Acta Mechanica Sinica最新文献

Insensitive explosive detonation has wide applications in compressing and driving inert materials, and thereby the interaction between detonation and inert materials has received more attention. In this paper, a two-dimensional numerical simulation based on the Euler multiphase flow framework is used to investigate the reflection behavior of the insensitive explosive detonation propagating around a cylinder. The results show that there is a critical incident angle, defined as transition angle for detonation propagating around the cylinder, below which the regular reflection (RR) on the cylinder surface is observed. When the incident angle is greater than the transition angle, RR changes to Mach reflection. This transition angle is larger than that obtained by polar curve theory and the change of incident angle is used to interpret above phenomenon. In addition, the influence of cylindrical radius and detonation reaction zone width on the reflection behavior is examined. As the cylindrical radius increases, the height of Mach stem increases while the transition angle decreases and gradually approaches the value in pole curve theory. Von Neumann reflection is observed when the reaction zone width is relatively small. This is because the energy release rate in the reaction zone is high for small reaction zone width, resulting in the formation of a series of compression waves near the cylindrical interface.

Overrunning clutches are unidirectional drive mechanisms that are widely used in transmission systems. However, existing overrunning clutches have complex structures, require high preparation accuracy, and fail after a certain degree of wear. To address these issues, we propose a new type of overrunning clutch consisting of a conical structure and novel compression-torsion conversion (CTC) metamaterial with curved plates. Theoretical calculations are employed to guide the material distribution and ensure the deformation coordination of the curved-plate CTC metamaterial for greater ultimate torque. The transmission mechanism of the proposed overrunning clutch is derived to guide the parameter selection of the CTC metamaterial and the conical structure. Experiments and finite element simulations reveal that the curved-plate CTC metamaterial features excellent CTC efficiency, flexibility, and transverse stiffness, which is conducive reducing the resistance of the overrunning state and ensures stability during operation. The unidirectional transmission system constructed with the new overrunning clutch shows reliable performances under working and overrunning states. The constructed overrunning clutch provides an effective one-way transmission method. The clutch with simple construction and self-compensated ability for wear exhibits great potential in miniaturized and lightweight equipment or robots.

A two-dimensional panel in subsonic flow with stochastic excitation is studied by assuming that the aerodynamic pressure contains random pressure fluctuations. Based on the global properties, the sensitivities of system parameters and noise intensities are presented. Firstly, the parameter region with multiple coexisting attractors under different dynamic pressures is obtained. It is found that the coexistence of multiple attractors extensively appears and the basin structure may be complex. Then the periodic time history diagrams are calculated by simulating the random pressure fluctuation as Poisson white noise. The results show that under typical bistable conditions, the noise sensitivity of the subsonic panel system is related to the basin structures and the disposition of the coexisting attractors to the saddle. The transition between two attractors diffuses along the unstable manifold and tends to the position where the basin boundary curvature is relatively large. The findings underscore the importance of global analysis in assessing the noise load carrying capacity, which provides some valuable insights into the safety design of subsonic panel systems.

It is well known that coarse-grained super-elastic NiTi shape memory alloys (SMAs) exhibit localized rather than homogeneous martensite transformation (MT), which, however, can be strongly influenced by either internal size (grain size, GS) or the external size (geometric size). The coupled effect of GS and geometric size on the functional properties has not been clearly understood yet. In this work, the super-elasticity, one-way, and stress-assisted two-way shape memory effects of the polycrystalline NiTi SMAs with different aspect ratios (length/width for the gauge section) and different GSs are investigated based on the phase field method. The coupled effect of the aspect ratio and GS on the functional properties is adequately revealed. The simulated results indicate that when the aspect ratio is lower than about 4:1, the stress biaxiality and stress heterogeneity in the gauge section of the sample become more and more obvious with decreasing the aspect ratio, which can significantly influence the microstructure evolution in the process involving external stress. Therefore, the corresponding functional property is strongly dependent on the aspect ratio. With decreasing the GS and the aspect ratio (to be lower than 4:1), both the aspect ratio and GS can affect the MT or martensite reorientation in each grain and the interaction among grains. Thus, due to the strong internal constraint (i.e., the constraint of grain boundary) and the external constraint (i.e., the constraint of geometric boundary), the capabilities of the functional properties of NiTi SMAs are gradually weakened and highly dependent on these two factors.

The prime objective of this work is to analyze the motion of magnetic domain walls (DWs) in a thin layer of magnetostrictive material that is perfectly attached to the upper surface of a thick piezoelectric actuator. In our analysis, we consider a transversely isotropic hexagonal subclass of magnetostrictive materials that demonstrate structural inversion asymmetry. To this aim, we utilize the one-dimensional extended Landau-Lifshitz-Gilbert equations, which describe the magnetization dynamics under the influence of various factors such as magnetic fields, spin-polarized electric currents, magnetoelastic effects, magnetocrystalline anisotropy, Rashba fields, and nonlinear dry-friction dissipation. By employing the standard traveling wave ansatz, we derive an analytical expression of the most relevant dynamic features: velocity, mobility, threshold, breakdown, and propagation direction of the DWs in both steady and precessional dynamic regimes. Our analytical investigation provides insights into how effectively the considered parameters can control the DW motion. Finally, numerical illustrations of the obtained analytical results show a qualitative agreement with the recent observations.

The primary impediments impeding the implementation of high-order methods in simulating viscous flow over complex configurations are robustness and convergence. These challenges impose significant constraints on computational efficiency, particularly in the domain of engineering applications. To address these concerns, this paper proposes a robust implicit high-order discontinuous Galerkin (DG) method for solving compressible Navier-Stokes (NS) equations on arbitrary grids. The method achieves a favorable equilibrium between computational stability and efficiency. To solve the linear system, an exact Jacobian matrix solving strategy is employed for preconditioning and matrix-vector generation in the generalized minimal residual (GMRES) method. This approach mitigates numerical errors in Jacobian solution during implicit calculations and facilitates the implementation of an adaptive Courant-Friedrichs-Lewy (CFL) number increasing strategy, with the aim of improving convergence and robustness. To further enhance the applicability of the proposed method for intricate grid distortions, all simulations are performed in the reference domain. This practice significantly improves the reversibility of the mass matrix in implicit calculations. A comprehensive analysis of various parameters influencing computational stability and efficiency is conducted, including CFL number, Krylov subspace size, and GMRES convergence criteria. The computed results from a series of numerical test cases demonstrate the promising results achieved by combining the DG method, GMRES solver, exact Jacobian matrix, adaptive CFL number, and reference domain calculations in terms of robustness, convergence, and accuracy. These analysis results can serve as a reference for implicit computation in high-order calculations.

The measurement of wing dynamic deformation in morphing aircraft is crucial for achieving closed-loop control and evaluating structural safety. For variable-sweep wings with active large deformation, this paper proposes a novel videogrammetric method for full-field dynamic deformation measurement. A stereo matching method based on epipolar geometry constraint and topological constraint is presented to find the corresponding targets between stereo images. In addition, a new method based on affine transformation combined with adjacent closest point matching is developed, aiming to achieve fast and automatic tracking of targets in time-series images with large deformation. A calculation model for dynamic deformation parameters is established to obtain the displacement, sweep variable angle, and span variation. To verify the proposed method, a dynamic deformation measurement experiment is conducted on a variable-sweep wing model. The results indicate that the actual accuracy of the proposed method is approximately 0.02% of the measured area (e.g., 0.32 mm in a 1.6 m scale). During one morphing course, the sweep variable angle, the span variation and the displacement increase gradually, and then decrease. The maximum sweep variable angle is 36.6°, and the span variation is up to 101.13 mm. The overall configuration of the wing surface is effectively reconstructed under different morphing states.

The behaviors of unsteady flow structures and corresponding hydrodynamics for a pitching hydrofoil are investigated numerically and theoretically in the present paper. The aims are to derive the total lift by finite-domain impulse theory for sub-cavitating flow (σ = 8.0) and cavitating flow (σ = 3.0), and to quantify the distinct impact of individual vortex structures on the transient lift to appreciate the interplay among cavitation, flow structures, and vortex dynamics. The motion of the hydrofoil is set to pitch up clockwise with an almost constant rate from 0° to 15° and then back to 0°, for the Reynolds number, 7.5 × 10⁵, and the frequency, 0.2 Hz, respectively. The results reveal that the presence of cavities delays the migration of the laminar separation bubble (LSB) from the trailing edge (TE) to the leading edge (LE), consequently postponing the hysteresis in the inflection of lift coefficients. The eventual stall under the sub-cavitation regime is the result of LSB bursting. While the instabilities within the leading-edge LSB induce the convection of cavitation-dominated vortices under the cavitation regime instead. Having validated the lift coefficients on the hydrofoil through the finite-domain impulse theory using the standard force expression, the Lamb vector integral emerges as the main contribution to the generation of unsteady lift. Moreover, the typical vortices’ contributions to the transient lift during dynamic stall are accurately quantified. The analysis indicates that the clockwise leading-edge vortex (−LEV) contributes positively, while the counterclockwise trailing-edge vortex (+TEV) contributes negatively. The negative influence becomes particularly pronounced after reaching the peak of total lift, as the shedding of the concentrated wake vortex precipitates a sharp decline due to a predominant negative lift contribution from the TEV region. Generally, the vortices’ contribution is relatively modest in sub-cavitating flow, but it is notably more significant in the context of incipient cavitating flow.

Convection driven by a spatially non-uniform internal heat source between two horizontal isothermal walls is studied by theoretical analysis and numerical simulation, in order to explore the bounds of the temperature and the vertical heat flux. Specifically, the rigorous lower bound of the weighted average temperature ⟨QT⟩ is derived analytically, by decomposing the temperature field into a background profile and a fluctuation part. This bound obtained for the first time to consider non-uniform heat sources is found to be compatible with the existing bound obtained in uniform internal heat convection. Of physical importance, an analytical relationship is derived as an inequality connecting ⟨QT⟩ and the average vertical heat flux ⟨wT⟩, by employing the average heat flux on the bottom wall (q_b) as an intermediary variable. It clarifies the intrinsic relation between the lower bound of ⟨QT⟩ and the upper bound of ⟨wT⟩, namely, these two bounds are essentially equivalent providing an easy way to obtain one from another. Furthermore, the analytical bounds are extensively demonstrated through a comprehensive series of direct numerical simulations.

In this work, the spray behaviors of a rotary atomizer with round-shaped injection orifices are experimentally investigated to study the breakup mechanism and spray characteristics using RP-3 as the liquid fuel. The breakup process of the liquid is visualized by the backlight shadow imaging method, which also provides the measurements of liquid breakup length and penetration height. The injection mode of the liquid film is observed using the front-light illumination method. The droplet size and distribution are measured using the laser particle size analyzer at various radial locations. Three typical breakup modes are identified: the ligament breakup mode, bag breakup mode, and shear breakup mode. Aerodynamic Weber number (We_d) and momentum flux ratio (q) are used to elaborate the liquid breakup regimes. Results of droplet sizing indicate that the Sauter mean diameter decreases with a higher rotational speed and slightly varies with the volume flow rate. A correlation between liquid breakup modes and non-dimensional droplet size is established based on We_d and q. This study presents some significance for understanding the impacts of the rotational speed and volume flow rates on the spray performance of actual aviation fuels in rotary atomizers.