Radial Expansion Velocity Research Articles

Characteristics of laser-induced plasma under reduced background pressure with Doppler spectroscopy of excited atomic species near the shockwave front

We present measurements of Laser Induced Plasma expansion relying on classical, laterally resolved spectroscopy. Easy observable Doppler splitting of Cu I 324.75 nm spectral line provides measurement of radial expansion velocity in a straightforward way. The measurements are conducted in atmosphere of air, argon and hydrogen at low pressure in the range 20–200 Pa. We found that expansion velocity is linearly decreasing if pressure of surrounding gas increases, with velocity/pressure slope nearly the same for all three gases. Copper atoms have the highest expansion speed in argon ( ∼ 50 km/s) and the smallest speed in air ( ∼ 42 km/s). It is found that expansion velocity increases linearly with irradiance, while intensity of the spectral line is quite insensitive to the laser irradiance.

Monitoring the dynamics of the surface deformation prior to the onset of plasma emission during femtosecond laser ablation of noble metals by time-resolved reflectivity microscopy

The generation of an expanding plasma during laser ablation is preceded at early times by the formation and evolution of a subsurface melted front. The monitoring of such transient event can't be studied by conventional spectroscopic techniques. Pump-probe femtosecond microscopy allows the following of the surface changes during femtosecond laser ablation taking advantage of the formation of a number of dynamic Newton rings that evolve with time. Measurements at different times allow the quantification of the radial expansion velocity of the molten material. For Au and Ag, expansions in the range of 7000–12,000m/s for Au and 3000–21,000m/s have been calculated depending on the pump energies. Such values correspond to hypersonic velocities with Mach number between 3 and 6.

NUMERICAL SIMULATION OF THE SVS 13 MICROJET AND BOW SHOCK BUBBLE

ABSTRACT Numerical simulations of the SVS 13 microjet and bow shock bubble are performed using the WENO method that reproduces the main features and dynamics of data from the Keck Telescope/OSIRIS velocity-resolved integral field spectrograph: an expanding, cooler bow shock bubble, with the bubble center moving at approximately 50 km s−1 with a radial expansion velocity of 11 km s−1, surrounding the fast, hotter jet, which is propagating at 156 km s−1. Contact and bow shock waves are visible in the simulations both from the initial short jet pulse that creates the nearly spherical bow shock bubble and from the fast microjet, while a terminal Mach disk shock is visible near the tip of the continuous microjet, which reduces the velocity of the jet gas down to the flow velocity of the contact discontinuity at the leading edge of the jet. At 21.1 years after the launch of the initial bubble pulse, the jet has caught up with and penetrated almost all the way across the bow shock bubble of the slower initial pulse. At times later than about 22 years, the jet has penetrated through the bubble and thereafter begins to subsume its spherical form. Emission maps from the simulations of the jet—traced by the emission of the shock-excited 1.644 μm [Fe ii] line—and the bow shock bubble—traced in the lower excitation 2.122 μm H2 1–0 S(1) line—projected onto the plane of the sky are presented, and are in good agreement with the Keck observations.

Dynamic expansion modeling of a thick-walled cylinder under internal high strain rate loading

ABSTRACTThis article presents analysis of the dynamic behavior of a thick-walled cylinder under the assumption of nonlinear strain rate hardening behavior under high strain rate loading before any fracture on the surface. The theoretical model applies both to direct and indirect use of dynamic strength of material and instant boundary conditions to establish a differential equation for radial expansion velocity. Further, detailed discussion will be given with emphasis on the main aspects of the cylinder behavior, i.e., radial displacement, internal pressure, strain rate, flow stress, radial and tangential stress, and the influence of the different material rate sensitive exponent.

Modeling of dynamic expansion of thick‐walled cylinder under high strain rate loading with strain rate sensitivity analysis

This paper presents an analysis of the dynamic behavior of thick‐walled cylinder using Levy‐Mises flow rule in the assumption of a non‐linear strain rate hardening behavior under high strain rate loading. The theoretical model to be developed in this work applies indirect use of dynamic strength of material characterized by a non‐linear relation and instant boundary condition based on Jones‐Wilkins‐Lee equation of state to establish differential equation for radial expansion velocity. Detailed discussion will be given with emphasis on the main aspects of the cylinder behaviour, i.e. radial displacement, internal pressure, strain rate, flow stress, radial and tangential stress and the influence of different material rate sensitive exponent.Results show a good agreement with the analytical solutions proposed here.

12CO emission from EP Aquarii: Another example of an axi-symmetric AGB wind?

The CO(1-0) and (2-1) emission of the circumstellar envelope of the AGB star EP Aqr has been observed using the IRAM PdBI and the IRAM 30-m telescope. The line profiles reveal the presence of two distinct components centered on the star velocity, a broad component extending up to ~10 km/s and a narrow component indicating an expansion velocity of ~2 km/s. An early analysis of these data was performed under the assumption of isotropic winds. The present study revisits this interpretation by assuming instead a bipolar outflow nearly aligned with the line of sight. A satisfactory description of the observed flux densities is obtained with a radial expansion velocity increasing from ~2 km/s at the equator to ~10 km/s near the poles. The angular aperture of the bipolar outflow is ~45 deg with respect to the star axis, which makes an angle of ~13 deg with the line of sight. A detailed study of the CO(1-0) to CO(2-1) flux ratio reveals a significant dependence of the temperature on the star latitude, smaller and steeper at the poles than at the equator at large distances from the star. Under the hypothesis of radial expansion and of rotation invariance about the star axis, the effective density has been evaluated in space as a function of star coordinates. Evidence is found for an enhancement of the effective density in the northern hemisphere of the star at angular distances in excess of ~3" and covering the whole longitudinal range. The peak velocity of the narrow component is observed to vary slightly with position on the sky, a variation consistent with the model and understood as the effect of the inclination of the star axis with respect to the line of sight. While the phenomenological model presented here reproduces well the general features of the observations, significant differences are also revealed, which would require a better spatial resolution to be properly described.

Single-drop impingement onto a wavy liquid film and description of the asymmetrical cavity dynamics.

The present paper is devoted to an experimental investigation of the cavity formed upon a single-drop impingement onto a traveling solitary surface wave on a deep pool of the same liquid. The dynamics of the cavity throughout its complete expansion and receding phase are analyzed using high-speed shadowgraphy and compared to the outcomes of drop impingements onto steady liquid surface films having equal thickness. The effects of the surface wave velocity, amplitude and phase, drop impingement velocity, and liquid viscosity on the cavity's diameter and depth evolution are accurately characterized at various time instants. The wave velocity induces a distinct and in time increasing inclination of the cavity in the wave propagation direction. In particular for strong waves an asymmetrical distribution of the radial expansion and retraction velocity along the cavity's circumference is observed. A linear dependency between the absolute Weber number and the typical length and time scales associated with the cavity's maximum depth and maximum diameter is reported.

Stress Analysis of Gun Barrel Subjected to Dynamic Pressure

In the optimal design of a modern gun barrel, there are some aspects to be considered. One of the main factor is internal ballistic which consist of pressure-time, pressure-distance, velocity-time and distance-time curves. In this paper, a simple analytical solution for the plastic stress of an internally pressurized open-ended thick-walled cylinder made of hardening steel which is the closest model to gun barrel is obtained in perfectly plastic and plane stress condition by using energy method and the yield criterion of Von Mises and adding rifle grooves and choosing stress components as basic unknowns and ballistic pressure equation as known. Then results of analytical solution are compared to a numerical model and verified a very well and reliable accuracy. So the resultant can be used easily in calculation of radial expansion velocity and compressive pressure.

Dynamic Fragmentation Tests of LY12 Aluminum Alloy Using a SHPB Based Expanding Ring Technique

A new experimental technology based on the Split Hopkinson Pressure Bar (SHPB) is developed for conducting expanding ring tests. The technique is useful for the studies of the dynamic tensile deformation and the fracture (fragmentation) properties of ductile metallic materials. The loading fixture includes a hydraulic cylinder filled with the incompressible fluid, which is pushed by a piston connected to the input bar. As the liquid is driven, it transfers the pressure to the specimen, compressing and expanding the metallic ring specimen in the radial direction. The approximately incompressible property of the liquid makes it possible to transfer a relatively low piston-driving velocity to a very high radial-expansion velocity of the specimen, as the sectional area of the cylinder narrows significantly. Using this experimental technology the ring specimens made of LY12 aluminum alloy were dynamically expanded and fragmentized. Results show apparent increases of the fragment numbers and fracture strain of the specimen with the increase of the impact velocity. The recovered ring fragments were examined and measured to obtain the average fragment sizes (lengths) during each test. These fragment sizes were compared with the estimates from the classical fragmentation theory that was initiated by N.F. Mott and later developed by D.E. Grady. It was found that the Mott-Grady theory provides reasonably good predictions of the fragment sizes for ductile materials. This conclusion agrees with the recent findings of ours based on the numerical simulations and physical arguments.

Determination of High Strain-Rate Behavior of Metals: Applications to Magnetic Pulse Forming and Electrohydraulic Forming

The design of processes like magnetic pulse forming and electrohydraulic forming involves multiphysical couplings that require numerical simulation, and knowledge on dynamic behaviour of metals. The forming process is completed in about 100 μs, so that the workpiece material deforms at strain-rates between 100 and 10 000 s-1. In this range, the mechanical behaviour can be significantly different than that in quasi-static conditions. It is often noticed that the strength and the formability are higher. The main goal of this study is to use an electromagnetically driven test on tubes or sheets to identify the constitutive behaviour of the workpiece material. In the case of tube, an industrial helix coil is used as inductor. Simulations with the code LS-Dyna® permit to find a configuration where the tube deforms homogeneously enough to allow axisymmetric modelling of the setup. The coil current is measured and used as an input for the simulations. The radial expansion velocity is measured with a Photon Doppler Velocimeter. The parameter identification is lead with the optimization software LS-Opt®. LS-Dyna axisymmetric simulations are launched which different set of parameters for the constitutive behaviour, until the computed expansion velocity fits the experimental velocity. The optimization algorithm couples a gradient method and a global method to avoid local minima. Numerical studies show that for the Johnson-Cook constitutive model, two or three experiments at different energies are required to identify the expected parameters. The method is applied to Al1050 tubes, as received and annealed. The parameters for the Johnson-Cook and Zerilli-Armstrong models are identified. The dynamic constitutive behaviour is compared to that measured on quasi-static tensile tests, and exhibits a strong sensitivity to strain-rate. The final strains are also significantly higher at high velocity, which is one of the major advantages of this kind of processes.

DIFFERENTIAL PROPER-MOTION MEASUREMENTS OF THE CYGNUS EGG NEBULA: THE PRESENCE OF EQUATORIAL OUTFLOWS

We present the results of differential proper-motion analyses of the Egg Nebula (RAFGL 2688, V1610 Cyg) based on the archived two-epoch optical data taken with the Hubble Space Telescope. First, we determined that the polarization characteristics of the Egg Nebula is influenced by the higher optical depth of the central regions of the nebula (i.e., the "dustsphere" of about 1000 AU radius), causing the nebula illuminated in two steps -- the direct starlight is first channeled into bipolar cavities and then scattered off to the rest of the nebula. We then measured the amount of motion of local structures and the signature concentric arcs by determining their relative shifts over the 7.25 yr interval. Based on our analysis, which does not rely on the single-scattering assumption, we concluded that the lobes have been excavated by a linear expansion along the bipolar axis for the past 400 yr, while the concentric arcs have been generated continuously and moving out radially at about 10 km/s for the past 5,500 yr, and there appears to be a colatitudinally-increasing trend in the radial expansion velocity field of the concentric arcs. There exist numerical investigations into the mass-loss modulation by the central binary system, which predict such a colatitudinally-increasing expansion velocity field in the spiral-shock trails of the mass-loss ejecta. Therefore, the Egg Nebula may represent a rare edge-on case of the binary-modulated circumstellar environs, corroborating the previous theoretical predictions.

Development mechanism of cathode surface plasmas of high current pulsed electron beam sources for microwave irradiation generation

AbstractThis paper presents the development mechanism of surface plasmas of carbon-fiber-cathode electron beam source and its effects on the operation of a high-power microwave source, reflex triode vircator powered by about 400 kV, 9 kA, about 350 ns pulsed power accelerator. Based on the current and voltage characteristics of diodes using carbon fiber cathode, the axial expansion velocity is 1.2 cm/μs and the delay time of explosive emission is 2 ns. Further, the comparison of carbon fiber and stainless steel cathodes is made. It was found that the threshold electric field for carbon fiber cathode is about 25 kV/cm, and the delay time of explosive emission and threshold electric field for stainless steel cathode is, respectively, 4.5 ns and 40 kV/cm. The radial expansion velocity of individual emitting centers is estimated to be 1.2 cm/μs, equal to the axial expansion velocity, and this shows the cathode plasma spots spherically expand. In the optimal diode gap for microwave irradiation or at the average current density of 230 A/cm2using carbon fiber cathode, the screening radius was 0.67 cm, the lifetime of cathode emitting centers was about 60 ns, the cathode plasma density was 5 × 1015 cm−3, and the Debye radius of cathode plasma was <3 × 10−5 cm−3. The self-quenching behavior of explosive emission centers occurs, due to the process of cathode surface material release and cooling. The generation and self-quenching of emitting centers, and screening effect of cathode plasmas determine the increase and decrease of cathode emitting area, which is independent of the current density and background pressure. The relation between the lifetime of virtual cathode and background pressure was discussed. It was found, both theoretically and experimentally, that a lower background pressure indicates a longer microwave pulse or a better microwave waveform. It was observed by comparison that the temporary behavior of cathode emitting area is similar to the development process of microwave pulse. The changes of emitting area affects the stability of beam current injected into the virtual cathode region, further leading to the fluctuation of microwave pulse of vircator.

Scattering and absorption of resonant radiation in an expanding sphere

The radiative transfer in a freely expanding gaseous sphere, illuminated by external radiation, has been numerically studied. The absorption coefficient, spectral and spatial characteristics of the absorbed and scattered radiations, and the radial dependences of excited atomic concentration have been determined, and the influence of the optical thickness and the atomic radial expansion velocity on them has been investigated.

Relativistic radial expansion: do we need dark energy?

The Hubble law, for which the radial expansion velocity is linear in the distance r, implies a visible Universe limited to a horizon at r = c/H. No information beyond this horizon can reach us.Here we suggest a modified Hubble law for the measured radial expansion velocity that does not put a limit on the radius of the visible Universe. The modified Hubble law is based on Einstein's well known addition of relativistic velocities, and differs from the (conventional linear) Hubble law in orders of Hr/c that are higher than the first order. The modified Hubble law solves the Olbers's paradox (as the linear Hubble law does), and the existence of dark energy becomes unnecessary for explaining existing astronomical data on dependence of luminosity as a function of redshift for type Ia supernovas.

Study on Expansion Process and Interaction of High Speed Twin Combustion-Gas Jets in Liquid

The multilevel stepped-wall and rectangular observation chambers are designed to study the multipoint ignition process and the combustion stability control mechanism of the bulk-loaded liquid propellant gun. The expansion process and interaction of high-speed twin combustion-gas jets in liquid are studied by means of a high-speed digital camera system. The influence of the nozzle diameter, dual-orifice interval, jet pressure, and chamber structure on the jet expansion shape is discussed. The results indicate that a larger ratio of diameter-to-length can suppress the jet instability in stepped-wall chambers. Higher axial expansion velocity is found under the larger injection pressure, which it increases the instability of jet expansion process. Compared with a rectangular chamber, the axial expansion velocity is smaller, and the radial expansion velocity is larger in stepped-wall chambers under the same conditions. The theoretical studies of interaction of the gas jet with liquid were developed based on the experiment. Two-dimensional unsteady models are used to get the pressure, density, and velocity contours. The numerical simulation results coincide well with the experiment.

Characteristics of flow from an oxy-fuel burner with separated jets: influence of jet injection angle

This paper presents an experimental study of flow development and structure on a separated jet burner in reacting and non-reacting flows. Effects of deflection jets in an aligned configuration of three round jets are emphasized. The idea is based on the confinement of a central jet of fuel by two side jets of oxygen to improve mixing, to control flame stability, and to reduce pollutant emissions. The fields of mean velocity and fluctuation intensity were measured using Particle Image Velocimetry. The deflection of jets has a considerable effect on the dynamic behavior and on the flame characteristics. Results showed that the deflection of jets favors mixing and accelerates merging and combining of jets to a single one. Measurements in reacting flow showed a high influence of combustion on dynamic fields. Compared to non-reactive case, in combustion, larger radial expansion and higher velocity were observed, particularly, above the stabilization point of the flame.

THE SUB-PARSEC SCALE RADIO PROPERTIES OF SOUTHERN STARBURST GALAXIES. II. SUPERNOVA REMNANTS, THE SUPERNOVA RATE, AND THE IONISED MEDIUM IN THE NGC 4945 STARBURST

Wide-field VLBI observations of the nearby starburst galaxy NGC 4945, obtained with the Australian LBA, have produced 2.3 GHz images over two epochs with a maximum spatial resolution of 0.3 pc. 15 sources were detected, 13 of which correspond to sources identified in higher frequency (3 cm and 12 mm) ATCA images. Four of the sources are resolved into shell-like structures ranging between 1.1 to 2.1 pc in diameter. From these data the spectra of 13 compact radio sources in NGC 4945 were modelled; nine were found to be consistent with free-free absorbed power laws and four with a simple power law spectrum. The free-free opacity is highest toward the nucleus but varies significantly throughout the nuclear region, implying that the overall structure of the ionised medium is clumpy. Of the 13 sources, 10 have steep intrinsic spectra associated with synchrotron emission from supernova remnants, the remaining sources have flat intrinsic spectra which may be associated with thermal radio emission. A non-thermal source with a jet-like morphology is detected ~1" from the assumed location of the AGN. A type II supernova (SN) rate upper limit of 15.3/yr is determined for the inner 250 pc region of the galaxy at the 95% confidence level, based on the lack of detection of new sources in observations spanning 1.9 years and a simple model for the evolution of supernova remnants. A type II SN rate of >0.1(v/1e4)/yr is implied from estimates of supernova remnant source counts, sizes and expansion rates, where v is the radial expansion velocity of the supernova remnant in km/s. A SFR of 2.4(v/1e4)<SFR(M>=5Msun)<370 Msun/yr has been estimated directly from the SN rate limits and is of the same order of magnitude as rates determined from integrated FIR and radio luminosities.

Velocity of Radial Expansion of Contrast-enhancing Gliomas and the Effectiveness of Radiotherapy in Individual Patients: a Proof of Principle

Aims The initial aims were to use recently available observations of glioblastomas (as part of a previous study) that had been imaged twice without intervening treatment before receiving radiotherapy in order to obtain quantitative measures of glioma growth and invasion according to a new bio-mathematical model. The results were so interesting as to raise the question whether the degree of radio-sensitivity of each tumour could be estimated by comparing the model-predicted and actual durations of survival and total numbers of glioma cells after radiotherapy. Materials and methods The gadolinium-enhanced T1-weighted and T2-weighted magnetic resonance imaging volumes were segmented and used to calculate the velocity of radial expansion ( v) and the net rates of proliferation (ρ) and invasion/dispersal ( D) for each patient according to the bio-mathematical model. Results The ranges of the values of v, D and ρ show that glioblastomas, although clustering at the high end of rates, vary widely one from the other. The effects of X-ray therapy varied from patient to patient. About half survived as predicted without treatment, indicating radio-resistance of these tumours. The other half survived up to about twice as long as predicted without treatment and could have had a corresponding loss of glioma cells, indicating some degree of radio-sensitivity. These results approach the historical estimates that radiotherapy can double survival of the average patient with a glioblastoma. Conclusions These cases are among the first for which values of v, D and ρ have been calculated for glioblastomas. The results constitute a ‘proof of principle’ by combining our bio-mathematical model for glioma growth and invasion with pre-treatment imaging observations to provide a new tool showing that individual glioblastomas may be identified as having been radio-resistant or radio-sensitive.

Cell kinetics underlying grading of gliomas

The development of a bio‐mathematical model of the dynamics of glioma growth (Tracqui et al 1995, Swanson 1999, Murray 2003) based on the net rates of proliferation (ρ) and dispersal (D) of glioma cells led to the discovery of the inevitability of “progression” of low‐grade gliomas to death (Pallud et al 2006) and provided a basis for predicting the development of contrast‐enhancement and the duration of survival given the size and velocity of expansion (Alvord &amp; Swanson 2007). The addition of hypoxia‐driven neo‐angiogenesis and necrosis provides a dynamic picture of the continuum of changes in cell concentration, mitoses, vascular proliferation and necrosis, criteria commonly used in grading gliomas. We have applied these criteria to random high‐power fields of each of 15 D's x 15 ρ's = 225 ρ and D combinations (virtual gliomas) and generated a map of probabilities of a particular grade being associated with each combination of D and ; at each point in time/size. The grade progressively changes with time and size, consistent with about 30% of grade II gliomas “progressing” to grade III and essentially all of these “progressing” to grade IV. At the moment measurement of the velocity of radial expansion and calculation of D and ρ requires two MRI observations without intervening treatment, but the accuracy of the prognosis for the individual seems clear.

Using mathematical modeling to predict survival of low‐grade gliomas

Pallud and colleagues have presented some interesting data concerning the long-term follow-up of low-grade gliomas, but why did they not pursue the implications of the mathematical model of glioma behavior that they championed in their previous work. If they had, they would have seen that there can be no dichotomy, no distribution of velocities that “results in two groups,” but rather a continuous gradation of velocities of glioma expansion contributing to survival differences. The two independent variables that Pallud and colleagues found to be statistically significant (p 0.034), velocity and size, are exactly what the mathematical model predicts for survival. Actually, these two variables combine seamlessly to produce a predicted survival for each patient; but in the available space and without access to the original data, we can illustrate only a sampling of predictions from the data provided in the manuscript (Table). For example, a tumor 80 mm in diameter would require only 2.5 years to grow enough to kill if expanding at 11 mm/year but would not yet have reached a fatal size within 11 years if only expanding at 2–3 mm/year. A few questions require clarification. First, if “patients were excluded when an oncological treatment was administered,... or when anaplastic transformation occurred...histologically... or when a contrast enhancement appeared on MRI,” who was left to die? Were all 143 patients “pure” low-grade gliomas until they died or were last seen alive? Second, what was “time zero” in this study? That is, what is the relation between the time for “the median tumor volume (estimated on MRI before surgery)” and the time to the beginning of “the overall median duration of clinical follow-up since radiological diagnosis”? Were there not magnetic resonance images on some patients being followed before any surgery (and after a radiological diagnosis), and were these excluded? Third, rather than a p value of 0.619 for the “random histological review...in 40 cases (28%),” would it not have been more interesting to review and compare all 22 cases in the “high-rate” group (ie, velocity 8mm/yr) with the 16 cases in the lowest of the “low-rate” group (ie, velocity 1mm/yr)? Can not neuropathology do better than to lump velocities of greater than 10-fold differences together? Finally, we offer a suggestion: We prefer the phrase “velocity of radial expansion” (which is more euphonious and would be half that of “diametric expansion”) to “growth rate” because the latter ambiguously implies proliferation, perhaps forgetting the cell loss factor, perhaps even including diffusion, whereas the mathematical model clearly separates net proliferation ( ) and net dispersal or diffusion (D), the product of which relate to the velocity.