Radial Expansion Velocity Research Articles

Behavior of Water Jet Accompanied with Air Suction

In order to atomize a liquid, the authors have investigated the behavior of air-water jets. In a series of experiments, we have discovered a strange phenomenon that the water jet accompanied with air suction from the free surface has made a periodic radial splash of water drop. The purpose of the present paper is to clear out the origin of this phenomenon and the behavior of water jet accompanied with air suction. The behavior of water jet has been photographed by a digital camera aided with a flashlight and high-speed video camera. Those experiments enable us to find the origin of a periodic radial splash due to a formation of single air bubble at the flow separation region inside the nozzle and due to explosive expansion of the bubble after injected in the free space. In order to analyze the radial splash of water, we have conducted the equation of spherical liquid membrane. The numerical results obtained have been compared with the experimental results and good agreement has been obtained in radial expansion velocity.

Comparison of average radial expansion velocity from impacted liquid filled cylinders

In this paper, the average radial expansion velocity of an impacted fluid filled cylindrical target is investigated. Theoretical and numerical predictions of the radial expansion velocity are compared to the experimentally measured radial expansion velocity. The primary objective of this work is to assess the ability of these theoretical and numerical techniques to predict the radial expansion velocity. A secondary objective of this work is to quantify the effect of changes in dimensional scale on the radial expansion velocity and to construct a simple physics based model which incorporates these scaling effects. Two-dimensional numerical hydrocode simulations accurately predict the measured ejection velocity for tests with low projectile–target misalignment. However, three-dimensional numerical calculations, which account for this misalignment, accurately predict all experimental tests. A theoretical formulation, based on a simple conservation of energy principle, yields a zero-dimensional model which accurately predicts the two-dimensional hydrocode simulations. Thus for experimental simulations which have low projectile–target misalignment, the simple theoretical model developed here accurately predicts the average radial expansion velocity. A dimensional analysis of this theoretical formulation yields a scaling relationship which accurately predicts the effect of dimensional scale between two different experimental test series.

A study of the lightning channel corona sheath

Dynamics of lightning channel corona sheath surrounding thin channel core is examined on the basis of three transmission‐line‐type models of the return stroke that specify different attenuation of longitudinal current with height. The corona sheath conductivity is estimated using measured radial electric field in the immediate vicinity of the lightning channel and measured channel base current. The corona sheath radius, velocity of corona sheath radial expansion, and corona current are examined using a multiexponential approximation of the lightning channel base current waveform. Additionally, energy dissipated in the corona sheath is estimated and compared to that dissipated in the channel core.

Dynamics of giant volcanic ash clouds from supervolcanic eruptions

The largest explosive volcanic eruptions that have occurred on Earth generated giant ash clouds from rising plumes that spread in the stratosphere around a height of neutral buoyancy, with estimated supply rates that are in the range 1011 to 1013 m3/s. These giant ash clouds are controlled by a balance between gravity and Coriolis forces, forming spinning bodies of nearly fixed proportions after a few hours and are initially insensitive to stratospheric winds. In contrast, volcanic plumes from eruptions with small to intermediate magnitude spread as inertial intrusions under the influence of stratospheric winds, with the Earth's rotation being unimportant. In the largest eruptions the giant spinning ash clouds typically develop diameters greater than 600 km and up to 6000 km in the most powerful super‐eruptions, thus explaining why areas of continental size can be covered with volcanic ash. The radial expansion and spinning velocities are calculated at tens of metres per second and increase with eruption intensity. Higher spreading velocities carry larger ash grain sizes to a given radius, so that grain size at a given distance from the source increases with eruption intensity, consistent with geological observations.

Heterogeneous Plasma‐Producing Structures at Current Implosion of a Wire Array

AbstractCharacteristic properties of the plasma production process have been considered for the case of megampere currents flowing through hollow cylindrical wire arrays of the Angara‐5‐1 facility. In 3‐4 nanoseconds after voltage applying to the wire surfaces there appear a plasma layer. The system becomes heterogeneous, i.e. consisting of a kernel of metal wires and a plasma layer. In several nanoseconds the current flow goes from metal to plasma, which results in reducing the electric field strength along the wire.The Joule heat energy delivered to the metal before the moment of complete current trapping by plasma is insufficient for the whole mass transition to a hot plasma state. The X‐ray radiography techniques made it possible to detect and study dense clusters of substance of ∼1g/cm3 at a developed discharge stage. The radial expansion velocity of ∼104 cm/s measured at the 70‐th nanosecond after the current start allows treating the dense core at a late stage in the form of a submicron heterogeneous structure from its liquid and slightly ionized gas phase. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Behavior of Water Jet Accompanied with Air Suction

In order to atomize a liquid, the authors have investigated the behavior of air-water jets. In a series of experiments, we have discovered a strange phenomenon that the water jet accompanied with an air suction from the free surface has made a periodic radial splash of water drop. The purpose of the present paper is clear out the origin of this phenomenon and the behavior of water jet accompanied with an air suction. The behavior of water jet has been photographed by a digital camera aided with a flash light and high-speed video camera. Those experiments enable us to find the origin of a periodic radial splash due to a formation of single air bubble at the flow separation region inside the nozzle and due to explosive expansion of the bubble after injected in the free space. In order to analyze the radial splash of water, we have conducted the equation of spherical liquid membrane. The numerical results obtained have been compared with the experimental results and good agreement has been obtained in radial expansion velocity.

An observation of a nascent fractal pattern in MD simulation for a fragmentation of an fcc lattice

To seek for a possible origin of fractal pattern in nature, we perform a molecular dynamics simulation for a fragmentation of an infinite fcc lattice. The fragmentation is induced by the initial condition of the model that the lattice particles have the Hubble-type radial expansion velocities. As time proceeds, the average density decreases and density fluctuation develops. By using the box counting method, it is found that the frequency-size plot of the density follows instantaneously a universal power-law for each Hubble constant up to the size of a cross-over. This cross-over size corresponds to the maximum size of fluctuation and is found to obey a dynamical scaling law as a function of time. This instantaneous generation of a nascent fractal is purely of dynamical origin and it shows us a new formation mechanism of a fractal patterns different from the traditional criticality concept.

Dynamics of pedestal perturbations by ELMs and edge harmonic oscillations in DIII-D

Edge density profile modifications associated with edge localized modes (ELMs) and edge harmonic oscillations (EHOs) have been measured using fast profile reflectometry in DIII-D H-mode plasmas. Specifically, high time (down to 10 µs) and spatial (∼4 mm) resolution density profile measurements from 0 × 1019 to 3.1 × 1019 m−3 provide us with new tools to study the dynamics of pedestal perturbations. During ELMs, it has been observed that the SOL density profile expands outwards to the vessel wall while the pedestal density reduces. These measurements provide direct evidence for enhanced particle radial transport to the vessel wall at the onset of ELMs. A large SOL density profile radial expansion velocity is observed during ELMs. A comparison of the velocities for different pedestal densities is made. The density scale length, density at the wall, and density fluctuations during ELMs are determined as a function of pedestal density, as well as particle transport during ELMs. Measurements in quiescent H-mode plasmas indicate that the pedestal density profile is modulated at the fundamental frequency of the EHO. A comparison between reflectometer and beam emission spectroscopy measurements regarding the EHO density fluctuation profile is presented.

Synaptic fusion pore structure and AMPA receptor activation according to Brownian simulation of glutamate diffusion.

The rising phase of fast, AMPA-mediated Excitatory Post Synaptic Currents (EPSCs) has a primary role in the computational ability of neurons. The structure and radial expansion velocity of the fusion pore between the vesicle and the presynaptic membrane could be important factors in determining the time course of the EPSC. We have used a Brownian simulation model for glutamate neurotransmitter diffusion to test two hypotheses on the fusion pore structure, namely, the proteinaceous pore and the purely lipidic pore. Three more hypotheses on the radial expansion velocity were also tested. The rising phases of the EPSC, computed under various conditions, were compared with experimental data from the literature. Our present results show that a proteinaceous fusion pore should produce a more marked foot at the beginning of the rising phase of the EPSC. They also confirm the hypothesis that the structure of the fusion pore and its radial expansion velocity play significant roles in shaping the fast EPSC time course.

The role of the electrical polarization field in magnetic plasma confinement

The radial expansion of an axially flowing plasma column in a constant axial magnetic field is considered for the following cases: (a) magnetized electrons and unmagnetized ions, and (b) magnetized electrons and partially magnetized ions . In case (a), radially escaping ions produce a radial electric polarization field. For a directed axial ion velocity of v0 = 1.5×104 m s−1, particle concentration n0 = 1018 m−3 and magnetic field B = 0.02 T, the azimuthal electron drift velocity in the crossed electric and magnetic fields is 4×105 m s−1, i.e. near to their thermal velocity with Te = 3×104 K: (kTe/me)1/2≈5×105 m s−1 and it together with the thermal velocity determines the electron collision frequency. With a neutral gas pressure of 1.3 Pa and a magnetic field strength of B = 0.01 T, the radial electron velocity reaches the radial ion expansion velocity within a time of ⩽8 × R/v0, where R is the plasma column radius.If the ions are partially magnetized, the electric polarization field is caused by radially escaping unmagnetized ions, while the electrons, and magnetized ions drift in the crossed electric, and magnetic field. The thermal, and axially directed ion velocities are much less than their azimuthal drift velocity, and thus the azimuthal velocity determines the collision frequency.

Propagation velocities of gas rings in collisional ring galaxies

The propagation velocity of the first gas ring in collisional ring galaxies, i.e. the velocity at which the maximum in the radial gas density profile propagates radially in the galactic disk, is usually inferred from the radial expansion velocity of gas in the first ring. Our numerical hydrodynamics modeling of ring galaxy formation however shows that the maximum radial expansion velocity of gas in the first ring (vgas) is invariably below the propagation velocity of the first gas ring itself (). Modeling of the Cartwheel galaxy indicates that the outer ring is currently propagating at km s-1, while the maximum radial expansion velocity of gas in the outer ring is currently km s-1. The latter value is in marginal agreement with the measurements of Higdon ([CITE]) based on HI kinematics. Modeling of the radial color gradients of the Cartwheel ring galaxy also indicates that the outer ring is propagating at km s-1 for the adopted distance to the galaxy of 140 Mpc. On the other hand, the azimuthally averaged surface brightness profile of the Cartwheel's outer ring does not peak exterior to those in K- and B-bands, contrary to what would be expected for such a high propagation velocity. We show that a combined effect of 41° inclination, finite thickness, and warping of the Cartwheel's disk might be responsible for the lack of angular difference in the peak positions. Indeed, the radial surface brightness profiles obtained along the Cartwheel's major axis, where effects of inclination and finite thickness are minimized, do peak exterior to those at K- and B-bands. The angular difference in peak positions implies vring = 110 km s-1, which is in agreement with the model predictions. We briefly discuss the utility of radio continuum emission and spectral line equivalent widths for determining the propagation velocity of gas rings in collisional ring galaxies.

Soft X-Ray Images of Krypton Gas-Puff Z-Pinches

A series of experiments has been carried out on Qiang-guang I generator to study the dynamics of krypton gas-puff Z-pinches. The generator was operated at a peak current of 1.5 MA with a rise-time of 80 ns. The specific linear mass of gas liner was about 20 μg/cm in these experiments. In the diagnostic system, a four-frame x-ray framing camera and a pinhole camera were employed. A novel feature of this camera is that it can give time-resolved x-ray images with four frames and energy-resolved x-ray images with two different filters and an array of 8 pinholes integrated into one compact assemble. As a typical experimental result, an averaged radial imploding velocity of 157 km/s over 14 ns near the late phase of implosion was measured from the time-resolved x-ray images. From the time-integrated x-ray image an averaged radial convergence of 0.072 times of the original size was measured. An averaged radial expansion velocity was 130 km/s and the maximum radial convergence of 0.04 times of the original size were measured from the time-resolved x-ray images. The dominant axial wavelengths of instabilities in the plasma were between 1 and 2 mm. The change in average photons energy was observed from energy spectrum- and time-resolved x-ray images.

P Cygni type Ly from starburst galaxies

P Cygni type Lya emission is exhibited in the spectra of nearly half the starburst galaxies in either the nearby Universe or the high-z Universe. Such emission is presumably formed by an expanding supershell surrounding star-forming regions. We assume that the Lya photons from the central star-forming regions transfer in a spherical galactic supershell of uniform neutral hydrogen that is expanding radially in a bulk flow. We apply the Monte Carlo code developed previously for static and plane-parallel media. We consider typical cases in which the supershell has Lya line-centre optical depth τ 0 = 10 5 -10 7 , a radial expansion velocity V e x p 300 km s - 1 and turbulence b ≃ 40 km s - 1 . We find that a series of emission peaks appear redward of the systemic redshift, their frequencies being given by (2N - 1)V e x p , where the number of back-scatterings N is greater than zero. We also investigate how the emergent profiles are changed in accordance with the expansion velocity and the column density of the supershell. We find that the number and the flux ratios of emission peaks are determined by interplay of the two parameters. It is very important that the kinematics of the supershell is imprinted on both the widths of the peaks and the velocity differences between peaks. We discuss the effects of dust extinction and the implication of our work in relation to recent spectroscopic observations of starburst galaxies.

Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 1. Methods

Two inversion methods are developed to reconstruct eruption column dynamics from granulometric data of Plinian deposits. They are based on the steady tephra dispersal model from umbrella clouds which spread as gravity currents. The methods provide (1) expansion rate of an umbrella cloud, (2) grain‐size distribution at the top of the eruption column, and (3) total amount of ejecta including very fine particles which are extensively dispersed in the atmosphere. The first quantity is correlated to the magma discharge rate. The second quantity provides information about sorting processes during column rise, and hence, about deceleration in the “gas thrust region” of the eruption column and the timing of column collapse. The third quantity, together with the estimated magma discharge rate, provides the effective duration of the eruption. The two methods are based on the same dispersal model but use different averaging. Combining the two methods can provide additional information about eruption dynamics, such as minor flow structures of the umbrella cloud and fluctuations of eruption rate. The methods are applicable to large‐scale eruptions where the radial expansion velocity of the umbrella cloud exceeds wind velocity. The applicability of the methods is tested from the viewpoint of comparison with real‐time observations in the companion paper (paper 2).

The Violent Interstellar Medium of Dwarf Irregular Galaxies

High-resolution observations of nearby dwarf irregular galaxies (in particular, IC 2574 and DDO 47) in the 21 cm line of neutral hydrogen (H I) obtained with the Very Large Array (VLA) reveal that their neutral interstellar medium (ISM) is dominated by holes and shells, most of which are expanding (the ““ violent II ISM). These features range in size from about 100 (a limit set by the size of the beam) to about 1000 pc, dominating the appearance of the H I surface brightness maps and inNuencing the velocity Ðelds. Current star formation, as traced by Ha emission, is predominantly found along the rims of the larger H I holes, suggesting propagating star formation. On linear scales of B100 pc, star formation occurs if the H I surface density reaches values higher than 1021 cm~2. The scale height of the H I layer in the dwarf irregular galaxies under study is found to be of order 400 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed one-dimensional velocity dispersion of B7 km s~1). This pu†ed-up disk implies a rather low H I volume density (0.15 cm~3). This, combined with the reduced gravitational pull, solid-body rotation throughout the disk and the absence of density waves, explains why the diameter distribution in dwarf galaxies extends to substantially larger values than in spiral galaxies. The radial expansion velocities of the H I holes (8E12 km s~1), the indicative ages [(10E60)] 106 yr], and the energy requirements for their formation (1050E1053 ergs) can be understood in terms of the combined e†ects of stellar winds and multiple supernova explosions of the most massive stars formed during a recent phase of active star formation. A comparison with other galaxies spanning a wide range of masses (M31, M33, and Holmberg II) shows that, somewhat surprisingly, the energies needed to create these structures are the same for all types of galaxies, at least to Ðrst order. The overall statistical properties of the H I holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities, and ages. The origin of the H I holes and shells is still not fully settled. In the standard picture, young star-forming regions and their aftermaths (strong stellar winds and supernova explosions) are held responsible for their formation. This picture, however, is not without its critics, and other mechanisms such as the infall of high-velocity clouds, turbulent motions, or even gamma-ray bursters have recently been proposed. A promising region that sheds light on the processes which lie at origin of the H I holes is the recently discovered supergiant shell in IC 2574 (IC 2574-SGS). Here, a multiwavelength study (optical broad and narrow band, H I, radio continuum, and X-ray) provides evidence that a young stellar cluster is driving the expansion of this particular H I shell. IC 2574-SGS measures roughly 1000] 500 pc in size and is expanding at about 25 km s~1. The H I data suggest an age of about 1.4] 107 yr ; the energy input must have been of order 2.6^ 1] 1053 ergs. Massive starforming regions, as traced by Ha emission, are situated predominantly on the rim of this H I shell. This supports the view that the accumulated H I on the rim has reached densities which are high enough for secondary star formation to commence. VLA radio continuum observations at j 6 cm show that these star-forming regions are the main sources of radio continuum emission in this galaxy. This emission is mainly thermal in origin. Soft X-ray emission coming from inside the H I shell is believed to be generated by an X-rayE emitting plasma keV) 1.6^ 0.5] 1038 ergs [L X(0.1E2.4 s~1]. The X-ray data are compatible with emission coming from a Raymond and Smith plasma at a temperature of about log T [K] 6.8 and a density of 0.03 cm~3. Deep R-band observations reveal the presence of an interior stellar cluster in IC 2574-SGS. It is believed to be the powering source for the formation and expansion of the H I shell as well as for the heating of the X-ray gas. These observations support the standard picture that at least some H I holes that are found in the ISM of galaxies are created as a result of massive star formation and the subsequent rapid evolution of the most massive stars.

Low-pressure switching plasmas with axial magnetic field stabilization

Low pressure gas discharge switches of the usual pseudospark design are known to have rather large electrode erosion rates when operated at high current levels and long switching times. In order to improve the plasma confinement and to reduce the electrode erosion in high-current applications, an axial magnetic field has been used in a low-pressure pulsed switching device. For a pulse duration of 28 /spl mu/s and current amplitudes of up to 50 kA the plasma expands quickly, leading to a diffuse discharge mode. The radial expansion velocity increases with current amplitude and reaches up to 10 km/s at a peak current of 53 kA. With a further increase in current, a constricted plasma is superimposed onto the expanded diffuse part. These findings are compared with the case in which no magnetic field is applied.

Coronal plasma behavior of the Z pinch produced from carbon and cryogenic deuterium fibers

A series of fiber pinch experiments has been carried out on the MAGPIE (mega-ampere generator for plasma implosion experiments) generator (1.8 MA, 150 ns) [Mithell et al., Rev. Sci. Instrum. 67, 1533 (1996)] to study the temporal evolution of the coronal plasma. Analysis of schlieren photographs, axial streak images and gated x-ray photographs gives the radial and axial motion of the coronal plasma. The influence of a current pulse (prepulse) of 30 kA applied 200 ns before the main discharge was also studied. Radial expansion velocities of 5.5×106 cm/s for carbon fiber shots without prepulse and 3.6×106 cm/s for carbon fibers with prepulse were measured. Axial wavelengths (λz) of dominant instabilities in the corona were between 0.05 and 0.2 cm corresponding to ka∼10–20. Comparison of the results obtained with carbon fibers with and without current prepulse and cryogenic deuterium fibers are presented.

Nonthermal direct photons in Pb+Pb at 160A GeV from microscopic transport theory

Direct photon production in central Pb+Pb collisions at CERN-SPS energy is calculated within the ultrarelativistic quantum molecular dynamics model (UrQMD), and within distinctly different versions of relativistic hydrodynamics. We find that in UrQMD the local momentum distributions of the secondaries are strongly elongated along the beam axis initially. Therefore, the pre-equilibrium contribution dominates the photon spectrum at transverse momenta above $\ensuremath{\approx}1.5$ GeV. The hydrodynamics prediction of a strong correlation between the temperature and radial expansion velocities on the one hand, and the slope of the transverse momentum distribution of direct photons on the other hand thus is not recovered in UrQMD. The rapidity distribution of direct photons in UrQMD reveals that the initial conditions for the longitudinal expansion of the photon source (the meson ``fluid'') resemble boost invariance rather than Landau-like flow.

Expansion of spark channels in air and possibilities of plasma crowbar in high pulsed current and magnetic field technique

The radial expansion velocities and diameters of spark channels in air are measured by the plasma-metal contact method. An LC discharge circuit with energy up to 50 kJ and with current frequencies ranging from ∼102 to ∼104 Hz served as the source of the channels. It is concluded on the basis of the results obtained that the previously proposed automatic plasma crowbar based on the expansion of spark channels in gases is a universal switch for an inductive load in LC discharge circuits that could find very wide applications in high pulsed current and magnetic field technology.

Optical measurements of plasma dynamics in carbon fiber Z-pinches

A series of experiments has been carried out on the Mega Ampere Generator for Plasma Implosion Experiments (MAGPIE) generator in order to study the dynamics of carbon fiber Z-pinches. The generator was operated at 1.4 MV, with a peak current of 1 MA, and a rise time of 150 ns. In some shots, a current prepulse of about 30 kA was provided to study its influence on the dynamics of the fiber pinch. Carbon fibers of 7, 33, and 300 /spl mu/m diameter were used during these experiments. The diagnostics employed were a self-referencing interferometer, a two-frame Schlieren system, an optical streak camera, and a four-frame X-ray framing camera. A novel feature of these measurements is the employment of an optical streak camera with a set of four slits arranged along the fiber axis and displaced in the radial direction. This permitted the study of the temporal evolution (axial and radial) of the plasma regions emitting in the visible part of the spectra. Correlation between these regions of the plasma and the location of X-ray hot spots is discussed. In carbon fibers of 33 pm diameter, the radial expansion velocity measured from Schlieren images was 3.6/spl times/10/sup 6/ cm/s and 5.5/spl times/10/sup 6/ cm/s for shots with and without prepulse, respectively. The dominant axial wavelengths of instabilities in the coronal plasma were between 0.05 and 0.2 cm, which correspond to ka values between 10 and 20, where k is the wavenumber of the instability and a is its amplitude. The dynamics of carbon fibers of different diameters are compared.