Velocity Redistribution Research Articles

Experimental and numerical investigation of turbulent convective heat transfer deterioration of supercritical water in vertical tube

Abstract The heat transfer phenomenon at supercritical pressure is different from the normal pressure due to the strong variation of thermal physical properties near the pseudo-critical point. The strong change of density leads to a great buoyancy effect at low flow rate, and flow acceleration at high flow rate, both of which influence heat transfer characteristics significantly. In this paper an experimental study is performed to analysis the heat transfer behavior of supercritical water. The test section is a vertically 10-mm ID tube with uniformly heating. Heat transfer deterioration phenomenon caused by the buoyancy effect and the acceleration effect is identified. At the same time, the heat transfer character of upward and downward flow is numerical studied by an in-house CFD– code developed by Shanghai Jiao Tong University. An improved k – ɛ – k t – ɛ t model is proposed and installed in this code. The wall temperature, especially near the heat transfer deterioration, is well predicted by this new model, compared with the traditional turbulence models. It is shown that the buoyancy effect causes shear stress and radial flow velocity redistribution which leads to heat transfer deterioration and recovery. Moreover, buoyancy can influence the turbulent kinetic energy dramatically, which will affect heat transfer capacity. In the downward flow case with high flow rate, the effect of buoyancy on heat transfer is not so obvious. However, as found in numerical study, the heat transfer deterioration caused by flow acceleration can occur both in upward and downward flow.

Analysis of secondary current effect in the modeling of shallow flow in open channels

The 3D information of a secondary flow evolved in curved or confluent channels, which is usually lost in conventional shallow water models due to the depth-averaging process, can be partially retrieved by incorporating the secondary current velocity profile in the vertical direction. In the present study, the depth-averaged shallow water equations with dispersion stresses were solved by the SU/PG scheme among the various numerical methods of the FEM. In the proposed model, to quantify the strength of the secondary flow, the comprehensive factors such as the deformation of the vertical profile of the streamwise velocity, the roughness coefficient, and the curvature ratio were embedded into the dispersion stress terms. Two sets of experimental data, one with a sharply curved channel by Rozovskii (1961) [40], and the other with a confluent channel by Shumate (1998) [51] were used to validate the proposed model. The computed values of the water surface profile and the depth-averaged velocity across the channel showed good agreement with experimental data, which indicated that secondary velocity profiles were preserved properly. The proposed model was applied to a natural stream with moderate curvature to test the field applicability. The simulation results obtained by the proposed model with dispersion terms matched quite well with the ADCP data, whereas the model without dispersion terms produced excessive velocities at both banks, and the commercial RMA-2 model yielded uniform span-wise velocity distributions at all sections. The analysis of the momentum balance and relative dominance of the secondary current demonstrated that the convection and pressure gradient terms played major roles in achieving equilibrium in momentum equations, and the bottom friction ranked next, followed by the remaining dispersion stresses and viscous term of a similar order. It was also found that the pressure gradient term was the primary factor that triggered velocity redistribution. As the Fr increased, the convective acceleration that formed along the channel curvature was activated as a secondary factor in velocity redistribution, whereas the viscous stress term lost its influence. The bottom friction term had minor exertion on the redistribution mechanism.

Phase analysis of non-coherent processes in a moving CDW

Earlier it has been found that the signal produced by a moving CDW contains along with fundamental of narrow band frequency (ω1) and its harmonics (ωn) a non-coherent part—a lot of non-coherent oscillations. Due to a rather broad frequency range the power associated with non-coherent oscillations is comparable to or even greater than that for fundamental frequency, and hence, can play an essential role in CDW dynamics.Now we report on the phase analysis of non-coherent part of the signal produced by moving CDW in thin samples of o-TaS3.To obtain frequency resolved phase characteristics of a signal with fast varying amplitude and abrupt phase jumps the wavelet transform has been performed using complex Morlet functions as a base.It was found that the frequencies from a rather narrow band around ω1 (typically Δω/ω1≈0.25) are tightly locked together and form the main harmonic of narrow band signal. Similarly, the frequencies close to ω2 are also tightly locked. Outside these narrow bands the oscillations are essentially non-coherent. The characteristic feature for these non-coherent oscillations is the following very specific phase synchronization process. Two very close frequency components manifest simultaneous abrupt phase shifts. For one component the phase is shifted by +π (forward), while for the other it is shifted by −π (backward).A most remarkable feature of this locking process is that the resultant phase shift between very close frequency components of non-coherent signal is every time exactly 2π.These observations are considered in terms of CDW velocity redistribution accompanying its motion in an electrical field.

Processes governing the flow redistribution in sharp river bends

Insight is provided in hydrodynamic processes governing the velocity redistribution in sharp river bends based on simulations of three recent experiments by means of Blanckaert and de Vriend's (2003, 2010) reduced-order nonlinear model without curvature restrictions. This model successfully simulated the flow redistribution and the secondary flow in all three experiments. The results indicate that the flow redistribution is primarily governed by topographic steering, curvature variations and secondary flow, in a broad range of different configurations, including mildly to sharply curved bends, narrow to shallow bends, smooth to rough bends, bends with additional complexities such as horizontal recirculation zones or patches of riverbed vegetation. The relative importance of these three dominant processes is case dependent, and controlled by the parameters Cf−1H/B, R/B and streamwise curvature variations. The first parameter characterizes a river reach, whereas the second and third parameters are characteristics of individual bends. Major differences exist between the hydrodynamic processes in mildly and sharply curved bends. First, velocity redistribution induced by curvature variations is negligible in mildly curved bends, but the dominant process in sharp bends. This result is relevant, because most meander models are based on the assumption of weak-curvature variations. Second, nonlinear hydrodynamic interactions play a dominant role in sharp bends, where mild-curvature models overpredict the secondary flow and in some cases even falsely identify it as the dominant process governing the velocity redistribution, which leads to unsatisfactory flow predictions. The reduction in secondary flow strength provoked by the nonlinear hydrodynamic interactions is accompanied by a reduction in the transverse bed slope, which reduces the effect of topographic steering.

The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three‐dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193° bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to 1.3. The large channel curvature and the point bar induce flow separation near the inner bank and the formation of several strong separated shear layers (SSLs), where production by mean shear dominates. DES shows that in addition to the main cell of cross‐stream circulation developing in the deeper part of the bend, several streamwise‐oriented vortices (SOV) form at the inner bank. DES satisfactorily captures the distribution of the streamwise velocity and streamwise vorticity in relevant cross sections compared to experiment. Comparison with a Reynolds‐averaged Navier‐Stokes (RANS) simulation shows that DES predicts more accurately the velocity redistribution and cross‐stream motions in the channel. This is because RANS significantly underpredicts the circulation and turbulence amplification inside the cores of the SOV vortices. DES is then used to clarify the influence of the SOV vortices and SSLs on the boundary shear stress. DES reveals the presence of several regions of large amplification of the pressure RMS fluctuations near the inner and outer banks, which can locally increase the bed erosion and affect the bank stability in the case of a bend with erodible banks. The mean flow bed shear stress distribution predicted by DES is significantly different than that predicted by RANS, while DES predictions of the mean flow are more accurate. This means that use of eddy‐resolving techniques like DES in mobile bed simulations of flow in curved alluvial channels should result in more accurate predictions of bathymetry.

Correction to “Topographic steering, flow recirculation, velocity redistribution, and bed topography in sharp meander bends”

[1] In the paper “Topographic steering, flow recirculation, velocity redistribution, and bed topography in sharp meander bends” by K. Blanckaert (Water Resources Research, 46, W09506, doi:10.1029/2009WR008383, 2010) the affiliations were given incorrectly. The correct affiliations are as follows: K. Blanckaert1,2,3 1Laboratory of Hydraulic Constructions, Ecole Polytechnique Federale Lausanne, Lausanne, Switzerland. 2Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, Netherlands. 3State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China.

Meander dynamics: A nonlinear model without curvature restrictions for flow in open‐channel bends

Despite the rapid evolution of computational power, simulation of meander dynamics by means of reduced and computationally less expensive models remains practically relevant for investigation of large‐scale and long‐term processes, probabilistic predictions, or rapid assessments. Existing meander models are invariantly based on the assumptions of mild curvature and slow curvature variations and fail to explain processes in the high‐curvature range. This article proposes a nonlinear model for meander hydrodynamics without curvature restrictions. It provides the distribution of the main flow, the magnitude of the secondary flow, the direction of the bed shear stress, and the curvature‐induced additional energy losses. It encompasses existing mild curvature models, remains valid for straight flow, and agrees satisfactorily with experimental data from laboratory experiments under conditions that are more demanding than sharp natural river bends. The proposed model reveals the mechanisms that drive the velocity redistribution in meander bends and their dependence on the river's roughness Cf, the flow depth H, the radius of curvature R, the width B, and bathymetric variations. It identifies Cf−1H/R as the major control parameter for meander hydrodynamics in general and the relative curvature R/B for sharp curvature effects. Both parameters are small in mildly curved bends but O(1) in sharply curved bends, resulting in significant differences in the flow dynamics. Streamwise curvature variations are negligible in mildly curved bends, but they are the major mechanisms for velocity redistribution in sharp bends. Nonlinear feedback between the main and secondary flow also plays a dominant role in sharp bends: it increases energy losses and reduces the secondary flow, the transverse bed slope, and the velocity redistribution.

Topographic steering, flow recirculation, velocity redistribution, and bed topography in sharp meander bends

The bed topography and associated flow field are investigated in a laboratory configuration with parameters that are representative for sharp natural meander bends. Zones of inward mass transport are characterized by a quasi‐linear transverse bed profile, whereas zones of outward mass transport, induced by pronounced curvature variations, are characterized by a quasi‐horizontal shallow point bar at the inside of the bend, a deep pool at the outside, and an increase in overall cross‐sectional area. These quasi‐bilinear bed profiles can be attributed to the curvature‐induced secondary flow that is confined to the pool. Topographic steering, mainly due to mass conservation, concentrates the major part of the discharge over the deepest zones of the bend. But the pattern of depth‐averaged velocities, which is relevant with respect to the development of the bed topography, does not show maximum values over the deepest zones. A term‐by‐term analysis of the depth‐averaged streamwise momentum equation reveals that the water surface gradient is the principal mechanism with respect to flow velocity redistribution, although inertia and secondary flow are also processes of dominant order of magnitude. A required condition for the occurrence of adverse pressure gradients and flow recirculation due to planform curvature variations is established. A different type of flow recirculation, due to a subtle feedback between the flow and the bed topography, occurs over the point bar. The neglect of the influence of vertical velocities impinging on the bed in models for sediment transport is identified as a major shortcoming in the modeling of the morphodynamics of meandering river channels.

Large-eddy simulation of a curved open-channel flow over topography

Large-eddy simulation (LES) is performed of a curved open-channel flow over topography based on the laboratory experiment by Blanckaert [“Topographic steering, flow circulation, velocity redistribution and bed topography in sharp meander bends,” Water Resour. Res., doi:10.1029/2009WR008303 (in press)]. In the experiment, the large-scale bed topography had developed to a more or less stationary shape which was prescribed in the LES model as boundary conditions neglecting the small-scale dune forms by means of a straightforward immersed boundary scheme in combination with a simple wall-modeling approach. The small-scale dunes are accounted for in the numerical model by means of parametrization. Sensitivity of the flow to this roughness parametrization is examined by simulating the flow for three different roughness heights. It was found that, notwithstanding the coarse method of representing the dune forms, the qualitative agreement of the experimental results and the LES results is rather good. Comparison of the LES results with the Reynolds averaged numerical simulation results of Zeng et al. [“Flow and bathymetry in sharp open-channel bends: Experiments and predictions,” Water Resour. Res. 44, W09401, doi:10.1029/2007WR006303 (2008)] reveals surprisingly good agreement. This good agreement is explained by the minor importance of turbulence stress gradients in the contribution to the transverse and streamwise momentum balance. Moreover, it is found that in the bend the structure of the Reynolds stress tensor shows a tendency toward isotropy which enhances the performance of isotropic eddy viscosity closure models of turbulence. This observation is remarkable since highly anisotropic turbulence might well be expected considering the complex nature of the geometry. Furthermore, the LES results reveal a pronounced recirculation zone near the convex inner bank of the flume due to the shallowness of the flow and strong curvature of the flume. At the interface between the recirculation zone and the main flow, a curved mixing layer is identified as well as strong upwelling flow motion that is accompanied with large production of turbulent kinetic energy.

Free-surface aeration and momentum exchange at a bottom outlet

This study aims to provide some new understanding of the air–water flow properties in highvelocity water jets discharging past an abrupt drop. Such a setup has been little studied to date despite the relevance to bottom outlets. Downstream of the step brink, the free-jet entrains air at both upper and lower air–water interfaces, as well as along the sides. An air–water shear layer develops at the lower nappe interface. At the lower nappe, the velocity redistribution was successfully modelled and the velocity field was found to be similar to that in two-dimensional wake flow. The results highlighted further two distinct flow regions. Close to the brink (Wex < 5000), the flow was dominated by momentum transfer. Further downstream (Wex > 5000), a strong competition between air bubble diffusion and momentum exchanges took place

A physically based method for triangulated surface flattening

This paper introduces a new method of generating 2D flat patterns from a 3D triangulated surface by opening the bending configuration of each winged triangle pair. The flattening can be divided into four steps. First, a 3D triangulated surface is modeled with a mass–spring system that simulates the surface deformation during the flattening. Second, an unwrapping force field is built to drive the mass–spring system to a developable configuration through the numerical integration. Third, a velocity redistribution procedure is initiated to average velocity variances among the particles. Finally, the mass–spring system is forced to collide with a plane, and the final 2D pattern is generated after all the winged triangle pairs are spread onto the colliding plane. To retain the size and area of the original 3D surface, a strain control mechanism is introduced to keep the springs from over-elongation or over-shrinkage at each time step.

Whistleron gas in magnetized plasmas

The nonlinear dynamics of whistler waves in magnetized plasmas is studied. Since the plasmas and beam-plasma systems considered here are assumed to be weakly collisional, the point of reference for the analysis performed in the present paper is the system of hydrodynamic and field equations. The renormalization group method is applied to obtain dynamical equations for the slowly varying amplitudes of whistler waves. Further, it has been shown that the amplitudes of eigenmodes satisfy an infinite system of coupled nonlinear Schrödinger equations. In this sense, the whistler eigenmodes form a sort of a gas of interacting quasiparticles, while the slowly varying amplitudes can be considered as dynamical variables heralding the relevant information about the system. An important feature of the approach is that whistler waves do not perturb the initial uniform density of plasma electrons. The plasma response to the induced whistler waves consists in velocity redistribution which follows exactly the behavior of the whistlers. In addition, selection rules governing the nonlinear mode coupling have been derived, which represent another interesting peculiarity of the description presented here.

On the Formation of the Resonance Lines of Helium in the Sun

To investigate the cause of anomalously bright resonance lines of helium in the Sun, we have studied the magnitude of the enhancements and some proposals made to explain them. Calculations in new semiempirical solar models indicate that the resonance lines of helium are enhanced by factors of 2-5 for He I and between 2 and 9 for He II, depending on the elemental abundances assumed. These enhancements are substantially less than earlier work has suggested, with the differences arising from radiative transfer not only in helium lines but also in lines of less abundant elements. Photon scattering, even with small line center optical depths (of order unity or less) throughout the transition region, is shown to significantly modify line intensities and their center-to-limb variation. This effect has important consequences both for our analysis and also for the analysis of solar EUV lines in general, including emission measure analyses. We have re-examined some proposals to explain the enhancements based on the plasma'' picture. The proposals include explicit heating (the burst'' picture) and heating via advection (diffusive and/or flowing models and the velocity redistribution'' [VR] proposal). We argue that the original VR mechanism must be modified to include kinetic effects for helium atoms, which reduce the effects of VR. The VR mechanism also naturally predicts helium lines that are blue-shifted relative to lines less sensitive to VR, which contradicts observations for the He I 584 Angstrom line. Ionizing plasma models also potentially lead to enhancement of Deltangreater than or equal to1 transitions (n is the principal quantum number) in other atoms and ions. Existing observational evidence for enhanced Deltangreater than or equal to1 transitions in other ions is weak. We conclude that the ionizing plasma scenarios are not the sole cause of the helium enhancement. Instead, in a companion paper, we propose that the thermodynamic properties of noble gases in the upper chromosphere will lead to enhancement of their spectral lines if cross-field diffusion into coronal plasma is important. Finally, we show that, surprisingly, multifluid flows of the kind computed by Fontenla and colleagues can reproduce observed intensities of several lines of C and Si and may contain a resolution to the helium enhancement problem. (Less)

On the Formation of Extreme‐Ultraviolet Helium Lines in the Sun: Analysis ofSOHOData

The resonance lines of in the Sun are several times stronger than expected, relative to lines of other ions. To explore the origins of this helium enhancement,'' we have studied data from the SUMER, CDS, MDI, and EIT instruments on the Solar and Heliospheric Observatory (SOHO). Time series data obtained in a quiet region and a coronal hole indicate that the spatio-temporal properties of the He I 584 Angstrom, and He II 304 and 1084 Angstrom lines are qualitatively unlike other chromospheric and transition region lines. Helium line intensities vary slowly compared to chromospheric oscillations and to transient transition region brightenings seen in other lines, yet they vary rapidly (both spatially and temporally) compared to the coronal radiation. This suggests that photoionization/recombination plays a minor excitation role in these lines. The Doppler shift of the 584 Angstrom line shows a remarkably clear 4-8 mHz oscillation, with no accompanying line intensity oscillation. Transient brightenings are used to show that nonradial photon scattering might explain the previously found diffuse appearance of the network but cannot account for the reduced network/internetwork intensity contrast in the 584 Angstrom line. We propose a new enhancement mechanism, based on arguments in a companion paper, in which predominantly neutral species such as diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. The enhanced internetwork emission may arise from the diffusion of atoms across the chromospheric canopy; the enhanced network emission may have contributions from diffusion from spicules. This mechanism may naturally explain why the 584 Angstrom line shows Doppler shift, but not intensity, oscillations. Finally, if a mechanism such as velocity redistribution'' dominates emission, spectroheliograms of lines will be bright in regions of large temperature gradients parallel to the magnetic field. Differences between and other spectroheliograms may then reveal the relative contributions of classical'' and cool loop'' models to the observed emission. (Less)

Erratum: “Velocity redistribution of excited atoms by radiative excitation transfer. II. Theory of radiation trapping in collimated beams” [J. Chem. Phys. 119, 7094 (2003)

Erratum: “Velocity redistribution of excited atoms by radiative excitation transfer. II. Theory of radiation trapping in collimated beams” [J. Chem. Phys. <b>119</b>, 7094 (2003)

Velocity redistribution of excited atoms by radiative excitation transfer. II. Theory of radiation trapping in collimated beams

We have developed a theory of resonance radiation imprisonment in collimated atomic beams. Treating the integral master equation describing imprisonment as a generalized wave (Schrödinger) equation and using the geometrical quantization technique for its solution, we obtained analytical representations for the effective radiative lifetime, mean scattering number, and trapping factors. We apply this theory to explain the recent observation of a dramatic velocity redistribution of excited atoms by radiative excitation transfer after the photofragmentation of Na2. In this process, the fast Na(3p) photofragments transfer their excitation energy efficiently via radiation to the abundant Na(3s) atoms from the primary particle beam. The influence of the hyperfine splitting of the ground state of Na atoms on this process is discussed. The ratio of the number of Na(3p) atoms produced by the radiative excitation transfer to the number of Na(3p) photofragments was found to be 0.13 and 0.19 for photodissociation of Na2 molecules in the vibrational levels v″=17 and v″=23, respectively. This is in good agreement with the corresponding experimental values of 0.16 and 0.22.

Velocity redistribution of excited atoms by radiative excitation transfer. I. Experimental demonstration by photodissociation of Na2 and field-free imaging

Photodissociation of state-selected sodium molecules, Na2(X 1Σg+,v″)+hν→Na2*(B 1Πu)→Na*(3p3/2)+Na(3s1/2) has been studied theoretically and experimentally using a novel “field-free” ion imaging design. The experiment uses a supersonic Na/Na2 beam in combination with the stimulated Raman adiabatic passage technique to prepare Na2 molecules in selected rovibronic levels of the electronic ground state. The Na(3p3/2) fragments are photoionized (or excited to high Rydberg states) in a permanently field-free reaction zone. The fragments enter the ion optics because of the flow velocity of the beam and are focused onto a position sensitive detector, which provides an energy resolution of about 50 meV. The measured anisotropic photofragment angular distributions reflect the alignment of the molecules prior to dissociation and are well explained by the anisotropic nature of the photodissociation by polarized laser light. The measured images show not only the expected relatively fast photodissociation fragments, but also the efficient formation of slow Na(3p3/2) atoms. Fast and slow refer to the atomic velocity relative to the center of-mass of the dissociating molecule. The ratio of the numbers of slow atoms and fast photofragments is 0.16 and 0.22 for the dissociation of Na2 from levels v″=17 and v″=23, respectively. Several models are analyzed to explain the observations. Calculations show that the dramatic velocity redistribution is caused by radiation trapping: the excitation is efficiently radiatively transferred from the fast Na(3p) photofragments to the abundant Na(3s) atoms from the primary beam, whereby the hyperfine splitting of the 3s state must be taken into account. Analytical formulas describing this mechanism show a ratio of slow to fast Na(3p) atoms of 0.13 for v″=17 and 0.19 for v″=23, which is in very good agreement with the experimental observations.

A model of the Lyman‐α line profile in the proton aurora

The Lyman‐α auroral emission is characterized by a broad line profile whose shape depends on the energy and pitch angle distributions of the initial proton beam, whereas its total brightness reflects the proton energy flux precipitated into the auroral upper atmosphere. Global remote sensing of the proton aurora through its ultraviolet signature makes it is increasingly important to relate the characteristics of the Lyman‐α emission to the physical properties of the precipitated proton flux. We present a numerical model of proton and hydrogen flux transport and kinetics based on the direct simulation Monte Carlo method. In this approach, all elastic and inelastic processes are stochastically simulated as well as is the production of Lyman‐α photons with the associated Doppler velocity component. The model also includes collisional, geomagnetic, and geometric spreading of the proton‐hydrogen beam. We show that consideration of the stochastic character of the H atom velocity redistribution after collisions produces line profiles different from those obtained in the strictly forward or mean scattering angle approximations previously used in proton transport codes. In particular, the predicted fraction of photons due to backscattered particles is considerably larger when stochastic collision scattering is considered than in the strictly forward or mean scattering angle approximations. In contrast to the median wavelength, the position of the peak in the line profile shows a weak inverse dependence on the proton energy. The efficiency of the Lyman‐α photon production per unit incident energy flux significantly drops as the mean proton energy increases. The line profile and the amount of blue‐shifted (for downward viewing) emission depends in a complex way on the initial energy and pitch angle distribution of the protons. The line profiles expected for the noon cusp and midnight proton aurora are shown to be significantly different.

The Role of Velocity Redistribution in Enhancing the Intensity of the Heii304 A Line in the Quiet‐Sun Spectrum

We present observational evidence of the effect of small scale (microturbulent) velocities in enhancing the intensity of the He II lambda304 line with respect to other transition region emission lines, a process we call redistribution. We first show results from the 1991 and 1993 flights of SERTS (Solar EUV Rocket Telescope and Spectrograph). The spectral resolution of the SERTS instrument was sufficient to infer that, at the spatial resolution of 5, the line profile is nearly gaussian both in the quiet Sun and in active regions. We were then able to determine, for the quiet Sun, a lower limit for the amplitude of non-thermal motions in the region of formation of the 304 A line of the order of 10 km/s. We estimated that, in the presence of the steep temperature gradients of the solar Transition Region (TR), velocities of this magnitude can significantly enhance the intensity of that line, thus at least helping to bridge the gap between calculated and observed values. We also estimated the functional dependence of such an enhancement on the relevant parameters (non-thermal velocities, temperature gradient, and pressure). We then present results from a coordinated campaign, using SOHO/CDS and H-alpha spectroheliograms from Coimbra Observatory, aimed at determining the relationship between regions of enhanced helium emission and chromospheric velocity fields and transition region emission in the quiescent atmosphere. Using these data, we examined the behavior of the He II lambda304 line in the quiet Sun supergranular network and compared it with other TR lines, in particular with O III lambda600. We also examined the association of 304 A emission with the so-called coarse dark mottle, chromospheric structures seen in H-alpha red wing images and associated with spicules. We found that all these observations are consistent with the velocity redistribution picture.

Influence of interstrand current redistribution on acceleration of quench propagation in eight-strand Rutherford cables

In order to study the relationship between quench propagation velocity and current redistribution during a quench, heater quench tests were performed. The cable used was eight-strand Rutherford cable coated with Sn-5Ag, which was subjected to a uniform magnetic field of 4 T. The quench process was measured by using voltage taps and sets of pickup coils. The authors found that, over a certain transport current, the quench propagation velocity accelerated as the normal zone propagated. The relationship between the quench propagation velocity and the current redistribution is discussed.