Unstable Jet Research Articles

Influence of Time-Modulation of Applied Current on Arc Stability in DC Pulsed Plasma Spray Torch

Conventional direct current arc plasma spray torches generate highly unstable plasma jets. This leads to some difficulties with control of coating properties especially when injecting liquid feedstock for the elaboration of finely structured coatings. At constant applied current, a properly designed arc plasma torch can work in self-sustained pulsed mode that produces large amplitude oscillations of the arc, which are interrupted by very repeatable restrikes. This paper investigates the influence of the modulation of the applied current on the arc stability. Electrical and acoustic diagnostics permit to show that amplification of arc oscillation and a stabilizing mechanism occur when the arc current frequency approaches the torch resonance frequency. Time-resolved optical emission spectroscopy is used to determine temperature and enthalpy profiles close to the resonance and the enthalpy modulation.

Numerical simulation of high inertial liquid-in-gas droplet in a T-junction microchannel

Two new flow regimes named unstable dripping and unstable jetting are identified in aqueous droplet generation within high inertial air flow inside a T-Junction microchannel.

Effect of interactions between spray jets on liquid distribution in a fluidized bed

AbstractIn Fluid Cokers™, banks of spray nozzles are used to inject oil into a bed of hot coke particles. The purpose of this study is to determine whether interactions between spray jets could enhance liquid distribution on hot coke particles, which is crucial to improve the operability and performance of Fluid Cokers. A low temperature experimental model of Fluid Coking was used to measure the liquid distribution. Preliminary screening of nozzle positions employed conductance measurements. A binder solution was utilized to further investigate the most interesting nozzle interactions, by simulating at low temperature the formation of agglomerates during high temperature coking. Adding different dyes to the binder solutions injected by the different nozzles helped determine how nozzles interacted.With two synchronized nozzles of the same size, the liquid distribution is greatly improved when the spray jets slightly merge due to the expansion time being significantly reduced. The merged spray jets result in an unstable single jet, which allows for bubbles to be released faster from the spray jet. Because the volume of the released bubble is about the same for individual and merged jets, the jets do not contract as much upon bubble release: this enhances the jets' ability to capture gas bubbles from the bed, accelerating their expansion. Fewer agglomerates are also produced due to the improved liquid distribution.

Flow patterns and mixing characteristics of horizontal buoyant jets at low and moderate Reynolds numbers

Laminar horizontal buoyant jets have been found to exhibit peculiar behavior of unstable nature, e.g., bifurcation in the form of a vertical secondary plume deriving from the primary jet, due to the unstable stratification on one half of circumferential interface between the jet and the ambient fluid. However, whether and how this exceptional unstable jet behavior persists into the transitional regime with moderate Reynolds numbers (500<Re<2000) remains an open question. In this study, we explored the flow pattern and stability, i.e., the tendency to bifurcate, of horizontal buoyant jets with different densimetric Froude number (Fr) extending from low to moderate Reynolds numbers in the laboratory. The non-intrusive laser imaging technique of Planar Laser Induced Fluorescence (PLIF) was employed for the flow visualization and measurements. Three distinct flow patterns characterized with unstable small-scale structures, namely, ‘primary jet with vertical secondary plume’, ‘primary jet with pseudo secondary jet’ and ‘quasi-turbulent horizontal buoyant jet’, were identified in the Re–Fr parameter space determined from the visualization results. The substantive bifurcation reported earlier in the literature based on shadowgraph imaging was however not observed within the presumptive space in the present experiments. Quantitative concentration measurements using PLIF were further analyzed to study the jet mixing and spreading characteristics in the laminar and transitional regimes, including the decay of maximum concentration along the jet centerline, the cross-sectional concentration distribution at different downstream locations, as well as the concentration spread. Remarkably, the concentration decay/spreading rates exhibited a first-decreasing-then-increasing/first-increasing-then-decreasing behavior respectively with Re within the test range, reconciling the contradictory trends reported in the literature and suggesting the existence of a Re threshold below which the viscous force will prevail and suppress the potentially enhanced mixing and entrainment of low Re jet due to its less uniformity and coherence. Beyond the threshold, strong correlation of the concentration decay and spreading coefficients with Fr also confirmed that the jet mixing and spreading are dependent on the buoyancy-generated turbulence. Overall, the experimental results provided new insight on the flow patterns and mixing characteristics of horizontal buoyant jets in the laminar and transitional regimes.

Scalar Flux Kinematics

The first portion of this paper contains an overview of recent progress in the development of dynamical-systems-based methods for the computation of Lagrangian transport processes in physical oceanography. We review the considerable progress made in the computation and interpretation of key material features such as eddy boundaries, and stable and unstable manifolds (or their finite-time approximations). Modern challenges to the Lagrangian approach include the need to deal with the complexity of the ocean submesoscale and the difficulty in computing fluxes of properties other than volume. We suggest a new approach that reduces complexity through time filtering and that directly addresses non-material, residual scalar fluxes. The approach is “semi-Lagrangian” insofar as it contemplates trajectories of a velocity field related to a residual scalar flux, usually not the fluid velocity. Two examples are explored, the first coming from a canonical example of viscous adjustment along a flat plate and the second from a numerical simulation of a turbulent Antarctic Circumpolar Current in an idealized geometry. Each example concentrates on the transport of dynamically relevant scalars, and the second illustrates how substantial material exchange across a baroclinically unstable jet coexists with zero residual buoyancy flux.

Effects of basin bottom slope on jet hydrodynamics and river mouth bar formation

River mouth bars are strategic morphological units primarily responsible for the development of entire deltaic systems. This paper addresses the role of receiving basin slope in the hydrodynamics of an exiting sediment-laden turbulent jet and in resulting mouth bar morphodynamics. We use Delft3D, a coupled hydrodynamic and morphodynamic numerical model, along with a theoretical formulation to reproduce the physics of the problem, characterized by a fluvially dominated inlet free of waves and tides. We propose an updated theoretical model with a slope-dependent entrainment coefficient, showing that the rate at which ambient fluid is incorporated into a jet increases with higher basin slopes. Transient results reveal that the magnitude of a basin slope can alter the stability of a jet, favoring the formation of an unstable meandering jet. While a stable jet gives rise to “middle-ground” bars accompanied by diverging channels, a “lunate” mouth bar results from unstable jets. Additional morphodynamic simulations demonstrate that the time required for mouth bar stagnation in its final position increases linearly with the basin slope. In contrast, the distance at which the mouth bar eventually forms decreases until reaching an asymptotic value for slopes higher than 2%. Moreover, the basin slope highly influences sedimentary processes responsible for bar formation: for milder slopes, progradation processes prevail, while in steeper basins aggradation is more relevant. Finally, the minimum relative water depth over a bar crest that forces the flow to bifurcate around a fully developed bar decreases with the basin slope.

On the composite response of the MLT to major sudden stratospheric warming events with elevated stratopause

AbstractBased on a climate‐chemistry model (constrained by reanalyses below ~50 km), the zonal‐mean composite response of the mesosphere and lower thermosphere (MLT) to major sudden stratospheric warming events with elevated stratopauses demonstrates the role of planetary waves (PWs) in driving the mean circulation in the presence of gravity waves (GWs), helping the polar vortex recover and communicating the sudden stratospheric warming (SSW) impact across the equator. With the SSW onset, strong westward PW drag appears above 80 km primarily from the dissipation of wave number 1 perturbations with westward period of 5–12 days, generated from below by the unstable westward polar stratospheric jet that develops as a result of the SSW. The filtering effect of this jet also allows eastward propagating GWs to saturate in the winter MLT, providing eastward drag that promotes winter polar mesospheric cooling. The dominant PW forcing translates to a net westward drag above the eastward mesospheric jet, which initiates downwelling over the winter pole. As the eastward polar stratospheric jet returns, this westward PW drag persists above 80 km and acts synergistically with the return of westward GW drag to drive a stronger polar downwelling that warms the pole adiabatically and helps reform the stratopause at an elevated altitude. With the polar wind reversal during the SSW onset, the westward drag by the quasi‐stationary PW in the winter stratosphere drives an anomalous equatorial upwelling and cooling that enhance tropical stratospheric ozone. Along with equatorial wind anomalies, this ozone enhancement subsequently amplifies the migrating semidiurnal tide amplitude in the winter midlatitudes.

A Geometric Interpretation of Eddy Reynolds Stresses in Barotropic Ocean Jets

AbstractBarotropic eddy fluxes are analyzed through a geometric decomposition of the eddy stress tensor. Specifically, the geometry of the eddy variance ellipse, a two-dimensional visualization of the stress tensor describing the mean eddy shape and tilt, is used to elucidate eddy propagation and eddy feedback on the mean flow. Linear shear and jet profiles are analyzed and theoretical results are compared against fully nonlinear simulations. For flows with zero planetary vorticity gradient, analytic solutions for the eddy ellipse tilt and anisotropy are obtained that provide a direct relationship between the eddy tilt and the phase difference of a normal-mode solution. This allows a straightforward interpretation of the eddy–mean flow interaction in terms of classical stability theory: the initially unstable jet gives rise to eddies that are tilted “against the shear” and extract energy from the mean flow; once the jet stabilizes, eddies become tilted “with the shear” and return their energy to the mean flow. For a nonzero planetary vorticity gradient, ray-tracing theory is used to predict ellipse geometry and its impact on eddy propagation within a jet. An analytic solution for the eddy tilt is found for a Rossby wave on a constant background shear. The ray-tracing results broadly agree with the eddy tilt diagnosed from a fully nonlinear simulation.

The Instabilities and Multiscale Energetics Underlying the Mean–Interannual–Eddy Interactions in the Kuroshio Extension Region

AbstractUsing a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 144° and 154°E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual variabilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

Dependence of Model Energy Spectra on Vertical Resolution

AbstractMany high-resolution atmospheric models can reproduce the qualitative shape of the atmospheric kinetic energy spectrum, which has a power-law slope of −3 at large horizontal scales that shallows to approximately −5/3 in the mesoscale. This paper investigates the possible dependence of model energy spectra on the vertical grid resolution. Idealized simulations forced by relaxation to a baroclinically unstable jet are performed for a wide range of vertical grid spacings Δz. Energy spectra are converged for Δz 200 m but are very sensitive to resolution with 500 m ≤ Δz ≤ 2 km. The nature of this sensitivity depends on the vertical mixing scheme. With no vertical mixing or with weak, stability-dependent mixing, the mesoscale spectra are artificially amplified by low resolution: they are shallower and extend to larger scales than in the converged simulations. By contrast, vertical hyperviscosity with fixed grid-scale damping rate has the opposite effect: underresolved spectra are spuriously steepened. High-resolution spectra are converged except for the stability-dependent mixing case, which are damped by excessive mixing due to enhanced shear over a wide range of horizontal scales. It is shown that converged spectra require resolution of all vertical scales associated with the resolved horizontal structures: these include quasigeostrophic scales for large-scale motions with small Rossby number and the buoyancy scale for small-scale motions at large Rossby number. It is speculated that some model energy spectra may be contaminated by low vertical resolution, and it is recommended that vertical-resolution sensitivity tests always be performed.

Comment on: Structure, transport, and vertical coherence of the Gulf Stream from the Straits of Florida to the Southeast Newfoundland Ridge, by Meinen and Luther

Remarkably low vertical coherences in downstream velocities have been reported recently for three alongstream locations in the Gulf Stream, a Western Boundary Current with known three-dimensional dynamics of a baroclinically and barotropically unstable jet. This comment addresses the possibility that spatial averaging of coherences may be contributing to the low values estimated.

Large eddy simulation predictions of absolutely unstable round hot jet

The paper presents a novel view on the absolute instability phenomenon in heated variable density round jets. As known from literature the global instability mechanism in low density jets is released when the density ratio is lower than a certain critical value. The existence of the global modes was confirmed by an experimental evidence in both hot and air-helium jets. However, some differences in both globally unstable flows were observed concerning, among others, a level of the critical density ratio. The research is performed using the Large Eddy Simulation (LES) method with a high-order numerical code. An analysis of the LES results revealed that the inlet conditions for the velocity and density distributions at the nozzle exit influence significantly the critical density ratio and the global mode frequency. Two inlet velocity profiles were analyzed, i.e., the hyperbolic tangent and the Blasius profiles. It was shown that using the Blasius velocity profile and the uniform density distribution led to a significantly better agreement with the universal scaling law for global mode frequency.

Observation of Electrospinning Behavior of Nanoscale Fibers by a High-Speed Camera

Electrospinning is regarded as an effective method to generate nanoscale fibers. However, we believe that the mechanism of the formation of nanoscale fibers by Electrospinning has not yet been fully investigated. In this study, the jet behavior under various conditions was observed with a high-speed camera with short exposure periods. The behaviors of stable and unstable jets were compared. The effect of bending instability on the deposition pattern of jets was examined at different voltages and concentrations. In addition, the diameters of nanoscale fibers were analyzed as a function of sampling position.

Measurement of the dispersion relation of a convectively unstable capillary jet under confinement

The dispersion relation of a confined capillary jet with negligible inertia is measured by Fourier analysis of the jet radius. The real part of the dispersion relation, obtained through the spatiotemporal Fourier spectrum, demonstrates that the phase velocity is independent of the perturbation frequency, at least in the accessible range of frequencies. The imaginary part of the dispersion relation, i.e., the spatial growth rate, is frequency dependent. To measure this dependence, an external forcing is used to amplify modes with different frequencies. The spatial growth rate is then obtained from the exponential growth of the corresponding Fourier mode. The phase velocity and the spatial growth rate are found to increase and decrease, respectively, with the capillary number and to depend only weakly on the degree of confinement. These observations are consistent with a spatial instability that convects the perturbations downstream with velocity proportional to the capillary number. The frequency of the dominant mode in the absence of forcing, on the contrary, depends mainly on the degree of confinement, i.e., on the jet diameter, and only weakly on the capillary number, which is consistent with the mode selection of the Rayleigh-Plateau instability. Experimental measurements are compared to predicted dispersion relations reported in the literature, and good qualitative agreement is observed.

High-frequency sea level oscillations in the Mediterranean and their connection to synoptic patterns

This paper contains a basin-wide assessment of high-frequency sea level oscillations in the Mediterranean Sea for the period from 2010 to 2014. Sea level series with temporal resolutions of 1min were taken from 29 tide gauges that had been operational for at least 4years with high-quality time series. The contribution of high-frequency sea level variance (2min–6h) to the total and residual sea level variance was estimated to vary between 0.4% and 9.5% and between 0.6% and 12.1%, respectively, but to become dominant at some stations during extreme high-frequency events. A total of 36 high-frequency sea level events were extracted from the series, with some occurring locally in one of the four selected regions of Spain, Sardinia, Sicily and the Greece–Ionian Sea (6 events), some occurring over two or three regions (19 events), and some occurring in all of the selected regions within a period of 1–3days (11 events). The basin-wide events normally propagate from the western to the eastern parts of the basin, with an average velocity of approximately 30km/h. Synoptic patterns associated with high-frequency events were analysed, and a high resemblance to patterns associated with meteotsunamis was observed: (i) a cyclone with a centre W-NW from the affected area, (ii) a strong thermal front in the lower troposphere, and (iii) a forefront of an unstable strong mid-tropospheric jet placed over the affected area. The strong coherence between high-frequency sea level events and synoptic patterns introduces the possibility of a timely forecast of these events.

Additional Value of Intra-Aneurysmal Hemodynamics in Discriminating Ruptured versus Unruptured Intracranial Aneurysms.

Hemodynamics are thought to play an important role in the rupture of intracranial aneurysms. We tested whether hemodynamics, determined from computational fluid dynamics models, have additional value in discriminating ruptured and unruptured aneurysms. Such discriminative power could provide better prediction models for rupture. A cross-sectional study was performed on patients eligible for endovascular treatment, including 55 ruptured and 62 unruptured aneurysms. Association with rupture status was tested for location, aneurysm type, and 4 geometric and 10 hemodynamic parameters. Patient-specific spatiotemporal velocities measured with phase-contrast MR imaging were used as inflow conditions for computational fluid dynamics. To assess the additional value of hemodynamic parameters, we performed 1 univariate and 2 multivariate analyses: 1 traditional model including only location and geometry and 1 advanced model that included patient-specific hemodynamic parameters. In the univariate analysis, high-risk locations (anterior cerebral arteries, posterior communicating artery, and posterior circulation), daughter sacs, unstable inflow jets, impingements at the aneurysm body, and unstable complex flow patterns were significantly present more often in ruptured aneurysms. In both multivariate analyses, only the high-risk location (OR, 3.92; 95% CI, 1.77-8.68) and the presence of daughter sacs (OR, 2.79; 95% CI, 1.25-6.25) remained as significant independent determinants. In this study population of patients eligible for endovascular treatment, we found no independent additional value of aneurysmal hemodynamics in discriminating rupture status, despite high univariate associations. Only traditional parameters (high-risk location and the presence of daughter sacs) were independently associated with ruptured aneurysms.

Geometric Decomposition of Eddy Feedbacks in Barotropic Systems

AbstractIn oceanic and atmospheric flows, the eddy vorticity flux divergence—denoted “F” herein—emerges as a key dynamical quantity, capturing the average effect of fluctuations on the time-mean circulation. For a barotropic system, F is derived from the horizontal velocity covariance matrix, which itself can be represented geometrically in terms of the so-called variance ellipse. This study proves that F may be decomposed into two different components, with distinct geometric interpretations. The first arises from variations in variance ellipse orientation, and the second arises from variations in the kinetic energy of the anisotropic part of the velocity fluctuations, which can be seen as a function of variance ellipse size and shape. Application of the divergence theorem shows that F integrated over a closed region is explained entirely by separate variations in these two quantities around the region periphery. A further decomposition into four terms shows that only four specific spatial patterns of ellipse variability can give rise to a nonzero eddy vorticity flux divergence. The geometric decomposition offers a new tool for the study of eddy–mean flow interactions, as is illustrated with application to an unstable eastward jet on a beta plane.

Fast transient microjets induced by hemispherical cavitation bubbles

AbstractWe report on a novel method to generate fast transient microjets and study their characteristics. The simple device consists of two electrodes on a substrate with a hole in between. The side of the substrate with the electrodes is submerged in a liquid. Two separate microjets exit through the tapered hole after an electrical discharge is induced between the electrodes. They are formed during the expansion and collapse of a single cavitation bubble. The cavitation bubble dynamics as well as the jets were studied with high-speed photography at up to 500 000 f.p.s. With increasing jet velocity they become unstable and spray formation is observed. The jet created during expansion (first jet) is in most cases slower than the jet created during bubble collapse, which can reach up to $400~\text{m}~\text{s}^{-1}$. The spray exiting the orifice is at least in part due to the presence of cavitation in the microchannel as observed by high-speed recording. The effect of viscosity was tested using silicone oil of 10, 50 and 100 cSt. Interestingly, for all liquids the transition from a stable to an unstable jet occurs at $We\sim 4600$. We demonstrate that these microjets can penetrate into soft material; thus they can be potentially used as a needleless drug delivery device.

ICOPE-15-C084 Experimental study on pressure oscillation caused by unstable steam jet in subcooled water flow

ICOPE-15-C084 Experimental study on pressure oscillation caused by unstable steam jet in subcooled water flow

Ribbon turbulence

We investigate the non-linear equilibration of a two-layer quasi-geostrophic flow in a channel with an initial eastward baroclinically unstable jet in the upper layer, paying particular attention to the role of bottom friction. In the limit of low bottom friction, classical theory of geostrophic turbulence predicts an inverse cascade of kinetic energy in the horizontal with condensation at the domain scale and barotropization in the vertical. By contrast, in the limit of large bottom friction, the flow is dominated by ribbons of high kinetic energy in the upper layer. These ribbons correspond to meandering jets separating regions of homogenized potential vorticity. We interpret these results by taking advantage of the peculiar conservation laws satisfied by this system: the dynamics can be recast in such a way that the initial eastward jet in the upper layer appears as an initial source of potential vorticity levels in the upper layer. The initial baroclinic instability leads to a turbulent flow that stirs this potential vorticity field while conserving the global distribution of potential vorticity levels. Statistical mechanical theory of the 112 layer quasi-geostrophic model predicts the formation of two regions of homogenized potential vorticity separated by a minimal interface. We explain that cascade phenomenology leads to the same result. We then show that the dynamics of the ribbons results from a competition between a tendency to reach the equilibrium state and baroclinic instability that induces meanders of the interface. These meanders intermittently break and induce potential vorticity mixing, but the interface remains sharp throughout the flow evolution. We show that for some parameter regimes, the ribbons act as a mixing barrier which prevents relaxation toward equilibrium, favouring the emergence of multiple zonal (eastward) jets.