Instability Of Jet Research Articles

A high-adaptability nozzle-array printing system based on a set covering printing planning model for printed display manufacturing

Inkjet printing technology is expected to enhance printed display mass production technology in the future. Nozzle-array printheads form the basis for printed display mass production applications. However, jet instability caused by air bubble entrapment and nozzle wettability changes during the printing process is a major challenge in the application of this technology. To adapt to possible nozzle abnormalities, a high-adaptability nozzle-array printing system based on a set covering printing planning (SCPP) model for printed display manufacturing is designed in this study. The study consists of two parts. First, a printing system based on multistep visual inspection and closed-loop feedback is proposed to accurately detect and screen abnormal nozzle positions. Notably, the inkjet printing system can identify nozzles with abnormal ejection characteristics and ensure that the remaining nozzles work accurately and stably. Then, an SCPP model is established for display pixel printing planning by using the remaining normal nozzles on the nozzle-array printhead. This model can output the most efficient printing path and nozzle printing action and can adapt to any pixel pattern, nozzle type, and abnormal nozzle distribution. The system and technology are highly adaptable and scalable for fabricating large-area printed display devices.

DES AND RANS MODELING OF PRIMARY ATOMIZATION IN A COAXIAL SWIRLING LIQUID-GAS JET

This study assesses different turbulence modeling approaches for simulation of two-phase coaxial annular swirling jet flows. The problem selected from literature for comparison involves an analytical inlet profile for an annular liquid sheet sandwiched between two coaxial annular gaseous jets. The liquid-gas interface is resolved using the volume-of-fluid model with continuum surface force approximation. Reynolds-averaged Navier-Stokes simulations and detached eddy simulations (DES) are conducted to obtain transient multiphase numerical solutions. Different turbulence models explored include the <i>k-ε</i> renormalization group (RNG) with swirl modification, the Reynolds stress model (RSM), RSM with scale-adaptive simulation (RSM-SAS), and DES. Comparisons with the direct numerical results from literature suggest that the <i>k-ε</i> RNG and RSM approaches simulate only the streamwise shear of the liquid jet and are inadequate in capturing the swirling aspect of the jet flow and expected instabilities. DES can predict several expected features such as radial asymmetry, surrounding gas vortices causing jet instabilities, and eventual jet breakup with reasonable accuracy. While RSM-SAS predicts radial asymmetry, some jet instability, and is much more accurate than <i>k-ε</i> RNG and RSM, it fails to predict instabilities as good as DES and does not predict a complete jet breakup. RSM-based methods are found to be computationally very expensive compared to the <i>k-ε</i> RNG model, suggesting DES as the better alternative than RSM methods for such applications if resources are available.

Features of Dynamics and Instability of Plasma Jets Expanding into an External Magnetic Field in Laboratory Experiments with Compact Coaxial Plasma Generators on a Large-Scale “Krot” Stand

Features of Dynamics and Instability of Plasma Jets Expanding into an External Magnetic Field in Laboratory Experiments with Compact Coaxial Plasma Generators on a Large-Scale “Krot” Stand

Instability of wall jet produced by two-dimensional inclined jet

Instability of wall jet produced by two-dimensional inclined jet

Features of Dynamics and Instability of Plasma Jets Expanding into an External Magnetic Field in Laboratory Experiments with Compact Coaxial Plasma Generators on a Large-Scale “Krot” Stand

Structural instabilities that develop during pulsed injection of dense plasma jets into vacuum in the presence of an external quasi-homogeneous magnetic field are studied by high-speed photography using ICCD cameras. The experiments are carried out in the chamber of the “Krot” stand, which has record-breaking dimensions in its class of installations (diameter—3 m, length of the working section—10 m), and makes it possible to study plasma dynamics by various diagnostic methods at scales of more than 1 m both along the magnetic field and in the direction transverse to the magnetic field. During injection along the magnetic field, a transverse collimation of the flow of ionized matter and the development of a flute instability of the plasma boundary are observed, which, at the late stages of expansion, leads to the plasma leaving the injection region in the form of several jets across the field. During transverse injection, the formation of a collimated flow, a “plasma sheet,” is observed, in which, as the plasma moves across the field, inhomogeneous structures develop in the direction of injection.

Chemically reacting mixing in coaxial miscible liquid jets under variable viscosities and reaction rates

We study the mixing and reactions between two fluids with disparate viscosities in a co-axial flow at high Schmidt number, through numerical simulations. The chemical reactions chosen in this work are competitive-consecutive in nature. Numerical implementation of the reactions proceeds as given by Li and Toor (1986), in a Large-Eddy Simulation (LES) framework. The aim of the present study is to investigate the effect of reaction rate constant ratio on the product and byproduct formations under distinct viscosity ratios between the annular and central jets. Flow features promoting mixing and reactions such as turbulence, engulfment, jet instability, waves and break-up are described. The effect of viscosity difference between the jet and co-flow on the mixing and reactions are also delineated. It is found that selectivities of the undesirable product decrease with reducing viscosity ratio as well as increasing reaction rate constant ratio.

Computational study of the Transmission pole during downburst using ANSYS, considering fluid-structure interactions

A downburst is described as a strong downdraft that causes a devastating wind outburst on or near the earth surface. Such occurrences are more common during thunderstorms. Downbursts generally impinge on the ground and convect radially in all directions from the point of contact. Downbursts have been widely documented as a regular cause of high voltage electrical transmission tower and pole failures in numerous nations. The flow dynamics of impinging jets are investigated using numerical models, with applicability to downburst-related high intensity winds. The downburst is simulated using three primary viscous models (, , and RANS). These three viscous models are used to simulate downburst in two dimensions. The RANS viscous model is used for further investigation. To determine the temporal variant characteristics of a downburst, both steady and transient state simulations are performed. The flow is quasi-periodic, with vortex rings formed by the initial jet instability impinging on the surface, where they find an unstable separation reattachment of the boundary layer. Fluid-structure interactions (FSI) are multiphysics problems that cannot be handled using single physics equations. To determine the impacts of downburst on transmission poles, a 2-D steel pole 8 metres high with a fixed base is put near the downburst. Downburst pressure is measured on a transmission pole. The stresses and deflections caused by the pressure distribution on the tower due to varying fluid domain dimensions and jet velocities on the transmission poles are determined using one way coupling, from 2-D transmission pole geometry to 3-D transmission pole geometry.

Wavenumbers 3 and 4 Quasi 2‐day Wave Activities Observed by Multiple Meteor Radars in the Two Hemispheres During Austral Summer

AbstractCombining observations from three meteor radars in the Northern Hemisphere (NH) and a meteor radar in the Southern Hemisphere (SH) and reanalysis data, we present successive wavenumbers 3 and 4 (W3 and W4) quasi 2 day wave activities in austral summer. In the mesosphere and lower thermosphere, the wave exhibits two burst activities. The W3 dominates the first burst, while the W4 is the predominate mode in the second burst. The mode transition arises in the late January and is completed around the end of January. The W3 and W4 have the nearly same dominate period of about 46 hr and vertical wavelengths of 110 km in the SH and about 55–65 km in the NH, and the similar latitude and altitude distributions in the zonal and meridional winds and temperature, except stronger magnitude and farther extension in the NH for the W3. The analysis indicates that the unstable regions of the summer jets in the lower mesosphere and stratopause region have the boundaries which toward lower latitudes are largely consistent with the critical layers, and Eliassen‐Palm (EP) fluxes of the W3 and W4 increase rapidly near the unstable regions. This demonstrates that both the W3 and W4 originate from the jet instabilities. Nevertheless, the EP flux of the W3 shows a stronger transfer across the hemispheres than that of the W4. The W3 and W4 have the same period and vertical scale, which is different from previous reports.

Electrohydrodynamic instability and disintegration of low viscous liquid jet

The simple-jet mode can be developed from dripping, dripping faucet (transition), and jetting when subjected to a sufficient strong electric field caused by a needle-plated electrode. In the present work, the instability and disintegration of an electrified jet of anhydrous ethanol in the electrohydrodynamic simple-jet mode are visualized by a high-speed camera. A systematic analysis and description on the evolution of the breakup morphology of the charged jet in the simple-jet mode was carried out. In the dripping mode at low Weber number, the unstable simple-jet occurs, while the stable simple-jet mode happens as Weber number higher than critical value. In a dripping faucet and jetting regime, the simple-jet mode can easily take place, and jet undergoes typical breakup modes including varicose, whipping, whipping assisted bifurcation, and ramified instabilities as electric potential increases. The growth rate of the maximum perturbation of the zero-, first-, and second-order surface waves determines the type of the instabilities. The operating window of the simple-jet mode is presented to indicate that the simple-jet mode only operates in the suitable range of Weber number and electric Bond number. The spray characteristics, including envelope angle, droplet size, and the stable length of the electrified jet, have been explored to demonstrate that the uniform drops could be generated in the simple-jet regime. The evolution from the cone-jet to the simple-jet mode is also observed when a special hemispherical nozzle is used. For an almost stable electric potential, the cone-jet can gradually transform into the simple-jet with an increase in the liquid flow rate.

A study on the effect of the collector properties on the fabrication of magnetic polystyrene nanocomposite fibers using the electrospinning technique

AbstractThe role of the collector properties in changing the fiber morphology in electrospinning has not been completely understood yet. In this work, we studied the effect of different collectors on the helicity of the magnetic polystyrene nanocomposite fibers containing superparamagnetic magnetite nanoparticles. Aluminum and ice were used as solid collectors. Ethanol, ethanol‐deionized (DI) water mixture, sodium dodecyl sulfate, Triton X‐100, deionized water, and NaCl baths were used as liquid collectors. Ice, when surrounded by a conductive foil, produced excellent fiber alignment, but increased local humidity, inhibiting the continuous jet formation. The deionized water and NaCl bath exhibited minimum helicity. The surfactant bath produced different structures such as coiled, helical, and zig‐zag with varied diameters. Ethanol and DI/Ethanol baths were found to retain the structure of the deformed fiber formed due to jet instability. The helicity of the fibers was observed to increase with decreasing surface tension. Incorporation of the magnetic phase affected the viscosity of the polymer solution and the hydrophobicity of the polystyrene fibers further influencing the obtained morphologies.

Instability of a Surface Jet over Rough Topography

Abstract The instability of a surface-trapped jet over rough bottom topography is examined using a linearized quasigeostrophic model. The jet is laterally sheared and thus susceptible to both barotropic and baroclinic instability. The relative magnitude of the two depends on the jet width and on the spectral characteristics and amplitude of the bathymetry. The most unstable eddies in the upper layer are typically smaller over bathymetry than with a flat bottom. Topography also alters momentum flux convergence in the upper layer and causes the perturbations to resemble eddies in a 1.5-layer flow. But as long as the jet is wider than the deformation radius, baroclinic instability is present, yielding deep eddies that are phase-locked to those at the surface. In addition, topography facilitates scattering of energy at depth to other scales. So, instability over rough topography could be an efficient, and largely overlooked, means of transferring mesoscale energy to the dissipative scales. Significance Statement This study investigates the effects of bottom roughness on large-scale ocean currents and their associated eddies. Roughness affects the eddy size and speed and how they exchange energy with the mean flow. Roughness also facilitates energy transfer to smaller scales where it can be dissipated. Thus, instability over rough topography could be an important part of the oceanic energy balance.

Mitigating jitter in droplet stream by uniform charging

Monodisperse droplets induced by Plateau–Rayleigh instability of liquid jet are widely applied. Due to spatial jitter, the spacing between droplets becomes uneven as the working distance increases. We found that the jitter can be ameliorated by uniformly charging the droplets. Under the electrostatic forces, the droplets align at uniform spacing over a long distance. Nevertheless, radial jitter emerges when the charging voltage is too high. The effect of charging on the jitter was modeled and validated by experiments. A recommended charging parameters configuration is given considering a trade-off between axial and radial jitter to obtain evenly distributed droplets.

Dynamics of a liquid sheet with a broken time reversal

With the presence of the odd viscosity, the linear instability of a planar jet (sheet) surrounded by an inviscid gas medium is explored for varicose and sinuous modes. The previous studies on sheet mechanics did not consider the role of the odd viscosity when measuring the maximum growth rate, dominant wave number, instability limit, and breakup length. Semi-dominant characteristic of the odd viscosity is that it is a stabilizing factor for the state of a liquid sheet. It is an unstable factor in a few cases as dictated by the model parameters, such as the odd viscosity number, aerodynamic force, Reynolds number, ambient gas density, and Weber number. In the production of metal alloys, metal powders, and the cooling and descaling stages of downstream metal manufacturing processes, particularly in the steel industry, sprays may be employed in a variety of ways.

Analysis of the oscillations induced by a supersonic jet applied to produce nanofibers

High-performance fibers are key components for enhancing the mechanical properties of composite materials. The development of high strength nanofibers augurs the production of new nano-composites with outstanding features. However, the robust production of continuous glass nanofibers that can be feasible processed for efficiently manufacturing nanocomposites is still challenging. Recently, Cofiblas (Continuous Fiberizing by Laser melting and Supersonic dragging) was demonstrated as a technique capable of producing continuous glass nanofibers with unlimited length. Cofiblas process has some similarities with the widely known melt blowing technique for the production of polymeric fibers. In both techniques, the design of the gas nozzle is key to ensure the feasibility of the process since the turbulences of the gas jet may induce strong whipping of the filament.This paper gives novel experimental evidences on the correlation of the supersonic gas jet instabilities with the oscillation of the filament in the melt-blowing and Cofiblas processes, relating these oscillations with the presence of shock waves and unsteadiness in the flow, and gives valuable insight into the use of supersonic jets in the melt blowing process as an effective approach for the formation of nanofibers. A thin 3D-axisymmetric model in OpenFOAM® was put to test by comparing the performance of different solvers which were validated by flow visualization of the exit jet using digital holography (DH). In order to perform a realistic and thorough validation, we simulated the optical measurements of the flow from the CFD simulations of the mass density by Abel transform and numerical differentiation. The application of digital holography as the flow visualization technique makes possible both a precise validation of the density maps obtained from the Abel transformation of the 2D-alike results, and the analysis of the shockwave pattern in the air jet. Conversely, the numerical reconstruction of time-averaged holograms is employed to detect unsteadiness in the flow and to analyze the fiber oscillation, which is essential to assess the stability of the process. Lastly, the analysis and comparison of the vibration of the filament using the basic design and the optimized nozzle demonstrates a clear influence of the shock waves and flow unsteadiness in the stability of the filament.

Fragmentation of faceted crystalline wires

The reduction of surface energy is a well-known driving force for the fragmentation of nanowires. Fragmentation can limit the service life of those small low-dimensional building blocks or can be controlled to induce beneficial shape changes. For isotropic surface energy, fragmentation is analogous to the classic Rayleigh-Plateau instability of liquid jets. However, commonly synthesized crystalline nanowires have strongly anisotropic surface energies and exhibit facets coinciding with cusps in the Wulff plot. Depending on growth orientation, different fragmentation behaviors have been seen in nanowires. Using phase-field simulations, we show that fragmentation of faceted nanowires with cubic crystal symmetry may occur by a finite-amplitude nonlinear instability, as opposed to a Rayleigh-Plateau-like linear instability, depending on nanowire growth orientation. We carry out a weakly nonlinear analysis based on sharp interface theory to characterize the faceted nanowire shape corresponding to a surface-energy saddle point. The analysis predicts that the minimum amplitude of a periodic shape perturbation to trigger fragmentation increases with cusp strength but decreases inversely proportionally to the perturbation wavelength for long wavelengths, in good quantitative agreement with phase-field predictions. The results provide the theoretical foundation to predict nanowire stability as a function of length and surface energy in diverse applications.

The formation process of annular starting jets

The formation and evolution process of the outer vortex ring (denoted as OV0) in annular starting jets has been studied numerically at stroke ratio $3\leqslant L_m/D\leqslant 8$ , $\beta \leqslant 0.8$ and Reynolds number $400\leqslant Re_{\triangle R}\leqslant 1400$ , where $D$ is the outer diameter of orifice, $L_m$ is the length of the fluid column, $\beta$ and $\triangle R$ are the ratio and difference between inner and outer radii of the annular orifice, respectively. The initial formation process of OV0 is found to be similar to that of the leading vortex ring in a circular starting jet, but the circulation strength is relatively lower and decreases gradually with increasing $\beta$ . The pinch-off process will also appear after OV0 grows to a certain size but is different from that of the circular starting jet. On one hand, OV0 is stretched and deformed by the strain field generated by the trailing jet's instability, and vorticity would be shed. On the other hand, OV0 accelerates the irrotational fluids around it under viscous action so as to form the induced velocity field, which would consume energy causing it to shed vorticity further under the limit of the Kelvin–Benjamin variational principle. Furthermore, OV0 would eventually become a stable vortex ring whose non-dimensional circulation ( $\varGamma /(U_{0max}D$ )) is found to be between 0.59–0.62 and it would not change with different $L_m/D$ at $\beta =0.5$ , which is much less than the 2–2.5 of circular starting jets.

Design and evaluation of a linear nozzle array with double auxiliary electrodes for restraining cross-talk effect in parallel electrohydrodynamic jet printing

It is attractive and challenging to fabricate micro-nano scale patterns in large area by parallel electrohydrodynamic jet printing (E-jet printing) with nozzle array. However, cross-talk effect (i.e. interference phenomenon) between nozzles may probably lead to poor positioning accuracy and cause jet instability. In this paper, design and evaluation of a special linear nozzle array with double auxiliary electrodes at both ends were proposed to restrain the cross-talk effect. Firstly, the numerical simulation model of multi-nozzle E-jet printing was established considering with electro-hydraulic coupling effect. The corresponding multi-nozzle E-jet printing processes in several geometry configurations were successfully simulated based on a commercial multi-physical software ‘COMSOL’. Secondly, the unique design of new type linear nozzle array with double auxiliary electrodes was proposed after accomplishing theoretical analysis and comparison study of simulation results. Thirdly, several experiments were carried out on a constructed multi-nozzle E-jet printing system to verify feasibility and effectiveness of the provided design. The experiments show that the proposed manner has obvious advantages in restraining cross-talk compared with traditional suppressing method ‘with single flat-head auxiliary electrode at both ends’. When nozzle space is 0.5 mm, the jet deflection angle 2.8° and Taylor cone angle 3.3° could be reduced to 0° and 0.05° with two methods respectively. Finally, the printability of fabricated new multi-nozzle was also characterized. Microscale droplets array (mean diameter about 200–240 μm) could be achieved with no deflection and the gap between them is equal with the distance between adjacent nozzles. This study can provide a reference for the design of high integrated printhead and E-jet printing system.

Instability of an active fluid jet

Instabilities of an active fluid jet (Center) are investigated. Left: For an active fluid of pullers, the jet breaks up into droplets in a varicose manner reminiscent of a Newtonian fluid. Right: For pushers, on the other hand, the jet buckles and undergoes a waving instability.

Jet instability of suspensions of different shaped particles exhibiting discontinuous shear thickening

This work is devoted to the detailed study of jet instability occurring in concentrated aqueous mixtures of calcium carbonate (CC) isotropic-shaped particles and rigid polyamide (PA) fibers. These mixtures exhibit very sharp discontinuous shear thickening (DST). The jets were subjected to a free fall under gravitational stretching at a constant flow rate. In the absence of PA fibers, we observed relatively strong lateral oscillations occurring for jet lengths L≳6cm and accompanied by small periodic undulations of the jet diameter. Two-dimensional Direct Fourier Transform analysis reveals approximately linear dispersion relations for propagation of lateral oscillations and diameter undulations with similar wave speeds ∼1m/s. This instability is ascribed to complex rheological behavior in an extensional flow above the DST transition. Theoretical modeling reveals abrupt jumps of the tensile stress along the jet likely leading to fluctuation of longitudinal and transverse velocity fields within the jet perceived through jet diameter and centerline undulations. The addition of PA fibers to CC suspension damps lateral oscillations but favors ruptures along the jet. This is tentatively explained by the interplay between growing lower and decreasing upper DST threshold stresses with increasing fiber volume fraction φf along with the thinning of the jet diameter down to the size of fiber flocs. Quantitatively, the stabilizing effect of PA fibers is manifested through an abrupt decrease in the lateral drift amplitude at φf≳0.75vol%.

Numerical Simulations of a Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

Abstract A turbulent lifted H2/N2 jet flame in a vitiating coflow environment is numerically investigated, using the Flamelet generated manifold (FGM) combustion model with large eddy simulations (LES). Due to the hot vitiated H2/air coflow, the primary stabilization mechanism is the auto-ignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the auto-ignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at auto-ignition location but premixed in the postflame. A series of LES/FGM simulations are completed in this work by reducing the coflow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the coflow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction at multiple locations, which have also been correctly captured. It is noted that for a high coflow temperature (and hence a low lift-off distance), the flame's lift-off is highly sensitive to the inlet boundary conditions and the mesh resolution near the jet entry. A relatively coarse mesh is used for all the simulations, which is generated using a careful strategy that not only resolves the jet instabilities near the fuel inlet but also keeps the overall mesh count low and allows for a large computational time step. A systematic sensitivity analysis of the computational speed is also performed. This work provides some useful guidelines for simulating the H2 diluted flames using the FGM model, which may be valuable to the gas turbine industry.