Radial Propagation Velocity Research Articles

Theory of Magnetic Switchbacks Fully Supported by Parker Solar Probe Observations

Magnetic switchbacks are rapid high-amplitude reversals of the radial magnetic field in the solar wind that do not involve a heliospheric current sheet crossing. First seen sporadically in the 1970s in Mariner and Helios data, switchbacks were later observed by the Ulysses spacecraft beyond 1 au and have been recently discovered to be a typical component of solar wind fluctuations in the inner heliosphere by the Parker Solar Probe spacecraft. While switchbacks are now well understood to be spherically polarized Alfvén waves thanks to Parker Solar Probe observations, their formation has been an intriguing and unsolved puzzle. Here we provide a simple yet predictive theory for the formation of these magnetic reversals: the switchbacks are produced by the distortion and twisting of circularly polarized Alfvén waves by a transversely varying radial wave propagation velocity. We provide an analytic expression for the magnetic field variation, establish the necessary and sufficient conditions for the formation of switchbacks, and show that the proposed mechanism works in a realistic solar wind scenario. We also show that the theoretical predictions are in excellent agreement with observations, and the high-amplitude radial oscillations are strongly correlated with the shear of the wave propagation speed. The correlation coefficient is around 0.3–0.5 for both encounter 1 and encounter 12. The probability of this being a lucky coincidence is essentially zero with p-values below 0.1%.

Two-dimensional reconstruction of filament temperatures and densities with the thermal helium beam at ASDEX Upgrade

In the scrape-off layer (SOL) of fusion plasmas, radial particle and energy transport is mainly carried by plasma filaments. The plasma parameters of such filaments can be measured by the thermal helium beam diagnostic (THB). By means of an extended collisional radiative model (CRM) used for the evaluation of the diagnostic data, the light intensity response of neutral helium transitions from filaments is studied, which depends on the parameters of the filament and the background. The electron density of the filament is found to be the dominant parameter impacting the line intensities. By applying a numerical reconstruction algorithm based on the CRM, electron temperatures and densities as well as the sizes of experimentally measured filaments are obtained. This method allows for the first time measurements of the temporal decay rates of filament density and temperature in the co-moving frame of the filament and its radial propagation velocity.

Effect of natural gas injection timing on performance and emission characteristics of marine low speed two-stroke natural gas/diesel dual-fuel engine at high load conditions

The scavenging mode, injector position and combustion chamber type of marine low-speed two-stroke dual-fuel engines are different from those of small and medium-sized engines. The combustion process and emission laws of marine low-speed two-stroke dual-fuel engines cannot fully replicate small and medium-sized engines. The aim of this paper is to numerically investigate combustion process and emissions of marine low-speed two-stroke dual-fuel engine with different natural gas injection timings (NGIT). Four NGIT (8 BTDC, 6BTDC, 4BTDC and 2BTDC) were employed. The results showed that the advance of NGIT increased the power, thermal efficiency and NO emission, but decreased methane (CH4) emission. Each combustion stage was affected by the NGIT in different rules. The combustion process had two unstable periods, a stable period and a long tail heat release. The NGIT had an optimal value (6°CA BTDC ∼ 4°CA BTDC) to reduce CH4 and NO emissions. With the NGIT advanced ahead every 2°CA, the power increased by 117 kW, the thermal efficiency increased by 0.7%, the peak cylinder pressure increased by 7%, the peak high temperature volume increased by 0.8%, the peak radial flame propagation velocity increased by 3.7 m/s, the peak axial flame propagation velocity increased by 2.7 m/s, and NO emission increased by 15.4%, while the combustion interruption factor decreased by 1.18%, and CH4 emission decreased by 7.6%.

Performance of combustion process on marine low speed two-stroke dual fuel engine at different fuel conditions: Full diesel/diesel ignited natural gas

In order to deeply understand the combustion process of marine low speed two-stroke dual fuel engine under the background of “carbon neutrality”, MAN B&W 6S50ME-C-GI was taken as the research object, combustion processes of the full diesel (FD) and diesel ignited natural gas (DING) at 100 % load were tested and calculated. The results show that the total combustion duration period (CDP) under DING is 15.5°CA shorter than that under FD. The slow combustion period (SCP) and post combustion period (PCP) under DING were 1.75°CA and 15°CA lower than those under FD, respectively. At the same combustion time, the local high temperature region under DING is reduced by an average of 8 % compared with FD. The radial flame propagation velocity under DING and FD showed a “single peak” law, and the axial flame propagation velocity shows a “fluctuation” law. The difference is that the radial flame propagation velocity under FD shows attenuation to zero twice at 8°CA ATDC and 14°CA ATDC, and the flame propagation velocity of natural gas combustion is faster than that of diesel combustion. The maximum radial flame propagation velocity under DING is 2.5 m/s faster than that under FD. The maximum axial flame propagation velocity under DING is 0.7 m/s faster than that under FD.

The Formation and Evolution of Turbulent Swirling Vortex Rings Generated by Axial Swirlers

The present work investigates the formation process and early stage evolution of turbulent swirling vortex rings, by using planar Particle Image Velocimetry (PIV) and Large Eddy Simulation (LES). Vortex rings are produced in a piston-nozzle arrangement with swirl generated by 3D-printed axial swirlers in experiments. Idealised solid-body rotation is applied in LES to evaluate the effect of nozzle exit velocity profile in experiments. The Reynolds number (Re) based on the nozzle diameter D and the slug velocity U0 in the nozzle is 20,000. The swirl number S generated ranges from 0 (zero-swirl vortex ring) and 1.1, covering the two critical swirl numbers previously identified in a swirling jet. Both PIV and LES results show that the formation number F decreases linearly as S increases, with the maximum F ≈ 2.6 at S = 0 (produced by the swirler with straight vanes) and minimum F = 1.9 at S = 1.1. The corresponding maximum attainable circulation in the nozzle axis parallel plane also diminishes with increasing S. Evolution of compact rings produced by a stroke ratio L/D = 1.5 reveals that circulation decay rate is largely proportional to S. The trajectory of the vortex core in the axial direction, hence the ring axial propagation velocity, decreases as S, while that in the radial direction and the radial propagation velocity, increase with S. An empirical scaling function is proposed to scale these variables.

Propagation Dynamics Associated with Resonant Magnetic Perturbation Fields in High-Confinement Mode Plasmas inside the KSTAR Tokamak.

The propagation dynamics of resonant magnetic perturbation fields in KSTAR H-mode plasmas with injection of small edge perturbations produced by a supersonic molecular beam injection is reported for the first time. The results show that the perturbation field first excites a plasma response on the q=3 magnetic surface and then propagates inward to the q=2 surface with a radially averaged propagation velocity of resonant magnetic perturbations field equal to 32.5 m/ s. As a result, the perturbation field brakes the toroidal rotation on the q=3 surface first causing a momentum transport perturbation that propagates both inward and outward. A higher density fluctuation level is observed. The propagation velocity of the resonant magnetic perturbations field is larger than the radial propagation velocity of the perturbation in the toroidal rotation.

Automatic ELM detection using gSPRT on the COMPASS tokamak

This paper contains the description of the generalized Sequential Probability Ratio Test (gSPRT) method used for automatic ELM detection in different diagnostic signals collected on the COMPASS tokamak. After determination of H-mode region based on Dα signal in a given shot, ELMs are automatically detected in different diagnostic time traces (e.g. magnetic signals and also in Dα signals). The onset time, the maximum location and the peak value, and the duration of each detected ELM is determined. Analyzed diagnostic signals came from different radial positions of the studied plasma volume, thus from arrival times of given ELM into different detectors, the average radial propagation velocity of the ELM event can be also estimated. Comparison of results from gSPRT method with two different, commonly used automatic ELM detection method (threshold technique and correlation based technique) is also reported.

Flammability and oxidation kinetics of hydrophobic silica aerogels

Silica aerogels (SAs) present great application prospects especially on thermal insulation, but their flammability is usually ignored. A combined study on the combustion behaviors and oxidation kinetics of hydrophobic silica aerogels prepared by ambient pressure drying (SA-apd) and supercritical drying (SA-sd) was performed by employing cone calorimeter and thermal analysis. The whole combustion process for SAs could be divided into three stages in which a fire propagation phenomenon was observed with the radial propagation velocity of 6.6⿿8.3cms⿿1. Current investigations forcefully demonstrated that hydrophobic SAs were combustible and easy to flashover when exposed to a heat flux higher than 25kWm⿿2. Compared between the two SAs, the SA-sd owned a less fire risk with presenting a less fire hazard and a lower smoke toxicity than those of SA-apd. The oxidation kinetics by Ozawa-Flynn-Wall method revealed that SA-sd had larger apparent activation energies than those of SA-apd which conformed to the thermal stability analysis by TG-DSC. Furthermore, a two-step combustion mechanism was proposed to explain the combustion behaviors of SAs.

The influence of plasma edge dynamics on blob properties in the stellarator TJ-K

Blob properties are studied in the scrape-off layer of the stellarator TJ-K. Langmuir probes and a fast camera are used to investigate the generation rate as well as the blob size and velocity scaling. Discharges with different ion species and magnetic field strengths provide access to a large range of plasma parameters. It was found that almost every large amplitude drift wave in the edge of the confined plasma triggers blob generation in the scrape-off layer, which implies that the birth rate of blobs is determined by the turbulence in the edge. Furthermore, the cross-field size of the blobs seems to correlate with the size of the generating drift waves. Since the observed radial propagation velocity of the blobs is well described by a size dependent blob velocity model, the size coupling between the drift waves and the blobs also has an impact on the blob velocities. Thus, the presented results imply that the dynamics in the edge have a large influence on the blob properties in the scrape-off layer.

The experimental research and analysis on the quench propagation of YBCO coated conductor and coil

Quench propagation characteristics of YBCO tapes and coils were studied in this paper. Quench propagation in each test was initiated by a NiCr wire heater mounted on the sample tape and coil. Quench propagation velocity was measured through voltage taps and temperature sensors across different zones of the tapes and coil. Voltage taps and temperature sensors are arranged in longitudinal and radial direction of the coil. The thermal stability margin, the longitudinal quench propagation velocity for the tape and coil, radial quench propagation velocity for coil and the minimum propagation current were measured. In this study, we investigated the relationship between heat inputs and normal zone propagation velocities and the impact of operating currents on normal zone propagation velocities. We also compared the longitudinal quench propagation velocity and the radial quench propagation velocity. Furthermore, we performed numerical simulations by using a computer program based on a one-dimensional heat flow equation model for YBCO tape. The temperature longitudinal distributions of YBCO tape were simulated. The simulation results also prove the relation between heat input and normal zone propagation velocities and agree with the experimental results.

Quench Characteristics of bi2212 Solenoid Insert Coils in Background Field up to 20 T

The use of wind and react bi2212 wire for high field ( >;20 T) insert coils has been demonstrated to be a promising technology. While cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets there is little data for the high field application. The present work shows data from systematic quench measurements at 4.2 K on two 6-layer 300 mm high bi2212 solenoid coils at different currents I between 50% and 95% of IC in different background fields of 15, 18 and 20 T from a wide bore compact NbTi-Nb3Sn magnet developed by Oxford Instruments. The quench was induced by localized heat pulses (20-100 ms) and recorded with temporal-spatial resolved voltage taps and thermometers. By precise control of the power delivered in the heat pulse, the minimum quench energy MQE and the time constant for the quasi-stationary minimum propagation zone MPZ was obtained for each test condition. MQE was found to follow a scaling law of IC2/I4 in three different coils. The MPZ exhibited the expected anisotropy, extending predominately in the tangential direction and confined in a single winding layer. The propagation was also predominantly along the winding layer with a relatively slow velocity less than 50 cm/s, with an even lower radial propagation velocity of ~ 3 mm/s. In the present study, the interplay between the HTS coils and with the LTS background field were also measured and the results are highly relevant to the design of 20 T plus high field magnets with integrated LTS and HTS coils. The results were analysed in the context of a wide current temperature range for current sharing in the bi2212 wire and the T3 dependence of heat capacity at low temperatures.

Application of Ultrasonic Wave and Resistograph for Nondestructive Testing of Defects in Standing Trees

In order to carry out non-destructive testing on standing trees, explore the propagation characteristics and the influence factors of the ultrasonic wave spreading in the standing trees. Take simon poplar as the research object, Resistograph were employed to test 30 sample (simon polar) standing trees and it gave a preliminary estimate of the internal defects in the samples, and RSM-SYS5 ultrasonic device were employed to test the ultrasonic propagation velocity spreading in simon poplar standing trees, with the statistics of the ultrasonic propagation velocity spreading separately in the health trees and standing trees with defects, and did a regression analysis of the relationship between the size of the defacts and the ultrasonic propagation velocity. The results show that: 1) When the diameter of sample trees are between 30 and 55cm, the radial velocity of ultrasonic spreading in healthy simon poplar standing trees are in the range of 1029.96m/s ~ 1200m/s, all more than 1000m/s; however, when there are defects in simon poplar standing trees, the ultrasonic radial velocity are less than 1000m/s. 2) The defacts in standing trees have an significant influence on the ultrasonic radial propagation velocity spreading in simon poplar standing trees, defect size and the radial velocity is significantly negatively correlated, and the correlation coefficient R is 0.887.

Investigation of statistical characterization of radial turbulence in the edge of HuanLiuqi-2A plasma with visible spectroscopy diagnostic

The radial motion and structure of edge turbulence in the HuanLiuqi-2A tokamak [Y. Liu et al., Nucl. Fusion 45, S239 (2005)] was measured with visible spectroscopy diagnostic system; the radial turbulence propagation velocity and correlation length were evaluated by time-delay correlation technique and there were typically Vr=0.4–1 km/s and Lr=2–6 cm, respectively. The edge intermittent events accompanied with turbulence have been observed in plasma edge and propagated outward radially; especially through the scrape-off layer, the fluctuation signals of Dα exhibit strong asymmetric probability distribution functions (PDFs). Their propagation trajectories and radial velocity PDFs were inferred from a special correlation technique and the results indicated the radial velocity of intermittent event depends on its amplitude. Some comparisons with previous measurements and models were also presented.

Displacement time history analysis and radial wave propagation velocity in pressurized multiwall carbon nanotubes

In this paper, radial wave propagation velocity in multiwall carbon nanotubes (MWCNTs) is investigated using continuum model as the multiple elastic isotropic shells. The effect of van der Waals forces between tubes is taken into account as a nonlinear function of inter-layer spacing of MWCNTs. To solve the governing equations, the finite element method is used. By using time history of displacement, the mean velocity of radial wave propagation from outermost tube to innermost tube is calculated. Also, the effects of geometric parameters such as inner radius and thickness of nanotubes on dynamic behaviors are investigated. Furthermore, dynamic load factors for each tube of MWCNTs are calculated using presented solution. The effects of hydrostatic pressures applied on bounding surfaces of MWCNTs on radial wave propagation are discussed. The presented solution and results are verified by comparing against those reported in published literatures.

Radial propagation velocity of energetic particle injections according to measurements onboard the Cluster satellites

Injections of energetic electrons with a dispersion over energies were observed during the February 23, 2004 (at about 03:20 UT) substorm onboard the Cluster satellites in the vicinity of perigee near the midnight meridian. The delays in the particle observation caused by the energy dependence of the magnetic drift velocities made it possible to determine the position and time of the beginning of the drift, tracing the trajectories of the leading center of particles back in time in the magnetospheric model. The comparisons of the measurements of four satellites allowed us to determine the radial propagation of the injection front with a velocity of 100–150 km/s at a distance of 7–9 RE. The comparison with a few previous measurements shows a substantial slowing down of injections as they approached the Earth, and this confirms the prospects of this method for more detailed study of propagation of plasma injection into the inner magnetosphere.

Experimental observation of the radial propagation of ELM induced filaments on ASDEX Upgrade

The ASDEX Upgrade tokamak has recently been equipped with a new filament probe for filament studies. The probe features a set of radially separated Langmuir pins which are suited for measuring the radial propagation velocity of impinging filaments. A series of type-I ELMy H-mode discharges has been analysed, and the radial propagation velocity, size and density of 466 filaments have been derived at different distances from the separatrix. Mean radial velocity, size and density have been derived and various parameter dependences have been studied with respect to different models for filament propagation. The radial evolution and density decay have also been investigated.

Dynamic response and radial wave propagation velocity in thick hollow cylinder made of functionally graded materials

PurposeThis paper seeks to obtain the dynamic behavior of cylinders made of functionally graded materials (FGMs). The cylinder should be analyzed subjected to dynamic and shock loads.Design/methodology/approachThe functionally graded cylinder is assumed to be made of many subcylinders. The material properties within a subcylinder are assumed to vary linearly in the thickness direction. The material properties in subcylinders are chosen as linear functions. The properties are controlled by volume fraction that is an exponential function of radius. The shell is assumed to be in plane strain condition, and is subjected to axisymmetric dynamic loading. The Navier Equation is solved by Galerkin finite element and Newmark methods. By using the Fast Fourier Transform, the time response is transferred to frequency domain and natural frequencies are illustrated.FindingsThe dynamic behavior of functionally graded thick hollow cylinder is discussed. The radial wave propagation due to an internal pressure unloading is studied. The time history of radial stresses are discussed and the mean velocity of radial stress wave propagation for different exponent “n” of FGM are determined.Originality/valueThis paper presents the high strength technique to studying and analyzing the functionally graded thick hollow cylinders subjected to dynamic loads and the mean velocity of radial wave propagation is obtained using the proposed method.

Vibration and radial wave propagation velocity in functionally graded thick hollow cylinder

This article presents the analysis of functionally graded thick hollow cylinders under dynamic load. The functionally graded cylinder is assumed to be made of many subcylinders. Each subcylinder is considered as an isotropic layer. Material’s properties in each layer are constant and functionally graded properties are resulted by suitable arrangement of layers in multilayer cylinder. The properties are controlled by volume fraction that is an exponential function of radius. The shell is assumed to be in plane strain condition, and is subjected to axisymmetric dynamic loading. The Navier equation is solved by Galerkin finite element and Newmark methods. In each interface between two layers, stress and displacement continuity are satisfied. By using the fast Fourier transform (FFT), the time response is transfered to frequency domain and natural frequencies are illustrated. Then the dynamic behavior of functionally graded thick hollow cylinder is discussed. The radial wave propagation due to an internal pressure unloading is studied. The time history of radial stresses are discussed and the mean velocity of radial stress wave propagation for different exponent “ n” of functionally graded material (FGM) are determined. Finally the functionally graded cylinder is assumed to be isotropic and the results are compared with the analytical results.

Computations of intermittent transport in scrape-off layer plasmas.

Two-dimensional fluid simulations of interchange turbulence for geometry and parameters relevant for the scrape-off layer of magnetized plasmas are presented. The computations, which have distinct plasma production and loss regions, reveal bursty ejection of particles and heat from the bulk plasma in the form of blobs. These structures propagate far into the scrape-off layer where they are dissipated due to transport along open magnetic field lines. From single-point recordings it is shown that the blobs have asymmetric conditional wave forms and lead to positively skewed and flattened probability distribution functions. The radial propagation velocity may reach one-tenth of the sound speed. These results are in excellent agreement with recent experimental measurements.

Radial propagation of Type-I ELMs on JET

The propagation of edge localized modes (ELMs) towards the main chamber could lead to potentially serious consequences in ITER (Federici G et al 2001 Nucl. Fusion 41 1967). This effect was studied in JET by examining the interaction of a large sample (>200) of Type-I ELMs with an array of Langmuir probes on the outboard limiter. High clearance discharges were used with a separatrix to wall distance of 8–18 cm. The average ELM duration was measured as ∼ 200 ± 50 µs, the average radial propagation velocity of the ELM plasmoid in the scrape-off layer as ⟨v⊥⟩ ∼ 0.45 ± 0.1 km s−1 or as a fraction of the sound speed, ⟨v⊥/cs⟩ ∼ 0.2%, and its diffusion coefficient, D⊥, as 500 ± 100 m2 s−1—a significant increase over inter-ELM levels. The above velocities are in good agreement with the lower (∥ loss) limit of the sheath-limited model of plasmoid propagation (Krasheninnikov S I 2001 Phys. Lett. A 283 368, Krasheninnikov S I et al 2002 29th EPS Conf. on Controlled Fusion and Plasma Physics (Montreux, Switzerland)).