveform modeling shows source mechanisms like those of previously known glacial earthquakes, a result consistent with available imagery. The seismic events thus do not represent a failure of the high subaerial ice cliff like that expected to trigger large‐scale calving and a marine ice‐cliff instability but, rather, rotational, buoyancy‐driven calving events, likely of the full glacier thickness. A limited investigation of the prevalence of smaller seismic events at times outside glacial‐earthquake windows identifies several additional events. However, we find that calving at the three glaciers we study—Jakobshavn Isbræ, Helheim Glacier, and Kangerdlugssuaq Glacier—often occurs as sequences of discrete buoyancy‐driven events in which multiple icebergs ranging in size over as much as three orders of magnitude are all lost within ∼30 min. We demonstrate a correlation between glacial‐earthquake magnitude and iceberg size for events with well‐constrained iceberg‐area estimates. Our results suggest that at least 10–30% more dynamic mass loss occurs through buoyancy‐driven calving at Greenland's glaciers than previously appreciated. Key Points: Seismic data show ice motion prior to half of large‐scale calving events we study and following one third of eventsSeismogenic ice motion prior to large calving events represents not subaerial ice‐cliff failure but rotational, buoyancy‐driven calvingWe present the first empirical demonstration of a relationship between the seismic magnitude of a glacial earthquake and iceberg size [ABSTRACT FROM AUTHOR]

Highlights • The effect of moisture content on natural convection flow in a cavity with differentially heated vertical and conducting horizontal walls was investigated. • Natural convection flow and heat transfer are very sensitive to the level of moisture present in the air and even at low temperature gradient between the cavity vertical walls. • The presence of moisture causes changes in the energy equation, which has a significant effect on the heat transfer coefficient and influences the weak natural convection flows inside the cavity. • The vertical walls have been found to show greater sensitivity for heat transfer with changes in moisture content, while both horizontal walls are almost insensitive to moisture content variation. • The heat transfer coefficient increases proportionally with increasing moisture concentration, while the average relative humidity inside the cavity significantly reduces with increasing temperature gradients between the vertical walls of the cavity. Abstract This paper reports a numerical investigation of low turbulence buoyancy-driven flow of moist air and heat transfer inside a rectangular cavity with differentially heated vertical walls. The variations of the flow, temperature and moisture inside the cavity has been analysed together with heat transfer coefficients for a range of mass fraction of water vapour and temperature gradients between the vertical walls of the cavity. The accuracy of the numerical methodology was also scrutinised by conducting a rigorous validation study of benchmark experimental data for the average Nusselt number for similar buoyancy driven cavity flow. The results of this investigation showed that during the natural convection process, the change in moisture content in the moist air has a significant influence on the flow and temperature fields inside the enclosure and the variation of the vertical wall temperature gradients have also shown to affect the moisture concentration inside the cavity. The percentage change in the average heat transfer varied significantly depending on the mass fraction of moisture in the air and the temperature gradient between the vertical walls. The results also showed a 3.5% increase in the average heat transfer for every 0.02 kg/kg increment in the mass fraction of water vapour. The findings from the study are significance to scientists and practitioners who are responsible for moisture management in enclosed space at the design threshold and for optimisation of energy used in buildings. [ABSTRACT FROM AUTHOR]

In geologic repositories for nuclear waste located in crystalline rocks, the waste is surrounded by a bentonite buffer that in practice is not permeable to water flow. The nuclides must escape by molecular diffusion to enter the seeping water in the fractures of the rock. At high water-seepage rates, the nuclides can be carried away rapidly. The seepage rate of the water can be driven by the regional hydraulic gradient as well as by buoyancy-driven flow. The latter is induced by thermal circulation of the water by the heat produced by radionuclide decay. The circulation may also be induced by salt exchange between buffer and water in the fractures. The main aim of this paper is to explore how salt exchange between the backfill and mobile water in fractures, by buoyancy effects, can increase the escape rate of radionuclides from a repository. A simple analytical model has been developed to describe the mass transfer rate induced by buoyancy. Numerical simulations support the simple solution. A comparison is made with the regional gradient-driven flow model. It is shown that buoyancy-driven flow can noticeably increase the release rate. [ABSTRACT FROM AUTHOR]

Entropy generation is an important aspect of modern thermal polymer processing optimization. Many polymers exhibit strongly non‐Newtonian effects and dissipation effects in thermal processing. Motivated by these aspects in this study, a numerical analysis of the entropy generation with viscous dissipation effect in an unsteady flow of viscoelastic fluid from a vertical cylinder is presented. The Reiner‐Rivlin physical model of grade 2 (second‐grade fluid) is used, which can envisage normal stress variations in polymeric flow‐fields. Viscosity variation is included. The obtained governing equations are resolved using implicit finite difference method of Crank‐Nicolson type with well imposed initial and boundary conditions. Key control parameters are the second‐grade viscoelastic fluid parameter (β), viscosity variation parameter (γ), and viscous dissipation parameter (ε). Also, group parameter (BrΩ−1), Grashof number (Gr), and Prandtl number (Pr) are examined. Numerical solutions are presented for steady‐state flow variables, temperature, time histories of friction, wall heat transfer rate, entropy, and Bejan curves for distinct values of control parameters. The results specify that entropy generation decreases with augmenting values of β, γ, and Gr. The converse trend is noticed with increasing Pr and BrΩ−1. Furthermore, the computations reveal that entropy and Bejan lines only occur close to the hot cylinder wall. [ABSTRACT FROM AUTHOR]

Highlights • Turbulence effects during the buoyancy-driven mixing were investigated. • Additional buoyancy terms were included in buoyancy-modified turbulence models. • Vortical oscillations showed good agreement between simulations and experiment. Abstract The turbulence effects during the buoyancy-driven mixing was investigated at a vertical mixing (VeMix) test facility, which was developed to investigate the mixing of high borated and low borated coolant in nuclear reactor. Additional buoyancy terms are included in buoyancy-modified turbulence models, which have been implemented in the CFD code ANSYS CFX and validated with experimental data captured by optical methods and conductivity measurement technology. The physicality of the flow phenomena and the vortical oscillations analyzed by Fourier transformation in both the experiments and simulations show good agreement under different flow conditions. The influence of different buoyancy models were investigated in detail and optimal models for simulations at similar flow conditions have been selected. [ABSTRACT FROM AUTHOR]

Buoyancy-driven flow inside a superposed enclosure filled with composite porous-hybrid nanofluid layers was investigated numerically using a local thermal nonequilibrium model for the heat transfer between the fluid and the solid phases. The bottom wall of the enclosure was partly heated to provide a heat flux, while the other parts of the wall were thermally insulated. The top and vertical walls of the enclosure were maintained at constant cold temperatures. The Darcy-Brinkman model was adopted to model the flow inside the porous layer. The Galerkin finite element method was used to solve the governing equations using the semi-implicit method for pressure linked equations algorithm. The selected parameters are presented for the Rayleigh number (Ra), 103 ≤ Ra ≤ 107, the Darcy number (Da), 10−7 ≤ Da ≤ 1, the porous layer thickness (S), 0 ≤ S ≤ 1, the modified conductivity ratio (γ), 10−1 ≤ γ ≤ 104, the interphase heat transfer coefficient (H), 10−1 ≤ H ≤ 1000, the heat source length (B), 0.2, 0.4, 0.6, 0.8 and 1, and the nanoparticle volume fraction (ϕ), 0 ≤ ϕ ≤ 0.2. It has been concluded that the rate of heat transfer of hybrid nanofluid (Cu−Al2O3/water) is higher than with the pure fluid. Furthermore, at Ra ≤ 105, the heat transfer rate maintains its maximum value when S reaches the critical value (S = 0.3). The values of S, Da, and B were found to have a significant effect on the heat removal from the heat source. Increasing the values of γ and H can strongly enhance the heat transfer rate and satisfy the thermal equilibrium case. [ABSTRACT FROM AUTHOR]

Highlights • Effects of eleven baffling structures on buoyancy-induced turbulence flow were evaluated. • The models were simulated and solved using a commercial CFD software program. • Both buoyancy-decoupled and buoyancy-coupled turbulence models were applied. • Turbulent kinetic energy difference and densimetric Froude number can evaluate the baffling effects. • Results of the baffling structre can be divided into three zones by the turbulent kinetic energy. Graphical abstract Abstract Computational fluid dynamics (CFD) models are applied to understand the effects of baffling structures on the buoyancy-induced turbulence flow in secondary settling tanks (SSTs). Eleven SST geometries, including four circular SSTs and seven rectangular SSTs, are evaluated in this study. The main research tool is a Fluent-based two-dimensional SST model. The model is verified with observations from literature. The validated model is then applied to compare the predictions of the effluent suspended solids (ESS) by the buoyancy-decoupled and buoyancy-coupled turbulence models on each SST geometry. Next, the effects of the SST baffling structures on the turbulence properties are compared and quantified. Additionally, stress tests are simulated on the SSTs, to predict their performances using both the buoyancy-decoupled and buoyancy-coupled turbulence models. The results for the original flow conditions show that the effects of the SST baffling structures on the buoyancy-induced turbulence can be divided into three zones: Zones A, where the baffling structures have negligible damping effects, and only the buoyancy-coupled turbulence model provides accurate predictions; Zone B, where the baffling structures partially dissipate the effect of buoyancy on turbulence, and the buoyancy-coupled model continues to provide accurate predictions but the buoyancy-decoupled turbulence model only provides qualitative but similar predictions; and Zone C, where the baffling structures fully dissipate the effect of buoyancy on turbulence, and both models provide similar, accurate predictions. Two indicators, the densimetric Froude number and the differences in turbulent kinetic energy can be used to predict the need for buoyancy coupling. [ABSTRACT FROM AUTHOR]

Highlights • For immiscible binary, the fluids' yield stress can have a stabilizing effect on the Rayleigh-Taylor instability. • The stabilizing effect of yield stress depends on the surface tension, contact angle, and fluid's viscosity. • Surface tension combined with an appropriate contact angle can make the interface stable. • Shaking the vessel in the vertical direction can have a stabilizing effect provided its frequency and amplitude are carefully chosen. Abstract We have numerically studied the buoyancy-driven flow of two immiscible yield-stress fluids in a vertical closed-ended pipe. Having assumed that the fluids of interest obey the bi-viscous model, we have relied on the finite-volume-method (FVM) in order to capture the late-time shape of the interface assumed to be initially flat. We have been able to recover a variety of well-established fluid-flow phenomena observed experimentally in this particular exchange flow for both Newtonian and yield-stress fluids. We have reached to the conclusion that, for immiscible fluids, the yield stress and/or the viscosity of the fluids can have a stabilizing effect on the interface under certain conditions. Our numerical results also suggest that surface tension and contact angle can play a key role in stabilizing the interface if properly chosen. A threshold curve is obtained for the "no-flow" zone and its variation with the fluids' rheology and the contact angle is determined for bi-viscous fluids. In cases where fluids' rheology and/or the wall's wetting property cannot be altered, shaking the vessel in the vertical direction is proposed as an active means for stabilizing the interface provided that its amplitude and frequency are within a certain range. [ABSTRACT FROM AUTHOR]

Highlights • CFD using large eddy simulation with variable density and viscosity is developed for the cold leg mixing benchmark. • Taylor length scales and integral time scales are calculated. • Sensitivity of initial conditions, Schmidt numbers, Atwood numbers, and fluid models are numerically studied. • Influences of nozzle geometry and presence of small perturbations on downcomer are presented. • Recommendations are made for similar buoyant mixing problems. Abstract The influences of the Schmidt number and fluid models of the Boussinesq approximation and variable density (low-Mach formulation) are investigated using large eddy simulation (LES) where fluid naturally advects between two tanks connected by a pipe. The geometry resembles a reactor-like vessel which comprises of a cold leg-downcomer region. The closed system consists of two miscible fluids with different densities and viscosities separated by a valve. Analysis with Reynolds-Averaged Navier–Stokes (RANS) is used for calculating Taylor length scales to establish minimum grid resolution for LES. Using particle image velocimetry (PIV) data, validation is conducted for four different magnitudes of Schmidt numbers. Evidence indicates that capturing the effects from the highest Schmidt number is essential for properly predicting fluctuations and instabilities in the flow. Although downcomer comparisons with PIV consist of larger errors, they are explained through additional sensitivity analyses involving initial conditions, geometry, and residual energy. Lastly, the influence of larger density differences is demonstrated using simulations with a higher Atwood number, temporally scaled using the fluid front velocity. Recommendations are made regarding future experiments and simulations for design of similar buoyant mixing problems. [ABSTRACT FROM AUTHOR]

The work presented in this paper deals with combined heat and mass transfer by natural convection in porous media. The aim is to investigate numerically the effect of control parameters on the flow behavior as well as the enhancement of heat transfer in vertical porous enclosure. The side wall temperature is periodic function of time which is the case in several physical problems. The Darcy's flow model coupled with the energy and mass equations is considered. The numerical results show that the flow's behavior is strongly dependent on the buoyancy ratio values. Three types of flows take place: chimney, reversal and top aspiration flow. Further, the effect of buoyancy ratio (N) at which the different flow types occur is significantly influenced by the values of control parameters. The relative heat transfer enhancement between constant and periodical wall temperature is profoundly affected by thermal Rayleigh number (Ra), buoyancy ratio and dimensionless amplitude (XA). In the case of opposing double diffusive flow, the relative difference becomes to the favor of the stationary case for high Ra and XA. [ABSTRACT FROM AUTHOR]

We investigate the coupled behaviour of two oscillatory flames produced by separate bundles of candles, referred to as candle-flame oscillators, as the distance between them is varied. Each bundle consists of four candles whose individual flames are fused so that the resultant flame produces self-sustained limit cycle oscillations. The recent study by Manoj et al. [Scientific Reports 8, 11626 (2018)] has reported the occurrence of four different modes of coupled behaviour, which include in-phase synchronization, amplitude death, anti-phase synchronization, and desynchronization by observing the flame dynamics of such coupled candle-flame oscillators. Here, we investigate the physical mechanism behind the occurrence of these different dynamical modes. Towards this purpose, we perform simultaneous measurements of the flow field around the candle flames using high-speed shadowgraph and of the reaction zone of each flame using high-speed CH* chemiluminescence imaging. We notice that these modes are distinguished by the distinct features of the flame dynamics and the corresponding buoyancy-induced flows surrounding the flames. We observe that the difference in the interaction of vortices, formed due to the instability of buoyancy-induced flows around each flame at various distances, plays a significant role in inducing different modes of coupled dynamics between the oscillators. Furthermore, we find that the change in the length scales of vortices shed around the flames is a contributing factor in increasing the frequency of the oscillators during the transition from in-phase to anti-phase mode of synchronization. [ABSTRACT FROM AUTHOR]

To consider the effect of cross diffusion more rigorously, the onset and the growth of the gravitational instabilities in a Hele-Shaw cell saturated with ternary solution are analyzed by considering all cross diffusion coefficients. Through the asymptotic analysis, we identify the double-diffusive (DD)-, diffusive-layer convection (DLC)- and extended double diffusive (EDD)-type instability regimes. To support the asymptotic stability analysis, new linear stability equations are derived in the global domain and then transformed into the similar domain. In the similar domain, we prove that initially the system is unconditionally stable. For transient stability analysis, the linear stability equations are solved by employing quasi-steady state approximations (QSSA’s). To avoid the limit of the conventional QSSAz, we obtain the critical time for the onset of instability motion using the QSSA in the similar domain (QSSAζ). [ABSTRACT FROM AUTHOR]

Highlights • The effect of isolated blockages proximity on natural convection flow inside an enclosure was investigated. • Natural convection inside an enclosure are very sensitive to the proximity of blockages. • The Heat transfer becomes very significant when blockages are positioned within the hydrodynamic boundary layer. • Blockages proximity can affect the overall heat transfer via flow field. • Blockages positioned outside the hydrodynamic boundary layer resulted in 7.6% increase in the heat transfer. Abstract This paper considers the natural convection inside an air filled enclosure having several isolated cylindrical blockages distributed within it, and arranged parallel to two vertical walls of the enclosure. These two walls are maintained at different temperatures resulting in a natural convection process inside the enclosure. The objectives are to provide experimental temperature data and to investigate the influence of the blockages proximity within and outside the active vertical wall hydrodynamic boundary layer of the natural convection flow and heat transfer. Experiments were designed with high degree of accuracy to provide reliable temperature data-bank for a range of blockages proximity to the active vertical walls. The test enclosure is an air filled rectangular cavity fixed at 0.97 m × 0.4 m × 1 m, corresponding to the height, width and depth respectively. The top and bottom walls are conducting surface and the temperature difference between the active vertical walls was maintained at 42.2 °C resulting in a characteristic Rayleigh number of 4.04 × 109 based on the enclosure height. All the walls of the enclosure were insulated externally while the inner surfaces were covered with conducting plate. The test cavity capability of establishing low turbulence natural convection flows was verified. All temperature data were obtained at steady state and it was verified to be reproducible by repeating the experiment at different times. Also, two-dimensionality was verified via rigorous temperature readings over a period of time. Additional temperature readings were recorded for the air and cylinder surfaces at several distinct locations. Further investigations were conducted using a numerical approach to supplement and validate the experiments. Experimental temperature data collated at various locations within the enclosure show excellent comparison with numerical results and as such provide a useful experimental benchmark temperature data for the validation of low turbulence natural convection flow in enclosure partially filled with isolated solid objects. Our result shows that a significant increment or reduction in air temperature and wall heat transfer could be achieved by varying the blockages proximity, and especially when the blockages are positioned within the hydrodynamic boundary layers of the active vertical walls. [ABSTRACT FROM AUTHOR]

• This paper presents numerical results of the buoyancy-driven flow of a nanofluid in a cavity in the presence of magnetic field. • To encounter the non-Newtonian layer influence, the power-law model is considered. • The solution of the governing equations is obtained by the Galerkin FEM method. • The effects of geometrical parameters are discussed and illustrated. • By simultaneously using non-Newtonian nanofluid and porous media, the heat exchange is positively affected. Magnetohydrodynamic (MHD) mixed convection in a 3D (three dimensional) lid-driven cavity loaded with a power-law nanofluid is examined. The bottom wavy wall is maintained at a hot temperature, while the upper lid is at a uniformly cold temperature. The vertical walls are kept in adiabatic conditions. The steady and three-dimensional flow of nanofluids is quantitatively studied utilizing thermophysical properties and the Galerkin Finite Element Method (GFEM). The findings were shown for a variety of Grashof numbers (Gr = 103-105), Hartmann numbers (Ha = 0–20), Reynolds numbers (Re = 10–500), power-law indexes (n = 0.8,1,1.6), and undulation numbers (N = 1–4). The influence of the various parameters on flow, heat transfer, and entropy generation is illustrated by the streamlines, isotherms, and isentropic contours. Higher Re, γ , N, φ , and lower Ha enhance the heat transfer. Entropy generation is mostly due to heat transfer but also fluid-friction and magneto effects contribute. Also, the increase in Re from 50 to 500 gives an enhancement in Nu av up to 87.5 %. Furthermore, the increase in power-law index (n) from 0.8 to 1 gives a reduction in Nusselt number up to 10.58 %. [ABSTRACT FROM AUTHOR]

• A 3D reservoir numerical model is built to investigate the nature of hydrogen buoyant flow behavior in storage aquifers. • A series of numerical sensitivity simulations are performed to examine the influence of storage formation heterogeneity and fluid-rock interaction parameters. • Geologic storage containment efficiency is evaluated by quantifying the 'escape ratio' (i.e., mass fraction of hydrogen in a presumed escape region overall total hydrogen) for various scenarios. Understanding buoyancy-driven flow is essential for ensuring the safety, reliability, and efficiency of underground hydrogen storage in saline aquifers. In this study, we develop a three-dimensional (3D) numerical model of an aquifer for hydrogen storage, with an isothermal system at 323.15 K, to investigate the behavior of hydrogen buoyant flow. We perform a series of sensitivity simulations to examine the influence of storage formation heterogeneity, including Dykstra-Parsons coefficient, autocorrelation length, and permeability anisotropy, as well as fluid-rock interaction parameters, such as capillary pressure and relative permeability, on storage efficiency. We evaluate the efficiency of geologic storage containment by quantifying a defined metric "escape ratio," which is the mass fraction of hydrogen in a presumed escape region over the total mass of hydrogen. Our results indicate that hydrogen escape ratios and migration characteristics are primarily influenced by formation heterogeneities. The escape ratio decreases as Dykstra-Parsons coefficient increases and permeability anisotropy ratio decreases. The specific hydrogen escape path is controlled by spatial structures (i.e., autocorrelation length). Inclusion of capillary heterogeneity results in a decrease in the escape ratio, and the escape region exhibits a smoother appearance in the absence of capillary heterogeneity trapping as compared to when it is modeled. As for fluid-rock interactions, the hydrogen escape ratio is highly susceptible to the exponent of Brooks-Corey relative permeability function. Nevertheless, the magnitude of the escape ratio variation is predominantly determined by formation heterogeneity. [ABSTRACT FROM AUTHOR]

In this study, laminar natural circulation and heat transfer in a tall rectangular enclosure with disconnected vertical partitions inside were investigated. Analytical expressions were developed to predict the circulation flow rate and the average Nusselt number in a partially partitioned enclosure with isothermal side walls at different temperatures and insulated top and bottom walls. The proposed formulas are then validated against numerical results for modified Rayleigh numbers of up to 106. The impacts of the governing parameters are also examined along with a discussion of the heat transfer regimes.

Highlights • Experimental analysis of buoyancy-driven flow inside a vertical tube. • Elasto-viscoplastic thixotropic material positioned above a less dense Newtonian oil. • Effect of governing parameters on interface stability, front velocity at terminal motion, and flow morphology are analyzed. • Three different flow regimes are observed: unstable, quasi-stable, and stable, when there is no flow. • Unstable regime presents a wavy core-annular flow and quasi-stable regime shows a slow plug flow starts after a time delay. Abstract We perform an experimental analysis of the exchange flow between an elasto-viscoplastic thixotropic material above a less dense Newtonian oil inside a vertical tube. This situation is usually found in cementing plug operations, performed in the oil industry when there is a need to abandon an oil well. The experiments show that three different flow regimes are observed, namely unstable, quasi-stable, and stable (no flow). The unstable regime is a wavy core-annular flow with the denser liquid in the core region. In the quasi-stable regime a slow plug flow starts after a time delay, which is a consequence of thixotropic and elastic effects. Through analysis of the experimental results it is possible to identify, for a given pair of fluids, the region in the governing parameter space, within which the exchange speed is sufficiently low (or zero). [ABSTRACT FROM AUTHOR]

Highlights • Stable triple layer core annular flows. • Viscoplastically sculpted skin layer controlled through flow rate variations. • Axisymmetric 2D simulations demonstrates feasibility and computes developing flow. • Parametric study reveals that skin yields when yield stress is too small: extensional yielding. • Extensional flow model developed allows quick simulation and estimation of necessary yield stresses for rigid skin. Abstract In Sarmadi et al. (2017) [1] we introduced a novel methodology for efficient transport of heavy oil via a triple-layer core-annular flow. Pumping pressures are significantly reduced by concentrating high shear rates to a lubricating layer, while ideas from Visco-plastic lubrication are used to eliminate the possibility of interfacial instabilities. Specifically, we purposefully position a shaped unyielded skin of a visco-plastic fluid between the transported oil and the lubricating fluid layer. The shaping of the skin layer allows for lubrication forces to develop as the core settles under the action of transverse buoyancy forces: adopting an eccentric position where buoyancy and lubrication forces balance. In Sarmadi et al. (2017) [1] we focused on a steady periodic length of established flow, to establish feasibility for the pipelining application. Here we address the equally important issue of how in practice to develop a triple layer flow with a sculpted/shaped viscoplastic skin, all within a concentric inflow manifold. First, we use a simple 1D model to control layer thickness via flow rates of the individual fluids. This is used to give the input flow rates for an axisymmetric triple-layer computation using a finite element discretization with the augmented Lagrangian method to represent the yield surface behavior accurately and a Piecewise Linear Interface Calculation (PLIC) method to track the interface motion. This establishes that these flows may be stably established in a controlled way with a desired interface shape. The shaped interface induces extensional stresses in the skin layer. We study this directly by developing a long-wavelength/quasi-steady analysis of the extensional flow. This allows us to predict the minimal yield stress required to maintain the skin rigid, for a given shape, all while maintaining a constant flow rate of the transported oil. [ABSTRACT FROM AUTHOR]

In this experimental investigation the influence of non-uniform heat flux distributions on the internal heat transfer coefficient in a horizontal circular tube was studied for liquid water. The tube had an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions were studied for Reynolds numbers ranging from 650 to 2600 at a Prandtl number of approximately 6.5. Heat flux distributions included fully uniform heating (which had a circumferential angle span of 360°) and different partial uniform heat flux distributions with angle spans of 180° or 90° at different circumferential positions. Depending on the angle span, local heat flux intensities ranging from 1658 W/m 2 to 6631 W/m 2 were tested. Results indicate that the average steady state Nusselt number is greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for cases where the applied heat flux was positioned on the lower half (in terms of gravity) of the tube circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Smaller angle spans produced lower heat transfer coefficients. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for angle spans of 180° and 90° which correlated 92% and 96% of the data respectively within 3% of the measured Nusselt number. [ABSTRACT FROM AUTHOR]

Abstract The transport of buoyancy-driven contaminants in narrow inclined spaces, for example underground tunnels, is frequently seen in engineering. We carry out a series of experiments to investigate the transient evolution of buoyancy-driven flow in a narrow inclined tank. The negative buoyancy is introduced by releasing brine through a localized source located at the ceiling of the tank which is immersed in fresh water. We adopt a light attenuation technique to measure the transient distribution of the reduced gravity in the tank. In each of the experiments, the reduced gravity of the downstream current is stratified in the transverse direction of the tank but almost keeps constant in the longitudinal direction. A model is established to calculate the average reduced gravity at the downstream side of the buoyant source after the flow reaches steady state. The reduced gravity of the backlayering flow is significantly larger than that of the downward current. A simple model is proposed to estimate the maximum transient backlayering length. Moreover, the non-dimensional time t b , max / H / g 0 ' sin θ for the transient backlayering flow to reach its maximum length is determined. The propagation velocity of the front of the current depends on the buoyancy flux of the front. The buoyancy-driven contaminates propagate downstream with constant velocity in the steady state. Highlights • Buoyancy-driven flow in a narrow inclined space is experimentally investigated. • The transient evolution of the reduced gravity is described. • Both the maximum transient backlayering length and the time required to reach the maximum backlayering length are determined. • The propagation velocity of the front of the buoyancy-driven flow is analyzed. • The results are useful for the control of buoyancy-driven contaminants. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of ambient pressure on smoke back-layering in subway tunnel fires with and without train blockage. A series of numerical simulations were conducted in a 1/4 small-scale tunnel with different heat release rates (40–160 kW), longitudinal ventilation velocities (0.2–0.8 m/s) and ambient pressures (60–100 kPa). The smoke back-layering lengths under different conditions are analyzed, and the results show that under the same heat release rate and ventilation velocity, the back-layering length increases with decreasing ambient pressure due to the weak inertial force of longitudinal airflow led by the low air density. The Li’s and Zhang’s models, which can well predict the smoke back-layering length under regular pressure, are modified for the reduced pressure. The constant increment of back-layering length between adjacent ambient pressures, which mainly depends on the heat release rate, is used to correct Li’s model. The smoke back-layering length under the low ambient pressure can be predicted by this modified model without train blockage. With train blockage considered, new models are developed by introducing both equivalent and virtual fire sources for predicting the smoke back-layering length under the low ambient pressure, which is shown to well reproduce the simulation results. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study of the mixing of a passive scalar in a bubble column at high Reynolds number and average gas volume fractions ranging from 2.0 % to 7.5 % . Starting from a homogeneous bubble column, the bubbly flow is progressively destabilized by imposing a gradient of gas volume fraction at the bottom of the tank. In that way, a single recirculation is produced, which allows to investigate the impact of a large-scale buoyancy-driven flow on the mixing of a passive scalar. It is shown that, as long as the shear-induced turbulence generated by the recirculation is negligible, mixing results from two main mixing mechanisms: the transport by the mean liquid velocity and the mixing induced by the bubbles. While the transport by the liquid recirculation can be accounted for by an advection term, the mixing induced by the bubbles is a diffusive process, the effective diffusivity of which has been measured in a homogeneous bubble column by Alméras et al. (2015). However, once the shear-induced turbulence produced by the shear develops, its role upon the mixing has to be taken into account too. [ABSTRACT FROM AUTHOR]

A linear stability analysis is conducted for the onset of natural convection driven by a concentration gradient in a horizontal layer of a near-critical binary fluid mixture. The problem is regarded as a limiting case of double-diffusive convection. The governing equations for small perturbations after normal-mode expansion are solved numerically with finite difference discretization to obtain the critical concentration Rayleigh number. It is found that, when the height of the fluid layer is small, the initial density stratification is negligible and the theoretical criterion developed under Boussinesq approximation with the modified Rayleigh number is accurate even extremely close to the critical point. However, for a large height, the initial density stratification makes the fluid layer become more unstable, and deviations from theoretical predictions are observed. We further propose a method to estimate these deviations, which can be used to check the applicability of the theoretical criterion. As the second part of the study, we apply the criterion to interpret the onset of convection for a transient problem: a near-critical binary fluid mixture confined in a two-dimensional cavity submitted to concentration increases at the bottom wall. The numerical results demonstrate four typical behaviours of the concentration boundary layer: onset of convection, collapse of the concentration boundary layer, return to stability, and remaining stable. Comparisons between numerical results and the stability criterion are made, where consistencies are found except for the behaviour of return to stability. We attribute the inconsistency to the existence of lateral walls, whose stabilizing effect is strong when the return to stability happens. [ABSTRACT FROM AUTHOR]

This paper introduces the scientific part of a large-scale study in the Upper Peninsula (U.P.) of Michigan, a historical mining area, for exploring the water in deep abandoned copper mines as a geothermal energy resource. The main focus of the paper is placed on the scientific understanding of the natural mine water-geologic formation system, especially the transport of heat and mass in this large-scale natural system, which is critical to the efficiency and sustainability of the energy renovation. For this purpose, a field study involving measurements of temperatures and chemicals in a local mine shaft in the U.P. is conducted to reveal the major issue in recovering geothermal energy in the water from the shaft, i.e., the temperature distribution. Water samples are also collected in situ to investigate the distribution and concentrations of major chemicals. Afterward, a theoretical framework for the thermo-hydrodynamic process in the mine water coupled with heat transfer in the surrounding geologic formations is developed to outline a mathematical description for studying the scientific issue. Simulations are finally conducted, based on the real geologic information, to preliminarily investigate the quasi-equilibrium water movement in this local mine shaft due to geothermal gradients to provide insights into the phenomena observed in the field study. [ABSTRACT FROM AUTHOR]

Existing studies on sediment retention ponds (SRPs) have examined the effects of pond layout, inlet and outlet geometry and the installation of baffles on the performance of the SRPs. However, the effects of a temperature difference between the ambient water in the pond and the inflow are often neglected, and the buoyancy forces arising from these differences in temperature can potentially change the flow in the pond and hence its hydraulic performance. This study has experimentally evaluated the effect of these temperature differences on the flow field and residence time in a retention pond for a range of temperature differences. As expected a cold inflow sinks to the bottom of the pond while a hot inflow remains at the surface, but in both cases the inflow flows more rapidly towards the outlet than is the case for isothermal inflow. A counter-current was observed at the bottom or the surface of the pond for colder or hotter influents, respectively. These thermally induced flows significantly reduced the residence time of the pond, reducing the hydraulic performance of the pond and causing severe short-circuiting. The results have also shown that the temperature differences in the pond decrease with time, yet small temperature differences persist with the pond remaining thermally stratified. [ABSTRACT FROM AUTHOR]

In this study, a series of experiments are performed to investigate the mixing and stratification of buoyancy-driven flows in a narrow inclined tank. The buoyancy-driven flow is produced by continuously releasing dense brine into an inclined tank filled with fresh water. Two distinct mixing regimes are identified. In one of the regimes, fresh water enters the tank because of the negative pressure induced by the brine plume. In the other regime, the fresh water inflow is mainly caused by the stack effect. A light attenuation technique is used to measure the density stratification and distribution in the tank. The effects of the buoyant source volume flow rate, source buoyancy flux, and source location on the mixing and stratification are investigated. The results indicate that the mixing between the buoyant and ambient fluids is strengthened by the increase in the source volume flow rate and the height difference between the source location and the lower end of the tank. However, the current downstream of the source becomes more stratified as the source buoyancy flux increases. A dimensionless parameter, λ , is proposed to evaluate the overall mixing intensity in the tank. As λ increases, the flow changes from a bidirectional one to a unidirectional one. The evolution of reduced gravity along the longitudinal direction is also investigated. The results indicate that fresh water is entrained into the brine layer if the thickness of the brine layer is less than the tank height. Otherwise, the reduced gravity remains constant along the longitudinal direction of the tank. [ABSTRACT FROM AUTHOR]

Due to several engineering applications of heat transfer in confined enclosures, it has been an attractive topic for the researchers. The presented research is also an important contribution in this massive research area. Effects of a bottom heated rotating cylinder in a vertical annulus has been experimentally studied with respect to heat transfer parameters. Bottom heating of inner cylinder triggers a buoyancy-driven flow of air within the annulus and heat transfer due to this heating is studied against the rotation of the inner cylinder. Thus, a simultaneous effect of bottom heating and rotation of inner cylinder on heat transfer is inspected. The experimental setup of presented research consists of two seamless mild steel cylinders, a mild steel bottom heating plate, wireless data acquisition system and a rotational mechanism for the inner cylinder. A series of experiments are carried out for bottom heated plate temperature ranging from 313 K to 333 K and rotational Reynolds number of inner cylinder from 0 - 1660. The experimental results are presented in the form of a quantitative analysis of measured steady-state temperatures and non-dimensional parameters of heat transfer such as local Nusselt number, local Rayleigh number and Gr/ΩRe2. It is noted that the rotation of inner cylinder is effectual to enhance the heat transfer within the annulus by altering heat transport mechanisms. Moreover, the rotational effects deteriorate the influence of buoyancy at high rotational Reynolds number. [ABSTRACT FROM AUTHOR]

Buoyancy-driven, incompressible, two-dimensional flow of a micropolar fluid inside an inclined porous cavity in the presence of magnetic field is investigated. The nonlinear partial differential equations are solved by employing a robust Galerkin finite element scheme. The pressure term in this scheme is eliminated by using the penalty method. The results are exhibited in the form of streamlines, isotherms, and local and average Nusselt numbers for two cases, namely, the constant and the sinusoidal heated lower wall of the conduit. In both cases, the side walls of the cavity are cold and the upper side is insulated. The main difference between the two cases is observed from temperature contours. For constant heated bottom wall a finite discontinuity appears in the temperature distribution at the corners of the bottom wall. In contrast, no such discontinuity appears in the temperature distribution for non-uniform heated bottom wall. The quantitative changes in temperature contours in different portions of the cavity are identified by comparing the results for both cases. The code is also validated and benchmarked with the previous numerical data available in the literature. It is found that the magnetic field inclined at a certain angle either suppresses or enhances the intensity of primary circulations depending on the inclination of the cavity. Further, the average Nusselt number at the bottom wall is higher when magnetic field is applied vertically irrespective of the inclination of cavity. The analysis presented here has potential application in solar collectors and porous heat exchangers. [ABSTRACT FROM AUTHOR]

Contaminant transport induced by buoyancy-driven flows is crucial for proper ventilation design in buildings. Compared to conventional single-zone spaces, considerably less research has been conducted on the dynamics of buoyant contaminated flows in narrow horizontal spaces with large length-to-width ratios. Experiments were performed to explore the characteristics of natural convection produced by a brine source in a narrow horizontal tank immersed in fresh water. The source was located at the center of the tank ceiling; hence, the buoyancy-driven flow consisted of a plume region and a symmetrical horizontal current. A light attenuation technique was used to measure the density distribution and vertical profile of the horizontal velocity of the current. The current density was vertically stratified, but declined very slowly in the horizontal direction. The head of the horizontal current advanced toward the two ends of the tank at a constant velocity that depended only on the source buoyancy flux. The average thickness of the horizontal current was correlated to the tank height and the source Froude number. In our experiments, the interface between the tail of the horizontal current and the fresh water was very stable, and nearly no mass exchange through the interface was observed. However, mixing between the fresh water and brine occurred in both the plume region and the rearward side of the horizontal current head. Both the total entrainment and the proportion of the entrainment occurring in the head were determined. The latter increased with the source Froude number. [ABSTRACT FROM AUTHOR]

Onset of natural convection due to transient diffusion in porous media has attracted considerable attention for its applications to CO2 sequestration. Stability analyses typically investigate the onset of such convection using an initial value problem approach in which a perturbation is introduced to the concentration field at an initial time t = tp. This leads to debate concerning physically appropriate perturbations, the critical time tc for linear instability and the counter-intuitive notion of an optimal initial time tp that maximizes perturbation growth. We propose an alternate approach in which transient diffusion is continuously perturbed by small porosity variations. With this approach, instability occurs immediately (tc = 0) without violating any physical constraints, such that the concepts of initial time tp and critical time tc become irrelevant. We also argue that the onset time for nonlinear convection is a more physically relevant parameter, and show that it can be predicted using a simple asymptotic expansion. Using the expansion, we explore the onset of nonlinear convection due to porosity perturbations that vary sinusoidally in the horizontal and vertical directions, and show there are optimal wavelengths that minimize the onset time. Finally, we find simple relationships for these wavelengths as functions of perturbation magnitude. These show that even small porosity perturbations, typically considered negligible in previous literature, are sufficient to trigger nonlinear convection and thereby influence the rate of CO2 dissolution within time scales comparable to previous analyses. [ABSTRACT FROM AUTHOR]

Mesoscopic numerical simulations have been carried out to learn about the flow and heat transfer characteristics of a 2-D permeable triangular cylinder, aligned at two different orientations under the influence of aiding buoyancy. Objective of this study is to investigate the effects of Darcy number and forebody shape on the hydrodynamic and thermal behaviour of the porous cylinder, under forced convection ( i . e . Ri = 0 ) and aiding buoyancy conditions ( Ri  = 0.5 and 1) for a Prandtl number value of 0.71 (air). The ranges of Reynolds number ( Re ) and Darcy number ( Da ) considered in this study are 1 ⩽ Re ⩽ 40 and 10 - 6 ⩽ Da ⩽ 10 - 2 , respectively. Lattice Boltzmann method with two distribution functions is employed to perform the numerical experiments. Alongwith BGK collision operator, a body force term with viscous and inertial effects of the porous medium is employed at the porous zone. Detailed results are exhibited in the form of wake length, drag coefficient, streamlines, isotherm contours, heat transfer enhancement ratio and mean Nusselt number. Furthermore, a comparative investigation of drag coefficient and mean Nusselt number of permeable triangular cylinder (apex and side facing flow) with that of the square cylinder is carried out at Da = 10 - 6 for different buoyancy levels. Under aiding buoyancy condition ( i . e . Ri > 0 ), the side facing triangular cylinder experiences less drag force than the apex facing for all values of Da . A significant thermal dissipation is observed for increasing values of non-dimensional permeability (or Da ) and Richardson number. Furthermore, simple expressions for mean Nusselt number, valid for the range of parameters embraced in the present study, are also provided. The appropriate selection of non-dimensional permeability under different buoyancy conditions is important while applying porous media modeling technique in diverse fields of engineering. [ABSTRACT FROM AUTHOR]

This paper is aimed at assessing the ability of the Lattice-Boltzmann Method (LBM) in reproducing the fundamental features of lock-exchange gravity currents. Both two- and three-dimensional numerical simulations are presented at different Reynolds numbers ( 1000 ≤ R e ≤ 30,000 ). Turbulence has been accounted for by implementing an equivalent Large Eddy Simulation (LES) model in the LBM framework. The advancement of the front position and the front velocity obtained by LBM numerical simulations are compared with laboratory experiments appositely performed with similar initial and boundary conditions and with previous results from literature, revealing that the dynamics of the gravity current as a whole is correctly reproduced. Lobes and clefts instabilities arising in three-dimensional simulations and the entrainment parameter are also analysed and comparisons with previous studies are presented. [ABSTRACT FROM AUTHOR]

A computational analysis of the natural ventilation process and entropy generation in 3-D prismatic greenhouse was performed using CFD. The aim of the study is to investigate how buoyancy forces influence air-flow and temperature patterns inside the greenhouse having lower level opening in its right heated façade and also upper level opening near the roof top in the opposite cooled façade. The bottom and all other walls are assumed to be perfect thermal insulators. Rayleigh number is the main parameter which changes from 10³ to 106 and Prandtl number is fixed at Pr = 0.71. Results are reported in terms of particles trajectories, iso-surfaces of temperature, mean Nusselt number, and entropy generation. It has been found that the flow structure is sensitive to the value of Rayleigh number and that heat transfer increases with increasing this parameter. Also, it have been noticed that, using asymmetric opening positions improve the natural ventilation and facilitate the occurrence of buoyancy induced upward cross air-flow (low-level supply and upper-level extraction) inside the greenhouse. [ABSTRACT FROM AUTHOR]

This study investigated the transient natural convection phenomenon in an underground water pit thermal storage with heat losses through the surrounding walls. An experimental test rig was built up, and a numerical model was developed to obtain the characteristics of the thermal stratification in the water pit thermal storage. Fluid properties are assumed to be constant, except for the density changes with temperature which is treated using the Boussinesq approximation. The simulations of temperature profiles are reasonably proved by experiments with the maximum relative errors of ±9.77%. The results show that water temperature decreases close to the walls due to the heat losses, which leads to a downward flow along the inclined sidewalls. A slight upward flow occurs at the center of the water pit thermal storage, which lifts the warmer water to a higher level. As a result, the buoyancy-driven flow gradually builds up the thermal stratification in the water pit thermal storage. The modelling results also show that the values of the average Nusselt numbers on the inner surface of the inclined sidewalls and the bottom of the tank are much higher than that of the top thermal insulation layer. The thermal energy storage efficiency decreases rapidly in the first five minutes from 100% to 83.19% due to the intense heat transfer, and then tends to level off at the end of 40 min. The maximum velocity of the natural convection appears close to the upper part of the inclined sidewalls, and it decreases with the cooling process evolved. The present work has a valuable attempt to fill the gap in the existing studies and is useful for guiding the water pit thermal storage design. [ABSTRACT FROM AUTHOR]

In this paper, we deal with the time periodic problem to coupled chemotaxis-fluid models. We prove the existence of large time periodic strong solutions for the full chemotaxis-Navier-Stokes system in spatial dimension $$N=2$$ , and the existence of large time periodic strong solutions for the chemotaxis-Stokes system in spatial dimension $$N=3$$ . On the basis of these, the regularity of the solutions can be further improved. More precisely speaking, if the time periodic source g and the potential force $$\nabla \varphi $$ belong to $$C^{\alpha , \frac{\alpha }{2}}(\overline{\Omega }\times \mathbb R)$$ , the solutions we obtained are also classical solutions. [ABSTRACT FROM AUTHOR]

Geometrically, the upper pointed surface of an aircraft and bonnet of a car are examples of upper horizontal surfaces of a paraboloid of revolution ( uhspr ). The motion of these objects strongly depends on the boundary layer that is formed within the immediate space on it. However, each of these surfaces is neither a horizontal/vertical nor cone/wedge and neither a cone nor a wedge. This article presents the motion of 2-dimensional Blasius flow of Carreau fluid on the surface of such object. The case in which the reaction between the Carreau fluid and catalyst at the surface produces significant temperature differences which consequently set up buoyancy-driven flows within the boundary layer is investigated. Single first-order Arrhenius kinetics is adopted to model the reaction on the surface of the catalyst situated on uhspr which initiates the free convection. Suitable similarity variables are applied to non-dimensionalized, parameterized and reduce the governing partial differential equations to a coupled ordinary differential equations (BVP). The BVP is solved numerically using the shooting technique. Temperature distribution in the flow of viscoelastic Carreau fluid is greater than that of a Newtonian fluid. Local heat transfer rate decreases faster when the Carreau fluid is characterized as shear-thinning. Maximum concentration is guaranteed at a small value of power-law index n and large value of thickness parameter. [ABSTRACT FROM AUTHOR]

A novel formulation of the direct forcing immersed boundary (IB) method, that treats it as an integral part of a SIMPLE method is presented for the simulation of incompressible flows. The incompressibility and no-slip kinematic constraints are treated implicitly as distributed Lagrange multipliers and are fully coupled with each other by combining them into a single Poisson-body forces system of equations that constitutes a regularized saddle point problem. A physically justified approximation of the system, resembling a technique typical of the penalty method, is carried out to facilitate the solution. By utilizing the Schur complement approach, the approximated system is decomposed into two separate systems of equations, allowing us to compute the values for the volumetric force and pressure corrections. The first system, which is characterized by a small (only O (10)) value of the condition number, is conveniently solved by the BiCgStab method [1] , converging within 2-3 iterations, while the second system is addressed by the direct TPF solver [2] , characterized by O (N 4 / 3) complexity. The entire methodology is designed to be highly portable, which facilitates the use of any available solver that was designed to simulate incompressible flows governed by the Helmholtz and Laplace operators but could not benefit from the immersed boundary formalism. The capabilities of the developed methodology applied to the simulation of representative shear- and buoyancy-driven confined flows developing in the presence of stationary immersed bodies are demonstrated. A further application of the developed approach to moving boundary and two-way coupled fluid-structure interaction problems is discussed. • A novel formulation of the based on the SIMPLE method, is presented. • The formulation efficiently solves a regularized saddle point system of equations. • The incompressibility and no-slip kinematic constraints are simultaneously met. • A further adaptation of the methodology to the FSI simulations is discussed. [ABSTRACT FROM AUTHOR]

• CI cyano was used to study cyanoHABs in Lake Pontchartrain (LP) and Mississippi Sound (MSS). • CyanoHABs were observed in 2018 and 2019 following 2 out of 3 Bonnet Carré Spillway openings. • Wind- and buoyancy-driven flows advected blooms from LP and Breton Sound into the MSS. • Unprecedented 2019 freshwater inflow into the MSS fueled a 3-month coastal cyanoHAB. • CI cyano and wind data showed potential for CyanoHAB monitoring from freshwaters to coastal waters. During the spring and summer of 2019, an unprecedented cyanobacterial harmful algal bloom (cyanoHAB) was responsible for beach advisories on 25 beaches along the Mississippi Sound for over 3 months. Due to the preceding heavy rainfall and flooding within the Mississippi River watershed, for the first time in history, the Bonnet Carré Spillway (BCS) opened twice in one year during 2019. The coastal cyanoHAB coincided with the second BCS opening. The main objectives of this study were: (1) to investigate the potential for using the National Aeronautics and Space Administration (NASA) ocean color standard Cyanobacteria Index (CI cyano) algorithm to characterize the spatial and temporal extent of the 2019 cyanoHAB; (2) to couple the CI cyano data with river discharge, salinity, and modeled-wind data to study the conditions leading to the cyanoHAB and factors aiding the advection and persistence of the bloom within the Mississippi Sound, including a possible relationship to the BCS; (3) to further investigate the relationship with the BCS by repeating the methods using data from 2018, which was a year when the BCS was opened but no evidence of cyanoHABs was reported along the Mississippi coast. Weekly means and monthly frequency CI cyano images, river discharge, salinity, and modeled-wind data from February to September of 2018 and 2019 were analyzed, which coincide with three BCS openings. In March 2018, a cyanobacteria bloom was observed within Lake Pontchartrain coinciding with the BCS opening; however, the month-long bloom was contained to the lake. Two distinct cyanoHABs were observed in 2019 and both blooms were advected into the Mississippi Sound, and likely contributed to the 3-month-long beach water advisories of 2019 along the Mississippi coastline. From March to mid-July 2019, salinity at stations within the Mississippi Sound was consistently near zero indicating high levels of freshwater. During that time, winds were predominantly northwestward, preventing the BCS waters from flushing into the Mississippi Shelf and resulting in BCS waters remaining longer within the estuarine lakes and Mississippi Sound. Although the BCS had an undeniable impact on the presence of the coastal cyanoHAB of 2019, other variables including wind direction, water flow, mixing, and persistence of freshwater within the Sound can determine the intensity and extent of the cyanoHABs. Coupling in situ phytoplankton information from freshwater water bodies to the marine continuum along with water flow, wind data, and satellite imagery could help identify cyanoHABs at early stages and forecast their trajectory and potential impacts on coastal areas. [ABSTRACT FROM AUTHOR]

The buoyancy-driven circulation is studied in an idealized domain using two models, one based on the linearized planetary geostrophic equations and a fully nonlinear GCM. The surface buoyancy is specified (relaxed) to a chosen function of latitude in the linear (nonlinear) model. The models yield similar baroclinic flow in the interior, where the surface velocities are primarily zonal, and near the southern boundary, where a westward surface flow feeds the western boundary current (WBC). They differ however in the north. While the linear model has westward surface flow of limited vertical extent near the northern wall, the GCM has baroclinic flow which extends to the bottom with eastward surface velocities. The difference is due to convection, which weakens the stratification in the nonlinear model, amplifying the thermal wind transport associated with the surface buoyancy gradient. The WBC flow in the GCM feeds this, with northward surface flow over its entire length (as seen in nearly all previous similar studies). This in turn determines the meridional overturning circulation, since the WBC accounts for the largest meridional velocities. Roughly half the upwelling occurs in the WBC in the GCM and half in the interior, while the upwelling occurs in the interior and near the southern wall in the linear model. The study highlights the role of convection in modifying the response to the surface buoyancy gradient.

Energy efficient vehicles are essential for a sustainable society and all car manufacturers are working on improved energy efficiency in their fleets. In this process, an optimization of aerodynamics and thermal management is most essential. The objective of this work is to improve the energy efficiency using encapsulated heat generating units by focusing on predicting temperature distribution inside an engine bay. The overall objective is to make an estimate of the generated heat inside an encapsulation and consecutively use this heat for climatization purposes. The study presents a detailed numerical procedure for predicting buoyancy-driven flow and resulting natural convection inside a simplified vehicle underhood during thermal soak and cool-down events. The procedure employs a direct coupling of one-dimensional and three-dimensional methods to carry out transient one-dimensional thermal analysis in the engine solids synchronized with sequences of steady-state three-dimensional simulations of the fluid flow. The boundary heat transfer coefficients and averaged fluid temperatures in the boundary cells, computed in the three-dimensional fluid flow model, are provided as input data to the one-dimensional analysis to compute the resulting surface temperatures which are then fed back as updated boundary conditions in the flow simulation. The computed temperatures of the simplified engine and the exhaust manifolds during the thermal soak and cool-down period are in favorable agreement with experimental measurements. The present study illustrates the capabilities of the coupled thermal-flow methodology to conduct accurate and fast computations of buoyancy-driven heat transfer. The methodology can be potentially applied to design and analysis of multiple demand vehicle thermal management systems in hybrid and electrical vehicles.

A numerical model is developed for simulating a single or multi–story Double Skin Façade integrating Photovoltaics (DSF-PV). The DSF-PV can co-generate solar electricity and heat, while it also allows daylight to be transmitted to the interior space. The buoyancy-driven air flow inside the cavity may be assisted by a fan to cool down the photovoltaics while providing natural or hybrid ventilation to adjacent zones. Automated roller shades are also implemented in the model and help regulate heating and cooling loads but also control the daylight levels in the indoor space. A parametric analysis for different control strategies for the airflow within the cavity and the roller shading devices is performed with the purpose to apply the proposed methodology to minimize the heating and cooling demand of the DSF-PV system. In addition, a parametric analysis for different adjacent zones floor areas is performed. The simulations show that a DSF-PV system can supply approximately 120kWh/façade area/year covering the yearly electricity demand of the adjacent office if the floor area is approximately less than 3 times larger than the floor area. [ABSTRACT FROM AUTHOR]

We demonstrate the use of the ‘Direct-Adjoint-Looping method’ for the identification of optimal buoyancy-driven ventilation flows governed by Boussinesq equations. We use the incompressible Reynolds-averaged Navier-Stokes (RANS) equations, derive the corresponding adjoint equations and solve the resulting sensitivity equations with respect to inlet conditions. We focus on a displacement ventilation scenario with a steady plume due to a line source. Subject to an enthalpy flux constraint on the incoming flow, we identify boundary conditions leading to ‘optimal’ temperature distributions in the occupied zone. Our results show that depending on the scaled volume and momentum flux of the inlet flow, qualitatively different flow regimes may be obtained. The numerical optimal results agree with analytically obtained optimal inlet conditions available from classical plume theory in an ‘intermediate’ regime of strong stratification and two-layer flow. [ABSTRACT FROM AUTHOR]