Instability Of Jet Research Articles

Temporal instability analysis of a confined non-Newtonian liquid jet with heat and mass transfer

In this study, the instability behavior of a confined non-Newtonian liquid jet in an annular gas layer is investigated theoretically. The viscosity of the non-Newtonian liquid is described using the power law model, and the corresponding dispersion relation is obtained by conducting a linear stability analysis. Additionally, the effects of heat and mass transfer, gas-to-liquid axial velocity ratio, power law index, and gas layer thickness on the instability of the power law liquid jet are investigated. The results demonstrate that the heat and mass transfer at the liquid–gas interface destabilizes the power law liquid jet flow. The results also indicate that an increase in the thickness of the gas layer makes the confined power law liquid jet unstable. Furthermore, the confined shear-thickening liquid jet is the most difficult to breakup in comparison with the shear-thinning and Newtonian liquid jets.

Transition in the flow due to a rotating sphere: Simulating isolated roughness

We present results of a study on the roughness-induced receptivity of the boundary layer that exists in the flow due to a rotating sphere. Our approach exploits a computational methodology that allows us to capture the full range of fundamental fluid physics that arises in the flow from the early collision of boundary layers due to the sphere’s rotation to the development of a radial jet and subsequent instabilities in the spatially and temporally developing flow. We demonstrate that the flow can exhibit a transient convective instability in the form of a spiral vortex, but ultimately the boundary layer returns to a laminar state. Consideration is also given to the impact of this transient instability on the radial jet, which when advected into the radial jet at the sphere’s equator serves to render the jet turbulent.

Numerical Investigation of Gas‐Liquid Reacting Flow in a Jet‐Type Singlet Oxygen Generator

AbstractA numerical simulation was carried out to investigate the process of gas‐liquid reaction in a jet‐type singlet oxygen generator by using volume‐of‐fluid coupled with large eddy simulation. The coupled hydrodynamic and reaction simulations provide detailed information about the reacting flow. Results reveal that the reaction aggravates the jet instability and affects the jet structure significantly due to the differential pressure around the liquid jet. The effects of gas velocity on chlorine conversion and the yield of singlet oxygen were investigated. The key is the increasing of the interface area rather than the convection. This numerical simulation study is convenient for the design of singlet oxygen reactors and the research of gas‐liquid two‐phase flow with reaction.

Particle Acceleration in Kink-unstable Jets

Abstract Magnetized jets in gamma-ray bursts and active galactic nuclei are thought to be efficient accelerators of particles; however, the process responsible for the acceleration is still a matter of active debate. In this work, we study the kink instability in non-rotating force-free jets using first-principle particle-in-cell simulations. We obtain similar overall evolution of the instability as found in magnetohydrodynamics simulations. The instability first generates large-scale current sheets, which at later times break up into small-scale turbulence. Reconnection in these sheets proceeds in the strong guide field regime, which results in a formation of steep power laws in the particle spectra. Later evolution shows heating of the plasma, which is driven by small-amplitude turbulence induced by the kink instability. These two processes energize particles due to a combination of ideal and non-ideal electric fields.

Collapse dynamics of spherical cavities in a solid under shock loading

Extraordinary states of highly localised pressure and temperature can be generated upon the collapse of impulsively driven cavities. Direct observation of this phenomenon in solids has proved challenging, but recent advances in high-speed synchrotron radiography now permit the study of highly transient, subsurface events in real time. We present a study on the shock-induced collapse of spherical cavities in a solid polymethyl methacrylate medium, driven to shock states between 0.49 and 16.60 GPa. Utilising multi-MHz phase contrast radiography, extended sequences of the collapse process have been captured, revealing new details of interface motion, material failure and jet instability formation. Results reveal a rich array of collapse characteristics dominated by strength effects at low shock pressures and leading to a hydrodynamic response at the highest loading conditions.

A molten metal jet impingement on a flat spreading surface

ABSTRACT In the event of a severe accident, past experiences such as Three Mile Island and Fukushima Daichi have shown that the reactor core of a light-water nuclear reactor, if not properly safeguarded, could go through a meltdown. This will be followed by the formation of a corium, a mix of molten fuel elements, and liquid metals from the Reactor Pressure Vessel (RPV). In the worst-case scenario, a melt through from the RPV can occur and lead to the spreading of the corium, in the form of a molten element’s jet impinging on a flat concrete structure of the Primary Containment Vessel (PCV). To enhance the decommissioning and the safety procedure, scope of the present article is to deepen the understanding of the phenomena involved in the mentioned scenario, mainly jet-instability and molten material spreading. In the present study, experiments were carried out, by using corium simulant materials such as Copper and Tin, to investigate the link between the instability of the gravity-driven molten metal jet and the impinging followed by its spreading over a flat area.

Capillary Instability of a Cylindrical Ferrofluid Jet in a Uniform Longitudinal Magnetic Field

The formulated problem makes it possible to study the influence of magnetic forces on capillary instability of a ferrofluid jet with given physical characteristics in both weak and strong fields. The field intensity range includes values that correspond to the ferrofluid magnetization curve interval in which the curve reaches the value of saturation magnetization. The obtained growth rates and wavelengths of the fastest growing perturbations of the jet surface shape are compared for strong and weak fields.

Instability growth and fragment formation in air assisted atomization

This paper reports an extensive study on the morphology of wave formation on the liquid core of atomizing sprays. The gas velocity, liquid jet velocity and liquid jet size are varied for two different fuels resulting in a range of liquid jet Reynolds numbers, aerodynamic Weber numbers and mass flux ratios. The liquid jet Reynolds number can be used to predict the initiation of jet instabilities, with coaxial air-flow velocity controlling their subsequent growth. A categorization of waves on the surface of the liquid according to their amplitude and wavelength has enabled (i) the identification of a threshold that leads to breakup, and (ii) the isolation of waves that lead to ligament formation from waves that result in droplets. The probability distribution of measured wavelength reasonably matches that of the ligament length, with no requirement for empirical constants. This confirms a direct link between interfacial instabilities and ligament formation in air assisted primary atomization.

Influence of nozzle geometry on primary and large-scale instabilities in coaxial injectors

Detailed knowledge on liquid jet instabilities is important to improve the performance of coaxial injectors, which find wide range of industrial applications. The present paper aims to investigate the influence of nozzle exit geometry on jet instabilities during the unsteady primary jet breakup process in liquid-centered coaxial injectors. Time resolved images of the liquid jet were processed to characterize Kelvin–Helmholtz instability close to the nozzle exit and flapping instability near the jet breakup location for three different nozzles with different liquid-to-gas cross-sectional area ratio (AR) and lip thickness to diameter ratio (LTR) of the central tube. The results showed considerable effect of AR and LTR on both primary and large scale instabilities as well as jet breakup length and morphology. It is found that the injector exit geometry plays a crucial role in triggering the interfacial disturbances as it conditions the liquid and air flow prior to their interaction.

Kink instabilities in relativistic jets can drive quasi-periodic radiation signatures

ABSTRACT Relativistic jets are highly collimated plasma outflows emerging from accreting black holes. They are launched with a significant amount of magnetic energy, which can be dissipated to accelerate non-thermal particles and give rise to electromagnetic radiation at larger scales. Kink instabilities can be an efficient mechanism to trigger dissipation of jet magnetic energy. While previous works have studied the conditions required for the growth of kink instabilities in relativistic jets, the radiation signatures of these instabilities have not been investigated in detail. In this paper, we aim to self-consistently study radiation and polarization signatures from kink instabilities in relativistic jets. We combine large-scale relativistic magnetohydrodynamic (RMHD) simulations with polarized radiation transfer of a magnetized jet, which emerges from the central engine and propagates through the surrounding medium. We observe that a localized region at the central spine of the jet exhibits the strongest kink instabilities, which we identify as the jet emission region. Very interestingly, we find quasi-periodic oscillation (QPO) signatures in the light curve from the emission region. Additionally, the polarization degree appears to be anticorrelated to flares in the light curves. Our analyses show that these QPO signatures are intrinsically driven by kink instabilities, where the period of the QPOs is associated with the kink growth time-scale. The latter corresponds to weeks to months QPOs in blazars. The polarization signatures offer unique diagnostics for QPOs driven by kink instabilities.

Numerical Modeling of Instability and Breakup of Elliptical Liquid Jets

Numerical simulations are performed to provide an in-depth insight into the effect of instabilities on liquid jets discharging from elliptical orifices. The axis-switching phenomenon and breakup are simulated and characterized under the effect of disturbances imposed at the nozzle exit. The simulations are based on the volume of fluid approach and an adaptive meshing. A range of orifice aspect ratios from 1 to 4 at the Rayleigh breakup regime is considered. The evolution of the jet cross section and axis switching under the influence of disturbances is compared with that of nonperturbed elliptical jets. It is found that the axis-switching repetition and breakup length exponentially decrease with the initial amplitude of disturbances. For a fixed amplitude, the optimum wavenumber responsible for minimum breakup length is observed to be dependent on ellipticity of the orifice. Moreover, it is found that the form of final breakup leads to elimination of the satellite droplets at the optimum wavenumbers, while the droplet size decreases and becomes more uniform.

Linear analysis and energy budget of viscous liquid jets in both axial and radial electric fields

In their countless industrial applications, axisymmetric and non-axisymmetric instabilities are respectively responsible for electrospraying and electrospinning. A linear method and energy budget have been applied in this study to investigate the instability of viscous jets under both the axial and radial electric fields; the liquid was taken to be a leaky dielectric and the gas a perfect dielectric; the effect of a parabolic velocity profile was considered and compared to that of a uniform velocity, and the energy analysis explained the physical mechanisms to an extent. The liquid viscosity and parabolic velocity profile had a combined effect on jet instability. Work induced by the parabolic velocity profile consisted of two parts: the energy transferred from the basic flow to the disturbances, and the influence of the corresponding shear stresses. At low viscosities, these influences were positive enough to prevail over the viscous dissipation, enhancing axisymmetric instability. However, the parabolic velocity profile functioned differently in small and large wavenumber regions, and the response to the axial electric fields varied in different regions, accounting for the dual effects of axial electric fields on axisymmetric instability. Also, under the interplay between the strong axial electric fields and the parabolic velocity profile, two distinct unstable regions emerged for the non-axisymmetric mode. The effects of the radial electric fields were less sensitive, whether or not the parabolic velocity profile was considered. In summary, the parabolic velocity profile was significant–especially for charged jets with weak viscosity and strong axial electric intensity. The effects of axial electric fields in the atomization zone, and the effects of fluid permittivity coupled with the axial electric fields, were also investigated here.

2D turbulence closures for the barotropic jet instability simulation

AbstractIn the present work the possibility of turbulence closure applying to improve barotropic jet instability simulation at coarse grid resolutions is considered. This problem is analogous to situations occurring in eddy-permitting ocean models when Rossby radius of deformation is partly resolved on a computational grid. We show that the instability is slowed down at coarse resolutions. As follows from the spectral analysis of linearized equations, the slowdown is caused by the small-scale normal modes damping arising due to numerical approximation errors and nonzero eddy viscosity. In order to accelerate instability growth, stochastic and deterministic kinetic energy backscatter (KEBs) parameterizations and scale-similarity model were applied. Their utilization led to increase of the growth rates of normal modes and thus improve characteristic time and spatial structure of the instability.

Hot Jupiter atmospheric flows at high resolution

ABSTRACT Global circulation models (GCMs) of atmospheric flows are now routinely used to interpret observational data on hot Jupiters. Localized ‘equatorial β-plane’ simulations have revealed that a barotropic (horizontal shear) instability of the equatorial jet appears at horizontal resolutions beyond those typically achieved in global models; this instability could limit wind speeds and lead to increased atmospheric variability. To address this possibility, we adapt the computationally efficient, pseudo-spectral PlaSim GCM, originally designed for Earth studies, to model hot Jupiter atmospheric flows and validate it on a reference benchmark. We then present high-resolution global models of HD209458b, with horizontal resolutions of T85 (128×256) and T127 (192×384). The barotropic instability phenomenology found in β-plane simulations is not reproduced in these global models, despite comparably high meridional resolutions. Nevertheless, high-resolution models do exhibit additional flow variability on long time-scales (of the order of 100 planet days or more), which is absent from the lower resolution models. It manifests as a breakdown of the north–south symmetry of the equatorial wind. By post-processing the atmospheric flows at various resolutions (assuming a cloud-free situation), we show that the stronger flow variability achieved at high resolution does not translate into noticeably stronger dayside infrared flux variability. More generally, our results suggest that high horizontal resolutions are not required to capture the key features of hot Jupiter atmospheric flows.

Barotropic instability of a zonal jet on the sphere: from non-divergence through quasi-geostrophy to shallow water

Two common approximations to the full Shallow Water Equations (SWEs) are non-divergence and quasi-geostrophy, and the degree to which these approximations lead to biases in numerical solutions are explored using the test bed of barotropic instability. Specifically, we examine the linear stability of strong polar and equatorial jets and compare the growth rates obtained from the SWEs along with those obtained from the Non-Divergent barotropic vorticity (ND) equation and the Quasi-Geostrophic (QG) equation. The main result of this paper is that the depth over which a layer is barotropically unstable is a crucial parameter in controlling the growth rate of small amplitude perturbations and this dependence is completely lost in the ND equation and is overly weak in the QG system. Only for depths of 30 km or more are the growth rates predicted by the ND and QG systems a good approximation to those of the SWEs, and such a convergence for deep layers can be explained using theoretical considerations. However, for smaller depths, the growth rates predicted by the SWEs become smaller than those of the ND and QG systems and for depths of between 5 and 10 km they can be smaller by more than . For polar jets, and for depths below 2 km the mean height in geostrophic balance with the strong zonal jet becomes negative and hence the barotropic instability problem is ill-defined. While in the SWEs an equatorial jet becomes stable for layer depths smaller than 3–4 km, in the QG and ND approximations it is unstable for layer depths down to 1 km. These result may have implications for the importance of barotropic instability in Earth's upper stratosphere and perhaps also other planets such as Venus.

Energy transfers between aerodynamic and acoustic fields in a rectangular impinging jet

Ventilation systems in buildings are of vital importance for the provision of acceptable thermal conditions and air quality while meeting stringent energy requirements. When a rectangular jet hits a slotted plate, an acoustic disturbance can be generated and self-sustained tones produced. Self-sustaining tones appear when a feedback loop develops between the surface of impact and the jet instabilities near to the jet exit. In order to control this phenomenon, more attention should be given to the source of energy that amplifies the resulting noise. Few studies have considered the fluctuation of the aerodynamic field using High Speed 3D Tomographic-PIV in the presence of self-sustaining tones. In this work, we investigate energy transfer between the aerodynamic and acoustic fields for different Reynolds numbers (Re=5294 and Re=5956) to better understand the noise generated by a rectangular jet impinging on a slotted plate. High Speed 3D Tomographic-PIV was performed at a sampling rate of 2 KHz and main flow vortices frequencies below 0.4 KHz. It was found that in the case of an optimal configuration for self-sustaining tones, the fluctuation of the Z-component of velocity presented higher amplitudes near the jet exit region.

ELECTRODYNAMIC INSTABILITY OF A SELF-GRAVITIONAL COMPOUND LIQUID JET

Ponte Academic JournalMar 2020, Volume 76, Issue 3 ELECTRODYNAMIC INSTABILITY OF A SELF-GRAVITIONAL COMPOUND LIQUID JETAuthor(s): Nasser S. Elazab ,Ahmed Mohamed Elmoasry, Jawdat AlebraheemJ. Ponte - Mar 2020 - Volume 76 - Issue 3 doi: 10.21506/j.ponte.2020.3.11 Abstract:The electrodynamic instability of a self-gravitating liquid jet (radius R) of density ambient with a finite liquid (radius qR with ) of density has been developed. The problem is formulated and upon The electric field is strongly stabilizing as (the dielectric constant). The gravitational force is destabilizing only for small axisymmetric perturbed wave length. The densities ratio and radii ratio q play important roles in stabilizing the compound liquid jet. It is found that whatever are the strong effects of the stabilizing different factors, the gravitational destabilizing character of the model will never be suppressed. Download full text:Check if you have access through your login credentials or your institution Username Password

Absolute instability of free-falling viscoelastic liquid jets with surfactants

The effect of surfactants on the absolute instability of a viscoelastic liquid jet falling under gravity is examined for axisymmetrical disturbances. In general, the inclusion of surfactants to the interface of a viscoelastic liquid jet allows for the possibility of further processing droplet sizes and breakup lengths. We use the upper-convected Maxwell model to provide a mathematical description of the dynamics of the jet. An asymptotic approach, based on the slenderness of the jet, is used to render the problem more tractable and obtain steady-state solutions and then perform a linear analysis of the convective and absolute instability on these base solutions. By considering travelling wave modes, we derive a dispersion relationship, which is then solved numerically using the Newton-Raphson method. We show the effect of changing a number of dimensionless parameters, including the initial surfactant concentration, on convective and absolute instability. In this work, we use a mapping technique known as the cusp map method to explore absolute instability. The convective/absolute instability boundary is identified for various parameter regimes.

Study on instability of circular liquid jets at subcritical to supercritical conditions using dynamic mode decomposition

The thermodynamic and fluid-dynamic properties associated with the injectant jet significantly influence its fluid dynamic characteristics. In most of the earlier studies on the behavior of liquid jet at high pressure and supercritical environment, the liquid jet is often injected at subcritical initial temperature. In the present study, the effect of variation in the fluid and flow properties of the injecting jet on the nature of instabilities and mixing behavior is investigated. The fluid properties of the injected jet are varied by preheating. High-speed imaging techniques are used to capture the jet behavior. To gain further insight into the instability nature of the jet, linear instability analysis was performed. Dynamic mode decomposition analysis was applied to the obtained high-speed images to extract and understand the relevant dynamic features.

Bubbling to Jetting Transition during Argon Injection in Molten Steel

Bubbling to Jetting Transition is of the outmost importance in metallurgical processes given that the flow regime influences the refining rates, the refractory erosion, and the blockage of injection nozzles. Bubbling to jetting transition during subsonic bottom injection of argon in molten steel is studied here. The effect of the molten steel height, the injection velocity, the nozzle diameter, and the molten steel viscosity on the jet height and the bubbling to jetting transition is numerically analyzed using Computational Fluid Dynamics. Five subsonic argon injection velocities are considered: 5, 25, 50, 100 and 150 m/s. Three values of the metal height are taken into account, namely 1.5 m, 2 m and 2.5 m. Besides, three values of the nozzle diameters are considered: 0.001 m, 0.005 m and 0.01 m. Finally, three values of the molten steel viscosity are supposed: 0.0067, 0.1 and 1 kg/(m·s). It is observed that for the argon-molten steel system, the bubbling to jetting transition occurs for an injection velocity less than 25 m/s and that for the range of viscosities considered, the molten steel viscosity does not exert significant influence on the jet height and the bubbling to jetting transition. Due to the jet instability at subsonic velocities, a second transition, namely jetting to bubbling, is appreciated.