A combined airfoil with secondary feather inspired by the golden eagle and its influences on the aerodynamics

Bird flight is a remarkable adaption that has allowed thousands of species to colonize all terrestrial habitats. A golden eagle has impressive flying abilities, such as hovering, perching, preying and attacking. To reveal the flying abilities, avian geometry of a golden eagle was extracted based on noncontact surface measurements using a ROMBER three-dimensional laser scanner. Distributions of a camber line, thickness and a secondary feather line of the extracted point cloud were fitted using convenient analytical expressions. A traditional airfoil was established with the camber line and thickness, then a combined airfoil was constructed by combining the traditional airfoil with a secondary feather. Oscillations of an airfoil as well as rapid pitch up were simplified as a sine wave around the quarter chord axis. Thereafter, both steady and unsteady aerodynamic performances of the airfoil are computed, the influences of the secondary feather on the steady and unsteady aerodynamics were further studied.

Ultra-Broadband Infrared Metamaterial Absorber for Passive Radiative Cooling

Infrared metamaterial absorber(MMA) based on metal-insulator-metal(MIM) configuration with flexible design,perfect and selective absorption,has attracted much attention recently for passive radiative cooling applications.To cool objects passively,broadband infrared absorption(i.e.8-14 μm) is desirable to emit thermal energy through atmosphere window.We present a novel MMA composed of multilayer MIM resonators periodically arranged on a PbTe/MgF_(2) bilayer substrate.Verified by the rigorous coup led-wave analysis method,the proposed MMA shows a relative bandwidth of about 45%(from 8.3 to 13.1 μm with the absorption intensity over 0.8).The broadband absorption performs stably over a wide incident angle range(below 50°) and predicts 12 K cooling below ambient temperature at nighttime.Compared with the previous passive radiative coolers,our design gets rid of the continuous metal substrate and provides an almost ideal transparency window(close to 100%)for millimeter waves over 1 mm.The structure is expected to have potential applications in thermal control of integrated devices,where millimeter wave signal compatibility is also required.

Influence of Assembly Parameters on Tensile Behavior of Hybrid CMC and Superalloy Bolted Joints by Progressive Damage Analysis

With the considerable applications of ceramic matrix composites(CMC) in aircraft engineering, the design of CMC bolted joint gains paramount attention because of its capacity to to improve load-bearing efficiency of aircraft key structure. In this work, a 3 D finite element model was established to predict tensile performance and failure modes of single-lap, single-bolt 2 D C/SiC composite, and superalloy joint, which considers the progressive damage behavior of 2 D woven C/SiC composites. On the basis of the developed progressive damage model, a parametric study was carried out to illustrate the effects of bolt preload and bolt-hole clearance on mechanical behaviors of the hybrid bolted joint. It was found that the increase in the value of bolt preload made the failure load grow first and then drop, and the optimum value of bolt preload 5.00 kN generated 56.47% rise in the initial failure load and 22.83% rise in the final failure load for the bolted joint in comparison with zero preload case. As the clearance increased from 0 to 2.00%, the initial and final failure loads respectively declined by 45.88% and 24.02% for 2.00% bolt-hole clearance relative to the neat-fit case. The loss in failure loads can be reduced to compressive stress concentration around the fastening hole-edge area, leading to the appearance of earlier damages by the introduction of increasing bolt hole clearance.

Biomimetic porous silicon oxycarbide ceramics with improved specific strength and efficient thermal insulation

Considering the challenge of aerodynamic heating,the development of high-performance insulating ce-ramic materials with lightweight and low thermal conductivity is crucially important for aerospace vehi-cles to achieve flight at high speed for a long time.In this work,macro-porous silicon oxycarbide(SiOC)ceramics with directional pores(DP-SiOC)(mean pore size of 88.1μm)were prepared using polysiloxane precursors via freeze casting and photocrosslinking,followed by pyrolysis.The DP-SiOC samples were lightweight(density∼0.135 g cm^(-3))with a porosity of 90.4%,which showed good shapability through the molding of polysiloxane precursors.The DP-SiOC samples also exhibited an ultra-low thermal con-ductivity of 0.048 W(m K)^(-1)at room temperature,which can also withstand heat treatment at 1200°C for 1 h.In addition,scaffolds with triply periodic minimal surfaces(TPMS)were fabricated using digital light processing(DLP)printing,which was further filled with polysiloxane precursors for increasing the strength of DP-SiOC.The TPMS scaffolds filled with macro-porous SiOC ceramics(TPMS-DP-SiOC)showed good integration between TPMS and macro-pore structures,which had a porosity∼75%and high specific strength of 9.73×10^(3)N m kg^(-1).The thermal conductivity of TPMS-DP-SiOC samples was 0.255 W(m K)^(-1)at room temperature.The biomimetic TPMS-DP-SiOC ceramics developed in this study are likely used for thermal protection systems.

Corrosion behavior of an Al-6Mg-Sc-Zr alloy

The corrosion behavior of an Al-6Mg-Sc-Zr alloy was studied and compared with that of an Al-6Mg-Zr alloy.The addition of scandium into the Al-6Mg-Zr alloy reduced the susceptibility to exfoliation corrosion. By using the constantload tensile method in a 3.5 wt.% NaCl solution, the resistance to SCC of the Al-6Mg-Sc-Zr alloy was higher than that ofthe Al-6Mg-Zr alloy. When the specimens were not applied with an anodic current, the Al-6Mg-Sc-Zr alloy was resistanceto SCC and no brittle cracking was found on the fracture surface. When an anodic current was applied, the Al-6Mg-Sc-Zralloy specimens failed as a result of accelerated corrosion rather than SCC. It was believed that the addition of scandium re-sulted in (Al3Sc, Zr) particles that greatly refined grains and promoted the formation of homogeneous discontinuous distri-bution of β-phase in the alloy base, which much contributed to good corrosion resistance of the Al-6Mg-Sc-Zr alloy.

Flutter analyses of complete aircraft based on hybrid grids and parallel computing

A tightly coupled method was developed to analyse aeroelasticity by constructing subiterative schemes for fluid and structural equations of motion,respectively.With MPI partition parallel computing,the fluid was solved by Navier-Stokes equations based on hybrid grids.A new unstructured background grid deformtion method was used for the CFD grid deformation.The transonic flutter wind tunnel model of a complete aircraft was simulated to validate the developed method.The flutter characteristics of the aircraft was analysed and compared with the test results.It indicates that the devoloped method has a relatively higher precision and can be used for aeronautical engineering application.

选区激光熔化与定向能量沉积NiTi形状记忆合金组织与性能的对比研究

利用激光增材制造技术成形了Ni50.8Ti49.2形状记忆合金,对比分析选区激光熔化(SLM)成形试样与激光定向能量沉积(DED)成形试样的物相分布、微观结构、力学性能与变形机理。结果表明SLM试样具有100 MPa的屈服强度和8%的伸长率,而DED试样的屈服强度高达700 MPa但仅有2%的伸长率。SLM试样的变形行为主要体现为形状记忆效应,DED试样的变形行为体现为伪弹性。XRD结果表明SLM基体为马氏体态而DED试样为奥氏体态。分析发现非平衡凝固和激光定向凝固过程导致激光增材试样存在明显的熔池行为,但由于SLM工艺的扫描速度明显高于DED工艺的,其存在更高的温度梯度和生长速度,形成了更小的晶粒尺寸分布和更强的织构强度,且形成的气孔缺陷较为均匀,使得材料的均匀变形更易发生,具有较高的伸长率。Ni元素的挥发以及更强的晶界界面能使得SLM试样具有足够的驱动力发生马氏体相变,改变了SLM试样的强织构方向和变形方式。

Optimization on conventional and electric air-cycle refrigeration systems of aircraft: A short-cut method and analysis

The air-cycle refrigeration system is widely used in commercial and military aircraft,and its efficiency greatly affects aircraft performance.Nowadays,this system requires a more efficient design and optimization method.In this paper,a short-cut optimization method with high efficiency and effectiveness is introduced for both conventional and electric air-cycle refrigeration systems.Based on the system characteristics,a four-layer parameter matching algorithm is designed which avoids computational difficulty caused by simultaneous equations.Fuel penalty is chosen as the objective function of optimization;design variables are reduced based on sensitivity analysis to improve optimization efficiency.The results show that the 3-variable optimization of the conventional air-cycle refrigeration system can obtain almost the same results as the traditional 6-variable optimization in that these two optimizations can both significantly reduce the fuel penalty.However,the computer running time of the 3-variable optimization is much shorter than that of the 6-variable optimization.The optimal fuel penalty of the electric air-cycle refrigeration system is lower than that of the conventional one.This study can provide reference for optimizing the aircycle refrigeration system of aircraft.

Planar Magnetic Metamaterial Slabs for Magnetic Resonance Imaging Applications

平面磁性的 metamaterial 的一种类型为磁性的回声成像(MRI ) 作为 superlens 与方形的弯屈微观结构被建议应用程序。印射测量的一个直接磁场证明通过 superlens 的无线电频率磁场被增加由在在 superlens 后面的大约 3 厘米的位置象 46.9% 一样高。制作平板的回声频率被发现为一个 1.5T MRI 系统在对目标频率(63.6 MHz ) 的好同意。MRI 与图象和 SNR (signal-to-noise 比率) 的紧张两个都被提高的磁性的 superlens 表演试验，暗示答应我们的平面磁性的 superlens 的 MRI 应用程序。

Mission decision-making method of multi-aircraft cooperatively attacking multi-target based on game theoretic framework

Coordinated mission decision-making is one of the core steps to effectively exploit the capabilities of cooperative attack of multiple aircrafts. However, the situational assessment is an essential base to realize the mission decision-making. Therefore, in this paper, we develop a mission decision-making method of multi-aircraft cooperatively attacking multi-target based on situational assessment. We have studied the situational assessment mathematical model based on the Dempster-Shafer(D-S) evidence theory and the mission decision-making mathematical model based on the game theory. The proposed mission decision-making method of antagonized airfight is validated by some simulation examples of a swarm of unmanned combat aerial vehicles(UCAVs)that carry out the mission of the suppressing of enemy air defenses(SEAD).

Simultaneous enhancement of mechanical and shape memory properties by heat-treatment homogenization of Ti_(2)Ni precipitates in TiNi shape memory alloy fabricated by selective laser melting

The excellent shape memory and mechanical properties of Ti Ni shape memory alloys(SMAs) fabricated using selective laser melting(SLM) are highly desirable for a wide range of critical applications. In this study, we examined the simultaneous enhancement of mechanical and shape memory properties using heat-treatment homogenization of Ti_(2)Ni precipitates in a Ti_(50.6)Ni_(49.4)SMA fabricated using SLM. Specifically, because of the complete solution treatment, nanoscale spherical Ti_(2)Ni precipitates were homogeneously dispersed throughout the grain interior. Interestingly, the resultant SMA exhibited an ultrahigh tensile strength of 880 ± 13 MPa, a large elongation of 22.4 ± 0.4%, and an excellent shape memory effect, with a recovery rate of > 98% and ultrahigh recoverable strain of 5.32% after ten loading–unloading cycles. These simultaneously enhanced properties are considerably superior than those of most previously reported Ti Ni SMAs fabricated using additive manufacturing. Fundamentally, the enhancement in tensile strength is ascribed to precipitation strengthening and work hardening, and the large plasticity is mainly attributed to the homogeneous nanoscale globular Ti_(2)Ni precipitates, which effectively impeded the rapid propagation of microcracks. Furthermore, the enhanced shape memory properties are derived from the suppression of dislocation movement and formation of retained stabilized martensite by the presence of high-density dislocations, nanoscale Ti_(2)Ni precipitates, and abundant interfaces. The obtained results provide insight into the enhancement of the two types of properties in Ti Ni SMAs and will accelerate the wider application of SMAs.

Parameter studies and evaluation principles of delamination damage in laminated composites

Delamination represents one of the most severe failure modes in composite laminates,especially when they are subjected to uniaxial compression loads.The evaluation of the delamination damage has always been an essential issue of composite laminates for durability and damage tolerance in engineering practice.Focusing on the most typical and representative elliptical delamination issue,an analytical model simultaneously considering the conservative buckling process and non-conservative delamination propagation process is implemented.Various computational cases considering different delamination depths,directions,aspect ratios,and areas are established,and the predicted results based on the analytical model are carefully compared.Effects of these geometrical delamination parameters on the buckling,delamination propagation,and failure behaviors of composite laminates are thoroughly analyzed,and innovative evaluation principles of the delamination damage have been concluded.It is found that the delamination area is the key factor that truly affecting the failure behaviors of delaminated composites,and the local/global buckling and failure loads show clear linearity with the delamination area,whilst the delamination depth and direction only have slight effects.

Flow field reconstruction and shock train leading edge position detection of scramjet isolation section based on a small amount of CFD data

Scramjet is the main power device of hypersonic vehicles. With the gradual expansion of wide velocity domain, shock wave/shock wave and shock wave/boundary layer are the main phenomena in scramjet isolator. When the leading edge of the shock train is pushed out from the inlet of the isolator, the engine will not start. Therefore, it is very important to detect the flow field structure in the isolator and the leading edge position of the shock train. The traditional shock train detection methods have low detection accuracy and slow detection speed. This paper describes a method based on deep learning to reconstruct the flow field in the isolator and detect the leading edge of the shock train. Under various back pressure conditions, the flow field images of computational fluid dynamics (CFD) data and the corresponding upper and lower wall pressure data were obtained, and a data set corresponding to pressure and flow field was constructed. By constructing and training convolutional neural networks, a mapping model with pressure information as input and flow field image as output is obtained, and then the leading edge position of shock train is detected on the output flow field image. The experimental results show that the average structure similarity (SSIM) between the reconstructed flow field image and the CFD flow field image is 0.902, the average peak signal-to-noise ratio (PSNR) is 25.289, the average correlation coefficient (CORR) is 0.956, and the root mean square error of shock train leading edge detection is 3.28 mm. Moreover, if the total pressure input is appropriately reduced, the accuracy of flow field reconstruction does not decline significantly, which means that the model has a certain robustness. Finally, in order to improve the detection accuracy of the leading edge position, we fine tuned the model and obtained another detection method, which reduced the root mean square error of the detection results to 1.87 mm.

Drogue detection for autonomous aerial refueling via hybrid pigeon-inspired optimized color opponent and saliency aggregation

Drogue detection is one of the challenging tasks in autonomous aerial refueling due to the requirement for accuracy and rapidity.Saliency detection based on image intrinsic cues can achieve fast detection,but with poor accuracy.Recent studies reveal that optimization-based methods provide accurate and quick solutions for saliency detection.This paper presents a hybrid pigeon-inspired optimization method,the optimized color opponent,that aims to adjust the weight of color opponent channels to detect the drogue region.It can optimize the weights in the selected aerial refueling scene offline,and the results are applied for drogue detection in the scene.A novel algorithm aggregated by the optimized color opponent and robust background detection is presented to provide better precision and robustness.Experimental results on benchmark datasets and aerial refueling images show that the proposed method successfully extracts the saliency region or drogue and exhibits superior performance against the other saliency detection methods with intrinsic cues.The algorithm designed in this paper is competent for the drogue detection task of autonomous aerial refueling.

Multi-objective clustering analysis via combinatorial pigeon inspired optimization

Multi-objective data clustering is an important issue in data mining, and the realization of data clustering using the multiobjective optimization technique is a significant topic. A combinatorial multi-objective pigeon inspired optimization(CMOPIO)with ring topology is proposed to solve the clustering problem in this paper. In the CMOPIO, a delta-locus based coding approach is employed to encode the pigeons. Thus, the length of pigeon representation and the dimension of the search space are significantly reduced. Thereby, the computational load can be effectively depressed. In this way, the pigeon inspired optimization(PIO) algorithm can be discretized with an auxiliary vector to address data clustering. Moreover, an index-based ring topology with the ability of contributing to maintain flock diversity is adopted to improve the CMOPIO performance. Comparative simulation results demonstrate the feasibility and effectiveness of our proposed CMOPIO for solving data clustering problems.

Foam structure to improve microwave absorption properties of silicon carbide/carbon material

Foam structure materials are well known for their lightweight,efficient,and broadband microwave absorption properties compared to bulk material.However,little has been understood about the effect of a foam structure on the absorption performance of the foam material.In this study,the role of foam structure properties of the silicon carbide/carbon(SiC/C)foam material on microwave absorption is explored using experiment and simulation.We find that the foam structure of SiC/C foam material causes diffraction,multiple reflections,improves the interfacial polarization,and compatibilization.The absorption performance of SiC/C foam material is also studied.The-10 dB effective absorption bandwidth can be adjusted from 4.0 GHz to 18 GHz by tuning SiC/C foam material thickness to 3-7 mm.Therefore,the foam structure design is an effective way to improve the absorption performance of the SiC/C foam material.

A fatigue damage-cumulative model in peridynamics

While the present structural integrity evaluation method is based on the philosophy of assumed similitude, Fatigue and Damage Tolerance(F&DT) evaluations for next generation of air-vehicles require high-fidelity physical models within cyberspace. To serve the needs of F&DT evaluation in digital twin paradigm, a fatigue damage-cumulative model within peridynamic framework is proposed in this paper. Based on the concept of fatigue element block and damage accumulation law in form of Coffin-Manson relationship, the proposed model applies to both fatigue crack initiation and fatigue crack growth;fatigue crack growth rates under constant-amplitude and simple variable-amplitude block loading cases can be well predicted for three common structural materials without inputs of Paris law parameters. Additionally, the proposed model can also be easily extended to a probabilistic version;for verification, multiple-site-damage problems are simulated and the statistic nature of fatigue process in experiments can be well captured. In the end, main features of the proposed model are summarized, and distinctions from the other models are discussed. There may be a potential for the peridynamic damage-cumulative model proposed in this work to numerically predict fatigue problems in digital twin paradigm for future generations of aerospace vehicles.

Pigeon-Inspired Circular Formation Control for Multi-UAV System with Limited Target Information

The problem of cooperative circular formation with limited target information for multiple Unmanned Aerial Vehicle(UAV)system is addressed in this paper.A pigeon-inspired circular formation control method is proposed to form the desired circular distribution in a plane based on the intelligent pigeon behavior during hovering.To reach the goal of prescribed radius and angular distribution,the controller is designed consisting of a circular movement part and a formation distribution part.Therein,the circular movement part is designed to make each UAV rotate around the speci-ed circle at the same angular speed only using the relative position between the UAV and the target.The formation distribution part could adjust the angular distance between each UAV and its neighbors with the jointly connected network to reduce communication cost.To smooth the speed variation,nonlinear PID-type method is delivered throughout the evolution of the system.The convergence analysis of the proposed control protocol is presented using Lyapunov theory and graph tools.The e®ectiveness of the proposed control strategies is demonstrated through numerical simulations.

Quality Control:Internal Defects Formation Mechanism of Selective Laser Melting Based on Laser-powder-melt Pool Interaction:A Review

Selective laser melting(SLM)is a 3D printing technology with a high near-net-shape ability and forming accuracy.However,the inevitable internal defects significantly hinder its development.Therefore,it is essential to fully understand the causes of internal defects in SLM processing and minimize the defects to achieve quality control accordingly.This work reviews the recent studies on internal defects in SLM,presenting the main internal defects of SLM as impurities,lack of fusion,gas pores,and micro-crack.These internal defects occur on the various phenomena in the laser-powder-melt pool(LPMP)stage.The formation of SLM internal defects is mainly affected by oxidation,denudation,balling,spatter,and keyholes;here,balling,spattering,and the keyhole phenomenon are the main factors causing internal defects in LPMP.Hence,this paper focuses on reviewing the balling effect,spatter behavior,and keyhole phenomenon,introducing the action mechanism of the above three phenomena under different process conditions.Additionally,the spatter behavior when forming internal defects is proposed.This review also considers the correlation between the spatter behavior and keyhole phenomenon and makes an important contribution to understanding and reducing SLM internal defects.It presents a reliable opinion on real-time monitoring and machine intelligent learning for SLM processing in the future,as well as supporting a systematic thinking for the suppression of defect formation in SLM.

CFD/CSD-based flutter prediction method for experimental models in a transonic wind tunnel with porous wall

To predict the flutter dynamic pressure of a wind tunnel model before flutter test,an accurate Computational Fluid Dynamics/Computational Structural Dynamics(CFD/CSD)-based flutter prediction method is proposed under the conditions of a 2.4 m×2.4 m transonic wind tunnel with porous wall.From the CFD simulations of the flows through an inclined hole of this wind tunnel,the Nambu's linear porous wall model between the flow rate and the differential pressure is extended to the porous wall with inclined holes,so that the porous wall can be conveniently modeled as a boundary condition.According to the flutter testing approach for the current wind tunnel,the steady CFD calculation is conducted to achieve the required inlet Mach number.A timedomain CFD/CSD method is then employed to evaluate the structural response of the experimental model,and the critical flutter point is obtained by increasing the dynamic pressure step by step at a fixed Mach number.The present method is applied to the flutter calculations for a vertical tail model and an aircraft model tested in the current transonic wind tunnel.For both models,the computed flutter characteristics agree well with the experimental results.