Showing papers on "Critical ionization velocity published in 1995"

Fingering instabilities in vertical miscible displacement flows in porous media

Abstract: The fingering instabilities in vertical miscible displacement flows in porous media driven by both viscosity and density contrasts are studied using linear stability analysis and direct numerical simulations. The conditions under which vertical flows are different from horizontal flows are derived. A linear stability analysis of a sharp interface gives an expression for the critical velocity that determines the stability of the flow. It is shown that the critical velocity does not remain constant but changes as the two fluids disperse into each other. In a diffused profile, the flow can develop a potentially stable region followed downstream by a potentially unstable region or vice versa depending on the flow velocity, viscosity and density profiles, leading to the potential for ‘reverse’ fingering. As the flow evolves into the nonlinear regime, the strength and location of the stable region changes, which adds to the complexity and richness of finger propagation. The flow is numerically simulated using a Hartley-transform-based spectral method to study the nonlinear evolution of the instabilities. The simulations are validated by comparing to experiments. Miscible displacements with linear density and exponential viscosity dependencies on concentration are simulated to study the effects of stable zones on finger propagation. The growth rates of the mixing zone are parametrically obtained for various injection velocities and viscosity ratios.

A magnetized coaxial source facility for the generation of energetic plasma flows

Abstract: The design features and characterization of a new magnetized coaxial plasma source facility for the generation of energetic plasma stream flows are presented. Careful, yet simple design of a low-inductance ( or approximately=130 kA) pulses to the plasma source with fast rise ( approximately 26 mu s). This allows for the production and acceleration of dense ( approximately 2*1015 cm-3), high-quality (10-40 eV), self-field plasmas to approximately 105 m s-1. We are then afforded the laboratory study of plasma streams with a wide variety of applications including, but not limited to, advanced thrusters for electric propulsion, astrophysical jets, critical ionization velocity, magnetic fusion, large-scale plasma etching and deposition, etc. Comparison to an ideal MHD model is made indicating reasonably good agreement in these self-field discharges, while elucidating the advantages of strong applied magnetization to provide nozzling in a magnetically constricted flow.

Collision of solitary waves in media with a second-order nonlinearity.

Abstract: We consider the collision behavior of self-trapped optical beams in media with a quadratic nonlinearity. It is found that the solitary waves behave similarly to true solitons above a critical velocity. Below this velocity they merge and form oscillating states.

Regime transitions affecting gas-solids suspensions and fluidized beds : Chemical Reaction Engineering

Abstract: Type A choking velocities, which set the boundary between fast fluidization and cocurrent upward transport, have been determined experimentally in a 152×152 mm riser based on differential pressure fluctuation and local voidage fluctuation measurements. The overall volumetric solids concentration at Type A choking velocities, i.e. the saturation concentration, rarely exceeds 0.01 for large Group B and Group D particles, but can be as high as 0.03 for fine Group A particles. A critical velocity U se , which marks the onset of significant particle entrainment from the riser, delineates the boundary between turbulent fluidization and high velocity fast fluidization. The critical velocity is much higher than the particle terminal settling velocity for Group A particles, while the two velocities are almost equal for Group D particles. Recent information is incorporated on an extended flow regime map

Surface Alfvén waves of negative energy

Abstract: The stability of an MHD tangential discontinuity is studied in an incompressible plasma where viscosity is taken into account at one side of the discontinuity. When the shear velocity is smaller than the threshold value for the onset of the Kelvin-Helmholtz (KH) instability, two surface waves can propagate along the discontinuity. There is a critical value for the shear velocity, which is smaller than the threshold value for the onset of the KH instability. When the shear velocity is smaller than the critical value, the two surface waves propagate in opposite directions. When the shear velocity is larger than the critical velocity, the two waves propagate in the same direction, and the wave with smaller phase velocity is a negative-energy wave. Viscosity causes this negative-energy wave to be unstable, and the instability increment is proportional to the viscosity coefficient.

Two‐dimensional magnetohydrodynamic simulation of a flowing plasma interacting with an externally imposed magnetic field

Abstract: The problem of plasma flow relative to a modulated magnetic field has been the subject of several studies. One motivation for studying this problem is the possibility of using a deliberately imposed surface of magnetic islands as a means of velocity profile control. This subject is also of importance for the study of stability against ideal and resistive magnetohydrodynamic (MHD) modes and the topic of locked modes. A two‐dimensional (2‐D) MHD simulation code is used to examine the behavior of a plasma flowing, in steady state, past a modulated magnetic field in ‘‘slab geometry.’’ It is shown that at ‘‘low’’ velocities the stress is dominated by the Maxwell and the viscosity terms and that forces are exchanged between the plasma and the magnetic field in a narrow boundary surrounding the island. It is found that the island is suppressed when the viscous force at the separatrix exceeds the maximum force that can be supported by an island. For ‘‘high’’ velocities (velocities beyond the critical velocity for...

Vortex nucleation in superfluid 4He.

Abstract: We have determined the features of a universal energy barrier which mediates the thermal nucleation of quantized vortices. The barrier is deduced from measurements of the intrinsic phase slip critical velocity using both dc flow and single phase slip experiments. It appears that at a given temperature a single curve can predict the outcome of all intrinsic vortex nucleation experiments for flow through small apertures.

Intrinsic critical velocities in superfluid 4 He flow through 12-μm diameter orifices near T λ : Experiments on the effect of geometry

Abstract: We report super fluid4He flow measurements at temperatures from 1.2 K up to Tλ — 3 mK in three orifices of different mesoscopic geometry. Under conditions of our experiments, the flow usually reaches a temperature-dependent intrinsic critical velocity, where dissipation is believed to occur by thermal (or quantum) nucleation of individual quantized vortex rings or loops. The nucleation rate should be sensitive to the wall geometry of the flow channel and to any local velocity enhancement at the most favorable nucleation site. According to the Iordanskii-Langer-Fisher (ILF) theory, the radius of the “critical” vortex ring, the threshold size which can grow freely by extracting energy from the flow, increases inversely as the superfluid density on approach to the superfluid onset temperature, Tλ. Thus sufficiently near Tλ the critical ring should be large enough that the geometry relevant to the nucleation process and local velocity enhancement can be studied by scanning electron microscope (SEM). We examined our three orifices by SEM. One, a standard optical pinhole, has a relatively smooth taper on one side and a sharp lip on the other. The second is similar, but contains a 1-μm flake perpendicular to the flow, which should provide additional velocity enhancement at its edge. In the third, the sharp lip is beveled to reduce the velocity enhancement at that site. Contrary to expectation, the intrinsic critical velocities are the same, within a relative calibration error of 10%, in all three cases. Thus, local sites of enhanced velocity do not appear to be active in nucleating vortices. This raises a question whether the classical two-fluid model which underlies the ILF calculation is adequate to describe the superfluid hydro-dynamics near walls, as it affects the vortex nucleation process.

The determination of the energy barrier for phase slips in superfluid4He

Abstract: The velocity dependence of the energy barrier for vortex creation in microscopic apertures is determined. When compared to results from other laboratories, the energy barrier seems to be a universal function of velocity. This universality suggests that the vortex nucleation process is independent of the microscopic surface structure of the aperture. In DC flow experiments, the vortices are nucleated at rates up to 700 kHz. In single phase slip experiments, the rate of nucleation is on the order of 10 Hz. Each of these types of experiments gives the energy barrier in a different velocity regime. The energy barrier has more curvature, as a function of velocity, than can be accounted for by the half-ring model of vortex nucleation.

Chapter 1 The landau critical velocity

Abstract: Publisher Summary This chapter reviews the saga of the Landau critical velocity, describing the way it has been measured and discussing in some detail the process of roton creation that sets in for velocities. A precise experimental value of the Landau critical velocity is in good agreement with the theoretical prediction, the latter being based on the dispersion curve as determined by inelastic neutron scattering. Roton creation has been shown to be a relatively weak dissipative process in the sense that quite modest electric fields are sufficient to propel the ions to drift velocities in the excess of Landau critical velocity. The measurements of v(E) over a very wide range of electric fields are consistent with the hypothesis that the rotons are created by the moving ion in pairs, rather than singly. The fast ion was discovered as the result of a serendipitous observation by Doake and Gribbon (1969). While using a chopped-dc Cunsolo (1961) ion cell for the measurement of ionic mobilities, they became aware that there were two separate species of negative ion present in the cell, with very different low electric field mobilities and strikingly different behaviors in strong electric fields,

On Ba+ production in the CRIT II Experiment

Abstract: Analysis of partical data from the CRIT 2 experiment, studying Alfven's critical ionization velocity (CIV) effect, shows that the density of newly created ions (presumably Ba(+) from the shaped-charge beam) is consistent with the increase in total plasma density measured by the independent RF plasma probe on board (Swenson et al., 1990) at the most active time period. We model this ion production using the measured electron flux data and the neutral barium model of Stenbaek-Nielsen et al. (1990a). To identify the main source mechanisms which may contribute most to the barium ionization, a simple model for the barium ion density at the payload location is developed based on Liouvilles theorem. We estimate that the electron impact ionization is responsible for 90% of the barium ion production observed by CRIT 2 in the first release and up to 45% in the second release. By employing a two-state approximation calculation (Rapp and Francis, 1962), the Ba-O(+) charge exchange cross section is found to range from about 2.0 X 10(exp -17) sq cm at a velocity of 4 km/s to 2.0 X 10(exp -15) sq cm at a velocity of 20 km/s. This result suggests that the Ba-O(+) charge exchange is probably dominant among all the non-CIV ionization processes. By considering the charge exchange process in our density model, the barrium ion densities are calculated for the two releases on CRIT II. The comparison between the model results and the observed data is found to be resonably consistent if the cross sections, as calculated above, are multiplied by 0.3 for the first release and 1.0 for the second release. Our result suggests that the charge exchange process could be the most important non-CIV ionization mechanism in the CRIT II experiment and it should be considered carefully case by case in CIV experiments.

Measurements of velocities of single particles for steady and oscillatory flows in plain and baffled tubes

Abstract: Preliminary research shows that velocities of single particles in plain pipes are dependent on the particle Froude number, and can be modelled by correlations. When fluid oscillation was applied to a plain pipe, the particle velocities increased, especially for low fluid velocities. This effect was found to be more pronounced with the increase of the specific gravity of the particles, which suggests that narrower residence time distributions can be achieved for particles with a density distribution. A twofold increase in particle velocities in a baffled tube was measured in the absence of fluid oscillation. However a critical flow velocity existed for this case, below which a particle will be obstructed by the presence of baffles. Such a critical velocity can be reduced by superimposing fluid oscillation in a baffled tube

Flame Stabilization and Blowoff Over a Single Droplet

Abstract: The burning behavior of a liquid fuel droplet under forced convection has been investigated numerically. The normalized governing system makes up the complete Navier-Stokes momentum, energy, species, and continuity equations in r-z coordinates with a one-step overall chemical reaction and finite-rate global kinetics. The evaporation process obeys the Clausius-Clapeyron law. The effects of incoming velocity (ū∞) and droplet diameter (d¯) are investigated separately. The envelope flame exists when free stream velocity is low. When ū∞ increases gradually, a critical velocity can be reached, at which the flame suddenly converts into a wake flame. No side flame is found. The envelope flames are retained throughout the decrement of the droplet diameter under the low-speed flow regime. The d2 law is found to still hold in such an environment.

Finite element analysis and optimization of a fluid-conveying pipe

Abstract: In this paper, a Finite element model of a cantilevered, fluid-conveying pipe is considered. When the flow rate through a uniform pipe exceeds a critical value, the stability of the undeformed configuration is lost through a Hopf bifurcation; i.e., the critical velocity corresponds to the onset of just one oscillatory mode. To maximize the critical velocity, a design optimization is performed. During successive design improvements, the evolution of the eigenvalues of the linearized system changes drastically, and special attention is paid to these changes. For the ‘optimal’ designs obtained, the critical velocity corresponds to the onset of two different oscillatory modes; i.e., a double Hopf bifurcation occurs. Lastly, numerical simulations of the post-critical behavior of the ‘optimal’ pipes are considered.

Ions and the landau critical velocity in He II

Abstract: Investigations of the motion of conventional negative ions (electron bubbles) in He II under pressures above 11 bar have provided the only means of measuring the Landau critical velocity for roton creation,vL, and for studying supercritical dissipation at higher velocities. Earlier work on roton creation is reviewed and it is pointed out that there is still no generally agreed explanation of the fact that the rotons seem to be emitted from the moving ionin pairs; nor is it known why the matrix element characterising the pair emission process should decrease rapidly with pressure. The possibility of studying these phenomena through use either of the “fast ion” (whose nature remains unknown), or of selected ions from the large variety of species that can be injected into He II by the recently developed technique of laser ablation, is discussed.

Particle velocity distribution and ionization processes in a gas‐puff Z‐pinch

Abstract: We have measured the time‐dependent radial velocity distributions of singly to five times ionized ions in an imploding plasma shell by observing the spectral shapes and intensities of emission lines in various directions. Higher charge states are found to be produced behind the lower charge states and to move faster radially inward. An ionization wave propagating much faster than the local radial ion velocities is observed. The ionization front velocity is found to be consistent with estimates of electron heat conduction from the plasma into the plasma‐neutral layer. The ionization and velocity histories of the particles are experimentally determined. The mechanisms of momentum transfer to the particles are also determined and compared with existing models.

Perforation of woven fabric by spherical projectiles

Abstract: Rectangular specimens of Twaron{reg_sign} fabric, clamped on two opposite sides, are subjected to impact perforation by 9.5 mm diameter spherical steel projectiles at speeds ranging from 140 m/s to 420 m/s. This plain woven fabric, comprising PPTA (poly-paraphenylene terepthalamide) fibers, is commonly employed in flexible an-nor applications. Its perforation response is examined in terms of residual velocity, energy absorbed and resulting deformation patterns. The existence of a critical or transition impact velocity, beyond which there is a significant reduction in energy absorbed by perforation, is observed. Differences in creasing and deformation induced in specimens are also demarcated by this transition impact velocity. Effects of difference in boundary conditions (clamped and free) on yarn breakage are also noted. A numerical model, based on an initially orthogonal network of pin-jointed bars interconnected at nodes, is formulated to simulate the fabric. Fiber yam mechanical properties are represented via a three-element spring-dashpot model which encapsulates viscoelastic behavior and fiber failure. Numerical results exhibit good correlation with experimental observations in terms of prediction of threshold perforation velocity, energy absorbed, occurrence of a transition critical velocity and fabric deformation characteristics.

Studies of the critical velocity of superfluid4he in a 2 μm by 2 μm aperture in a thin foil at various frequencies

Abstract: Measurements of the critical velocity behavior of oscillatory superfluid4He flow through a 2 μm by 2 μm square aperture in a 0.1 μm thick titanium foil are being made at temperatures between 0.36 K and 2.1 K and at pressures of less than 0.4 bar at various frequencies between 50 Hz and 1000 Hz. The purpose of this work is to study a micron-size aperture for possible frequency-dependent deviations from the critical velocity behavior seen in submicron-size apertures. Preliminary results show a nearly linear decrease of critical velocity with increasing temperature that is similar to the temperature dependences seen in smaller apertures and that is approximately independent of frequency. However, at frequencies above 500 Hz, a region appears at the lowest temperatures in which supercritical behavior is dominated by large energy-loss events requiring a number of half-cycles for completion, a region that extends up to 1.1 K at 970 Hz.

Thermal conductivity study of thin He films adsorbed in porous glasses with well controlled pore sizes

Abstract: Thermal conductivity study of thin He films adsorbed in porous glasses with well controlled pore sizes[2] is reported. The measurements are performed in a cell where a torsional oscillator monitors the superfluid density change, for the same films for comparison. Since the heat flux through the superfluid is proportional to the superfluid velocity, we discuss the possible observation of the intrinsic critical velocity vsl = h/ml, inherent in superfluid He films in such porous systems with unit pore length l, as discussed by Minoguchi and Nagaoka[3], which marks the velocity at which phase slippages start to occur over macroscopic scales.

Solutions near the hydraulic control point in a gravity current model

Abstract: A steady‐state gravity current model that incorporates entrainment and friction is used to describe large‐scale gravity currents and channel flows. When the model includes pressure effects from varying current thickness, critical points occur when the current velocity is equal to the phase velocity of waves on the interface. Some solutions have the possibility to pass from super to subcritical flow, or vice versa. These solutions pass through a hydraulic control point and the objective is to analyze the behavior of the solutions in the vicinity of such points. Using a phase space in which the hydraulic control points occur as equilibrium points, and performing Taylor expansion to the first order, the result is a system of autonomous differential equations with constant coefficients that can describe the behavior of the solutions for different parameter regimes near a hydraulic control point. If an equilibrium point in phase space represents a saddle point, it is distinguished between three different solut...

Numerical Simulation for Prediction of Saltation Velocity for Gas-Solid Two-Phase Flow in a Horizontal Pipe

Abstract: The saltation velocity (or the critical velocity) for coarse particles is predicted numerically, where the particle motion is traced by the Lagrangian approach by considering the fluid drag, gravity force, Magnus force, and Saffman force acting on the particles, whereas the gas flow is calculated by the marching method. The interaction between gas and particle is considered on the level of mean gas flow. It is assumed that the total pressure drop in a region of terminal velocity of particles consists of two parts, one due to the gas alone and the other due to the fluid drag. The fluid drag consists of the particle source term in the equation of motion for the gas, where the change in fluid drag due to particle-particle collision as well as particle-wall collision is considered. The result is quantitatively compared with several well-known empirical formulae for the saltation behavior.

Non-equilibrium ionization and electron density in the coronal plasma during solar flares

Abstract: The Li-like to He-like ion population ratio for calcium at the onset of solar flares is observed to be about 60% higher than in steady-state ionization balance. The measurement of the duration of this initial period of transient ionization and of the population ratios of adjacent ionization states allows an estimate of the electron density of the coronal plasma at flare onset. The density found in this study, within 1·109 and 7·109 cm−3 is comparable with the density typical of pre-flare active regions.