Merging of different streams in channel junctions represents a common mixing process that occurs in systems ranging from soda fountains and bathtub faucets to chemical plants and microfluidic devices. Here, we report a spontaneous trapping of colloidal particles in a merging flow junction when the merging streams have a salinity contrast. We show that the particle trapping is a consequence of non-equilibrium interactions between the particles, solutes, channel, and the freestream flow. A delicate balance of transport processes results in a stable near-wall vortex that traps the particles. We use 3-D particle visualization and numerical simulations to provide a rigorous understanding of the observed phenomenon. Such trapping mechanism is unique from the well-known inertial trapping enabled by vortex breakdown [Proc. Natl. Acad. Sci. U.S.A. 111, 4770 (2014)], or the solute-mediated trapping enabled by diffusiophoresis [Phys. Rev. X 7, 041038 (2017)], as the current trapping is facilitated by both the solute and the inertial effects, suggesting a new mechanism for particle trapping in flow networks.

A new experimental and methodological approach dedicated to the analysis of the development of a turbulent mixing zone produced by the Richtmyer-Meshkov instability is proposed. It relies on an innovative experimental device for the generation of well-defined initial gaseous interfaces in a multi-parametric and controlled way. This device ensures high repeatability and allows to shape the initial condition following various patterns, from monotonous to non monotonous ones. An overview of the main results obtained by means of strioscopic, Particle Image Velocimetry and tomoscopic measurements are provided. They unravel the fast transition of the mixing zone to a fully turbulent state, for a given set of control parameters of the device, as the imprint of the initial condition is lost and the dynamical spectral content covers a wide range of scales, compatible with the achievement of a self-similar trend.

The dynamics of an oscillating shear layer when confined is enriched by retarded actions whose physical modelling is not trivial. We present a non-linear delayed saturation feedback model, which allows to correctly reproduce the complex shear layer spectra observed experimentally in open cavity flows in the incompressible limit. The model describes the evolution of the amplitude of the shear layer instabilities and considers two hydrodynamic feedback mechanisms directly related to the confinement introduced by the walls. One is associated with reflections of instability waves on the vertical cavity walls and the other to intracavity recirculation flow. These feedback mechanisms provide retarded actions with time lags that are used in the delay differential equation and allow the computation of the model parameters on physical grounds. The frequency components of six experimental cases in different flow regimes are well recovered by the dynamical model. The results show that the model with a single feedback mechanism produces monoperiodic oscillations of the amplitude, while the interplay of two, purely hydrodynamic feedback mechanisms, allow for quasi-periodicity to develop.

The anisotropy and structure of turbulence simulated by large eddy simulations with and without Stokes-drift forcing are analyzed, with an emphasis on the linkage between the distinctive structure of Langmuir turbulence near the surface where cellular vortices aligned with the wind and wave propagation direction are apparent and the Langmuir-enhanced mixed layer entrainment at the base of the ocean surface boundary layer (OSBL) where turbulent structures differ. The tensor invariants of the Reynolds stresses, the variance of vertical velocity and buoyancy, and the velocity gradient statistics are used to categorize turbulence structures as a function of depth, including an extension of the barycentric map to show the direction as well as the magnitude of turbulence anisotropy and a vector-invariant extension of the Okubo-Weiss parameter. The extended anisotropic barycentric map and the velocity gradient statistics are demonstrated to be useful, providing compact information of the anisotropy, orientation, and structure of turbulent flows. It is found that the distinctive anisotropy and structures of Langmuir turbulence are quickly lost below regions where Stokes drift shear is significant and vortices are apparent, consistent with past observations and model results. As a result, the turbulent structures near the base of the OSBL are not significantly affected by the presence of Stokes drift above but are instead dominated by local Eulerian shear, except in one important manner. Langmuir turbulence does affect the mixed layer entrainment by providing extra available turbulent kinetic energy (TKE) via enhanced near-surface TKE production and higher vertical TKE transport energizing the turbulent structures near the base of the OSBL. The additional TKE is utilized by structures similar to those that exist without Stokes drift forcing in terms of anisotropy of their Reynolds stresses, but they are more energetic because of the Langmuir turbulence. Thus, parameterizing the major aspects of Langmuir turbulence on entrainment at the base of the OSBL can be incorporated through enhancing available energy without other modifications.

We put forward the idea of defining vortex boundaries in planar flows as closed material barriers to the diffusive transport of vorticity. Such diffusive vortex boundaries minimize the leakage of vorticity from the fluid mass they enclose when compared to other nearby material curves. Building on recent results on passive diffusion barriers, we develop an algorithm for the automated identification of such structures from general, two-dimensional unsteady flow data. As examples, we identify vortex boundaries as vorticity diffusion barriers in two flows: an explicitly known laminar flow and a numerically generated turbulent Navier–Stokes flow.

We study the development of interfacial magnetoelastic patterns when an initially circular droplet of a field-activated fluid (ferrofluid, or a magnetorheological fluid), surrounded by a nonmagnetic fluid, is subjected to a radial magnetic field in a Hele-Shaw cell. Elasticity takes effect when the fluids are brought into contact, and due to a chemical reaction, the interface separating them becomes a gel-like elastic layer. By modeling the interface as an elastic membrane having a curvature-dependent bending rigidity, a perturbative mode-coupling theory is employed to investigate the weakly nonlinear dynamics of the system. In this context, we examine how the interface responds to the influence of magnetic, elastic, and yield stress forces. Our findings support the relevance of a curvature weakening effect, in the sense that magnetoelastic fingering structures tend to arise and protrude in regions of lower bending rigidity. We contrast the magnetoelastic patterns with the corresponding usual shapes of magnetic fluid interfaces without bending rigidity, but with surface tension.

When a dipolar spheroid is subjected to a shear flow in the presence of an external field, there is a torque due to the dipole-field interaction which tends to align the spheroid in the direction of the field, and a torque due to the shear flow which tends to rotate the particle in closed `Jeffrey orbits’. When the external field is in the flow plane, depending on the strength and orientation of the field, the phase portrait in orientation space could have 2-6 stationary nodes and/or a limit cycle. When the external field strength is low, there are two stationary points off the flow plane which are both stable/unstable, and an unstable/stable limit cycle on the flow plane. When the external field strength is high, there is one stable node where the particle orientation is parallel to the field, and one unstable node where the particle orientation is anti-parallel to the field. As the external field strength is increased, the manner in which the phase portrait evolves depends on the external field orientation with respect to the flow direction and the particle shape factor. It is shown that complex phase portraits result from the relatively simple trajectories of the stationary nodes in the three-dimensional space spanned by the two orientation angles and the dimensionless parameter Σ, which is the ratio of the torques due to the shear flow and the external field. Depending on the aspect ratio of the particle, there are up to two saddle-node bifurcations, two sub-critical bifurcations on the flow plane and one reverse saddle-node merger of two stationary points off the flow plane. The limits of an ideal thin rod/disk are shown to be singular limits, where there are no stationary points off the flow plane even for low external field, and the orientation of the particle changes discontinuously as the cross-stream component of the external field changes sign.

Recent studies of rotating Rayleigh–B'enard convection at high rotation rates and strong thermal forcing have shown a significant discrepancy in total heat transport between experiments on a confined cylindrical domain on the one hand and simulations on a laterally unconfined periodic domain on the other. This paper addresses this discrepancy using direct numerical simulations on a cylindrical domain. An analysis of the flow field reveals a region of enhanced convection near the wall, the sidewall circulation. The sidewall circulation rotates slowly within the cylinder in anticyclonic direction. It has a convoluted structure, illustrated by mean flow fields in horizontal cross-sections of the flow where instantaneous snapshots are compensated for the orientation of the sidewall circulation before averaging. Through separate analysis of the sidewall region and the inner bulk flow, we find that for higher values of the thermal forcing the heat transport in the inner part of the cylindrical domain, outside the sidewall circulation region, coincides with the heat transport on the unconfined periodic domain. Thus the sidewall circulation accounts for the differences in heat transfer between the two considered domains, while in the bulk the turbulent heat flux is the same as that of a laterally unbounded periodic domain. Therefore, experiments, with their inherent confinement, can still provide turbulence akin to the unbounded domains of simulations, and at more extreme values of the governing parameters for thermal forcing and rotation. We also provide experimental evidence for the existence of the sidewall circulation that is in close agreement with the simulation results.

We consider the drainage of blood plasma across the capillary wall, focusing on the flow through the endothelial glycocalyx layer that coats the luminal surface of vascular endothelial cells. We investigate how the presence of a sub-glycocalyx space between the porous glycocalyx and the impermeable endothelial cells affects the flow, using the Darcy and Stokes equations to model the flow in the glycocalyx and sub-glycocalyx space, respectively. Using an asymptotic analysis, we exploit the disparity of lengthscales to reduce the problem complexity to reveal the existence of several asymptotic regions in space. We provide a detailed characterisation of the flow through the glycocalyx layer in terms of the microscale system parameters, and we derive analytic macroscale results, such as for the flux through and hydraulic conductivity of the glycocalyx layer. We show that the presence of a sub-glycocalyx space results in a higher flux of blood plasma through the glycocalyx layer, and we use our theoretical predictions to suggest experiments that could be carried out to shed light on the extent of the layer.

This work reports a numerical study of mixing in stratified fluids induced by the motion of monodispersed swarm of bubbles in a thin gap. Simulations are run for void fractions between 3.35 strength of stratification is varied by changing the Froude number between 4.5 to 12.74. The confinement prevents turbulent production and mixing occurs primarily due to transport by the bubble wake. We observe a zigzag motion of the bubbles attributed to the periodic vortex shedding behind the bubbles. We report the formation of horizontal clusters and establish a direct correlation between the size of clusters and the rise velocity of the bubbles. We report an increase in the buoyancy flux across the isopycnals as the void fraction increases. The fraction of energy production due to the buoyancy flux increases with the strength of stratification, giving rise to a higher mixing efficiency.

In many simulations of turbulent flows, the viscous forces $\nu\nabla^2 \bu$ are replaced by a hyper-viscous term $-\nu_p(-\nabla^2)^{p}\bu$ in order to suppress the effect of viscosity at the large scales. In this work we examine the effect of hyper-viscosity on decaying turbulence for values of p ranging from p=1 (ordinary viscosity) up to p=100. Our study is based on direct numerical simulations of the Taylor-Green vortex for resolutions from 5123 to 20483. Our results demonstrate that the evolution of the total energy E and the energy dissipation ϵ remain almost unaffected by the order of the hyper-viscosity used. However, as the order of the hyper-viscosity is increased, the energy spectrum develops a more pronounced bottleneck that contaminates the inertial range. At the largest values of p examined, the spectrum at the bottleneck range has a positive power-law behavior E(k)∝kα with the power-law exponent α approaching the value obtained in flows at thermal equilibrium α=2. This agrees with the prediction of Frisch et al. [Phys. Rev. Lett. 101, 144501 (2008)] who suggested that at high values of p, the flow should behave like the truncated Euler equations (TEE). Nonetheless, despite the thermalization of the spectrum, the flow retains a finite dissipation rate up to the examined order, which disagrees with the predictions of the TEE system implying suppression of energy dissipation. We reconcile the two apparently contradictory results, predicting the value of p for which the hyper-viscous Navier-Stokes goes over to the TEE system and we discuss why thermalization appears at smaller values of p.

The Betz limit expresses the maximum proportion of the kinetic energy flux incident on an energy conversion device that can be extracted from an unbounded flow. The derivation of the Betz limit requires an assumption of steady flow through a notional actuator disk that is stationary in the streamwise direction. The present derivation relaxes the assumptions of steady flow and streamwise actuator disk stationarity, which expands the physically realizable parameter space of flow conditions upstream and downstream of the actuator disk. A key consequence of this generalization is the existence of unsteady motions that can, in principle, lead to energy conversion efficiencies that exceed the Betz limit not only transiently, but also in time-averaged performance. Potential physical implementations of those unsteady motions are speculated.

Depending on the reflectional and rotational symmetries of annular combustors for aeroengines and gas turbines, self-sustained azimuthal thermoacoustic eigenmodes can be standing, spinning or mix of these two types of waves. These thermoacoustic limit cycles are unwanted because the resulting intense acoustic fields induce high-cycle fatigue of the combustor components. This paper presents a new theoretical framework for phenomenologically describing the dynamics of the slow-flow variables, which define the state of an eigenmode, i.e. if the latter is standing, spinning or mixed. The acoustic pressure is expressed as a hypercomplex field and this ansatz is inserted into a one dimensional wave equation that describes the thermoacoustics of a thin annulus. Slow-flow averaging of this wave equation is performed by adapting the classic Krylov-Bogoliubov method to the quaternion field in order to derive a system of coupled first order differential equations for the four slow-flow variables, i.e. the amplitude, the nature angle, the preferential direction and the temporal phase of the azimuthal thermoacoustic mode. The state of the mode can be conveniently depicted by using the first three slow-flow variables as spherical coordinates for a Bloch sphere representation. Stochastic forcing from the turbulence in annular combustors is also accounted for. This new analytical model describes both rotational and reflectional symmetry breaking bifurcations induced by the non-uniform distribution of thermoacoustic sources along the annulus circumference and by the presence of a mean swirl.

We use numerical simulations to study the dynamics of three dimensional vesicles in unconfined and confined Poiseuille flow. Previous numerical studies have shown that when the fluid viscosity inside and outside the vesicle is same (no viscosity contrast), a transition from asymmetric slippers to symmetric parachutes takes place as viscous forcing or capillary number is increased. At higher viscosity contrast, an outward migration tendency has also been observed in unconfined flow simulations. In this paper, we study how the presence of viscosity contrast and confining walls affect the dynamics of vesicles and present phase diagrams for confined Poiseuille flow with and without viscosity contrast. To our knowledge, this is the first study that provides a phase diagram for 3D vesicles with viscosity contrast in confined Poiseuille flow. The confining walls push the vesicle towards the center while the viscosity contrast has the opposite effect. This interplay leads to important differences in the dynamics like bistability at high capillary numbers.

Using mixtures of soap, water, and long chain polymers, free-floating soap bubbles can be formed with volumes approaching 100 m3. Here we investigate how such thin films are created and maintained over time. We show how the extensional rheology is the most important factor in creating the bubble, and how polydispersity in molecular weight of the solvated polymers leads to better performance at lower concentrations. Additionally, using IR absorption, we measure soap film thickness profiles and film lifetimes. Although the initial thickness mostly depends on the choice of detergent, polymers can dramatically increase film lifetime at high molecular weights and high concentrations, although such high concentrations can inhibit the initial film formation. Thus, the ideal concentration of polymer additives for making giant bubbles requires a robust viscoelastic rheology during extension, and is aided by long film lifetimes during gravitational drainage and evaporation.

We study the mixing of a passive scalar in homogeneous turbulent flow with and without anisotropic external force. It is assumed that no scalar source exists and the scalar spectrum at an initial time instant t0 is given by the form Ck2+o(k2) at the wavenumber k→0, where C is independent of k. We have performed direct numerical simulations (DNSs) of the mixing, in which the initial integral length scales of the scalar field are comparable to those of the velocity field and the Schmidt number is unity. The DNSs show that even though the large-scale anisotropy of the velocity field grows with time owing to the external force, its scalar field counterpart remains almost unchanged, i.e., frozen with respect to time, in a state where the scalar field evolves in a self-similar manner. The degree of the anisotropy of each of the velocity and scalar fields is measured by the ratios of the integral length scales in different Cartesian directions. The DNSs also suggest that the scalar spectrum keeps the form Ck2+o(k2) at small k for time t(≥t0), and that C is time-independent.

We study the dynamics of fragmentation for Newtonian and viscoelastic liquids in rotary atomization. In this common industrial process centripetal acceleration destabilizes the liquid torus that forms at the rim of a spinning cup or disk due to the Rayleigh-Taylor instability. The resulting ligaments leave the liquid torus with a remarkably repeatable spacing that scales inversely with the rotation rate. The fluid filaments then follow a well-defined geometrical path-line that is described by the involute of a circle. Knowing the geometry of this phenomenon we derive the detailed kinematics of this process and compare it with the experimental observations. We show that the ligaments elongate tangentially along the involute of the circle and thin radially as they separate from the cup. We use these kinematic conditions to develop an expression for the spatial variation of the filament deformation rate and show that it decays away from the spinning cup. Once the ligaments are sufficiently far from the cup, they are not stretched sufficiently fast to overcome the critical rate of capillary thinning and consequently undergo capillary-driven break up forming droplets. We couple these kinematic considerations with the known properties of several Newtonian and viscoelastic test liquids to develop a quantitative understanding of this commercially-important fragmentation process that can be compared in detail with experimental observations. We also investigate the resulting droplet size distributions and observe that the appearance of satellite droplets during the pinch-off process lead to the emergence of bidisperse droplet size distributions. These binary distributions are well described by the superposition of two separate Gamma distributions that capture the physics of the disintegration process for the main and satellite droplets, respectively. Furthermore, as we consider more viscous Newtonian liquids or weakly viscoelastic test fluids, we show that changes in the liquid viscosity or viscoelasticity have a negligible effect on the average droplet size. However, incorporation of viscous/viscoelastic effects delays the thinning dynamics in the ligaments and thus results in broader droplet size distributions. The ratio of the primary to satellite droplet size increases monotonically with both viscosity and viscoelasticity. We develop a simple physical model that rationalizes the observed experimental trends and provides us a better understanding of the principal dynamical features of rotary fragmentation for both Newtonian and weakly viscoelastic liquids.

In the context of the analysis of the chaotic properties of homogeneous and isotropic turbulence, direct numerical simulations are used to study the fluctuations of the finite time Lyapunov exponent (FTLE) and its relation to Reynolds number, lattice size and the choice of the steptime used to compute the Lyapunov exponents. The results show that using the FTLE method produces Lyapunov exponents that are remarkably stable under the variation of the steptime and lattice size. Furthermore, it reaches such stability faster than other characteristic quantities such as energy and dissipation rate. These results remain even if the steptime is made arbitrarily small. A discrepancy is also resolved between previous measurements of the dependence on the Reynolds number of the Lyapunov exponent. The signal produced by different variables in the steady state is analyzed and the self decorrelation time is used to determine the run time needed in the simulations to obtain proper statistics for each variable. Finally, a brief analysis on MHD flows is also presented as an extension to recent work, which shows that the Lyapunov exponent is still a robust measure in the simulations, although the Lyapunov exponent scaling with Reynolds number is significantly different from that of magnetically neutral hydrodynamic fluids.

This study investigates the Reynolds-number dependence of shock-induced flow through particle clouds at 10% volume fraction, using ensemble-averaged results from particle-resolved large eddy simulations. The advantage of using large eddy simulations to study this problem is that they capture the strong velocity shears and flow separation caused by the no-slip condition at the particle surfaces. The shock particle cloud interaction produces a reflected shock wave, whose strength increases with decreasing particle Reynolds number. This results in important changes to the flow field that enters the particle cloud. The results show an approximate proportionality between the mean flow velocity and the flow fluctuation magnitudes. Maximum particle drag forces are in excellent agreement with previous inviscid studies, and we complement these results with statistics of time-averaged particle forces as well as the variation of temporal oscillations. The results of this work provides a basis for development of improved simplified dispersed flow models.

We investigate the impact of buoyancy on the solute mass transport in an evaporating liquid mixture (non-volatile solute + solvent) confined in a slit perpendicular to the gravity. Solvent evaporation at one end of the slit induces a solute concentration gradient which in turn drives free convection due to the difference between the densities of the solutes and the solvent. From the complete model coupling mass transport and hydrodynamics, we first use a standard Taylor-like approach to derive a one dimensional non-linear advection-dispersion equation describing the solute concentration process for a dilute mixture. We then perform a complete analysis of the expected regimes using both scaling analysis and asymptotic solutions of this equation. The validity of this approach is confirmed using a thorough comparison with the numerical resolution of both the complete model and the 1D advection-dispersion equation. Our results show that buoyancy-driven free convection always impacts solute mass transport at long time scales, dispersing solutes in a steadily increasing length scale along the slit. Beyond this confined drying configuration, our work also provides an easy way for evaluating the relevance of buoyancy on mass transport in any other microfluidic configuration involving concentration gradients.

In order to provide new insights into the energetics of turbulent boundary layer flows with separation, we compute and characterize energy transport lines, which are defined as the lines tangent to the total mechanical energy transport vector field. Separation is induced in a Reθ=490 turbulent boundary layer by imposing two types of transpiration velocity profiles at the top boundary of the computational domain: one with suction and blowing, and the other with suction only. For both separation bubbles, we find that energy transport lines can exhibit attractors, which are a manifestation of local dissipation in the flow. We identify several attracting sets along the bottom wall and attracting spiral nodes inside the separation bubbles and their corresponding basins of attraction. The size and positions of the energy basins of attraction help us understand where the available total mechanical energy is transported to due to the mean flow, and turbulent Reynolds and viscous stresses, before it is fully dissipated. The suction-and-blowing separation bubble leads to slightly smaller losses of total mechanical energy than those for a reference non-separated boundary layer, while the suction-only case shows larger losses than the non-separated boundary layer case.

The directional motion of sessile drops can be induced by slanted mechanical vibrations of the substrate. As previously evidenced , the mechanical vibrations induce drop deformations which combine axisymmetric and antisymmetric modes. In this paper, we establish quantitative trends from experiments conducted within a large range of parameters, namely the amplitude A and frequency f of the forcing, the liquid viscosity η and the angle between the substrate and the forcing axis α. These experiments are carried out on weak-pinning substrates. For most parameters sets, the averaged velocity $$ grows linearly with $A$. We extract the mobility, defined as $s=\frac{\Delta }{\Delta A}$. It is found that $s$ can show a sharp maximal value close to the resonance frequency of the first axisymmetric mode $f_p$. The value of $s$ tends to be almost independent on $\eta$ below 50 cSt, while $s$ decreases significantly for higher $\eta$. Also, it is found that for peculiar sets of parameters, particularly with $f$ far enough from $f_p$, the drop moves in the reverse direction. Finally, we draw a relationship between $$ and the averaged values of the dynamical contact angles at both sides of the drop over one period of oscillation.

Quantum mechanics places significant restrictions on the hydrodynamics of superfluid flows. Despite this it has been observed that turbulence in superfluids can, in a statistical sense, share many of the properties of its classical brethren; coherent bundles of superfluid vortices are often invoked as an important feature leading to this quasi-classical behaviour. A recent experimental study~ inferred the presence of these bundles through intermittency in the pressure field, however direct visualization of the quantized vortices to corroborate this finding was not possible. In this work, we performed detailed numerical simulations of superfluid turbulence at the level of individual quantized vortices through the vortex filament model. Through course-graining of the turbulent fields, we find compelling evidence supporting the conclusions of Ref.~ at low temperature. Moreover, elementary simulations of an isolated bundle show that the number of vortices inside a bundle can be directly inferred from the magnitude of the pressure dip, with good theoretical agreement derived from the HVBK equations. Full simulations of superfluid turbulence show strong spatial correlations between course-grained vorticity and low pressure regions, with intermittent vortex bundles appearing as deviations from the underlying Maxwellian (vorticity) and Gaussian (pressure) distributions. Finally, simulations of a decaying random tangle in an ultra-quantum regime show a unique fingerprint in the evolution of the pressure distribution, which we argue can be fully understood using the HVBK framework.

Lubricant-infused surfaces (LIS) make drops deposited on them remarkably mobile. However, the dynamics of those drops proved to be subtle, due to the numerous phases at stake (lubricant, drop, air, textures). In this Letter, we highlight the unique role played by a feature specific to LIS: drops are surrounded by a foot of oil, drawn by surface tension from the lubricating film. As a consequence, viscous dissipation can be localized in four distinct regions, which we tune independently through various experimental set-ups. Despite this complexity, we evidence a universal scaling for the friction law and thus rationalize previous results recently obtained in the literature.

In Large Eddy Simulations (LES), the large scales of turbulent motion are resolved directly, whereas the small scales that are computationally expensive to resolve are modeled. The scale separation is performed either in the physical space (space or time) or Fourier space, which involves application of an explicit or implicit scale filtering operation, parting the resolved and subfilter-scales (well known as subgrid-scales). Proper Orthogonal Decomposition (POD) of a turbulent flow also leads to the scale separation, where the POD modes represent characteristic scales of motion. The similarity between the physical/Fourier scales and the POD modes in terms of their kinetic energy content and the interchange of energy among the scales/modes is used to model the effect of subfilter-scales of motion for the LES. The subfilter- scales stress tensors, namely Leonard, cross and Reynolds are expressed directly in terms of the POD modes, and are modeled by accounting the energy contribution of the subfilter-scale POD modes. In addition to the mathematical properties of the subfilter-scale stress tensor, the model accurately predicts the near wall asymptotic behavior and phenomenon of the backward transfer of subfilter-scale energy (the backscatter).

Immiscible displacement of fluids with large viscosity mismatch is inherently unstable due to viscous fingering, even in porous media where capillary forces dominate. Adding polymer to the displacing fluid reduces the viscosity mismatch and suppresses the viscous fingering instability thereby increases the fluid displacement leading to extensive use in applications such as oil recovery. Surprisingly, however, an increase in displacement occur even for very large viscosity mismatches. Moreover, significant additional displacement is observed when the polymer solution is followed by additional water flow. Thus, the fundamental physics of this phenomenon remains unclear. To understand this behavior, we use confocal microscopy to visualize the displacement of oil in a 3D micromodel of a porous medium and simultaneously measure the local flow velocities of the displacing fluid . We find that the increased displacement results from a counterintuitive effect: Polymer retention in the medium and the resultant local changes in flow. Typically retention is avoided since it reduces the permeability of the medium; instead, we find that large and heterogeneous local changes in flow lead to sufficiently large enough viscous forces at the interface of the immiscible fluids resulting in increased displacement.

Evaporation along a curved liquid vapor interface, such as that of a wetting meniscus is a classic multi-scale problem of vital significance to many fields of science and engineering. However, a complete description of the local evaporative flux at all length scales, especially without arbitrary tuning of boundary conditions, is lacking. A multi-scale method to model evaporation from steady meniscus is described such that a need for tuning of boundary conditions and additional assumptions are alleviated. A meniscus submodel is used to compute evaporation flux in the bulk meniscus while a transition film submodel is used to account for enhanced evaporation near the contact line. A unique coupling between the meniscus and transition film submodels ensures smooth continuity of both film and mass flux profiles along the meniscus. The local mass flux is then integrated over the interfacial area to investigate the contribution from the different regions on the surface. The model is evaluated with data from cryo-neutron phase change tests conducted previously at NIST . It is found that the peak mass flux in the transition region is 2 orders of magnitude greater than the flux at the apex. Despite the enhanced evaporation in the thin film, it was found that 78-95% of the evaporation occurs in the bulk meniscus due to the large area. The bulk meniscus contribution increases with increase in vapor pressure and Bond number but decreases with an increase in thermal conductivity of the substrate. Using a non-uniform temperature boundary suggests that there is a possibility that the adsorbed film may have a non-zero mass flux.

We consider a drop of constant density and uniform interfacial tension rising in a linearly density stratified fluid. In the limits of weak inertia and stratification effects, we calculate the drag acting on the drop, the flow fields inside and outside the drop, the drop deformation, and the drift volume associated with the drop using the method of matched asymptotic expansions. Stratification or inertia increase the drag and this enhanced drag acting on a drop is equal to (3λ+23(λ+1))2 times the enhanced drag acting on a rigid sphere, where λ is the viscosity ratio. This relation between the enhanced drags of a drop and a rigid sphere holds even in the presence of both these effects (stratification and inertia). On the other hand, stratification does not result in any deformation of the drop up to the first order of approximation. At zero inertia and small advective transport rate of density, the drift volume associated with the drop rising in a stratified fluid is finite (but large compared to the drop’s volume) unlike the drift volume in a homogeneous fluid which is infinite.

Motivated by the observation of sediments being carried by meltwater plumes originating at the base of a marine terminating glacier, laboratory experiments are performed to examine the transport and deposition of particles settling out from a buoyant line-plume rising along a sloping upper boundary. If the plume source has relatively high momentum and is located near the bottom of the domain, a strong recirculating region develops near the source. Emanating from this region is a particle bearing buoyant plume that moves at near constant speed along the slope. Particles are observed to settle within the plume itself and then descend from the plume toward the tank bottom being drawn back in the direction of the source through a return flow driven by the plume’s entrainment of the underlying ambient fluid. A light attenuation technique is employed to measure non-intrusively the depth of the sediment bed after the source is turned off and all the particles settled out. Sediments are found to accumulate near the source over the extent of the recirculating region and then decrease approximately linearly with distance from the source. Conceptual theoretical models suggest that the linearly sloping bed results from a combination of vertical mixing in the plume near the recirculation region and the return flow acting to detrain from the plume particles more effectively near the plume source where the shear between the plume and the return flow is largest. In many aspects the experiments are not representative of a steady glacial meltwater plume due to restrictions of the experimental setup, notably the relatively low Reynolds number of the flow. Nonetheless the experiments are suggestive of the complicated dynamics and sediment deposition patterns that may occur near the base of a marine terminating glacier, with a crude estimate of mean clay deposition at a rate of 6,cm per year over a distance of 2,km from the source.

The hydrodynamic viscous fingering instability can be influenced by a simple viscosity changing chemical reaction of type A+B→C, when a solution of reactant A is injected into a solution of B and a product C of different viscosity is formed by reaction. We investigate here numerically such reactive viscous fingering in the case of a reaction decreasing the viscosity to define the optimal conditions on the chemical and hydrodynamic parameters for controlling fingering. In particular, we analyze the influence of the P{' e}clet number ${\rm Pe}$ on the efficiency of the chemical control of fingering. We show that the viscosity decreasing reaction has an increased stabilizing effect when ${\rm Pe}$ is decreased. On the contrary, fingering is more intense and the system more unstable when ${\rm Pe}$ is increased. The related reactive fingering patterns cover then respectively a smaller (larger) area than in the non-reactive equivalent. Depending on the value of the P{' e}clet number, a given chemical reaction may thus either enhance or suppress a fingering instability. This stabilization and destabilization at low and high ${\rm Pe}$ are shown to be related to the ${\rm Pe}$-dependent characteristics of a minimum in the viscosity profile that develops around the miscible interface thanks to the effect of the chemical reaction.