Features of the occurrence of the additional stratification on the bottom-side F region over the equatorial location of Trivandrum

The general features of occurrence of an additional layer on the bottom side of F region, referred to as F0.5 layer in the pre noon period, over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip lat of 0.5° N) in India during the period from 2004 to 2007 are presented using ionosonde observations. The F0.5 layer has a June (northern summer) solsticial maximum probability of occurrence with secondary maxima during December (northern winter) solstice. The seasonal as well as the day-to-day variability in the occurrence of F0.5 layer as mentioned in this paper seems to be a result of the variations in the amplitude and phases of the tides and gravity waves, and inventory of the metallic ions of meteoric origin. This study brings out an important manifestation of morning time F layer base region dynamics.     

Stability of the seasonal variations in diurnal and semidiurnal components of mid-latitude F2 layer parameters

We report work utilizing 15-min resolution ionospheric data obtained with DPS-4 digisonde in 2003–2011 to study the seasonal variations in amplitudes and phases of the most powerful spectral components of the F2 layer critical frequency (foF2) and peak height (hmF2) fluctuations over Irkutsk (52.5°N, 104.0°E). We show that fluctuations of both parameters contain quasi-harmonic components with periods of T_n = 24/n h (n = 1–7). The number of distinct spectral peaks varies from 3 in summer to 7 in winter. Amplitude and phase characteristics of the diurnal (n = 1) and semidiurnal (n = 2) components is studied using the data sets extracted from the original data sets with band-pass filter. It has been found that the amplitudes of diurnal/semidiurnal foF2 and diurnal hmF2 components are maximum in winter and minimum in summer. Amplitudes of the diurnal components vary gradually; those of the foF2 semidiurnal one, abruptly, thus forming a narrow winter maximum in November–January. The phase (local time of maximum) of the diurnal foF2 component increases gradually by 4–6 h from winter to summer. The phase of the semidiurnal foF2 component is nearly stable in winter/summer and sharply decreases (increases) by 2–3 h near the spring (autumn) equinox. The phase of the diurnal component of hmF2 (local time of minimum) varies slightly between 1130 and 1300 LT; that of the semidiurnal one decreases (increases) by 4–6 h from January to March (from September to November). The results obtained show that the main features of seasonal variations in the diurnal and semidiurnal components of the mid-latitude F2 layer parameters recur consistently during the solar activity growth and decline phases.     

A comparative sporadic-E layer study between two mid-latitude ionospheric stations

Hourly systematic measurements of the highest frequency reflected by the sporadic-E layer (foEs) recorded from January 1976 to June 2009 at the ionospheric stations of Rome (Italy, 41.8°N, 12.5°E) and Gibilmanna (Italy, 37.9°N, 14.0°E) were considered to carry out a comparative study between the sporadic E layer (Es) over Rome and Gibilmanna. Different statistical analysis were performed taking into account foEs observations near the periods of minimum and maximum solar activity. The results reveal that: (1) independently from the solar activity, Es develops concurrently over extended regions in space, instead of being a spatially limited layer which is transported horizontally by neutral winds over a larger area; especially during summer months, when an Es layer is present at Rome, there is a high probability that an Es layer is also present over Gibilmanna, and vice versa; (2) Es layer lifetimes of 1–5 h were found; in particular, Es layers with lifetimes of 5 h both over Gibilmanna and Rome are observed with highest percentages of occurrence in summer ranging between 80% and 90%, independently from the solar activity; (3) latitudinal effects on Es layer occurrence emerge mostly for low solar activity during winter, equinoctial, and summer months, when Es layers are detected more frequently over Gibilmanna rather than Rome; (4) when the presence of an Es layer over Rome and Gibilmanna is not simultaneous, Es layer appearance both over Rome and Gibilmanna confirms to be a locally confined event, because drifting phenomena from Rome to Gibilmanna or vice versa have not been emphasized.     

武汉上空钠层与钾层的同时激光雷达观测

简要介绍了(WIPM)双波长共振荧光激光雷达系统和数据处理方法, 并利用其对武汉上空钠层和钾层进行了多天的同时观测, 通过对钠层和钾层的绝对数密度、柱密度以及质心高度的计算, 同时与中纬度其他地区的相关参数进行比较,得出武汉上空钠层的绝对数密度、柱密度以及质心高度与其他中纬度地区十分相近的结论, 并且发现了一个钠层突发和钾层突发不同时出现的现象.      

低雷诺数涡轮叶片边界层转捩及分离特性测量

低雷诺数工作条件下涡轮流场特征及其控制设计,是航空发动机低压涡轮部件设计的难点和重点。针对低雷诺数涡轮叶栅流场开展了实验研究工作,利用油流显示、表面静压、边界层压力探针等测量手段研究了涡轮叶片边界层的分离和转捩。结果表明雷诺数降低导致了流动损失的增大,且存在一个临界雷诺数。当雷诺数小于临界雷诺数时,发生在吸力面的流动分离是开式的层流分离泡,不会再附与叶片;当雷诺数大于临界雷诺数时,分离流会在尾缘前重新附着于叶片吸力面,形成闭式分离泡。随着雷诺数的减小,出口尾迹变宽,出口流动损失、出口速度亏损和出口气流角偏离增大,尾迹中心向吸力面方向移动。     

立楔诱导高超声速分离流动的被动控制研究

双模态超燃冲压发动机由于压力扰动可能发生不起动现象,造成推力严重下降,对飞行稳定性与飞行安全具有很强的破坏性.不起动初始阶段主要受到激波与边界层相互作用引起的流动分离影响,希望通过控制分离达到改善流动的目的.采用5阶特征型WENO(weighted essentially non-oscillator)格式与3阶TVD(total variation diminishing)型Runge-Kutta(R-K)格式的高精度数值方法,求解三维Navier-Stokes(N-S)方程,研究与分析了凸起物和被动吹吸两种被动控制方法对激波与边界层相互作用导致的高超声速流动分离现象的控制效果.结果表明:凸起物通过诱导流向涡形成,改变空间压力分布,减弱二次分离,影响分离结构;吹吸方式的被动控制技术通过平衡分离区与再附区之间的高低压差,降低逆压梯度,使压力分布与分离区域发生改变.      

Ionospheric disturbances on 8 September, 2010: Was it connected with the incoming moderate Chongqing earthquake?

Earthquake prediction stimulates the searches for a correlation between seismic activity and ionospheric anomalies. Contrary to common focuses on strong earthquakes, we report the ionospheric disturbances, 2 days before a moderate Ms = 4.7 Chongqing earthquake (29.4°N, 105.5°E, depth = 7.0 km, occurred at 21:21 LT, 10 September, 2010) with the data of ground-based ionosondes and IGS receivers. The data covering the period under the quiet geomagnetic conditions and a geomagnetic storm was analyzed with upper and lower bounds. It is found that there were significant enhancements of foF2 and total electron content (TEC) on the afternoon of 8 September, 2010, with a limited area close to the epicentre, which was different from the feature of ionospheric perturbations triggered by the geomagnetic storm on 15 September. Taking into account the heliogeomagnetical condition, we conclude that the observed ionospheric enhancements were very likely associated with the forthcoming moderate Chongqing earthquake, which implies that the relationship between the amplitudes of ionospheric disturbances and earthquakes is very complicated.     

A novel virtual material layer model for predicting natural frequencies of composite bolted joints

A novel virtual material layer model based on the fractal theory was proposed to predict the natural frequencies of carbon fiber reinforced plastic composite bolted joints. Rough contact surfaces of composite bolted joints are modeled with this new proposed approach. Numerical and experimental modal analyses were conducted to validate the effectiveness of the proposed model. A good consistence is noted between the numerical and experimental results. To demonstrate the necessity of accurately modeling the rough contact surfaces in the prediction of natural frequencies, virtual material layer model was compared with the widely used traditional model based on the Master-Slave contact algorithm and experiments, respectively. Results show that the proposed model has a better agreement with experiments than the widely used traditional model (the prediction accuracy is raised by 8.77% when the pre-tightening torque is 0.5 N∙m). Real contact area ratio A* of three different virtual material layers were calculated. Value of A* were discussed with dimensionless load P*, fractal dimension D and fractal roughness G. This work provides a new efficient way for accurately modeling the rough contact surfaces and predicting the natural frequencies of composite bolted joints, which can be used to help engineers in the dynamic design of composite materials.     

非均匀气动加热下隔热层结构的优化设计

为获得非均匀气动加热条件下高超声速飞行器不同部位的合适的热防护系统厚度,发展了一种基于网格变形技术（ASD）的隔热层结构优化方法。以高超声速飞行器的复合材料隔热层结构为研究对象,采用有限元法在局部均布热流载荷、三角形均变热流载荷和二次函数热流载荷等作用条件下建立隔热层结构的热固耦合分析模型,提出了基于ASD技术与热固耦合分析相结合的结构轻量化设计方法。结果表明,基于网格变形技术能够快速有效地解决优化过程中的网格自动更新问题,并得到了光滑柔顺的厚度形状曲线,优化后更充分发挥了隔热层结构各层材料的承载能力。     

翼型湍流尾缘噪声半经验预测公式改进

改进了传统的翼型湍流边界层尾缘噪声BPM半经验预测公式。传统的BPM半经验湍流边界层尾缘噪声预测公式对高攻角和厚翼型在高频范围的预测结果大于实验结果,通过分析比较传统BPM半经验预测公式和Howe翼型尾缘噪声理论模型发现:这主要是由于传统BPM半经验预测公式对压力面声源噪声辐射高估引起的。因此将压力面声源噪声辐射与吸力面声源噪声辐射的幅值比由原来的边界层位移厚度一次方比值改进为二次方比值,进而得到了改进后的BPM半经验预测公式;使用改进后的BPM半经验预测公式对NACA0012翼型在不同来流不同攻角下的噪声辐射进行了预测比较,发现对于NACA0012翼型,改进后的BPM半经验预测公式具有较高精度;另外也预测了较厚的风力机翼型DU-96-W-180,预测结果明显改善。