Showing papers on "Critical ionization velocity published in 1991"

Instability in dynamic fracture

Abstract: Cracks in brittle materials have terminal velocities far below theoretical predictions. To address this problem we have investigated the propagation of cracks in a brittle plastic (polymethylmethacrylate). Velocity measurements with resolution an order of magnitude better than past experiments reveal the existence of a critical velocity at which the velocity begins to oscillate, the mean acceleration drops sharply, and a pattern is formed on the fracture surface. Thus the dynamics of cracks may be governed by a dynamical instability.

The stability of countercurrent mixing layers in circular jets

Abstract: A spatially developing countercurrent mixing layer was established experimentally by applying suction to the periphery of an axisymmetric jet. A laminar mixing region was studied in detail for a velocity ratio R = ΔU/2U between 1 and 1.5, where ΔU describes the intensity of the shear across the layer and U is the average speed of the two streams. Above a critical velocity ratio Rr = 1.32 the shear layer displays energetic oscillations at a discrete frequency which are the result of very organized axisymmetric vortex structures in the mixing layer. The spatial order of the primary vortices inhibits the pairing process and dramatically alters the spatial development of the shear layer downstream. Consequently, the turbulence level in the jet core is significantly reduced, as is the decay rate of the mean velocity on the jet centreline. The response of the shear layer to controlled external forcing indicates that the shear layer oscillations at supercritical velocity ratios are self-excited. The experimentally determined critical velocity ratio of 1.32, established for very thin axisymmetric shear layers, compares favourably with the theoretically predicted value of 1.315 for the transition from convective to absolute instability in plane mixing layers (Huerre & Monkewitz 1985).

Critical velocity of stick-slip motion.

Abstract: that women with occult proliferative lesions in the control group pool preferentially refused to volunteer for our study. This seems unlikely. The difference in cytologic results was significant when corrected for age, as indicated in our reference 19. Page and DuPont suggest that we should have removed from the experimental group the two patients with PBD in whom breast cancer was detected during our study. We reported that, in one case, PBD was detected 1 year before the diagnosis of cancer and, in the other, the classification of PBD (on the basis of fine-needle aspirates) was made independent of the suspicion ofcancer (as detected by mammography). Such individuals should be considered as affected with PBD in genetic and statistical analysis. These examples strengthen our contention that cytologic analysis can be an effective diagnostic tool in studies ofgenetic susceptibility to breast cancer. We used a one-tailed test ofsignificance because our hypothesis stated that TBD is more frequent in the clinically normal relatives of two closely related women with breast cancer than in controls.' A two-tailed test, which would be appropriate for detecting positive or negative associations between PBD and breast cancer, also revealed a significant correlation (P < 0.04). We would like to reemphasize that systematic sampling ofthe breast by fine-needle aspiration is strictly a research tool, and we discourage its use as a clinical screening method for asymptomatic women. We have no specific clinical recommendations to make to women who have histologically or cytologically defined PBD (4) other than to follow established screening guidelines. MARK SKOLNICK C. JAY MARSHALL WILLIAM MCWHORTER DAVID GOLDGAR LISA CANNON-ALBRIGHT JOHN H. WAR HARMON EYRE University of Utah Medical Center, Salt Lake City, UT 84132 G. BERRY SCHUMANN University ofAlberta, Edmonton T6G 2R7, Canada D. TIm BISHOP Imperial Cancer Research Fund, Leeds LS2 9LU, United Kingdom

Model for steady state friction

Abstract: A model for dynamic steady state friction between two rough surfaces is developed in which the transfer of momentum from the horizontal to the vertical direction by collisions between asperities on opposing surfaces leads to a friction law which is independent of the detailed mechanism of energy dissipation. For nonfractal surfaces the model applies above a lower critical velocity which increases exponentially with smoothness. At high velocities there is velocity weakening and, as the smoothness of the surfaces increases, the velocity dependence rapidly approaches the experimentally observed logarithmic dependence, in agreement with phenomenological state variable friction laws. For fractallike surfaces the model applies over the whole velocity range above V = 0, showing velocity strengthening at low velocities and velocity weakening at high velocities, implying the existence of a stick-sliplike instability.

Interpretation of the electric fields measured in an ionospheric critical ionization velocity experiment

Abstract: The quasi-dc electric fields measured in the CRIT I ionospheric release experiment are studied. In the experiment, two identical barium shaped charges were fired toward a main payload, and three-dimensional measurements of the electric field inside the streams were made. The relevance of proposed mechanisms for electron heating in the critical ionization velocity (CIV) mechanism is addressed. It is concluded that both the 'homogeneous' and the 'ionizing front' models probably are valid, but in different parts of the streams. It is also possible that electrons are directly accelerated by a magnetic field-aligned component of the electric field. The coupling between the ambient ionosphere and the ionized barium stream is more complicated that is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes.

Redistribution of particles during casting of composite melts: Effects of buoyancy and particle pushing

Abstract: When a melt containing a dispersion of second phase particles is solidified, the initial distribution of the particles can change due to three phenomena, namely, buoyant motion of the particles, pushing of the particles by the moving solidification front, and by convection currents in the melt This paper presents a computer simulation model using which, the net redistribution due to the combined effect of the first two phenomena can be predicted the existing theory for calculating the critical velocity for particle pushing is extended to include the effect of the buoyancy force and a numerical correlation is developed for easy calculation of the critical velocity This correlation is incorporated into a computer programme which tracks the position, velocity and direction of the solidification front as well as the position of each particle in the melt as a function of time The final positions of the particles describe the distribution of the particles in the solidified material Predicted distributions for various heat extraction rates and particle sizes are presented for a system of silicon carbide particles in a pure aluminium melt solidifying unidirectionally as well as multidirectionally in cylindrical moulds It is shown that for any heat extraction rate there is an optimum particle size which gives the maximum uniformity of distribution in the solidified material

Speed of rotation in a vacuum arc centrifuge

Abstract: Results of measurements made on a vacuum arc centrifuge are reported. The rotational velocity of the plasma column has been deduced by cross correlating the floating potential detected by two Langmuir probes inserted into the plasma. The main result is that the rotational velocity is slightly lower than the Alfven critical velocity, for the four different elements used as cathode material to form the plasma (Mg, Zn, Cd, Pb). A tentative explanation for the results is proposed based on an extension of the global energy balance argument which describes a conventional plasma centrifuge.

Enhanced radial dispersion in open tubular column chromatography

Abstract: Radial diffusion of solutes in the mobile phase limits the practical linear velocities that can be used to increase speed of analysis in conventional open tubular column chromatography. Several methods have been devised to enhance radial dispersion that do not depend only on molecular radial diffusion. In this review, the methods of enhancing dispersion found in the chromatographic literature are discussed. These methods include the use of deformed columns, and spinning band and static mixer columns. These techniques have been found to be of little value in analytical chromatography. Electroosmotic flow has been used to generate a plug-like flow profile in LC and generates one to two orders of magnitude lower plate height than produced at laminar flow conditions. Turbulent and secondary flow have been used to enhance radial diffusion in open tubular columns for gas, liquid, and supercritical fluid chromatography. These techniques are described from their theoretical and practical aspects. Turbulent flow has been investigated as a means for enhancing radial dispersion in open tubular column chromatography for several years. At low linear velocities (Reynolds numbers < 2000), the Golay equation accurately predicts that increasing velocities generate higher plate heights. However, turbulent flow experimental results have shown that unretained solutes have decreasing plate heights with increasing velocity after a critical velocity is achieved. Up to this time, turbulent flow chromatography has not been shown to be useful for retained solutes, and applications in analytical separations are absent in the literature. Secondary flow has also been studied by several investigators for the enhancement of radial dispersion. A circular, radial flow profile can be generated by centrifugal forces in tightly coiled columns, and after a critical velocity is achieved, columns coiled to appropriate aspect ratios yield lower plate heights than straight columns for unretained solutes as the velocity is increased. Although retained solutes yield higher plate heights in coiled rather than conventional straight columns in some cases due to the resistance to mass transfer at the interphase region, higher speeds of analysis are obtained. Secondary flow chromatography has been successfully applied in gas and supercritical fluid chromatography for the rapid separation of alkanes having low retention. Nevertheless, applications using secondary flow in chromatographic separations are limited.

Critical velocity correlations for slurry transport with non‐Newtonian fluids

Abstract: Transport properties of various uncross-linked and cross-linked non-Newtonian slurries in horizontal pipes are studied. Flow data are gathered in three long, different diameter, transparent horizontal pipes. Investigated are the effects of the pipe size, polymer concentration, fluid rheological properties, cross-linking effects, particle size and density, solids concentration, fluid density, and slurry rate on the critical deposition velocities and particle resuspension velocities. It is found that critical deposition and resuspension velocities for the non-Newtonian carrier fluids tested are significantly lower than those for water. Both velocities depend greatly on pipe size and particle density. Higher critical velocities are required to minimize settling (1) in larger pipe sizes and (2) for pumping solids denser than sand. For less viscous fluids, critical deposition and resuspension velocities increase slightly with increasing solids concentration. They are, however, independent of solids concentration for more viscous fluids.

Charge exchange contamination of CRIT-II barium CIV experiment. [critical ionization velocity in ionosphere]

Abstract: Experiments have been recently performed which attempted to confirm Critical Ionization Velocity (CIV) ioinzation by deploying chemicals at high velocity in the ionosphere. Specifically, the CRIT-II Rocket performed a barium release in the ionosphere where observations of Ba+ resonant emissions following the release are believed to have resulted from the CIV process. Calculations are presented here which suggest a significant fraction (if not all) of the Ba+ observed likely resulted from charge exchange with the thermosphere ions and not through CIV processes. The results presented here are pertinent to other CIV experiments performed in the ionosphere. It is recommended that laboratory measurements of the charge exchange cross section between O+ and Ba as well as other metal vapors used in CIV experiments be made.

The effect of shear and rotary inertia of a rotor at its critical speeds

Abstract: This paper deals with the eigenfrequencies of flexural vibration of a homogeneous rotor model which includes shear, rotary inertia and the gyroscopic effect A formula for the critical speeds of this rotor has also been found It appears that the effect of these factors is smaller when the critical speed increases and the ratio of slenderness of a rotor decreases

Phase-Slippage Studies of the Critical Velocity in Helium-4

Abstract: The problem of the critical velocity for the flow of superfluid 4He through small orifices has been long standing.1–6 We reexamine it here in the light of the results obtained by phase slippage experiments. The occurence of phase slips 7–11 when the flow velocity v s exceeds a certain threshold v c marks the onset of dissipation in the superfluid. According to current views,12 dissipative slips of the phase by 2π are caused in 4He by vortex filaments crossing all the streamlines passing through the orifice. The initial stages of this process are, however, still unknown.

Critical velocities and mutual friction in 3He-4He mixtures at temperatures below 100 mK

Abstract: • A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.

Experimental and numerical investigation of soot deposition in laminar stagnation point boundary layers

Abstract: Soot deposition from a hot gas flow to a cooled solid wall is investigated numerically and experimentally for laminar stagnation point flows. Numerical predictions of soot deposition are made by solving the coupled momentum, energy, and soot transport equations, which include the effects of variable transport properties and thermophoretic transport (i.e. mass transport down a temperature gradient) of soot particles. The results of the numerical computations indicate that the deposition rate, expressed in terms of the deposition velocity, increase with an increasing potential flow velocity gradient and decrease as the wall temperature approaches the freestream temperature. In addition, an insitu laser diagnostic technique is developed which provides simultaneous measurement of the soot deposit thickness and freestream soot concentration. The technique uses two laser beams (488 and 632 nm wavelength) coincident at a point adjacent to the deposition surface to measure the thickness of the deposit and the light extinction by airborne soot particles. Comparison of the experimental results to the numerical model indicates a critical velocity gradient beyond which the “sticking” fraction for the soot significantly decreases. This velocity gradient, which corresponds to maximum deposition, has been found to be approximately 2000 sec −1 for soot produced from fuel rich air/ethylene mixtures.

On the vibrations of three-dimensional angled piping systems conveying fluid

Abstract: The vibrations of three dimensional angled pipe systems conveying fluid are studied by using the finite element method. Extended Hamilton's principle is applied to derive the equations of motion. The characteristics matries consisting of inertia, stiffness, and Coriolis terms are derived by variational method, in which the effects of the internal flow velocity and pressure are considered. The change of dynamic characteristics of the piping system due to the variation of flow velocity, pressure and the geometry of the system is investigated. As a result, it can be found that the natural frequency of the system decreases generally as the flow velocity and pressure increase and that the tendency is more significant as the geometry of the system is similar to the straight pipe.

CIV interaction: laboratory experiments on the magnetic field and neutral density limits

Abstract: Laboratory experiments are reported which determine the magnetic field and neutral density limits for critical ionization velocity (CIV) interaction in the impact configuration. A combination of microwave interferometry and spectroscopy has been used to measure how the electron energy distribution varies with the neutral density and the magnetic field strength. The efficiency of the CIV process is evaluated in terms of the efficiency factor eta of energy transfer to the electrons. This efficiency is studied as a function of the ratio VA/V0 between the Alfven velocity and the plasma stream velocity and the ratio nu i/ omega gi between the ionization frequency and the ion gyro frequency. With other parameters kept constant, VA/V0 is proportional to the square root of the magnetic field, while nu i/ omega gi is proportional to the neutral density. The authors have found that these two dimensionless parameters are coupled in such a fashion that a stronger magnetic field can compensate for a lower neutral density. For their strongest magnetic field, corresponding to VA/V0=4, CIV interaction is found to occur for a comparatively low value nu i/ omega gi approximately=0.1. For VA/V0=1, we found a clear absence of CIV interaction even for nu i/ omega gi approaching unity.

Critical transport velocity in two-phase, horizontal pipe flow

Abstract: This paper reports on the suspension of solid particles or entrainment of liquid droplets in two- phase flow. Theoretical and empirical relationships have been derived for both instances without any consideration to the similarities between the two. However, a general relation for two-phase flow is desirable since there are systems that cannot be readily defined due to the dual (solid/liquid) nature of the transported material, such as colloids, pulp, slurries, and sludge. Using turbulence theory, one general equation can be derived to predict critical transport velocities for two-phase horizontal flow.

Interface instabilities in liquid crystals

Abstract: Segregation of impurities at a nematic/isotropic interface induces the Mullins-Sekerka instability. Above a critical velocity tne interface initially flat destabilizes to a wavy pattern. As the velocity is increased a series of dynamical modes are observed. Those which break the parity of the pattern, travelling waves and solitary modes, also select the wavelength.