The Rayleigh–Taylor instability for inviscid and viscous fluids

The classical Rayleigh–Taylor instability occurs when two inviscid fluids, with a sharp interface separating them, lie in two horizontal layers with the heavier fluid above the lighter one. A small sinusoidal disturbance on the interface grows rapidly in time in this unstable situation, as the heavier upper fluid begins to move downwards through the lighter lower fluid. This paper presents a novel numerical method for computing the growth of the interface. The technique is based on a spectral representation of the solution. The results are accurate right up to the time when a curvature singularity forms at the interface and the inviscid model loses its validity. A spectral method is then presented to study the same instability in a viscous Boussinesq fluid. The results are shown to agree closely with the inviscid calculations for small to moderate times. However, the high interface curvatures that develop in the inviscid model are prevented from occurring in viscous fluid by the growth of regions of high vorticity at precisely these singular points. This leads to over-turning of the interface, to form mushroom-shaped profiles. It is shown that different initial interface configurations can lead to very different geometrical outcomes, as a result of the flow instability. These can include situations when detached bubbles form in the fluid.     

Development of the Rayleigh-Taylor instability at the boundary of a friable medium layer accelerated by a compressed air flow

The results of experiments with a friable medium layer driven by a compressed gas flow are reported. A layer of polypropylene particles was accelerated in a square channel under the action of air pressure. The initial perturbation on the unstable layer boundary had a nearly sinusoidal shape. Since the medium possesses no tensile strength (and exhibits no surface tension), the unbound particles occurring in the protruding front regions (spikes) of the unstable layer boundary fall into the gas. In the depressed front regions (bubbles), the medium probably exhibits a certain densification and the resulting compressive strength hinders the bubble growth in the initial stage of instability development. The gas penetrating into the layer leads to expansion and fluidization of the layer.     

Simulation of single mode Rayleigh–Taylor instability by SPH method

A smoothed particle hydrodynamics (SPH) solution to the Rayleigh–Taylor instability (RTI) problem in an incompressible viscous two-phase immiscible fluid with surface tension is presented. The present model is validated by solving Laplace’s law, and square bubble deformation without surface tension whereby it is shown that the implemented SPH discretization does not produce any artificial surface tension. To further validate the numerical model for the RTI problem, results are quantitatively compared with analytical solutions in a linear regime. It is found that the SPH method slightly overestimates the border of instability. The long time evolution of simulations is presented for investigating changes in the topology of rising bubbles and falling spike in RTI, and the computed Froude numbers are compared with previous works. It is shown that the numerical algorithm used in this work is capable of capturing the interface evolution and growth rate in RTI accurately.     

相界面上超声空化气泡聚并、滑移促进的传质

根据超声空化泡在相间特殊运动时周围流体流动特性,以相间传质渗透理论为依据,结合流体动力学原理,在充分研究相界面上气泡行为的基础上,根据界面上超声空化气泡生长过程的动力学行为,导出了相界面上超声空化气泡聚并和滑移促进的传质模型。在此基础上,建立了相界面上气泡滑移、聚并及脱离界面逸出这三种气泡行为共同促进的传质模型,该模型可用于描述超声空化界面等相界面上的传质行为,为超声波强化传质过程提供了理论依据。     

Synthesis of composite emulsions and complex foams with the use of microfluidic flow-focusing devices

A method is described for the formation of stable, composite aqueous emulsions of 1) combinations of distinct families of bubbles of nitrogen, 2) combinations of distinct families of droplets of an organic fluid (either perfluoro(methyl)decalin or hexadecane), and 3) combinations of bubbles and droplets. A system of two or three microfluidic flow-focusing units is coupled to a single outlet channel. The composite emulsions can be precisely tuned, both in their composition and in the number fraction of components--either bubbles or droplets--of different types. The use of microfluidic technology, with closely coupled flow-focusing units, guarantees that the emulsions are mixed locally at a controlled local stoichiometry. The emulsions self-assemble in a nonequilibrium process to form a wide variety of highly organized periodic lattices.     

Homogeneous Nucleation of 4He Crystals by Acoustic Waves

We have recently found experimental evidence for the homogeneous nucleation of crystals in metastable liquid ⁴He at high pressure. For this we combined the focusing of a high intensity spherical acoustic wave with a simple light scattering technique. We discuss the analysis used to distinguish between nucleation of bubbles in the negative pressure swings of the wave from nucleation of crystals in the positive swings. We also discuss the interest of our results and future developments of our experiment in the general context of the study of nucleation and instability limits in phase diagrams.     

The emission of sound by statistically homogeneous bubble layers

This paper is concerned with the flow of a bubbly fluid along a wavy wall, which is one Fourier component of a linearized hydrofoil. The bubbles are dispersed, not throughout the whole of the liquid, but only over a certain distance from the wall, as occurs in practice with cavitation bubbles. Outside the bubbly regime there is pure liquid.The interface between the bubbly fluid and pure liquid fluctuates for various reasons. One of these is the relative motion between bubbles and liquid. This is considered here in detail. A calculation is made of the sound emitted by the bubbly layer into pure liquid as a result of this stochastic motion of the interface.     

Fluid flow and heat transfer characteristics of separation and reattachment flow over a backward-facing step

In this study, a direct numerical simulation of the fluid flow and heat transfer characteristics of separation and reattachment flow at a backward-facing step is presented. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the Reynolds number and expansion ratio on the fluid flow and heat transfer characteristics are investigated. The size of the primary recirculation zone increases with the reduction of expansion ratio and the fluctuation of isotherms increased with the increase of Reynolds number. The periodic characteristics and the dissimilarity between Nu and Cf appear in the transitional flow regime. The rotating fluids in the reattachment region increase the flow instability and the interchange of the hot and cold fluids increases heat transfer instability. The combined effects of flow instability and heat transfer instability play an important role in the formation of the dissimilarity between Nu and Cf.     

Convective Instabilities in Two Liquid Layers

We perform linear stability calculations for horizontal fluid bilayers, taking into account both buoyancy effects and thermocapillary effects in the presence of a vertical temperature gradient. To help understand the mechanisms driving the instability, we have performed both long-wavelength and short-wavelength analyses. The mechanism for the large wavelength instability is complicated, and the detailed form of the expansion is found to depend on the Crispation and Bond numbers. The system also allows a conventional Rayleigh-Taylor instability if heavier fluid overlies lighter fluid, and the long-wavelength analysis describes this case as well. In addition to the asymptotic analyses for large and small wavelengths, we have performed numerical calculations using materials parameters for a benzene-water system.     

Bubble growth rate and phenomenon of degenerate boiling of a fluid in the form of film vaporization

An analysis of results of an investigation of vaporization of a thin fluid film under vacuum is presented. The vaporization process has basic features characteristic of bubble boiling of a fluid, except for formation of bubbles. This makes it possible to classify the phenomenon as degenerate boiling of a fluid in the form of film evaporation during which local thinning of the layer takes place and funnel- and crater-shaped structures are formed. Funnels and craters are vapor sources with different powers. Dependences of the bubble growth rate above vapor sources of both types that generalize available experimental data are obtained.     

On the Correlation between Properties of Multielectron Dimples and Bubbles

Characteristics of multielectron dimples on the surface of liquid helium and those of multielectron bubbles arising in its bulk when the charged surface develops electrohydrodynamic instability are strongly interrelated. This circumstance allows one to explain a number of observable properties of multielectron bubbles in helium.      

Nonlinear radial oscillations of micro-bubbles in a compressible UCM medium

The nonlinear oscillations of acoustically forced spherical gas bubbles in an upper-convected Maxwell (UCM) compressible fluid are investigated. The nonlinear viscoelastic model used is suitable for large-amplitude excitation of bubbles that cannot be captured by linear models. The effects of acoustic excitation are studied for compressible nonlinear viscoelastic media, which increases the complexity and nonlinearity of the behavior. The Keller–Miksis equation is used to model the dynamics of a single bubble. The constitutive equations of compressible UCM are used for viscoelastic media. These governing equations are non-dimensionalized and coupled to determine the bubble dynamic behavior. The set of derived non-dimensionalized integro-differential equations developed are numerically solved simultaneously using the fourth-order Runge–Kutta method featured by the automatic variable time step-size module. The combined effects of compressibility and viscoelasticity of the fluid on bubble radius are investigated. The results show that the combination of compressibility and nonlinear viscoelasticity for bubble radial oscillations makes forced bubble dynamics more applicable for human needs, especially for large deformations in highly non-Newtonian fluids like industrial polymers or even tissue-like media. It can be seen that compressibility controls the oscillations at higher forcing amplitudes. The relevance and importance of these bubble dynamics to biomedical ultrasound applications and light emissions by sonoluminescence and other industries are evident.     

Sliding and sticking vapour bubbles under inclined plane and curved surfaces

In flow boiling heat is transferred by the combined effects of nucleate boiling, with local generation of bubbles, and evaporative and convective cooling by the passage of bubbles generated elsewhere. In this study, nucleate boiling was eliminated by measuring the heat transfer near injected steam bubbles sliding under an inclined plate heated to low superheats, using liquid crystal thermography combined with high speed video recording and computerised image analysis. Heat was transferred by evaporation of the thin liquid film between the bubble and the wall and by enhanced convection in a wake region wider than the bubble and many bubble diameters long. Evaporation was the dominant mechanism for large, easily deformed, slow-moving bubbles. For small, faster-moving bubbles the reduction in evaporation was offset by an improvement in convection.     

Fission of Multielectron Bubbles in Liquid Helium Under Electric Fields

Multielectron bubbles (MEBs) are cavities in liquid helium which contain a layer of electrons trapped within few nanometres from their inner surfaces. These bubbles are promising candidates to probe a system of interacting electrons in curved geometries, but have been subjected to limited experimental investigation. Here, we report on the observation of fission of MEBs under strong electric fields, which arises due to fast rearrangement of electrons inside the bubbles, leading to their deformation and eventually instability. We measured the electrons to be distributed unequally between the daughter bubbles which could be used to control the charge density inside MEBs.     

Interaction and ordering of bubbles levitated in vortical flow

Interaction of suspended bubbles with the flow and with other bubbles is encounted in numerous ground-based natural and technological processes and plays essential role in space applications. In experiments conducted on board spacecraft, bubbles remain suspended for a long time while at normal gravity they tend to rise due to buoyancy. However, in intensive flow bubbles may be trapped by vortices and maintain their vertical position, exhibiting the behavior similar to microgravity conditions. This type of behavior was observed e.g. bubble motions in the Couette-Taylor flow. Air bubbles injected to the viscous fluid sheared in a Couette device form a highly ordered structure. In a short time scale, the bubbles are getting entrapped in the secondary rotating flow of the Taylor vortices and are pushed to the center streamline of the vortex defying buoyancy. During a longer time scale the inertial interaction between the bubbles in the primary shear flow drives the bubbles away from each other. Equal size bubbles eventually assume an ordered string with equal separation distances between all neighbors. This phenomenon repeats itself regardless of the number of bubbles in the flow. Hence, doublets will assume opposite positions, triplets will form a triangle, four bubbles will arrange in a square and so on. We present visualizations, measurements and a model inviscid 2D analysis     

Bubble Dynamics in Nucleate Pool Boiling on Thin Wires in Microgravity

A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behavior and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. The fluid is R113 at 0.1 MPa and subcooled by 26°C nominally for all cases. Three modes of heat transfer, namely single-phase natural convection, nucleate boiling, and two-mode transition boiling, are observed in the experiment both in microgravity aboard the 22nd Chinese recoverable satellite and in normal gravity on the ground before and after the space flight. Dynamic behaviors of vapor bubbles observed in these experiments are reported and analyzed in the present paper. In the regime of fully developed nucleate boiling, the interface oscillation due to coalescence of adjacent tiny bubbles is the primary reason of the departure of bubbles in microgravity. On the contrary, in the discrete bubble regime, it’s observed that there exist three critical bubble diameters in microgravity, dividing the whole range of the observed bubbles into four regimes. Firstly, tiny bubbles are continually forming and growing on the heating surface before departing slowly from the wire when their sizes exceed some value of the order of 10⁻¹ mm. The bigger bubbles with about several millimeters in diameter stay on the wire, oscillate along the wire, and coalesce with adjacent bubbles. The biggest bubble with diameter of the order of 10 mm, which was formed immediately after the onset of boiling, stays continuously on the wire and swallows continually up adjacent small bubbles until its size exceeds another critical value. The same behavior of tiny bubbles can also be observed in normal gravity, while the others are observed only in microgravity. Considering the Marangoni effect, a mechanistic model about bubble departure is presented to reveal the mechanism underlying this phenomenon. The predictions are qualitatively consistent with the experimental observations.     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

Shape oscillation of bubbles in a row excited by ultrasonic vibration

Behaviors of bubble(s) exposed to an ultrasonic vibration are focused. Size of the bubbles interested in the present study is of O(1 mm) in diameter. The bubbles were injected through the micro syringe to the test fluid (water or water/surfactant mixture) filled in the rectangular tank. Ultrasonic vibration was irradiated to the bubble(s) after the detachment of the bubble from the tip of the syringe; thus the bubbles were exposed to the periodic oscillation in rising the test fluid. The authors clearly detect radial and shape oscillations under the large-amplitude vibration by use of high-speed camera. Preferable mode number of the shape oscillation, and the transition process from the radial to the shape oscillation are discussed.     

Non‐Equilibrium Self‐Assembly of Monocomponent and Multicomponent Tubular Structures in Rotating Fluids

When suspended in a denser rotating fluid, lighter particles experience a cylindrically symmetric confining potential that drives their crystallization into either monocomponent or unprecedented binary tubular packing. These assemblies form around the fluid's axis of rotation, can be dynamically interconverted (upon accelerating or decelerating the fluid), can exhibit preferred chirality, and can be made permanent by solidifying the fluid. The assembly can be extended to fluids forming multiple concentric interfaces or to systems of bubbles forming both ordered and “gradient” structures within curable polymers.