Chinese Journal of Aeronautics最新文献

Shock wave focusing is an effective way to create a hot spot or a high-pressure and high-temperature region at a certain place, showing its unique usage in detonation initiation, which is beneficial for the development of detonation-based engines. The flame propagation behavior after the autoignition induced by shock wave focusing is crucial to the formation and self-sustaining of the detonation wave. In this study, wedge reflectors with two different angles (60° and 90°) and a planar reflector are employed, and the Mach number of incident shock waves ranging from 2.0 to 2.8 is utilized to trigger different flame propagation modes. Dynamic pressure transducers and the high-speed schlieren imaging system are both employed to investigate the shock-shock collision and ignition procedure. The results reveal a total of four flame propagation modes: deflagration, DDT (Deflagration-to-Detonation Transition), unsteady detonation, and direct detonation. The detonation wave formed in the DDT and unsteady detonation mode is only approximately 75%−85% of the Chapman-Jouguet (C-J) speed; meanwhile, the directly induced detonation wave speed is close to the C-J speed. Transverse waves, which are strong evidence for the existence of detonation waves, are discovered in experiments. The usage of wedge reflectors significantly reduces the initial pressure difference ratio needed for direct detonation ignition. We also provide a practical method for differentiating between detonation and deflagration modes, which involves contrasting the speed of the reflected shock wave with the speed of the theoretically nonreactive reflected shock wave. These findings should serve as a reference for the detonation initiation technique in advanced detonation propulsion engines.

Electrically Assisted Forming (EAF) technology has obvious advantages in material forming. To develop an effective constitutive model considering electrical effects, room temperature and electrically assisted quasi-static uniaxial tensile tests were conducted using ultrathin nickel-based superalloy plates with a thickness of 0.25 mm. The research focused on the two most widely recognized effects: the Joule thermal and the electric athermal effects. The mechanism of current action can be divided into two scenarios: one considering the Joule thermal effect only, and the other considering both effects simultaneously. Two basic constitutive models, namely the Modified-Hollomon model and the Johnson-Cook (J-C) model, were selected to be optimized through the classification of two different situations, and four optimized constitutive models were proposed. It was found that the J-C model with simultaneous consideration of the Joule thermal effect and electric athermal effect had the best prediction effect by comparing the results of these four models. Finally, the accuracy of the optimization model was verified by finite element simulation of the electrically assisted stretching optimization model. The results show that the constitutive model can effectively predict the temperature effect caused by the Joule heat effect and the athermal effect of current on the material.

As the controlled research of Dynamic Installation (DI) and Static Installation (SI), a new interference installation method was developed based on electromagnetic loading to enhance the mechanical properties of composite structures. Four different interference-fit sizes were considered, ranging from a net fit to 2.0%. The experiments were conducted to evaluate the installation resistance and the mechanical behavior of the joint under external loads. Meanwhile, an FEA model to model the stress distribution and damage behavior of the bolt-hole contact interface was established. The load–displacement curve and damage modes of experiments were used to verify the FEA results. The results show that the installation resistance during DI process was remarkably lower than that of SI process corresponding to all interference-fit sizes, and the stress amplitudes induced by interference were larger and widely distributed. The damage of the hole wall was positively correlated with interference fit size, but DI can significantly reduce the damage compared to SI. In performance tests, DI enhanced the static bearing capacity and extended longer fatigue life of the joints than SI. DI methods can be an effective way to achieve highly reliable interference joints in composite structures.

Liquid film cooling as an advanced cooling technology is widely used in space vehicles. Stable operation of liquid film along the rocket combustion inner wall is crucial for thermal protection of rocket engines. The stability of liquid film is mainly determined by the characteristics of interfacial wave, which is rarely investigated right now. How to improve the stability of thin film has become a hot spot. In view of this, an advanced model based on the conventional Volume of Fluid (VOF) model is adopted to investigate the characteristics of interfacial wave in gas–liquid flow by using OpenFOAM, and the mechanism of formation and development of wave is revealed intuitively through numerical study. The effects from gas velocity, surface tension and dynamic viscosity of liquid (three factors) on the wave are studied respectively. It can be found that the gas velocity is critical to the formation and development of wave, and four modes of droplets generation are illustrated in this paper. Besides, a gas vortex near the gas–liquid interface can induce formation of wave easily, so changing the gas vortex state can regulate formation and development of wave. What’s more, the change rules of three factors influencing on the interfacial wave are obtained, and the surface tension has a negative effect on the formation and development of wave only when the surface tension coefficient is above the critical value, whereas the dynamic viscosity has a positive effect in this process. Lastly, the maximum height and maximum slope angle of wave will level off as the gas velocity increases. Meanwhile, the maximum slope angle of wave is usually no more than 38°, no matter what happens to the three factors.

Geometric error, mainly due to imperfect geometry and dimensions of machine components, is one of the major error sources of machine tools. Considering that geometric error has significant effects on the machining quality of manufactured parts, it has been a popular topic for academic and industrial research for many years. A great deal of research work has been carried out since the 1970 s for solving the problem and improving the machining accuracy. Researchers have studied how to measure, detect, model, identify, reduce, and compensate the geometric errors. This paper presents a thorough review of the latest research activities and gives an overview of the state of the art in understanding changes in machine tool performance due to geometric errors. Recent advances in measuring the geometrical errors of machine tools are summarized, and different kinds of error identification methods of translational axes and rotation axes are illustrated respectively. Besides, volumetric geometric error modeling, tracing, and compensation techniques for five-axis machine tools are emphatically introduced. Finally, research challenges in order to improve the volumetric accuracy of machine tools are also highlighted.

Adhesively Bonded Carbon Fibre Reinforced Plastic (CFRP) and titanium alloy have been extensively used as a hybrid structure in modern aircrafts due to their excellent combination of mechanical properties and chemical stabilities. This study utilised NaOH anodising method to create micro-rough titanium surfaces for enhancing adhesive bonding between titanium alloy and CFRP laminates. A special and simple technique named Resin Pre-Coating (RPC) was also employed to improve the surface wetting of anodised titanium and grinded CFRP substrates. The influences of anodising temperature and duration on the surface morphology, wettability and adhesive bond strength were investigated. The single lap shear test results showed that the bond strength of specimens anodised at 20 °C for 15 min improved by 135.9% and 95.4%, respectively, in comparison with that of acid pickled and grinded specimens (without RPC treatment). Although increasing the anodising temperature and duration produced rougher titanium surfaces, the adhesively bonded joints were not strong enough due to relatively friable titanium oxide layers.

Non-destructive testing of composites is an important issue in the modern aircraft industry. Composites are susceptible to the barely visible impact damage which can affect the residual strength of the material and occurs both during production and operation. The continuum model for describing the damaged zone is presented. The slip theory relations used for a continuous distribution of slip planes are applied. At the initial stage, the isotropic background model is used. This model allows the material slippage along the fractures based on the Coulomb friction law with the small viscous addition. In this regime, the govern system of equations becomes rigid. To overcome this difficulty, the explicit–implicit grid-characteristic scheme is proposed. The standard ultrasound diagnostic procedure of damaged composite materials is successfully simulated. Compared with the trivial free-surface fracture model, different reactions on the compression and stretch waves are registered. This approach provided an effective way for the simulation of complex dynamic behavior of damage zones.