Chinese Journal of Aeronautics最新文献

Space truss structures are essential components for space-based remote sensing loads with high spatial and temporal resolutions. To achieve high-precision vibration control, an accurate and efficient dynamics model is essential. In addition to the current equivalent beam model (EBM) based on the classical continuum theory, an improved equivalent beam model (IEBM) is proposed that considers the impact of the distinction between trusses and beams on torsional and shear deformations, as well as the impact of shear deformation on flexural rigidity. According to the displacement expressions of spatial beams, torsional, shear, and bending correction coefficients are introduced to derive expressions of strain energy and kinetic energy. The energy equivalence principle is then utilized to calculate the elasticity and inertia matrices, and dynamics equations are established using the finite element method. Subsequently, an IEBM is constructed by employing the particle swarm optimization approach to determine the correction coefficients with the truss natural frequency as the optimization target. The natural vibration characteristics of the structure are estimated for various material properties. Compared with the full-scale finite element model, the EBM reaches a maximum error of 80% for a low modulus of elasticity, while the maximum error of the IEBM is less than 2% for any given parameters, indicating its superior accuracy to the EBM.

The internal friction of floating spline can cause self-excited vibration of a supercritical flexible rotor system. To address this issue, a high-efficiency dynamic modeling method is proposed to investigate the self-excited vibration behavior and instability evolution of the rotor. Experiments are conducted to validate the theoretical results. The coupled dynamic equations for the rotor system connected with the floating spline are derived through the combination of finite element method and lumped parameter model. A hybrid numerical approach of precise integration and Runge-Kutta method is adopted to examine the effects of the friction coefficient of spline’s tooth surface, torque, and eccentricity on the self-excited vibration of the rotor system. The results show that the spline friction leads to negative damping and inputs energy into the rotor system under supercritical conditions, triggering self-excited vibration when the input energy exceeds a specific level. With the same parameters, the experimentally obtained axial trajectory and primary frequency components are consistent with the theoretical results, verifying the accuracy of the proposed theoretical model. This study can serve as a useful theoretical guide for the dynamic stability design of flexible rotor systems with the floating spline.

For the second-order finite volume method, implicit schemes and reconstruction methods are two main algorithms which influence the robustness and efficiency of the numerical simulations of compressible turbulent flows. In this paper, a compact least-squares reconstruction method is proposed to calculate the gradients for the distribution of flow field variables approximation. The compactness of the new reconstruction method is reflected in the gradient calculation process. The geometries of the face-neighboring elements are no longer utilized, and the weighted average values at the centroid of the interfaces are used to calculate the gradients instead of the values at the centroid of the face-neighboring elements. Meanwhile, an exact Jacobian solving strategy is developed for implicit temporal discretization. The accurate processing of Jacobian matrix can extensively improve the invertibility of the Jacobian matrix and avoid introducing extra numerical errors. In addition, a modified Venkatakrishnan limiter is applied to deal with the shock which may appear in transonic flows and the applicability of the mentioned methods is enhanced further. The combination of the proposed methods makes the numerical simulations of turbulent flow converge rapidly and steadily with an adaptive increasing CFL number. The numerical results of several benchmarks indicate that the proposed methods perform well in terms of robustness, efficiency and accuracy, and have good application potential in turbulent flow simulations of complex configurations.

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.

Scallop ice is a special phenomenon that occurs during swept wing aircraft passing through icing clouds. It poses a great challenge for the icing safety assessment that the complex scallop ice shape feature and its mechanism are still unclear. In this work, a large-scale icing wind tunnel experiment of swept wing designed by NACA0012 airfoil is conducted in the Icing Wind Tunnel of China Aerodynamics Research and Development Center. The detailed three-dimensional ice shapes under 0°, 15°, 30° and 45° swept angles are obtained by laser scanning technology. The experimental results show that with the swept angle increasing from 0° to 45°, the 2D double ice horn structures show certain spanwise variation, and finally transform into complete scallop ice with ice thickness greatly enhanced in the stagnation line region. The empirical mode decomposition of the spanwise ice curve captures the high-frequency fluctuation on the scallop ice caused by the small-scale roughness element, while the trend with low frequency is not obvious. Based on the experimental data, a new complete scallop ice geometric model, named 5Points-5Lines-2Arcs (5P-5L-2A) model, is proposed, which can provide important basis for the quantitative description of complex scallop ice shape.

Carbon fiber reinforced silicon carbide (C_f/SiC) composites are widely used in aerospace for their excellent mechanical properties. However, the quality of the machined surface is poor and unpredictable due to the material heterogeneity induced by complex removal mechanism. To clarify the effects of fiber orientation on the grinding characteristics and removal mechanism, single grit scratch experiments under different fiber orientations are conducted and a three-phase numerical modelling method for 2.5D C_f/SiC composites is proposed. Three fiber cutting modes i.e., transverse, normal and longitudinal, are defined by fiber orientation and three machining directions i.e., MA (longitudinal and normal), MB (longitudinal and transverse) and MC (normal and transverse), are selected to investigate the effect of fiber orientation on grinding force and micro-morphology. Besides, a three-phase cutting model of 2.5D C_f/SiC composites considering the mechanical properties of the matrix, fiber and interface is developed. Corresponding simulations are performed to reveal the micro-mechanism of crack initiation and extension as well as the material removal mechanism under different fiber orientations. The results indicate that the scratching forces fluctuate periodically, and the order of mean forces is MA > MC > MB. Cracks tend to grow along the fiber axis, which results in the largest damage layer for transverse fibers and the smallest for longitudinal fibers. The removal modes of transverse fibers are worn, fracture and peel-off, in which normal fibers are pullout and outcrop and the longitudinal fibers are worn and push-off. Under the stable cutting condition, the change of contact area between fiber and grit leads to different removal modes of fiber in the same cutting mode, and the increase of contact area results in the aggravation of fiber fracture.