小型飞行器空气动力学

对小型飞行器设计中涉及的空气动力学问题进行了综述。描述了雷诺数和展弦比对固定翼飞行器的设计以及飞行性能的影响。在低雷诺数飞行范围。翼型上边界层的特性对飞行器的设计尤为关键。本文讨论了大量有关层流边界层(包括层流分离泡影响)的实验,作为例子,列举了几个此飞行雷诺数范围的小型低空无人驾驶飞行器(UAVs)。此外,对扑动翼推进的理论模型进行了简述;其范围涵盖了早期的准定常附着流模型,以及后来计及非定常尾涡、流动分离以及气动弹性等效应的模型。文中还介绍了那些与理论互补并最终导致扑翼机设计成功的实验。

OEDGE modeling of plasma contamination efficiency of Ar puffing from different divertor locations in EAST

Modeling with OEDGE was carried out to assess the initial and long-term plasma contamination efficiency of Ar puffing from different divertor locations, i.e. the inner divertor, the outer divertor and the dome, in the EAST superconducting tokamak for typical ohmic plasma conditions. It was found that the initial Ar contamination efficiency is dependent on the local plasma conditions at the different gas puff locations. However, it quickly approaches a similar steady state value for Ar recycling efficiency >0.9. OEDGE modeling shows that the final equilibrium Ar contamination efficiency is significantly lower for the more closed lower divertor than that for the upper divertor.

Attitude control of multi-spacecraft systems on SO(3)with stochastic links failure

In this paper,for Multi-Spacecraft System(MSS)with a directed communication topology link and a static virtual leader,a controller is proposed to realize attitude consensus and attitude stabilization with stochastic links failure and actuator saturation.First,an MSS attitude error model suitable for a directed topology link and with a static virtual leader based on SO(3)is derived,which considers that the attitude error on SO(3)cannot be defined based on algebraic subtraction.Then,we design a controller to realize the MSS on SO(3)with attitude consensus and attitude stabilization under stochastic links failure and actuator saturation.Finally,the simulation results of a multi-spacecraft system with stochastic links failure and a static virtual leader spacecraft are demonstrated to illustrate the efficiency of the attitude controller.

Adaptive Modeling for Downwash E®ects in Multi-UAV Path Planning

This pape developa a novel mettad to model the air flow downwash faree ganated by the quadrotor un manmed arial vehiele(UAV)and itsefket on the neigh boring UAVs.Rach UAV shaped by a wirtual stracture for oollision-tree path plan ning.The shape is modified from a standard apberical body to a prapased adaptive eylinder to optimine the path planning while mani mloing the dowuwash impact.The eylinder beight varies based on the UAV dzumstanae and the paedieted downwash impact.Purthermare,the downwash model can aid in the eylinder beightextreme valae appointment.A flock-basedpath planning algorithm is invastigated in this study to ompare the spberical UAV shape model with the prapased eylindrical UAV shape model.The UAV with the adaptve eylindrical model is simulated and verified via Gaseho and Robat Operating System(ROS)simulation platfarm.

A Mesh Adjustment Scheme for Embedded Boundaries

An adaptive meshing technique and solution method is proposed in which a two-dimensional body-fitted multi-block mesh is locally adjusted to arbitrarily embedded boundaries that are not necessarily aligned with the mesh.Not only does this scheme allow for rapid and robust mesh generation involving complex embedded boundaries,it also enables the solution of unsteady flow problems involving bodies and interfaces moving relative to the flow domain.This scheme has been implemented within a block-based adaptive mesh refinement(AMR)numerical framework which can ease computational expense while maintaining a detailed representation of the embedded boundary and providing an accurate resolution of the spatial characteristics of the fluid flow.Rigid body motion and evolving motion due to physical processes are considered.A block-based AMR level set method is used to deal with evolving embedded boundaries.Numerical results for various test problems are presented to verify the validity of the scheme as well as demonstrate the capabilities of the approach for predicting complex two-dimensional inviscid and laminar fluid flows.

Multi-UAV Cooperative Hunting in Cluttered Environments Considering Downwash E®ects

This paper presents a novel solution to the three-dimensional (3D) cooperative hunting ofmultiple drones that deals with surrounding a target simultaneously while navigating aroundobstacles in the cluttered dynamic 3D environment. Meanwhile, drones avoid the air°owdownwash force created by the spinning propellers on unmanned aerial vehicles (UAVs) andtheir e®ect on the other UAVs. This solution consists of a 3D Simultaneous Encirclementstrategy, the cooperative hunting objective with a novel revised particle swarm optimization(PSO*) path planning algorithm, a °ocking theory-inspired obstacle avoidance algorithm, and acascade PI controller. Simulation results with varying conditions were carried out to validatethe e®ectiveness of the proposed solution by successfully taking care of the downwash e®ects,and having multiple hunter UAVs hunt and encircle a moving or stationary target in a dynamicor static obstacle-rich cluttered environment.

基于主动热载荷管理技术的涡轮盘疲劳寿命

为降低航空发动机涡轮盘失效风险并提高使用寿命,将主动热载荷管理技术应用于预置裂纹的涡轮盘模型,采用通用权函数法计算裂纹应力强度因子进而分析轮盘寿命,研究经主动热载荷管理的轮盘上能量分布对轮盘寿命的影响,探索轮盘上热边界载荷与寿命的关联性和变化规律,并通过有限元仿真模拟初步分析其作用机理。结果表明:主动热载荷管理技术通过优化轮盘的温度分布,可以有效地降低裂纹附近的应力,延缓裂纹的扩展,显著的提高寿命和安全性,当热边界载荷系数分别取0.05和0.10时,轮盘相应的寿命分别增加12.2%和26.1%。

Consensus based on learning game theory with a UAV rendezvous application

Multi-agent cooperation problems are becoming more and more attractive in both civilian and military applications. In multi-agent cooperation problems, different network topologies will decide different manners of cooperation between agents. A centralized system will directly control the operation of each agent with information flow from a single centre, while in a distributed system, agents operate separately under certain communication protocols. In this paper, a systematic distributed optimization approach will be established based on a learning game algorithm.The convergence of the algorithm will be proven under the game theory framework. Two typical consensus problems will be analyzed with the proposed algorithm. The contributions of this work are threefold. First, the designed algorithm inherits the properties in learning game theory for problem simplification and proof of convergence. Second, the behaviour of learning endows the algorithm with robustness and autonomy. Third, with the proposed algorithm, the consensus problems will be analyzed from a novel perspective.

Machine learned interatomic potentials using random features

We present a method to model interatomic interactions such as energy and forces in a computationally efficient way.The proposed model approximates the energy/forces using a linear combination of random features,thereby enabling fast parameter estimation by solving a linear least-squares problem.We discuss how random features based on stationary and non-stationary kernels can be used for energy approximation and provide results for three classes of materials,namely two-dimensional materials,metals and semiconductors.Force and energy predictions made using the proposed method are in close agreement with density functional theory calculations,with training time that is 96%lower than standard kernel models.Molecular Dynamics calculations using random features based interatomic potentials are shown to agree well with experimental and density functional theory values.Phonon frequencies as computed by random features based interatomic potentials are within 0.1%of the density functional theory results.Furthermore,the proposed random features-based potential addresses scalability issues encountered in this class of machine learning problems.

Combustion enhancement in rearward step based scramjet combustor by air injection at step base

Numerical simulations were performed to model the non-reacting and reacting flow behind a rearward step flameholder in Mach 1.6 supersonic flow with fuel injection at the step base.The combustor geometry was based on the University of Florida scramjet experimental facility.Turbulence was modeled using k-u shear stress transport(SST),laminar flamelet was used for combustion modeling.Wall static pressure showed good agreement with experimental data for non-reacting and reacting flow.For non-reacting flow,dummy fuel helium mole fraction distribution in the recirculation region behind the step was validated with planar laser induced fluorescence(PLIF)images in experiments.To improve the combustion characteristics,air was injected in tandem with hydrogen at step base using various configurations.With all fuel injection as baseline,the case with 2 air jets around each fuel jet and air injected at 2 times the stagnation pressure of fuel showed the most improvement compared to other cases.It was most effective in reducing the local fuel richness,shortening the flame length and increasing combustion efficiency.

Effects of aspect ratio and inclination angle on aerodynamic loads of a flat plate

Large Eddy Simulations are carried out to analyze flow past flat plate in different configurations and inclinations.A thin flat plate is considered at three inclination angles(α=30°,60°and 90°)and three aspect ratios(AR=0.5,2 and 5).The Reynolds number based on the free stream velocity and chord length of the plate at different inclination angles varies between 75,000 to 150,000.An increase in the inclination angle while the aspect ratio(span to chord)is constant results in higher drag and lower lift on the plate.Increasing the aspect ratio at a constant inclination angle increases the mean aerodynamic loading except for theα=30°and AR=0.5 case where the mean forces are larger than the other aspect ratios for this specific inclination angle.The small aspect ratio suppresses and blocks the separation of the flow from the top and bottom edges causing larger aerodynamic forces relative to AR=2,5.Visualization of the flow structures shows the tip vortices have a significant role in controlling the shedding vortices from the top and bottom edges.Atα=30°and AR=0.5,the two tip vortcies control and suppress the flow separation from the top and bottom edges.A stable wake was found for this case with no fluctuation.As the aspect ratio increases,the influence of the tip vortices on flow separation from the top and bottom edges reduces.As a result,larger fluctuations were found for cases with higher aspect ratios.