用光学记录速度干涉仪测量自由面速度

 建立了一套光学记录速度干涉仪系统(ORVIS),用于测量强激光产生的冲击波状态方程中的自由面速度。该光学记录速度干涉仪系统的时间分辨率为179 ps,可以测量自由面速度随时间变化的整个过程。在天光KrF高功率准分子激光装置上进行激光打靶实验,激光波长248.4 nm,脉冲宽度25 ns,最大输出能量158 J。在激光功率密度为6.24×10¹¹W·cm^-2的条件下,测得厚20 μm铁膜的自由面速度可达3.86 km/s;在激光功率密度为7.28×10¹¹W·cm^-2条件下,100 μm铝膜（靶前有100 μm的CH膜作为烧蚀层）的自由面速度可以达到2.87 km/s。     

Group velocity of solitons

It is shown that, in addition to the well-known phase accumulation of a traveling soliton, which may be interpreted as a change of phase velocity as a result of the Kerr nonlinearity, there is a change in the speed of travel of the envelope, the group velocity. This analysis is extended to dispersion-managed solitons, for which it is shown that the discrepancy between phase- and group-velocity changes is generally smaller.     

The stability analysis of the full velocity and acceleration velocity model

The stability analysis is one of the important problems in the traffic flow theory, since the congestion phenomena can be regarded as the instability and the phase transition of a dynamical system. Theoretically, we analyze the stable conditions of the full velocity and acceleration difference model (FVADM), which is proposed by introducing the acceleration difference term based on the previous car-following models (the optimal velocity model and the full velocity difference model, OVM and FVDM). By numerical simulations, it is found that when the traffic flow is unstable, the traffic jam in the FVADM is weaker than that in the FVDM. Also it is observed that the spreading speed of the jam is slower in the FVADM than that in the FVDM and the fluctuations of vehicles in the FVADM are smaller than those in the FVDM. Therefore, the acceleration difference term has strong effects on traffic dynamics and plays an important role in stabilizing the traffic flow.     

超声波在液体中相速与群速的测量

简要叙述了相速与群速的定义,并用实验测量了超声波在不同液体中的相速与群速.     

The velocity of gravitational waves

We examine the propagation of gravitational waves in the new field theory of gravitation recently proposed by Novello-De Lorenci-Luciane (NDL). This examination is done on a solvable case corresponding to a spherically symmetric static configuration. We show that in NDL theory the velocity of gravitational waves is lower than light velocity. We point out some consequences of this result and suggest a possible scenario for its verification.     

Direct measurement of superluminal group velocity and signal velocity in an optical fiber

We present an easy way of observing superluminal group velocities using a birefringent optical fiber and other standard devices. In the theoretical analysis, we show that the optical properties of the setup can be described using the notion of "weak value." The experiment shows that the group velocity can indeed exceed c in the fiber; and we report the first direct observation of the so-called "signal velocity," the speed at which information propagates and that cannot exceed c.     

Self-gated Fourier velocity encoding

Self-gating is investigated to improve the velocity resolution of real-time Fourier velocity encoding measurements in the absence of a reliable electrocardiogram waveform (e.g., fetal magnetic resonance or severe arrhythmia). Real-time flow data are acquired using interleaved k-space trajectories which share a common path near the origin of k-space. These common data provide a rapid self-gating signal that can be used to combine the interleaved data. The combined interleaves cover a greater area of k-space than a single real-time acquisition, thereby providing higher velocity resolution for a given aliasing velocity and temporal resolution. For example, this approach provided velocity spectra with a temporal resolution of 19 ms and velocity resolution of 22 cm/s over an 818 cm/s field-of-view. The method was validated experimentally using a computer-controlled pulsatile flow apparatus and applied in vivo to measure aortic-valve flow in a healthy volunteer.     

Energy and phase velocity considerations required for attenuation and velocity measurements of anisotropic composites

Viscoelastic fibre-reinforced composite materials have a number of possible advantages for use in underwater acoustic applications. In order to exploit these materials it is important to be able to measure their complex stiffness matrix in order to determine their acoustic response. Ultrasonic transmission measurements on parallel-sided samples, employing broadband pulsed transducers at 2.25 MHz and an immersion method, have been used to determine the viscoelastic properties of a glass-reinforced composite with uniaxially aligned fibres. The composite measured was constructed from Cytecfiberite's CYCOM 919 E-glass. The theory of acoustic propagation in anisotropic materials shows that the direction of energy propagation is, in general, different from that given by Snell's Law. At 15 degrees incidence, Snell's Law implies a refracted angle of 40 +/- 2 degrees, whereas the energy direction is observed to be 70 +/- 2 degrees. Despite this, the experimental data indicates that the position of the receiving transducer has relatively little effect on the apparent phase velocity measured. The phase velocities measured at positions determined from the refracted angle and energy direction are 3647 and 3652 +/- 50 m s(-1), respectively. However, the amplitude of the received signal, and hence estimate of attenuation, is highly sensitive to the receiver position. This indicates that the acoustic Poynting vector must be considered in order to precisely determine the correct position of the receiving transducer for attenuation measurements. The beam displacement for a 17.6 mm sample at 15 degrees incidence is 9.5 and 40 mm by Snell's Law and Poynting's Theorem, respectively. Measured beam displacements have been compared with predictions derived from material stiffness coefficients. These considerations are important in recovering the complex stiffness matrix.     

Solar-wind velocity decreases

Summary Considering the solar-wind plasma as fully ionized, electrically neutral and radially expanding spherically symmetric, we solve the equations of mass and energy conservation. The plasma under consideration is assumed as one-fluid and its parameters are estimated according to the measured data at 1 AU during the period of the observed very low temperatures. In this way, we could obtain an analytical relation between the temperature and the velocity of the solar-wind plasma. Solar-wind velocity and temperature observations, made aboard the Vela 5, 6 and IMP 6 satellites during the period August 1969–May 1974, suggest that on the average a solar-wind velocity decrease correlates with a proton and electron temperature depression. This depression could possibly originate from the expansion of the solar wind between a fast-moving plasma stream and a following slower one. The above correlation of data, which is also in accordance with the conservation of energy in the solar wind, is compared with the analytically derived relation. It seems, at least within the limits of errors, that the theoretical relation fits the experimental observations. Consequently, we could propose that the observed very low temperatures in the solar wind may be due to the fact that the temperature decrease of the expanding plasma exceeds the temperature increase due to the heat conduction of the electrons from the hot corona to the plasma along the open interplanetary-magnetic-field lines, so that on the whole the plasma is cold.

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Riassunto Considerando il plasma del vento solare completamente ionizzato, elettricamente neutro e in espansione radiale a simmetria sferica, si risolvono le equazioni di conservazione di massa ed energia. Si considera il plasma in questione come un solo fluido e i suoi parametri sono valutati in base ai dati misurati ad 1 AU durante il periodo delle temperature molto basse osservate. In questo modo si potrebbe ottenere una relazione analitica tra la temperatura e la velocità del plasma del vento solare. Le osservazioni sulla velocità e sulla temperatura del vento solare, fatte a bordo dei satelliti Vela 5, 6 e IMP 6 durante il periodo agosto 1969–maggio 1974, suggeriscono che in media una diminuzione della velocità del vento solare è connessa con la depressione della temperatura protonica e elettronica. Questa depressione potrebbe trarre origine dall'espandersi del vento solare tra una corrente di plasma che si muove velocemente e una successiva più lenta. Si confronta la suddetta correlazione tra i dati, che è anche in accordo con la conservazione di energia nel vento solare, con la relazione dedotto analiticamente. Sembra, almeno entro i limiti dell'errore, che la relazione teorica sia in accordo con le osservazioni sperimentali. Di conseguenza, si potrebbe proporre che le temperature molto basse osservate nel vento solare possano essere dovute al fatto che la diminuzione di temperatura del plasma in espansione supera l'aumento di temperatura dovuto alla conduzione di calore degli elettroni dalla corona caldissima al plasma lungo le linee aperte del campo magnetico interplanetario, cosicché nel complesso il plasma è freddo.

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Резюме Рассматривая плазму солнечного ветра, как полностью ионизованную, электрически нейтральную и расширяюшуюся по радиусу сферически симметричным образом, мы решаем уравнения сохранения массы и энергии. Предполагается, что рассматриваемая плазма является одножидкостной и параметры плазмы оцениваются из измеренных данных при 1 AU в течение периода наблюденных очень низких температур. Таким образом, мы можем получить аналитическое соотношение между температурой и скоростью плазмы солнечиого ветра. Определения температуры и скорости солнечного ветра, проведенные на борту космических аппаратов VELA 5,6 и IMP 6 в течение августа 1969 г.—мая 1974 г., предполагают, что, в среднем, уменьшение скорости солнечного ветра коррелирует с уменьшением температуры протонов и электронов. Указанное уменьшение температуры, по-видимому, возникает из-за разделения солнечного ветра на поток быстродвижущейси плазмы и поток медленнодвижущейся плазмы. Выэеуказанные данные по корреляции, которые согласуются с сохранением энергии в солнечном ветре, сравниваются с аналитически выведенным соотноэением. По крайней мере, в пределах погреэностей теоретическое соотноэение соответствует экспериментальным результатам. Таким образом, мы можем предположитя, что наблюденные очень низкие температуры в солнечном ветре могут возникать из-за того, что уменьшение температуры расширяюшейся плазмы превышает увеличение температуяы, обусловленное теплопроводностью электронов из горячей короны в плазму вдоль незамкнутых линий межпланетного магнитного поля, так что в целом плазма является холодной.

     

Noninvariant one-way velocity of light

After discussing in the first five sections the meaning and the difficulties of the principle of relativity we present a new sel of spacetime transformations between inertial systems (inertial transformations), based on three assumptions: (1) The two-way velocity of light is c in all inertial systems and in all directions; (2) Time dilation effects take place with the usual relativistic factor; (3) Clocks are synchronized in the way chosen by nature itself, e.g., in the Sagnac effect. We show that our new transformation laws can explain the available experimental evidence in spite of the implied noninvariance of the one-way velocity of light.     

On the velocity tensors

A new object, called the velocity tensor, is introduced. It allows to formulate a generally covariant mechanics. Some properties of the velocity tensor are derived. The text was submitted by the authors in English.     

Acoustic particle velocity horns

The paper considers receiving acoustic horns designed for particle velocity amplification and suitable for use in vector sensing applications. Unlike conventional horns, designed for acoustic pressure amplification, acoustic velocity horns (AVHs) deliver significant velocity amplification even when the overall size of the horn is much less than an acoustic wavelength. An AVH requires an open-ended configuration, as compared to pressure horns which are terminated at the throat. The appropriate formulation, based on Webster's one-dimensional horn equation, is derived and analyzed for single conical and exponential horns as well as for double-horn configurations. Predicted horn amplification factors (ratio of mouth-to-throat radii) were verified using numerical modeling. It is shown that three independent geometrical parameters principally control a horn's performance: length l, throat radius R(1), and flare rate. Below a predicted resonance region, velocity amplification is practically independent of frequency. Acoustic velocity horns are naturally directional, providing maximum velocity amplification along the boresight.     

Group velocity, phase velocity, and dispersion in human calcaneus in vivo

Commercial bone sonometers measure broadband ultrasonic attenuation and/or speed of sound (SOS) in order to assess bone status. Phase velocity, which is usually measured in frequency domain, is a fundamental material property of bone that is related to SOS, which is usually measured in time domain. Four previous in vitro studies indicate that phase velocity in human cancellous bone decreases with frequency (i.e., negative dispersion). In order to investigate frequency-dependent phase velocity in vivo, through-transmission measurements were performed in 73 women using a GE Lunar Achilles Insight commercial bone sonometer. Average phase velocity at 500 kHz was 1489 +/- 55 m/s (mean +/- standard deviation). Average dispersion rate was -59 +/- 52 m/sMHz. Group velocity was usually lower than phase velocity, as is expected for negatively dispersive media. Using a stratified model to represent cancellous bone, the reductions in phase velocity and dispersion rate in vivo as opposed to in vitro can be explained by (1) the presence of marrow instead of water as a fluid filler, and (2) the decreased porosity of bones of living (compared with deceased) subjects.     

Electron velocity in superlattices

Calculations of the electron velocity in superlattices based on the miniband dispersion relation, and the velocity defined through the tunneling time are discussed. The former definition is based on the intrinsically infinite modified Kronig-Penney model, while the latter rests upon the transfer matrix method and takes the finiteness of the superlattice into account. The main result is that the velocities differ: for weakly coupled structures where the tunneling time can be defined through the linewidth, the transfer matrix method predicts a smaller velocity than the modified Kronig-Penney model.Received: 7 August 2003, Published online: 24 October 2003PACS: 73.21.Cd Superlattices - 73.20.At Surface states, band structure, electron density of states - 73.40.Gk Tunneling - 03.65.Xp Tunneling, traversal time, quantum Zeno dynamics     

Shock-wave velocity of a femtosecond-laser-produced plasma

Fluorescence measurements have been used to characterize the velocity of atoms in a femtosecond-laser-produced plasma. Nanogram amounts of a copper sample were ablated by the focused radiation (λ=775 nm) of an all-solid-state laser. The laser was operated at a pulse rate of 10 Hz with an energy of 200μJ per pulse. The microplasma expanded into a defined argon atmosphere of pressures between 0.02 and 850 mbar. Atomic fluorescence was excited in the laser plume by a dye-laser pulse with the wavelength set to the line Cu I 282.4 nm. The narrowed beam of the dye-laser was directed into the plasma at different heights above the sample surface. The fluorescence radiation was measured with an échelle-spectrometer, equipped with an intensified-charge-coupled device as the detector. The velocity depends strongly on the pressure of the ambient atmosphere and the distance from the sample surface. The highest velocity found at an argon pressure of 0.02 mbar was 1.0×10⁶ cm s⁻¹.     

Loading velocity and brittleness of solids

The ductile-brittle transition observed in many solids due to either a decrease in temperature or an increase in the loading velocity can be supplemented with a reversible brittle-ductile transition under loading at a hypervelocity exceeding the velocity of propagation of fracture cracks. This phenomenon occurs in the course of high-velocity penetration of solids into organic and inorganic glasses, as well as into ceramic materials.     

Measurement of the laboratory's absolute velocity

The report is given on a local measurement of the absolute velocity of a laboratory. This is the resultant velocity due to all types of motion in which the laboratory takes part (about the Earth's axis, about the Sun, about the galactic center, about the center of the cluster of galaxies).