High Propagation Velocity Research Articles

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.

Preparation and characterization of energetic Al-Mg mechanical alloy powders

Metals such as Al and Mg have high combustion enthalpies and they are widely used as additives in energetic materials for propellants, explosives, and pyrotechnics. However, long ignition delays and slow combustion kinetics limit their current applications. An approach suggested in this work is to design new metal-based materials in which pre-determined phase changes will occur and trigger ignition at a desired temperature and also accelerate the rate of heat release during combustion. As a first step, metastable solid solutions of Mg in Al (10–50% of Mg) have been produced by mechanical alloying. The ignition temperatures of the produced alloys in air were determined using digital imaging and three-color pyrometry of the electrically heated filaments coated with different alloy powders. Combustion of mechanical alloys in air was studied using a laminar, premixed flame aerosol burner. The ignition temperatures were around 1,000 K, much lower than the pure aluminum ignition temperature of about 2,300 K. The steady flames of mechanical alloy powders were produced at lower equivalence ratios and had higher propagation velocities than similar pure aluminum powder flames. Phase compositions of the combustion products were determined using X-ray diffraction. In addition to Al 2O 3 and MgO, significant amounts of Al 2MgO 4 were found in experiments.

Nanotechnology for SAW devices on AlN epilayers

Hexagonal aluminum nitride (AlN) has revealed as an excellent candidate to be used as substrate for surface acoustic wave (SAW) devices. Its high SAW propagation velocity, electromechanical coupling constant and an almost negligible temperature coefficient of delay (TCD) promise the fabrication of low-cost SAW devices operating in the microwave region of the spectrum, useful under harmful environment conditions of temperature and radiation. In this paper, a revision of the excellent SAW-related characteristics of AlN grown by molecular beam epitaxy on sapphire is presented. Several technological issues related to the fabrication of high frequency (>1 GHz) SAW devices in this semiconductor using an e-beam lithographic system are discussed. On the other hand, design issues like the effect introduced by the generation of bulk acoustic waves (BAW) will be analyzed using a novel method based on the inverse Fourier transform. These approaches will be presented both from theoretical and experimental perspectives.

Analytical and experimental insights into fast deflagrations, detonations, and the deflagration-to-detonation transition process

The present paper reports on experimentally as well as analytically obtained results of fast propagating deflagrations, the deflagration-to-detonation transition process, and detonations. Due to the high propagation velocity of these highly transient combustion waves, experimental data were mainly achieved by means of conventional measuring devices in past. These data were used to develop analytical models describing the physical processes of these fast combustion waves. Due to the progress in the development of new laser-optical measuring-techniques, detailed insight into these combustion waves can nowadays be achieved. These measuring-techniques facilitate the validation of analytical models and, furthermore, it is now possible to understand the physical processes of these fast combustion waves. This paper describes the analytical models proposed by Mayinger et al. and new experimental results obtained by means of the color-schlieren technique and the laser-induced predissociation fluorescence.

Pulsed breakdown at high overvoltage: development, propagation andenergy branching

This paper presents a review of investigations of detailed spatialand temporal structures of high-voltage pulsed nanosecond discharges in theform of fast ionization waves. The most distinctive features of this type ofdischarge are a high propagation velocity (109-1010 cm s-1),good reproducibility of the discharge parameters at a moderate (tens of hertz)repetition rate and spatial homogeneity over a large gas volume. The dischargewas initiated by voltage pulses of negative polarity with an amplitude of10-15 kV, a duration at half maximum of 25 ns and a rise time of the front of3-5 ns. The behaviour of the electric field and electron andexcited-stateconcentrations were analysed on the basis of experimental data within theframe of the unified kinetic approach. It was found that the longitudinalcomponent of the electric field has a sharp (2-3 ns) maximum and that theelectrons and excited particles are produced preferentially behind the frontin relatively weak electric fields. The peak field value was close to or evenstronger than the threshold for the generation of runaway electrons in asteady-state uniform electric field. An analysis based on absolutetime-resolved measurements of the spectrum of two molecular bands showed that,behind the breakdown front, the EEDF should be substantially overpopulatedwith high-energy electrons. Energy branching in the discharge was analysed.Possibilities of application of the fast ionization wave as a source of auniform pulsed plasma were suggested and justified.

The Chronaxie and Propagation Velocity of Canine Cervical Vagus Nerve Fibers In Vivo

A recent medical development is the stimulation of the left afferent vagus nerve to control seizures in man. It is well known that vagus nerves innervate the heart; however, when afferent fibers are stimulated so are efferent fibers. Basic to the understanding of afferent fiber stimulation is the fundamental excitability properties of the vagus nerve. In this study, we measured chronaxie, rheobase and propagation velocity in the intact dog cervical afferent vagus nerve. Five adult mongrel dogs of both sexes, weighing 22–29 kg, were sedated with thiopental sodium, intubated, and connected to an anesthesia machine delivering isoflurane and oxygen. The ventral side of the neck was dissected and two bipolar electrodes were then placed on the vagus nerve, as far from one another as possible, and data were collected in the form of the strength–duration curve using single rectangular current pulses. Two different fiber types, which we designated A and B, were identified electrophysiologically. The average values for chronaxie were 75.4 ± 24.5 μs for A fibers and 82.3 ± 23.3 μs for B fibers. Values for rheobase were 0.63 ± 0.18 mA for A fibers and 0.66 ± 0.22 mA for B fibers. Propagation velocities were 59.0 ± 9.6 m/s for A fibers and 43.4 ± 8.0 m/s for B fibers. With an effective electrode area of 5 mm2, this yields current densities of 13.0 mA/cm2 and 12.9 mA/cm2 for A and B fibers respectively. We also identified fibers in the right vagus, which we called BH, with propagation velocity of 5.75 ± 0.35 m/s, which appears to be responsible for cardiac slowing. Fibers with a high propagation velocity appeared to have a short chronaxie and fibers with a low propagation velocity appeared to have a long chronaxie; however, no definitive relationship between propagation velocity and chronaxie was found.

Lithium niobate thin films on diamond/silicon substrates for surface acoustic wave filter device applications

Piezoelectricity and surface acoustic wave (SAW) propagation of a LiNbO3/diamond/silicon multilayer structure are reported for the first time. Theoretical calculations predict lithium niobate thin films on diamond/silicon substrates have both high SAW propagation velocities and high electromechanical coupling coefficients. The metal-organic decomposition (MOD) technique is used to grow lithium niobate thin films on diamond/silicon substrates. X-ray diffraction analysis shows that the lithium niobate films are polycrystalline. SAW devices are fabricated by depositing interdigital transducers (IDTs) onto the LiNbO3/diamond/silicon films. The SAW activities are observed by measuring the frequency response of the SAW device.

Current and electromagnetic field associated with lightning-return strokes to tall towers

An analysis of electric and magnetic fields radiated by lightning first and subsequent return strokes to tall towers is presented. The contributions of the various components of the fields, namely, static, induction, and radiation for the electric field, and induction and radiation for the magnetic field are illustrated and discussed. It is shown in particular that the presence of a tower tends, in general, to increase substantially the electric and magnetic field peaks and their derivatives. This increase is mainly caused by the presence of two oppositely propagating current wavefronts originating from the tower top and by the very high propagation velocity of current pulses within the tower, and depends essentially on the wavefront steepness of the channel-base current. Because of the last factor, the increase of the field magnitudes is found to be significantly higher for subsequent return strokes, which are characterized by much faster risetimes compared to first return strokes. The presented results are consistent with experimental observations of current in lightning strokes to the Toronto CN Tower and of the associated electric and magnetic fields measured 2 km away. These findings partially explain the fact that subsequent return strokes characterized by lower current peaks but higher front steepnesses and return stroke speeds may result in higher field peaks. The results obtained have important implications in electromagnetic (EM) compatibility. It is found that lightning strokes to tall metallic objects lead to increased EM field disturbances. Also, subsequent return strokes are to be considered an even more important source of EM interference than first return strokes. Indeed, EM fields from subsequent strokes are characterized by faster fronts and additionally, they may reach greater peaks than first strokes. Lastly, findings of this study emphasize the difficulty of extracting reliable lightning return stroke current information from remote EM field measurements using oversimplified formulae.

High-Performance Surface Transverse Wave Resonators in the Lower GHz Frequency Range

Since the first successful surface transverse wave (STW) resonator was demonstrated by Bagwell and Bray in 1987, STW resonant devices on temperature stable cut orientations of piezoelectric quartz have enjoyed a spectacular development. The tremendous interest in these devices is based on the fact that, compared to the widely used surface acoustic waves (SAW), the STW acoustic mode features some unique properties which makes it very attractive for low-noise microwave oscillator applications in the 1.0 to 3.0 GHz frequency range in which SAW based or dielectric resonator oscillators (DRO) fail to provide satisfactory performance. These STW properties include: high propagation velocity, material Q-values exceeding three times those of SAW and bulk acoustic waves (BAW) on quartz, low propagation loss, unprecedented 1/f device phase noise, extremely high power handling ability, as well as low aging and low vibration sensitivity. This paper reviews the fundamentals of STW propagation in resonant geometries on rotated Y-cuts of quartz and highlights important design aspects necessary for achieving desired STW resonator performance. Different designs of high- and low-Q, low-loss resonant devices and coupled resonator filters (CRF) in the 1.0 to 2.5 GHz range are characterized and discussed. Design details and data on state-of-the-art STW based fixed frequency and voltage controlled oscillators (VCO) with low phase noise and high power efficiency are presented. Finally, several applications of STW devices in GHz range data transmitters, receivers and sensors are described and discussed.

HIGH-PERFORMANCE SURFACE TRANSVERSE WAVE RESONATORS IN THE LOWER GHz FREQUENCY RANGE

Since the first successful surface transverse wave (STW) resonator was demonstrated by Bagwell and Bray in 1987, STW resonant devices on temperature stable cut orientations of piezoelectric quartz have enjoyed a spectacular development. The tremendous interest in these devices is based on the fact that, compared to the widely used surface acoustic waves (SAW), the STW acoustic mode features some unique properties which makes it very attractive for low-noise microwave oscillator applications in the 1.0 to 3.0 GHz frequency range in which SAW based or dielectric resonator oscillators (DRO) fail to provide satisfactory performance. These STW properties include: high propagation velocity, material Q-values exceeding three times those of SAW and bulk acoustic waves (BAW) on quartz, low propagation loss, unprecedented 1/f device phase noise, extremely high power handling ability, as well as low aging and low vibration sensitivity. This paper reviews the fundamentals of STW propagation in resonant geometries on rotated Y-cuts of quartz and highlights important design aspects necessary for achieving desired STW resonator performance. Different designs of high- and low-Q, low-loss resonant devices and coupled resonator filters (CRF) in the 1.0 to 2.5 GHz range are characterized and discussed. Design details and data on state-of-the-art STW based fixed frequency and voltage controlled oscillators (VCO) with low phase noise and high power efficiency are presented. Finally, several applications of STW devices in GHz range data transmitters, receivers and sensors are described and discussed.

Effect of structure doping profile on the current switching-off process in power semiconductor opening switches

Using a physicomathematical model, the process of current breaking in power semiconductor opening switches was investigated in p +-p-n-n + structures with different doping profiles. The model takes account of the actual doping profile of a structure, diffusion and drift of current carriers in a strong electric field, recombination via deep impurities and Auger recombination, and impact ionization in a dense plasma. The calculation of the electrical circuit of an opening switch is based on solution of Kirchhoff’s equations. It has been shown that in the nanosecond regime of breaking superhigh current densities with densities of the interrupted currents from a few to tens of kA/cm2, the dominant factor in the current breaking process is the width of the p-region in the initial doping profile of a structure. An increase in the p-region width from 100 to 200 µm makes the velocity of the excess plasma front propagating in the p-region in the reverse pumping stage higher by a factor of 5–7. Higher propagation velocity of the plasma front makes the current breaking process more intensive, which is manifested in the shorter current breaking time and higher overvoltage across the opening switch.

High Velocity Acoustical Head Wave on Surface of ST Quartz

According to surface transient excitation theory of elastic wave for piezoelectric crystal, in the surface of a piezoelectric crystal can exist acoustical high velocity head wave except the general surface acoustic wave (SAW). Such a mode is to propagate with the group velocity of the longitudinal bulk wave along the crystal surface. For ST quartz its value is 5.744×103 m/s, which agrees with our experiments. The fact that the longitudinal head wave mode has a high propagation velocity offers potential applications for high frequency SAW devices.

Review article: colonic sensorimotor physiology in health, and its alteration in constipation and diarrhoeal disorders.

To review the physiology of colonic motility and sensation in healthy humans and the pathophysiological changes associated with constipation and diarrhoea. Medline Search from 1965 using the index terms: human, colonic motility, sensation, pharmacology, neurohormonal control, gastrointestinal transit, constipation, diarrhoea and combinations of these. In health, the ascending and transverse regions of colon function as reservoirs to accommodate ileal chyme and the descending colon acts as a conduit; the neuromuscular functions and transmitters control colonic motility and sensation and play pivotal roles in disorders associated with constipation and/or diarrhoea. Disorders of proximal colonic transit contribute to symptoms in idiopathic constipation, diarrhoea-predominant irritable bowel syndrome and carcinoid diarrhoea. Colonic function in patients presenting with constipation is best assessed clinically by colonic transit time using radiopaque markers ingested orally. Measurements of colonic contractility are less useful clinically but they can help identify motor abnormalities including colonic inertia; in some patients with irritable bowel syndrome, abdominal pain, urgency and diarrhoea are temporally associated with high amplitude contractions, which originate in the proximal colon and traverse the distal conduit at very high propagation velocities. Visceral hypersensitivity contributes to the urgency and tenesmus in irritable bowel syndrome and inflammatory bowel disease. Colonic motility and sensation can be reduced by anticholinergic agents, somatostatin analogues and 5HT3 antagonists. Physiological and pharmacological studies of the human colon have provided new insights into the pathophysiology of colonic disorders, and offer possibilities of novel therapeutic approaches for constipation or diarrhoea associated with colonic motor or sensory dysfunction.

Nonlinear propagation of dust-acoustic waves in a magnetized dusty plasma with vortex-like ion distribution

A theoretical investigation has been made of the nonlinear propagation of dust-acoustic waves in a magnetized three-component dusty plasma consisting of a negatively charged dust fluid, free electrons and vortex-like distributed ions. It is found that, owing to the departure from the Boltzmann ion distribution to a vortex-like one, the dynamics of small- but finite-amplitude dust-acoustic waves in a magnetized dusty plasma is governed by the modified Korteweg–de Vries equation. The latter admits a stationary dust-acoustic solitary-wave solution that has larger amplitude, smaller width and higher propagation velocity than that involving adiabatic ions. The effects of external magnetic field, trapped ions and free electrons on the properties of these dust-acoustic solitary waves are briefly discussed.

Obliquely propagating electrostatic solitary structures in a magnetized hot dusty plasma with trapped ions

A theoretical investigation has been made of obliquely propagating electrostatic solitary structures in a hot magnetized three-component dusty plasma, which consists of a negatively charged hot dust fluid, Blotzmann distributed electrons, and free and trapped ions. It has been found that owing to the departure from the Boltzmann ion distribution to a trapped ion one, the dynamics of small but finite-amplitude electrostatic waves in a magnetized hot dusty plasma is governed by the modified Korteweg–de Vries (KdV) equation. The latter admits a stationary solitary wave solution that has a larger amplitude, a smaller width, and higher propagation velocity, than that involving Boltzmann distributed ions. It has been shown that the effects of obliqueness, external magnetic field, trapped ions, and free electrons change the behavior of these electrostatic solitary structures that have been found to exist with only negative potential in this dusty plasma model.

Origin, propagation and attenuation of pressure waves in gas—solid fluidized beds

Recently reported results on the origin, propagation and attenuation of pressure waves in bubbling gas—solid fluidized beds are re-evaluated and the results are compared with additional experiments reported here. It is found that the measured pressure fluctuations are a result of slow and fast propagating pressure waves. Pressure waves with high propagation velocities (> 10 m/s) are unambiguously identified as compression waves, which move upwards and downwards. Upward moving compression waves originate from gas bubble formation and gas bubble coalescence. The amplitude of upward moving pressure waves is linearly dependent on the distance to the bed surface. Downward moving compression waves are caused by gas bubble eruptions at the fluidized bed surface, bubble coalescence and by changes in bed voidage. In this case the pressure wave amplitude is independent of the distance to the bed surface. Pressure waves with propagation velocities of less than 2 m/s are caused by rising gas bubbles. These pressure waves move upwards only, with an amplitude proportional to the bubble size. The average wave propagation velocity measured in a freely bubbling bed is lower than that predicted from the pseudo-homogeneous compressible wave theory owing to the presence of slowly rising gas bubbles. The average propagation velocity of pressure waves in a gas—solid circulating fluidized bed is adequately described as a function of local voidage by pseudo-homogeneous compressible wave theory. At low voidages in the bottom of the riser, the propagation velocity is lowered by the presence of gas bubbles or large gas voids.

Nonlinear effects in dynamic loading of a material with defect zones

An analysis is made of some characteristic features of the interaction between solitary pulses (soliton-like waves) generated under high-velocity loading and structural defects in an Al crystallite. Calculations are made using the molecular dynamics method. It is shown that in materials undergoing shear loading and those undergoing compression different types of solitary pulses are formed which interact differently with regions containing structural defects. The waves formed under compression have a higher propagation velocity in the material, and their profile recovers fairly rapidly after propagating through defect zones. These phenomena are of interest for developing methods of nondestructive testing.

Solitary Waves in a Three-component Dusty Plasma withTrapped Ions

A theoretical investigation has been made of dust-acoustic solitarywaves in an unmagnetized three-component dusty plasma which consistsof a negatively charged dust fluid, free electrons and trapped as wellas free ions. It is found that owing to the departure from theBoltzmann ion distribution to a trapped ion one, the dynamics of smallbut finite amplitude dust-acoustic waves is governed by a modifiedKorteweg-de Vries equation. The latter admits a stationarydust-acoustic solitary wave solution which has larger amplitude,smaller width, and higher propagation velocity, than that involvingadiabatic ions. The effects of trapped ions and free electrons onthese dust-acoustic solitary waves are discussed. The presentinvestigation should be useful in understanding some nonlinearfeatures of the dust-acoustic waves which have been observed in arecent numerical simulation study.

Nonlinear propagation of ion-acoustic waves in a hot magnetized plasma with vortexlike electron distribution

Effects of external magnetic field, ion temperature, and vortexlike electron distribution are incorporated in the study of nonlinear ion-acoustic waves in a hot magnetized plasma that consists of a positively charged, hot ion fluid and trapped, as well as, free electrons. It is found that, owing to the departure from the Boltzmann electron distribution to a vortexlike one, the dynamics of small but finite amplitude ion-acoustic waves is governed by a nonlinear equation of K-dV (Korteweg-de Vries) type. The latter admits a stationary ion-acoustic solitary wave solution, which has larger amplitude, smaller width, and higher propagation velocity, than that involving isothermal electrons. The effects of external magnetic field and ion temperature on the properties of these ion-acoustic solitary structures are also discussed.

Supercritical Vapor Explosions: Comparisons Between Thermodynamic and Mechanistic Models

The thermal interaction of certain molten materials such as Al2O3 with water results in vapor explosions with very high (supercritical) pressures and propagation velocities. A quasi-steady-state analysis and a transient analysis of a supercritical vapor explosion in one-dimensional multiphase flow were applied to analyze experimental data of an Al2O3-water fuel/coolant interaction obtained from the KROTOS 26, 28, 29, and 30 tests. The shock adiabatic thermodynamic model and the TEXAS mechanistic model were used to perform this analysis. The predicted results of the initial vapor void fraction and explosion conversion ratio from both models, together with the estimated experimental results, for the KROTOS 26 test were compared.