Showing papers on "Critical ionization velocity published in 1996"

Dynamic crack processes via molecular dynamics

Abstract: From large-scale molecular-dynamics simulations of dynamic crack propagation, we find that cracks accelerate quickly to a relatively steady velocity. Energy released by bond breaking accumulates in a local phonon field that moves with the crack tip and promotes the emission of dislocations. Branching follows dislocation emission along a slip plane. The branching instability requires the crack to achieve a critical velocity, as well as in induction time for energy buildup at the crack tip. {copyright} {ital 1996 The American Physical Society.}

Chaotic oscillations in pipes conveying pulsating fluid

Abstract: Chaotic motions of a simply supported nonlinear pipe conveying fluid with harmonie velocity fluetuations are investigated. The motions are investigated in two flow velocity regimes, one below and above the critical velocity for divergence. Analyses are carried out taking into account single mode and two mode approximations in the neighbourhood of fundamental resonance. The amplitude of the harmonic velocity perturbation is considered as the control parameter. Both period doubling sequence and a sudden transition to chaos of an asymmetric period 2 motion are observed. Above the critical velocity chaos is explained in terms of periodic motion about the equilibrium point shifting to another equilibrium point through a saddle point. Phase plane trajectories, Poincare maps and time histories are plotted giving the nature of motion. Both single and two mode approximations essentially give the same qualitative behaviour. The stability limits of trivial and nontrivial solutions are obtained by the multiple time scale method and harmonic balance method which are in very good agreement with the numerical results.

Melt-extracted oxide ceramic fibres — the fundamentals

Abstract: A melt-extraction technique, using a sharpened molybdenum wheel, has been used to produce fine oxide ceramic fibres. Wetting of the molybdenum wheel by molten ceramic is a key parameter in the melt-extraction process. Two types of fibre are generally obtained, depending on the extraction speed. At very low wheel speed, fine and uniform fibres of high quality are produced. However, when the wheel speed exceeds a critical velocity, Rayleigh waves are formed on the free surface of the fibres. Moreover, the average fibre thickness first increases with the wheel velocity, then passes through a maximum, decreasing at high velocity. This thickness variation is discussed in terms of both surface tension and viscosity of the liquid ceramics.

Three‐dimensional electromagnetic Monte Carlo particle‐in‐cell simulations of critical ionization velocity experiments in space

Abstract: Although the existence of the critical ionization velocity (CIV) is known from laboratory experiments, no agreement has been reached as to whether CIV exists in the natural space environment. In this paper we move towards more realistic models of CIV and present the first fully three-dimensional, electromagnetic particle-in-cell Monte-Carlo collision (PIC-MCC) simulations of typical space-based CIV experiments. In our model, the released neutral gas is taken to be a spherical cloud traveling across a magnetized ambient plasma. Simulations are performed for neutral clouds with various sizes and densities. The effects of the cloud parameters on ionization yield, wave energy growth, electron heating, momentum coupling, and the three-dimensional structure of the newly ionized plasma are discussed. The simulations suggest that the quantitative characteristics of momentum transfers among the ion beam, neutral cloud, and plasma waves is the key indicator of whether CIV can occur in space. The missing factors in space-based CIV experiments may be the conditions necessary for a continuous enhancement of the beam ion momentum. For a typical shaped charge release experiment, favorable CIV conditions may exist only in a very narrow, intermediate spatial region some distance from the release point due to the effects of the cloud density and size. When CIV does occur, the newly ionized plasma from the cloud forms a very complex structure due to the combined forces from the geomagnetic field, the motion induced emf, and the polarization. Hence the detection of CIV also critically depends on the sensor location.

Effect of the separation ratio on the transition between travelling waves and steady convection in the two-component Rayleigh-Benard problem

Abstract: It has been predicted theoretically that the critical Peclet number for the transition between travelling waves and steady overturning convection in the two component Rayleigh-Benard problem depends on the so-called separation ratio, with an exponent equal to 4/7. Since the Peclet number is proportional to the velocity amplitude, we have measured the critical velocity in the rolls system at the transition point. The experimental technique is laser-Doppler Velocimetry. By repeating the experiments for several systems or several separation ratios, we are thus able to confirm or invalidate the theoretical prediction. In fact, we did not find the predicted 4/7 exponent law. Even worse, for large separation ratios, the transition is not observed, and the velocity field contains higher harmonics.

Method and apparatus for starting a hard disk drive having separate landing and data zones

Abstract: A contact start/stop architecture for disk drives overcomes problems introduced by low flying MR (magnets resistive) heads and textured landing zones. Takeoff velocity for heads vary from the textured landing zone to the data zone. Contact start/stop operations are controlled relative to a critical takeoff velocity between takeoff velocity in the data zone and takeoff velocity in the landing zone, which critical velocity is less than that required to achieve negligible contact between the head and disk surface while the head is within the landing zone, but greater than takeoff velocity within the data zone. An initial seek from the landing zone begins at critical takeoff velocity. The head drags along the disk surface in the landing zone, since it has not reached takeoff velocity within the landing zone, but will immediately be at fly height as soon as the head reaches the data zone, minimizing the total drag distance of the head in the landing zone. A similar control scheme is used during a contact stop operation with respect to the head's landing velocity, which is equal to takeoff velocity. During contact stop operations, the actuator arm retracts the head into the landing zone at critical takeoff velocity. The servo control for initial seek from the landing zone utilizes back EMF (elecromative force) from the actuator voice coil motor without relying on servo information read by the MR head while the head is within the landing zone, since the MR head cannot accurately transduce magnetic transitions on rough surfaces.

Extinction of turbulent premixed flames by small-scale turbulence at Kolmogorov microscale

Abstract: By using the counterflow, premixed double flame stabilized in the forward stagnation region of the porous cyclinder, we have accurately determine the extinction limits of laminar and turbulent propane-air premixed flames as a function of the equivalence ratio and the stretch rates. The flow-field and turbulence characteristics were measured along the stagnation streamline by a hot-wire anemometer for the case without combustion. The mean velocity profile is found to be independent of the turbulence characteristics and coincides with the theoretical one obtained assuming the potential flow. The turbulence intensity and Kolmogorov microscale are nearly constant along the stagnation strealine up to the proximity of the stagnation point. For the laminar flames, the critical velocity gradient at which the extinction occurs agrees well with the previous data obtained by differents flame configurations. In the present study, the stretch rate induced by mixture turbulence is assumed to be the inverse of the Kolmogorov time scale, and the total stretch rate is estimated by the sum of the bulk stretch rate induced by the mean velocity gradient and the turbulence stretch rate. Results show that the total stretchs rate at which the extinction of the turbulent premixed flame occurs coincides with the critical velocity gradient at which the laminar premixed flame extinction occurs. Therefore, the small-size eddies with the Kolmogorov microscale play an important role in extinguishing the turbulent premixed flame. However, these eddies do not affect the combustion reaction at molecular scale.

Payload environment and gas release effects on sounding rocket neutral pressure measurements

Abstract: The Space Physics Experiments Aboard Rockets-3 sounding rocket carried a payload that performed active experiments to diagnose the physical mechanisms and test the effectiveness of several grounding schemes, to study high-voltage bias effects on the performance of solar cells, and to monitor the undisturbed plasma and neutral gas environment of the payload. As a part of that payload, the neutral pressure gauge obtained measurements of the pressure surrounding the payload both during and between the various active experiments. Neutral pressure results show a pressure elevated by as much as two orders of magnitude over atmospheric model-derived pressures for the entire flight. Neutral pressure measurements indicate that gas released during neutral gas (argon) releases, which was one of the mechanisms for grounding the payload, and attitude control system thruster firings (nitrogen) may be displacing the ambient gas, at least from the vantage point of the neutral pressure gauge. The prerelease ambient pressure appears to be the determining factor for whether a gas release causes an increase or decrease in the ambient pressure. Also, the decay time associated with these gas releases, or the time it takes for the measured pressure to return to ambient, is on the order of a few tenths to a few hundredths of a second once the gas valves are shut.

Momentum coupling in the “CRIT II” critical ionization velocity experiment

Abstract: A unified theory has been developed to explain the formation of a quasi-dc electric pulse (ω ≤ Ω i ) induced by an ionizing neutral barium beam across an ionospheric plasma. We obtained a generalized form for the dc electric field in the plane perpendicular to the magnetic field within a simplified slab barium cloud moving perpendicular to the geomagnetic fields. A current system associated with the quasi-dc electric field was also proposed to provide a way to transfer the momentum between the streaming barium cloud and the ambient plasma. The characteristic time derived by using this model for momentum coupling between the streaming barium cloud and the ambient plasma is found to be in agreement with the previous result. The quasi-dc electric field predicted by this model is reasonably consistent with the CRIT II critical ionization velocity measurements. On the basis of the constrains of the conservation of energy and momentum, we found that Alfven's critical ionization velocity (CIV) effect is a self-limiting ionization process in a finite extent neutral cloud. It may be the reason why the CIV effect took place in CRIT II but lasted only for a very short period, and it may have resulted in low ionization yields in most of space CIV experiments.

Critical velocities and phase slips in superfluid 4 He flow through a 2×2 μm aperture

Abstract: Ac measurements of the critical velocity behavior of superfluid4He flow through a 2×2 μm square aperture in a 0.1 μm thick titanium foil have been made at temperatures between 0.36 and 2.10 K at frequencies which vary with temperature from 19 to 11 Hz. Single 2π phase slips are clearly resolved over the entire temperature range. These slips define a critical velocity that decreases nearly linearly with increasing temperature. Whereas below 1.0 K the width of the observed critical velocity distribution is relatively temperature independent, above 1.0 K it decreases linearly with temperature toward zero near Tλ. We also observe a smaller, less temperature-dependent, transient critical velocity induced by high drive levels.

Sliding Friction of Lubricated Surfaces

Abstract: I discuss the sliding motion of an elastic block on a substrate with a mono-layer of lubrication molecules. This model of boundary lubrication illustrates how the surface stress σ generated by the lubrication layer at the block-substrate interface is transmitted to the upper surface of the block. The relation between σ and the drift velocity υ of the lubrication layer exhibit hysteresis and a velocity “gap” giving rise to “stick-and-slip” motion at low sliding velocities.

The yield of ionization in critical ionization velocity space experiments

Abstract: We compare the conditions in laboratory and space critical ionization velocity (CIV) experiments. One significant difference that comes to light is the rapid expansion of the neutral cloud in space experiments, which does not take place in the laboratory. This has important ramifications for the ultimate ionization yield if there is a time delay in the ignition of the CIV discharge. We find that a simple kinetic model implies that the delay time of CIV ignition is a critical factor in determining the ultimate yield of the experiment. Although the delay time is difficult to calculate precisely, we provide some estimates that predict low CIV yield for typical space experimental geometries, densities and expansion rates. We examine the possibility of the variation of experimental conditions to maximize yield, but find that natural limitations in the design of space experiments may lead to low yields in the best of circumstances. This implies that experiments to date neither prove nor disprove the relevance of the CIV process to cosmology.

Foreign matter collision damage evaluation method in gas turbine blade

Abstract: PURPOSE: To enable it to evaluate the critical impact characteristic of foreign matter collision with both gas turbine rotor and stator blades accurately and nondestructively by means of limited operations on tests. CONSTITUTION: Bump tests are executed, measuring a deformed variable <5 and an impact value Fp produced in a gas turbine blade, and then work δby a pellet in this bump test is calculated fran the deformed variable 3 and the impact load value Fp, and the work W is devided by the volume of a deformed part of the gas turbine blade, finding the normalized work Ed by means of the pellet in time of the bump test. Next, with general relations between the normalized work of a colliding material and a ratio of collision velocity in the colliding material to a critical velocity piercing through the gas turbine blade, a critical velocity Vc of the pellet to a pellet diameter (d) in the bump test and gas turbine blade thickness (h) is found out of the normalized work Ed by means of the pellet in the bump test, and further a calculation expression of the critical velocity of the colliding material in various collisional conditions is found out of the critical velocity Vc of the pellet.

Critical velocity of continuous vortex formation in rotating3He-A

Abstract: Vortex lines with continuous (singularity-free) core structure are usually formed when superfluid3He-A is accelerated to rotation. The critical counterflow velocityvc(T), at which vortex formation starts, has been measured with cw NMR. The onset of vortex formation is found to depend on the order parameter texture and differs from that of singular core vortices in4He-II or3He-B due to much larger length scale.

Method for taking samples of gas-liquid flow and device for its embodiment

Abstract: FIELD: oil industry; applicable in taking samples for determination of gas factor and well production rate. SUBSTANCE: zone of sample taking, conditions for critical flow of main flow are provided, and sample is taken from this zone with critical velocity of flow of sampled flow. Critical conditions of flows are provided by installation of calibrated pipe unions on the way of flows and taking sample into sampling tube from the zone of critical efflux of main flow. EFFECT: higher efficiency. 4 cl, 4 dwg

The intrinsic critical velocity near T λ

Abstract: We present preliminary measurements of the intrinsic critical velocity for vortex production due to flow through a submicron aperture at temperatures from 1.65K toT λ−T=3×10−3 K and fluid drive pressures (Josephson frequencies) from 0.5mPa (f j=35Hz) to 250mPa (f j=17kHz). The critical velocity is probed using a constant pressure drive technique which maintains a constant phase slip nucleation rate for several minutes. The measured critical velocity qualitatively agrees with a model of thermally activated vortex half rings.

Effect of vibrations on microaperture flow experiments in superfluid helium

Abstract: It is shown in a simple model that low frequency mechanical vibrations of the experimental apparatus can have a strong effect on the superfluid flow in a microaperture between two volumes of superfluid helium. For superfluid3He flowing through a submicron aperture one can easily get a random flow velocity of the order of the critical velocity. Conclusions are made on how to avoid this effect by the proper design of the experimental cell.

Critical velocity of gas in heat and mass transfer equipment

Abstract: A formula for calculating the critical gas velocity is found that is in good correlation with experimental data. Its generalization to the case of the presence of regular roughness on the surface of a packed bed is obtained.

Dynamical Stability and Dynamical Evolution of the Disks of SC Galaxies

Abstract: We give a detailed description of the dynamical interaction of the stellar and gaseous components of a galactic disk. The stability of the two-component system against axisymmetric density perturbations is analyzed and the critical velocity dispersions of the stars and the gas, which control the instability, are determined. By comparison with the observed velocity dispersions it is shown that NGC 6946, a typical Sc galaxy, seems to be stable in the outer parts of the disk, but is dynamically unstable in the inner parts. The transition occurs exactly at the HII region disk boundary, so that the onset of gravitational instability appears to be related to the threshold of massive star formation (Kennicutt 1989). We study the dynamical evolution of the instability by numerical simulations using a code (cf. Casoli & Combes 1982), which implements both the stellar and gaseous components. It is shown that the gas forms after a short initial period of growing ring-like perturbations large cloud complexes, which induce by ‘swing-amplification’ (Toomre 1990) multi-armed spiral structures in the stellar and gas disks, exactly as observed.