Instability Of Sheet Research Articles

Meltwater discharge during the Holocene from the Wilkes subglacial basin revealed by beryllium isotope analysis of marine sediments

ABSTRACTUnderstanding Antarctic Ice Sheet dynamics related to global climate change is of scientific and societal interest as the future behaviour of the ice sheet under the currently changing climate is unknown. We present beryllium‐10 (10Be) analysis of a high‐resolution marine sediment core from the Adélie Basin near the eastern Wilkes Land margin, which is susceptible to marine ice sheet instability due to the low‐lying nature and down‐sloping trough of the Wilkes Subglacial Basin. Combined with a newly constructed age model using compound specific radiocarbon dates, the data reveal three events associated with high meteoric 10Be at ca. ~10 ka, ca. ~6.5 ka and from ca. ~4 ka. We interpret these high meteoric 10Be events to be derived from the deposition of 10Be released from the ice sheet during meltwater discharge. In particular, the shift to higher meteoric 10Be concentration at~4 ka may correspond to changes in climate patterns at this time. Copyright © 2019 John Wiley & Sons, Ltd.

Fast Recursive Reconnection and the Hall Effect: Hall-MHD Simulations

Abstract Magnetohydrodynamic (MHD) theory and simulations have shown that reconnection is triggered via a fast “ideal” tearing instability in current sheets whose inverse aspect ratio decreases to , with S as the Lundquist number defined by the half-length L of the current sheet (of a thickness of 2a). Ideal tearing, in 2D sheets, triggers a hierarchical collapse via stretching of X-points and recursive instability. At each step, the local Lundquist number decreases, until the subsequent sheet thickness either approaches kinetic scales or the Lundquist number becomes sufficiently small. Here we carry out a series of Hall-MHD simulations to show how the Hall effect modifies recursive reconnection once the ion inertial scale is approached. We show that as the ion inertial length becomes of the order of the inner, singular layer thickness at some step of the recursive collapse, reconnection transits from the plasmoid-dominant regime to an intermediate plasmoid+Hall regime and then to the Hall-dominant regime. The structure around the X-point, the reconnection rate, the dissipation property, and the power spectra are also modified significantly by the Hall effect.

Linear instability of viscoelastic planar liquid sheets in the presence of gas velocity oscillations

The linear temporal instability of a viscoelastic liquid sheet has been studied here, in the presence of gas velocity oscillations. The Floquet theory was applied to solve this problem. More than one unstable region appeared and these regions were divided into the Kelvin–Helmholtz (K–H) and the parametric unstable regions. In the K–H unstable region, the disturbance grew in the form of travelling waves; but in the parametric unstable regions, the disturbance was in the form of standing waves. An increase in oscillation amplitude contributed to the growth of instability in both regions, due to the stronger hydrodynamic force and parametric resonance, respectively; and the parametric unstable regions were more sensitive to the increased amplitude. The main effect of the oscillation frequency was to change the location of the parametric unstable regions. A viscoelastic liquid sheet can behave with greater stability than its Newtonian counterpart in all unstable regions; however, zero shear viscosity, stress relaxation time, and deformation retardation time all affect the parametric regions more dramatically than the K–H unstable regions. The parametric unstable region is more sensitive to viscous dissipation than the K–H unstable regions, resulting in a shift in the leading role between the different unstable regions. The competition between the K–H and parametric unstable regions is discussed in detail here.

Dynamic Evolution of Current Sheets, Ideal Tearing, Plasmoid Formation and Generalized Fractal Reconnection Scaling Relations

Abstract Magnetic reconnection may be the fundamental process allowing energy stored in magnetic fields to be released abruptly, with solar flares and coronal mass ejection being archetypal natural plasma examples. Magnetic reconnection is much too slow of a process to be efficient on the large scales, but accelerates once small enough scales are formed in the system. For this reason, the fractal reconnection scenario was introduced to explain explosive events in the solar atmosphere; it was based on the recursive triggering and collapse via tearing instability of a current sheet originally thinned during the rise of a filament in the solar corona. Here we compare the different fractal reconnection scenarios that have been proposed, and derive generalized scaling relations for the recursive triggering of fast, “ideal” —i.e., Lundquist number independent—tearing in collapsing current sheet configurations with arbitrary current profile shapes. An important result is that the Sweet–Parker scaling with Lundquist number, if interpreted as the aspect ratio of the singular layer in an ideally unstable sheet, is universal and does not depend on the details of the current profile in the sheet. Such a scaling, however, must not be interpreted in terms of stationary reconnection, rather it defines a step in the accelerating sequence of events of the ideal tearing mediated fractal cascade. We calculate scalings for the expected number of plasmoids for such generic profiles and realistic Lundquist numbers, showing that in ideal tearing scenarios a smaller number of plasmoids, by orders of magnitude, is generated compared to the original fractal model.

Effect of heat and mass transfer on the instability of an annular liquid sheet

A temporal linear instability analysis has been conducted to investigate the effect of heat and mass transfer on the instability of an annular liquid sheet axially moving in the gas medium. The dispersion relation is obtained and solved numerically. Energy budget is calculated to provide physical insights of various instability driving mechanisms. The results show that heat and mass transfer promotes the wave growth rate mainly at small wave numbers. In contrast to the case without heat and mass transfer, the wave growth rate with strong heat and mass transfer peaks at zero wave number. Liquid viscosity is found to have a minimal stabilizing effect on the sheet instability. Regardless of the heat and mass transfer, increasing liquid Weber number suppresses the sheet instability below a crossover point of wave number at 1.15, and promotes the sheet instability above the crossover point. In the presence of strong heat and mass transfer, increasing gas-to-liquid density ratio reduces the wave growth rate at small wave numbers below 0.28, and beyond which increases the wave growth rate. Reducing sheet thickness promotes the sheet instability for all wave numbers. The energy budgets show that the inner gas disturbance is most responsible for promoting the wave growth rate at large liquid Weber numbers; and the inner interface dominates the outer one in destabilizing the annular liquid sheet.

Spray dynamics simulations for pulsatile injection at different ambient pressure and temperature conditions

A numerical study on the transient characteristics of a pulsatile, iso-octane spray issuing from a pressure-swirl atomizer is presented. The effects of system pressure and temperature, as well as the initial fuel temperature on spray dispersion and evaporation, are highlighted. The computations were carried out using ANSYS FLUENT-15.0, assuming the spray dispersion to be axisymmetric. Gas phase turbulence is simulated using the renormalized group k- ε model, while the discrete phase model is used for tracking fuel droplets. The linear instability sheet atomization model is adopted for the primary breakup of the liquid sheet, and the Taylor Analogy Breakup and Wave Breakup models are adopted for the secondary breakup, depending upon the operating conditions. The drag force on the droplet is evaluated, after incorporating the effects of evaporation and neighbouring droplets, along with droplet shape distortion. The significance of droplet collision on the evolution of droplet size distribution is examined. The local mean drop sizes and spray penetration length are in agreement with the experimental results of the literature. The predicted results indicate that the spray is narrower and penetrates less at higher ambient pressure. In this respect, the additional force on droplets due to local static pressure gradient is examined in detail. The effect of ambient conditions on the spray evaporation process is studied based on the spatio-temporal evolution of the equivalence ratio of the mixture of fuel vapour and air.

Marine ice sheet instability amplifies and skews uncertainty in projections of future sea-level rise

Sea-level rise may accelerate significantly if marine ice sheets become unstable. If such instability occurs, there would be considerable uncertainty in future sea-level rise projections due to imperfectly modeled ice sheet processes and unpredictable climate variability. In this study, we use mathematical and computational approaches to identify the ice sheet processes that drive uncertainty in sea-level projections. Using stochastic perturbation theory from statistical physics as a tool, we show mathematically that the marine ice sheet instability greatly amplifies and skews uncertainty in sea-level projections with worst-case scenarios of rapid sea-level rise being more likely than best-case scenarios of slower sea-level rise. We also perform large ensemble simulations with a state-of-the-art ice sheet model of Thwaites Glacier, a marine-terminating glacier in West Antarctica that is thought to be unstable. These ensemble simulations indicate that the uncertainty solely related to internal climate variability can be a large fraction of the total ice loss expected from Thwaites Glacier. We conclude that internal climate variability alone can be responsible for significant uncertainty in projections of sea-level rise and that large ensembles are a necessary tool for quantifying the upper bounds of this uncertainty.

Instability of an annular liquid sheet exposed to compressible gas flows

A temporal linear stability analysis was performed for a viscous annular liquid sheet exposed to compressible gas flows. An analytical dispersion relation illustrating the disturbance growth characteristics at the gas–liquid interfaces was formulated and validated, and a parametric study was performed to explore the mechanism that affects the sheet instability. The results reflect that para-sinuous disturbances on the liquid sheet always predominate over para-varicose disturbances under the considered flow conditions. The aerodynamic interaction at the gas–liquid interfaces of the liquid sheet is the decisive factor on sheet instability. As for the main factors affecting the aerodynamic effects, the contribution of the gas–liquid velocity difference to aerodynamic interactions is more significant than that of the gas–liquid density ratio. Neglecting the gas compressibility markedly underestimates the aerodynamic effects at the gas–liquid interfaces for flow conditions of high gas Mach number, which results in decreased disturbance growth rate and dominant wave number. Moreover, both the liquid viscous force and the surface tension force hold back the disturbance growing and stabilize the liquid sheet under the considered conditions. Note that the destabilizing effect of the viscous force is limited for high liquid Reynolds number cases. In general, it is very necessary to consider the compressibility of the gas flows in the instability study of practical twin-fluid atomization.

Effects of air swirler geometry on air and spray droplet interactions in a spray chamber

In this work, interactions between cold spray and swirl airflow are characterized experimentally and numerically for various air swirler geometries. The number of vanes, vane angle, vane curvature, and air velocity are varied for the swirling airflow. Spray visualization and phase Doppler interferometry techniques have been employed to obtain important parameters such as spray cone angle and mean drop size. In the numerical work, the Realizable [Formula: see text] turbulence model has been employed along with the Linearized Instability Sheet Atomization and Taylor Analogy Breakup to simulate spray field. The predicted results for the airflow field compare well with available experimental results in published literature. Good match is also found between the present predictions and measured spray data. The radial distribution of Sauter mean diameter exhibits peak values at the periphery of the spray near the injector, due to liquid swirl. The peak Sauter mean diameter shifts to the spray axis, beyond the recirculation zone, due to entrainment of droplets by the swirling air stream. The volume fraction of droplets exhibits multi-modal distribution, due to the interactions between spray droplets and recirculating airflow. A curved swirler gives rise to finer spray compared to a flat swirler, due to lower losses and better utilization of momentum.

Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula

Lack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ18O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m.

Subarcsecond Blobs in Flare-related Coronal Jets

Abstract In this paper, we report multiwavelength observations of subarcsecond blobs in coronal jets. In AR 12149, a C5.5 circular-ribbon flare occurred at ∼04:55 UT on 2014 August 24, which consisted of a discrete circular ribbon and a short inner ribbon inside. Two jets (jet1 and jet2) were related to the flare. Jet1 appeared first and experienced untwisting motion during its early propagation along a closed coronal loop. Jet2 appeared 6 minutes later and propagated upward along another closed loop. During its initial phase, a big plasmoid was ejected out of jet2 at a speed of ∼150 km s−1. After the flare peak time (05:02 UT), multiple bright and compact blobs appeared in the lower part of jet2, which were observed by the Slit-Jaw Imager (SJI) on board the Interface Region Imaging Spectrograph. The blobs observed by SJI in 1330 Å have sizes of 0.″45–1.″35, nearly 84% of which are subarcsecond (<1″). The mean value and standard deviation of the sizes are 0.″78 and 0.″19, respectively. The velocities of the blobs range from 10 to more than 220 km s−1, some of which decelerate and disappear during the upward propagation. Three of the blobs had their counterparts in extreme-ultraviolet (EUV) wavelengths observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory spacecraft. The velocities are almost identical in ultraviolet (UV) and EUV wavelengths. We propose that the blobs observed in 1330 Å are the cool component (∼0.025 MK), while the blobs observed in EUV are the hot component of several MK. In jet1, only one blob was present, with a size of ∼1″ and a velocity of ∼40 km s−1. We conclude that the blobs are created by the tearing-mode instability of the current sheet at the base or inside the coronal jets. Our results have important implications for uncovering the fine structures of coronal jets and understanding the relationship between the blobs observed at UV and EUV wavelengths.

Elastic instabilities of thin annular sheets due to partial shrinkage

Elastic instabilities of thin annular sheets due to partial shrinkage

Marine ice sheet instability and ice shelf buttressing of the Minch Ice Stream, northwest Scotland

Abstract. Uncertainties in future sea level projections are dominated by our limited understanding of the dynamical processes that control instabilities of marine ice sheets. The last deglaciation of the British–Irish Ice Sheet offers a valuable example to examine these processes. The Minch Ice Stream, which drained a large proportion of ice from the northwest sector of the British–Irish Ice Sheet during the last deglaciation, is constrained with abundant empirical data which can be used to inform, validate, and analyse numerical ice sheet simulations. We use BISICLES, a higher-order ice sheet model, to examine the dynamical processes that controlled the retreat of the Minch Ice Stream. We perform simplified experiments of the retreat of this ice stream under an idealised climate forcing to isolate the effect of marine ice sheet processes, simulating retreat from the continental shelf under constant “warm” surface mass balance and sub-ice-shelf melt. The model simulates a slowdown of retreat as the ice stream becomes laterally confined at the mouth of the Minch strait between mainland Scotland and the Isle of Lewis, resulting in a marine setting similar to many large tidewater glaciers in Greenland and Antarctica. At this stage of the simulation, the presence of an ice shelf becomes a more important control on grounded ice volume, providing buttressing to upstream ice. Subsequently, the presence of a reverse slope inside the Minch strait produces an acceleration in retreat, leading to a “collapsed” state, even when the climate returns to the initial “cold” conditions. Our simulations demonstrate the importance of the marine ice sheet instability and ice shelf buttressing during the deglaciation of parts of the British–Irish Ice Sheet. We conclude that geological data could be applied to further constrain these processes in ice sheet models used for projecting the future of contemporary ice sheets.

Suppression of marine ice sheet instability

A long-standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean (marine ice sheets) to destabilise under buoyancy. This paper addresses the processes by which a buoyancy-driven mechanism for the retreat and ultimate collapse of such ice sheets – the marine ice sheet instability – is suppressed by lateral stresses acting on its floating component (the ice shelf). The key results are to demonstrate the transition between a mode of stable (easily reversible) retreat along a stable steady-state branch created by ice-shelf buttressing to tipped (almost irreversible) retreat across a critical parametric threshold. The conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice-shelf profile and can be largely independent of basal stress, in contrast to principles established from studies of unbuttressed grounding-line dynamics. The stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding-line stability (bifurcation) diagrams, which provide a rapid assessment of the steady-state positions, their natures and the conditions for secondary grounding, giving clear visualisations of global stabilisation conditions. A further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components. The region forms an extended grounding area in which the ice sheet lies very close to flotation, and there is no clearly distinguished grounding line. The formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line. The results of this paper provide conceptual insight into the phenomena controlling the stability of the West Antarctic Ice Sheet, the collapse of which has the potential to dominate future contributions to global sea-level rise.

Analysis of viscous fluid flow in a pressure-swirl atomizer using large-eddy simulation

A computational fluid dynamics study is carried out on the inner nozzle flow and onset of liquid sheet instability in a large-scale pressure-swirl atomizer with asymmetric inflow configuration for high viscosity fluids. Large-eddy simulations (LES) of the two-phase flow indicate the unsteady flow character inside the nozzle and its influence on liquid sheet formation. A novel geometric volume-of-fluid (VOF) method by Roenby et al. (2016), termed isoAdvector, is applied for sharp interface capturing. We carry out a Reynolds number sweep (420 ≤ Re ≤ 5300) in order to investigate the link between the asymmetric inner nozzle flow and liquid sheet characteristics in laminar, transitional and fully turbulent conditions. Inside the nozzle, the numerical simulations reveal counter-rotating Dean vortices, flow impingement locations, and strong asymmetric flow features at all investigated Reynolds numbers. A helical, rotating gaseous core is observed when Re ≥ 1660. For laminar flow (Re=420), an S-shaped liquid film is observed, while the gas core presence at Re ≥ 1660 results in a hollow cone liquid sheet. For the intermediate value Re=830, the numerical simulations indicate a liquid sheet of mixed type. Consequences of the inflow asymmetry and Reynolds number to the uniformity of the injected liquid mass distribution and liquid sheet instability are pointed out.

Linear and Non-linear Analysis of Breakup of Liquid Sheets: a Review

Atomization primarily involves interaction between liquid and surrounding gaseous medium resulting in the growth of unstable waves on a distorted liquid surface. There is a huge volume of work surrounding various models and theories involved in investigation of interface dynamics during sheet breakup. The present work summarizes the linear and nonlinear studies for different geometrical shapes of liquid sheet such as planar, annular, and swirling. The techniques involved in deriving nonlinear governing equations such as perturbation technique, multiple scales approach, and long wave approximation have been outlined. Effects of various factors on sheet instability such as viscosity, compressibility, surface temperature difference, unequal velocity, lateral waves and gas swirl have been listed. Research lacunae and scope of future work are presented in the summary.

Features of ice-rafted debris (IRD) at IODP site U1312 and their palaeoenvironmental implications during the last 2.6 Myr

In this paper, we build an age model by tuning the G. ruber δ18O curve to the LR04 stack and U1308 benthic δ18O curve with the aid of the high-resolution lightness parameter L*, which mirrors the marine oxygen isotope stages, and we analyse the major palaeoenvironmental events indicated by different proxies, particularly the >250 μm fraction and the 150–250 μm fraction of ice-rafted debris (IRD) at IODP Site U1312. In general, the input of IRD at Site U1312 can be divided into two stages. In Stage 1, ranging from ~2.6 Ma to ~1 Ma, the number of lithic grains was small, the frequency of ice rafting events was low, and the components were diverse and mainly comprised quartz, feldspar and clasts. In Stage 2, ranging from ~1 Ma to the present, the number and frequency of lithic grains considerably increased and were accompanied by detrital carbonate additions derived from the Hudson Strait after ~650 ka. Regarding the IRD features in the study area, the distinctive layers are as follows. IRD did not occur until 2.52 Ma at Site U1312, implying that icebergs discharged by the Northern Hemisphere glaciers bearing lithic grains were involved in pelagic sedimentation for the first time. Interestingly, a monotonic component of debris, magnetite with some ilmenite and quartz, was observed in a single layer at approximately 1.4 Ma. The source of this component may simply be the erosion of volcanic formations in Iceland during the glacial period. Detrital carbonate played an important role in the IRD component after ~650 ka, and the input and frequency of IRD increased dramatically, revealing glacier enhancements and the instability of the Laurentide ice sheet. The IRD characteristics at Site U1312 during MIS 22, MIS 12 and MIS 6 were different from those at other sites near the IRD belt and may be closely related to the European ice sheets. During MIS 10-8, the input of IRD was high, the dominant warm species G. ruber was too rare for the isotope test to be conducted, and the abundance of planktonic foraminifera severely declined, which may indicate that extreme conditions occurred at Site U1312 during this interval.

Stopping the flood: could we use targeted geoengineering to mitigate sea level rise?

Abstract. The Marine Ice Sheet Instability (MISI) is a dynamic feedback that can cause an ice sheet to enter a runaway collapse. Thwaites Glacier, West Antarctica, is projected to be the largest individual source of future sea level rise and may have already entered MISI. Here, we use a suite of coupled quasi-2-D ice–ocean simulations to explore whether targeted geoengineering using either a continuous artificial sill or isolated artificial pinning points could counter a collapse. Successful interventions occur when the floating ice shelf regrounds on the structure, increasing buttressing and reducing ice flux across the grounding line. Regrounding is more likely with a continuous sill that is able to block warm water transport to the grounding line. The smallest design we consider is comparable in scale to existing civil engineering projects but only has a 30 % success rate, while larger designs are more effective. There are multiple possible routes forward to improve upon the designs that we considered, and with decades or more to research designs it is plausible that the scientific community could come up with a plan that is both effective and achievable. While reducing emissions remains the short-term priority for minimizing the effects of climate change, in the long run humanity may need to develop contingency plans to deal with an ice sheet collapse.

Vortex sheet roll-up revisited

The classical problem of roll-up of a two-dimensional free inviscid vortex sheet is reconsidered. The singular governing equation brings with it considerable difficulty in terms of actual calculation of the sheet dynamics. Here, the sheet is discretized into segments that maintain it as a continuous object with curvature. A model for the self-induced velocity of a finite segment is derived based on the physical consideration that the velocity remain bounded. This allows direct integration through the singularity of the Birkhoff–Rott equation. The self-induced velocity of the segments represents the explicit inclusion of stretching of the sheet and thus vorticity transport. The method is applied to two benchmark cases. The first is a finite vortex sheet with an elliptical circulation distribution. It is found that the self-induced velocity is most relevant in regions where the curvature and the sheet strength or its gradient are large. The second is the Kelvin–Helmholtz instability of an infinite vortex sheet. The critical time at which the sheet forms a singularity in curvature is accurately predicted. As observed by others, the vortex sheet strength forms a finite-valued cusp at this time. Here, it is shown that the cusp value rapidly increases after the critical time and is the impetus that initiates the roll-up process.

Response of Marine‐Terminating Glaciers to Forcing: Time Scales, Sensitivities, Instabilities, and Stochastic Dynamics

Recent observations indicate that many marine‐terminating glaciers in Greenland and Antarctica are currently retreating and thinning, potentially due to long‐term trends in climate forcing. In this study, we describe a simple two‐stage model that accurately emulates the response to external forcing of marine‐terminating glaciers simulated in a spatially extended model. The simplicity of the model permits derivation of analytical expressions describing the marine‐terminating glacier response to forcing. We find that there are two time scales that characterize the stable glacier response to external forcing, a fast time scale of decades to centuries, and a slow time scale of millennia. These two time scales become unstable at different thresholds of bed slope, indicating that there are distinct slow and fast forms of the marine ice sheet instability. We derive simple expressions for the approximate magnitude and transient evolution of the stable glacier response to external forcing, which depend on the equilibrium glacier state and the strength of nonlinearity in forcing processes. The slow response rate of marine‐terminating glaciers indicates that current changes at some glaciers are set to continue and accelerate in coming centuries in response to past climate forcing and that the current extent of change at these glaciers is likely a small fraction of the future committed change caused by past climate forcing. Finally, we find that changing the amplitude of natural fluctuations in some nonlinear forcing processes, such as ice shelf calving, changes the equilibrium glacier state.