Surface strengthening and grain refinement of AZ31B magnesium alloy by ultrasonic cavitation modification

Ultrasonic cavitation modification (UCM) employs cavitation effect to induce strong plastic deformation on the material surface and improve surface properties. To explore the surface strengthening and grain refinement of materials by UCM, the UCM orthogonal experiments of AZ31B magnesium alloy were carried out in water and kerosene, respectively. The effects of ultrasonic amplitude, distance from the sample, and processing time on Vickers hardness and grain size of the material were studied. The results showed that the Vickers hardness of samples increased to 1.5–3 times after UCM in water, which was 23.77–48.19% higher than that in kerosene. The metallographic observation indicated the grains on the surface of AZ31B were refined after UCM. The maximum fluctuation of grain size on the material surface was not more than 10 μm after UCM in water, and most of them were concentrated between 1.5 μm and 2.5 μm, while the former was more than 40 μm and the latter were concentrated between 2 μm and 10 μm in kerosene. This reflected that the grain refinement effect of UCM in water was better than that in kerosene. Ultrasonic cavitation can be used as a benign means to improve the surface properties of materials.     

二阶抛物型方程Dirichlet问题粘性解的存在性

利用Perron方法,讨论抛物型方程Dirichlet问题粘性解的存在性,随后给出实例。     

Real-time trajectory optimization for powered planetary landings based on analytical shooting equations

Traditionally, numerical trajectory integration for shooting equation calculation and iterations for shooting with randomly guessed initial solutions deteriorate the real-time performance of indirect methods for on-board applications. In this study, the indirect method is improved to achieve real-time trajectory optimization of fuel-optimal powered planetary landings with the help of analytical shooting equation derivations and a practical homotopy technique. Specifically, the contributions of t...     

计及横向剪切效应的环形谐振子理论研究CSCD

经典环形谐振子理论是一种基于忽略横向剪切变形假设建立的环理论,因而无法准确求解高径比较大的环问题。为了克服上述不足,基于Timoshenko理论和哈密顿原理,建立考虑横向剪切效应的环形谐振子新理论,包括广义本构关系、平衡方程和周期性条件等。利用经典的布勃诺夫-伽辽金法,并结合有限元法,获得谐振子在缓慢、匀速转动过程中二阶弯曲角频率和进动系数的理论解。研究表明,横向剪切变形会在一定程度上影响环形结构二阶弯曲角频率和进动系数的计算结果;环形谐振子的进动系数不是恒定的,它会随着高径比h/r的增大而缓慢地减小。     

Closed-form solution of attitude command generation for spin-to-spin maneuver

In recent earth observing missions, agile satellites enable various imaging modes beyond the traditional along-track strip imaging. However, it requires maneuvering with boundary conditions of considerable angular velocity, i.e., spin-to-spin maneuvering. This paper proposes an attitude command generation method for spin-to-spin maneuvering that can provide feedforward commands for the attitude control loop. A general solution for arbitrary flight time is provided which steers a satellite to the given final attitude and angular velocity at the prescribed time. In addition, an alternative method is proposed that further improves the maneuvering speed, which is applicable to small-angle maneuvering cases. The proposed solutions are both closed-form which are more intuitive and easier to comprehend than numerical solutions. It also has a great advantage in computational efficiency, which could enable its use on-board in real time. Numerical examples demonstrate the performance of the proposed methods in a single maneuvering case as well as in a consecutive maneuvering case integrated with a realistic earth observing scenario.     

Symmetric Mach reflection configuration with asymmetric unsteady solution

Symmetric Mach reflection in steady supersonic flow has been usually studied by solving a half-plane problem with the symmetric line treated as reflecting surface, thus losing the opportunity to discover antisymmetric flow structures. Here in this paper we treat this problem as an entire-plane problem. Using an unsteady numerical approach, we find that the two sliplines exhibit antisymmetric unsteadiness if the Mach stem height is small while the flow remains symmetric if the Mach stem height is large. The mechanism by which disturbance, generated in the downstream of the flow duct between the two sliplines, propagates upstream is identified and it is also shown that the interaction between the transmitted expansion waves and the sliplines increases the amplitude of the unstable modes. The present study suggests a new type of compressible jet that deserves further studies.     

平流层飞艇三叶螺旋桨结构优化方法

为实现螺旋桨轻质量和高固有频率之间的权衡设计，发展了1种桨叶对称削层结构的分区优化方法。为拓宽其高效率的速度和高度范围，应采用变桨距技术，需要设计圆柱形桨叶根部。该桨叶与不同桨距角的桨毂组合装配，可实现人工变桨距，在地面试验中达到高空转速。该螺旋桨采用组合分体式桨毂布局、桨叶内部填充泡沫和碳纤维混合结构，基于NSGA-Ⅱ（non-dominated sorting genetic algorithm），完成了支座固支的桨叶铺层参数优化，得到桨叶质量和频率的Pareto解集，在±10%频率安全裕度外选取最优铺层方案，并与实物测试值对比，结果表明：桨叶质量相对误差2.09%；支座固支的单桨叶频率相对误差9.30%；桨毂固支的组合体频率相对误差2.76%，避开了工作转速共振区间，证明该结构优化方法是合理有效的。     

某大型飞机后缘襟翼气动/机构综合优化设计

大型飞机增升装置的研制不仅需要其提供足够的气动性能,也需要对噪声、舒适性等进行综合考量设计,而增升装置是提高大型飞机综合性能的重要系统,也是目前技术发展亟待研究和解决的重要问题。对于增升装置机构设计,通过设计较为简单的铰链襟翼机构来引导襟翼达到较优位置,然后选择气动性能较好的位置作为新的优化位置,求得较优机构位置。机构位置改变结合气动性能验证需要进行大量的迭代计算以及结果优化,因此以机构设计为基础,探究多目标优化计算的新方法,最终实现气动结构一体化设计目标。选用铰链式后缘襟翼为研究对象,综合考量后缘襟翼旋转与扰流板下偏联合运动对于气动性能影响,利用所研究方法,对其进行气动机构一体化设计并得出设计结果。     

Higher-order vibration of thick composite and sandwich plates based on an alternative higher-order model

The transverse stretching vibration of thick sandwich plates, which is attributed to largely different stiffness at the adjacent layers, is a challenging issue, and efficient approach for such issue is less reported in the published literature. Thus, natural frequencies corresponding to stretching vibration modes are generally neglected in engineering design, which might impact structural safety as frequencies of the exciting force are close to transverse stretching vibration frequencies. This paper proposes an alternative higher-order model for dynamic analysis corresponding to the higher-order vibration modes. The proposed model is classified in the displacement-based equivalent single-layer theory, as the number of displacement parameters in the proposed model is independent of the layer number. The continuity of displacements and transverse shear stresses can be fulfilled at the interfaces between the adjacent layers of structures. To demonstrate the capability of the proposed model, typical examples are analyzed by utilizing the proposed model, the three-dimensional finite element method and the chosen higher-order models. By comparing with the exact three-dimensional elasticity solutions, it is found that the proposed model can yield more accurate natural frequencies corresponding to the higher-order displacement modes than the selected models. Moreover, the factors influencing reasonable prediction of the higher-order frequencies are investigated in detail, which can provide a reference for the accurate prediction of the higher-order frequencies.