Acta Mechanica Sinica最新文献

The lamellar microstructure is one of the most typical microstructures of TiAl alloys. There are three γ/γ interfaces with different microstructures in lamellar γ-TiAl alloys. In this work, we investigated the deformation processes of lamellar γ-TiAl alloys with different interfacial spacing (λ) via uniaxial tensile loading using molecular dynamics simulations, including true twin (TT), pseudo-twin (PT), rotational boundary (RB), and the mixed structure (TT ∥ PT ∥ RB). The results show that in all lamellar γ-TiAl samples, the Shockley partial dislocation prefers to nucleate in the region between two neighboring interfaces. Then, dislocations move towards, crossing the γ/γ interface. Finally, the dislocation slippage leads to the destruction of the interface, resulting in cracks and structural failure. With the decrease of λ, the ultimate strength slightly increases in the TT or PT structure of γ-TiAl, which follows the Hall-Petch relation. But in general, the interfacial spacing has a slight effect on the ultimate strengths of these four structures of γ-TiAl.

This work investigates the dynamic response of a monocrystalline nickel-titanium (NiTi) alloy at the atomic scale. The results deduced from non-equilibrium molecular dynamics modeling demonstrate that no shear deformation band (SDB) appears in the sample at an impact velocity of less than 0.75 km/s. As this velocity increases, shear deformations become pronouncedly localized, and the average spacing between SDBs decreases until it stabilizes. Combining shear stress and particle velocity profiles, the survival of SDBs is found to be closely associated with plastic deformation. The dislocations clustering around SDBs predominantly exhibit 〈100〉 partial dislocations, whereas 1/2〈111〉 full dislocations are dominant in those regions without SDBs. Void nucleation always occurs on SDBs. Subsequently, void growth promotes a change in the SDB distribution characteristic. For the case without SDB, voids are randomly nucleated, and the void growth exhibits a non-uniform manner. Thus, there is an interaction between shear localization and void evolution in the NiTi alloy subjected to intensive loading. This study is expected to provide in-depth insights into the microscopic mechanism of NiTi dynamic damage.

Nanomaterials have garnered recognition for their notable surface effects and demonstration of superior mechanical properties. Previous studies on the surface effects of nanomaterials, employing the finite element method, often relied on simplified two-dimensional models due to theoretical complexities. Consequently, these simplified models inadequately represent the mechanical properties of nanomaterials and fail to capture the substantial impact of surface effects, particularly the curvature dependence of nanosurfaces. This study applies the principle of minimum energy and leverages the Steigmann-Ogden surface theory of nanomaterials to formulate a novel finite element surface element that comprehensively accounts for surface effects. We conducted an analysis of the stress distribution and deformation characteristics of four typical 2D and 3D nanomaterial models. The accuracy of the developed surface element and finite element calculation method was verified through comparison with established references. The resulting finite element model provides a robust and compelling scientific approach for accurately predicting the mechanical performance of nanomaterials.

Periodic isolator is well known for its wave filtering characteristic. While in middle and high frequencies, the internal resonances of the periodic isolator are evident especially when damping is small. This study proposes a novel aperiodic vibration isolation for improving the internal resonances control of the periodic isolator. The mechanism of the internal resonances control by the aperiodic isolator is firstly explained. For comparing the internal resonances suppression effect of the aperiodic isolator with the periodic isolator, a dynamic model combing the rigid machine, the isolator, and the flexible plate is derived through multi subsystem modeling method and transfer matrix method, whose accuracy is verified through the finite element method. The influences of the aperiodicity and damping of the isolator on the vibration isolation performance and internal resonances suppression effect are investigated by numerical analysis. The numerical results demonstrate that vibration attenuation performances of the periodic isolator and aperiodic isolator are greatly over than that of the continuous isolator in middle and high frequencies. The aperiodic isolator opens the stop bandgaps comparing with the periodic isolator where the pass bandgaps are periodically existed. The damping of the isolator has the stop bandgap widening effect on both the periodic isolator and the aperiodic isolator. In addition, a parameter optimization algorithm of the aperiodic isolator is presented for improving the internal resonances control effect. It is shown that the vibration peaks within the target frequency band of the aperiodic isolator are effectively reduced after the optimization. Finally, the experiments of the three different vibration isolation systems are conducted for verifying the analysis work.

We propose a hybrid quantum-classical method, the quantum-enriched large-eddy simulation (QELES), for simulating turbulence. The QELES combines the large-scale motion of the large-eddy simulation (LES) and the subgrid motion of the incompressible Schrödinger flow (ISF). The ISF is a possible way to be simulated on a quantum computer, and it generates subgrid scale turbulent structures to enrich the LES field. The enriched LES field can be further used in turbulent combustion and multi-phase flows in which the subgrid scale motion plays an important role. As a conceptual study, we perform the simulations of ISF and LES separately on a classical computer to simulate decaying homogeneous isotropic turbulence. Then, the QELES velocity is obtained by the time matching and the spectral blending methods. The QELES achieves significant improvement in predicting the energy spectrum, probability density functions of velocity and vorticity components, and velocity structure functions, and reconstructs coherent small-scales vortices in the direct numerical simulation (DNS). On the other hand, the vortices in the QELES are less elongated and tangled than those in the DNS, and the magnitude of the third-order structure function in the QELES is less than that in the DNS, due to the different constitutive relations in the viscous flow and ISF.

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.