Showing papers on "Critical ionization velocity published in 1993"

Dynamic response of a beam on elastic foundation of finite depth under a moving force

Abstract: In this paper, dynamic response of an infinitely long beam resting on a foundation of finite depth, under a moving force is studied. The effect of foundation inertia is included in the analysis by modelling the foundation as a series of closely spaced axially vibrating rods of finite depth, fixed at the bottom and connected to the beam at the top. Viscous damping in the beam and foundation is included in the analysis. Steady state response of the beam-foundation system is obtained. Detailed numerical results are presented to study the effect of various parameters such as foundation mass, velocity of the moving load, damping and axial force on the beam. It is shown that foundation inertia can considerably reduce the critical velocity and can also amplify the beam response.

Impaction and rebound of particles at acute incident angles

Abstract: When a particle impacts on a surface at an angle < 90°, only the normal component of the impact velocity is usually considered to contribute to deformation. A new model has been developed in which the impact velocity tangential to the surface contributes towards lateral rotation during impact, and the remaining tangential kinetic energy and the stored elastic deformation energy provide the energy for particle rebound. Calculations with the model for conditions used in published experiments compare well with the data and explain observed dependencies. It was found that the rebound velocity and also the critical velocity, at which rebound starts, are strongly dependent on the incident impact angle. Aspects of the new model have been used to calculate and explain the performance of a newly developed particle bounce monitor.

Stability of metal/electrolyte interface in Hall-Héroult cells: Effect of the steady velocity

Abstract: An instability of the metal-bath interface in Hall-Heroult cells is described The instability is hydrodynamic rather than magnetohydrodynamic in nature Above a critical value of the velocity of the fluids (which are assumed to have the same steady velocity), the interface becomes unstable and waves grow; this result is calculated without any specific electromagnetic mechanism The critical velocity depends on the cell’s flow pattern; for example, a one loop pattern will have a critical velocity different than that of a two loop pattern The critical velocities calculated are near the velocities measured for aluminum in commercial cells Also, the form of the waves which are generated is a set of traveling waves whose pattern depends on the pattern of the steady velocity The variety of observed motions is reminiscent of the variety seen in interface topographical animations generated from measured anode currents In this analysis, increasing the anode-to-cathode distance decreases the cell’s stability, as indicated by the value of the steady velocity at which flow would become unstable Two possible technical explanations are developed for this difference from conventional wisdom Additional study in this area is recommended Finally, the application of this analysis to an operating cell is illustrated, showing the potential of the method in assessing the stability effects of various operating or design parameters

CIV Experiments on ATLAS‐1

Abstract: A test of the Critical Ionization Velocity (CIV) theory was made with neutral xenon releases from the Space Experiments with Particle Accelerators hollow cathode plasma contactor onboard the Shuttle Orbiter Atlantis during the ATLAS-1 mission. The gas velocity perpendicular to the Earth's magnetic field was essentially the orbital velocity (7.5 km/s), and thus it exceeded the CIV for xenon. The releases were observed with onboard instrumentation. A factor of 60 enhancement was seen in the Langmuir probe current. Calculations confirmed that release conditions generally satisfied criteria for CIV and predicted a maximum factor of 20 increase in plasma density. Thus, CIV effects were likely to have occurred during the ATLAS-I experiments.

Analytical and Experimental Investigation of Entrainment in Capillary Pumped Wicking Structures

Abstract: Analytical and experimental investigations were conducted to identify and better understand the parameters that govern the entrainment of liquid droplets in high-velocity gas streams flowing over capillary wicking structures. Using a flow visualization technique, two modes of entrainment were identified and described for high-velocity gas flows over an intermittently interrupted liquid surface. These two modes, roll-wave entrainment and stripping entrainment, were found to correspond to the lower and upper critical gas velocities, respectively. Measurement of the critical gas velocities and the droplet size distribution (Sauter mean diameter) of the entrained sprays were made as a function of the capillary pore size for three different mesh sizes and were compared with several analytical models developed in previous investigations. The flow visualization results indicate that the upper critical velocity is insensitive to variations in the capillary pumping rate provided the capillary pores are properly primed. The experimental results also indicate that the critical velocity for a given mesh is strongly influenced by the mesh dimensions, but that the previously developed criteria for estimating the critical velocity results in an underestimation of the upper critical velocity for all but very small pore sizes. Finally, to resolve this problem a new analytical model formore » predicting the critical velocity was developed and shown to be accurate for a wide range of capillary pore sizes.« less

Numerical quasi‐linear study of the critical ionization velocity phenomenon

Abstract: The critical ionization velocity (CIV) for a neutral barium (Ba) gas cloud moving across the static magnetic field is studied numerically using quasi-linear equations and a parameter range which is typical for the shaped-charge Ba gas release experiments in space. For consistency the charge exchange between the background oxygen ions and neutral atoms and its reverse process, as well as the excitation of the neutral Ba atoms, are included. The numerical results indicate that when the ionization rate due to CIV becomes comparable to the charge exchange rate the energy lost to the ionization and excitation collisions by the superthermal electrons exceeds the energy gain from the waves that are excited by the ion beam. This results in a CIV yield less than the yield by the charge exchange process.

Measurements of the time constant for steady ionization in shaped-charge barium releases

Abstract: Quantitative measurements of three solar illuminated shaped-charge barium releases injected at small angles to the magnetic field were made using a calibrated color television camera. Two of the releases were from 1989. The third release, a reanalysis of an event included in Hallinan's 1988 study of three 1986 releases, was included to provide continuity between the two studies. Time constants for ionization, measured during the first 25 s of each release, were found to vary considerably. The two 1989 time constants differed substantially, and both were significantly less than any of the 1986 time constants. On the basis of this variability, we conclude that the two 1989 releases showed evidence of continuous nonsolar ionization. One release showed nonsolar ionization which could not he attributed to Alfven's critical ionization velocity process, which requires a component of velocity perpendicular to the magnetic field providing a perpendicular energy greater than the ionization potential.

Upper bound estimates of anomalous ion production in space‐based critical ionization velocity experiments

Abstract: The critical ionization velocity (CIV) is an anomalous ionization mechanism first proposed by Alfven. Experiments have confirmed the existence of a critical velocity in laboratory experiments, but sounding rocket experiments have been ambiguous as to the existence of the critical velocity in the ionosphere. The purpose of this paper is to produce upper bound estimates of anomalous ion production in space-based experiments of the critical ionization velocity. The analysis relies on the results of implicit particle-in-cell simulations and a simple rate model to predict the number of ions produced as a neutral cloud traverses a point in space. The model assumes a point release of neutral gas in the ionosphere which is meant to represent a typical sounding rocket experiment. The results of the model suggest why strong evidence of CIV is not observed in space-based experiments. Space-based experiments require the anomalous ionization process to be initiated through seed ionization provided by charge exchange or other mechanisms. This seed ionization process is too slow in space experiments to ignite CIV. The results also indicate that some of the space based results can be accounted for by assuming a large barium-oxygen charge exchange cross section instead of invoking anomalous ionization mechanisms.

Air Viscous Fingers in Isotropic Fluid and Liquid Crystals Obtained in Lifting Hele-Shaw Cell Geometry(*).

Abstract: The air viscous fingers in glycerol, nematic and smectic liquid crystals are studied using the so-called lifting Hele-Shaw cell geometry. It is shown experimentally that in this case a critical velocity of lifting exists, below which the viscous fingers do not grow. The dependences of this velocity and critical wavelength upon various parameters are measured. The obtained experimental data in the case of small lifting angles are in satisfactory agreement with the phenomenological estimations for linear Hele-Shaw cells.

Momentum coupling in ionospheric critical ionization velocity experiments

Abstract: The critical ionization velocity (CIV) effect is a process that can rapidly ionize a neutral gas which moves through a magnetized plasma. The process has been studied for several decades in laboratory experiments, but presently the emphasis has moved to ionospheric injection experiments. In these experiments, the neutral gas component is released at high velocity, with respect to the ionosphere, from a rocket or a satellite. Efficient momentum coupling between the injected cloud and the ambient ionosphere is achieved by means of Alfven waves that are launched along the magnetic field. A computer model is presented for the momentum exchange between a cloud of injected ions and the ionosphere, and the model electric fields and particle spectra are shown to agree in detail with measurements from the Critical Ionization Test II (CRIT II); [Swenson et al., Geophys. Res. Lett. 17, 2337 (1990)] ionospheric injection experiment.

Onset of vorticity in a rotating annulus of helium II

Abstract: We present measurements of the critical angular velocity for the onset of vorticity in a rotating annulus. The measurements of the critical velocity were made over a temperature range from 1.5 to 2.165 K, and are consistently lower than theoretical predictions by about 20%. Nor do our results agree within experimental uncertainty with previous measurements of the critical velocity in an annulus. It appears that the discrepancies can be explained by the presence and growth of high concentrations of residual vorticity which prevent the system from achieving the lowest free energy state. We find that vorticity perpendicular to the axis of rotation grows as the angular velocity is increased and that the axial mutual friction coefficientB″ is 0.

Possibility of the Critical Velocity Phenomena in the First SEPAC Experiment

Abstract: The contributions of critical velocity ionization and ions production at charge exchange are compared in space experiment. The results of Space Shuttle experiment SEPAC are discussed. Observed phenomena are explained by electron heating during the development of a lower hybrid instability in beam-beam plasma interactions.

Dynamic plastic stability of cylindrical shell

Abstract: The paper gives a general review of plastic stability of cylindrical shell under impulsive load. Solution methods for shells under uniform radial impulsive velocity and under axial impact are reviewed. Special emphasic is given to work done on axial impact in the Department of Mechanics of Peking University in the last few years. One special feature is that there exists a second critical impact velocity which render the shell to non-uniform and/or non-axisymmetric buckling. It is about twice the threshold velocity obtained in previous literature for axisymmetric buckling. Its relation to radius/thickness ratio is given and the strain rate effect, stress rate effect are also discussed. A sufficient condition based on energy consideration has also been given for the evaluation of threshold velocity and the second critical velocity.

Critical Velocities and Nucleation of Vortices in a Model of Superflow

Abstract: We study the superfluid flow around an obstacle and observe a transition to dissipation which is explained by the nucleation of quantized vortices. This transition between a potential flow state, dominated by sounds waves (phonons), toward a vortical state occurs at a well determined velocity threshold. We study the dependence of the critical velocity in different geometries in 2 or 3 spaces dimension. We also report a new mechanism of nucleation of a pair of vortices (vortex-antivortex) from the collision between (very short wavelength) sound waves.

Laser anemometer measurements and computations for transonic flow conditions in an annular cascade of high turning core turbine vanes

Abstract: An advanced laser anemometer (LA) was used to measure the axial and tangential velocity components in an annular cascade of turbine stator vanes operating at transonic flow conditions. The vanes tested were based on a previous redesign of the first-stage stator in a two-stage turbine for a high-bypass-ratio engine. The vanes produced 75 deg of flow turning. Tests were conducted on a 0.771-scale model of the engine-sized stator. The advanced LA fringe system employed an extremely small 50-micron diameter probe volume. Window correction optics were used to ensure that the laser beams did not uncross in passing through the curved optical access port. Experimental LA measurements of velocity and turbulence were obtained at the mean radius upstream of, within, and downstream of the stator vane row at an exit critical velocity ratio of 1.050 at the hub. Static pressures were also measured on the vane surface. The measurements are compared, where possible, with calculations from a three-dimensional inviscid flow analysis. Comparisons were also made with the results obtained previously when these same vanes were tested at the design exit critical velocity ratio of 0.896 at the hub. The data are presented in both graphical and tabulated form so that they can be readily compared against other turbomachinery computations.

Possible Negative Collisional Damping and Instability of a Drifting Two-Dimensional Quantum Plasma

Abstract: The high-frequency conductivity/resistivity of a two-dimensional quantum plasma embedded in a random distribution of impurities is calculated with the use of a semiclassical kinetic description. The plasma is drifting at speed vD relative to the impurities and the system is under the influence of an electromagnetic radiation of wave number q and frequency ω. It is shown that the collisional damping rate exhibits some anomalous oscillatory behavior due to the electron fluctuations which disturb the system from thermodynamic equilibrium when vD ~ Fermi velocity vF. As a consequence, there exists a frequency-dependent critical velocity vc(ω), above which the system behaves like a negative resistive medium.

Critical velocity ionization in the environment of large spacecraft

Abstract: Summary form only given. The authors discuss some potential problems of critical velocity ionization in the environment of large spacecraft, such as the Space Station. Outgassing, exhaust, and reflection of ambient species may form a significant environment of a large spacecraft. As a result of beam plasma interactions, Alfven's critical ionization velocity (CIV) phenomenon may occur. CIV experiments in space have been inconclusive. The authors point out that there exist restrictive conditions which may explain the low yields in the CIV space experiments. The finite width of the beam allows energetic electrons to escape along magnetic field lines easily without ionization, thus reducing the probability of ionization chain reaction. The finite longitudinal length of a beam pulse, or cloud, may not allow enough time for electron heating along magnetic field lines. The parallel component of the lower hybrid plasma wavelength may be too long to fit into the beam. These conditions may become less restrictive for the environment of large spacecraft. If CIV ionization really occurs in the spacecraft environment, the vicinity of the Space Station may be a more likely place. As a consequence, the transport of ions and electrons may pose contamination and radiation problems on the instruments aboard.