Accurately predicting hypersonic transitional flow on cone via a symmetry approach

A new algebraic transition model is proposed based on a Structural Ensemble Dynamics (SED) theory of wall turbulence, for accurately predicting the hypersonic flow heat transfer on cone. The model defines the eddy viscosity in terms of a two-dimensional multi-regime distribution of a Stress Length (SL) function, and hence is named as SED-SL. This paper presents clear evidence of precise predictions of transition onset location and peak heat flux of a wide range of hypersonic Transitional Boundary Layers (TrBL) around straight cone at zero incidence, to an unprecedented accuracy as validated by over 70 measurements for varying five crucial influential factors (Mach number, temperature ratio, cone half angle, nose Reynolds number and noise level). The results demonstrate the universality of the postulated multi-regime similarity structure, in characterizing not only the spatial non-uniform distribution of the eddy viscosity in hypersonic TrBL on cone, but also the dependence of the transition onset location on the five influential factors. The latter yields a novel correlation formula for transition center Reynolds number which takes similar functional form as the SL function within the symmetry approach. It is concluded that the SED-SL model simulates TrBL around cone with uniformly high accuracy, and then points out to an optimistic alternative way to construct hypersonic transition model.     

喷注方案对圆形超燃冲压发动机性能的影响研究

在飞行马赫数Ma=6，总当量比为1.0条件下，采用三维数值模拟研究了不同喷注位置煤油当量比分布对双凹腔圆形发动机推力性能和壁面热流的影响。喷注位置包括支板壁面喷注K1，隔离段出口壁面喷注K2，第一凹腔尾缘壁面喷注K3以及第一扩张段壁面喷注K4。结果表明，K1注油当量比大小直接影响燃烧室内的燃烧模态和流道中心燃烧。为了保证发动机推力性能，K1注油须达到一定量，促使流道燃烧处于亚燃模态，且流道内具有较强的中心燃烧。为优化发动机壁面热流环境，剩余燃料需要在K2，K3和K4分散注入。K2和K3注油当量比大小同时影响第一凹腔燃烧性能，其中K2注油当量比降低，推力性能下降，但壁面热流性能提高，而适当增加K3喷注煤油，有利于提高推力性能。增加K4注油，第二凹腔及其之后流道区域燃烧增强，发动机推力性能和热流性能均提高。通过分析各注油位不同当量比分布对发动机力热性能的影响规律，最终获得了力热性能较优的注油当量比分配方案，此时K1～K4注油当量比大小依次为0.6，0.1，0.1，0.2。     

A numerical study of noontime bite-outs in F2-region electron density

Noontime bite-outs in ionospheric F₂-region electron density in the geomagnetic equatorial, low, and middle latitudes have been reproduced in this study using the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM). The different driving mechanisms of F₂-region noontime bite-outs at different latitudes were further verified by modeling simulation. In the geomagnetic equator, the upward ExB plasma drifts are the main process to produce the noontime bite-outs in ionospheric electron density. In the geomagnetic low latitudes, both the electrical fields and poleward meridional winds play a crucial role in forming the noontime bite-outs. In contrast to the case at the geomagnetic equator, a weaker fountain effect might be an favorable condition for the noontime bite-outs to occur at low latitudes. For geomagnetic middle latitudes, an upward plasma flux and poleward meridional winds are the dominated drivers in producing the noontime bite-outs. Modeling results show that a large upward plasma flux and poleward meridional winds make the noontime bite-outs to occur and observable over middle latitudes.     

考虑约束的高超声速飞行器制导与控制一体化设计

针对高超声速飞行器上升段飞行过程中强耦合、强非线性同时要求满足过程约束的特点，提出了一种结合级联控制方法和控制障碍函数的新型三维制导控制一体化算法。首先通过对速度子系统设计控制障碍函数约束算法来满足飞行器的过程约束要求，然后利用反步法、动态逆控制方法设计其余子系统的控制器，两者共同组成制导控制一体化控制器。考虑到飞行器在上升过程中容易遭遇阵风扰动的问题，设计非线性干扰观测器以增强算法的鲁棒性。最后通过李雅普诺夫函数证明了系统的稳定性，并且通过仿真验证了该新算法能够在满足高超声速飞行器上升段过程约束的同时，实现飞行器的三维跟踪控制。     

Static magnetic field analysis of hollow-cup motor model and bow-shaped permanent magnet design

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.     

Distributed fixed-time attitude coordinated control for multiple spacecraft with actuator saturation

This paper investigates the distributed fixed-time attitude coordinated control problem for multiple spacecraft subject to actuator saturation under the directed topology. First, a distributed fixed-time observer is presented for each follower spacecraft to estimate the leader spacecraft’s states. Compared with the commonly used fixed-time observer, the settling time of the proposed fixed-time observer can be easily adjusted by some free design parameters. Next, a distributed fixed-time control ...     

随机扰动下的航天器姿轨保持自抗扰控制

空间飞行器在轨运行过程中除受空间摄动外，还因飞行器任务需要产生随机扰动力和扰动力矩。针对空间飞行器受随机扰动产生的耦合运动控制问题，提出了利用自抗扰方法进行轨道保持和姿态稳定的控制方法。通过引入二阶线性扩展状态观测器，对系统总扰动和状态进行观测。结合PD控制方法结合总扰动前馈补偿，克服空间主要摄动及飞行器本身产生的随机扰动，实现轨道保持和姿态稳定。仿真试验结果表明:该方法可以有效克服总扰动的影响，实现姿轨协同控制。     

低速巡飞弹非匹配干扰观测控制器设计

针对低速巡飞弹倾斜转弯控制系统的非线性、强耦合、不确定的特征,研究了一种基于干扰观测器的自适应反演控制方法。将参数摄动、外界干扰和执行机构的响应偏差分别等效进成系统的匹配干扰和非匹配干扰,并设计在有限时间内估计误差收敛的非线性干扰观测器进行估计。利用反演控制的思想,在虚拟控制量中抵消非匹配干扰,在实际控制量中抵消匹配干扰。利用李雅普诺夫理论设计自适应调节律补偿干扰观测器的估计误差,改进控制系统的瞬态性能。仿真结果表明,干扰观测器估计误差在有限时间内收敛到一定区间,系统能够有效地克服干扰的影响,快速准确地跟踪滚转角、攻角、侧滑角参考指令。     

Exploring the effect of COVID-19 pandemic lockdowns on urban cooling: A tale of three cities

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.     

遥感卫星多路数据传输系统同频共用方法

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。