Acta Mechanica Sinica最新文献

Understanding the dynamic characteristics of deep-sea explosions is essential to improve the survivability and combat capability of deep-sea equipment. In this paper, by considering the practical underwater conditions, we investigated the mechanical effects of the deep-sea 1-kg-trinitrotoluene (TNT) explosion with charge depths ranging from 1 to 10 km through numerical simulation and dimensional analysis. The shock wave overpressure, the positive overpressure pulse, the bubble pulse, and the energy distribution for various depth explosions were analyzed systematically. The simulation results showed that the charge depth was negligible for the peak overpressure of the shock wave. However, the positive overpressure pulse, the shock wave energy, the maximum bubble radius, the bubble energy, and the bubble period decrease significantly with increasing the charge depth. Then, the dimensional analysis for deep-sea TNT explosion was performed to reveal the key dimensionless parameters, from which the scaling laws of the shock wave overpressure and the overpressure pulse were obtained. By fitting the simulation results, the dimensionless equations were proposed, providing an effective method for predicting the peak overpressure and the positive overpressure pulse of shock wave for underwater TNT explosion over a wide range of water depths.

Hybrid energy harvesting systems are broadly applied in various fields due to the advantage of improving energy harvesting efficiency. In actual environment, there are many complex phenomena exhibiting jump, flights, rare transition features, and intermittent features, which can be described by systems subjected to non-Gaussian Lévy process. Sometimes, it is difficult to build mathematical models of complex hybrid energy harvesting systems precisely, especially for those driven by non-Gaussian Lévy noise. With the development of simulation capabilities and observing techniques recently, massive noise measurement data or simulating data can be feasibly obtained and there are many existing techniques devoted to discovering governing laws from abundant data. In this paper, we aim to extract the system equations from observed data influenced by non-Gaussian Lévy noise via using a data-driven method. The expressions of drift term, diffusion term and Lévy term can be approximated with the help of Fokker-Planck equation and non-local Kramers-Moyal formulae, and the coefficients of the expressions are learned by utilizing a sparse regression approach in the least square sense. Three examples are given to demonstrate the effectiveness of the method. Results show that the approach can well be applied to not only hybrid energy harvesting systems under Gaussian Brownian process but also the systems subjected to non-Gaussian Lévy process. Additionally, the relations between the demarcation parameter and an indicator denoted as Ratio for different time steps are analyzed, and results demonstrate that the indicator can be regarded as the criterion of selecting the appropriate demarcation parameter.

Topology optimization is a common approach for material distribution in continuous structure due to its rigorous mathematical theory. However, with the increase of material types in design domain, the computational efficiency of traditional topology optimization for multiple materials problem is greatly decreased. In this paper, a novel deep learning-based topology optimization method is proposed to achieve multi-material structural design for improving computational efficiency. A large number of multi-material topological configurations are simulated by solid isotropic material with penalization (SIMP), to construct multi-material topology optimization dataset. Subsequently, ResUNet involved generative adversarial network (ResUNet-GAN) is developed for high-dimensional mapping from design parameters to the corresponding multi-material topological configuration. Finally, the ResUNet-GAN, trained by the multi-material dataset, is utilized to design multi-material topological configuration. Numerical simulations verify that the well-trained ResUNet-GAN is successfully applied to three types of cases: the cantilever beam with double materials, the cantilever beam with triple materials, and the half-MBB with triple materials. The deep learning-based topology optimization approach is superior to the conventional methods in terms of higher computational efficiency, performing the potential of such a data-driven method to accelerate the calculation of structural optimization design.

Accurately predicting the nonlinear dynamic response of shaft-disk rotors is crucial in the design of rotary machines. In available studies, the disk is commonly treated as rigid body to simplify the analysis of the nonlinear problem. This paper aims to clarify the effect of disk flexibility on the nonlinear dynamic response of shaft-disk rotors. Both gyroscopic and centrifugal effects are considered. The shaft is considered as a Euler-Bernoulli beam with the von Kármán nonlinearity, and the disk is considered as a Kirchhoff plate. The differential equations of motion are derived utilizing the Lagrange equation. The pseudoarclength continuation method is employed to numerically analyze the nonlinear forced vibration response of the rotor under the unbalanced force. By comparing with the existing reference, the accuracy of the present model is verified. The effects of the flexibility, position, radius, and thickness of the disk, as well as the wall thickness and length of the shaft on the nonlinear vibration of the rotor are explored. Results show that ignoring the disk flexibility is acceptable only in very limited cases, in most cases, however, this simplification will overestimate the nonlinear vibration amplitude of the rotors, which makes the design of the shaft-disk rotors conservative.

Based on the theory of complex variable function, the propagation properties of anti-plane shear wave in inhomogeneous right-angle space are analysed. The inhomogeneity of the medium is reflected in the fact that the shear modulus is a function related to spatial coordinates. A displacement auxiliary function and a pair of mapping functions are introduced to assist in deriving the governing equation. Meanwhile, the transformation between coordinates can be more convenient in the complex coordinate system. The wave field expressions of reflected and scattering waves are constructed by using the image method. The unknown coefficients in the scattering waves are solved according to the boundary condition of the circular hole, and the analytical expressions of the wave field in the right-angle space are given. In dynamic analysis, rigidity can be expressed by shear modulus. In the analysis of numerical examples, the dependence of displacement amplitude and dynamic stress concentration factor (DSCF) on the inhomogeneous parameter, reference wave number, and the position of circular hole are discussed.

To solve nonlinear dynamic problems in three-dimensional space, this investigation presents a new isogeometric solid element and nonlinear strain field of the proposed element through coordinate transformation in tensor analysis. In order to make the proposed element can be used seamlessly in computer aided design (CAD) and computer aided engineering (CAE) systems, trivariate T-spline is adopted as the basis function of the isogeometric solid element. Local mesh update algorithms are developed for T-spline based isogeometric solid element in virtue of Bézier projection method. Mesh is refined locally in important regions to obtain accurate results and redundant elements are coarsened beyond important regions to improve computational efficiency. Therefore, the computational efficiency and the computational accuracy are balanced in nonlinear dynamics analysis. Three statics numerical examples are set to test the correctness of the elemental elastic model. A flexible pendulum simulation is performed to study the dynamics characteristic of the proposed element. An experiment is implemented to test the proposed element’s ability to solve nonlinear dynamics problems. A simulation that a flexible pendulum contacts with a stick is performed and local mesh update algorithms are employed. The computation results and computation time are analyzed to research the effectiveness of the proposed locally varying mesh T-spline based solid element in nonlinear dynamics problems.

In order to comprehensively understand the mechanical behavior of biological entities and aerospace applications subjected to hypergravity environments, we delve into the impact of hypergravity on the equivalent compliance of cubic lattice structures. Capitalizing on the periodic spatial distribution, we employ a unit cell methodology to deduce the homogenized stress-strain relationship for the lattice structures, subsequently obtaining the associated equivalent compliance. The equivalent compliance can be conveniently reduced to instances without hypergravity influence. Furthermore, numerical simulations are executed to validate the derivations and to illustrate that hypergravity indeed affects the mechanical properties of lattice structures. We introduce a non-dimensional hypergravity factor, which quantifies the impact of hypergravity magnitude relative to the Young’s modulus of the base material. Our findings reveal that the hypergravity factor influences perpendicular compliance quadratically and parallel compliance linearly. Simultaneously, the perpendicular shear compliance remains unaffected, whereas the parallel shear compliance experiences an inverse effect. Additionally, the lattice structure transforms into a gradient material oriented in the hypergravity direction, consequently generating a scale effect.

Research is carried out on the prediction of aircraft surface noise during flight with high maneuverability and high overload. The dual hybrid Reynolds-averaged Navier-Stokes/large eddy simulation (RANS/LES) calculation model based on zonal mixing and turbulent scale mixing is modified, and the new hybrid model is established to simulate complex separated flow accurately. A calculation method for simultaneously calculating aerodynamic/motion, internal/external flow, and compressible/incompressible flow is employed. The dual hybrid RANS/LES model combined with six-degree-of-freedom motion equations is used to simulate the maneuvering process of the aircraft and calculate the surface acoustic load. The calculation results reveal that the acoustic load at the inlet of the engine and tail nozzle is large. During positive overload flight, the surface noise is greater than negative overload due to the stronger turbulent fluctuation on the aircraft surface and the impact of jet noise at the tail nozzle. The contribution rate of jet noise to the overall sound pressure level (OASPL) of the fuselage gradually increased from front to rear. In the maneuvering state, the vortex motion on the aircraft surface is enhanced, and the pressure fluctuation is stronger than that in the cruise state, which makes the surface acoustic load greater than that in the cruise state.

The instability and breakup of liquid jets under static or alternating electric fields are involved in numerous industrial applications. Unlike under electrostatic fields, far fewer investigations have been conducted to analyze the instability of liquid jets in alternating electric fields. Thus, the electric and viscous correction of viscous potential flow (EVCVPF) is applied here to describe the linear instability of leaky-dielectric liquid jets subjected to alternating electric fields. The effects of alternating electric fields, fluid electric properties, and other parameters are investigated. The capillary instability response is like that of the jets under electrostatic fields. Under a sufficiently strong alternating electric field, the resonance instability dominates surface disturbances, leading to the resonant atomization. Viscous damping makes the resonance weaker–even vanishing with the increasing frequency. Furthermore, the conductive charge–largely dependent on fluid conductivities–has the opposite effect of the surface charge. Thus, when the charge relaxation time approaches the imposed period, the parametric resonance is strongly inhibited. In addition, when aerodynamic effects are sufficiently strong, the resonance is covered.

In this paper, a design strategy to keep a pipe away from resonance by adjusting retaining clips is presented. The pipe natural frequency is within the band of vibration frequencies generated by mechanical operation, which can cause the resonance phenomenon. The resonance of pipes could bring serious disaster to the mechanical operation. Therefore, the band of vibration frequencies generated by the mechanical operation is defined as the preserved frequency band (PFB). The pipe in engineering has been simplified to a multispan pipe conveying fluid. An integral pipe model of a multispan pipe conveying fluid is established by simplifying the retaining clips. The natural frequencies of a pipe with clips are obtained. The resonance range and safety range of pipe length are defined as the natural frequencies inside and outside the PFB, respectively. The absolute safety length and the absolute resonance length with respect to clip number are derived. Moreover, the influence of the clip stiffness and location on the safety range is determined. The results show that the safety range is significantly changed by adjusting the vertical stiffness, torsional stiffness, and location of the clips. Based on the results, a pipe design strategy is proposed to stay away from resonance and make for a smaller number of clips. Subsequently, a design for an aircraft pipe is carried out as an example. As a result of the design, the natural frequencies of the aircraft pipe are kept away from the PFB. In summary, a design strategy is obtained to avoid resonance for pipes by adjusting the clip stiffness and/or location.