Stability Of Jet Research Articles

Fluid Viscosity and Corresponding Effects on Fluid flow, Velocity Magnitude and Electric Field Distribution in Electrohydrodynamic Jetting.

The phenomenon of electrohydrodynamic (EHD) jetting is affected by both the flow and electric properties of the dielectric fluid. A computational fluid dynamics (CFD) approach has been used to analyse the resultant effect of fluid viscosity on EHD flows. This study looks at the unique effect of fluid viscosity on the flow profile, velocity magnitude and the electric field distribution. It is very difficult to experimentally study these relationships, but CFD provides insights that can open the world of Electrohydrodynamics to new levels of applications; as this will give an idea of how to manipulate the jet formation. Viscosity has been highlighted as one of the key parameters that aid jet elongation and stability. Therefore, the necessity to study its role. Most studies have been carried out experimentally, but this paper provides computation insights. To solve the multiphase problem, a finite volume method using the Volume of Fluid approach for capturing the shape of the interface was used for the investigation. The leaky dielectric model which describes the process solves the combination of the Charge Transport Model and Navier-Stokes equations simultaneously. The transient liquid-gas interface tracking was achieved using the VOF technique. Among many other features observed, the results showed that it takes more time for the electric field to overcome the opposing surface tension of the solution at higher viscosity than at lower viscosity. Higher electric field magnitudes/strength were observed for the fluid of lower viscosity than that of the higher viscosity. Also, an increase in the viscosity reduces the droplet size/jet diameter.

Effect of fiber suspension jet stability on alignment quality of discontinuous carbon fiber tapes

A hydrodynamic alignment process has been developed for converting discontinuous random carbon fibers into tapes with a highly aligned orientation distribution to greatly improve the applicability of recovered fibers to composite parts. In hydrodynamic alignment processes short fibers are aligned by the velocity gradient along the flow direction in a convergent nozzle. Thereafter the jet of fiber suspension is deposited on a nylon mesh and the now redundant dispersion medium is drained away to leave an aligned fiber tape. The fundamental physical principles at work in the process have not been widely studied and are shown in the present work to greatly influence the properties of the resulting materials. In this work, the influence of suspension jet stability on the fiber orientation distribution was examined and the liquid jet break-up regime was determined. To explore the factors which can affect the suspension jet stability, different nozzle geometries, viscosities of dispersion media, fiber lengths and Reynolds numbers were applied in experimental work. The shear rate profiles inside different nozzles were simulated by Computational Fluid Dynamics methods and the results described in this paper.

Direct writing of liquids by micro dispensing: Stability and shape of laminar jets with high Froude numbers

Nowadays, a wide range of industrial manufacturing processes require high precision for product manufacturing. Micro dispensing is concerned with jetting of liquids through nozzles with a typical diameter on the order of magnitude of 100 microns. In this work, various aspects of micro dispensing of a Newtonian liquid are studied, including jet break-up and the (equilibrium) profile of a printed line on a substrate. Contrary to the investigations of others, this work is concerned with jets of high Froude (Weber) numbers in the range of 40≲Fr≲1500(1≲We≲100). Phenomenological correlations between the liquid properties, dispensing conditions and the jet break-up ratio are inferred based on dimensional reasoning and a comprehensive set of experiments. These correlations are used to create maps that indicate stable dispensing regimes. Furthermore, when the liquid is dispensed from the nozzle, the jet diameter usually contracts over the jet length. An analytical solution of the Navier-Stokes equation, valid to describe the shape of gravity-driven jets, is modified to obtain a correlation for inertia-driven jets. Finally, a correlation is derived to (numerically) obtain the three-dimensional profile of a liquid that is dispensed on a flat substrate.

Adjoint sensitivity and optimal perturbations of the low-speed jet in cross-flow

The tri-global stability and sensitivity of the low-speed jet in cross-flow are studied using the adjoint equations and finite-time horizon optimal disturbance analysis at Reynolds number $Re=2000$, based on the average velocity at the jet exit, the jet nozzle exit diameter and the kinematic viscosity of the jet, for two jet-to-cross-flow velocity ratios $R=2$ and $4$. A novel capability is developed on unstructured grids and parallel platforms for this purpose. Asymmetric modes are more important to the overall dynamics at $R=4$, suggesting increased sensitivity to experimental asymmetries at higher $R$. Low-frequency modes show a connection to wake vortices. Adjoint modes show that the upstream shear layer is most sensitive to perturbations along the upstream side of the jet nozzle. Lower frequency downstream modes are sensitive in the cross-flow boundary layer. For $R=2$, optimal analysis reveals that for short time horizons, asymmetric perturbations dominate and grow along the counter-rotating vortex pair observed in the cross-section. However, as the time horizon increases, large transient growth is observed along the upstream shear layer. When $R=4$, the optimal perturbations for short time scales grow along the downstream shear layer. For long time horizons, they become hybrid modes that grow along both the upstream and downstream shear layers.

On the viscous instabilities and transitions of two-layer model with a layered topography

In this article, the viscously-damped instability arising in the shear jet of west boundary layer governed by the two-layer quasi-geostrophic equation with a layered topography is analyzed. First, the nonlinear stability and the exponential stability of the shear jet is studied. More precisely, we derive an upper bound on the Reynolds number Re below which the shear jet is not only locally nonlinearly stable but also globally exponentially stable. Second, it is shown that there exists a critical value of the Reynolds number Re above which the shear jet will become linearly unstable and there exists a dynamic transition in the west boundary layer. To shed light on the type of the dynamic transition, we reduce the two-layer quasi-geostrophic equation to a system of ODEs by making use of the technique of center manifold reduction. Then, we infer from this system of ODEs that the dynamic transition is of continuous type, leading to a stable periodic oscillation of west boundary layer currents. Finally, we investigate the effect of the slope of the bottom topography on the stability and transition of the shear jet. We find that although a large slope stabilizes the shear jet, it has no impact on the transition type.

Linear stability of Landau jet: non-parallel effects

We perform linear stability analysis of Landau jet using quasi-parallel approximation and self-similar approach. While the former detects only sinusoidal unstable modes, the latter also captures axisymmetric disturbances in agreement with previous results. The direct comparison of dispersion curves and eigenmode shapes for m = 1 and ReD ≈ 56 indicates that low frequencies are poorly described within quasi-parallel approximation making this approach questionable for a laminar-turbulent transition model.

Wall Effect on the Stability of Two-Dimensional Laminar Jet

Wall Effect on the Stability of Two-Dimensional Laminar Jet

Experimental study on working characteristics of direct current plasma jet igniter

The flameout of the combustor at high altitude poses a great threat to the flight safety of the aircraft. The low temperature and pressure as well as thin air at high altitude make it difficult to successfully and reliably reignite. The direct current plasma jet, one of the promising plasma assisted ignition technologies, has been applied in the different engines. In this paper, the working characteristics (mainly including discharge characteristics and jet characteristics) of direct current plasma jet igniter is investigated experimentally by using electric signal, synchronously acquired arc movement pictures, and high speed schlieren pictures of plasma jet in both static flow field and cross flow field. The influence of the gas flow rate and cross flow velocity on the working characteristics of direct current plasma jet igniter is also studied. Results show that there are three stages (jet formation stage, jet expansion stage, and jet stability stage) form the breakdown of the igniter electrodes to the stable distribution of the jet in the combustion section. The operating voltage and fluctuation frequency rises significantly (from 92.55 V to153.11 V, and from 9.2 kHz to17.3 kHz), the influence area and penetration depth of plasma jet increased in cross flow environment with the increase of the gas flow rate. As the cross flow velocity increases, the three-dimensional fluctuation of arc movement is more obvious, the influence area and penetration depth of plasma jet decreases quickly. Both the discharge characteristics and jet characteristics affect the ignition capability of plasma jet igniter, and this study has certain guiding significance for the design of direct current plasma jet igniter.

Harsh environment resistant - antibacterial zinc oxide/Polyetherimide electrospun composite scaffolds.

Strategies for developing materials with the functionality to combat bacterial infection are targets for applications such as smart bandages and bone tissue integration. This work milestone was to develop ZnO-polyetherimide (ZnO/PEI) composite scaffolds with antibacterial activity against gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacteria. The electrospinning process using suspensions of PEI with different ZnO nanoparticles content were heightened to promote spinnability, jet stability, and fibers with morphological homogeneity. Simulating harsh environments (laser ablation and solvent corrosion) was employed onto specimens and antibacterial functionality, morphology variations, contact angle, and tensile strength variability were evaluated. The antibacterial outcomes, accessed by a modified version of the Japanese Industrial Standard (JIS) Z 2801, presented an inhibition rate up to 100 and 99% after 24 h for S. aureus and E. coli, respectively. The treated samples presented alike responses against both bacteria, highlighting the robustness of the developed scaffolds.

Jovian Vortices and Jets

We explore the theory of isolated vortices in strongly sheared, deep zonal flows and the stability of these banded jets, as occur in Jupiter’s atmosphere This is done using the standard 2-layer quasigeostrophic model with the lower layer depth becoming infinite; however, this model differs from the usual layer model because the lower layer is not assumed to be motionless but has a steady configuration of alternating zonal flows. Steady state vortices are obtained by a simulated annealing computational method as generalized to fluid problems with constraints and also used in the used in the context of magnetohydrodynamics. Various cases of vortices with a constant potential vorticity anomaly atop zonal winds and the stability of the underlying winds are considered using a mix of computational and analytical techniques.

Effects of drive amplitude on continuous jet break-up

We develop a one-dimensional model of jet breakup in continuous inkjet printing to explore the nonlinear behavior caused by finite-amplitude modulations in the driving velocity, where jet stability deviates from classic (linear) “Rayleigh” behavior. At low driving amplitudes and high Weber numbers, the spatial instability produces drops that pinch-off downstream of the connecting filament, leading to the production of small satellite droplets between the main drops. On the other hand, we identify a range of driving amplitudes where pinching becomes “inverted,” occurring upstream of the filament connecting the main drops, rather than downstream. This inverted breakup is preferable in printing, as it increases the likelihood of satellite drops merging with the main drops. We find that this behavior can be controlled by the addition of a second harmonic to the driving signal. This model is in quantitative agreement with a full axisymmetric simulation, which incorporates nozzle geometry.

Enhancing the penetration stability of plasma jet in liquid by improving the chamber structure

Two kinds of chamber structures were designed to improve the penetration stability of plasma jet in the working fluid. They are the cone-truncated chamber with an inclined wall structure and the stepped-wall chamber with an abrupt expanding structure. Based on the experiment, the penetration characteristics of plasma jet in liquid were investigated by the numerical calculation, especially the multiphase flow field structure and the jet penetration stability. In the cone-truncated chamber, the arc-like pressure wave does not evolve into the plane wave like the wave evolution rule in the traditional cylindrical chamber, and an alternating structure of arc-like wave and the plane wave is formed in the stepped-wall chamber. The stability control of stepped wall is more significant than that of inclined wall because the low-pressure vortex formed at the corner of step enhances the radial induction effect. Besides, the stepped-wall structure can restrain the moving randomness of the big vortex's core. The axial expansion speed of plasma jet in the ambient fluid is decreased exponentially and the speed estimation formula was obtained in this work.

Finite element analysis of nozzle selection based on high pressure water jet technology

High-pressure water jet technology is an emerging type of welding joint strengthening process in recent years, which has unique advantages such as non-polluting, high efficiency, and can strengthen some complicated welds that in dead end position such as fillet welds. However, there is no supporting water jet strengthening nozzle domestically, so this paper uses the computational fluid dynamics method to simulate the internal and external flow fields of the nozzle, which is the most commonly used four conical convergence nozzles with different structure of the current water jet technology, making the flow field velocity distribution of the four nozzles and the impact pressure on the target surface compared and analysed. The results show that the impact pressure and axial speed of the conical short-line nozzle are slightly smaller than that of the conical (13°) nozzle, but conical short-line nozzle’s jet collection property and stability are superior to the other types of nozzles. Therefore, the conical short-line nozzle should be selected when the high-pressure water jet technology is utilized for welding joint reinforcement.

Acoustic modes in jet and wake stability

Motivated by recent studies that have revealed the existence of trapped acoustic waves in subsonic jets (Towneet al.,J. Fluid Mech., vol. 825, 2017, pp. 1113–1152), we undertake a more general exploration of the physics associated with acoustic modes in jets and wakes, using a double vortex-sheet model. These acoustic modes are associated with eigenvalues of the vortex-sheet dispersion relation; they are discrete modes, guided by the vortex sheet; they may be either propagative or evanescent; and under certain conditions they behave in the manner of acoustic-duct modes. By analysing these modes we show how jets and wakes may both behave as waveguides under certain conditions, emulating ducts with soft or hard walls, with the vortex-sheet impedance providing effective ‘wall’ conditions. We consider, in particular, the role that upstream-travelling acoustic modes play in the dispersion-relation saddle points that underpin the onset of absolute instability. The analysis illustrates how departure from duct-like behaviour is a necessary condition for absolute instability, and this provides a new perspective on the stabilising and destabilising effects of reverse flow, temperature ratio and compressibility; it also clarifies the differing symmetries of jet (symmetric) and wake (antisymmetric) instabilities. An energy balance, based on the vortex-sheet impedance, is used to determine stability conditions for the acoustic modes: these may become unstable in supersonic flow due to an energy influx through the shear layers. Finally, we construct the impulse response of flows with zero and finite shear-layer thickness. This allows us to show how the long-time wavepacket behaviour is indeed determined by interaction between Kelvin–Helmholtz and acoustic modes.

Chitosan & Conductive PANI/Chitosan Composite Nanofibers - Evaluation of Antibacterial Properties

Background: Within the healthcare industry, including the care of chronic wounds, the challenge of antimicrobial resistance continues to grow. As such, there is a need to develop new treatments that can reduce the bioburden in wounds. Objective: The present study is focused on the development of polyaniline (PANI) / chitosan (CH) nanofibrous electrospun membranes and evaluates their antibacterial properties. Methods: To this end, experimental design was used to determine the electrospinning windows of both pure chitosan and PANI/CH blends of different ratios (1:3, 3:5, 1:1). The effect of key environmental and process parameters (relative humidity and applied voltage) was determined, as well as the effect of the PANI/CH ratio in the blend and the molecular interactions between PANI and chitosan that led to jet stability. Results: The nanofibrous mats were evaluated regarding their morphology and antibacterial effect against model gram positive and gram negative bacterial strains, namely B. subtilis and E. coli. High PANI content mats show increased bactericidal activity against both bacterial strains. Conclusion: The blend fibre membranes combine the materials’ respective properties, namely electrical conductivity, biocompatibility and antibacterial activity. This study suggests that electrospun PANI/CH membranes are promising candidates for healthcare applications, such as wound dressings.

Impacts of Free-falling Spheres on a Deep Liquid Pool with Altered Fluid and Impactor Surface Conditions.

Vertical impacts of spheres on clean water have been the subject of numerous water entry investigations characterizing cavity formation, splash crown ascension and Worthington jet stability. Here, we establish experimental protocols for examining splash dynamics when smooth free-falling spheres of varying wettability, mass, and diameter impact the free surface of a deep liquid pool modified by thin penetrable fabrics and liquid surfactants. Water entry investigations provide accessible, easily assembled and executed experiments for studying complex fluid mechanics. We present herein a tunable protocol for characterizing splash height, flow separation metrics, and impactor kinematics, and representative results which might be acquired if reproducing our approach. The methods are applicable when characteristic splash dimensions remain below approximately 0.5 m. However, this protocol may be adapted for greater impactor release heights and impact velocities, which augurs well for translating results to naval and industry applications.

Measurement and Time Response of Electrohydrodynamic Direct-Writing Current.

The micro/nano current is an important characteristic to reflect the electrohydrodynamic direct-writing (EDW) process. In this paper, a direct-written current measurement system with a high signal to noise ratio was proposed to monitor the charged jets, providing the data basis for the promotion of stability and precision of the EDW jet. The electrical characteristics of the printing process were studied, the electrohydrodynamic direct-written current was associated with the stability of charged jet and the accuracy of direct-written patterns. There was an impulse current when the front end of the jet reached the collector and then a stable jet could be gained. With the increase of applied voltage, the severe fluctuation of measured current increased, the charged jet became more unstable and the accuracy of direct-written parallel lines was lower. The effects of processing parameters on direct-written current were also investigated. The average direct-written current at the stable stage increased as the applied voltage and polymer concentration increased, and it decreased as the distance from the nozzle to the collector increased. This research will promote the development and applications of EDW technology in the fields of micro/nano manufacturing.

Micro-machining with abrasive slurry-jets: effects of dissolved polymer concentration and nozzle design

The resolution of features machined with abrasive slurry jets is dependent on the diameter of the erosion footprint on the workpiece and on the stability of the jet. Previous work has shown that the footprint size decreases, but the jet oscillation increases, with increasing polymer concentration. The present study investigated the effect of nozzle design on jet stability in abrasive slurry-jet micro-machining of glass using slurries with dissolved polymers. A variety of transparent acrylic nozzles were made with different geometries and a 180-μm sapphire orifice to study the oscillation of jets of water mixed with high molecular-weight polyethylene oxide. Fluorescent streak photography of the flows within the nozzles confirmed the presence of unstable vortices that can lead to jet oscillation. However, the dependence of these vortices on nozzle geometry and PEO concentration was unclear. Jet oscillation was then measured photographically and found to increase nonlinearly with polymer concentration for all nozzle geometries. These results were used to design and manufacture a new steel nozzle with a 30° tapered entrance region which reduced jet oscillation. Micro-channels were then machined in glass plates using a range of polymer slurries containing aluminium oxide particles. The polymer concentration that produced the best combination of a relatively small footprint while simultaneously minimizing the jet oscillation amplitude resulted in channels that were 11% narrower than reference channels machined with a pure water slurry and a standard nozzle.

Mechanisms and modeling of electrohydrodynamic phenomena.

The purpose of this paper is to review the mechanisms of electrohydrodynamic (EHD) phenomenon. From this review, researchers and students can learn principles and development history of EHD. Significant progress has been identified in research and development of EHD high-resolution deposition as a direct additive manufacturing method, and more effort will be driven to this direction soon. An introduction is given about current trend of additive manufacturing and advantages of EHD inkjet printing. Both theoretical models and experiment approaches about the formation of cone, development of cone-jet transition and stability of jet are presented. The formation of a stable cone-jet is the key factor for precision EHD printing which will be discussed. Different scaling laws can be used to predict the diameter of jet and emitted current in different parametrical ranges. The information available in this review builds a bridge between EHD phenomenon and three-dimensional high-resolution inkjet printing.

Molecular Dynamics Simulation of a Jet in a Binary System at Supercritical Environment.

With the development of large-thrust liquid rocket engines, the behavior of liquid in supercritical conditions arouses increasing public interest. Due to the high pressure and temperature of the combustion chamber, fuel reaches its critical point much more easily, and enters supercritical conditions. Due to the drastic changes in the physical properties of the fluid near the critical point, it is usually difficult to simulate the fluid motion using traditional computational fluid dynamic methods; but molecular dynamics (MD) can simulate fluid motion at the molecular level. In view of the engineering application, the physical properties of a binary system consisting of argon and nitrogen, and the stability of subcritical jets sprayed into supercritical environment, has been studied here using the MD method. First, the molecular dynamic simulation of the equation of state (EOS) of the mixture was put forward. Four conditions, with different mixing ratios of nitrogen, were designed. The results showed that the mixing ratio of nitrogen noticeably affected the results; these results were compared with the Soave-Redich-Kwong (SRK) EOS. Second, a simulation was conducted of subcritical nitrogen jet sprayed into a supercritical argon environment. After analyzing the results, the jet density and temperature distributions were obtained and the disturbance growth rate of the shear layer was analyzed.