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We present a practical numerical framework for incompressible interfacial multiphase flows on unstructured grids with arbitrary and hybrid elements. The numerical framework is constructed by combining VPM (volume-average/point-value multi-moment) and UMTHINC (unstructured multi-dimensional tangent of hyperbola interface capturing) schemes. To facilitate accurate and reliable simulations for interfacial multiphase flows on arbitrary and hybrid unstructured grids, we have made the following major new efforts in this work. (1) UMTHINC scheme on prismatic and pyramidal elements to facilitate computations on hybrid arbitrary unstructured grids; (2) Consistent numerical formulation for mass and momentum transports to simulate multiphase flows of large density ratio; (3) Combined FVM-FEM for accurate solution to diffusion equation; (4) Pressure-projection formulation in consistent with the balanced-force model. Integrating all these numerical techniques effectively enhances the accuracy and robustness in interface capturing and numerical solution of multiphase fluid dynamics, which results in a numerical framework of great significance for practical applications. Numerical verifications have been carried out through benchmark tests ranging from surface tension dominant flows of small scale to large scale flows with violently-changing interfaces. Numerical results demonstrate that the present framework is robust with adequate accuracy for simulating multiphase flows in complex geometries. [ABSTRACT FROM AUTHOR]

Two air entrainment mechanisms driven by vortex instability are reported in the unstable relaxation of a horizontal shear layer below a free surface. This flow is experimentally investigated by means of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) coupled with surface profilometry. PLIF identifies counter-rotating vortex pairs (CRVP) emanating from the surface following the growth of high steepness two-dimensional millimetre-size waves for Reynolds and Weber numbers based on the momentum thickness of 177 to 222 and 7.59 to 13.9, respectively. High spatio-temporal resolution PIV reveals the role of surface-generated vorticity and flow separation in the highly curved trough of the waves on the injection of a CRVP. Air bubbles are entrapped in the wake of these CRVPs at Reynolds number above 190. PIV data and spanwise PLIF images show two initiation mechanisms: primary vortex instability modulating the spanwise location where the flow separates, resulting in the pinch off of an air ligament, and secondary vortex instability turning a CRVP into Ω-shaped loops pulling the surface down. Instability wavelengths agree with linear stability analysis, and models for these new air entrainment mechanisms are proposed. [ABSTRACT FROM AUTHOR]

In liquid rope coiling, a slender jet of viscous fluid falling onto a rigid surface builds a rotating corkscrew-like structure. Here, I use a numerical continuation method to construct a complete regime diagram for liquid rope coiling. I first consider the onset of coiling, and show that a suitable onset criterion is that the radius a1 of the rope itself be just equal to the radius R of the coil. Numerical calculation of the critical surface a1=R in the space of the dimensionless fall height ΠH, flow rate ΠQ and nozzle diameter Πd shows that the surface has four distinct asymptotic limits corresponding to a viscous (V) mode, a gravitational (G) mode and two inertial modes (I1, I2) which are distinguished by how much the tail of the jet is stretched by gravity. Exact expressions for the onset frequencies in each of these four modes are determined. Finally, the regime diagram is constructed in the form of contour plots of the dimensionless coiling frequency as a function of ΠH and ΠQ for several values of Πd. The diagram exhibits a total of six modes: V, G, I1, I2, a multivalued inertio-gravitational (IG) mode and a third inertial mode I3 with viscosity-dominated stretching of the tail. The regime diagram permits prediction of the coiling frequency for given values of the fall height, flow rate, viscosity and nozzle diameter, and should therefore be useful in practical applications ranging from non-woven textile production to 3D printing. [ABSTRACT FROM AUTHOR]

An analytical theory has been developed for properties of a steady, axisymmetric liquid–gas capillary bridge that is present between two identical, perfectly wettable, rigid spheres. In this theory the meridional profile of the capillary bridge surface is represented by a part of an ellipse. Parameters in this geometrical description are determined from the boundary conditions at the three-phase contact circle at the sphere and at the neck (i.e. in the middle between the two spheres) and by the condition that the mean curvature be equal at the three-phase contact circle and at the neck. Thus, the current theory takes into account properties of the governing Young–Laplace equation, contrary to the often-used toroidal approximation. Expressions have been developed analytically that give the geometrical parameters of the elliptical meridional profile as a function of the capillary bridge volume and the separation between the spheres. A rupture criterion has been obtained analytically that provides the maximum separation between the spheres as a function of the capillary bridge volume. This rupture criterion agrees well with a rupture criterion from the literature that is based on many numerical solutions of the Young–Laplace equation. An expression has been formulated analytically for the capillary force as a function of the capillary bridge volume and the separation between the spheres. The theoretical predictions for the capillary force agree well with the capillary forces obtained from the numerical solutions of the Young–Laplace equation and with those according to a comprehensive fit from the literature (that is based on many numerical solutions of the Young–Laplace equation), especially for smaller capillary bridge volumes. [ABSTRACT FROM AUTHOR]

Microfluidic platforms capable of complex on-chip processing and liquid handling enable a wide variety of sensing, cellular, and material-related applications across a spectrum of disciplines in engineering and biology. However, there is a general lack of available microscale sensors capable of non-optically monitoring and quantifying on-chip fluid motion. Hence, many microfluidic systems are confined to the laboratory because their use requires optical microscopy. Here, we present a method for dynamically tracking laminar interfacial flows in microfluidic channels non-optically using impedance spectroscopy. Using a microfluidic T-channel, we generate a liquid interface by co-flowing two different electrolyte streams side-by-side. The interface is driven through an array of “displacement” electrodes where it is electrokinetically deflected across the microchannel. The interfacial flow is monitored downstream using an array of interdigitated “impedance” electrodes which dynamically measure the electrochemical impedance near the surface of the flow channel. We demonstrate that the impedance spectrum is sensitively influenced by the position of the deflected interface. While laminar fluid interfaces are ubiquitous to microfluidic flows and used extensively in rheology and biomolecular detection, it is currently difficult to measure their position non-optically. The sensing method presented here enables the interfacial position to be dynamically determined without a microscope and provides a new tool for lowering the barriers to operating microfluidic devices outside the confines of a traditional laboratory. [ABSTRACT FROM AUTHOR]

This work aims to investigate synergy in anionic and zwitterionic surfactant mixtures, as they result in better interfacial properties and micellization behavior. Various mixtures of the pH-insensitive zwitterionic surfactant 3-(decyldimethylammonio) propanesulfonate (Zwittergent 3-10) and sodium dodecylsulfate (SDS) were prepared in aqueous solution at a range of pH values between 2 and 13. The thermodynamic parameters during mixed surfactant adsorption at the air/water interface are obtained and the results show the mixed surfactant systems having superior properties to the constituent surfactants. Experimentally, the mixed surfactant solutions clearly improve the surface activities by lowering the critical micelle concentration (CMC) and lowering the surface tension at the air/water interface. The synergisms are investigated through the interaction parameters estimated from regular solution theory that is used to quantitatively describe the nonideality of surfactant mixtures. High negative interaction parameters are obtained from these surfactant mixtures. Experimental precipitation phase boundaries of SDS in the presence of CaCl were also investigated in mixtures containing pH-insensitive zwitterionic surfactant at different pH levels from 2 to 13 and SDS mole fractions of 0.25, 0.50, 0.75, and 1.00. Changes in the precipitation phase boundaries are due to the changes in the speciation or activities of the major components both below and above the CMC. As a result, the precipitation phase boundaries are pH dependent. In addition, mixed micellization and counterion binding to the micelle also change the precipitation phase boundary above the CMC. The activity-based solubility product of calcium dodecylsulfate is also determined from the precipitation phase boundaries below the CMC. X-ray diffraction patterns and SEM images confirm that only calcium dodecylsulfate precipitates in the soap scum for all pH and surfactant compositions studied. [ABSTRACT FROM AUTHOR]

The effect of viscosity and thermal conduction on the acoustics in a shear layer above an impedance wall is investigated numerically and asymptotically by solving the linearised compressible Navier-Stokes equations (LNSE). It is found that viscothermal effects can be as important as shear, and therefore including shear while neglecting viscothermal effects by solving the linearised Euler equations (LEE) is questionable. In particular, the damping rate of upstream-propagating waves is found to be under-predicted by the LEE, and dramatically so in certain instances. The effects of viscosity on stability are also found to be important. Short wavelength disturbances are stabilised by viscosity, greatly altering the characteristic wavelength and maximum growth rate of instability. For the parameters considered here (chosen to be typical of aeroacoustic situations), the Reynolds number below which the flow stabilises ranges from 105 to 107. By assuming a thin but non-zero-thickness boundary layer, asymptotic analysis leads to a system of boundary layer governing equations for the acoustics. This system may be solved numerically to produce an effective impedance boundary condition, applicable at the wall of a uniform inviscid flow, that accounts for both the shear and viscosity within the boundary layer. An alternative asymptotic analysis in the high-frequency limit yields a different set of boundary layer equations, which are solved to yield analytic solutions. The acoustic mode shapes and axial wavenumbers from both asymptotic analyses compare well with numerical solutions of the full LNSE. A closed-form effective impedance boundary condition is derived from the high-frequency asymptotics, suitable for application in frequency domain numerical simulations. Finally, surface waves are considered, and it is shown that a viscous flow over an impedance lining supports a greater number of surface wave modes than an inviscid flow. [ABSTRACT FROM AUTHOR]

A model is developed for the extrusion in the direction of gravity of a slender fluid cylinder from a die of arbitrary shape. Both gravity and surface tension act to stretch and deform the geometry. The model allows for an arbitrary but prescribed viscosity profile, while the effects of extrudate swell are neglected. The solution is found efficiently through the use of a carefully selected axial Lagrangian coordinate and a transformation to a reduced-time variable. Comparisons between the model and extruded glass microstructured optical fibre preforms show that surface tension has a significant effect on the geometry but the model does not capture all of the behaviour observed in practice. Experimental observations are used in conjunction with the model to argue that some deformation, due neither to surface tension nor gravity, occurs in or near the die exit. Methods are considered to overcome deformation due to surface tension. [ABSTRACT FROM AUTHOR]

This experimental and analytical study investigates the double-averaged (DA) turbulent flow characteristics within an array of large gravel obstacles found atop a porous gravel bed. Analysis of the experimental data reveals that the DA streamwise velocity preserves the logarithmic law above the form-induced sublayer, while a linear law and a third-degree polynomial function apply within the form-induced and interfacial sublayers, respectively. The form-induced shear stress is 70% of the DA Reynolds shear stress (RSS) occurring at the virtual bed level. The DA turbulent kinetic energy (TKE) components, streamwise and vertical, attain their peak values at the obstacle crest level, while they diminish sharply below the virtual bed level. The fluxes of the TKE streamwise and vertical components, however, change their signs slightly below the crest level, indicating a changeover of the dominance of the bursting events. For the TKE budget, the TKE production, diffusion, and pressure energy diffusion rate terms attain their peak values at the crest level, while the TKE dissipation rate has its peak value at the virtual bed level. Third-order moments of velocity fluctuations follow the linear relationship, and their signs change slightly below the crest level. The quadrant analysis suggests that the sweep events are the governing mechanism at the near-bed flow region, while the ejection events become predominant with an increase in vertical distance. The quadrant plots of the form-induced velocity components display a pseudo-elliptical scatter within the interfacial sublayer and a small circular cluster above the form-induced sublayer. [ABSTRACT FROM AUTHOR]

The nonaqueous-phase liquid (NAPL)-water interfacial area and the mass transfer rate across the NAPL and water interface are often key factors in in situ groundwater pollution treatment. In this study, the NAPL-water interfacial area and residual NAPL saturation were measured using interfacial and partitioning tracer tests in a two-dimensional flow cell. The results were compared with previous column and field experiment results. In addition, the mass transfer rates at various NAPL-water interfacial areas were investigated. Fe2+-activated persulfate was used for in situ chemical oxidation remediation to remove NAPL gradually. The results showed that the reduction of NAPL-water interfacial areas as well as NAPL saturation by chemical oxidation caused a linear decrease in the interphase mass transfer rates (R2 = 0.97), revealing the relationship between mass transfer rates and interfacial areas in a two-dimensional system. The NAPL oxidation rates decreased with the reduction of interfacial areas, owing to the control of NAPL mass transfer into the aqueous phase. [ABSTRACT FROM AUTHOR]

Multiphase flow in porous media is important in many natural and industrial processes, including geologic CO2 sequestration, enhanced oil recovery, and water infiltration into soil. Although it is well known that the wetting properties of porous media can vary drastically depending on the type of media and pore fluids, the effect of wettability on multiphase flow continues to challenge our microscopic and macroscopic descriptions. Here, we study the impact of wettability on viscously unfavorable fluid-fluid displacement in disordered media by means of high-resolution imaging in microfluidic flow cells patterned with vertical posts. By systematically varying the wettability of the flow cell over a wide range of contact angles, we find that increasing the substrate's affinity to the invading fluid results in more efficient displacement of the defending fluid up to a critical wetting transition, beyond which the trend is reversed. We identify the pore-scale mechanisms--cooperative pore filling (increasing displacement efficiency) and corner flow (decreasing displacement efficiency)--responsible for this macroscale behavior, and show that they rely on the inherent 3D nature of interfacial flows, even in quasi- 2D media. Our results demonstrate the powerful control of wettability on multiphase flow in porous media, and show that the markedly different invasion protocols that emerge--from pore filling to postbridging-- are determined by physical mechanisms that are missing from current pore-scale and continuum-scale descriptions. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of drop impact on a thin liquid film at short times in order to identify the mechanisms of splash formation. Using numerical simulations and scaling analysis, we show that it depends both on the inertial dynamics of the liquid and the cushioning of the gas. Two asymptotic regimes are identified, characterized by a new dimensionless number $J$: when the gas cushioning is weak, the jet is formed after a sequence of bubbles are entrapped and the jet speed is mostly selected by the Reynolds number of the impact. On the other hand, when the air cushioning is important, the lubrication of the gas beneath the drop and the liquid film controls the dynamics, leading to a single bubble entrapment and a weaker jet velocity. [ABSTRACT FROM AUTHOR]

The use of mixed ligand layers including poly(ethylene glycol)-based ligands for the functionalization of nanoparticles is a very popular strategy in the context of nanomedicine. However, it is challenging to control the composition of the ligand layer and maintain high colloidal and chemical stability of the conjugates. A high level of control and stability are crucial for reproducibility, upscaling, and safe application. In this study, gold nanoparticles with well-defined mixed ligand layers of α-methoxypoly(ethylene glycol)-ω-(11-mercaptoundecanoate) (PEGMUA) and 11-mercaptoundecanoic acid (MUA) were synthesized and characterized by ATR-FTIR spectroscopy and gel electrophoresis. The colloidal and chemical stability of the conjugates was tested by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and UV/vis spectroscopy based experiments, and their interactions with cells were analyzed by elemental analysis. We demonstrate that the alkylene spacer in PEGMUA is the key feature for the controlled synthesis of mixed layer conjugates with very high colloidal and chemical stability and that a controlled synthesis is not possible using regular PEG ligands without the alkylene spacer. With the results of our stability tests, the molecular structure of the ligands can be clearly linked to the colloidal and chemical stabilization. We expect that the underlying design principle can be generalized to improve the level of control in nanoparticle surface chemistry. [ABSTRACT FROM AUTHOR]

In this work, the interfacial instability and transition of a two-fluid flow from a stratified state to large amplitude waves or slugs is considered. By combining an asymptotic approximation of the linear Orr-Sommerfeld analysis with nonlinear resonant wave interaction theory, a novel nonlinear slug-transition criterion is derived. This criterion corresponds to a bounding condition on the upper fluid's velocity in order to limit the amount of energy (provided by the linear instability) which is transferred to long waves through resonant wave interactions. It is proposed that such a condition can predict the formation of large-amplitude long waves and/or slugs. Quantitative comparisons of the onset of slugging are made between the prediction by the nonlinear transition criterion and the experimental measurements carried out in a horizontal square channel. Good agreement is observed. An additional heuristic model is developed which generalizes the transition criterion to flow through horizontal pipes. Comparisons are made for flows through different pipe diameters and over a wide range of fluid properties. Good agreement between the present theoretical predictions and the experimental measurements is also observed. [ABSTRACT FROM AUTHOR]

Production of particle stabilized oil in water emulsions has been investigated both theoretically and experimentally under oscillatory shear conditions using different stabilizing particles (SPs). The investigation included analysis of the interaction between particles interfacial stability and droplets breakage and coalescence. For hydrophobic SPs, droplets maintained their sizes as determined by torque balance (TB) without significant breakage or coalescence. For the more hydrophilic SPs, larger droplets formed that broke by eddies in the inertial subrange. At higher fluid shear stresses, loss of the SPs occurred during droplet formation leading to near bare droplet surface and coalescence to much larger sizes with subsequent fragmentation by capillary instabilities. The final droplet size in both cases was very different from TB model predictions. A modeling approach is proposed that combined both TB and droplet breakage and coalescence mechanisms. Comparison between the experimental results and the models predictions showed satisfactory agreement. [ABSTRACT FROM AUTHOR]

When two microemulsions are put in contact in the gravity field along a horizontal contact line, cross-diffusion can trigger the transport of one species in the presence of a gradient in concentration of another species. We show here theoretically that such cross-diffusion effects can induce buoyancy-driven convective instabilities at the interface between two solutions of different compositions even when initially the less dense solution lies on top of the denser one. Two different sources of convective modes are identified depending whether positive or negative cross-diffusion is involved. We evidence the two predicted cross-diffusion driven instabilities experimentally using a two-layer stratification of Aerosol-OT (AOT) water-in-oil microemulsions solutions with different water or AOT composition. [ABSTRACT FROM AUTHOR]

A Lagrangian–Eulerian advection scheme (LEAS) with Moment-of-Fluid (MoF) interface reconstruction is presented for interfacial flows. In MoF method, in addition to the volume fraction field, material centroid is used for interface reconstruction. Physically material centroid indicates the material location inside a mixed cell. Therefore, MoF method reconstructs linear interfaces exactly and is second order accurate for curved interfaces. Despite being second order accurate for static interface reconstruction, MoF reconstruction is inaccurate when centroid is not properly advected. An accurate centroid advection method based on Barycenter of centroid is discussed here. A comparison of this centroid advection with the pure Lagrangian advection based on Runge–Kutta integrator is demonstrated for two different advection tests. The superiority of MoF is established by comparing the volume error norm with other interface reconstructions. Significant improvement in accuracy is observed when LEAS material advection is used in conjuction with accurate centroid advection for MoF reconstruction. [ABSTRACT FROM PUBLISHER]

We examine experimentally the effects of surface-active impurities on the small-amplitude free oscillations of axisymmetric liquid bridges. The surface tension, oscillation frequency, and damping rate are measured at different instants from the free surface formation. The experiments with n-hexadecane and n-dodecane show that none of these interfacial quantities is significantly affected by the free surface age. The damping rates exceed by a $$\mathcal{O}(1)$$ quantity their corresponding values for a clean free surface. This extra-damping can be modeled in terms of the monolayer shear viscosity exclusively. Similar values of this quantity are obtained for n-hexadecane and n-dodecane, although the impurity effects on the surface tension are much greater in the first case. We conducted experiments with deionized water liquid bridges to analyze the impurity effects on free surfaces with high elasticity numbers, where Marangoni convection is expected to increase the monolayer dissipation. In this case, the damping rates are up to three times as those of a clean free surface. The monolayer dissipative effects do not increase as the free surface ages, although the surface tension decreases considerably during this process. Similar impurity effects are observed when an anionic surfactant is dissolved in deionized water. For a fixed value of the liquid bridge slenderness, both the oscillation frequency and damping rate are functions of the liquid bridge volume exclusively, independently of the free surface age and the liquid-ambient combination. Extra-dissipation increases sharply as the liquid bridge volume decreases. [ABSTRACT FROM AUTHOR]

This article deals with the interactions between a central crack and a pair of outer cracks situated at the interface of orthotropic elastic half planes under thermo-mechanical loading. The mixed boundary value problem has been reduced to a pair of singular integral equations which has been solved numerically using Jacobi polynomial method. The interaction effects have been obtained in terms of stress magnification factors depending on the crack spacing and crack length. The phenomena of crack shielding and crack amplification have been depicted through graphs for different particular cases. [ABSTRACT FROM AUTHOR]

The stability of thin electrically conducting viscoelastic fluid film flowing down on a non-conducting inclined plane in the presence of electromagnetic field is investigated under induction-free approximation. Surface evolution equation is derived by long-wave expansion method. The stabilizing role of Hartman number M (magnetic field) and the destabilizing role of the viscoelastic property $${\varGamma}$$ and the electric parameter E on such fluid film are established through the linear stability analysis of the surface evolution equation. Investigation shows that at small values of Hartman number ( M), the influence of electric parameter ( E) on the viscoelastic parameter $${(\varGamma)}$$ is insignificant, while for large values of M, E introduces more destabilizing effect at low values of $${\varGamma}$$ than that at high values of $${\varGamma }$$ . An interesting result also perceived from our analysis is that the stabilizing effect of Hartman number ( M) is decreasing with the increase of the values of $${\varGamma}$$ and E, even it gives destabilizing effect after a certain high value of the electric field depending on the high value of $${\varGamma}$$ . The weakly nonlinear study reveals that the increase of $${\varGamma}$$ decreases the explosive and subcritical unstable zones but increases the supercritical stable zone keeping the unconditional zone almost constant. [ABSTRACT FROM AUTHOR]

Pressurization and pressure control in cryogenic storage tanks are to a large extent affected by heat and mass transport across the liquid–vapor interface. These mechanisms are, in turn, controlled by the kinetics of the phase change process and the dynamics of the turbulent recirculating flows in the liquid and vapor phases. In this paper, the effects of accommodation coefficient and interfacial turbulence on tank pressurization and pressure control simulations are examined. Comparison between numerical predictions and ground-based measurements in two large liquid hydrogen tank experiments, performed in the K-site facility at NASA Glenn Research Center (GRC) and the Multi-purpose Hydrogen Test Bed (MHTB) facility at NASA Marshall Space Flight Center (MSFC), are used to show the impact of accommodation coefficient and interfacial and vapor phase turbulence on evolution of pressure and temperatures in the cryogenic storage tanks. In particular, the self-pressurization comparisons indicate that: (1) numerical predictions are essentially independent of the magnitude of the accommodation coefficient; and (2) surprisingly, laminar models sometimes provide results that are in better agreement with experimental self-pressurization rates, even in parametric ranges where the bulk flow is deemed fully turbulent. In this light, shortcomings of the present CFD models, especially, numerical treatments of interfacial mass transfer and turbulence, as coupled to the Volume-of-Fluid (VOF) interface capturing scheme, are underscored and discussed. [ABSTRACT FROM AUTHOR]

We propose an efficient formulation of the scale-separation approach which has been developed by Han et al. [10] for multi-scale sharp interface modeling of multi-phase flows based on the level-set technique. Instead of shifting the entire level-set field twice as in the original method, the improved method identifies the non-resolved interface structures from two auxiliary level-sets close to the interface. Non-resolved structures are separated from the interface by a localized re-distancing method, which increases the computational efficiency considerably compared to the original global reinitialization procedure. Several tests for two-phase flow problems, involving simple and complex interface structures, are carried out to show that the present method maintains sharper interface structures than the original method, and achieves effective scale-separation. [ABSTRACT FROM AUTHOR]

Explicit analytical expressions for the drag and diffusion coefficients of a spherical particle attached to the flat interface between two immiscible fluids are constructed for the case of a vanishing viscosity ratio between the fluid phases. The model is designed to account explicitly for the dependence on the contact angle between the two fluids and the solid surface. The Lorentz reciprocal theorem is applied in the context of geometric perturbations from the limiting cases of 90° and 180° contact angles. The model agrees well with the experimental and numerical data from the literature. Also, an advantage of the method utilized is that the drag and diffusion coefficients can be calculated up to one order higher in the perturbation parameter than the known velocity and pressure fields. Extensions to other particle shapes with known velocity and pressure fields are straightforward. [ABSTRACT FROM AUTHOR]

Motivated by the fabrication of microstructured optical fibres, a model is presented for the extension under gravity of a slender fluid cylinder with internal structure. It is shown that the general problem decouples into a two-dimensional surface-tension- driven Stokes flow that governs the transverse shape and an axial problem that depends upon the transverse flow. The problem and its solution differ from those obtained for fibre drawing, because the problem is unsteady and the fibre tension depends on axial position. Solutions both with and without surface tension are developed and compared, which show that the relative importance of surface tension depends upon both the parameter values and the geometry under consideration. The model is compared with experimental data and is shown to be in good agreement. These results also show that surface-tension effects are essential to accurately describing the cross-sectional shape. [ABSTRACT FROM AUTHOR]

The turbulence structure of the gaseous phase in a stratified air/water pipe flow is investigated through a combination of three experimental approaches: PIV, conductance probes and hot-wire anemometry. Original velocity field measurements in the air phase were successfully achieved by using water droplets as tracer particles. The resulting measurements from this campaign describe a wide range of underlying mechanisms that govern the complex interplay between the flow dynamics of the gaseous phase and the interfacial flow pattern. One of such mechanisms is the generation of strong vortical structures in the wake of a wave-crest. The intensity, the size and the occurrence frequency of these structures depend strongly on the two-phase flow rate combination and their presence imposes changes to the turbulent energy cascade. [ABSTRACT FROM AUTHOR]

Prior modal stability analysis (Kojima et al., Phys. Fluids, vol. 27, 1984, pp. 19-32) predicted that a rising or sedimenting droplet in a viscous fluid is stable in the presence of surface tension no matter how small, in contrast to experimental and numerical results. By performing a non-modal stability analysis, we demonstrate the potential for transient growth of the interfacial energy of a rising droplet in the limit of inertialess Stokes equations. The predicted critical capillary numbers for transient growth agree well with those for unstable shape evolution of droplets found in the direct numerical simulations of Koh & Leal (Phys. Fluids, vol. 1, 1989, pp. 1309-1313). Boundary integral simulations are used to delineate the critical amplitude of the most destabilizing perturbations. The critical amplitude is negatively correlated with the linear optimal energy growth, implying that the transient growth is responsible for reducing the necessary perturbation amplitude required to escape the basin of attraction of the spherical solution. [ABSTRACT FROM AUTHOR]

Hyporheic zone is considered as a dynamic hydrologic ecotone that critical for maintaining the health of river systems. The hyporheic flux occurs generally in response to variations in river bedforms. In this study, we first generated several typical bedforms. The turbulent flow over the 3D dunes and hyporheic flow in the sediment are simulated through a computational fluid dynamics (CFD) approach. Turbulent flow in the water column is simulated by solving the Reynolds-averaged Navier-Stokes (RANS) equations with the k-ω turbulence closure model, and a steady state groundwater flow model is applied for the underlying porous media. These two sets of equations are coupled through the pressure distribution at the sediment-water interface (SWI).Each case was subjected to different hydraulic conditions, i.e., increasing open channel Reynolds Numbers (Re). Results show that the pressure gradient along the SWI is highly controlled by the spatial structure of bedform, which consequently determines flow dynamics in the porous media. The interfacial flux is dominated by the pressure configuration over the SWI which is a function of Re via a power-law trend. The mean fluid residence time is related to Re by an inverse-power law relationship. This study has led to the basic understanding of hyporheic flow induced by more natural 3D dunes. [ABSTRACT FROM AUTHOR]

Cavitation in a liquid moving past a constraint is numerically investigated by means of a free-energy lattice Boltzmann simulation based on the van der Waals equation of state. The fluid is streamed past an obstacle, and depending on the pressure drop between inlet and outlet, vapor formation underneath the corner of the sack-wall is observed. The circumstances of cavitation formation are investigated and it is found that the local bulk pressure and mean stress are insufficient to explain the phenomenon. Results obtained in this study strongly suggest that the viscous stress, interfacial contributions to the local pressure, and the Laplace pressure are relevant to the opening of a vapor cavity. This can be described by a generalization of Joseph's criterion that includes these contributions. A macroscopic investigation measuring mass flow rate behavior and discharge coefficient was also performed. As theoretically predicted, mass flow rate increases linearly with the square root of the pressure drop. However, when cavitation occurs, the mass flow growth rate is reduced and eventually it collapses into a choked flow state. In the cavitating regime, as theoretically predicted and experimentally verified, the discharge coefficient grows with the Nurick cavitation number. [ABSTRACT FROM AUTHOR]

We present a theoretical study on the role of elasticity in causing fingering or fracturing instability during the immiscible displacement process of a viscoelastic fluid by another viscoelastic fluid in a rectilinear Hele-Shaw cell. Upper convected Maxwell (UCM) models are used for both fluid layers and linear stability analysis is performed in the regime of moderate to large Deborah number. This is a generalization of Hai and Daripa (2022) where the case of a Newtonian fluid displacing an UCM fluid is considered. The elastic effect of the displacing layer has a significant impact on the overall flow stability. The dispersion relation is implicitly given by a quartic polynomial equation with coefficients depending on a modified wavenumber k ˆ , viscosity contrast η r / η l (displaced/displacing fluid), relaxation time contrast λ r / λ l and a composite parameter β inversely proportional to the flow speed U (depth averaged). Viscous effect is still the dominant mechanism in determining long wave stability (unstable if η r / η l > 1). The elastic effect of the displacing layer always destabilizes short waves (unstable if wavelength is shorter than λ l U). In addition, three types of singular behaviors are found all of which are associated with elastic effects: (i) velocity becomes singular at infinitely many isolated wavenumbers (precise values are inversely proportional to λ l U or λ r U); (ii) stress becomes singular if wavenumber exceeds certain value (can happen even for slow flow); and (iii) growth rate becomes singular at up to two wavenumbers if η r / η l , λ r / λ l and β fall within a certain range but this can always be avoided if flow is slow enough. The special cases of an UCM fluid displacing air or a viscous Newtonian fluid are also considered. • Analytic solutions for perturbations. • Explicit formulas for the dispersion relations. • Viscous effect dominates long wave stability. • The elastic effect of the displacing fluid destabilizes short waves. • Singularities associated with elasticity in connection with fracturing instability. [ABSTRACT FROM AUTHOR]

Backwater curves denote the depth profiles of steady flows in a shallow open channel. The classification of these curves for turbulent regimes is commonly used in hydraulics. When the bottom slope I is increased, they can describe the transition from fluvial to torrential regimes. In the case of an infinitely wide channel, we show that laminar flows have the same critical height hc as that in the turbulent case. This feature is due to the existence of surface slope singularities associated to plug-like velocity profiles with vanishing boundary-layer thickness. We also provide the expression of the critical surface slope as a function of the bottom curvature at the critical location. These results validate a similarity model to approximate the asymptotic Navier-Stokes equations for small slopes I with Reynolds number Re such that Re I is of order 1. [ABSTRACT FROM AUTHOR]

An exploratory study of two-phase physics was undertaken in a slow moving tank containing liquid. This study is under the regime of conjugate heat and mass transfer phenomena. An experiment was designed and performed to estimate the interfacial mass transfer characteristics of a slowly moving tank. The tank was swayed at varying frequencies and constant amplitude. The experiments were conducted for a range of liquid temperatures and filling levels. The experimental setup consisted of a tank partially filled with water at different temperatures, being swayed using a six degrees-of-freedom (DOF) motion actuator. The experiments were conducted for a frequency range of 0.7-1.6 Hz with constant amplitude of 0.025 m. The evaporation of liquid from the interface and the gaseous condensation was quantified by calculating the instantaneous interfacial mass transfer rate of the slow moving tank. The dependence of interfacial mass transfer rate on the liquid-vapor interfacial temperature, the fractional concentration of the evaporating liquid, the surface area of the liquid vapor interface and the filling level of the liquid was established. As sway frequency, filling levels, and liquid temperature increased, the interfacial mass transfer rate also increased. The interfacial mass transfer rate estimated for the swaying tank compared with the interfacial mass transfer rate of stationary tank shows that vibration increases the mass transfer. [ABSTRACT FROM AUTHOR]

Analytical solutions for both a finite assembly and a periodic array of bubbles steadily moving in a Hele-Shaw channel are presented. The particular case of multiple fingers penetrating into the channel and moving jointly with an assembly of bubbles is also analysed. The solutions are given by a conformal mapping from a multiply connected circular domain in an auxiliary complex plane to the fluid region exterior to the bubbles. In all cases the desired mapping is written explicitly in terms of certain special transcendental functions, known as the secondary Schottky-Klein prime functions. Taken together, the solutions reported here represent the complete set of solutions for steady bubbles and fingers in a horizontal Hele-Shaw channel when surface tension is neglected. All previous solutions under these assumptions are particular cases of the general solutions reported here. Other possible applications of the formalism described here are also discussed. [ABSTRACT FROM AUTHOR]

This work is inspired by the expansion of the plasma bubble in a narrow gap reported by Chu and Lee [Phys. Rev. Lett. 107, 225001 (2011)]. We report the unstable phenomena of the plasma-liquid interface with different curvature in a Hele-Shaw cell. Dielectric barrier discharge is produced in the cell at atmospheric pressure which is partially filled with silicone oil. We show that the Saffman-Taylor-like instability is observed on the bubble-type, channel-type, and drop-type interfaces. The Schlieren observation of the plasma-drop interaction reveals that there is a vapor layer around the drop and the particle image velocimetry shows the liquid flow inside the drop. We propose that the thermal Marangoni effect induced by the plasma heating is responsible for the unstable phenomena of the plasma-liquid interaction. The fluctuation of the interface is shown consistently with the Saffman-Taylor instability modified by the temperature-dependent velocity and surface tension. [ABSTRACT FROM AUTHOR]

In this paper we report on numerical studies of unsteady, gravity-driven flow of a subaqueous erodible granular bed on an inclined plane. According to our simulations, the evolution of the flow can be partitioned in three phases. In the first phase, due to the onset of an interfacial instability, the material interface deforms into a series of long waves. In the second phase, these waves are transformed to skewed vortex ripples that grow in time and eventually coalesce. The computed wavelengths of these ripples are in good agreement with previously reported experimental measurements. In the third phase of the flow evolution, the high fluid velocities wash out the vortex ripples and a layer of rapidly moving particles is formed at the material interface. The predicted granular velocities comprise two segments: a concave one at the vicinity of the material interface, where the maximum is attained, followed by a slightly convex one, where they decrease monotonically to zero. The same trend has been reported in experimental results for the corresponding steady flows. Finally, we investigate via a parametric study the effect of the configuration stresses, which represent contact forces between grains. As it turns out, such stresses have a stabilizing effect, in the sense that increasing their magnitude inhibits the formation of vortex ripples. Graphical abstract: [ABSTRACT FROM AUTHOR]

The finite element discretization of a variable interfacial stress tensor σГ(x) is considered; variable interfacial tension is included as a special case. Interfacial stress, for example, occurs in two-phase flow problems. In the weak formulation, the interfacial stress gives rise to a functional which is supported on the interface Г. A finite element discretization of this functional is presented and analyzed. The discretization admits almost independent meshes for the approximation of the interface and the approximation of the flow variables. The main result is an O(hk+1/2)-errorbound for the interfacial stress functional in a natural norm if the discrete interface is an O(hk+1)- approximation of Г. The bound is shown to be sharp in numerical experiments [ABSTRACT FROM AUTHOR]

Probe-tack measurements evaluate the adhesion strength of viscous fluids confined between parallel plates. This is done by recording the adhesion force that is required to lift the upper plate, while the lower plate is kept at rest. During the lifting process, it is known that the interface separating the confined fluids is deformed, causing the emergence of intricate interfacial fingering structures. Existing meticulous experiments and intensive numerical simulations indicate that fingering formation affects the lifting force, causing a decrease in intensity. In this work, we propose an analytical model that computes the lifting adhesion force by taking into account not only the effect of interfacial fingering, but also the action of wetting and viscous normal stresses. The role played by the system's spatial confinement is also considered. We show that the incorporation of all these physical ingredients is necessary to provide a better agreement between theoretical predictions and experiments. [ABSTRACT FROM AUTHOR]

We consider the axisymmetric arrangement of an annular liquid film, coating the inner surface of a narrow cylindrical tube, in interaction with an active core fluid. We introduce a low-dimensional model based on the two-phase weighted residual integral boundary layer (WRIBL) formalism (Dietze & Ruyer-Quil, J. Fluid Mech., vol. 722, 2013, pp. 348–393) which is able to capture the long-wave instabilities characterizing such flows. Our model improves upon existing works by fully representing interfacial coupling and accounting for inertia as well as streamwise viscous diffusion in both phases. We apply this model to gravity-free liquid-film/core-fluid arrangements in narrow capillaries with specific attention to the dynamics leading to flooding, i.e. when the liquid film drains into large-amplitude collars that occlude the tube cross-section. We do this against the background of linear stability calculations and nonlinear two-phase direct numerical simulations (DNS). Due to the improvements of our model, we have found a number of novel/salient physical features of these flows. First, we show that it is essential to account for inertia and full interphase coupling to capture the temporal evolution of flooding for fluid combinations that are not dominated by viscosity, e.g. water/air and water/silicone oil. Second, we elucidate a viscous-blocking mechanism which drastically delays flooding in thin films that are too thick to form unduloids. This mechanism involves buckling of the residual film between two liquid collars, generating two very pronounced film troughs where viscous dissipation is drastically increased and growth effectively arrested. Only at very long times does breaking of symmetry in this region (due to small perturbations) initiate a sliding motion of the liquid film similar to observations by Lister et al. (J. Fluid Mech., vol. 552, 2006, pp. 311–343) in thin non-flooding films. This kickstarts the growth of liquid collars anew and ultimately leads to flooding. We show that streamwise viscous diffusion is essential to this mechanism. Low-frequency core-flow oscillations, such as occur in human pulmonary capillaries, are found to set off this sliding-induced flooding mechanism much earlier. [ABSTRACT FROM PUBLISHER]

Self-induced internal boiling in burning functional droplets has been observed to induce severe bulk shape oscillations in droplets with characteristic bubble ejection events that corrugate the droplet surface. Such bubble–droplet interactions are characterized by a distinct regime of a single bubble growing inside the droplet where evaporative Darrieus–Landau instability occurs at the bubble–droplet interface. In this regime the bubble–droplet system behaves as a self-excited coupled oscillator . In this study, we report the external flame-acoustic interaction with bubbles inside the droplet resulting in controlled droplet deformation. In particular, by exciting the droplet flame in a critical, responsive frequency range (80 Hz ⩽ f P ⩽ 120 Hz) the droplet deformation cycle could be altered through suppression of these self-excited instabilities and intensity/frequency of bubble ejection events. This selective acoustic tuning also enabled the control of bubble dynamics, bulk droplet-shape distortion and the final precipitate morphology even in burning nanoparticle laden droplets. [ABSTRACT FROM AUTHOR]

To model the interfacial transfer terms more accurately in two-fluid model as well as to further comprehend the intrinsic mechanism for downward two-phase flows, more information that involves local interfacial parameters is required. The local interfacial characteristics in upward and downward bubbly flows were investigated separately in a 50.8 mm inner-diameter round pipe. A four-sensor optical probe was used in the measurement of local interfacial parameters, including void fraction, interfacial area concentration (IAC), bubble frequency, interfacial velocity and Sauter mean diameter. The radial profiles of these parameters in downward flows were presented and compared with that in upward flows. In general, the void fraction shows a core-peaked distribution for the downward flow at low void fraction, but shows a wall-peaked distribution for the upward flow; while at high void fraction, it has an off-center-peaked distribution for the downward flow but a core-peaked distribution for the upward flow. Also, under relative high void fraction conditions, the profile of Sauter mean diameter presents a pronounced maximum in the channel center for the upward flow whereas near the channel wall for the downward flow. Additionally, with an increase in the gas flow rate, the flat profiles of interfacial velocity transform to power-law shapes in the upward flows but to that with the velocity increasing along the radius for the downward flows. [ABSTRACT FROM AUTHOR]

In the volume-of-fluid algorithm, the calculations of the interface normal and curvature are crucially important for accurately simulating interfacial flows. However, few methods have been proposed for the high-precision interface calculation on an unstructured grid. In this paper, the authors develop a height function method that works appropriately on an unstructured grid. In the process, the definition of the height function is discussed, and the high-precision calculation method of the interface normal is developed to meet the necessary condition for a second-order method. This new method has highly reduced computational cost compared with a conventional high-precision method because the interface normal calculation is completed by solving relatively simple algebraic equations. The curvature calculation method is also discussed and the approximated quadric curve of an interface is employed to calculate the curvature. Following a basic verification, the developed height function method is shown to successfully provide superior calculation accuracy and highly reduced computational cost compared with conventional calculation methods in terms of the interface normal and curvature. In addition, the height function method succeeds in calculating accurately the slotted-disk revolution problem and the oscillating drop on unstructured grids. Therefore, the developed height function method is confirmed to be an efficient technique for the high-precision numerical simulation of interfacial flows on an unstructured grid. [ABSTRACT FROM AUTHOR]

We study the shape and dynamics of cavities created by the explosion of firecrackers at the surface of a large pool of water. Without confinement, the explosion generates a hemispherical air cavity which grows, reaches a maximum size and collapses in a generic w-shape to form a final central jet. When a rigid open tube confines the firecracker, the explosion produces a cylindrical cavity that expands without ever escaping the free end of the tube. We discuss a potential flow model, which captures most of these features. [ABSTRACT FROM PUBLISHER]

It is usually believed that wall slip contributes small effects to macroscopic flow characteristics. Here we demonstrate that this is not the case for the thermocapillary migration of a long bubble in a slippery tube. We show that a fraction of the wall slip, with the slip length $\lambda $ much smaller than the tube radius $R$, can make the bubble migrate much faster than without wall slip. This speedup effect occurs in the strong-slip regime where the film thickness $b$ is smaller than $\lambda $ when the Marangoni number $S= \tau _{T} R/\sigma _{0}~ (\ll 1)$ is below the critical value $S^* \sim (\lambda /R)^{1/2}$, where $\tau _{T}$ is the driving thermal stress and $\sigma _{0}$ is the surface tension. The resulting bubble migration speed is found to be $U_{b} \sim (\sigma _{0}/\mu )S^{3}(\lambda /R)$, which can be more than a hundred times faster than the no-slip result $U_{b} \sim (\sigma _{0}/\mu )S^{5}$ (Wilson, J. Eng. Math., vol. 29, 1995, pp. 205–217; Mazouchi & Homsy, Phys. Fluids, vol. 12, 2000, pp. 542–549), with $\mu $ being the fluid viscosity. The change from the fifth power law to the cubic one also indicates a transition from the no-slip state to the strong-slip state, albeit the film thickness always scales as $b\sim RS^{2}$. The formal lubrication analysis and numerical results confirm the above findings. Our results in different slip regimes are shown to be equivalent to those for the Bretherton problem (Liao, Li & Wei, Phys. Rev. Lett., vol. 111, 2013, 136001). Extension to polygonal tubes and connection to experiments are also made. It is found that the slight discrepancy between experiment (Lajeunesse & Homsy, Phys. Fluids, vol. 15, 2003, pp. 308–314) and theory (Mazouchi & Homsy, Phys. Fluids, vol. 13, 2001, pp. 1594–1600) can be interpreted by including wall slip effects. [ABSTRACT FROM PUBLISHER]

Effects of water vapor annealing on SiO2/4H-SiC structures formed following different routes were investigated using water isotopically enriched in 18O and ²H (D). Isotopic exchange between oxygen from the water vapor and oxygen from SiO2 films deposited on 4H-SiC was observed in the whole depth of the films, differently from the behavior of SiO2 films thermally grown on 4H-SiC. The highest amount of D was obtained in the sample with the highest negative fixed charge concentration, suggesting that the D incorporation occurs in defects in the structure that exist prior to the annealing. As a consequence of the water annealing, a significant reduction in the negative effective charge in metal-oxide-semiconductor capacitors and the removal of the SiO2/SiC interfacial region was observed, attributed to the reduction of the amount of SiOxCy compounds in the interfacial region. [ABSTRACT FROM AUTHOR]

Compared to the significant body of work devoted to surface engineering for promoting dropwise condensation heat transfer of steam, much less attention has been dedicated to fluids with lower interfacial tension. A vast array of low-surface tension fluids such as hydrocarbons, cryogens, and fluorinated refrigerants are used in a number of industrial applications, and the development of passive means for increasing their condensation heat transfer coefficients has potential for significant efficiency enhancements. Here we investigate condensation behavior of a variety of liquids with surface tensions in the range of 12 to 28 mN/m on three types of omniphobic surfaces: smooth oleophobic, re-entrant superomniphobic, and lubricant-impregnated surfaces. We demonstrate that although smooth oleophobic and lubricant-impregnated surfaces can promote dropwise condensation of the majority of these fluids, re-entrant omniphobic surfaces became flooded and reverted to filmwise condensation. We also demonstrate that on the lubricant-impregnated surfaces, the choice of lubricant and underlying surface texture play a crucial role in stabilizing the lubricant and reducing pinning of the condensate. With properly engineered surfaces to promote dropwise condensation of low-surface tension fluids, we demonstrate a four to eight-fold improvement in the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Against the common wisdom that wall slip plays only a minor role in global flow characteristics, here we demonstrate theoretically for the displacement of a long bubble in a slippery channel that the well-known Bretherton $2/3$ law can break down due to a fraction of wall slip with the slip length $\lambda $ much smaller than the channel depth R. This breakdown occurs when the film thickness $h_{\infty } $ is smaller than $\lambda $, corresponding to the capillary number $Ca$ below the critical value $Ca^{\ast } \sim (\lambda /R)^{3 / 2}$. In this strong slip regime, a new quadratic law $h_{\infty } /R \sim Ca^{2} (R/\lambda )^{2}$ is derived for a film much thinner than that predicted by the Bretherton law. Moreover, both the $2/3$ and the quadratic laws can be unified into the effective $2/3$ law, with the viscosity $\mu $ replaced by an apparent viscosity $\mu _{app}= \mu h_{\infty } /({\lambda } + h_{\infty })$. A similar extension can also be made for coating over textured surfaces where apparent slip lengths are large. Further insights can be gained by making a connection with drop spreading. We find that the new quadratic law can lead to $\theta _{d} \propto Ca^{1 / 2} $ for the apparent dynamic contact angle of a spreading droplet, subsequently making the spreading radius grow with time as $r \propto t^{1 / 8}$. In addition, the precursor film is found to possess $\ell _{f} \propto Ca^{ - 1 / 2}$ in length and therefore spreads as $\ell _{f} \propto t^{1 / 3}$ in an anomalous diffusion manner. All these features are accompanied by no-slip-to-slip transitions sensitive to the amount of slip, markedly different from those on no-slip surfaces. Our findings not only provide plausible accounts for some apparent departures from no-slip predictions seen in experiments, but also offer feasible alternatives for assessing wall slip effects experimentally. [ABSTRACT FROM PUBLISHER]

Abstract: Many physical problems arising in biological or material sciences involve solving partial differential equations in deformable interfaces or complex domains. For instance, the surfactant (an amphiphilic molecular) which usually favors the presence in the fluid interface may couple with the surfactant soluble in one of bulk domains through adsorption and desorption processes. Thus, it is important to accurately solve coupled surface-bulk convection–diffusion equations especially when the interface is moving. In this paper, we first rewrite the original bulk concentration equation in an irregular domain (soluble region) into a regular computational domain via the usage of the indicator function so that the concentration flux across the interface due to adsorption and desorption processes can be termed as a singular source in the modified equation. Based on the immersed boundary formulation, we then develop a new conservative scheme for solving this coupled surface-bulk concentration equations which the total surfactant mass is conserved in discrete sense. A series of numerical tests has been conducted to validate the present scheme. As an application, we extend our previous work [M.-C. Lai, Y.-H. Tseng, H. Huang, An immersed boundary method for interfacial flows with insoluble surfactant, J. Comput. Phys. 227 (2008) 7279–7293] to the soluble case. The effects of solubility of surfactant on drop deformations in a quiescent and shear flow are investigated in detail. [Copyright &y& Elsevier]