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551.
《中国航空学报》2022,35(9):306-313
The hollow-cup Permanent Magnet (PM) motors have the characteristics of low power consumption, and are widely used in the aerospace field. At present, the tile-shaped PMs used by hollow-cup PM motors have poor sinusoidal characteristics of the air gap magnetic flux density waveform, which will cause torque ripple. The existing method for improving the air gap magnetic flux density waveform is not very effective when applied to hollow cup, a special motor with no stator core and large air gap. A bow-shaped PMs structure is designed for the hollow-cup motor in this paper. First, the equivalent surface current method is used to calculate the analytical formula of the static magnetic field of the model. Then, the Finite Element (FE) method is used to calculate the static air gap flux density waveform of conventional tile-shaped PMs and bow-shaped PMs with different bow heights, and the corresponding harmonics and sine distortion are obtained by Fourier decomposition. The simulation results show that the bow-shaped PMs can effectively improve the sinusoidal characteristics of the static air gap flux density waveform. And the suitable bow height is determined. Finally, a prototype is made based on the suitable bow height for experiments, and compared with the analytical result and the FE result, and the accuracy and effectiveness of the bow-shaped PMs with the suitable bow height are verified. 相似文献
552.
553.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2023,71(1):1017-1033
COVID-19 pandemic has had a major impact on our society, environment and public health, in both positive and negative ways. The main aim of this study is to monitor the effect of COVID-19 pandemic lockdowns on urban cooling. To do so, satellite images of Landsat 8 for Milan and Rome in Italy, and Wuhan in China were used to look at pre-lockdown and during the lockdown. First, the surface biophysical characteristics for the pre-lockdown and within-lockdown dates of COVID-19 were calculated. Then, the land surface temperature (LST) retrieved from Landsat thermal data was normalized based on cold pixels LST and statistical parameters of normalized LST (NLST) were calculated. Thereafter, the correlation coefficient (r) between the NLST and index-based built-up index (IBI) was estimated. Finally, the surface urban heat island intensity (SUHII) of different cities on the lockdown and pre-lockdown periods was compared with each other. The mean NLST of built-up lands in Milan (from 7.71 °C to 2.32 °C), Rome (from 5.05 °C to 3.54 °C) and Wuhan (from 3.57 °C to 1.77 °C) decreased during the lockdown dates compared to pre-lockdown dates. The r (absolute value) between NLST and IBI for Milan, Rome and Wuhan decreased from 0.43, 0.41 and 0.16 in the pre-lockdown dates to 0.25, 0.24, and 0.12 during lockdown dates respectively, which shows a large decrease for all cities. Analysis of SUHI for these cities showed that SUHII during the lockdown dates compared to pre-lockdown dates decreased by 0.89 °C, 1.78 °C, and 1.07 °C respectively. The results indicated a high and substantial impact of anthropogenic activities and anthropogenic heat flux (AHF) on the SUHI due to the substantial reduction of huge anthropogenic pressure in cities. Our conclusions draw attention to the contribution of COVID-19 lockdowns (reducing the anthropogenic activities) to creating cooler cities. 相似文献
554.
为节省频率资源,遥感卫星通常采用扩频体制实现多路数据的同频共用传输,针对通道间容易引起相互干扰的问题进行了研究,通过改进码分多址干扰估算模型,对采用平衡Gold码序列扩频系统抗干扰容限上界进行预测。在满足国际电信联盟(ITU)对卫星辐射功率通量密度约束条件下,提出了一种基于直接序列扩频体制的两路业务数据同频共用传输方法。仿真结果表明:当载波频率2GHz左右,两路辐射源的EIRP之差不大于8~15dBw时可以实现同频共用传输;如对信号扩频带宽进行差异化处理,则其相互兼容性还可进一步提高。 相似文献
555.
556.
为了降低旋转爆震发动机燃烧室壁面温度,设计了陶瓷基复合材料燃烧室主动冷却结构。对燃烧室主动冷却结构的传热特性进行数值模拟,获得主动冷却燃烧室壁面温度响应和温度分布规律。对燃烧室主动冷却结构进行了模型简化,将模拟旋转爆震波获得的不同壁面温度下的热流密度参数加载在冷却模型上,提高了壁面温度模拟的计算效率。结果表明:燃烧室内壁面热流密度随着壁面温度的升高而降低,扩散区的平均热流密度最大;陶瓷基复合材料燃烧室主动冷却结构可以有效降低燃烧室壁面温度,在相同冷却流量下,矩形冷却截面的冷却效果优于圆形冷却截面,可以将燃烧室壁面的温度降到1 200 K以下;燃烧室壁面最高温度在燃烧室中段区域。 相似文献