Transactions of Nanjing University of Aeronautics & Astronautics
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    2019,36(1):3-19, DOI: 10.16356/j.1005-1120.2019.01.001
    Abstract:
    Laser ultrasonic technique has received increasing attentions in the past decade due to its contactless nature and a wide range of applications have been reported. In this review, applications of laser ultrasonic technique developed at Nanjing University of Aeronautics and Astronautics (NUAA) as well as elsewhere for non-destructive testing in composite laminates are presented. The principles of generating and detecting in a laser ultrasonic system are introduced, three different system configurations are also introduced with each configuration’s advantages and disadvantages explained. More importantly, two major applications developed at NUAA for composite laminates are presented including damage detection, stiffness reconstruction and fatigue life prediction. Both applications are realized by a fixed-point PZT sensor and scanning pulse laser based on the linear reciprocal theorem. Analytical method and numerical models are employed and developed to realize the functionalities.
    2019,36(1):20-28, DOI: 10.16356/j.1005-1120.2019.01.002
    Abstract:
    The local buckling of stiffened panels is one of possible failure modes and concerned by engineers in the preliminary design of lightweight structures. In practice, a simplified model, i.e., a rectangular plate with elastically restrained along its unloaded edges, is established and the Ritz method is usually employed for solutions. To use the Ritz method, however, the loaded edges of the plate are usually assumed to be simply supported. An empirical correction factor has to be used to account for clamped loaded edges. Here, a simple and efficient method, called the quadrature element method (QEM), is presented for obtaining accurate buckling behavior of rectangular plates with any combinations of boundary conditions, including the elastically restrained conditions. Different from the conventional high order finite element method(FEM), non-uniformly distributed nodes are used, and thus the method can achieve an exponential rate of convergence. Formulations are worked out in detail. A computer program is developed. Improvement of solution accuracy can be easily achieved by changing the number of element nodes in the computer program. Several numerical examples are given. Results are compared with either existing solutions or finite element data for verifications. It is shown that high solution accuracy is achieved. In addition, the proposed method and developed computer program can allow quick analysis of local buckling of stiffened panels and thus is suitable for optimization routines in the preliminary design stage.
    2019,36(1):29-38, DOI: 10.16356/j.1005-1120.2019.01.003
    Abstract:
    Deep space communication is quite different from conventional ground communication due to its time-varying, complexity and large signal delay, which consequently affects communication quality and system efficiency. Adjusting the transmission parameters when the channel environment changes during the communication can guarantee the performance index of the system, and therefore improve communication efficiency. An adaptive transmission scheme of transceiver based on Consultative Committee for Space Data Systems (CCSDS) protocols is proposed in this paper. According to the variation of the deep space channel, the symbol rate of transmission data is adjusted dynamically by estimating the signal-to-noise ratio (SNR) of the receiver in real time and adjusting the channel environment. This scheme can improve the channel utilization and system throughput under the premise of limiting the system bit error rate. Furthermore, this scheme is successfully implemented in Xilinx Virtex-5 FPGA board.
    2019,36(1):39-60, DOI: 10.16356/j.1005-1120.2019.01.004
    Abstract:
    Rotor wake analysis, a fundamental research of helicopter technology, has been widely applied for rotor aerodynamic analysis. This paper summarizes the research of different rotor wake models at home and abroad and reviews the development process of rotor wake methods as well as the research achievement obtained in each stage. Then, the new progress of helicopter rotor wake methods is described in detail. It includes constant circulation contours modeling method of rotor wake, pseudo-implicit relaxation iteration and time-accurate solution method, research on aerodynamic interaction characteristics of helicopter rotor/fuselage by wake method, research on the rotor blade-vortex interaction noise and interaction of coaxial rigid rotor aerodynamics by viscous vortex particle method, and application of free wake method to helicopter flight dynamics modeling. In the end, some prospects for the research of helicopter rotor wake method are put forward, which clarifies the ideas for the future development of rotor wake method.
    2019,36(1):61-75, DOI: 10.16356/j.1005-1120.2019.01.005
    Abstract:
    An implicit higher-order discontinuous Galerkin(DG) spatial discretization for the compressible Euler equations in a rotating frame of reference is presented and applied to a rotor in hover using hexahedral grids. Instead of auxiliary methods like grid adaptation, higher-order simulations (fourth- and fifth- order accuracy) are adopted. Rigorous numerical experiments are carefully designed, conducted and analyzed. The results show generally excellent consistence with references and vigorously demonstrate the higher-order DG method’s better performance in loading distribution computations and tip vortex capturing, with much fewer degrees of freedom (DoF). Detailed investigations on the outer boundary conditions for hovering rotors are presented as well. A simple but effective speed smooth procedure is developed specially for the DG method. Further results reveal that the rarely used pressure restriction for outlet speed has a considerable advantage over the extensively adopted vertical speed restriction.
    2019,36(1):76-84, DOI: 10.16356/j.1005-1120.2019.01.006
    Abstract:
    The effects of chemical reactions in the hypersonic reacting flow are investigated using an integrated algorithm considering simultaneously two different reaction mechanisms, i.e., including the high temperature air non-equilibrium chemical reactions and the H2-air combustion reactions. The program is validated by the air non-equilibrium flow at Mach number of 25.9 with the RAM C-II configuration and the shock-induced combustion flow at Mach number of 4.512 6 around a sphere, respectively. Furthermore, the mixed reacting flow with the Mach number of 10.0 with an opposing jet of hydrogen is numerically analyzed. The results show that the program is reliable, and the effects of chemical reactions engender in the decrease of peak temperature along characteristic lines, as well as on the surface. The production of water is augmented in the region with high ratio of oxygen to hydrogen and weakened in the area with low ratio of oxygen to hydrogen by the air chemical non-equilibrium effects.
    2019,36(1):85-92, DOI: 10.16356/j.1005-1120.2019.01.007
    Abstract:
    The flow boiling heat transfer characteristics of refrigerant R134a flowing inside two different kinds of mini-channels are investigated. One channel is multi-port extruded with the hydraulic diameter of 0.63 mm, and the other one is rectangular with offset fins and a hydraulic diameter of 1.28 mm. The experiments are performed with a mass flow rate between 68 and 630 kg/(m2·s), a heat flux between 9 and 64 kW/m2, and a saturation pressure between 0.24 and 0.63 MPa, under the constant heat flux heating mode. It is found that the effect of mass flow rate on boiling heat transfer is related to heat flux, and that with the increase of heat flux, the effect can only be efficient in higher vapor quality region. The effects of heat flux and saturation pressure on boiling heat transfer are related to a threshold vapor quality, and the value will gradually decrease with the increase of heat flux or saturation pressure. Based on these analyses, a new correlation is proposed to predict the boiling heat transfer coefficient of refrigerant R134a in the mini-channels under the experimental conditions.
    2019,36(1):93-102, DOI: 10.16356/j.1005-1120.2019.01.008
    Abstract:
    A two?dimensional axisymmetric numerical simulation was successfully carried out on the muzzle flow field of a 300 mm?caliber counter?mass propelling gun. Based on the FLUENT software, using the finite volume method (FVM) and the realizable k?ε turbulence model, we adopted the holistic movement of a partitioned mesh processing method coupled with the intermediate ballistic model and the six degree?of?freedom model (6?DOF). We compared the flow field characteristics at the velocity of 1 730.4, 978.3, and 323.4 m/s. The results indicate that the pressure of the hypersonic initial flow field is much higher than that of the subsonic and supersonic initial flow fields. In the case of the subsonic (323.4 m/s) flow field, the tiny disturbance spreads throughout the whole domain. But in the cases of the supersonic (978.3 m/s) and the hypersonic (1 730.4 m/s) flow fields, it cannot spread to the upstream disturbance source, and the disturbance domain of the supersonic flow field is wider than that of the hypersonic. It is noted that the subsonic flow field has a rounded shock wave before the projectile. However, in the supersonic and hypersonic flow fields, a shear layer is formed which begins from the head of the projectile and extends outward from the side of the projectile. Then a multi?layer shock wave is formed composed of coronal shock waves, bottom shock waves, reflected shock waves, and Mach disk.
    2019,36(1):103-113, DOI: 10.16356/j.1005-1120.2019.01.009
    Abstract:
    A robust adaptive control scheme is proposed for attitude maneuver and vibration suppression of flexible spacecraft in situations where parametric uncertainties, external disturbances, unmeasured elastic vibration and input saturation constraints exist. The controller does not need the knowledge of modal variables but the estimates of modal variables provided by appropriate dynamics of the controller. The requirements to know the system parameters and the bound of the external disturbance in advance are also eliminated by adaptive updating technique. Moreover, an auxiliary design system is constructed to analyze and compensate the effect of input saturation, and the state of the auxiliary design system is applied to the procedure of control design and stability analysis. Within the framework of the Lyapunov theory, stabilization and disturbance rejection of the overall system are ensured. Finally, simulations are conducted to study the effectiveness of the proposed control scheme, and simulation results demonstrate that the precise attitude control and vibration suppression are successfully achieved.
    2019,36(1):114-123, DOI: 10.16356/j.1005-1120.2019.01.010
    Abstract:
    A self-sensing test method for the temperature of piezoelectric stack, based on the high correlation between the static capacitance and the stack temperature, is proposed in order to construct a self-sufficient methodology of temperature measurement. Firstly, a theoretical model of static capacitance of the piezoelectric stack under preload was set up, and the influence of preload on the static capacitance was analyzed. Secondly, the correctness of the model was verified by static capacitance test experiments under various preloading conditions. Finally, the temperature measurement experiments at low-temperature stage for two kinds of piezoelectric stacks, namely the low-temperature-resistant piezoelectric stack and conventional piezoelectric stack, were conducted under various preloading conditions using a polynomial fitting method. The results, which validate the accuracy of the test method, show that the maximum temperature deviations of the two kinds of piezoelectric stack are 3.9 ℃ and 2.8 ℃, respectively, when the preload force is close to the specified value. The test method uses the piezoelectric stack itself as a temperature sensor, which does not require additional equipment for temperature sensing, so that the space and equipment cost could be economized. And the test for static capacitance is concise and convenient, which indicates that in the cooling process, a concise and efficient test of the temperature of the piezoelectric stack could be realized so as to grasp the current temperature change in time.
    2019,36(1):124-132, DOI: 10.16356/j.1005-1120.2019.01.011
    Abstract:
    An active fault tolerant control scheme is investigated for the attitude control systems of spacecraft with external disturbance and actuator faults by using the sliding mode technique. Firstly, the dynamic equations and kinematic equations of spacecraft are given. For the dynamic mode of spacecraft in faulty case, a fault diagnosis component is used for fault detection and estimation by using a nonlinear observer. According to the fault estimation information obtained during the fault diagnosis, the fault tolerant control scheme is developed by adopting the backstepping sliding mode control technique. Meanwhile, the Lyapunov theory is used to analyze the stability of the closed-loop attitude systems. Finally, simulation results for the attitude dynamics models show the feasibility of the proposed fault tolerant scheme.
    2019,36(1):133-143, DOI: 10.16356/j.1005-1120.2019.01.012
    Abstract:
    A method for predicting effective thermal conductivities (ETCs) of three-dimensional five-directional (3D5D) braided composites is presented. The effective thermal conductivity prediction method contains a digital image processing technology. Multiple scanning electron microscopy (SEM) images of composites are analyzed to obtain actual microstructural features. These actual microstructural features of 3D5D braided composites are introduced into representative volume element (RVE) modeling. Apart from applying actual microstructural features, compression effects between yarns are considered in the modeling of RVE, making the RVE more realistic. Therefore, the ETC prediction method establishes a representative unit cell model that better reflects the true microstructural characteristics of the 3D5D braided composites.The ETCs are predicted with the finite element method. Then thermal conductivity measurements are carried out for a 3D5D braided composite sample. By comparing the predicted ETC with the measured thermal conductivity, the whole process of the ETC prediction method is proved to be effective and accurate, where a relative error of only 2.9 % is obtained. Furthermore, the effects of microstructural features are investigated, indicating that increasing interior braiding angles and fiber fill factor can lead to higher transverse ETCs. Longitudinal ETCs decrease with increasing interior braiding angles, but increase with increasing fiber fill factor. Finally, the influence of variations of microstructure parameters observed in digital image processing are investigated. To explore the influence of variations in microstructural features on variations in predicted ETCs, the actual probability distributions of microstructural features obtained from the 3D5D braided composite sample are introduced into the ETC investigation. The results show that, compared with the interior braiding angle, variations in the fiber fill factor exhibit more significant effects on variations in ETCs.
    2019,36(1):144-151, DOI: 10.16356/j.1005-1120.2019.01.013
    Abstract:
    In order to study the dynamic behaviors of the thermal protection system (TPS) and dynamic strength of the strain-isolation-pad (SIP), a two degree-of-freedom dynamic theoretical model is presented under the acoustic excitation and base excitation. The tile and SIP are both considered as the elastic body and simplified as a mass point, a linear spring and a damping element. The theoretical solutions are derived, and the reasonability of theoretical model is verified by comparing the theoretical results with the numerical results. Finally, the influences on the dynamic responses of TPS by the structural damping coefficient of TPS, elasticity modulus and thickness of SIP are analyzed. The results show that the material with higher damping, and SIP with thicker size and lower elastic modulus should be considered to reduce the dynamic responses and intensify the security of TPS. The researches provide a theoretical reference for studying the dynamic behaviors of TPS and the dynamic strength of SIP. Besides, the dynamic theoretical model can be used as a quick analysis tool for analyzing the dynamic responses of TPS during the initial design phase.
    2019,36(1):152-159, DOI: 10.16356/j.1005-1120.2019.01.014
    Abstract:
    A passive compliant non-cooperative target capture mechanism is designed to maintain the non-cooperative target on-orbit. When the relative position between capture mechanism and satellite is confirmed, a pair of four-bar linkages lock the docking ring, which is used for connecting the satellite and the rocket. The mathematical model of capture mechanism and capture space is built by the Denavit-Hartenberg(D-H) method, and the torque of each joint is analyzed by the Lagrange dynamic equation. Besides, the capture condition and the torque of every joint under different capture conditions are analyzed by simulation in MSC. Adams. The results indicate that the mechanism can capture the non-cooperative target satellite in a wide range. During the process of capture, the passive compliant mechanism at the bottom can increase capture space, thereby reducing the difficulty and enhance stability of the capture.
    2019,36(1):160-167, DOI: 10.16356/j.1005-1120.2019.01.015
    Abstract:
    A linear acceleration sensor, which is inspired by the human balance organ, is designed and prepared. It uses a liquid mass-block and a symmetrical-electrodes metal-core polyvinylidene fluoride fiber (SMPF) as the sensor element. The output signal of the sensor has an exponential relationship with the excitation amplitude of the impacting vibration. It is capable of detecting the amplitude and the correct frequency for sinusoidal excitations using an exponential correlation. The experiments indicate that both the output signal of the sensor and the resonance frequency increase substantially with increasing diameter of the metal core. The first-order resonance frequencies of the sensors with 40, 60, and 80 μm diameter metal wires are below 10 Hz, which is near the range of human body motion frequencies.
    2019,36(1):168-180, DOI: 10.16356/j.1005-1120.2019.01.016
    Abstract:
    In view of the collapse of a deep excavated foundation pit of the Xianghu subway underground station in Hangzhou of China, the main features of the accident are analyzed, and the induced factors of the accident are summarized. Then, a 3-D FEM analysis model is created to demonstrate the soil-support structures interaction system, and the effect of the main factors, such as the volume replacement ratio of the bottom soil reinforcing, the asymmetric ground overload, the embedded depth of the diaphragm wall, the shear strength of the bottom soils disturbed by the construction, and the excessive excavation of the bottom soil, are analyzed and compared. The results show that the ineffective original reinforcement plan for the bottom soft soil is the most prominent factor for the accident, and the disturbance effect of the deep excavation on the shear strength of the bottom soft soil is another significant factor for the accident. Meanwhile, if the reinforcement of the bottom soft soil is canceled, an appropriate extension of the diaphragm retaining walls to the under lying harder soil layer can also effectively prevent the collapse of the deep excavated foundation pit. In addition, the partly excessive excavation in the process has a great influence on the axial force of the most nearby horizontal support but few effect on the stability of the diaphragm wall. Thus, the excessive excavation of the bottom soils should not be the direct inducing factor for the accident. To the asymmetric ground overload, it should be the main factor inducing the different damage conditions of the diaphragm walls on different sides. According to the numerical modeling and actual engineering accident condition, the development process of the accident is also identified.
    2019,36(1):181-187, DOI: 10.16356/j.1005-1120.2019.01.017
    Abstract:
    In order to study discrete nonconservative system, Hamilton’s principle within fractional difference operators of Riemann-Liouville type is given. Discrete Lagrange equations of the nonconservative system as well as the nonconservative system with dynamic constraint are established within fractional difference operators of Riemann-Liouville type from the view of time scales. Firstly, time scale calculus and fractional calculus are reviewed. Secondly, with the help of the properties of time scale calculus, discrete Lagrange equation of the nonconservative system within fractional difference operators of Riemann-Liouville type is presented. Thirdly, using the Lagrange multipliers, discrete Lagrange equation of the nonconservative system with dynamic constraint is also established. Then two special cases are discussed. Finally, two examples are devoted to illustrate the results.
    2019,36(1):188-195, DOI: 10.16356/j.1005-1120.2019.01.018
    Abstract:
    With the worsening of energy crisis and environmental pollution, electric vehicles with four in-wheel motors have been paid more and more attention. The main research subject is how to reasonably distribute the driving torque of each wheel. Considering the longitudinal motion, lateral motion, yaw movement and rotation of the four wheels, the tire model and the seven DOF dynamic model of the vehicle are established in this paper. Then, the torque distribution method is proposed based on road adhesion margin, which can be divided into anti-slip control layer and torque distribution layer. The anti-slip control layer is built based on sliding mode variable structure control, whose main function is to avoid the excessive slip of wheels caused by road conditions. The torque distribution layer is responsible for selecting the torque distribution method based on road adhesion margin. The simulation results show that the proposed torque distribution method can ensure the vehicle quickly adapt to current road adhesion conditions, and improve the handling stability and dynamic performance of the vehicle in the driving process.
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