翼型尾缘噪声源空间分布与辐射特性关系

详细研究了翼型湍流边界层尾缘宽频噪声源空间分布与辐射特性的关系.采用基于雷诺平均流场的翼型尾缘宽频预测方法研究了NACA0012翼型湍流边界层尾缘宽频噪声在4种不同工况下的噪声源空间分布与辐射特性.首先计算了NACA0012翼型湍流边界层尾缘噪声源在不同频率下的空间分布.计算结果发现:边界层中湍流是翼型湍流边界层尾缘噪声声源.随着频率的增加,噪声源强度和噪声源空间尺寸都是先增加后减小,噪声源位置不断靠近翼型尾缘.同时也计算了边界层内不同位置处的噪声源对远场噪声的辐射特性,结果表明:边界层内层区域,其噪声频谱能量集中在高频;边界层外部区域,其噪声能量集中在中低频;攻角增大或者来流速度减小,噪声能量向低频转移.      

计入带电粒子入射角度对次级电子效应影响时航天飞行器的带电特性

本文采用由实验得到的带电粒子入射角度对次级电子的影响关系,根据轨道限制电流收集理论,导出了不同几何形体航天器的次级电子数通量表达式;并按局部电流平衡模型,计算了航天器背阴面的带电电位。结果表明,计入带电粒子入射角的影响后,航天器表面负电位的数值有一定减少。因此,在计算航天器表面带电时,应当考虑这种影响。     

声光相关处理器中的付里叶光学分析

介绍了付里叶光学分析的基本原理,并将其应用于声光信号处理系统。分析了付里叶光学分析方法对于复相关运算具有的独特的方便性、对于空间及时间积分的两类声光处理系统,导出了复透射率的表达式。分析结果表明:采用付里叶光学分析声光信号处理系统是可行的,并对实践具有重要的指导意义。     

直升机机内PWM照明调光系统产生音频干扰问题的研究

在直升机机内照明调光领域，国外军机已广泛采用脉冲宽度调制（PWM）调光方式。PWM调光方式相比传统线性调光方式具有以下明显优点：效率高，调光范围宽，功耗小，带载能力强，附加损耗小等。但PWM调光系统的不足是电磁辐射，尤其会对机内通话系统产生音频干扰，在机通耳机中会出现明显干扰声，影响机内通话质量。本方案能有效抑制PWM...     

3D Imaging of the OH mesospheric emissive layer

A new and original stereo imaging method is introduced to measure the altitude of the OH nightglow layer and provide a 3D perspective map of the altitude of the layer centroid. Near-IR photographs of the OH layer are taken at two sites separated by a 645 km distance. Each photograph is processed in order to provide a satellite view of the layer. When superposed, the two views present a common diamond-shaped area. Pairs of matched points that correspond to a physical emissive point in the common area are identified in calculating a normalized cross-correlation coefficient (NCC). This method is suitable for obtaining 3D representations in the case of low-contrast objects. An observational campaign was conducted in July 2006 in Peru. The images were taken simultaneously at Cerro Cosmos (12°09′08.2″ S, 75°33′49.3″ W, altitude 4630 m) close to Huancayo and Cerro Verde Tellolo (16°33′17.6″ S, 71°39′59.4″ W, altitude 2272 m) close to Arequipa. 3D maps of the layer surface were retrieved and compared with pseudo-relief intensity maps of the same region. The mean altitude of the emission barycenter is located at 86.3 km on July 26. Comparable relief wavy features appear in the 3D and intensity maps. It is shown that the vertical amplitude of the wave system varies as exp (Δz/2H) within the altitude range Δz = 83.5–88.0 km, H being the scale height. The oscillatory kinetic energy at the altitude of the OH layer is comprised between 3 × 10⁻⁴ and 5.4 × 10⁻⁴ J/m³, which is 2–3 times smaller than the values derived from partial radio wave at 52°N latitude.     

Solar Isotopic Composition as Determined Using Solar Energetic Particles

Solar energetic particles (SEPs) provide a sample of the Sun from which solar composition may be determined. Using high-resolution measurements from the Solar Isotope Spectrometer (SIS) onboard NASA’s Advanced Composition Explorer (ACE) spacecraft, we have studied the isotopic composition of SEPs at energies ≥20 MeV/nucleon in large SEP events. We present SEP isotope measurements of C, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni made in 49 large events from late 1997 to the present. The isotopic composition is highly variable from one SEP event to another due to variations in seed particle composition or due to mass fractionation that occurs during the acceleration and/or transport of these particles. We show that various isotopic and elemental enhancements are correlated with each other, discuss the empirical corrections used to account for the compositional variability, and obtain estimated solar isotopic abundances. We compare the solar values and their uncertainties inferred from SEPs with solar wind and other solar system abundances and find generally good agreement.     

Jovian periodicities (∼10 h, ∼40 min) on Ulysses’ Distant Jupiter Encounter observations around the Halloween CIR events

We analyzed data from four different instruments (HI-SCALE, URAP, SWOOPS, VHM/FGM) onboard Ulysses spacecraft (s/c) and we searched for possible evidence of Jovian emissions when the s/c approached Jupiter during the times of Halloween events (closest time approach/position to Jupiter: February 5, 2004/R = 1683 R_J,θ = ∼49°). In particular, we analyzed extensively the low energy ion measurements obtained by the HI-SCALE experiment in order to examine whether low energy ion/electron emissions show a symmetry, and whether they are observed at north high latitudes upstream from the jovian bow shock, as is known to occur in the region upstream from the south bow shock as well ( Marhavilas et al., 2001). We studied the period from October 2003 to March 2004, as Ulysses moved at distances 0.8–1.2 AU from the planet at north Jovicentric latitudes <75°, and we present here an example of characteristic Jovian periodicities in the measurements around a CIR observed by Ulysses on days ∼348–349/2003 (R = 1894 R_J,θ = 72°). We show that Ulysses observed low energy ion (∼0.055–4.7 MeV) and electron (>∼40 keV) flux and/or spectral modulation with the Jupiter rotation period (∼10 h) as well as variations with the same period in solar wind parameters, radio and magnetic field directional data. In addition, characteristic strong ∼40 min periodic variations were found superimposed on the ∼10 h ion spectral modulation. Both the ∼10 h and ∼40 min ion periodicities in HI-SCALE measurements were present in several cases during the whole period examined (October 2003 to March 2004) and were found to be more evident during some special conditions, for instance during enhanced fluxes around the start (forward shock) and the end (reverse shock) of CIRs. We infer that the Jovian magnetosphere was triggered by the impact of the CIRs, after the Halloween events, and it was (a) a principal source of forward and reverse shock-associated ion flux structures and (b) the cause of generation of ∼10 h quasi-periodic magnetic field and plasma modulation observed by Ulysses at those times.     

Intermediate-term periodicities in relativistic solar electron fluences during solar cycles 22 and 23

In this paper, we have investigated the intermediate-term periodicities of the relativistic (E > 10 MeV) solar electron flares measured by IMP-8 satellite of NASA for the time period of 1986–2001. This period of investigation includes the entire solar cycle 22; ascending, maximum and a part of descending phase of the current solar cycle 23. To determine accurately the occurrence rate of electron flux, we have employed three different spectral decomposition techniques, viz. fast Fourier transformation (FFT); maximum entropy method (MEM) and Lomb–Scargle periodogram analysis method. For solar cycle 22, in the low frequency range, power spectrum analysis exhibits statistically significant periodicities at ∼706, ∼504 and ∼392 days. In the intermediate frequency range, we have found a series of significant periodicities ∼294, ∼221, ∼153, ∼86, ∼73 and ∼66 days. For short term, periodicities of ∼21–23, ∼31 and ∼37 days were found in power spectrum. When solar cycle 23 is considered the significant periodicities are ∼20, ∼23, ∼29, ∼39, ∼54, ∼63, ∼118, ∼133 and ∼154 days. These results provide evidence that the best known Rieger period (∼153 days), appeared in the high energetic electron flux data for cycle 22 and also likely during maxima of cycle 23. The existence of these periodicities has been discussed in the light of earlier results.     

Low ionization lines in high luminosity quasars: The calcium triplet

In order to investigate where and how low ionization lines are emitted in quasars we are studying a new collection of spectra of the CaII triplet at λ8498, λ8542, λ8662 observed with the Very Large Telescope (VLT) using the Infrared Spectrometer And Array Camera (ISAAC). Our sample involves luminous quasars at intermediate redshift for which CaII observations are almost nonexistent. We fit the CaII triplet and the OI λ8446 line using the Hβ profile as a model. We derive constraints on the line emitting region from the relative strength of the CaII triplet, OI λ8446 and Hβ.