金属间化合物TiAl(W,Si)合金的蠕变行为和机制

 研究了 Ti-47Al-2 W-0.5 Si合金在 650～ 750℃区间的蠕变行为和变形机制。结果表明,合金 650℃蠕变寿命与施加应力之间符合线性的双对数关系,可用表达式 lg^t_f=10 lg^R+30来描述。蠕变寿命与最小蠕变速率之间满足 Monkman-Grant关系的修正式。合金的比蠕变强度与抗热腐蚀镍基高温合金 K438G相当。在700℃变载荷下蠕变时具有与恒载荷下蠕变相类似的特征。 800℃长期时效粗化合金组织,降低蠕变寿命。位错滑移和形变孪生是合金蠕变的主要变形机制。     

Simple adaptive control for aircraft control surface failures

This work extends the so-called simple adaptive control approach to direct model reference adaptive control of multi-input multi-output systems to include loss of control effectiveness failures. It is proven that all signals are bounded for loss of control effectiveness failures during a bounded input disturbance. A state space approach is introduced for computing the feedforward compensator that is required by the stability result. The adaptive algorithm is applied to a three input model of the linearized lateral dynamics of the F/A-18 aircraft. Simulation results are obtained with single, double, and triple control effectiveness failures of 88% during the occurrence of a lateral gust. These results show that the adaptive controller exhibits improved model following as compared with a fixed gain eigenstructure assignment controller.     

Estimation techniques for GMSK using linear detectors in satellite communications

This paper describes data-aided signal level and noise variance estimators for Gaussian minimum shift keying (GMSK) when the observations are limited to the output of a filter matched to the first pulse-amplitude modulation (PAM) pulse in the equivalent PAM representation. The estimators are based on the maximum likelihood (ML) principle and assume burst-mode transmission with known timing and a block of L₀ known bits. While it is well known that ML estimators are asymptotically unbiased and efficient, the analysis quantifies the rate at which the estimators approach these asymptotic properties. It is shown that the carrier phase, amplitude, and noise variance estimators are unbiased and can achieve their corresponding Cramer-Rao bounds with modest combinations of signal-to-noise ratio and observation length. The estimates are used to estimate the signal-to-noise ratio. It is shown that the mean squared error performance of the ratio increases with signal-to-noise ratio while the mean squared error performance of the ratio in decibels decreases with signal-to-noise ratio. Simulation results are provided to confirm the accuracy of the analytic results.     

Reconnection Diffusion in Turbulent Fluids and Its Implications for Star Formation

Astrophysical fluids are turbulent a fact which changes the dynamics of many key processes, including magnetic reconnection. Fast reconnection of magnetic field in turbulent fluids allows the field to change its topology and connections. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. Plasma associated with a given magnetic field line at one instant is distributed along a different set of magnetic field lines at the next instant. This diffusion of plasmas and magnetic field is enabled by reconnection and therefore is termed “reconnection diffusion”. The astrophysical implications of this concept include heat transfer in plasmas, advection of heavy elements in interstellar medium, magnetic field generation etc. However, the most dramatic implications of the concept are related to the star formation process. The reason is that magnetic fields are dynamically important for most of the stages of star formation. The existing theory of star formation has been developed ignoring the possibility of reconnection diffusion. Instead, it appeals to the decoupling of mass and magnetic field arising from neutrals drifting in respect to ions entrained on magnetic field lines, i.e. through the process that is termed “ambipolar diffusion”. The predictions of ambipolar diffusion and reconnection diffusion are very different. For instance, if the ionization of media is high, ambipolar diffusion predicts that the coupling of mass and magnetic field is nearly perfect. At the same time, reconnection diffusion is independent of the ionization but depends on the scale of the turbulent eddies and on the turbulent velocities. In the paper we explain the physics of reconnection diffusion both from macroscopic and microscopic points of view, i.e. appealing to the reconnection of flux tubes and to the diffusion of magnetic field lines. We make use of the Lazarian and Vishniac (Astrophys. J. 517:700, 1999) theory of magnetic reconnection and show that this theory is applicable to the partially ionized gas. We quantify the reconnection diffusion rate both for weak and strong MHD turbulence and address the problem of reconnection diffusion acting together with ambipolar diffusion. In addition, we provide a criterion for correctly representing the magnetic diffusivity in simulations of star formation. We discuss the intimate relation between the processes of reconnection diffusion, field wandering and turbulent mixing of a magnetized media and show that the role of the plasma effects is limited to “breaking up lines” on small scales and does not affect the rate of reconnection diffusion. We address the existing observational results and demonstrate how reconnection diffusion can explain the puzzles presented by observations, in particular, the observed higher magnetization of cloud cores in comparison with the magnetization of envelopes. We also outline a possible set of observational tests of the reconnection diffusion concept and discuss how the application of the new concept changes our understanding of star formation and its numerical modeling. Finally, we outline the differences of the process of reconnection diffusion and the process of accumulation of matter along magnetic field lines that is frequently invoked to explain the results of numerical simulations.     

Lower and Upper Ionosphere of Mars

The ionosphere of Mars has been explored mostly with the radio occultation experiment onboard Mariners 6, 7, 9; Mars 2, 3, 4, 6; Viking 1, 2, and more recently on Mars Global Surveyor (MGS) and Mars Express (MEX). In addition to the radio occultation experiment, MEX also carried Mars Advanced Radar for the Subsurface and Ionosphere Sounding (MARSIS) experiment which provided electron density profiles well above the main ionospheric peak. The atmosphere of Mars was measured directly by the neutral mass spectrometer onboard Viking 1 and 2 Landers. Later, an accelerometer and radio occultation experiment on MGS provided large data sets of atmospheric density at various locations in the upper and lower atmospheres of Mars, respectively. In this paper we review results of these upper and lower atmospheric/ionospheric measurements. Results of these measurements have been compared with theoretical models by several workers; therefore, we also review various atmospheric and ionospheric models of Mars.     

What do Cosmogenic Isotopes Tell us About Past Solar Forcing of Climate?

In paleoclimate studies, cosmogenic isotopes are frequently used as proxy indicators of past variations in solar irradiance on centennial and millennial timescales. These isotopes are spallation products of galactic cosmic rays (GCRs) impacting Earth's atmosphere, which are deposited and stored in terrestrial reservoirs such as ice sheets, ocean sediments and tree trunks. On timescales shorter than the variations in the geomagnetic field, they are modulated by the heliosphere and thus they are, strictly speaking, an index of heliospheric variability rather than one of solar variability. Strong evidence of climate variations associated with the production (as opposed to the deposition) of these isotopes is emerging. This raises a vital question: do cosmic rays have a direct influence on climate or are they a good proxy indicator for another factor that does (such as the total or spectral solar irradiance)? The former possibility raises further questions about the possible growth of air ions generated by cosmic rays into cloud condensation nuclei and/or the modulation of the global thunderstorm electric circuit. The latter possibility requires new understanding about the required relationship between the heliospheric magnetic fields that scatter cosmic rays and the photospheric magnetic fields which modulate solar irradiance.     

Relationship Between Substorm Auroras and Processes in the Near-Earth Magnetotail

Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for monitoring of substorm intensity in terms of the magnetic flux and energy dissipation.     

Power anomaly effects and costs in low-voltage mobile power systems

Electric power anomalies or disturbances can disrupt the normal operation of equipment, accelerate aging, or even cause outright failures thus resulting in increased costs of maintenance and reduced system reliability. Past research on the effects caused by power anomalies has been mostly focused on industrial, commercial, or residential systems, or on power distribution equipment. A literature survey reveals that there is no comprehensive review related to low-voltage (LV) power systems and utilization equipment applicable to military combat vehicles, such as aircraft and ships. This paper summarizes the results of a new literature survey that focused on the causes, effects, and mitigation methods for power anomalies typical of LV mobile power systems. Electric power anomaly cost data collected from the literature are also presented, from which the costs of anomalies to the national defense are estimated using some simple rationales.     

An approach to knowledge-aided covariance estimation

This paper introduces a parametric covariance estimation scheme for use with space-time adaptive processing (STAP) methods operating in heterogeneous clutter environments. The approach blends both a priori knowledge and data observations within a parameterized model to capture instantaneous characteristics of the cell under test (CUT) and reduce covariance errors leading to detection performance loss. We justify this method using both measured and synthetic data. Performance potential for the specific operating conditions examined herein include: 1) averaged behavior within roughly 2 dB of the optimal filter, 2) 1 dB improvement in exceedance characteristic relative to the optimal filter, highlighting improved instantaneous capability, and 3) impervious ness to corruptive target-like signals in the secondary data (no additional signal-to-interference-plus-noise ratio (SINK) loss, compared with 10 dB or greater loss for the standard STAP implementation), with corresponding detections comparable to the optimal filter case     

Mechanism of error propagation from the subdaily Universal Time model into the celestial pole offsets estimated by VLBI

Within the analysis of space geodetic observations, errors of the applied subdaily Earth rotation model can induce systematic effects in different estimated parameters. In this paper, we focus on the impact of the subdaily Universal Time (UT1) model on the celestial pole offsets (CPO) estimated from very long baseline interferometry (VLBI) observations. We provide a mechanism that describes the error propagation from the subdaily UT1 into the daily CPO.In typical 24-h VLBI sessions the observed quasars are well distributed over the sky. But the observations, if looked at from the Earth-fixed frame, are not homogeneously distributed. The amount of observations performed in different terrestrial directions shows an irregularity which can be roughly compared to the case where the observations are collected in only one Earth-fixed direction. This peculiarity leads to artefacts in VLBI solutions, producing a correlation between the subdaily variations in UT1 and the position of the celestial pole. As a result errors in diurnal terms of the subdaily UT1 model are partly compensated by the estimated CPO. We compute for each 24-h VLBI session from 1990 until 2011 the theoretical response of the CPO to an error in the subdaily UT1 by setting up a least-squares adjustment model and using as input the coordinates of the observed quasars and observation epochs. Then real observed response of the estimated CPO derived from the VLBI session solutions is compared to the predicted one. A very good agreement between the CPO values estimated from VLBI and the predicted values was achieved. The presented model of error propagation from the subdaily UT1 into the daily CPO allows to predict and explain the behaviour of CPO estimates of VLBI solutions computed with different subdaily Earth rotation models, what can be helpful for testing the accuracy of different subdaily tidal models.