Theory Of Kelvin-Helmholtz Instability Research Articles

Modeling liquid droplet impact on a micropillar-arrayed viscoelastic surface via mechanically averaged responses

Droplet impact on a substrate is an intriguing phenomenon that widely exists in our daily life and a broad range of industrial processes. However, droplet impact dynamics on soft textured surfaces are less explored and the underlying mechanisms remain elusive. Here, we report numerical simulation of droplet impact dynamics on a micropillar-arrayed soft surface using BASILISK, which involves a multiscale geometric domain containing the micropillars and droplet that are in the order of μm and mm, respectively. As such, the volume of fluid (VOF) method is coupled with the finite volume method (FVM) to build the fluid fields and track their interface. From a conceptual point of view, the micropillared substrate is formed by imposing interstitial gaps into the otherwise intact soft material, whose viscoelastic properties can be quantified by gap density ϵ. Via a five-parameter generalized Maxwell model, the viscoelastic properties of the micropillared substrate can be approximated by its equivalent elastic response in the Laplace–Carson (LC) space, and the averaged bulk strain of the micropillared substrate in the real space is obtained by the inverse LC transform. Moreover, through parametric studies of splash extent, it turns out that for a specific ϵ, the splash is dramatically intensified with increasing impact velocity Ui. The splash also turns more violent with increasing ambient pressure Pa, which is evidenced by a larger splash angle of 114.44∘ between the ejected sheet and the horizontal substrate at 5 atm. Conversely, the splash becomes more depressed with increasing surface tension σ. Overall, the splash magnitudes of our simulations agree well with those predicted by the Kelvin-Helmholtz instability theory. By leveraging the LC transform in the fluid-viscoelastic solid interactions, our simulation methodology captures the main features of droplet impact dynamics on microstructured viscoelastic surfaces by means of the mechanically averaged responses while avoiding the predicament of domain scale inconsistency.

Study on Rubber Composite Armor Anti-Shaped Charge Jet Penetration

The process of rubber composite armor anti-shaped charge jet (SCJ) penetration was divided into four parts based on jet deformation that occurred when the SCJ penetrated the rubber composite armor. Results on the interference speed interval, interference frequency, and surplus penetration capability of the SCJ with the rubber composite armor were derived based on the stress wave and the Kelvin-Helmholtz instability theory. The effects of rubber layer thickness and obliquity of the armor for the composite armor anti-SCJ penetration were studied through theoretical, X-ray, and depth of penetration experiments. The results showed that when the obliquity was at 60° and the rubber layer thickness was in the range of 3 mm to 3.5 mm, the rubber composite armor seriously disturbed the stability of the SCJ. Thus, the rubber composite armor was found to have the best protection capability under these specifications.

Numerical Simulation of Spray Process of Fuel Nozzle

Control of exhaust emission and combustion efficiency depend directly on the atomization of the liquid. The research on the atomizing performance of fuel nozzle is certainly important. A model using VOF-LES method has been built to investigate sheet breakup behavior of the main circuit of a pressure swirl atomizer. LES which takes into account of turbulence of fuel and air, has been used to simulate the phenomenon of sheet breakup. Vivid gas-liquid interface, instability wave and primary breakup phenomenon have been captured. The patterns of the sheet breaking-up change with different fuel flux. The mechanism of sheet breakup has been analyzed by combining variations of velocity and turbulent kinetic energy with Kelvin-Helmholtz instability theory and turbulence on circumference. The breakup length captured by simulation agrees well with the results of the semi-empirical formulas.

On the linear theory of Kelvin-Helmholtz instabilities of relativistic magnetohydrodynamic planar flows

Aims. We investigate the linear stability properties of the plane interface separating two relativistic magnetized flows in motion with respect to each other. The two flows are governed by the (special) relativistic equations for a magnetized perfect gas in the infinite conductivity approximation.

Kelvin–Helmholtz instability in a weakly ionized layer

We study the linear theory of Kelvin-Helmholtz instability in a layer of ions and neutrals with finite thickness. In the short wavelength limit the thickness of the layer has a negligible effect on the growing modes. However, perturbations with wavelength comparable to layer's thickness are significantly affected by the thickness of the layer. We show that the thickness of the layer has a stabilizing effect on the two dominant growing modes. Transition between the modes not only depends on the magnetic strength, but also on the thickness of the layer.

LATTICE BOLTZMANN SIMULATION OF THE INTERFACIAL GROWTH OF THE HORIZONTAL STRATIFIED TWO-PHASE FLOW

The interfacial growth of the horizontal stratified two-phase flow, which is caused by the velocity difference between two phases, is simulated by the lattice Boltzmann scheme for two-phase fluid. In order to verify the validity of the interface generated by the lattice Boltzmann method, the critical velocity difference that cause the interfacial instability and growth is measured in 2D and 3D for several wave numbers. The measurement is nearly agreement with the analysis of the Kelvin-Helmholtz instability theory.

Instability of a two-phase loop thermosyphon

The instability of two-phase loop thermosyphons was investigated experimentally and analytically. Three orifice type inserts were used to study the effect of change in the pressure drop in the flow channel of the TLT on the flow instability and temperature fluctuation. It is observed that a decrease in the size of the orifice insert from 3.7 mm (no insert) to 0.71 mm drastically reduced the fluctuation of the temperature, especially at the evaporator section of the TLT. With the orifice type insert of 0.71 mm for the TLT, the overall temperature fluctuation was almost completely eliminated, especially at higher power input to the TLT. The analysis based on the Kelvin-Helmholtz instability theory seems to predict reasonable well the loop stability state of the TLT with experimentally determined constant factors.

The effect of mass transfer on Kelvin-Helmholtz instability at the gas-liquid interface of a sonic reacting and non-reacting gas jet submerged in a liquid

The theory of Kelvin-Helmholtz instability is employed to analyze the instability phenomena of non-reacting and reacting stratified gas flows injected sonically into a liquid. The effect of the mass transfer at the gas-liquid interface on the instability is investigated. It is shown that the mass transfer affects the pressure perturbation which acts to transfer energy from the gas phase to the liquid layer through its evaporating and condensing behavior, its wave-drag and lift components, against forces due to surface tension and liquid viscosity. The dimensionless wave frequency, amplification, and wavelength at the maximum instability are presented as a function of a dimensionless surface tension/viscous parameter and a blowing parameter due to the interfacial mass transfer. The interfacial evaporation is found to enhance the instability while the interfacial condensation is to reduce the instability. The results provide the theoretical explanation of the reported dynamic and instability behavior found in the reacting jet of a HCl gas submerged in the ammonia aqueous solution. Finally, an application to the prediction of the break-off plume length observed in submerged reacting jets is presented and the results are compared to exper imental data of the HCl(g)- NH 3(aq) system.

Behavior of the gas-liquid interface in a horizontal tube after the step change of air flow.

We studied the behavior of the air-water interface in a horizontal tube caused by the step change of air flow above still water. As to the critical condition of wave generation, the experimental results are well explained by the theory of Kelvin-Helmholtz instability. This tendency holds true also in the case of different surface tension. The lower limit of air velocity for slugging in a short time (1.75 sec) after commencement of the flow is about 3 m/s greater than the Wallis' results regarding the transition from the steady wavy flow.

Behavior of the Gas-liquid Interface in a Horizontal Tube after the Step Change of Air Flow

We studied the behavior of air-water interface in a horizontal tube caused by a step change of air flow above still water. As to the critical condition of wave generation, the experimental results are well explained by the theory of Kelvin-Helmholtz instability. This tendency remains true also in case of different surface tension. The lower limit of air velocity for slugging in a short time (1.75 sec) after the staring of the flow is about 3m/s greater than the results of Wallis which correspond to the transition from the steady wavy flow.

Discretized Simulation of Vortex Sheet Evolution with Buoyancy and Surface Tension Effects

The discrete vortex method previously used by Rosenhead and others to compute the roll-up of a vortex sheet in a single fluid is extended to the situation in which the vortex sheet is a horizontal interface separating a lighter inviscid fluid above from a heavier fluid below. The stabilizing effects of gravity and surface tension are included in a discretized circulation equation used to calculate the circulations of individual vortices during the course of the computation. Calculations show the evolution of the interface under stable, unstable, and marginally unstable conditions as given by linear Kelvin-Helmholtz instability theory. Under stable conditions, finite amplitude disturbances oscillate with a period slightly larger than predicted by classical theory for infinitesimal perturbations. For unstable conditions, the finite amplitude growth rate is much smaller than the corresponding linearized growth rate for infinitesimal disturbances. Under marginally unstable conditions, small irregularities along the interface cause numerical difficulties before the maximum disturbance amplitude and extent of roll-up has been computed.

Universal time seasonal variations of auroral zone magnetic activity and VHF scintillations

The AL index, which is the negative component of the auroral electrojet index AE, is utilized to study the universal time seasonal variations of northern hemisphere auroral zone magnetic activity in the midnight sector on quiet days. The solstitial AL patterns are found to be consistent with the predictions of the Kelvin-Helmholtz instability theory for geomagnetic activity which is based on the variation of dipole tilt angle Ψ with respect to the solar wind. Further, it is found that the best ordering of the seasonal variation is obtained by dividing the year into two broad seasons, one 6-month period centered around the June solstice, when the plane of symmetry of the plasma sheet in the magnetotail is above the solar magnetospheric equatorial plane, and the other 6-month period centered around the December solstice, when the plane of symmetry is below the solar magnetospheric equatorial plane. The observed AL patterns are thus interpreted as a seasonal modulation of low-energy particle precipitation caused by the asymmetric topology of the plasma sheet. The nature of this seasonal pattern is found to vary in different sectors of the auroral zone. Since auroral scintillations are associated with this particle precipitation, a comparative study between scintillations and AL index was undertaken. It is found that the long-term seasonal behavior of 137-MHz scintillation data from ATS 3 at an auroral station in the North Atlantic sector is consistent with the seasonal behavior of AL data in this sector.