Stability Of Jet Research Articles

Simulation of 3D vortex jets in plasma torch application

The gas tunnel type plasma jet is an effective heat source for thermal processing applications such as plasma spraying. The key concept of gas tunnel plasma is its torch configuration, especially the role of the vortex gas flow. This is very important for the stability and energy density of the plasma jet produced. This work studied the flow of gas vortex in 3 dimensions using a finite element simulation. The simulation is based on solving partial differential equations where the incompressible Navier-Stokes equation is used as a governing equation that describes the laminar flow. The geometry of the plasma torch investigated is based on the design by A. Kobayashi. Key parameters investigated were gas pressure, velocity and profile of the vortex. It can be shown that the simulation produced results that are better matched to the experimental result than the calculation done in previous work. The simulation can also show detailed pictures of the vortex and its properties within the plasma chamber. This study could be useful in the design optimization of the plasma torch in the future.

Correction to: Sediment waves and the gravitational stability of volcanic jets

Correction to: Sediment waves and the gravitational stability of volcanic jets

Addressing the Impact of Non-intrinsic Uncertainty Sources in the Stability Analysis of a High-Speed Jet

It is acknowledged by mechanical and aeronautical engineers that many important industrial fluid mechanics simulation-based decisions are made by analysing Reynolds-Averaged Navier-Stokes (RANS) simulations. However, despite that the use of RANS simulations has become a necessity, still unrealistic conditions such as not considering experimental variability or blindly trust a turbulence model happens in practice. This is an important concern in simulation of high-speed jet flows, as the reliability and the accuracy of RANS is still an issue. In this work non-intrusive Uncertainty Quantification (UQ) has been applied to the linear stability analysis of 3D RANS simulations of an under-expanded supersonic jet under uncertainty. The Parabolized Stability Equations (PSE) are utilised for the first time in the literature in a probabilistic approach for the propagation of uncertainty upon CFD simulations. As PSE does not include turbulence nor stagnation pressure as parameters in their mathematical formulation (named non-intrinsic parameters in this work), this framework enables a recommended practice to quantify the impact of uncertainty from earlier stages on the latter jet instability analysis. For this objective, an artificial stochastic base-flow is generated from UQ on RANS to develop the final quantification of uncertainties on jet stability characteristics with PSE. Spatial uncertainty quantification results show that the considered stagnation pressure uncertainty (equivalent to mass-flow rate uncertainty) affects mainly to the shock-cells area, which is in agreement with the jet flow mechanics. The tested turbulent viscosity ratio uncertainty from the Spalart-Allmaras turbulence model has been observed more influential on the shear layer region in general, and with a special role on the growth rate of the perturbations. Frequency (in terms of Strouhal number), has been found influential on the spatial localisation of the turbulent wave packets. This procedure is proposed as an standard approach in the field. The framework may help to understand how instability features are being affected by expected or unexpected uncertainty sources and can guide to future robust developments in the industry, for instance in jet noise reduction.

On the thermal instability of supercavitating liquid jet surrounded by coaxial rotary gas

Abstract Based on the jet stability theory, under the conditions of gas rotation, fluid compressibility and supercavitation, this paper gives the mathematical model describing the thermal instability of supercavitating liquid jet surrounded by a coaxial rotary gas, and the corresponding numerical method for solving the mathematical model is proposed and verified by the data in reference. Then, this paper analyzes the effects of gas–liquid temperature differences and temperature gradients on jet instability, and studies the thermal stability of supercavitating jet. The results show that the maximum disturbance growth rate, the dominant frequency and the maximum disturbance wave numbers increase linearly with the increase of gas–liquid temperature differences. The existence of temperature gradient inside the jet makes the effects of temperature differences on jet instability more obvious. The temperature gradient will inhibit the effect of supercavitation on jet instability, while gas–liquid temperature difference will promote the effect of supercavitation on jet instability.

Sediment waves and the gravitational stability of volcanic jets

Sediment waves and the gravitational stability of volcanic jets

Effect of phenolic antioxidants on the thermal oxidation stability of high-energy–density fuel

The addition of antioxidants is normally adopted to improve the thermal oxidation stability of jet fuel, but the detailed antioxidation mechanism is still ambiguous. Herein, the inhibition of exo-tetrahydrodicyclopentadiene (the main component of JP-10) toward thermal oxidation and deposition by phenolic antioxidants (2,6-di-tert-butyl-4-methyphenol (BHT), 6-tert-butyl-2,4-dimethyphenol (TBDP), 2,6-di-tert-butylphenol (DTBP), 2-tert-butyl-4-methoxyphenol (BHA) and 2-tert-butyl-4-methylphenol (TBMP)) was investigated to reveal antioxidation mechanism and screen the best antioxidant for high-energy–density fuel. The theoretical and experimental results show that the addition of all phenolic antioxidants can significantly inhibit fuel oxidation and prolong the shelf life. Importantly, TBDP and TBMP show better performance than other antioxidants since they can scavenge more radicals per antioxidant molecule. However, antioxidants form dimer products via the addition between phenoxyl radicals and alkyl radicals during short-term thermal stress, which has a higher tendency to produce deposits. Consequently, the addition of phenolic antioxidants causes more severe deposition when jet fuel serves as the coolant in aircrafts. This work would guide the rational design of antioxidant for practical application in high-energy–density fuel.

Convergence of Machine Vision and Melt Electrowriting.

Melt electrowriting (MEW) is a high-resolution additive manufacturing technology that balances multiple parametric variables to arrive at a stable fabrication process. The better understanding of this balance is underscored here using high-resolution camera vision of jet stability profiles in different electrical fields. Complementing this visual information are fiber-diameter measurements obtained at precise points, allowing the correlation to electrified jet properties. Two process signatures-the jet angle and for the first time, the Taylor cone area-are monitored and analyzed with a machine vision system, while SEM imaging for diameter measurement correlates real-time information. This information, in turn, allows the detection and correction of fiber pulsing for accurate jet placement on the collector, and the in-process assessment of the fiber diameter. Improved process control is used to successfully fabricate collapsible MEW tubes; structures that require exceptional accuracy and printing stability. Using a precise winding angle of 60° and 300 layers, the resulting 12 mm-thick tubular structures have elastic snap-through instabilities associated with mechanical metamaterials. This study provides a detailed analysis of the fiber pulsing occurrence in MEW and highlights the importance of real-time monitoring of the Taylor cone volume to better understand, control, and predict printing instabilities.

Numerical investigation of gas-liquid two-phase flow in horizontal pipe with orifice plate

Gas-liquid two-phase flow in horizontal or vertical pipeline significantly impacts on the transportation process. In fact, in the pipeline for regulating the flow flux or pressure, orifice plate and other current limiting device are frequently encountered. The flow behaviors of two-phase flow thus become more complicated in the pipeline with orifice plate. In order to explore the characteristics of gas-liquid two-phase flow in the pipeline accompanied by orifice plate, the computational fluid dynamics (CFD) simulation with volume of fluid (VOF) method and SST k−ω eddy viscosity model were imposed to probe the transient two-phase flow in a horizontal pipeline. The reliability of the numerical method was verified by a case of annular multiphase flow in a pipeline. In our research, two transient simulations related to different liquid volume ratios were carried out to explore the developments of the flow regimes. In addition, fluctuations of the area-average fluid pressures acting on the orifice plate surfaces were also monitored. The frequency characteristics of pressure pulsations were extracted via non-uniform fast Fourier transform (NUFFT). The main results show that the jet could be produced after the liquid passes through the orifice plate, and the liquid volume ratio has significant effects on the gas-liquid two-phase flow behaviors, a higher ratio leads to a faster stable and fuller jet. The stability of jet could be reflected via the dimensionless velocity ratio. The gas-liquid two-phase flow in the pipe with orifice plate induces typical broadband excitation.

Stability of a Viscous Liquid Jet in a Coaxial Twisting Compressible Airflow

Based on the linear stability analysis, a mathematical model for the stability of a viscous liquid jet in a coaxial twisting compressible airflow has been developed. It takes into account the twist and compressibility of the surrounding airflow, the viscosity of the liquid jet, and the cavitation bubbles within the liquid jet. Then, the effects of aerodynamics caused by the gas–liquid velocity difference on the jet stability are analyzed. The results show that under the airflow ejecting effect, the jet instability decreases first and then increases with the increase of the airflow axial velocity. When the gas–liquid velocity ratio A = 1, the jet is the most stable. When the gas–liquid velocity ratio A > 2, this is meaningful for the jet breakup compared with A = 0 (no air axial velocity). When the surrounding airflow swirls, the airflow rotation strength E will change the jet dominant mode. E has a stabilizing effect on the liquid jet under the axisymmetric mode, while E is conducive to jet instability under the asymmetry mode. The maximum disturbance growth rate of the liquid jet also decreases first and then increases with the increase of E. The liquid jet is the most stable when E = 0.65, and the jet starts to become more easier to breakup when E = 0.8425 compared with E = 0 (no swirling air). When the surrounding airflow twists (air moves in both axial and circumferential directions), given the axial velocity to change the circumferential velocity of the surrounding airflow, it is not conducive to the jet breakup, regardless of the axisymmetric disturbance or asymmetry disturbance.

Large-Eddy Simulations of Transverse Jet Mixing and Flame Stability in Supersonic Crossflow

Large-eddy simulations (LES) combined with the partially stirred reactor (PaSR) combustion model are employed to investigate the turbulent mixing, combustion mode, and flame stability of a sonic hydrogen jet injecting into high-enthalpy supersonic crossflow at three momentum flux ratios , in other words, 0.71, 2.11, and 4.00, respectively. The LES accuracy in terms of the turbulent kinetic energy, power spectra density, and subgrid Damköhler number is carefully addressed against various LES resolution criteria and the experimental mean pressure distribution on the upper wall. The ignition processes with autoignition and shock compression effects are identified and analyzed. At , the shock-induced ignition occurs behind the reflected shock wave, and the combustion heat release is dominated by the premixed combustion. While for the high jet-to-crossflow momentum flux ratios, for example, and 4.00, ignition happens around the jet orifice due to the strong bow shock and reflected shock effects, and the combustion heat releases are dominated by the nonpremixed combustion. Furthermore, the mechanisms of flame stabilization, local extinction, and reignition in the transverse jet combustion in supersonic crossflow are further analyzed with the chemical explosive mode analysis.

Sinusoidal AC-induced electrohydrodynamic direct-writing nanofibers on insulating collector

Printing orderly patterns on the insulating collector is the key for the development and application of flexible electronics. However, electrospinning on the insulating collector still has the problem of unstable jet due to the charge accumulation. The alternating current (AC)-induced electrohydrodynamic direct-writing (EDW) technology is a good way to decrease the interferences of charge repulsion, which is beneficial to printing orderly micro/nanostructures on the insulating collector. In this work, the sinusoidal AC-induced EDW is used to enhance the stability of charged jet and the deposition behaviors under AC electric field are also studied. The reciprocation transferring of charges induced by the AC electric field decreased the density of the accumulating charges on the insulating collector. The effect of AC electric field parameters on the direct-written micro/nanostructures are investigated to optimize the printing process. As the voltage peak increases, the fiber deposition bandwidth shows a trend of decreasing first and then increasing. Increasing the voltage frequency appropriately is beneficial to decrease the bandwidth of fiber deposition and to increase the stability of the jet. By improving the stability and controllability of the jet printing process, precise micro/nanopatterns can be direct-written on the insulating collector. This research provides a good foundation for expanding the application fields of EDW.

Interpreting Reynolds stress from perspective of eddy geometry

Geometric features in oceanic mesoscale eddies such as tilt and anisotropy can influence the properties of the Reynolds stress that provides feedback between the eddies and the background flow. By regarding an eddy as a wave, previous studies have parameterized the Reynolds stress based on the equivalence in the tilt angle between the phase of the eddy stream functions and the variance ellipse for the Reynolds stress (RS-ellipse). However, the wave assumption cannot predict the anisotropy of the RS-ellipse, and also largely simplifies the eddy geometry, which would naturally be an ellipsoid rather than a wave. The present study explores the shape relation between elliptical eddies and the RS-ellipse, by mathematically reformulating the Reynolds stress based on the eddy shape. The new formula reveals that the shape relation is regulated by the horizontal extent of the occurrence probability distribution (PDF) of the eddy, and that the shape of the eddy and RS-ellipse are identical at the place of maximum PDF when the horizontal scale of the PDF is sufficiently larger than the size of the eddy. A similar tendency is found in eddies detected by satellite altimetry in the Kuroshio Extension jet region. A detailed analysis of the PDF in this region shows that the tilts of the eddies are likely to be consistent with the destabilization effect on the jet, suggesting a strong relation between the eddy geometry and the jet's stability in this region. These findings may open a path toward a new method to parameterize the Reynolds stress with the background state, exploiting the shape equivalence between the eddies and the RS-ellipse.

Stability analysis of a thinning electrified jet under nonisothermal conditions.

The linear stability of a jet propagating under an electric field is analyzed under nonisothermal conditions. The electrified jet of a Newtonian fluid is modeled as a slender filament, and the leaky dielectric model is used to account for the Maxwell stresses within the fluid. The convective heat transfer from high-temperature jet to the surroundings results in formation of thicker fibers owing to increased viscosity upon cooling. The jet exhibiting substantial thinning under the action of tangential electric field is examined for stability toward axisymmetric nonperiodic disturbances. This is in contrast to most prior studies which analyzed the stability of a cylindrical jet of uniform radius without thinning under extensional flow by examining only periodic disturbances. Two case studies of reference fluids differing in viscosity and electrical properties are examined. The spectrum of discrete growth rates for axisymmetric disturbances reveal qualitatively distinct instabilities for the two fluids. For a fluid with high electrical conductivity, the conducting mode driven by the coupling of surface charges and an external electric field is found to be the dominant mode of instability. On the contrary, for low conductivity materials, the surface-tension-driven capillary mode is found to be the most critical mode. Heat transfer from the jet to the surroundings tends to stabilize both types of instability mode. Under sufficiently strong heat transfer, the axisymmetric instability, which is believed to be responsible for producing nanofibers with diametric oscillations in electrospinning process, is suppressed. The stabilization is attributed to the enhancement of viscous stress in the thinning jet upon cooling. It is observed that the stabilization effect is relatively more pronounced in a thinning jet compared to the cylindrical jet of uniform radius. The effects of various material and process parameters on the stability behavior is also examined.

High-Speed Video Analysis of the Process Stability in Plasma Spraying

In plasma spraying, instabilities and fluctuations of the plasma jet have a significant influence on the particle in-flight temperatures and velocities, thus affecting the coating properties. This work introduces a new method to analyze the stability of plasma jets using high-speed videography. An approach is presented, which digitally examines the images to determine the size of the plasma jet core. By correlating this jet size with the acquisition time, a time-dependent signal of the plasma jet size is generated. In order to evaluate the stability of the plasma jet, this signal is analyzed by calculating its coefficient of variation cv. The method is validated by measuring the known difference in stability between a single-cathode and a cascaded multi-cathode plasma generator. For this purpose, a design of experiment, covering a variety of parameters, is conducted. To identify the cause of the plasma jet fluctuations, the frequency spectra are obtained and subsequently interpreted by means of the fast Fourier transformation. To quantify the significance of the fluctuations on the particle in-flight properties, a new single numerical parameter is introduced. This parameter is based on the fraction of the time-dependent signal of the plasma jet in the relevant frequency range.

Coaxial helical gas assisted laser water jet machining of SiC/SiC ceramic matrix composites

• A novel coaxial helical gas atmosphere is firstly introduced to extend the stable length of water jet as well as expel the accumulated water layer during the machining of LWJ. • The influences of gas component and pressure on the stable length of water jet and accumulated water layer status on substrate surface are revealed. • Scribing and through cutting of CMCs substrates with thickness of 3 mm are realized by CGALWJ machining without the drawing of SiC fibers, formation of recast layer and delamination. SiC/SiC ceramic matrix composites (CMCs) offer an excellent combination of properties at high temperature such as high specific strength, chemical inertness and irradiation tolerance. Those superior properties make CMCs beneficial for use in high-temperature structural applications that are exposed to extreme environments such as aerospace and nuclear energy. However, well machining qualities can hardly be achieved by conventional machining techniques owing to these properties. Laser water jet (LWJ) machining is a promising solution, which is capable of ablating materials with less/no heat defects, well machining precision and consistency. Nevertheless, the machining capacity of LWJ is still limited by the stability of water jet to a great extent. A water layer may form on substrate surface during the impingement of LWJ, which also sets up obstacle for sufficient ablation. Therefore, a novel coaxial helical gas atmosphere is introduced to promote the machining capacity of LWJ in this paper. A theoretical model is established to describe the gas-water two-phase flow field during the ejection and impingement of coaxial gas assisted LWJ (CGALWJ). The influences of gas component and pressure on the stable length of water jet and surface water layer status are analyzed based on numerical simulations and experiments. Scribing experiments are further carried out on CMCs substrates with thickness of 3 mm. Groove with maximum depth-to-width ratio of 13.6 as well as through cutting are realized without the drawing of SiC fibers, formation of recast layer and delamination. The theoretical and experimental results provide solid foundation for the high-quality machining of ceramic matrix composites and other hard-to-process materials.

Numerical simulations of jets

When astrophysical jets were discovered one hundred years ago, the field of numerical simulations did not yet exit. Since the arrival of programmable computers though, numerical simulations have increasingly become an indispensable tool for dealing with “tough nut” problems which involve complex dynamic and non-linear phenomena. Astrophysical jets are an ideal example of such a tough nut, where multi-scale plasma physics, radiative and non-thermal processes, turbulence and relativity combine to present a formidable challenge to researchers.Highlighting major achievements obtained through numerical simulations concerning the validity and nature of the Blandford–Znajek mechanism, the launching, collimation, acceleration and stability of jets, their interaction with the surrounding plasma, jet-galaxy feedback mechanisms etc., we trace how the field developed from its first tentative steps into the age of “maturity”. We also give a brief and personal outlook on how the field may evolve in the foreseeable future.

Fabrication of durable corrosion-resistant polyurethane/SiO2 nanoparticle composite coating on aluminium

In this work, SiO2 nanoparticle–embedded polyurethane superhydrophobic coating on aluminium substrate was fabricated using spin coating technique. By embedding SiO2 nanoparticles into the polyurethane matrix, superhydrophobicity is successfully achieved with water static contact angle of 156 ± 3° and tilt angle of 6 ± 1°. Coating exhibits excellent self-cleaning and antifouling properties. Wetting stability of coating was further evaluated by performing water jet, bending, sand falling, tape peeling, floating, annealing, and chemical stability tests. It is found that coating is mechanically, thermally, and chemically stable. Water droplet impact test shows the bouncing, pinning, and splashing behaviour of water droplets at different impact velocities. The electrochemical experiment has demonstrated that the corrosion potential of aluminium after coating increases and the corrosion current density decreases, revealing the corrosion resistance property of coating. We envision that the self-cleaning, antifouling, and anticorrosive superhydrophobic coating of the polyurethane/SiO2 nanoparticle composite can effectively prevent the corrosion of aluminium and the aforesaid coating has great industrial applications.

Zonal jets at the laboratory scale: hysteresis and Rossby waves resonance

Abstract

Turbulent jet stability increased by ribs inside the nozzle – Stereo PIV measurement one diameter past the nozzle

We observe that decreasing the inner nozzle surface by adding longitudinal ribs increases the jet stability in terms of the amount of turbulent kinetic energy in the near shear layer. We try to explain our observation as a stabilization effect of secondary flow vortices emerging in the corners of the ribs. These stream-wise vortices damage the development of larger-scale structures in the near shear layer. This explanation is supported by autocorrelation function of the stream-wise velocity component, which displays slightly smaller integral length-scale in the case with ribs than in the case of smooth nozzle. The experiment is performed at Reynolds number 2.2 × 105 (based on the nozzle diameter 50 mm); the Stereo-PIV (Particle Image Velocimetry) measurement takes place at the plane perpendicular to the jet axis one diameter past the nozzle exit. Optical 3D scanner controls the real nozzle geometry. This article presents preliminary measurement at single position and single velocity only; further exploration of this problem is needed.

Turbulence and Particle Acceleration in Shearing Flows

Abstract We explore constraints imposed by shear-driven instabilities on the acceleration of energetic particles in relativistic shearing flows. We show that shearing layers in large-scale AGN jets are likely to encompass a sizeable fraction (≳0.1) of the jet radius, requiring seed injection of GeV electrons for efficient acceleration. While the diffusion process may depend on predeveloped turbulence if injection occurs at higher energies, electron acceleration to PeV and proton acceleration to EeV energies appears possible within the constraints imposed by jet stability.