Nonlinear magnetic reconnection models with separatrix jets

The basic plasma processes associated with magnetic reconnection in the Earth's magnetosphere are studied in our proposed project. In the past year, several problems associated with the 2-D and 3-D nonlinear reconnection processes were investigated. A new theory for nonlinear fast steady-state magnetic reconnection is proposed. The inflow region possesses highly curved magnetic field lines, which differs from that in the classical model of Petschek. A separatrix jet of plasma is ejected from the central diffusion region along the magnetic separatrix. Two types of outflow are studied, the simplest one possessing a potential outflow magnetic field and the other containing weak standing shock waves attached to the ends of the diffusion region. Across the separatrix jet, the outflow can be either slowed down by the fast-mode shocks or speeded up by the slow-mode shocks, depending on the downstream boundary conditions. In the 3-D MHD simulations, frayed magnetic flux ropes are found to be formed during the dayside magnetic reconnection process. Strong flux rope-aligned plasma flows are observed. Simulations of the driven reconnection process also indicate that the focusing of the Poynting flux has an important effect on the onset of reconnection and the location of reconnection sites.

Time evolutions of the electron diffusion region embedded in the ion-scale diffusion region and the reconnection rate associated with magnetic reconnection are investigated using 2-1/2 dimensional full kinetic simulations in a large system, so that any effects of the downstream boundary conditions are negligible. The simulation code employs the adaptive mesh refinement technique and the particle splitting algorithm to the conventional particle-in-cell code, which enable us to perform large-scale full particle simulations. It is shown that the reconnection rate increases associated with magnetic reconnection and reaches a peak value large enough for fast reconnection, but then it decreases as time goes on, even though the effects of the system boundary are negligible. The key process responsible for slowing the reconnection processes is the extension of the electron diffusion region in association with the enhancement of the polarization electric field directing toward the neutral sheet in the electron inflow region. The polarization electric field is caused by the inertia difference between ions and electrons, and enhanced by the meandering motions of the background ions. In order to confirm the role of the polarization electric field, we compare the simulation results with mi∕me=1 and 100. It is found that (1) a quasi-steady reconnection is achieved in the system where the polarization electric field does not arise, (2) a large reconnection rate is obtained, even in the system without the Hall effects. It is suggested that the anomalous resistivity due to the Buneman-type instability might be required to support a steady-state fast reconnection.

Magnetic reconnection is widely believed to be involved in dynamical phenomena such as solar flares or magnetospheric substorms. The Sweet–Parker model of magnetic reconnection in a Y-type geometry predicts a characteristic time scale proportional to S1/2 (where S is the Lundquist number), which is too slow to account for the observed time scales. The Petschek model, in contrast, predicts a time scale proportional to ln S in an X-point geometry. Numerical magnetohydrodynamic (MHD) simulations in the high-S regime generally validate the Sweet–Parker model, unless the resistivity is enhanced in the diffusion region to large values (such that typically S<103). It is demonstrated in this paper that nonlinear reconnection dynamics in a Harris sheet driven by inward boundary flows occurs on a nonlinear time scale that is proportional to S1/5 and thus has a weaker dependence on resistivity than the Sweet–Parker time scale. The current sheet amplitude at the separatrix (spanning Y points) grows algebraically in the linear regime but is suddenly enhanced after it makes a transition to the nonlinear regime. An analytical calculation is given for both the linear and the nonlinear regimes, and supported by two-dimensional resistive MHD simulations. The features of sudden current sheet enhancement and fast reconnection, controlled by boundary flows, are relevant to the phenomena of substorm onset or the impulsive phase of flares.

Magnetic reconnection is a fundamental process that can take place in astrophysical or laboratory plasmas. It occurs within regions of large magnetic gradient where the magnetic field is no longer frozen to the plasma but instead diffuses through it, releasing magnetic energy and causing a change in the connectivity of the field lines. In particular, on the Sun, magnetic reconnection is believed to play an important role in coronal heating, X ray bright points, solar flares and canceling magnetic features. Here, an exact time‐dependent solution of the MHD equations for magnetic annihilation in response to a time‐varying stagnation point flow is presented. The main assumptions in this model are that the magnetic field lines are straight (so that there is no magnetic tension acting on the plasma) and the flow that carries the field lines together is of stagnation point type. This is a reasonable model for the resistive MHD behaviour near the X point of a reconnecting field, especially when the central diffusion region is long, as in the flux pile up regime (Priest and Forbes 1986). The general solution is used to conduct a series of numerical experiments, namely, the evolution of different initial magnetic profiles in a steady flow; the effect of a sudden change in magnetic diffusivity on an initially steady state; the effects of a velocity that either increases linearly in time or ramps up from one steady value to another. The results exhibit the effects of subtle imbalances in diffusion and advection but have the following general features: (1) a diffusion layer is created, the thickness of which is determined by the nature of the plasma flow; (2) the magnetic field outside the diffusion region is determined by advection and will either exponentially increase, exponentially decay or remain in a steady state, depending on whether the initial magnetic profile B (y) ∼ y−n has n < 1, n > 1, or n = 1, respectively; and (3) the magnetic field within the diffusion layer tends to respond to the advected magnetic field at its edge.

Two-dimensional particle-in-cell simulations have been performed to study magnetic reconnection in low-density electron-positron plasmas without a guide magnetic field. Impulsive reconnection rates become of the order of unity when the background density is much smaller than 10% of the density in the initial current layer. It is demonstrated that the outflow speed is less than the upstream Alfvén speed, and that the time derivative of the density must be taken into account in the definition of the reconnection rate. The reconnection electric fields in the low-density regime become much larger than the ones in the high-density regime, and it is possible to accelerate the particles to high energies more efficiently. The inertial term in the generalized Ohm’s law is the most dominant term that supports a large reconnection electric field. An effective collisionless resistivity is produced and tracks the extension of the diffusion region in the late stage of the reconnection dynamics, and significant broadening of the diffusion region is observed. Because of the broadening of the diffusion region, no secondary islands, which have been considered to play a role to limit the diffusion region, are generated during the extension of the diffusion region in the outflow direction.

Alluvial fans and fan deltas can, in principle, have exactly the same upstream conditions, but fan deltas by definition have ponding water at their downstream boundary. This ponding creates effects on the autogenic behaviour of fan deltas, such as backwater adaptation, mouth bars and backward sedimentation, whereas alluvial fans may lack these effects. Hence the present authors hypothesize that morphodynamics on alluvial fans are determined primarily by upstream boundary conditions, whereas morphodynamics on fan deltas are determined by both the upstream and the downstream boundary condition and changes therein. To isolate the effects of the upstream and downstream boundaries, five new alluvial fan experiments are compared with the details of three fan deltas published earlier that were formed under very similar and simple conditions. Similar to the fan deltas, the alluvial fans build up by sheet flow, whilst quasi‐regular periods of incision cause temporary channelized flow. Incision is followed by channel backfilling, after which the fan returns to sheet flow. The channelization and backfilling in alluvial fans is markedly less pronounced and more prone to autogenic disturbance than in fan deltas. The difference is caused by morphodynamics at the downstream boundary. In a fan delta, the flow expansion of the channel causes deposition of all the sediment, which forms a mouth bar and causes strong backfilling. In an alluvial fan, on the other hand, the slope break at the fan perimeter causes some deposition, but transport is not reduced to zero. Consequently, the backfilling in alluvial fans is less pronounced than in fan deltas. Other published experiments support this trend: removal of the mouth bar by a river leads to permanent channelization, whilst pronounced mouth‐bar formation in highly channelized deltas promotes backward sedimentation. The experimental results for this study predict that, when alluvial fans prograde into lakes or deep rivers, they transition to fan deltas with increasingly deeper channels and thicker backfill deposits.

The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories.

As a coastal city which lies in lowland area, Jakarta is prone to flooding. One major river which flow through Jakarta is Ciliwung River. There are alternatives to reduce flood risk, such as: river capacity improvement, existing natural reservoir and polder system improvement, upstream reservoir construction, city drainage improvement, flood channel construction and flood diversion. This paper presents capacity analysis of a proposed flood diversion of Ciliwung River to Cipinang River. Cipinang River has its downstream end at Eastern Flood Canal (Kanal Banjir Timur, KBT). This diversion is based on the available capacity of KBT. A 1-D numerical hydraulic model using HEC-RAS based on a proposed design is used to assess the performance of the diversion system in any combination of upstream and downstream boundary condition. Simulations were done for steady condition. The results show that capacity of the system can be achieved for certain condition at upstream and downstream boundary. The effects at the downstream reach of Ciliwung and Cipinang River due to the diversion are also obtained.

Lagrangian generic second order traffic flow models for node

VISCOUS FLUTTER OF A FINITE ELASTIC MEMBRANE IN POISEUILLE FLOW

The paper will focus on one specific core‐compressor instability, rotating stall, because of the pressing industrial need to improve current design methods. The determination of the blade response during rotating stall is a difficult problem for which there is no reliable procedure. During rotating stall, the blades encounter the stall cells and the excitation depends on the number, size, exact shape and rotational speed of these cells. The long‐term aim is to minimize the forced response due to rotating stall excitation by avoiding potential matches between the vibration modes and the rotating stall pattern characteristics. Accurate numerical simulations of core‐compressor rotating stall phenomena require the modelling of a large number of bladerows using grids containing several tens of millions of points. The time‐accurate unsteady‐flow computations may need to be run for several engine revolutions for rotating stall to get initiated and many more before it is fully developed. The difficulty in rotating stall initiation arises from a lack of representation of the triggering disturbances which are inherently present in aeroengines. Since the numerical model represents a symmetric assembly, the only random mechanism for rotating stall initiation is provided by numerical round‐off errors. In this work, rotating stall is initiated by introducing a small amount of geometric mistuning to the rotor blades. Another major obstacle in modelling flows near stall is the specification of appropriate upstream and downstream boundary conditions. Obtaining reliable boundary conditions for such flows can be very difficult. In the present study, the low‐pressure compression (LPC) domain is placed upstream of the core compressor. With such an approach, only far field atmospheric boundary conditions are specified which are obtained from aircraft speed and altitude. A chocked variable‐area nozzle, placed after the last compressor bladerow in the model, is used to impose boundary conditions downstream. Such an approach is representative of modelling an engine.Using a 3D viscous time‐accurate flow representation, the front bladerows of a core compressor were modelled in a whole‐annulus fashion whereas the rest of bladerows are modelled in a single‐passage fashion. The rotating stall behaviour at two different compressor operating points was studied by considering two different variable‐vane scheduling conditions for which experimental data were available. Using a model with nine whole‐assembly models, the unsteady‐flow calculations were conducted on 32‐CPUs of a parallel cluster, typical run times being around 3–4 weeks for a grid with about 60 million points. The simulations were conducted over several engine rotations. As observed on the actual development engine, there was no rotating stall for the first scheduling condition while mal‐scheduling of the stator vanes created a 12‐band rotating stall which excited the 1st flap mode. Copyright © 2006 John Wiley & Sons, Ltd.

This paper describes the numerical investigation of the centrifugal impeller response to downstream static pressure distortions imposed by volutes at off-design operations. An unsteady 3D Euler solver with non-reflecting upstream and downstream boundary conditions and phase-lagged periodicity conditions is used for this purpose. The mechanisms governing the unsteady flow field are analysed. A parametric study shows the influence of the acoustic Strouhal number on the amplitude of the flow perturbations. Radial forces calculated on backward leaned and radial ending centrifugal impellers, show non-negligible influence of the impeller geometry.

The Nonuniform Magnetic Reconnection model presented by Priest and Lee (1990) was the first to incorporate several features observed in previous numerical models, in particular the highly curved field in the inflow region, which relaxes one of the key assumptions of the classical Almost-Uniform reconnection model of Petschek (1964). Also present is a strong jet of plasma emerging from the reconnection region along the separatrix and a negative current spike at the outflow from the central diffusion region. In this paper we present a generalisation of the Priest-Lee model to include pressure gradients in the inflow region, so relaxing the other main assumption of the Petschek and Priest-Lee models, namely that the inflow is current-free or potential. This produces several reconnection regimes similar to those described by Priest and Forbes (1986) in their Unified Almost-Uniform solutions, where significant pressure gradients are included, but their inflow, like Petschek's, is almost-uniform with s...

Computer simulations study the temporal dynamics of magnetic reconnection in a current sheet system under a continuous local plasma injection imposed on the inflow boundary. The basic reconnection process is strongly influenced by the resistivity model. For a uniform resistivity model, the field lines become flattened with time so that the resulting quasisteady configuration involves a long diffusion region and the reconnection rate strongly depends on the resistivity, in good agreement with the Sweet–Parker mechanism. For an anomalous resistivity model, magnetic reconnection drastically occurs, leading to development of a large-scale plasmoid. Once an effective resistivity is triggered, a quasisteady fast reconnection mechanism spontaneously develops even in the absence of the specified boundary condition, in good agreement with the spontaneous fast reconnection model. When the imposed plasma injection is larger than the one inherent to the spontaneous fast reconnection mechanism, the fast reconnection mechanism can no longer proceed steadily and multiple X points and plasmoids result. It is concluded that it is not the boundary condition that drives the fast reconnection mechanism; the boundary condition may simply initiate the onset of magnetic reconnection (or effective resistivity) but has no essential effect on the fast reconnection development.

Floods in a semi-arid region with flat terrain have been examined in this study. The Rel River is a river that runs through Dhanera town and vanishes in the little desert of Kutch (which is almost flat). There was a lot of rain in July of 2017, and huge damages were noted in the Dhanera town and surrounding areas. The floodwater covers many square kilometres in the Rel River Basin. The current study has been used two-dimensional (2D) hydraulic modelling to simulate the flood of 2017. For this event, the Hydraulic Engineering Centre-River Analysis System (HEC-RAS) model has been utilised to simulate 2D overland flow. The Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM), which has been acquired from the earth explorer, has been utilised for this study. Upstream boundary conditions are based on flow hydrograph of Dhanera highway bridge gauge station, whereas downstream boundary condition is based on normal depth. The HEC-RAS model has been calibrated and validated using field survey data collection. During the flood in the year 2017, the Dhanera town was 75–77% submerged. Due to the villagers' encroachment on the floodplains to conduct farming, the observed flood depths in rural areas were high. Streams and nullahs in urban areas had been blocked, resulting in greater flood depth and a longer flood recession time. To deal with severe flooding scenarios, it is believed that significant mitigating measures are necessary. The findings of this 2D hydraulic model may be used to recommend flood mitigation techniques.