Surface Breakup Research Articles

Azimuthal shear instability of a liquid jet injected into a gaseous cross-flow

AbstractWe investigate azimuthal instabilities which exist on the periphery of a non-turbulent liquid jet injected transversely into a gaseous cross-flow. We predict that the temporal growth of such instabilities may lead to the formation of interface corrugations, which are eventually sheared off of the jet surface (known as the jet ‘surface breakup’). In this study we employ temporal linear stability analyses to understand the nature of these instabilities. The analysis is based on a continuous formulation of momentum equations in which the jet and cross-flow are considered to be slightly miscible at the vicinity of the interface. We identify the shear instability as the primary destabilization mechanism in the flow. This inherently inviscid mechanism opposes the previously suggested mechanism of surface breakup (known as ‘boundary-layer stripping’), which is based on a viscous interpretation. The results show that the wavelengths of instabilities increase by moving away from the jet windward stagnation point toward the leeward point. We also investigate the influence of the jet-to-cross-flow density ratio on the flow stability and find that a higher ratio leads to formation of instabilities with higher wavenumbers on the jet surface. The results show that the density may have a non-monotonic stabilizing/destabilizing effect on the flow.

High-yield growth of carbon nanofilaments on nickel foam using nickel-tin intermetallic catalysts.

The integration of nanomaterials into macroscopic structures is of importance to their practical use. We report the direct synthesis of carbon nanofilaments on Ni foam using Ni-Sn intermetallic nanoparticles. The use of SnO2 nanoparticles was highly effective for the high-yield growth of carbon nanofilaments without the occurrence of surface breakup, resulting from excessive carbon accumulation in the Ni foam. Carbon nanofilaments with a diameter of 50 nm were synthesized and contained Ni3Sn nanoparticles at the tip, indicating a tip-growth mechanism. Higher vacuum conditions led to the growth of highly crystalline carbon nanofilaments. The results obtained using different sources of hydrocarbon revealed that in contrast to C2H2, CH4 or C3H8 did not induce carbon nanofilament formation on Ni foam.

Experimental Investigation of Liquid Jet Breakup in a Cross Flow of a Swirling Air Stream

This paper presents the results of an experimental investigation of liquid jet breakup in a cross flow of air under the influence of swirl (swirl numbers 0 and 0.2) at a fixed air flow Mach number 0.12 (typical gas turbine conditions). The experiments have been conducted for various liquid to air momentum flux ratios (q) in the range of 1 to 25. High speed (@ 500 fps) images of the jet breakup process are captured and those images are processed using matlab to obtain the variation of breakup length and penetration height with momentum flux ratio. Using the high speed images, an attempt has been made to understand the physics of the jet breakup process by identification of breakup modes—bag breakup, column breakup, shear breakup, and surface breakup. The results show unique breakup and penetration behavior which departs from the continuous correlations typically used. Furthermore, the images show a substantial spatial fluctuation of the emerging jet resulting in a wavy nature related to effects of instability waves. The results with 15 deg swirl show reduced breakup length and penetration related to the nonuniform distribution of velocity that offers enhanced fuel atomization in swirling fuel nozzles.

Numerical simulation of hydro-elastic problems with smoothed particle hydrodynamics method

Violent free surface flows with strong fluid-solid interactions can produce a tremendous pressure load on structures, resulting in elastic and even plastic deformations. Modeling hydro-elastic problems with structure deformations and a free surface breakup is difficult by using routine numerical methods. This paper presents an improved Smoothed Particle Hydrodynamics (SPH) method for modeling hydro-elastic problems. The fluid particles are used to model the free surface flows governed by Navier-Stokes equations, and the solid particles are used to model the dynamic movement and deformation of the elastic solid objects. The improved SPH method employs a Kernel Gradient Correction (KGC) technique to improve the computational accuracy and a Fluid-Solid Interface Treatment (FSIT) algorithm with the interface fluid and solid particles being treated as the virtual particles against their counterparts and a soft repulsive force to prevent the penetration and a corrective density approximation scheme to remove the numerical oscillations. Three typical numerical examples are simulated, including a head-on collision of two rubber rings, the dam break with an elastic gate and the water impact onto a forefront elastic plate. The obtained SPH results agree well with experimental observations and numerical results from other sources.

Direct synthesis of amorphous carbon nanotubes on Fe76Si9B10P5 glassy alloy particles

A mass of amorphous carbon nanotubes (ACNTs) have been synthesized by chemical vapor deposition of C2H2 directly on Fe76Si9B10P5 glassy alloy particles without the addition of an external catalyst at 550°C for 1h. The ACNTs is 1–5μm in length and 5–30nm in diameter and has a Brunauer–Emmett–Teller surface area of 81.4m2/g. Investigations on the ACNT growth process confirmed that the initial stage for appearance of isolated islands (α-Fe nanoparticles), which might be due to stress and surface breakup generated during high temperature CVD process, would be the decisive step for the ACNT growth.

Direct growth of MWCNTs on 316 stainless steel by chemical vapor deposition: Effect of surface nano-features on CNT growth and structure

Multi-walled carbon nanotubes were directly grown by chemical vapor deposition on as-received or pretreated 316 SS without application of an external catalyst. A detailed study of the size distribution of surface features formed by different steps of the synthesis process showed that the heating cycle and any complementary pretreatment may produce significant changes of the surface topography, thus suggesting that the influence of any primary characteristics of the original surface, as well as those caused by a pretreatment, should be assessed in conjunction with the effects of heating. Average lateral size of nano-features less than 60nm (after heating) were shown to favor mainly the carbon nanotube growth while a larger features size was associated predominantly to the carbon nanofiber synthesis. Scanning and transmission electron microscopy observations suggest two different mechanisms for nanotube/nanofiber growth: (1) base growth mode caused by nanosized hills on the surface catalyzing the nanotube/nanofiber synthesis, (2) tip growth mode requiring substrate surface break-up as a preliminary step to form catalytic particles, with similarities to the “metal dusting” mechanisms. While untreated steel showed the best results concerning carbon nanotube coverage and homogeneity, oxidized-reduced samples showed an almost exclusive growth of carbon nanofibers with a full coverage.

타원형 노즐의 내부유동 구조가 액주분열에 미치는 영향

An experimental study was performed to investigate the liquid jet breakup of a circular nozzle and elliptical nozzles. Furthermore Numerical simulation was attempted to investigate the internal flow structure in the circular and elliptical nozzles. This study showed that the disintegration characteristics of the liquid jet of elliptical nozzles were much different from those of the circular nozzle. The liquid jet issued from the elliptical nozzles became more unstable at the same injection pressure. Surface breakup was observed at the jet issued from the elliptical nozzles with the increase of injection pressure. The disintegration of the liquid jet of elliptical nozzles was related with the internal flow structure which is revealed from the numerical simulation.

Bubble oscillations and motion under vibration

Bubbles under vibration can behave in unusual ways, e.g., moving downward against the force of buoyancy. While the bubble downward motion due to the Bjerknes force is well known at acoustic frequencies close to the bubble resonant frequency, these experiments demonstrate that these effects can be observed at relatively low frequencies as well. Experiments were performed in a thin, quasi-two-dimensional rectangular acrylic box partially filled with 20-cSt PDMS silicone oil with overlying ambient air. The apparatus was subjected to sinusoidal axial vibration that produced breakup of the gas-liquid free surface, producing liquid jets into the air, droplets pinching off from these jets, gas cavities in the liquid from impacts of these droplets, and bubble transport below the interface. Vibration conditions for the attached videos are 280 Hz frequency, 15 g acceleration, and 94 micron peak-to-peak displacement. Behaviors shown in the videos include the following. 1. Free surface breakup into jets and droplets, and formation of bubbles under the free surface. 2. Bubbles thus generated moving downward in the cell. 3. Bubbles attracted to the first bubble deep in the cell and eventually merging to form a large bubble at the base of the cell. 4. Bubble cluster at the base of the cell merging to form a larger bubble, which stabilizes at a levitated location below the free surface and acts to damp out the surface breakup. 5. The levitated bubble interface and its breakup are similar to the free surface breakup into jets and droplets, but the jets in the bubble are facing downward. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

원형 및 타원형 노즐 내부유동과 외부유동의 상관관계

원형 및 타원노즐의 내부유동과 외부유동의 상관관계를 알아보기 위해 실험적 연구가 수행되었다. 분사압력에 따라 유량, 분무각, 액적크기 등의 외부유동에 관해 관찰하였고, 노즐 내부유동의 유속 및 압력분포 등을 수치해석을 통해 정량적인 결과를 도출하였다. 외부유동의 경우, 동일한 압력조건하에서 타원형 노즐의 경우, 원형 노즐에서 나타나지 않는 표면분열의 분무특성을 관찰할 수 있었고, 수치해석을 통해 노즐 내부의 유동을 분석한 결과, 원형의 경우와는 달리 타원형 노즐의 단축에서 내부유동의 재부착이 노즐 벽면에서 발생되었다. 타원노즐 외부유동의 표면분열이 내부유동에 따른 결과라고 판단된다.

EFFECT OF INTERNAL FLOW STRUCTURE IN CIRCULAR AND ELLIPTICAL NOZZLES ON SPRAY CHARACTERISTICS

An experimental study was performed to investigate the atomization characteristics of a circular nozzle and elliptical nozzles of small diameter under high injection pressure, which has a hydraulic flip condition for the nozzle internal flow structure. The flow rate and drop size characteristics were measured for various injection pressures. Numerical simulations were attempted to investigate the internal flow structure in the circular and elliptical nozzles because the experimental study was limited in its measurements of flow velocity distributions, pressure distributions, and streamlines in the relatively small orifices. This study showed that the disintegration characteristics of the liquid jet of the elliptical nozzles were very different from those of the circular nozzle. In the case of the elliptical nozzles, the liquid jet became more unstable at the same injection pressure, unlike that of the circular nozzle. Surface breakup was observed at the jet issued from the elliptical nozzles with the increase of injection pressure. Furthermore, the numerical simulations informed that the internal flow structure of the elliptical nozzle was quite different from that of the circular nozzle. In the case of the circular nozzle, as with much of the literature on the internal flow structure of the hydraulic flip, the flow detached from the orifice wall. However, the internal flow structure of the elliptical nozzle in hydraulic flip was reattached to the orifice wall of the minor axis, unlike the flow in the circular nozzle. It has been concluded that the internal flow structure of the elliptical nozzle has influence on the disintegration characteristics of the liquid jet issued from the elliptical nozzle.

Numerical simulation of sloshing in rectangular tank with VOF based on unstructured grids

A new method for sloshing simulation in a sway tank is present, in which the two phase interface is treated as a physical discontinuity, which can be captured by a well-designed high order scheme. Based on Normalized Variable Diagram (NVD), a high order discretization scheme with unstructured grids is realized, together with a numerical method for free surface flow with a fixed grid. This method is implemented in an in-house code General Transport Equation Analyzer (GTEA) which is an unstructured grids finite volume solver. The present method is first validated by available analytical solutions. A simulation for a 2-D rectangular tank at different excitation frequencies of the sway is carried out. A comparison with experimental data in literature and results obtained by commercial software CFX shows that the sloshing load on the monitor points agrees well with the experimental data, with the same grids, and the present method gives better results on the secondary peak. It is shown that the present method can simulate the free surface overturning and breakup phenomena.

Mechanisms of the Local Surface Deformation and Breakup of the Wave Breaking Jets

Mechanisms of the Local Surface Deformation and Breakup of the Wave Breaking Jets

Growth of filamentous carbon by decomposition of ethanol on nickel foam: Influence of synthesis conditions and catalytic nanoparticles on growth yield and mechanism

This paper presents nano-sized carbon filaments grown by catalytic decomposition of ethanol on polycrystalline nickel foam. Our study is focused on the effects of synthesis conditions on the formation of filamentous carbon. The formation rate of filamentous carbon increased as the supply of liquid ethanol into the furnace and the synthesis time increased during growth process, whereas H 2 concentration in carrier gas had the opposite effect. The most favorable temperature for the highest formation rate was around 600 °C. We investigated the role of catalytic nanoparticles in the growth mechanism of filamentous carbon. Tip growth was the primary mechanism responsible for growth of the filamentous carbon; octopus-like carbon filaments were formed on large nanoparticles of about 100 nm in diameter. It was also discovered that surface break-up on the nickel foam precipitated the formation of filamentous carbon. We detected the presence of Ni 3C as synthesis conditions after a long exposure to ethanol.

Surface breakup and air bubble formation by drop impact in the irregular entrainment region

Drop impact on a water surface can be followed by underwater sounds originating not at the drop impact but when the entrained bubbles oscillate. Although the sound mechanism in the regular bubble entrainment region is well-known, there is less knowledge on the impact phenomena in the irregular bubble entrainment region where various situations can exist, such as many types of bubble formation or even no bubble generation under some conditions. In the present study, the aim is to clarify the dynamics of the water surface after the impact of a primary drop, mainly with diameter 5.2, 5.7 and 6.2mm, each of which is accompanied by a single satellite drop. Special attention was paid to the breakup behaviour of the water surface for Froude number Fr < 300. It was found that three underwater sounds were generated for a single drop impact, besides the sound due to impact itself. The first two were audible to the human ear, but the third one was almost inaudible. The first underwater sound resulted from the oscillation of a single air bubble formed as a result of the satellite drop impact on the bottom of the contracting cavity, and the second sound was due to the oscillation of air bubbles generated during the collapse of the water column. The formation of these air bubbles strongly depends on the Froude number, Weber number (or Bond number) and the aspect ratio of the drop at impact, although involving probability characteristics. Furthermore it is suggested that an air bubble entrapped in a water column plays an important role in increasing the probability of contact between the column surface and the curved free surface. A Japanese Suikinkutsu was introduced as an application of drop-impact-induced sounds. Using an open-type Suikinkutsu an additional experiment was carried out with larger drops with average diameters of 6.2, 7.2 and 7.8, mm.

1835 インジェクタスプレーのLESにおける粒子分裂モデルの評価(J15-1 噴霧による混合気形成および燃焼(1),J15 噴霧による混合気形成および燃焼)

In this study, numerical simulation has been conducted for the spray injected from a slit nozzle injector for a direct gasoline engine. The droplets in the spray were investigated by the combination of Large Eddy Simulation (LES) and Lagrangian Discrete Droplet Model (DDM). Droplets are atomized through the primary breakup induced by the instability of the liquid surface and secondary breakup induced by relative velocity between droplet and surrounding gas. This paper applies three secondary breakup models to an injector spray flow and evaluates their results compared with experimental data.

Massively parallel modeling of the sawtooth instability in tokamaks

The sawtooth instability is one of the most fundamental dynamics of an inductive tokamak discharge such as will occur in ITER. The sawtooth occurs when the current peaks in a tokamak, creating a region in the core where the safety factor is less than unity, q<1. While this instability is confined to the center of the plasma in low-pressure, low-current, large-aspect-ratio discharges, under certain conditions it can create magnetic islands at the outer resonant surfaces or set off a sequence of events that leads to a major disruption. Sawtooth behavior is complex and remains incompletely explained. The SciDAC Center for Extended MHD Modeling (CEMM) has undertaken an ambitious campaign to model this periodic motion as accurately as possible using the most complete fluid-like description of the plasma - the Extended MHD model. The multiple time and space scales associated with the reconnection layer and growth time make this an extremely challenging computational problem. The most recent simulation by the M3D code used over 500,000 elements for 400,000 partially implicit time steps for a total of 2×1011 space-time points, and there still remain some resolution issues. However, these calculations are providing insight into the nonlinear mechanisms of surface breakup and healing. We have been able to match many features of a small tokamak and can now project to the computational requirements for simulations of larger, hotter devices such as ITER.

4225 イレギュラー気泡取り込み領域における気液界面分裂と水中過渡音響(G05-13 気泡・液滴(2),G05 流体工学)

4225 イレギュラー気泡取り込み領域における気液界面分裂と水中過渡音響(G05-13 気泡・液滴(2),G05 流体工学)

Borderline magic clustering: the fabrication of tetravalent Pb cluster arrays on Si(111)-(7x7) surfaces.

Well-ordered arrays of identical Pb clusters have been fabricated on a Si(111)-(7x7) substrate by the temperature-mediated surface clustering method. Interestingly, these clusters can easily transform into other forms when the growth temperature deviates slightly from the optimal values. In accord with experiments, first-principle total-energy calculations reveal several cluster structures centered on a mixed cluster model involving surface Pb and Si exchange. This borderline Pb/Si(111) system provides a unique, controlled way to study surface magic cluster formation and breakup dynamics.

Simulation of free‐surface flows induced by partially immersed moving body: Validation

AbstractPredictive performance of a free‐surface simulation technique is assessed using the volume‐of‐fluid (VoF) method, in the context of a challenging, yet simple and well‐defined, unsteady‐flow configuration mimicking the in‐mouth kinematic processes occurring during the consumption of liquid foods. An experimental “mouth‐analog” setup consisting of a cavity initially filled with a liquid and agitated by a ram moving periodically was modeled and simulated. Experimental and computational results are compared for a range of liquid viscosities (silicon oil and water) and over a range of processing conditions (low‐ and high‐agitation frequency) that trigger important effects such as surface reconnection and breakup, wall coating, filament formation, and air inclusion. Even with the large mesh and time‐step sizes used in the computations, the main relevant bulk kinematics of this complex transient free‐surface flow can be represented with a Euler volume‐tracking method such as the VoF method.

Liquid breakup at the surface of turbulent round liquid jets in still gases

An experimental study of liquid column breakup lengths and turbulent primary breakup properties at the surface of turbulent round liquid jets in still air at standard temperature and pressure is described. Jet exit conditions were limited to non-cavitating water and ethanol flows, long length/diameter ratio (greater than 40:1) constant-diameter round injector passages, jet exit Reynolds numbers of 5000–200,000, jet exit Weber numbers of 235–270,000 and liquid/gas density ratios of 690 and 860, at conditions where direct effects of viscosity were small (e.g., liquid jet Ohnesorge numbers were smaller than 0.0053). Three liquid column breakup modes were observed, as follows: a weakly turbulent Rayleigh-like breakup mode observed at small jet exit Weber and Reynolds numbers, a turbulent breakup mode observed at moderate jet exit Weber numbers, and an aerodynamic bag/shear breakup mode observed at large jet Weber numbers. The turbulent primary liquid column breakup mode was associated with conditions where drop diameters resulting from turbulent primary breakup along the liquid surface were comparable to the diameter of the liquid column itself. The bag/shear liquid column breakup mode was observed when liquid turbulence caused large deformations of the liquid column so that portions of it were in a gaseous cross flow; this resulted in bag or shear liquid column breakup, very similar to the breakup of non-turbulent liquid jets in gaseous cross flow. Mean streamwise drop velocities after breakup were comparable to mean streamwise velocities within the jet whereas mean cross stream drop velocities after breakup were comparable to cross stream velocity fluctuations within the liquid. Rates of primary breakup at the liquid surface are reported as surface efficiency factors (the fraction of the maximum possible cross stream drop liquid flux at the surface based on the mean relative cross stream velocity of the drops at the surface and the liquid density). The resulting surface efficiency factors varied from small values near the onset of liquid surface breakup to values of the order of unity as conditions near breakup of the liquid column as a whole were approached.