Showing papers on "Critical ionization velocity published in 1981"

The role of the critical ionization velocity phenomena in the production of inner coma cometary plasma

Abstract: The critical velocity triggering anomalous ionization of the neutral gas by plasma flow is calculated for the model based on the lower hybride instability. It depends strongly on the plasma and gas parameters, denning the instability development of the ionized atoms beam in the counter-streaming plasma. In particular, the possible role of the critical ionization mechanism for Halley's comet is examined. The fulfilment of both Townsend's condition for the selfustained beam plasma discharge and Alfven's condition for the critical velocity mechanism indicates that this mechanism may operate only within 104 km from the cometary nucleus and give an ion production rate close to that observed for Kohoutek's comet.

Dynamic Behavior of Continuous Cantilevered Pipes Conveying Fluid Near Critical Velocities

Abstract: The plane motion of a cantilevered pipe conveying fluid is examined when the flow velocity is in the neighborhood of that generating flutter In contrast to previous studies, the flow velocity is not prescribed as a constant, but is determined from the laws of motion A system of two nonlinear partial differential equations which are coupled through the nonlinear terms is thereby obtained The solution is found by the use of the Krylov-Bogoliubov averaging method and the results are discussed indicating the effect of nonlinearities

Attraction and repulsion of solid particles by solidification fronts. II. Dimensional analysis

Abstract: A scheme of dimensional analysis is developed for the description of pushing and engulfing of particles by solidification fronts. The nondimensional groups contain all the properties to be believed relevant, such as the critical velocity of engulfing Vc, the free energy of adhesion of the particle, viscosity, densities, thermal conductivities, temperature, specific heat, diffusion coefficient, and particle diameter. The determination of the parameters of the dimensional analysis is based on a substantial body of experimental critical velocity Vc data as a function of particle diameter. These experiments were performed with naphthalene and biphenyl as matrix materials and a range of polymer particles in the particle size range from 10–200 mm. The final relation obtained is tested for internal consistency and the resulting dependence of the critical velocity of engulfing on particle size is compared with experimental results.

Experiments on the critical ionization velocity interaction in weak magnetic fields

Abstract: The critical ionization velocity interaction is studied experimentally in a configuration with a magnetized plasma stream colliding with a stationary neutral gas cloud. In all previous experiments of this kind the magnetic field (1) has had a component transverse to the plasma flow and (2) has been strong in the sense that the electron gyro frequency, eB/mc, has exceeded or been approximately equal to the plasma frequency, wpc. Both these conditions play an important role in existing theories of the critical velocity interaction. The present experiments are performed to determine whether or not such interaction is possible when one of these conditions is not fulfilled, namely when the magnetic field is weak (eB/me>>wpc). Experiments have been performed both with a transverse and longitudinal (aligned with the plasma flow) magnetic field. It is found that in both cases the critical ionization velocity effect either disappears or becomes too small to be distinguishable among classical collisional processes.

Viability of the Critical Ionization Velocity Concept in Selected Space Situations

Abstract: When a magnetized plasma and a marginally ionized neutral gas penetrate each other, the gas is rapidly ionized and assimilated into common motion with the plasma if a critical relative velocity corresponding to the ionization energy of the neutrals is surpassed This effect was predicted by Alfven and confirmed in numerous laboratory experiments Via similarity transformations from the laboratory to typical space conditions the critical velocity mechanism can be shown to be viable in tenuous space plasmas, too This was verified by observations on the moon, by a recent rocket experiment in the ionosphere, and by numerical calculations of solar wind behaviour under the influence of the interstellar neutral gas drifting through the heliosphere — Based on these findings, earlier work is reassessed of other authors who invoked the critical velocity effect in connection with the ionization of cometary atmospheres, with regard to a comet-like interaction between Venus and the solar wind, and as a source mechanism for the ionosphere of Jupiter’s satellite Io Conceivably, critical velocity effects are also involved in the formation of circumstellar shells such as stellar bubbles and planetary nebulae, the deceleration of supernova remnants by interstellar matter, the expansion of H II regions into molecular clouds, and the physics of radio galaxies

New experimental results obtained with second-sound shock waves

Abstract: Second-sound shock waves are used to study the nonlinear dynamics of superfluid helium II and, especially, intrinsic interactions arising between the two-fluid components. Three new results are summarized: 1) formation of double-shock pulses; 2) formation of helium I boundary layers; and 3) identification of a fundamental critical velocity.

A steady‐state fluid model of a rotating plasma

Abstract: A steady‐state, three‐fluid model of the boundary layer in a rotating plasma is presented. Predictions of the model are compared with discharge voltage measurements from rotating plasma experiments. These measurements have previously been interpreted in terms of Alfven’s critical velocity interaction. The model here, which takes centrifuging in the rotating plasma into account, offers an alternative description.

The Critical Ionization Velocity

Abstract: The critical ionization velocity effect was first proposed in the context of space plasmas. This effect occurs for a neutral gas moving through a magnetized plasma and leads to rapid ionization and braking of the relative motion when a marginal velocity, “the critical velocity”, is exceeded. Laboratory experiments have clearly established the significance of the critical velocity and have provided evidence for an underlying mechanism which relies on the combined action of electron impact ionization and a collective plasma interaction heating electrons. There is experimental support for such a mechanism based on the heating of electrons by the modified two-stream instability as part of a feedback process. Several applications to space plasmas have been proposed and the possibility of space experiments has been discussed.

The interaction of a moving neutral gas with a stationary magnetized plasma

Abstract: In this paper it is shown that a stationary plasma can be accelerated by a moving neutral gas only if the velocity of the neutral gas exceeds Alfven's critical velocity. An expression for the terminal velocity of the interaction is given which shows that, in the limit of high incoming neutral gas speeds, the composite plasma is accelerated up to one quarter of the gas speed. We also discuss terminal velocities associated with the inverse problem, namely the deceleration of a magnetized plasma as a result of its motion through, and interaction with, a stationary neutral gas.

Critical velocity effects in superfluid 3HeB

Abstract: The oscillatory flow of superfluid 3 HeB has been studied through 5μ and 1μ thin film orifices. A critical velocity characterized by excess dissipation and increased ϱ n is observed. The temperature dependence of this critical velocity and the effects of steady d.c. flows superimposed on the oscillatory flow are discussed.

On the critical reduced velocity and the response characteristics of vortex-induced oscillations of bluff structural sections

Abstract: This paper presents an investigation on vortex-induced deflectional oscillations of various bluff structural sections.With aid of a number of formerly reported works, it is noted that the onset critical reduced velocity Vcr, 0 of vortex-induced oscillations is characterized by slenderness ratio B/D where B and D signify projection of chord length to mean flow and that of height to the plane normal to mean flow.Experimental results indicate that critical reduced velocity of vortex-induced oscillations can be classified into four types. In the onset reduced critical velocity and slenderness ratio diagram, the two of four types show that Vcr, 0 increases linearly with slenderness ratio B/D, to which a majority of road bridge deck sections belongs, while other types show that the onset reduced velocity Vcr, 0 tends to be controlled by the Strouhal number. It should be-however noted that the equivalent lift farce coefficient CL or the maximum amplitude of vortex-induced responses relates dispersedly with geometrical shape of cross sections.

Isothermal flow of superfluid helium in extremely long tubes

Abstract: Isothermal flow of superfluid helium in tubes of more than a hundred meter length has been investigated. The experimental results for gravitational flow and for flow in a closed circuit are found to be mutually consistent, and are in excellent agreement with isothermal flow data reported in the literature for shorter capillaries. Below a superfluid transport velocity of 2 cm/s the required pressure heads are found to drop sharply to a very low level, and become zero at a critical velocity somewhat below 2 mm/s, where persistent flow can be realized. From the fact that for very small pressure heads the resulting superfluid velocity becomes independent of the length of the flow tube, it is concluded that in this region the formation of turbulence is entirely located near the entrance of the tubes, from where it penetrates the tubes over a length of a few tenths of a meter.

Flowing States and Vortices in the Classical XY Model in an External Field

Abstract: Uniformly flowing states and vortices in the classical XY model in an external field are studied. This is done by using a continuum approximation and by paying attention to particular solutions to nonlinear partial differential equations for two angles 8 and cp of rotation of spins for which cp satisfies the Laplace equation. For these two states equations for 8 have forms similar to that in the classical Ising model in a transverse field. The uniformly flowing states are therefore described by kink-type excitations identical to those in the two-dimensional Ising model. Phonon modes associated with the uniformly flowing states are also studied, which are similar to Bogoliubov phonons. Vortex solutions and vortex formation energy are studied in close similarity to the case of liquid He'. By comparing the energies of these two states, an expression for critical velocity is obtained. By making correspondence to the case of liquid He', numerical values of the critical velocity and of the velocity of phonons around the uniformly flowing states are estimated. For the former the numerical value is in fair agreement with experimental data.

Vortex nucleation by negative ions in He4-rich HE3HE4 mixtures

Abstract: At high pressures at low temperatures the acceleration of a negative ion in He4, placed in an electric field, can be limited by vortex-ring nucleation. There exists a Landau criterion for calculating the critical velocity for vortex ring production. I calculate the dependence of the critical velocity on He3 concentration. Very small amounts of He3 dissolved in He4 dramatically affect the structure of both ions and vortex rings, and thus the critical velocity is considerably reduced.

Critical Velocity and Dissipation of Flow in Superfluid 3He–A Phenomenological Theory–

Abstract: Critical velocities and dissipation of a superflow in a superfluid 3 He are explained by the phase slippage due to the vortex ring like object through a narrower channel for the AC oscillatory flow. But for a DC superflow in a wider channel, they are explained by the depairing of the order parameter due to the supercurrent.

Impact Damage on Various Metal Leading Edges from Small Hard Objects.

Abstract: : Impact damage on various metal leading edge configurations was studied by performing a series of hard particle impact tests and characterizing the damage Fatigue tests were used to assess the damage and the concept of an equivalent elastic stress concentration factor was used to characterize severity of damage Notch fatigue specimens were also fatigue tested in tension to provide baseline data for each material investigated The materials investigated included 8Al-1Mo-1V titanium, 4130 steel in the annealed and heat-treated conditions, and 7075-T651 aluminum The data demonstrated good reproducibility and showed that the extent of a particular type of damage could be categorized in terms of an equivalent elastic stress concentration factor Based on the results, the 4130 steel was superior in performance with the highest critical velocity values and being less sensitive to fatigue degradation than the other materials No correlation could be established between the critical velocity and the target material parameters of density, modulus, or yield strength The concept of geometric scaling was investigated by performing a series of tests using different leading edge thickneses and projectile sizes Observation of the type of damage and plots of critical velocity versus particle size (in relation to leading edge thicknesses) appeared to validate the scaling concepts (Author)