Acta Mechanica Sinica最新文献

At low-Reynolds-number, the performance of airfoil is known to be greatly affected by the formation and burst of a laminar separation bubble (LSB), which requires a more precise simulation of the delicate flow structures. A framework based on the interior penalty discontinuous Galerkin method and large eddy simulation approach was adopted in the present study. The performances of various subgrid models, including the Smagorinsky (SM) model, the dynamic Smagorinsky (DSM) model, the wall-adapting local-eddy-viscosity (WALE) model, and the VREMAN model, have been analyzed through flow simulations of the SD7003 airfoil at a Reynolds number of 60000. It turns out that the SM model fails to predict the emergence of LSB, even modified by the Van-Driest damping function. On the contrary, the best agreement is generally achieved by the WALE model in terms of flow separation, reattachment, and transition locations, together with the aerodynamic loads. Furthermore, the influence of numerical dissipation has also been discussed through the comparison of skin friction and resolved Reynolds stresses. As numerical dissipation decreases, the prediction accuracy of the WALE model degrades. Meanwhile, nonlinear variation could be observed from the performances of the DSM model, which could be attributed to the interaction between the numerical dissipation and the subgrid model.

To meet the demand for air-breathing power for wide-range vehicles at Mach 0–10, two thermal cycles with ammonia as the fuel and coolant were analyzed, namely the precooled rocket-turbine cycle (PC-RT) and the precooled gas-turbine cycle. Firstly, the operating modes of the precooled cycle engines were divided into turbine mode, precooling mode, and ramjet mode. Secondly, a fluid-structure coupling heat transfer program was used to evaluate the cooling effects of different fuels on the incoming high-temperature air. The result shows that the equivalent heat sink of ammonia is higher than that of other fuels and can meet the cooling requirement of at least Mach 4 in the precooling mode. Thirdly, the performance of the PC-RT in the turbine and precooling modes was compared at Mach 2.5. The result shows that air precooling alleviates the restriction of the pumping pressure on the minimum required β and improves the specific thrust within a reasonable range of β. Fourthly, the performance of the precooled cycle engines was compared when using different fuels. The result shows that the specific thrust of ammonia is greater than that of other fuels, and the performance advantages of ammonia are the most obvious in the precooling mode due to its highest equivalent heat sink. To sum up, the precooled cycle engines with ammonia as the fuel and coolant presented in this study have the advantages of no carbon emissions, low cost, high specific thrust, and no clogging of the cooling channels by cracking products. They are suitable for applications such as the first-stage power of the two-stage vehicle, and high Mach numbers air-breathing flight.

We propose a suite of strategies for the parallel solution of fully implicit monolithic fluid-structure interaction (FSI). The solver is based on a modeling approach that uses the velocity and pressure as the primitive variables, which offers a bridge between computational fluid dynamics (CFD) and computational structural dynamics. The spatiotemporal discretization leverages the variational multiscale formulation and the generalized-α method as a means of providing a robust discrete scheme. In particular, the time integration scheme does not suffer from the overshoot phenomenon and optimally dissipates high-frequency spurious modes in both subproblems of FSI. Based on the chosen fully implicit scheme, we systematically develop a combined suite of nonlinear and linear solver strategies. Invoking a block factorization of the Jacobian matrix, the Newton-Raphson procedure is reduced to solving two smaller linear systems in the multi-corrector stage. The first is of the elliptic type, indicating that the algebraic multigrid method serves as a well-suited option. The second exhibits a two-by-two block structure that is analogous to the system arising in CFD. Inspired by prior studies, the additive Schwarz domain decomposition method and the block-factorization-based preconditioners are invoked to address the linear problem. Since the number of unknowns matches in both subdomains, it is straightforward to balance loads when parallelizing the algorithm for distributed-memory architectures. We use two representative FSI benchmarks to demonstrate the robustness, efficiency, and scalability of the overall FSI solver framework. In particular, it is found that the developed FSI solver is comparable to the CFD solver in several aspects, including fixed-size and isogranular scalability as well as robustness.

In this paper, an incremental contact model is developed for the elastic self-affine fractal rough surfaces under plane strain condition. The contact between a rough surface and a rigid plane is simplified by the accumulation of identical line contacts with half-width given by the truncated area divided by the contact patch number at varying heights. Based on the contact stiffness of two-dimensional flat punch, the total stiffness of rough surface is estimated, and then the normal load is calculated by an incremental method. For various rough surfaces, the approximately linear load-area relationships predicted by the proposed model agree well with the results of finite element simulations. It is found that the real average contact pressure depends significantly on profile properties.

The China-type cubic press (CCP) is widely used in the high-pressure field because of its simple operation and low cost. However, the low ultimate pressure inside the cavity of CCP has limited its application. In order to improve the ultimate pressure of the cavity, this paper simulates the pressure transfer efficiency and the Von Mises stress (VMS) of the tungsten carbide (WC) anvil. We find that the effect of the pretightening force of the WC anvil can be changed by changing the angle of the steel supporting ring. When the angle of the steel supporting ring is 1.2°, the pretightening force of the WC anvil is the most uniform, and the support effect of the WC anvil is the best. At the same time, this paper designs a double-beveled WC (D-WC) anvil. The D-WC anvil can not only improve the ultimate pressure of the cavity, but also ensure the stability of the cavity and the durability of the WC anvil. The design in this paper can also be used with the first-stage pressurization assembly to achieve better pressurization effect.

The extreme conditions severely constrain the dynamic characteristics of aircraft landing gear retraction mechanism (ALGRM). This paper proposes a dynamic modeling and analysis method for ALGRM considering the coupling effects of extreme conditions such as clearance joints, flexible rods, and salt spray corrosion. Firstly, the mathematical model for clearance joint and flexible rod is established and the dynamic model of ALGRM considering clearance joints and flexible rods is formulated based on Lagrangian equation. Furthermore, the salt spray corrosion model for clearance joint is developed using COMSOL simulation software. Finally, the effects of different temperatures and relative humidities on the corrosion depth of clearance joint and the dynamic characteristics of ALGRM under the coupling effects of extreme conditions are investigated. The results have found that the impact of extreme conditions on dynamics of system cannot be ignored. This study not only provides a theoretical foundation for predicting the dynamic characteristics of ALGRM under extreme conditions but also offers insights for the optimization design and corrosion protection efforts of landing gear.

The consistency of the dynamic behavior of the mechanical equivalent model of high-speed pantographs with that of actual high-speed pantographs under service conditions is crucial for the correctness and accuracy of the numerical simulation results of the pantograph-catenary interaction. Firstly, based on experimental data and the finite element method, models of a mass-point frame and reduced pan head were established, which can simulate the rolling and pitching motion characteristics of the dual-strip pan head. The correctness of the finite element numerical simulation of the pantograph-catenary system based on the model was verified through an industry standard and line tests. Then, the variation law of the standard deviation of the contact force (SDCF) in the speed range of 240–450 km/h was studied, and the mechanism of large fluctuation in SDCF was explained from the perspective of pantograph-catenary resonance. Finally, the influence of pan head degree of freedom and low-pass filtering frequency of the contact force time-domain signal on SDCF was studied, and the applicable speed level of the traditional three-mass model and 20 Hz filtering were provided.

With the increasing and refined applications of silicone rubber devices in the biomedical field, it is of great significance to accurately describe and predict the mechanical behavior of them under large deformation. This paper finds that after considering the influence of higher-order shear strain on the normal stress, the Poynting effect in ribbed silicone rubber tubes with certain cross-sectional shapes exhibits a new phenomenon—a non-monotonic trend between axial deformation and twist angle. This paper develops a nonlinear finite element program for simulating large deformations of hyperelastic materials, and studies the Poynting effect in ribbed circular tubes of twisted silicone rubber. The results show that in the ribbed circular tubes with a porosity between 12% and 40% (with the number of ribs ranging from 12 to 26), there appears a normal to reverse conversion of the Poynting effect, that is, the axial extension ratio first decreases and then increases during a monotonic loading process, indicating that the influence of higher-order shear strain on normal stress cannot be ignored when the cross-sectional shape is complex. Especially in ribbed circular tubes with about 20% porosity, a substantial change of axial normal strain from −0.035% to 0.035% can be achieved within a twist angle range of 180°. Based on this, the quantitative influence of higherorder shear strain on normal stress is studied. These research results provide a theoretical basis for accurately controlling the axial expansion and contraction of twisted parts and indicate that a normal to reverse conversion of the Poynting effect can be implemented by designing the cross-sectional shape under certain conditions.

Malignant ureteral obstruction may lead to renal function damage, renal colic, and infection. The impact of obstructive development on ureteral peristalsis was rarely studied, which requires further investigation. This study used theoretical biomechanical methods to study the motion characteristics of the ureteral wall and obtained the radial motion equation of the ureteral wall. The motion equation was solved by 4–5th order Runge Kutta method. Analyze the motion equation of the ureteral wall, derive the expression for malignant obstructive ureteral pressure, as well as the analytical expressions for radial displacement and circumferential stress of the ureteral wall. By analyzing the radial motion equation of the ureter, it can be found that peristalsis is influenced by the pressure difference between inside and outside. The analytical solutions for radial displacement and stress contained exponential terms. Under the condition of 50% obstruction, the displacement and stress of the ureter were reduced by 90.53% and 81.10%, respectively. This study established the radial motion equation of the ureter and provided analytical solutions for the radial displacement and stress of the obstructed ureter. Based on the radial motion equation of the ureter, the radial motion characteristics of the ureteral wall were explored, including peristalsis and disappearance of peristalsis. This study provided a quantitative relationship between ureteral obstruction and peristalsis. As the degree of obstruction increased, ureteral peristalsis gradually weakened or even disappeared.

This paper presents a deep learning-based topology optimization method for the joint design of material layout and fiber orientation in continuous fiber-reinforced composite structure (CFRCS). The proposed method mainly includes three steps: (1) a ResUNet-involved generative and adversarial network (ResUNet-GAN) is developed to establish the end-to-end mapping from structural design parameters to fiber-reinforced composite optimized structure, and a fiber orientation chromatogram is presented to represent continuous fiber angles; (2) to avoid the local optimum problem, the independent continuous mapping method (ICM method) considering the improved principal stress orientation interpolated continuous fiber angle optimization (PSO-CFAO) strategy is utilized to construct CFRCS topology optimization dataset; (3) the well-trained ResUNet-GAN is deployed to design the optimal structural material distribution together with the corresponding continuous fiber orientations. Numerical simulations for benchmark structure verify that the proposed method greatly improves the design efficiency of CFRCS along with high design accuracy. Furthermore, the CFRCS topology configuration designed by ResUNet-GAN is fabricated by additive manufacturing. Compression experiments of the specimens show that both the stiffness structure and peak load of the CFRCS topology configuration designed by the proposed method have significantly enhanced. The proposed deep learning-based topology optimization method will provide great flexibility in CFRCS for engineering applications.