Acta Mechanica Sinica最新文献

In this paper, the load characteristics of shock wave, bubble pulsating water jet and cavitation closure are studied by carrying out underwater explosion experiments and numerical simulation of fixed square plates with 2.5, 5 and 10 g trinitrotoluene. The results show that under the combined action of multiple loads, the impulse of bubble pulsation and water jet load plays a leading role in the process of underwater explosion, and the impulse of cavitation closure load is greater than that of shock wave. The damage to the structure cannot be ignored, and the pressure time-history curve presents a “multi-peak” state, and it is pointed out that the water jet is a concentrated load. Then, the dynamic response of the full-scale model of the ship under the combined action of multiple loads is studied, and the dynamic response of the ship under different cabin water depths and different explosion distances is discussed. The results show that when the ship is empty, the damage degree of the ship is the most serious, and the influence of cavitation effect on the half cabin is weaker than that of the empty cabin, so the damage degree is the second, and the damage degree is the smallest when the cabin is full. When the distance parameter is less than 0.68, the shock wave and the after flow play a leading role in the dynamic response of the ship. When the distance parameter is between 0.68 and 1.38, the combined action of the bubble pulsating water jet and the cavitation closure multi-load causes the main damage to the ship. When the distance parameter is greater than 1.38, the bubble pulsation and the cavitation closure load play a leading role.

Central pattern generators (CPGs) are neural circuits which are found in both invertebrates and vertebrates, and are capable of generating rhythmic patterns of neural activity without receiving any rhythmic input. The mechanism by which CPG controls alternating and turning movements in vertebrates has not yet been fully understood. In this paper, a new CPG model for rat hindlimbs is proposed to unravel the above mechanism based on recent physiological experimental results. The model is obtained by adding commissural interneurons (CINs, including V0, V3, and CINi populations) and chx10 Gi neurons into Deng’s CPG model. Based on our proposed CPG model, different alternating gaits in the rat hindlimb model can be achieved by ablating different populations of commissural interneurons, which is consistent with physiological experimental findings. Then, turning movements are studied by applying unilateral excitatory and inhibitory stimuli to chx10 Gi neurons which are connected to the flexion side of the CPG with inhibitory synapses. It is found that unilateral activation leads to turning towards the same side, while unilateral inhibition results in turning towards the opposite side, which is also consistent with the results in physiological experiments. Finally, the ion-channel control mechanism of CPG burst rhythms is investigated, and it is found that the CPG burst rhythm is most sensitive to sodium ion channels, with a significantly greater impact than the influence of reciprocal inhibition on the rhythm. The proposed CPG model provides a new perspective for understanding motor neural mechanism of vertebrates, and also could be adopted in motion control of hindlimb robots.

This study examines the hemodynamics of non-Newtonian blood flow in stenosed arteries, focusing on the roles of stenosis severity, guidewire presence, and various non-Newtonian constitutive models. Computational simulations using the generalized power-law, Casson, and Carreau-Yasuda models are conducted for stenosis severities of 50%, 70%, 80%, and 90%. Results indicate that stenosis severity exerts the greatest influence on pressure and wall shear stress (WSS), with increasing severity leading to higher pressure drops and WSS maxima. Guidewire presence reduces recirculation zone lengths by nearly 60% across different severities and raises the trans-stenotic pressure drop up to 120%. While the choice of constitutive model has minimal impact on hemodynamics within the stenotic region, it becomes crucial in healthy vessels, where non-Newtonian effects are more pronounced. In cases with a guidewire, pressure gradients in the healthy region show up to 18.8% differences between non-Newtonian models. These findings highlight the dominant roles of stenosis severity and guidewire presence in shaping hemodynamics within stenotic regions while emphasizing the need for precise constitutive modeling to capture flow characteristics in healthy vascular segments.

As a clean energy source, microwaves have significant potential to enhance the efficiency of breaking hard rock for mechanical excavation in tunnelling engineering. In this study, the variable angle shear tests were carried out on granite specimens after microwave irradiation. Two-dimensional digital image correlation (2D-DIC) and acoustic emission (AE) technologies were used to monitor the mechanical behavior change during loading. The results indicate as the microwave heating time increased from 0 to 5 min and then to 10 min, the cohesion of the granite specimen decreased from 32.603 to 29.355 MPa and then to 24.666 MPa. But there is only a slight increase in the internal friction angle. As the shear angle and microwave irradiation duration increase, the cumulative AE counts and AE energy significantly decrease, indicating that microwaves can effectively reduce the energy needed in hard rock breaking. The change in AE b-value during loading can be divided into five stages: a rapid rise period, a stable fluctuation period, a rapid downward period, a small rising period, and a final downward period. Notably, the small rising period of the b-value between two rapid downward periods can be used as a precursor to failure. However, microwave irradiation can shorten or even eliminate this period. As observed through 2D-DIC, the strain evolution and crack propagation process of granite specimens are significantly influenced by both the shear angle and microwave irradiation. Decreasing shear angle and increasing heating time leads to a more dispersed distribution of the maximum principal strain concentration zone during loading, and the final failure pattern of the sample becomes more complex. Through the quantitative calculation of the normal force on the hob, it is found that microwave pretreatment can effectively enhance excavation rates. The maximum penetration increased to 15.67 mm/r for the specimens after 10 min irradiation, representing a 46.18% improvement compared to untreated specimens.

During the past decades, the numerical methods based on Navier-Stokes (N-S) equations and direct simulation Monte Carlo (DSMC) methods have been proven effective in simulating flows in the continuum and rarefied regimes, respectively. However, as single-scale methods, they face challenges in addressing common multi-scale problems, which are essential to simulate hypersonic flows around near-space vehicles and the flows in the micro-electro-mechanical systems. Hence, there is an urgent need for a method to predict multi-scale flows. In this work, a quantified model-competition (QMC) mechanism for diatomic multi-scale flows is derived from the integral solution of the Rykov model equations. This mechanism encapsulates both continuum and rarefied behaviors in a cell, weighted according to its local physical scale. By building upon the QMC mechanism, the N-S solver and DSMC solver are directly integrated within a cell to devise a simplified unified wave-particle (SUWP) method for diatomic gases. Specifically, the two-temperature equations considering the rotational energy are introduced into the kinetic inviscid flux scheme and the N-S solver. As to the particle part, the collisionless DSMC solver is utilized to describe the non-equilibrium phenomenon. The proposed SUWP method for diatomic gases undergoes validation across a series of cases, including zero-dimensional homogeneous gas relaxation, one-dimensional normal shock structure, two-dimensional flows around a flat plate and cylinder, and three-dimensional flows past a sphere and blunt cone. Additionally, the implementation details of multi-scale wave-particle methods analysis and discussion are also undertaken in this work.

Various physiological diseases and injuries, such as stroke or brain tissue concussion, are closely associated with the rupture of blood vessels, so that blood pressure plays an important role in their progression. Predicting the pressure-induced rupture of soft tissue can be challenging, however, due to the mechanical nonlinearity, tension-compression asymmetry, and softness-induced large deformations. A phase-field model (PFM) in the finite deformation regime is thus herein proposed to study the blood pressure-induced rupture, considering the tissue’s tension-compression asymmetry and nonlinearity. A staggered scheme is proposed to solve the coupling of the deformation and the phase field problems, and implemented in commercial software. With a proposed regularization of the phase field inside the pressure-induced rupture region, the model’s accuracy is first verified in terms of its capability to predict the crack opening displacement, and compare well with direct numerical simulation performed under the assumption of no crack propagation. Pressure-induced rupture in soft brain tissue with multiple initial tears (seen as crack-like defects) is subsequently examined in detail. This proposed PFM further incorporates blood transport during the rupture process, showing potential in predicting the realistic behavior of soft tissue.

This study employs the direct simulation Monte Carlo method to investigate two-dimensional compressible decaying isotropic turbulence under high Mach number conditions, focusing on the effects of thermal non-equilibrium (TNE) and molecular thermal fluctuations. Simulations are performed for low-temperature cases involving rotational non-equilibrium, followed by high-temperature cases emphasizing vibrational non-equilibrium. The results demonstrate that the initial TNE state significantly impacts turbulence compressibility. Specifically, for initially rotationally hot cases, elevated translational temperatures strongly suppress turbulence compressibility, resulting in a slower decay of turbulent kinetic energy. These findings are also applicable to initially vibrationally hot cases, but the influence of TNE diminishes as the vibrational relaxation number Z_vib increases. Moreover, increasing Z_vib leads to a significant lag of vibrational temperature fluctuations relative to translational and rotational temperature fluctuations. Analysis of the turbulent energy and temperature spectra reveals that molecular thermal fluctuations dominate at length scales (i.e., crossover length scales) comparable to the turbulent dissipation length scale, causing the spectra to increase linearly with the wavenumber. For cases with initially rotationally or vibrationally hot conditions, the suppression of compressibility leads to a significant increase in the crossover length scale.

Metamaterials programmed with target rate-dependent mechanical properties are efficient platforms for realizing advanced functionalities. Yet, the loading rate-dependent mechanical property programming has received limited attention. Here, the “stair-building” strategy is employed in the rate domain by combining the bistability with viscoelasticity. An arbitrary target curve in the programmable space can be approximated by a “stair” built by two kinds of “bricks”. The “bricks” can be realized by a dual-bistable unit, constructed by two bistable structures in series. The dual-bistable unit can switch between two efficient stable phases without inducing changes in the global morphology. Such a unit exhibits N-shaped stress-strain curves at both efficient stable phases with different peak values, resulting in different heights of “bricks”. Moreover, the N-shaped curves have rate-dependent peak values, indicating that the heights of “bricks” change with loading rate. The “stair-building” strategy is realized by array-structured mechanical metamaterials based on dual-bistable units. Different stress-strain curves under various loading rates can be reprogrammed in the same piece of metamaterial by intentionally selecting the efficient stable phases of units. Besides, the rate effect of the metamaterial can also be tuned by reprogramming stress-strain curves under both low and high loading rates, respectively. This reprogrammable metamaterial is promising in smart vibration isolators and adaptive energy absorbers.

This study investigated the impact fragmentation behavior of projectiles with different yield strengths using 45 steel, 35CrMnSiA steel, and T12A steel as core materials. Through experimental and simulation analyses, the fracture mechanisms and damage characteristics of these materials under high strain rate impacts were examined. The results indicated that 45 steel and 35CrMnSiA steel exhibited noticeable ductility, with fracture surfaces showing prominent dimples and shear lips, experiencing significant plastic deformation and necking during impact. In contrast, T12A steel demonstrated brittle behavior, with fracture surfaces characterized by smooth, bright areas and fine radial streaks. In ballistic tests, T12A steel projectiles displayed distinct damage patterns compared to the other two materials due to their brittleness, resulting in shorter residual lengths. The force-time curves obtained from experiments and simulations showed that higher core strength shortened the time to reach peak force and reduced the overall contact duration. However, the high strength of T12A steel was not always associated with higher peak stresses, as its brittle fracture mode led to early instability. Fragment analysis revealed that smaller fragments (< 4 mm) primarily originated from the projectile head, while larger fragments came from the tail. Microscopic examination of the fragments revealed a mix of ductile and brittle fracture modes, with different particle sizes exhibiting distinct fracture surface features. Overall, the study provides insights into the impact performance and failure mechanisms of different core materials, highlighting the importance of material properties in determining projectile behavior under high strain rate conditions.