Showing papers on "Critical ionization velocity published in 2004"

Stability of laminar and turbulent flow of superfluid 4He at mK temperatures around an oscillating microsphere

Abstract: The flow of superfluid helium around a vibrating microsphere is investigated at temperatures between 1 K and 25 mK. At small oscillation amplitudes pure potential flow is observed, the linear drag force on the sphere being determined only by ballistic quasiparticle scattering below 0.7 K with phonons contributing exclusively below 0.5 K. At larger oscillation amplitudes a strongly nonlinear drag force gives evidence of stable turbulent flow if at least 0.6 pW are transferred from the sphere to the turbulent superfluid. In an intermediate range of amplitudes (or driving forces) both flow patterns are unstable and intermittent switching between both is observed below 0.5 K. We have recorded time series of this switching phenomenon at constant drives and temperatures lasting up to 36 hours. We have made a statistical analysis of the times series by means of reliability theory. The lifetime of the turbulent phases grows with increasing drive and diverges at a critical value (or at least becomes unmeasurably long). Stability of the laminar phases in the intermediate regime depends on the excess velocity of the sphere above the critical velocity. Metastable laminar phases are observed above the critical velocity having a mean lifetime limited to 25 minutes by natural background radioactivity which occasionally produces local vorticity due to ionization of the liquid. Finally, it is suggested that the breakdown of potential flow belongs to the class of “system failure” experiments which is well known in reliability testing and whose statistical properties are described by extreme-value theory.

Instability mechanisms in a premixed prevaporized combustor

Abstract: Instabilities in a premixed prevaporized 150-kW model scale combustor are investigated experimentally. The injector fed with liquid heptane and preheated air features two sets of swirling blades that induce flow rotation in the same direction (corotative) or in the opposite direction (counter-rotative). The flame is stabilized with swirl behind a dump. Instabilities occur in the low-frequency range around 400 Hz corresponding to a quarter-wave mode acoustic coupling of the system. Simultaneous measurements of pressure and heat-release oscillations and phase-locked CH chemiluminescence images are used to characterize the combustion dynamics. In both corotative (COS) and counter-rotative (CNS) cases, the reaction region moves closer to the injector when the flame becomes unstable by about one-third of the stabilization distance under normal operation. Experiments indicate that the two swirl configurations have distinct domains of instability. The instability boundary separating stable and unstable regions can be defined in terms of a critical velocity v c , which depends on the equivalence ratio Φ, air injection temperature T i n j , and swirl geometry. In the coswirl configuration, instabilities occur when the injection velocity is lower than the critical velocity [u v c (Φ, T i n j ; CNS)]. In a range of conditions corresponding to low injection velocities, reduced eqivalence ratio, and for the coswirl configuration, an unsteady flashback takes place in which the flame moves periodically in and out of the fuel premixer. This mechanism is related to the existence of a low-velocity region near the injector exit plane. Observations of the space-time development of the heat release under unstable operation indicate that the oscillations are significantly influenced by the swirl geometry and are caused by different mechanisms. The coswirl configuration features a central recirculation, which gives rise to periodic vortex roll-up, convection, and sudden release of heat when the vortices impinge on the lateral walls. In the counterswirl geometry there are no identifiable flow structure, but the heat-release pattern is convected periodically in the chamber. Estimates of the delay times associated with the two mechanisms support the view that coswirl instabilities are driven by vortex roll-up, whereas counterswirl instabilities are probably sustained by equivalence-ratio inhomogeneities.

Effect of the Nature of Different Polymeric Fibers on Steady-State Bed Coalescence of an Oil-in-Water Emulsion

Abstract: The effect of different polymeric fibers on non-Brownian oil drop steady-state bed coalescence was investigated. Three polymeric low-energy smooth fibers were used: polyethylene, polyester, and polyurethane. The oil-in-water system was used as the model of an unstable emulsion, with the drop size smaller than the pore size. Experiments were carried on geometrically similar beds over a wide range of bed permeability (porosity) and a wide range of fluid velocity, from 16 to 70 m h-1. The bed coalescence efficiency was followed via the effluent oil concentration and critical velocity. The unexpectedly large differences in critical velocities obtained on the investigated materials correlated well with the critical surface tension.

Visco-elastic flow past circular cylinders mounted in a channel: experimental measurements of velocity and drag

Abstract: Experimental results are described of a flow in a rectangular channel with a width of 20 mm and a height of 0.16 m. In the symmetry plane of this channel we can mount one or two cylinders each with a diameter of 10 mm. In the case of the two cylinders they are displaced with respect to each other in the streamwise direction. In this set-up, the velocity field is measured by means of Laser Doppler Velocimetry (LDV) under various flow conditions and along various traverses. First, a reference experiment is carried out with a Newtonian fluid. For this Newtonian case we have also carried out a numerical computation which can be used to test the accuracy of our measuring system. The computational results agree excellently with the experimental data. The computations also show that the flow in the channel cannot be considered as 2D. After this a series of experiments are discussed which have been carried out with visco-elastic polymer fluids. Material parameters of the fluids, such as the shear viscosity and the first normal stress difference, are obtained with help of rheometric measurements. For the visco-elastic polymer fluids our observations indicate differences with the Newtonian case, especially, in the wake of the cylinders when the velocity is increased. Eventually, at some critical velocity elastic instabilities occur. Finally, we note that, while in a Newtonian fluid the drag varies linearly with the flow rate, this is no longer the case in a visco-elastic fluid, where we find that the drag increases with the square of the flow rate.

Stratification of moving multicomponent Bose-Einstein condensates

Abstract: A mixture of the multicomponent Bose-Einstein condensate is considered, where each component moves with its own velocity. As a result of the relative motion, the mixture stratifies when the relative velocity reaches a critical value. Stability conditions for a binary moving mixture are derived and the critical velocity is found.

Modeling of water wetting in oil-water pipe flow

Abstract: Internal CO2 corrosion in crude oil transport pipelines is always associated with the presence of “free” water, and the likelihood of corrosion generally increases with the volume fraction of the water phase. A recently developed method is applied here to predict the critical velocity for entraining the water by the flowing oil phase. Entrainment of the water eventually eliminates the corrosion problem. The effects of pipe diameter, surface tension, oil viscosity and density on the critical velocity for water entrainment are discussed in detail in this paper. On the other side, if all the water is not entrained by the flowing oil phase, it is important to predict the thickness of the water film, the in-situ: water cut, film velocity and wetted area. A new model is proposed here that can be used to calculate these parameters, which are crucial for corrosion prediction in multiphase flow. A comparison is carried out between this new model and experiments conducted in large diameter horizontal pipe flow.

Liquid convection effects on the pushing-engulfment transition of insoluble particles by a solidifying interface: Part I. Analytical calculation of the lift forces

Abstract: During the solidification of a liquid containing insoluble particles, the particles can be instantaneously engulfed, or continuously pushed, or pushed and subsequently engulfed. A critical velocity for the pushing-engulfment transition is observed experimentally. Most models proposed to date ignore the complications arising from the liquid convection ahead of the solid-liquid interface. They simply solve the balance between the attractive drag force exercised by the liquid on the particle and the repulsive interfacial force. This work is an effort to calculate analytically the lift forces (Saffman and Magnus forces) under certain assumptions regarding the nature of fluid flow ahead of the solid/liquid interface. This makes possible the quantitative evaluation of the three experimentally observed regimes occurring during particle-interface interaction: (1) at low convection—no effect on the critical velocity for the particle engulfment transition; (2) at intermediate convection—increased critical velocity; (3) at high convection—no particle-interface interaction.

Apparatus and method for enhancing productivity of natural gas wells

Abstract: A natural gas production system prevents liquid accumulation in the wellbore and minimizes friction loading in the wellbore by maintaining production gas velocity above a critical minimum velocity. A pressurized gas is injected into the well to supplement the flow of production gas such that the velocity of the total gas flow up the well exceeds the critical velocity. A choke is fitted to the gas injection line, and total gas flows are measured by a flow meter. A flow controller compares the measured total gas flow rate against the critical flow rate, and determines a minimum gas injection rate required to maintain the total gas flow rate at or above the critical flow rate. The flow controller then adjusts the choke to achieve the desired gas injection rate. The injection gas may be recirculated production gas from the well, or a gas from a separate source.

Nonlinear investigation of the effect of primary suspension on the hunting stability of a rail wheelset

Abstract: The mathematical model of a single railway wheelset moving with constant speed on a smooth, level, and tangential track is used to numerically investigate the nonlinear dynamics of the wheelset. Nonlinearities in the wheelset model include the nonlinear wheel-rail profile and the friction-creep characteristics of the wheel-rail contact geometry. Wheelset numerical simulations consider single-point and two-point wheel-rail contact scenarios. Sensitivity of the critical hunting velocity to primary stiffness and damping parameters is examined. Results of the lateral stability study indicate that the critical hunting velocity of the wheelset is most sensitive to the primary longitudinal and lateral stiffness. Methods involving semi-active and active control of the primary longitudinal stiffness are developed to raise the critical velocity of hunting. These approaches are seen to considerably increase the wheelset critical hunting velocity.

The influence of flow acceleration on stone stability

Abstract: The stability of a bed of stones subject to a flow is often described in terms of a critical velocity or shear stress generated by the flow. These classical design methods like for example Shields, do not take the influence of flow acceleration into account. In experiments and practice, it appeared that when a flow is accelerated, stones start to move at a point where the so-called critical velocity is not reached yet. The movement of stones must have a second cause beside the velocity of flow. Only a little information is known on the influence of flow acceleration on stone stability of the bed. The objective of this thesis is to obtain more insight into the influence of acceleration of flow on the stability of stones. By carrying out experiments in a flume containing a local contraction, the stone stability in an accelerated flow is investigated. In the contraction the stability of two different stone sizes, subject to different velocity-acceleration combinations, is analysed. If the hypothesis is correct, than for some velocity-acceleration combinations movement occurs while for the same velocity combined with a lower acceleration no movement occurs. The shear stress occurring in the accelerated flow is determined using the shear velocity. According to the classical Shields method the shear velocity is responsible for the movement of the stones. Movement is detected for lower shear velocities then expected. According to the hypothesis this is a result of the extra generated force on the stones due to acceleration. After analysing the data it appeared that combinations of the same velocity and different accelerations showed differences in movement. The amount of movement goes up for an increase in acceleration combined with a constant or slightly decreasing velocity. This proves that there is a relation between the stability of the stones and a combination of the velocity and acceleration generated forces. The Morison equation is used to describe the relation between the forces acting on a stone. It combines the force generated by acceleration and the force generated by the peak velocities due to turbulence, as the sum of both forces. The extra force due to acceleration appeared to be of the same order as the force due to the velocity. Therefore, when looking at the stone stability in an accelerated flow, it is important to take the force generated by the acceleration into account. The resulting Morison force acting on a stone is proved to be responsible for the stability of the stones. Finally, a unique relation, valid for both stone diameters, between the force acting on the stone and the entrainment is found. This power relation consists of a dimensionless Morison-Shields parameter representing the force on a stone and a dimensionless entrainment parameter. The relation does not depend on stone size and is therefore expected to be universal in use.

Intrinsic pinning of vorticity by domain walls of l texture in superfluid 3He-A.

Abstract: We present the first observation of substantial persistent flow in superfluid 3He-A in thick simply connected slabs in a zero magnetic field, but only in l textures with domain walls. The flow is induced in a rotating cryostat using a torsional oscillator as a probe. The hysteretic dependences of the trapped vorticity on the maximal angular velocity of rotation are fairly universal for different densities of domain walls and slab thicknesses. A model of a critical state set by either the critical velocity for vortex nucleation or pinning strength explains all observations.

Active Plasma Experiment: North Star Particle Data

Abstract: We report on data from particle instruments making in situ sounding rocket measurements of the particle environment within and near an aluminum plasma jet caused by an explosion in the auroral ionosphere. The Active Plasma Experiment sounding rocket was launched to an altitude of 350 km from the Poker Flat Research Range in January 1999. The payload separated after launch into observing payloads and two explosive plasma jet generators. During the flight, the two explosive packages were detonated, and the observing payloads studied the surrounding environment. The particle instruments measured the resulting plasma jet from a distance of approximately 500 m. The instruments measured ions from 10 to 420 eV, electrons from 10 to 6000 eV, and low-energy electrons from 2 to 1200 eV. After each explosion, the particle instruments recorded the passage of a burst of material past the spacecraft. Analysis of mass-dependent effects, plasma β, and critical ionization velocity parameters are presented, together with a comparison to earlier experimental observations. In particular we note that the duration of the enhanced ion fluxes is controlled by the jet velocity and drops sharply when the jet velocity falls below the critical ionization velocity for each ion, with the peak ion fluxes only observed while v j e t > V c r i t (O+).

Introduction to the North Star Active Plasma-Jet Space Experiment

Abstract: T HE North Star Active Plasma Experiment mission was designed to investigate the dynamics and interactions of highspeed plasma jets in the high-latitude ionosphere. The artificial high-speed plasma jets produced during the North Star mission are relevant to plasma jets produced by electric propulsion thrusters, to electrodynamics associated with spacecraft tethers, and to advance spacecraft charging technologies. In addition, this experiment is relevant to the study of basic plasma physics problems such as momentum coupling between different plasma populations and how this coupling relates to the propagation of Alfven waves and electron acceleration. Specifically, the goals of the North Star mission were to address issues relating to the propagation of high-speed plasma jets across magnetic field lines, investigate the role of the neutral atmosphere in secondary jet ionization processes, and assess the Critical Ionization Velocity hypothesis. The North Star mission1 was launched on 22 January 1999 at 1358:03 UT using a Black Brant XII sounding rocket from Poker Flat, Alaska (Fig. 1). The experiment occurred just after an auroral breakup. The trajectory of the rocket crossed auroral arcs on its ascent prior to the execution of the two active plasma-jet experiments. The plasma jets were produced using a device known as an explosive-type generator (ETG) and resulted in the generation of a high-speed jet of both neutral and ionized aluminum. The neutral aluminum is formed from the rapid recombination of aluminum ions formed at the time of the initial jet initiation. The jets were directed perpendicular to the magnetic field toward instrumented subpayloads located 200–1500 m from the source. The plasma-jet experiments occurred at altitudes of 360 km (ETG-1) and 280 km (ETG-2). A canister of compressed air was released prior to ETG-1 injection to simulate the neutral density at a 150-km altitude in the vicinity of the payload. The North Star plasma-jet papers reviewed in this introduction address diverse topics, such as plasma-jet electrodynamics, plasma-jet magnetic field perturbations, plasma-jet propagation and neutralization, optical signatures of the plasma jet, and the Critical Ionization Velocity (CIV) theory. Erlandson et al.1 present an overview of the North Star mission and a review of the findings, and they discuss the high-speed optical measurements of the high-speed aluminum plasma-jet injection. The emissions were dominated by line emissions caused by neutral aluminum and a continuum thought to be from hot (1500–2000 K), micron-sized debris. The timing from detectors viewing the jet

Instability of a Simplified Model of a Free Hanging Riser Conveying Fluid

Abstract: Studying stability of a vertically suspended, fully submerged pipe conveying water (free-hanging water intake riser), researchers have found that if a critical flow velocity is reached, these pipes may flutter However, there are different predictions for the value of this critical velocity Researchers have mentioned values changing from infinitely small fluid velocities up to velocities which are unachievable in practice The nonlinear hydrodynamic damping caused by the surrounding water seems to be crucial for correct description of the stability of the submerged riser aspirating fluid In this paper, using the Morison’s equation, the nonlinear drag is taken into account as a function of the relative velocity between the current and the velocity of the riser itself The nonlinear system is studied employing the Galerkin method Ten-mode discretization turns out to be necessary to obtain an accurate result It is shown that the current has a strong stabilizing effect If the internal fluid flow exceeds a critical velocity the riser performs self-sustained oscillations with the amplitude smaller than one diameterCopyright © 2004 by ASME

Avoid turbulent entrainment (the critical velocity requirement)

Abstract: The avoidance of surface turbulence is probably the most complex and difficult rule to fulfill when dealing with gravity pouring systems. Above the critical velocity, there is the danger of surface entrainment leading to defect creation. The melt is safe from entrainment problems below the critical velocity. Experiments on the casting of aluminum have demonstrated that the strength of castings is significantly reduced if the critical velocity is exceeded. Getting the liquid metal out of the crucible or melting furnace and into the mould is a critical step when making a casting. It is likely that most casting scrap arises during a few seconds of pouring of the casting. Most castings are made by pouring the liquid metal into the opening of the running system, using gravity to effect the filling action of the mould. The varying cross-sectional areas of the metal as it rises in the mould pose a problem if the fill rate through the bottom gate is fixed—as it is approximately true for many counter-gravity filling systems that lack any sophistication of programmable control.

Critical velocity model for grain incipient motion in consideration of the randomness of grain location

Abstract: Incipient motion of solid grain in solid-liquid two\|phase flow is often concerned in many petroleum technologies, such as horizontal drilling, gravel-packing and sand washing. The random deposit location of grains on the bed surface causes the randomness of the incipient motion of solid grains. A random variable of dimensionless submergence was introduced to denote the location randomness. The probability of the variable distribution was counted as uniform. In consideration of buoyant weight, fluid thrust force, uplift force, cohesive force of grains, and the relationship between force arm and submergence, a calculation model for critical velocity of solid grain was established by means of moment equilibrium analysis. Integrating the critical velocity equation and submergence probability distribution equation, a quantitative relationship between the critical velocity and the incipient motion velocity of grains was built, which offers a practical formula for calculation of the critical velocity under certain incipient motion criterion. The results of this work agree well with the experimental data for the grain with diameter range from 0.001 mm to 10 mm.

3 – Introduction to mixing and dispersion in natural waterways

Abstract: This chapter presents an introduction to mixing and dispersion in natural waterways. Dispersion of matter in natural rivers is of considerable importance. Applications include sediment and salt dispersion, injection of heated water into a cooler stream, and storm water waste disposal during flood. With increasing flow velocity, there is a critical velocity above which the flow motion becomes characterized by an unpredictable behavior, strong mixing properties, and a broad spectrum of length scales. The effects of buoyancy on submerged air bubble in a liquid are often expressed in terms of the bubble rise velocity of a single bubble rising steadily in a fluid at rest. It is observed that for an individual air bubble rising uniformly in a fluid at rest and subjected to a hydrostatic pressure gradient, the rise velocity depends upon the value of the drag coefficient which is a function of the bubble shape and velocity. The bubble rise velocity in a non-hydrostatic pressure gradient is also elaborated.

Transition of Planar Rotational Motion of an Asymmetric Spacecraft into Oscillatory Motion at Its Reentry into the Atmosphere

Abstract: The motion of a spacecraft with small asymmetry relative to its center of mass is considered. The restoring aerodynamic moment of the spacecraft is described by the Fourier series in terms of the angle of attack with the two first sinusoidal and the first cosinusoidal terms. A solution for the angle of attack in the undisturbed rotational motion is found. The analytical expression is obtained for the integral of action taken along the separatrices that separate the rotational and oscillatory regions of the phase portrait of a system. The transition of the spacecraft's motion from planar rotational to oscillatory is investigated. This transition is caused by a slow variation of moment characteristic coefficients, as well as by the presence of small asymmetry and damping and slow variation of their coefficients. Analytical formulas are obtained for determining the times of transition from rotational to oscillatory motion, as well as for the critical angular velocity of beyond-the-atmosphere rotation. When this critical velocity is exceeded, body rotation proceeds for a long time interval (planar autorotation arises).

Theoretical Studies of dilute Bose-Einstein condensates in a double-well potential

Abstract: In this Thesis we apply the Gross-Pitaevskii equation (GPE) to describe properties of a dilute, near zero temperature Bose gas for various confining geometries. We start by reviewing some basic information about the density, the chemical potential and elementary excitations of a dilute atomic condensate confined in a single harmonic trap for a Bose condensate with repulsive and attractive interactions and we also discuss the stability in the case of attractive interactions. We extend our study to a one and three dimensional double-well trap. We investigate the eigenenergy levels and show that the nonlinearity leads to triangular structures which appear either in the ground or excited states for the case of a Bose condensate with attractive or repulsive interactions respectively. We apply the eigenenergy level picture to analyse Josephson effects induced when the barrier IS moved at a constant velocity across the trapping potential or by the application of a time-dependent potential gradient. The GPE simulations are compared to the predictions of a nonlinear two state model. Above a critical velocity there is a transition to a superposition of ground and excited states which leads to sudden changes in the population difference. The direction of Josephson flow depends critically on the initial state of the system and we discuss the feasibility of experimental control of the atomic flow using phase-imprinting. The stability of a low temperature Bose-Einstein condensate with attract interactions in one and three dimensional double-well potentiate is discussed. The condensate is shown to collapse at a critical potential gradient which corresponds to a critical number of atoms in one of the two wells. Finally we investigate the stability and tunnelling effects in a multi-well system.

The Analysis of Flow-Induced Vibration and Design Improvement in KSNP Steam Generators of UCN #5,6

Abstract: The KSNP Steam Generators (Youngkwang Unit 3 and 4, Ulchin Unit 3 and 4) have a problem of U-tube fretting wear due to Flow Induced Vibration (FIV). In particular, the wear is localized and concentrated in a small area of upper part of U-bend in the Central Cavity region. The region has some conditions susceptible to the FIV, which are high flow velocity, high void fraction, and long unsupported span. Even though the FIV could be occurred by many mechanisms, the main mechanism would be fluid-elastic instability, or turbulent excitation. To remedy the problem, Eggcrate Flow Distribution Plate (EFDP) was installed in the Central Cavity region of Ulchin Unit 5 and 6 steam generators, so that it reduces the flow velocity in the region to a certain level. However, the cause of the FIV and the effectiveness of the EFDP was not thoroughly studied and checked. In this study, therefore the Stability Ratio (SR), which is the ratio of the actual velocity to the critical velocity, was compared between the value before the installation of EFDP and that after. Also the possibility of fluid-elastic instability of KSNP steam generator and the effectiveness of EFDP were checked based on the ATHOS3 code calculation and the Pettigrew’ s experimental results. The calculated results were plotted in a fluid-elastic instability criteria-diagram (Pettigrew, 1998, Fig. 9). The plotted result showed that KSNP steam generator with EFDP had the margin of Fluid-Elastic Instability by almost 25%.

Metastable states in the planar two-dimensional XY model and dissipation in superfluid flow

Abstract: We use the Metropolis algorithm to study the stability of superfluid flow in a model system, namely the two-dimensional planar $XY$ model. Flow properties are examined by studying the behavior of the system in metastable ``twisted'' states. We demonstrate the stability of superfluidity in this model and we discuss the Meissner effect and velocity quantization. We also study the critical velocity and dissipation by vortex creation and rotational flow and their dependence on the geometry of the system. An expression for the average superfluid velocity as a function of time, ${\overline{v}}_{s}(t)$, is obtained and compared with experimental results.

A transfer matrix method for solving stability of pipes conveying fluid on elastic foundation with various end supports

Abstract: This paper investigates the stability of pipes conveying fluid on elastic foundation. The governing equations of the pipes are derived. Based on the initial parameter method, a concise relation between displacement vector and force vector at both ends of the pipe is obtained. For a pipe conveying fluid with arbitrary intermediate spring supports, a transfer matrix method is used to determine the characteristic equation, from which the critical velocity is calculated. Several numerical examples show that the present method may save computer time and achieve good accuracy since the characteristic equation is a two-order one. The influence of rigidity coefficient of the foundation on the critical velocity is discussed in detail.

Analyzing and Calculating the Critical Driving Velocity before Climbing Phenomenon

Abstract: The mechanism of self-excited viberation caus in g climbing phenomenon because of the low velocity of driving parts in a machin e’s feed system was discussed.After building motion model of the machine’s feed system,a formula for caculating the lowest critical velocity that decides climbing phenomenon was reasoned,and will be worthable in applications.