Unstable Jet Research Articles

Interaction of an Upwelling Front with External Vortices: Impact on Cross-shore Particle Exchange

Coastal upwellings, due to offshore Ekman transport, are more energetic at the western boundaries of the oceans, where they are intensified by incoming Rossby waves, than at the eastern boundaries. Western boundary upwellings are often accompanied by a local vortex field. The instability of a developed upwelling front and its interaction with an external vortex field is studied here with a three-dimensional numerical model of the hydrostatic rotating Navier-Stokes equations (the primitive equations). The baroclinic instability of the front leads to the growth of meanders with 100-200 km wavelength, in the absence of external vortex. On the $$f$$ -plane, these waves can break into a row of vortices when the instability is intense. The $$\beta$$ -effect is stabilizing and strongly decreases the amplitude of meanders. Simulations are then performed with a front initially accompanied by one or several external vortices. The evolutions in this case are compared with those of the unstable jet alone. On the $$f$$ -plane, when an external vortex is close to the front, this latter sheds a long filament which wraps up around the vortex. This occurs over a period similar to that of the instability of the isolated front. Cyclones are more efficient in tearing such filaments offshore than anticyclones. On the $$\beta$$ -plane, the filaments are short and turbulence is confined to the vicinity of the front. At long times, waves propagate along the front, thus extending turbulence alongshore. The initial presence of a vortex alley leads to a stronger destabilization of the front and to a larger cross-shore flux than for a single vortex, with many filaments and small vortices pushed far offshore. In the ocean, this cross-shore exchange has important consequences on the local biological activity.

Numerical investigation on the heat transfer characteristics of unstable steam jet under different operating conditions

Transient numerical study was performed to investigate the characteristics of interphase heat transfer in unstable steam jet under different operating conditions. Bubble radius change rate, interfacial heat transfer coefficient and overall condensation mass flux reached their maximum values during the bubble necking stage. The maximum peak value of interfacial heat transfer coefficient and bubble radius change rate decreased with the decrease of inlet pressure differential and water subcooling. The peak values of interfacial heat transfer coefficient and internal velocity were found in the bubble neck region. The effect of interphase heat transfer on bubble growth and necking stages was analyzed on the basis of overall force balance. Momentum force played a major role during the bubble growth stage, and the condensation force had a dominant effect on bubble necking stage. Condensation force first decreased and reached its minimum value during bubble rapid necking stage. Subsequently, the condensation force rapidly increased and promoted the movement of bubble necking. Moreover, the condensation force curves under different conditions had the same change law.

Towards launching a stable wide plasma jet from a single tube: I. The importance of controlling the gas dynamics

The success of plasma jets in broad fields is raising the demand to enlarge the jet size to be used in practical and industrial applications. However, conventional upscaling approaches that use multi-tubes have considerable drawbacks resulting from fluid-dynamics and electrical interactions among the jets. Therefore, developing a new upscaling approach to launch a wide plasma jet from a single tube is a great of interest. Thus, this study aims to unveil and address the challenges faced in obtaining a stable, uniform, and wide plasma jet from a single tube with a diameter of up to 30 mm. The focus is on the dynamic flow of the effluent gas, where its modification showed a significant influence on the jet formation in wide tubes. It was discovered that the presence of a sudden expansion flow at the inlet of wide tubes has a remarkable negative effect on the jet formation, owing to the increase in the air entrainment. Therefore, no discharge could be ignited at relatively low flow rates, and only an unstable jet with a brush-like shape could be launched at higher flow rates. Meanwhile, the mitigation of the sudden expansion influences by controlling the geometric parameters such as expansion ratio and length-diameter ratio at different Reynolds numbers indicated a significantly positive effect on the discharge ignition and stability of the formed jet. Furthermore, a new approach to address the challenges of upscaling the plasma jet is presented; in this approach, a porous material (a sponge) is used as a gas diffuser and embedded at the tube inlet. The gas diffuser greatly modified the gas flow inside the tube, thereby a stable, uniform plasma jet could be launched from a wide tube at relatively low flow rates. Finally, the interaction of the jet with different targets is presented to show the applicability of the launched jet. The treatment patterns from a wide single tube are similar to that observed using a narrow tube, where a wide diffused discharge spot found over a dielectric target, whilst intense discrete spots were created on a floating metallic target.

Simulating the dynamics and synchrotron emission from relativistic jets – II. Evolution of non-thermal electrons

ABSTRACT We have simulated the evolution of non-thermal cosmic ray electrons (CREs) in 3D relativistic magneto hydrodynamic (MHD) jets evolved up to a height of 9 kpc. The CREs have been evolved in space and in energy concurrently with the relativistic jet fluid, duly accounting for radiative losses and acceleration at shocks. We show that jets stable to MHD instabilities show expected trends of regular flow of CREs in the jet spine and acceleration at a hotspot followed by a settling backflow. However, unstable jets create complex shock structures at the jet head (kink instability), the jet spine-cocoon interface, and the cocoon itself (Kelvin–Helmholtz modes). CREs after exiting jet head undergo further shock crossings in such scenarios and are re-accelerated in the cocoon. CREs with different trajectories in turbulent cocoons have different evolutionary history with different spectral parameters. Thus, at the same spatial location, there is mixing of different CRE populations, resulting in a complex total CRE spectrum when averaged over a given area. Cocoons of unstable jets can have an excess build up of energetic electrons due to re-acceleration at turbulence driven shocks and slowed expansion of the decelerated jet. This will add to the non-thermal energy budget of the cocoon.

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.

Experimental investigation of bubbly flow by air injection on an inclined hydrofoil

Flow features resulting from air injection through a hole on an inclined hydrofoil in a free stream flow are characterized in this work. The experiments were carried out in a cavitation tunnel at the Chungnam National University. The hydrofoil was oriented at angles of inclination, α=±3.5° and 0°, in the test section with water flowing at the free stream velocity, V∞= 3, 5, and 7 m/s (Froude number, Fn∼ 30–70). The air jet emanating from the hole on the inclined hydrofoil along the streamwise direction [Reynolds number of air, Reair∼O(103)], at various air injection rates, Cq, was characterized by high-speed imaging. Geometric features of the air jet, such as jet width, w, jet angle, β, jet effective diameter, deff, and jet thickness, t, are measured from the high-speed images. A power-law relation is shown to exist between the non-dimensional geometric features of the jet and the non-dimensional parameters, Reair,Fn,Cq. Depending on the Froude number, Fn, and non-dimensional air injection rate, Cq, several jet domains such as unstable jet, cloudy puff, buoyant-bifurcating jet, and stable-buoyant jet domains were observed upon air injection at Reair∼O(103) for various hydrofoil inclinations.

The Dynamics of Globally Unstable Air-Helium Jets and Its Impact on Jet Mixing Intensity

The paper presents experimental investigations of the low-density air-helium jets. The paper is aimed at the analysis of the flow conditions promoting the local absolute instability leading to global flow oscillations. A number of the test cases are analysed with a wide range of the shear layer thickness showing conditions favorable for the global modes and also mixing intensity triggered by such a regime. It is shown that high mixing intensity is determined not only by the global regime but also by the vortex pairing process. The results are compared with a recently proposed universal scaling law for an onset into the global mode. The results turn out to be far from this scaling law and the reasons for such discrepancies are discussed. The measurements show also that if the shear layer at the nozzle exit is thin enough the global modes are suppressed. The mechanism leading to the global mode suppression under such conditions is carefully analysed.

Melt Electrospinning of Nanofibers from Medical-Grade Poly(ε-Caprolactone) with a Modified Nozzle.

Melt electrospun fibers, in general, have larger diameters than normally achieved with solution electrospinning. This study uses a modified nozzle to direct-write melt electrospun medical-grade poly(ε-caprolactone) onto a collector resulting in fibers with the smallest average diameter being 275±86nmunder certain processing conditions. Within a flat-tipped nozzle is a small acupuncture needle positioned so that reduces the flow rate to ≈0.1µL h-1 and has the sharp tip protruding beyond the nozzle, into the Taylor cone. The investigations indicate that 1-mm needle protrusion coupled with a heating temperature of 120°C produce the most consistent, small diameter nanofibers. Using different protrusion distances for the acupuncture needle results in an unstable jet that deposited poor quality fibers that, in turn, affects the next adjacent path. The material quality is notably affected by the direct-writing speed, which became unstable above 10mm min-1 . Coupled with a dual head printer, first melt electrospinning, then melt electrowriting could be performed in a single, automated process for the first time. Overall, the approach used here resulted in some of the smallest melt electrospun fibers reported to date and the smallest diameter fibers from a medical-grade degradable polymer using a melt processing technology.

Stability and transition on a Coandă cylinder

An experimental stability and transition investigation of a centrifugally unstable wall jet blown from a slot over a circular Coandă cylinder was conducted using flow visualization and particle image velocimetry. Clear and unambiguous observations of spontaneously generated stationary streamwise structures, never observed previously, were analyzed using standard image processing techniques. These structures ultimately exhibited a secondary time-dependent wavy instability that was followed by transition to turbulence. A modified Görtler number, based on the slot height, was used as a basis for comparison to linear stability theory and was directly related to the Reynolds number via geometric scaling. Direct observation of the vortices allowed the identification of an upper limit to the critical Görtler number of 6.3 that bracketed the value of 3.5 from the linear stability theory. The vortical shear layer grew exponentially along the azimuth and was characterized by a Reynolds number dependent growth rate parameter. Extrapolation of the growth rate parameter to zero furnished a critical Görtler number of 3.1 ± 1.25 (95% CI) that compared remarkably well with the linear stability theory. The shear layer separation angle did not vary monotonically with the Reynolds number: as the Reynolds number increased, the thicker boundary layer was more susceptible to separation, and thus, the separation angle, relative to the slot, decreased. However, following transition to turbulence at higher Reynolds numbers (≈400), the high-momentum fluid near the wall, resulting from turbulent mixing, produced a subsequent increase in the separation angle.

Nutrient‐Rich Gravity Current Formed by Upwelling in Barrow Canyon: High‐Resolution Observations

AbstractThe distribution of nutrients influences the Chukchi Sea's rich ecosystems and affectsthe biogeochemistry of the central Arctic. Nutrients that become limiting in the late summer can be replenished episodically by physical processes that are likely to change in concert with Arctic climate trends. Here we report on unique, simultaneous, physical, and biogeochemical measurements of one such process: upwelling in Barrow Canyon. High‐resolution transects of a towed vehicle, which was equipped with physical, turbulence, and biogeochemical sensors, captured the upwelling of dense, salty, and nitrate‐rich waters into the shallow regions of Barrow Canyon. Upwelling drastically modifies the nitrate distribution of Barrow Canyon through the vertical advection of Atlantic Water from the Canada Basin and through turbulent flux across the nutricline. Upwelled waters form a highly sheared gravity current that is susceptible to both baroclinic and symmetric instabilities. The current exhibits transverse circulation common to frictional gravity currents. Our observations suggest that rapid and dramatic nutrient changes in Barrow Canyon can be accomplished by upwelling; the subsequent formation of unstable jets and fronts enhances the irreversible flux of upwelled nutrients to less dense waters in the Chukchi sea.

Contrasting phase field method and pairwise force smoothed particle hydrodynamics method in simulating multiphase flow through fracture-vug medium

Strong heterogeneity and anisotropy result in complex flow and relatively low oil recovery in fractured-vuggy reservoirs. In this work, we assess the Cahn-Hilliard phase field method (PFM) and the pairwise force smoothed particle hydrodynamics (PF-SPH) method in modeling 2D multiphase flow through fracture-vug medium, in terms of different capillary numbers, mobility ratios, gravity, and wettability. Comparing the results of numerical simulations and physical experiments, the results of the PF-SPH method are closer to the physical experimental displacement process than those of the PFM, and the PF-SPH method is found to be computationally more efficient and accurate than the PFM. The experimental and numerical results reveal that when the injection velocity, represented via logarithm of the capillary number (LCN), is not greater than −4 (gas phase case) or −2.3 (water phase case) in the fracture-vug medium, the gravitative differentiation of the oil-gas or oil-water is significant, and the oil-gas or oil-water flow is a stable stratified flow. When the injection velocity LCN exceeds −3 (gas phase case) or −1.3 (water phase case) in the vertical section, the gravitative differentiation of the oil-gas or oil-water is not obvious, and the fluid flow pattern appears as an unstable jet flow.

Frequency selection in a gravitationally stretched capillary jet in the jetting regime

A capillary jet falling under the effect of gravity continuously stretches while thinning downstream. We report here the effect of external periodic forcing on such a spatially varying jet in the jetting regime. Surprisingly, the optimal forcing frequency producing the most unstable jet is found to be highly dependent on the forcing amplitude. Taking benefit of the one-dimensional Eggers & Dupont (J. Fluid Mech., vol. 262, 1994, pp. 205–221) equations, we investigate the case through nonlinear simulations and linear stability analysis. In the local framework, the WKBJ (Wentzel–Kramers–Brillouin–Jeffreys) formalism, established for weakly non-parallel flows, fails to capture the nonlinear simulation results quantitatively. However, in the global framework, the resolvent analysis, supplemented by a simple approximation of the required response norm inducing breakup, is shown to correctly predict the optimal forcing frequency at a given forcing amplitude and the resulting jet breakup length. The results of the resolvent analysis are found to be in good agreement with those of the nonlinear simulations.

Idealised simulations of cyclones with robust symmetrically unstable sting jets

Abstract. Idealised simulations of Shapiro–Keyser cyclones developing a sting jet (SJ) are presented. Thanks to an improved and accurate implementation of thermal wind balance in the initial state, it was possible to use more realistic environments than in previous idealised studies. As a consequence, this study provides further insight into SJ evolution and dynamics and explores SJ robustness to different environmental conditions, assessed via a wide range of sensitivity experiments. The control simulation contains a cyclone that fits the Shapiro–Keyser conceptual model and develops a SJ whose dynamics are associated with the evolution of mesoscale instabilities along the airstream, including symmetric instability (SI). The SJ undergoes a strong descent while leaving the cloud-head banded tip and markedly accelerating towards the frontal-fracture region, revealed as an area of buckling of the already-sloped moist isentropes. Dry instabilities, generated by vorticity tilting via slantwise frontal motions in the cloud head, exist in similar proportions to moist instabilities at the start of the SJ descent and are then released along the SJ. The observed evolution supports the role of SI in the airstream’s dynamics proposed in a conceptual model outlined in a previous study. Sensitivity experiments illustrate that the SJ is a robust feature of intense Shapiro–Keyser cyclones, highlighting a range of different environmental conditions in which SI contributes to the evolution of this airstream, conditional on the model having adequate resolution. The results reveal that several environmental factors can modulate the strength of the SJ. However, a positive relationship between the strength of the SJ, both in terms of peak speed and amount of descent, and the amount of instability occurring along it can still be identified. In summary, the idealised simulations presented in this study show the robustness of SJ occurrence in intense Shapiro–Keyser cyclones and support and clarify the role of dry instabilities in SJ dynamics.

Classification of dark pion multiplets as dark matter candidates and collider phenomenology

New confining sectors can contain a set of pseudo-Goldstone mesons that exhibit a complicated structure in terms of stability and relative masses. Stable ones can act as dark matter candidates, while their interactions with the unstable ones determine their relic abundances. The overall structure, by specifying which channels are kinematically forbidden or not, affects the cosmology, constraints and collider phenomenology. In this paper, we present a classification of these pseudo-Goldstone meson structures. We find that the structures can be classified into three categories, corresponding to strong, suppressed and essentially non-existent constraints from indirect detection. Limits on decay lengths of the unstable mesons and dark jet properties are presented for several benchmark models.

Effects of Moisture in a Two-Layer Model of the Midlatitude Jet Stream

AbstractThe effects of moisture on the energetics of a statistically stationary, baroclinically unstable jet representing the midlatitude atmosphere are examined using a two-layer, β-plane shallow-water model. Flow is driven by a relaxation of the interface between the two layers to a baroclinically unstable profile. Moisture is input to the lower layer by evaporation. When supersaturation occurs, precipitation is triggered and the related latent heat release drives a mass transfer between the two layers. A comparison between dry and moist reference atmospheres shows that precipitation reduces eddy kinetic energy. This is related to the meridional distribution of precipitation, which occurs on the poleward side of the jet (where the interface field is raised). This latitudinal structure of precipitation is related to a correlation between poleward flow and ascent, which is analyzed using a shallow-water analog to the ω equation. The precipitation effect on the energy budget is predominately due to zonal- and time-averaged terms. Because of this, dry simulations in which the thermal forcing is modified to mimic the effect of zonally averaged precipitation are carried out and compared with their precipitating counterparts. These simulations show a similar reduction of baroclinic eddy kinetic energy; however, the barotropic eddy kinetic energy response shows a larger difference.

Stability of a jet moving in a rectangular microchannel.

We study numerically the basic flow and linear stability of a capillary jet confined in a rectangular microchannel. We consider both the case where the interface does not touch the solid surfaces and that in which the jet adheres to them with a contact angle slightly smaller than 180^{∘}. Given an arbitrary set of values of the governing parameters, the fully developed (parallel) two-dimensional basic flow is calculated and then the growth rate of the dominant perturbation mode is determined as a function of the wave number. The flow is linearly stable if that growth rate is negative for all the wave numbers considered. We show that when the coflowing stream viscosity is sufficiently small in terms of that of the jet, there is an interval of the flow rate ratio Q for which the jet adheres to the walls or not depending on whether the flow is established by decreasing or increasing the value of Q. When the distance between the interface and the channel wall is of the order of the jet radius, the jet is unconditionally unstable. However, for sufficiently small interface-to-wall distances, the viscous stress can dominate the capillary pressure and fully stabilize the flow. Our results suggest that the capillary modes are suppressed and the flow becomes stable when the jet adheres to the channel walls. The combination of the above results indicates that, under certain parametric conditions, stable or unstable jets can be formed depending on whether the experimenter sets the flow rate ratio by decreasing or increasing progressively the jet flow rate while keeping constant that of the outer stream. Our theoretical predictions for the stablity of a coflow in a rectangular channel are consistent with previous experimental results [Humphry et al., Phys. Rev. E 79, 056310 (2009)PLEEE81539-375510.1103/PhysRevE.79.056310].

Numerical study on mechanism of condensation oscillation of unstable steam jet

Submerged steam jet condensation is widely used in many industries. However, this process may cause damage to equipment due to pressure oscillation, especially at low steam mass flux. The dynamic process of steam bubble and different pressure oscillation types were investigated in this numerical study. Five typical stages were observed, which were bubble growth stage, bubble slow necking stage, bubble rapid necking stage, bubble detachment stage and secondary bubble oscillation stage. Two pressure oscillation types were found by the synchronization analysis of bubble pattern and pressure field. The mechanisms of condensation oscillation of unstable steam jet were revealed. As for the first type, two pressure peaks existed in the fluctuation process of secondary bubble volume. The higher pressure peak was generated when secondary bubble was oscillated to its minimum volume, and the other was generated when secondary bubble was collapsed. As for the second type, only one pressure peak existed and secondary bubble was quickly contracted without volume fluctuation. Pressure peak was generated when secondary bubble was quickly contracted to its minimum volume. Moreover, analyses of flow field pressure at different locations were carried out. Results indicated that the pressure fluctuation was mainly induced by the condensation oscillation of secondary bubble for both types. The trends of pressure at different locations were the same as that of secondary bubble internal pressure.

Assessing Adaptive Mesh Refinement (AMR) in a Forced Shallow-Water Model with Moisture

AbstractTwo forced shallow-water flow scenarios are explored in a 2D fourth-order finite-volume dynamical core with adaptive mesh refinement (AMR) to investigate AMR’s ability to track and resolve complex evolving features. Traditional shallow-water test cases are mainly characterized by large-scale smooth flows that do not effectively test the multiscale abilities of variable-resolution and AMR models to resolve sharp gradients and small-scale flow filaments. Therefore, adding forcing mechanisms to the shallow-water system to model key atmospheric processes adds complexity and creates small-scale phenomena. These can serve as foci for dynamic grid refinement while remaining simple enough to study the numerical design of a model’s dynamical core. The first shallow-water flow scenario represents a strengthening, tropical cyclone–like, vortex that is driven by a Betts–Miller-like convection scheme. The second shallow-water test is built upon a barotropically unstable jet with an added Kessler-like warm rain scheme that leads to precipitating frontal zones. The key feature of both tests is that there is significant sensitivity to the model grid while converging (structurally) at high resolution. Both test cases are investigated for a series of uniform resolutions and a variety of AMR tagging criteria. The AMR simulations demonstrate that grid refinement can resolve local features without requiring global high-resolution meshes. However, the results are sensitive to the refinement criteria. Criteria that trigger refinement early in a simulation reproduce the uniform-resolution reference solutions most reliably. In contrast, AMR criteria that delay refinement for several days require careful tuning of the AMR thresholds to improve results compared with uniform-resolution simulations.

Experimental and theoretical study of pressure oscillation of unstable steam-air jet condensation in water in a rectangular channel

Pressure oscillation occurs during the steam jet condensation in water, notably in unstable flow pattern region. Consequently, pressure oscillation may risk the thermal system. In this work, we introduce a method of reducing the pressure oscillation intensity by adding a small amount of air into steam in unstable flow pattern region. The effects of the air content on the unstable flow patterns and pressure oscillation characteristics were studied experimentally. Notably, the air layer showed a periodic accumulation-and-dispersion phenomenon around the steam–water interface. This phenomenon resulted in a periodic and dramatic fluctuation of the interface at a relatively low air mass fraction. When the air mass fraction increased to a critical value, the flow pattern became stable due to a sharp reduction in condensation rate. Accordingly, the pressure oscillation intensity increased and then dropped at the critical air mass fraction, showing a convex-shaped distribution. The air mass fraction should exceed the critical value if air is added into steam to reduce the pressure oscillation intensity. Otherwise, the air could lead to a much stronger pressure oscillation. A theoretical model for the critical air mass fraction was established and could predict the critical air mass fraction within a deviation of ±14%.

Nonthermal ion acceleration by the kink instability in nonrelativistic jets

We investigate the self-consistent particle acceleration physics associated with the development of the kink instability (KI) in nonrelativistic, electron-ion plasma jets. Using 3D fully kinetic particle-in-cell simulations, we show that the KI efficiently converts the initial toroidal magnetic field energy into energetic ions. The accelerated ions form a nonthermal power-law tail in the energy spectrum, containing ≃10% of the initial magnetic field energy, with the maximum ion energy extending to the confinement energy of the jet. We find that the ions are efficiently accelerated by the concerted action of the motional electric field and the highly tangled magnetic field that develop in the nonlinear phase of the KI: fast curvature drift motions of ions across magnetic field lines enable their acceleration along the electric field. We further investigate the role of Coulomb collisions in the ion acceleration efficiency and identify the collisional threshold above which nonthermal ion acceleration is suppressed. Our results reveal how energetic ions may result from unstable nonrelativistic plasma jets in space and astrophysics and provide constraints on the plasma conditions required to reproduce this acceleration mechanism in laboratory experiments.