Stability Of Jet Research Articles

Motive Characteristics and Adhesion-Rebound Mechanism of Single Aggregate Based on Shotcrete

The theory of viscoelastic collision mechanics is used to explore the rebound and adhesion mechanisms of single aggregates in shotcrete at the meso-level in this study. Abrasive balls with different radiuses were used, fully considering the characteristics of the aggregates and the sprayed wall, to build multifactor physical models for the shotcrete rebound of the aggregate. The movement trajectories, velocities, and energy dissipations of abrasive balls are analyzed, and the shotcrete-rebound mechanical model of a single aggregate based on wet-mix shotcrete is proposed. The results of the study show that there are obvious differences in the rebound velocity of the aggregate in different collision forms, and the viscoelastic properties of aggregates affect the jet stability of wet-mix shotcrete. According to the quantitative and qualitative analyses, the energy loss during the collision-rebound of the aggregate is inversely proportional to the rebound velocity and proportional to the rebound resistance. The functional relationship between the resistance F and the particle radius R during the rebound and adhesion of aggregate can be expressed as F=CR1/5, where C is a constant.

Aerothermal comparison of a diffuser hole and a slot geometry: thermal performance and jet stability

Numerous experimental investigations, predominantly determining the adiabatic film cooling effectiveness and the heat transfer coefficient with film cooling dependent on parameters such as blowing ratio (M), density ratio (DR), velocity ratio (VR) and momentum flux ratio (IR) have been conducted in the past for a vast variety of geometries. To fully characterize a jet in cross-flow for its application in film cooling, thermal and aerodynamic parameters have to be analysed in conjunction. In the present work, detailed flow field measurements using Stereoscopic Particle Image Velocimetry (SPIV) are conducted at engine-realistic operating conditions in a test rig at the Institute of Thermal Turbomachinery (ITS) at the Karlsruhe Institute of Technology (KIT). Additionally, thermal measurements using Infrared Thermography (IRT) are carried out to determine the adiabatic film cooling effectiveness and the heat transfer coefficient with film cooling. Two film cooling hole geometries are analysed: A 10°-10°-10° laidback fan-shaped shaped film cooling hole (LFH10) and a slot geometry with a plenum-like inlet. Flow field data, and thermal film cooling quantities are analysed to quantify and characterize their aero-thermal behaviour. The significance of the stability and continuity of the exiting film cooling jets for the thermal performance is demonstrated to comprehend the differences in how efficiently the provided cooling air is used. It is shown that the absence of the counter-rotating vortex pair may not necessarily lead to an improved adiabatic film cooling effectiveness <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>η</mml:mi><mml:mrow><mml:mrow><mml:mi mathvariant="normal">a</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">W</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:math></inline-formula>.

Spatiotemporal signatures of elastoinertial turbulence in viscoelastic planar jets

The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertio-elastic instabilities. We show how fluid elasticity has a nonmonotonic effect on jet stability depending on magnitude, creating two distinct regimes. The nonlinear evolution of these instabilities generates a state of elasto-inertial turbulence (EIT) with different spatiotemporal features than Newtonian turbulence. We use high-speed digital schlieren imaging and dynamic mode decomposition to quantify EIT and identify two modes of instability which can lead to a transition to turbulence at a lower Reynolds number with flow-aligned structures in the turbulent region.

On the stability of relativistic two-component AGN jets

ABSTRACT A number of observations of astrophysical jets, at different scales, have shown that jets are often non-uniform outflows in their cross-section. Their structure is believed to play an important role in their overall stability. In this work, we combine analytical methods and numerical simulations to investigate the stability of non-uniform jets originating from active galactic nuclei. We adopt a standard ‘spine and sheath’ model, using a fast, light inner spine and a heavier, slower outer sheath. In the first part of this work, we conduct a linear stability analysis, finding the time-scales for the growth of the instabilities and the corresponding eigenfunctions. We focus on the nature of the physical processes that dominate and drive the destabilization of configurations. In the second part, we examine the evolution of the perturbed jets through relativistic 3D numerical simulations using the pluto code. Starting with the eigenfunctions found in the first part as initial conditions, we derive instability growth times and evolution which are in good agreement with the linear analysis.

Stability of jets and wakes confined by compliant walls

The absolute instability of jets and wake flows have often been studied when either unconfined in the flow normal direction, or when confined by solid, impermeable walls. In this study, we confine the flow by compliant, or flexible, walls that are only able to move in the flow normal direction. By using a local instability analysis, we find that wall compliance can have a significant effect on existing flow instabilities and can even induce new absolutely unstable modes.

Dynamic catcher for stabilization of high-viscosity extrusion jets.

A 'catcher' based on a revolving cylindrical collector is described. The simple and inexpensive device reduces free-jet instabilities inherent to high-viscosity extrusion injection, facilitating delivery of microcrystals for serial diffraction X-ray crystallography.

Study of the electric arc dynamics in a cascaded-anode plasma torch

In plasma spraying process, the plasma jet stability depends on the electric arc movement inside the plasma torch. For conventional plasma torches with rod cathode and a large anodic surface, arc roots are free to move along the anode wall, leading to high plasma jet fluctuations. To overcome these instabilities, cascaded-anode plasma torches have been developed, reducing considerably the electric arc movement and thus the plasma jet fluctuations. Unfortunately, a small number of studies reports on the behaviour of theses torches including the arc instabilities and on electrode erosion process as well. In this work, several diagnostics have been set up to further understand the cascaded-anode plasma torch behaviour under different sets of operating conditions. Influence of plasma forming gas composition on the electric arc dynamic has been studied through anodic imaging. The issue of the intense plasma radiation masking is discussed since the light emitted by the arc column interferes with anodic arc roots observations. Multiple constricted arc roots have been observed and a preferential site for anodic arc root has been identified for all experimental conditions. The addition of a secondary plasma forming gas promotes a higher number of reattachments. The influence of the electric arc movement on plasma jet stability is also studied by following the plasma jet brightness fluctuations. The low voltage fluctuation frequency is also observed on brightness spectra, which means that these fluctuations could have an impact on in-flight particle treatment.

Investigation of the stability of a planar Oldroyd-B jet

Investigation of the stability of a planar Oldroyd-B jet

Marangoni destabilization of bidimensional-confined gas–liquid co-flowing streams in rectangular microfluidic channels

In microchannels, the stability of a fluid jet injected into another immiscible fluid strongly depends on its degree of geometric confinement. When the width of the jet, w, is larger than the channel height, H, the surface tension driven Rayleigh–Plateau instability is suppressed so that the 2D (bidimensional)-confined jet is absolutely stable and never collapses into bubbles (or drops) in contrast to what occurs when w ≤ H [Dollet et al., “Role of the channel geometry on the bubble pinch-off in flow-focusing,” Phys. Rev. Lett. 100(3), 034504 (2008); Guillot et al., “Stability of a jet in confined pressure-driven biphasic flows at low Reynolds number in various geometries,” Phys. Rev. E 78(1), 016307 (2008)]. We here demonstrate both experimentally and theoretically that this picture is, indeed, no longer valid when Marangoni effects are considered. We experimentally show that the addition of small length alcohol molecules into the liquid phase destabilizes a 2D-confined gas–water microfluidic stream ( w > H), leading to the generation of steady non-linear waves and further to the production of bubbles. Using a simple hydrodynamic model, we show through a linear analysis that the destabilization of the gas stream may result from a Marangoni instability due to the fast adsorption of the alcohol molecules, which occurs on a timescale comparable to that of the microfluidic flow.

Fast on–off controlling of electrohydrodynamic printing based on AC oscillation induced voltage

Stability control of electrohydrodynamic (EHD) printing technology is urgent needed for efficient fabrication of flexible electronics. In this study, a new fast on–off controlling technology for micro droplets of EHD is proposed by applying an AC induced voltage. The suspending droplet interface is broken through quickly, and the impulse current can be significantly reduced from 527.2 to 50.14 nA, which greatly reduces its negative impact on jet stability. What’s more, time interval of jet generation can be shortened by a factor of three, while not only significantly improving the uniformity of the droplets, but effectively reducing the droplet size from 195 to 104 μm. Moreover, the controllable and mass formation of micro droplets are realized, but also the structure of each droplet is able to be controlled independently, which promoted the development of EHD printing technology in more fields.

The Elasticity of Polymer Melts and Solutions in Shear and Extension Flows.

This review is devoted to understanding the role of elasticity in the main flow modes of polymeric viscoelastic liquids-shearing and extension. The flow through short capillaries is the central topic for discussing the input of elasticity to the effects, which are especially interesting for shear. An analysis of the experimental data made it possible to show that the energy losses in such flows are determined by the Deborah and Weissenberg numbers. These criteria are responsible for abnormally high entrance effects, as well as for mechanical losses in short capillaries. In addition, the Weissenberg number determines the threshold of the flow instability due to the liquid-to-solid transition. In extension, this criterion shows whether deformation takes place as flow or as elastic strain. However, the stability of a free jet in extension depends not only on the viscoelastic properties of a polymeric substance but also on the driving forces: gravity, surface tension, etc. An analysis of the influence of different force combinations on the shape of the stretched jet is presented. The concept of the role of elasticity in the deformation of polymeric liquids is crucial for any kind of polymer processing.

High thermal performance of the solar air heater designs triggered by improved jet stability

In this experimental work, the thermal and thermohydraulic performance of two novel designs of solar air heater employing confined submerged circular air jet arrays is investigated. Design-I consists of a wavy transparent acrylic sheet for air jet impingement from the top side of the absorber plate, while Design-II consists of a wavy metallic impinging plate for air jet impingement from the bottom side of the absorber plate. Both solar air heaters are designed to induce stable air jets for the range of mass flow rates ranging from 0.01 kg/s to 0.04 kg/s. Obtained results presented the superiority of Design-I in terms of heat transfer augmentation to air and compactness in size compared to Design-II. The thermal and thermohydraulic efficiency of Design-I corresponding to the mass flow rate of 0.04 kg/s is obtained as 89.72% and 89.16%, which is found to be 24.33% and 24.52% high compared to Design-II, respectively. In addition, the recommended value of the mass flow rate is obtained as 0.03 kg/s on the basis of thermohydraulic efficiency and operating cost, that presents an operating cost of 0.109 INR/kWh for Design-I. This work is extended for the numerical investigation to reveal the associated physics of fluid and heat transfer in solar air heater designs. In numerical investigation, 3D structured O-grid mesh is generated and RNG k-ε turbulence model is employed using the commercial CFD tool.

Two-dimensional numerical study of effect of magnetic field on evolution of laser-driven jets

Astrophysical jets are highly collimated supersonic plasma beams distributed across various astrophysical backgrounds. The triggering mechanism, collimation transmission, and stability of jets have always been a research hotspot of astrophysics. In recent years, observations and laboratory research have found that the magnetic field plays a crucial role in jet collimation, transmission, and acceleration. In this work, the two-dimensional numerical simulation of the jet in front of the CH plane target driven by an intense laser is carried out by using the open-source MHD FLASH simulation program. We systematically investigate the dynamic behaviors of jet evolution caused by the Biermann self-generated magnetic field, the external magnetic field with different directions and initial strengths and compare them with each other. Simulation results show that the Biermann self-generated magnetic field does not affect the jet interface dynamics. The external magnetic field has a redirecting effect on the plasma outflow. The external magnetic field, which is parallel to the direction of the plasma outflow center in front of the target, is conducive to the generation and collimation of the jet. The evolution of the jet goes through three stages: antimagnetic ellipsoid cavity, conical nozzle, and collimated jet. Its formation process and evolution process result from competition among plasma thermal, magnetic, and ram pressure. In terms of force, plasma thermal pressure gradient and magnetic pressure forces play a decisive role in the jet evolution process. The presence of magnetic pressure significantly limits the radial expansion of the jet to achieve axial collimation transmission. The length-diameter ratio of the jet is positively correlated with the initial axial applied magnetic field intensity. In addition, we observe in the simulation that there are many node-like structures in the jet evolution zone, similar to the jet node in YSO. The results provide a reference for future experimental research related to jets and contribute to a more in-depth understanding of the evolution of celestial jets.

NONLINEAR EHD STABILITY OF ENCLOSED COAXIAL JETS WITH MASS AND HEAT TRANSMISSION IN THE EXISTENCE OF AN OSCILLATORY GAS VELOCITY PROFILE

In this paper, the present work investigates a nonlinear temporal instability of coaxial cylinders in porous media in the existence of an oscillating gaseous velocity. Heat and mass transfer (MHT), as well as a uniform axial electric field were all taken into account. The viscous potential flow (VPF) was employed to calculate liquid and gas velocities to make the mathematical manipulations easier. As a result of the analysis of the boundary-values problem, the cylindrical interface displacement provided a nonlinear characteristic equation. To achieve the stability inspection, a novel approach was created. The article designates both the oscillatory and uniform streaming gas. The nonlinear analysis was completed using the homotopy perturbation method (HPM), which resulted in a Klein-Gordan equation, to arrive the stability maps. Additionally, the resonance and non-resonance cases were accomplished. In the previous situations, the stability standards were theoretically derived and numerically proven using regular diagrams. It was found that the unchanging flowing had a twofold effect. The linear MHT parameter was identified to perform a twofold character in the stability setup. On the other hand, nonlinear parameters have opposing effects. The outcomes of the homogeneous gas velocities were substantially conforming.

Supersonic gas jet stabilization in laser–plasma acceleration

Abstract Supersonic gas jets generated via a conical nozzle are widely applied in the laser wakefield acceleration of electrons. The stability of the gas jet is critical to the electron injection and the reproducibility of the wakefield acceleration. Here we discussed the role of the stilling chamber in a modified converging–diverging nozzle to dissipate the turbulence and to stabilize the gas jets. By the fluid dynamics simulations and the Mach–Zehnder interferometer measurements, the instability originating from the nonlinear turbulence is studied and the mechanism to suppress the instability is proposed. Both the numerical and experimental results prove that the carefully designed nozzle with a stilling chamber is able to reduce the perturbation by more than 10% compared with a simple-conical nozzle.

Study on Dynamic Performance and Induced Breakdown of the Plasma-Triggered-Based Protective Gas Gap Switch

The plasma-triggered-based protective gas switch (PTGS) has obvious advantages in the control switch application of fast energy dissipation device used in the hybrid dc transmission projects. However, the high-pressure environment critically limits the plasma jet ability, easily leading to the trigger failure. Therefore, this article establishes a plasma-jet-triggered experimental platform to study the dynamic performance and induced breakdown laws of PTGS. It is shown that the evolution of plasma jet is divided into three stages: plasma jet of high-pressure, saturated stability, and dissipation. The induced breakdown mainly occurs in the high-pressure jet and saturated stability stage. The initial plasma jet velocity increases, as the energy storage capacitance and the charging voltage increase, making it faster to reach the jet length for induced breakdown, resulting in a significant reduction in switch-on time and improving the PTGS trigger stability. The increase in trigger gas pressure inhibits the plasma jet ability, but improves the electrical strength of PTGS. In order to achieve induce breakdown of PTGS, the plasma jet must reach the critical jet length. SF6 gas improves the insulation strength of PTGS and prevents the formation of plasma jet. To achieve stable trigger conduction, the trigger energy of PTGS must be increased. The findings can be used to develop theoretical guidance for improving the working performance of the high-voltage and large-capacity protective gas gap switches (GGSs).

A review of additives in abrasive water jet machining and their performance

Abrasive Water Jet Machining (AWJM) is an advanced machining process that has a variety of applications, including machining of hard-to-machine materials, drilling, milling, coating removal, polishing, etc. They are attractive because of their versatility, absence of heat-affected zones, low thermal-distortions, low cutting-forces, and environment-friendly nature. However, use of plain-water in AWJM poses some problems due to lack of jet coherence, poor suspension characteristics, and unsatisfactory performances in submerged-cutting and cutting of hollow surfaces. Countering these problems appropriately can lead to better cutting performances in terms of material removal rate, depth of cut, kerf-width, and kerf-taper. It can be achieved with addition of some additives in abrasive water-jet. These additives are mostly polymers of high molecular mass, which increases viscosity, suspension capability, and stability of jet, while reducing drag. In addition to AWJM, the effect of polymeric-additives in other water-based jets like Abrasive-Slurry-Jet (ASJ), High-pressure ASJ, etc. are also discussed. Therefore, this study aims to identify and list of different additives that are used in AWJM and other water-jet-based machining-processes over a past few decades, and to provide a run-through on their performances. It can provide the significant information to researchers working in fields related to waterjets.

Ultrasonic coupled abrasive jet polishing (UC-AJP) of glass-based micro-channel for micro-fluidic chip

The efficiency of abrasive jet polishing (AJP) is relatively low for glass-based micro-channels, and an ultrasonic coupled AJP (UC-AJP) approach was thus proposed. The fluid mechanic model of the UC-AJP was established to reveal the ultrasonic enhancement law and the optimal design parameters. The calculation results indicated that ultrasonic vibration could help significantly improve the turbulent kinetic energy and impact erosion that had an important role in jet stability and polishing efficiency. High-speed photograph and polishing experiments with different amplitudes were then performed to explore the performance of the proposed UC-AJP method. It was found that for glass-based micro-channel UC-AJP produced a smoother surface and higher polishing efficiency than AJP. There is an optimal amplitude range in the UC-AJP process, which can maximize the polishing efficiency for the micro-channel. Such improvements were ascribed to the synergistic effect of the pulse and cavitation as a result of the formation of pulsed abrasive jet.

Data-driven global stability of vertical planar liquid jets by dynamic mode decomposition on random perturbations

A data-driven approach to estimate the global spectrum of gravitational planar liquid jets (sheet or curtain flows) is presented in this work. The investigation is carried out by means of two-dimensional numerical simulations performed through the solver BASILISK, based on the one-fluid formulation and the volume-of-fluid approach. The dynamic mode decomposition technique is applied to extract the underlying linear operator, considering random perturbations of the base flow. The effectiveness of this procedure is first evaluated comparing results with those of a simplified one-dimensional curtain model in terms of spectrum and eigenfunctions. The methodology is then applied to a two-dimensional configuration obtaining the BiGlobal spectra for both supercritical (Weber number We > 1) and subcritical (We < 1) regimes. Results highlight that in supercritical regime, the spectrum presents three branches: the upper and lower ones exhibit a purely sinuous behavior with frequencies quite close to those predicted by the one-dimensional model; the middle branch presents a predominant varicose component, increasing with the frequency. The subcritical spectrum, instead, shows that the first two less stable eigenvalues, sorted by increasing frequency, exhibit, respectively, a sinuous and a varicose behavior, while their growth rate is almost the same. As expected, the subcritical regime does not reveal the slow branch. The effect of the density ratio, rρ, between the two phases is investigated, revealing that the flow system is unstable for rρ>0.05. Topological inspections of the leading modes in this unstable configuration show that the predominance of a varicose behavior is related to the rupture of the curtain.

Heat Transfer and Fluid Dynamics Study in Solar Air Heater Employing Impinging Circular Air Jet Array for Effective Jet Stability

Abstract This experimental and numerical investigation is intended to increase heat transfer, reduce pumping power, and present important guidelines on the use of confined submerged air jet impingement in solar air heater designs that employ an array of circular air jets for high thermohydraulic performance. In order to increase thermohydraulic performance, a novel approach is adopted, i.e., by improving jet stability and consequently jet deflection that gives origination to four solar air heater designs for this investigation. The obtained numerical results revealed that thermohydraulic performance improves significantly by stabilizing the air jet array, which means lowering the jet deflection. The thermohydraulic performance of design-I is high compared to design-II, design-III, and design-IV, both at equal mass flow rate and Reynolds number. Moreover, design-I presents 20%, 33%, and 52% relative improvement in deviation angle of the jet core compared to design-II, design-III, and design-IV, respectively, at a minimum mass flow rate of 0.01 kg/s and Reynolds number of Re=2000, respectively. In addition, correlations to define heat transfer and fluid dynamics characteristics are established. An important guideline upon the use of solar air heater with jet impingement is that a solar air heater of short length and height (jet and plate spacing), and large width should be used for high thermohydraulic performance.