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2021, 53(9): 2355-2356.   doi: 10.6052/0459-1879-21-428
With meshfree and fully Lagrangian features of particle methods, smoothed particle hydrodynamics (SPH) is suitable to achieve high-accurate simulations of multiphase flows with large interfacial deformations, discontinuities, and multi-physics. Multiphase flow simulations with SPH methods have been reported abundantly and the specific implementations are much different. In this review, the basic SPH method and issues about fluid pressure, surface tension and solid boundary are discussed. And various implementations of SPH for multiphase flow simulation are mainly summarized as: (1) Lagrangian solver for the two-fluid model (TFM): The two phases are discreterized into two independent groups of SPH particles and coupled by the explicit interphase interaction; (2) multiphase SPH: The multiphase SPH method is considered as the natural extension of SPH method on multiphase flow simulation, and the interphase interaction is implicitly described by SPH parameters; (3) coupling of SPH and other discrete methods: The two phases with large differences each adopt different discrete methods to give play to the advantages of different Lagrangian methods; and (4) coupling of SPH and grid-based methods: The grid method handles the simple main-flow to obtain the balance between accuracy and efficiency. Also, some issues associated with SPH simulations of multiphase flows, such as the physicalization of simulation parameters and the improvement of accuracy and efficiency, are suggested as requiring attention.
2021, 53(9): 2357-2373.   doi: 10.6052/0459-1879-21-270
Granular material is a complex multi-body interaction system which is composed of a large number of discrete particles and their surrounding free volume. Although the correlation between free volume and the mechanical properties as well as the deformation characteristics of granular materials has been proved, the local free volume of non-spherical particles is not fully understood at present due to the difficulties in characterizing. In this paper, the combined finite and discrete element method (FDEM) is used to simulate the triaxial tests of ellipsoidal particles with different principal axis lengths, and the Set Voronoi tessellation method is applied to construct the Voronoi cells of the particles during shearing. The statistical distribution and evolution of the local free volume of the granular systems during shearing are analyzed, and the influence of particle shape on the evolution of free volume is studied. Our results show the anisotropy of Voronoi cells gradually increases during shearing, and the degree of anisotropy increase will be intensified with the increase of particle shape asphericity, which means the granular assembly with a larger asphericity will experience more intense rearrangement during shearing. The local void ratio of ellipsoidal particle systems with different asphericity statistically complies with a k−Γ distribution, which is controlled by the global void ratio of granular assembly and not affected by particle shape and shear state. The local void ratio fluctuations follow an asymmetric laplace distribution (ALD), and its asymmetric parameter which has a linear relationship with the global void ratio of granular assembly describes the competition between contraction and dilatation of local free volume.
2021, 53(9): 2374-2383.   doi: 10.6052/0459-1879-21-255
The rolling resistance between particles plays an important role in the stability of the particulate systems. In a conventional discrete element method, the rolling resistance model between particles is usually made of springs, dashpots, and sliders in the rotational direction. The particles rolling kinetic energy is dissipated by the viscous (moment) and friction forces. With this model, the viscous force (moment) is directly related to the rolling velocity. Consequently, the dynamic dissipation capacity of particles close to the static state becomes weaker with the rolling velocity decreasing. It is known that the time required to simulate a particle rolling with a velocity close to zero by using the traditional discrete element method is longer than the experimental results. To solve this problem, the mechanism of rolling resistance caused by material hysteresis is analyzed based on tribological principle, and a new discrete element model of hysteresis rolling resistance (HDEM) is established. A hysteresis spring with velocity-independent kinetic energy dissipation is proposed, and its constitutive law’s formula is derived. To verify the new rolling resistance model, the free-rolling of a single round particle specimen on a flat surface is measured through a physical experiment. The measured data are compared with the results simulated by the new rolling resistance model HDEM and the conventional rolling resistance model. The results show that the results based on HDEM are more consistent with the experimental data, and the particle oscillation frequency is in better agreement with the experimental phenomenon observed.
2021, 53(9): 2384-2394.   doi: 10.6052/0459-1879-21-236
The study of wave propagation in granular materials is of great significance in metamaterial manufacturing. The boundary design of wave-conducting metamaterials needs to consider the reflection and absorption of stress waves. First, the wave propagation behavior in a one-dimensional particle chain has been studied. According to the difference in the maximum kinetic energy that the particles can obtain at different positions from the boundary, the definition of the boundary area is given. Then the stress wave propagation behaviors of multiple sets of two-dimensional particle samples under impact load are analyzed. The influences of different boundary shapes and particle arrangement on the propagation behavior of stress waves in the pro-border zone have been considered. The results show that the arrangement of particles in the pro-border zone mainly affects the relative position and local porosity of particles near the boundary. The stress wave reflected by the boundary propagates directly in the pro-border zone in the shape of the boundary line. The more complicated the boundary situation (high local porosity, random arrangement of particles), the more accurate the conclusion. The wave velocity mainly determines the shape of the wave-front outside the pro-border zone, i.e., in the material center area. The convergence effect of the arc boundary on the wave reflection and the dispersion effect caused by the arrangement of the particles in the pro-border zone are two competing factors, which together determine the reflection process of the wave in the pro-border zone. Finally, the changes of the force chain network in the pro-border zone before and after reflection are analyzed. The distribution of kinetic energy intuitively reflects the phenomenon of reflection hysteresis. The process of particle contact and rebound in the boundary area corresponds to the storage and release of energy. This research will provide reference for the handling of boundary problems in metamaterial design.
2021, 53(9): 2395-2403.   doi: 10.6052/0459-1879-21-242
2021, 53(9): 2404-2415.   doi: 10.6052/0459-1879-21-221
2021, 53(9): 2416-2426.   doi: 10.6052/0459-1879-21-240
The investigation of ice loads on polar ships and offshore engineering structures is very important for anti-ice structure design, safe operation and structural integrity management in ice-covered regions. Recently, the rapid developments on high-performance computing techniques and multi-media, multi-scale numerical methods provide an effective improvement on the determination of ice loads on polar ships and offshore engineering structures. The numerical methods represented by the discrete element method (DEM) achieved excellent contributions on the ice load predictions. Therefore, considering the engineering demands to forecast ice loads and mechanical responses of polar ships and offshore structures, and also based on the present state-of-the-art of the multi-media and multi-scale numerical methods for coupling of sea ice, engineering structures and fluid, the concept, frame and technique of numerical ice tank are discussed based on DEM simulations. The numerical ice tank has significant advantages in reliability, economy, rapidity, expansibility and scenario in determining the ice load on hulls and offshore engineering structures. Based on the concept and experience of numerical tank, this paper illustrates the feasibility and engineering application prospects of numerical ice tank with the DEM simulations on ice loads and structural mechanical responses of typical ship and offshore platform. The computational parameters in DEM simulations were calibrated with the mechanical properties of sea ice obtained with physical experiments. The ice loads on ship hull and jacket platforms simulated with DEM were compared with the model tests and filed measurements. Finally, the interaction between ice cover and structures of model tests in ice tank are repeated numerically with DEM. With the numerical ice tank, ice loads on ships and offshore structures can be simulated with DEM under various ice conditions on different scales. The necessity of combination of theoretical analysis, field measurement and model test with the numerical ice tank is also elaborated. The research above can be aided to develop the numerical software for ice load determination for polar ships and offshore engineering structures, and to promote the implementation of the polar ocean strategy in China.
2021, 53(9): 2427-2453.   doi: 10.6052/0459-1879-21-243
Gappy POD is a method of data reconstruction based on the proper orthogonal decomposition (POD). We study the applicability of gappy POD to the reconstruction of fluid turbulence configurations and focus mainly on two factors. The first factor is the complexity of the data, which mostly depends on the number of POD modes with non-zero eigenvalues. The second factor is the area and the geometry of the gap. By taking these factors into account, we reformulate the gappy POD reconstruction and derive a formula to compute the reconstruction error. Rotating turbulence data is used as a case study of gappy POD reconstruction, where the reconstruction error can be separated into two parts. The first contribution to the reconstruction error is from the truncation error during the POD expansion and it is amplified by the smallest eigenvalue of the matrix, which consists of POD modes at known indexes. This error mainly depends on the flow complexity, e.g. for flow of moderate complexity, this error decreases with the increase in number of POD modes employed during the reconstruction process. For flow of large complexity, a small POD truncation error can be detrimental and contribute signification to the reconstruction error. Therefore, all POD modes should be considered when utilizing Gappy POD reconstruction to eliminate the truncation error, especially for the turbulent flow field. The second part of the reconstruction error appears when the matrix composed of POD modes at the known points is not of full column rank. This part of error depends on the area and the geometry of the gap. The gap area determines the amount of the lost information. For the same gap area, the gap geometry determines the correlation of the lost information. Gappy POD reconstruction works well when both the amount and the correlation of the lost information are small.
2021, 53(10): 2703-2711.   doi: 10.6052/0459-1879-21-464
Wall immersed in fluid will form highly complex wake flow with specific features. Therefore, the extraction and analysis of flow feature has important research value. However, in the case of high Reynolds number, the wake flow field are complex, so it is difficult to identify and extract the flow features by traditional mathematical and statistical method. In this paper, a new flow field feature extraction and analysis method based on deep learning of wake time history data is proposed, and the shape recognition based on local time history is realized; At the same time, accuracy of different time history parameter is analyzed, and the optimal physical parameters suitable for target recognition are obtained. Research results on the flow field data of cylinder and square cylinder show that the model based on convolution neural network proposed in this paper has good training convergence and high prediction accuracy, and model using transverse velocity time history has highest accuracy. At the same time, it is proved that method proposed in this paper is a new high-precision method for target recognition immersed in fluid.
2021, 53(10): 2692-2702.   doi: 10.6052/0459-1879-21-332
Establishing a parameterized aeroelastic model is one of the obstacles in aeroelastic research of the variable-sweep wing. The local modeling technology is widely known as a practical method for constructing a linear parameter varying (LPV) model. However, there has been a lack of effective methods to deal with the incoherency of the local aeroelastic models. The inconsistency of the local aeroelastic models is reflected in the discontinuity of the local structural and aerodynamic models with the change of the system parameters. To solve this problem, this paper proposed a bottom-up coherent processing method to deal with the incoherent local aeroelastic models of the variable-sweep wing. Firstly, the Hungarian algorithm was used to track the structural modes and sort them according to the modal branches. In this way, the matched modes can ensure the coherency of the structural models; Next, the incoherent problem of the aerodynamic model was solved by introducing a scaling matrix in the expression of rational functional approximation, such that the aerodynamic coefficient matrices were written in a coherent form. After the above two steps, the resulting local state-space models have a coherent form, and the aeroelastic state-space model at arbitrary swept angle can be constructed quickly by interpolating the coherent local state-space models, so the computations for the aeroelastic stability and the slow time-varying responses can be performed effectively. Simulation results demonstrated that the model obtained by interpolating on the incoherent aeroelastic models will lead to great modeling errors, while the one obtained by interpolating on the coherent local models can produce an accurate aeroelastic model at any given swept angle of the wing. This paper provides a useful, accurate and efficient modeling method of the parameter-varying aeroelastic system for the variable-sweep wing.
2021, 53(11): 1-13.   doi: 10.6052/0459-1879-21-275
Based on the JF-24 high-enthalpy shock tunnel in Institute of Mechanics, Chinese Academy of Sciences, the current paper performed direct-connect combustion tests of a high-Mach-number scramjet engine to study high-Mach-number combustion enhancement methods and fuel types’ effects. The test-section was a circular cross-section scramjet combustor with cavity structures, and fuel injectors were arranged in the isolator. Hydrogen and ethylene fuels were severally used in current tests at the same equivalence ratio of 0.7. Fuel injection utilized two different test-section configurations without and with small struts, respectively. Some injection holes of the latter configuration were located on the strut tops. For each configuration, two adjacent rings of injecting holes were arranged for single-ring and dual-rings injections, respectively. Test results demonstrated that stabilized combustion performances of hydrogen and ethylene fuels in hypersonic flows under a Mach number $M{a_{\text{f}}}{\text{ = }}10$ flight condition. Meanwhile, compared to the single-ring fuel injection method, dual-rings fuel injections and adding injections on small-strut tops were beneficial for combustion enhancements. The reason was speculated that interactions of adjacent fuel jets and shock/separation structures probably could improve fuel-air mixing, and additional fuel injection on small-strut tops meant more available air for mixing. Under the same combustion enhancement methods of dual-ring injections and additional small-strut top injections, hydrogen fuel generated better thrust performance than ethylene fuel, while their combustion efficiencies were similar. This was possibly because that the hydrogen fuel had a higher caloricity, and thus it could generate more heat release. Besides, test also verified that under the current high-enthalpy high-speed inflow condition, combustion heat release was controlled by fuel-air mixing processes, and meanwhile the upper limits of heat release was limited by high-temperature dissociation effects. This was because that heat release led to decreases of local flow speeds and increase flow temperatures. Consequently, high-temperature dissociation endothermic reactions would be more remarkable, resulting in decrease of heat release.
2021, 53(11): 1-11.   doi: 10.6052/0459-1879-21-348
Most of the existing researches on deformation reconstruction of flexible structures with finite deformation are only based on the geometric relationship between curvature and strain, which ignores the longitudinal deformation and the coupling effect of the longitudinal deformation and the bending deformation. In order to construct a more accurate deformation reconstruction method which can be extended with the help of existing mechanical tools, this paper takes the plane beam as the object, partially inherits inverse finite element method developed by Tessler A, and regards the deformation reconstruction problem of plane beam as a kind of numerical optimization problem. Firstly, by introducing the absolute nodal coordinate formulation (ANCF) into the description of mapping relationship between strain and displacement, an inverse gradient reduced ANCF plane beam element is derived. Secondly, the inverse ANCF element is modified to simplify the degree of freedom of nodes and ensure the C2 continuity at nodes by introducing the penalty function, which not only ensures the problem is well-posed, but also improves the accuracy of the final result. Finally, based on the inverse ANCF element, the Newton method is used to develop two types of algorithms for deformation reconstruction under different working conditions, one is the element-by-element algorithm and the other is the multi-element algorithm. The numerical simulation results show that the reconstruction relative error of this method is less than 1% under the condition of large deformation, and it still maintains high accuracy under the condition of few measuring points. The convergence and computational efficiency of the method are verified by numerical simulation example.
2021, 53(10): 2791-2804.   doi: 10.6052/0459-1879-21-338
The initiation, steady propagation and failure mechanism of gaseous detonation wave in periodic inhomogeneous media are very complex, and many physical mechanisms are still unclear, which is an active topic in detonation physics. Numerical simulation of propagation of gaseous detonations in the inhomogeneous medium is studied by using the reactive Euler equations coupled with a two-step chemical reaction model. The inhomogeneity is generated by placing artificial temperature perturbations with different wavelengths and amplitudes. The influence of temperature disturbance with different wavelength and amplitude on the structure of wave front is analyzed. The results show that, the transition of ZND detonation to cellular detonation under artificial temperature disturbance is mainly controlled by two competitive factors: one is the intrinsic instability of detonation wave, the other is the wavelength and amplitude of artificial disturbance, the former is the internal factor, the latter is the external factor. The existence of artificial temperature disturbance delays the evolution of ZND detonation to cellular detonation by suppressing the development of shear wave, and the increase of internal instability can slow down this delay phenomenon. This shows that the artificial temperature disturbance can restrain the development of cell instability in a certain range, but it cannot stop the process. The discontinuity of temperature makes the detonation wave front more distorted, which leads to the existence of a weak triple-wave structure near the shear wave, which is, the artificial disturbance increases the inherent instability of detonation wave and changes the propagation mechanism of detonation wave front. The propagation of detonation and the instability of detonation are restrained by the artificial temperature disturbance with large amplitude. The formation of detonation front cellular structure depends on the artificial temperature disturbance and its own instability.
2021, 53(10): 2776-2790.   doi: 10.6052/0459-1879-21-069
By setting small electromechanical systems such as wireless sensors in the traffic environment to realize traffic condition monitoring, system management and facility health monitoring, etc., the traffic system can be operated in a safer, orderly and efficient manner. However, how to power these widely distributed small electromechanical systems? This paper proposes a magnetic coupling road energy harvesting design to collect vehicle rolling energy and convert it into electricity. The device transmits non-contact energy through magnetic coupling, which reduces the impact on the device and makes it have a good seal, so as to improve the robustness. The vehicle rolling excitation is converted into high-speed one-way rotation through the up-frequency gear and the ratchet mechanism, and the reversing gear mechanism can continue to collect the reset elastic potential energy, which improves the output power of the device. Based on the working principle of the system, the electromechanical coupling dynamics model is established. The numerical simulation explored the impact of key design parameters such as the limit distance of the speed bump and the stiffness of the resetting spring on the dynamics and electrical performance of the energy harvesting system. When the vehicle speed is 50 km/h, the maximum output voltage of the system is 76.28 V and the maximum power is 59.94 W. The magnetic coupling road energy harvesting device can become an important part of the intelligent traffic system in future, harvesting the energy of the traffic environment and providing sustainable green carbon-free power for small and medium electromechanical systems in the traffic environment.
2021, 53(11): 1-9.   doi: 10.6052/0459-1879-21-374
Impact of spheres on liquid surfaces is a universal phenomenon in nature and industrial processes. However, the relevant researches mainly focused on the impact of millimeter or larger spheres on the horizontal liquid surface. Further studies on the dynamic characteristics of submillimeter sphere impact process and the influence of curved interface on impact behavior is necessary. Herein, we presented the observation on the impact of submillimeter spheres on the curved surface of droplet by using high-speed microphotography technology. Owing to the existence of curved liquid surface, the impact phenomenon is different from those after impact horizontal liquid surface. The azimuthal angle of TPCL (three phase contact line) pinned point is positive linear correlation with the impact angle during wetting process, while the non-axisymmetric cavity is first formed on the higher side of TPCL pinned point and the curvature radius is larger. The evolution of the dominant forces and the energy conversion mechanism during the impact process were revealed. The influence of impact velocity and angle α on impact behavior were analyzed, and the impact pattern diagram was provided. The results show that the form drag dominates the motion of the sphere at the slamming stage, while the kinetic energy loss of the sphere is positive correlation with spheres velocity. The surface tension dominates the process at the cavity development stage, and the kinetic energy of the sphere is transformed into the surface energy that maintains the cavity. The cavity length of oscillation mode increases with the increase of Weber number We, and the cavity development velocity is basically consistent. According to the dimensional analysis and experimental results, the relationship between critical Weber number Wecr and α is $We_{cr}^{1/2}$ = α / 40 by fitting.
2021, 53(10): 2745-2751.   doi: 10.6052/0459-1879-21-351

High speed and shock compression behind the bow shock of an aircraft head result in very high temperature, which would subsequently lead to a conductivity plasma flowfield around the vehicle. The plasma gas provides a direct working environment for the application of magnetic field. The magnetohydrodynamic (MHD) flow control, which uses the magnetic field to alter the trajectory of ions or electrons, can improve the aerodynamic characteristics of hypersonic vehicles effectively. It has potential prospects on aerodynamic force control and aerodynamic heating management. Besides, the development of superconducting materials and electromagnetic technology contribute to a great upsurge of MHD flow control research significantly. Although research work has been carried out in the field of MHD flow control at home and abroad, its experimental investigation is still challenging. And for the measurement of pressure and heat flux, there is no systematic conclusion because of the limited test conditions and measurement techniques. The results of different researchers may be different from each other and from the theoretical results and numerical simulations. Thus, the influence on the shock stand-off distance, pressure and heat flux under MHD flow control deserves an in-depth investigation. Besides, the numerical simulations and theoretical methods do also need reliable experimental data for variation. The aim of this review paper is to summarize and discuss the developments on MHD flow control technology based on high temperature real gas effect, including the experimental technique, numerical method, and the influence rules and dynamics mechanism of MHD flow control. Its development trend is also discussed and prospected in the paper.

2021, 53(6): 1515-1531.   doi: 10.6052/0459-1879-21-067
Graphene and other two-dimensional (2D) materials possess various excellent properties and hold great promises for next generation of electronic devices and other applications. The mechanical properties are of fundamental importance in the research and application of 2D materials. Despite the fact that 2D materials have been extensively investigated in the past two decades, efforts on the mechanical properties are strikingly lacking and vastly needed. Atomic force microscopy (AFM) is one of the most widely used tools for the mechanical characterizations of low-dimensional materials. Particularly, the AFM-based nano-indentation technique has been extensively employed to explore the mechanical properties of 2D materials. In this review, we first introduce the basic backgrounds of 2D materials and atomic force microscopy. The mechanism and theoretical background of AFM-based nano-indentation are then demonstrated. In the second part, we review the research work by employing nano-indentation on studying the in-plane mechanical properties of 2D materials. The measurement errors of AFM-based nano-indentation and their origins are also discussed. Nano-indentation is perfectly suitable for the in-plane/intralayer mechanical measurement but also greatly limited in probing the out-of-plane/interlayer elasticity, due to the extreme anisotropy of 2D materials. Therefore, in the third part, we introduce an unconventional AFM-based technique - Angstrom-indentation which allows for sub-nm deformation on 2D materials. With such a shallow indentation depth comparable to the interlayer spacing of 2D materials, Angstrom-indentation is capable of measuring and tuning the interlayer van der Waals interactions in 2D materials. The interlayer elasticities of graphene and graphene oxide measured by Angstrom-indentation are discussed as examples in the third part. In the final part, we give a quick overview of a new type of 2D material - van der Waals heterostructure and its novel mechanical properties. We also discuss the potential application of Å-indentation in the investigation of the mechanical properties of van der Waals heterostructures.
2021, 53(4): 929-943.   doi: 10.6052/0459-1879-20-354
This paper experimentally and numerically investigates the fluid-structure interaction between a spark-induced bubble and a floating structure. The boundary integral method is adopted to simulate the bubble dynamic behaviors and the auxiliary function method is used to improve the computational accuracy of the nonlinear fluid-structure interaction. The double-node method is employed to maintain the computational stability of the gas-liquid-solid interaction line. Besides, we use the underwater electric discharge technique to generate bubbles and the high-speed photography to record the bubble dynamics and the structural responses. Firstly, we compare the numerical result with the experimental data and favorable agreement is achieved which validates this numerical model. Through parametric study with respect to the dimensionless distance $\gamma _{s}$ from the initial bubble center to the floating structure (the reference length is the maximum bubble radius), we then find that (1) as $\gamma_{s}$ increases from 0.2 to 2, five types of jetting pattern such as necking together with annular jet ($0.2\leqslant \gamma_{s} \leqslant 0.3)$, contacting jet ($0.4\leqslant \gamma_{s} \leqslant 0.6)$, non-contacting jet ($0.7\leqslant \gamma_{s} \leqslant 1)$, collision of a jet directed towards the floating body and a counter-jet ($1.1\leqslant \gamma_{s} \leqslant 1.3)$ and individual counter-jet ($1.4\leqslant \gamma_{s} \leqslant 2)$ can be formed; (2) it is also found that the velocity of the jet directed towards the structure first increases, then decreases and finally increases again as $\gamma_{s}$ increases; additionally, it may be in the order of $\sim$1000m/s when $\gamma _{s}$ varies from 0.7 to 0.9; as $\gamma_{s}$ increases, the counter-jet velocity increases; (3) under the conditions of the presented experiments, the bubble migrates towards the floating structure when $\gamma_{s} <\mbox{1.5}$ due to the stronger Bjerknes attraction of the floating structure than the Bjerknes repellence of the free surface on the bubble during the collapsing phase. When $\gamma_{s} \geqslant \mbox{1.5}$, however, the free surface has stronger effects on the migratory behavior of the bubble than the floating structure which causes the bubble to migrate away from the free surface at the collapse stage.
2021, 53(4): 944-961.   doi: 10.6052/0459-1879-20-357
Bubble directional transportation using the superhydrophobic surfaces of different specific geometry in the water has broad application prospects in the fields of mineral flotation and biological incubation. The surface orientation of the planar straight superhydrophobic surfaces is a crucial parameter for the related engineering structures. However, it is still unclear that the effect of surface orientation on the bubble slipping along the inclined surface. The high-speed shadowgraphy is used to study the movement characteristics of the slipping bubble ($D_{eq}=2.4$ mm, $Re=500$ $\sim$ 700, $We=7$ $\sim$ 13) on the superhydrophobic linear trajectory with the width of 2 mm under different surface orientations ($-90^\circ\leqslant \beta \leqslant 90^\circ$) and inclination angles ($45^\circ\leqslant \alpha \leqslant 75^\circ$). The slipping velocity of the bubble ($u)$ on the trajectory is approximately stable, and the shape like semi-bullet with multi-ridges. The slipping bubble can be divided into two shape types: the stable and the unstable according to the fluctuation level of the gas-liquid interface. Stable bubble only appear when the inclination angle is small and the azimuth angle is large ($45^\circ\leqslant \alpha <70^\circ$, $| \beta | \geqslant 45^\circ$). As $\alpha$ changes, two kinds of $u$-$\beta$ relations can be found: When $\alpha \leqslant 65^\circ$, the slipping velocity is approximately a unimodal distribution about $\beta =0^\circ$ (the maximum sliding velocity at $\beta =0^\circ$); When $\alpha \geqslant 70^\circ$, the azimuth angle has no significant influence on $u$. The maximum sliding velocity can be upto 0.66 m/s ($\beta =0^\circ$, $\alpha =70^\circ$), which is much higher than that of the free-rising bubble of the similar size ($\sim$0.25 m/s), mainly as a combined effect of the wall-wettability and the inertial force. Surface orientation ($\beta$) and trajectory inclination angle ($\alpha$) affect the slipping velocity and the stability of the gas-liquid interface by changing the driving force, as a buoyance component, of the bubble along the trajectory direction and the bubble frontal area.
2021, 53(4): 962-972.   doi: 10.6052/0459-1879-20-405
Water-air two-phase flow can be found in many practical engineering projects in various fields. To simulate water-air two-phase flow with high accuracy has always been a challenging problem and a highlight in the realm of computational fluid dynamics. Based on the assumption that both water and air can be considered as incompressible fluid, for free surface flow in open water areas, the WENO-THINC/WLIC model for water-air two-phase flow is therefore established. In the developed model, the fifth-order accurate weighted essentially non-oscillation (WENO) scheme is used to solve the Navier-Stokes equation for fluid flows, and the improved multi-dimensional tangent of hyperbola for interface capturing scheme with weighted line interface calculation method (THINC/WLIC) is adopted to track the interface. The fractional step method is applied to discretize and solve the governing equations, the pressure projection method is adopted to compute the pressure field, and the third-order accurate total variation diminishing (TVD) Runge-Kutta (RK) method is used to discretize the temporal terms. In order to verify the model, it is applied to simulate two benchmarks of interface evolution subjected to an external velocity field, Zalesak's disk and shearing vortex, the linear sloshing, and the dam-breaking flow problem. Through comparison of the simulated results with the analytical or experimental ones, adaptability and accuracy of the water-air two-phase model are discussed. The analysis indicates that the simulation outputs are in good accordance with theoretical or experimental results, which means the model is capable to simulate incompressible water-air two-phase flows. With the further improved WENO schemes and THINC schemes, more precise prediction results for water-air two phase flow problems can be achieved with the proposed combined WENO-THINC model.
2021, 53(4): 973-985.   doi: 10.6052/0459-1879-20-430
Shock - shock interference flow flied prediction is one of the most challenging problems in supersonic flow and even hypersonic flow. In particular, type IV shock interference has attracted more and more attention due to the extremely high thermal loads it generates in the vicinity of stagnation point. In this paper, we analyze meticulously the effect of high temperature gas effects on the geometric structure of the shock interference and the flow field parameters, especially of the type IV shock interference, based on the calorimetric perfect gas model and the thermal perfect gas model considering only vibration excitation, respectively, by numerically solving the viscous two-dimensional compressible Navier - Stokes equations for the cylindrical-induced bow shock wave and oblique shock wave interference problems. With the increase of free stream Mach number, the effect of high-temperature gas is gradually significant. And then, based on a new genetic algorithm with generalized separability (multi-level block building algorithm), mathematical models that can predict the characteristic geometric structures such as the location of the triple wave point and the geometry of the supersonic
With the rapid development of low-power electronic equipment and self-powered wireless sensor networks in engineering, vibration energy harvesting has been widely used in aerospace engineering, mechanical engineering, biomedical engineering, and sustainable energy engineering. Vibration energy harvesting can not only convert vibration energy into usable electrical energy to power microelectronic equipment, but also reduce harmful vibrations to protect instruments and equipment. According to the different conversion mechanisms of vibration energy, the vibration energy harvesting system can be divided into electrostatic type, electromagnetic type, piezoelectric type, magnetostrictive type, triboelectric type and their hybrid type. Among them, piezoelectric and electromagnetic vibration energy conversion mechanisms have been widely used in various engineering fields due to their simple structure, easy assembly, and high energy conversion performance. Due to extreme environmental interference, broadband, low frequency and other vibrations are easy to occur in the engineering. It forces the rapid development of vibration energy harvesting technology in the direction of nonlinearity, which further attracts many scholars to study the optimal design of the structure and circuit of vibration energy harvesting. Firstly, this article summarizes the research progress of nonlinear vibration energy harvesting technology in the past ten years. It mainly includes the research status of design technology basis, nonlinear structure design, dynamic analysis and so on. Secondly, it focuses on the main research results of the integration of vibration energy harvesting and vibration suppression, including the application of nonlinear quasi-zero stiffness and nonlinear energy sink in the field of vibration energy harvesting. Finally, the optimized design of external vibration energy harvesting circuit and active control strategy are summarized, and effective methods to further improve the efficiency of nonlinear vibration energy harvesting are analyzed.
The previous studies on the vertical penetration of structures through level ice mostly did not consider the water action, which was inconsistent with the actual application scenarios. In this paper, a numerical simulation method of ice-water-structure interaction based on S-ALE (Structured Arbitrary Lagrange Euler) method and penalty function fluid-structure coupling algorithm is established by using LS-DYNA finite element software.Eulerian algorithm is used to describe air and water areas, Lagrangian algorithm is used to describe cylinder structure and level ice structure, and elastic-plastic strain rate model is used to characterize the mechanical properties of ice materials. Self-built test bench for vertical penetration of cylinder through level ice verified the feasibility of finite element method to calculate the interaction between structure and level ice problem.By simulating the ice-breaking process of cylinder vertical upward water breakthrough, it is compared with the ice-breaking process of cylinder vertical penetration in waterless environment. The results show that there is "water cushion effect" in the interaction between structure and level ice in water environment; The extreme value of ice breakthrough load has no significant change with the presence or absence of water; The duration of ice load when the structure breaks through level ice in water environment is obviously longer than that in waterless environment.The elastic deformation stage of level ice in water environment is longer, and the deflection change of level ice is greater than that in waterless environment. The research results of this paper provide a research basis for strength calculation and optimization design of ice-breaking structure with vertical vertical upward water breakthrough in polar ice area.
Vibration energy harvesting technology can convert the vibration energy of equipment working conditions into electrical energy, which provides a new idea for realizing self-powered wireless monitoring nodes in coal mines. In this paper, we design a linear-arch composed beam tri-stable piezoelectric energy harvester by introducing nonlinear magnetic force, and analyse the influence of the horizontal distance, vertical distance and excitation acceleration on dynamic characteristics. The nonlinear magnetic force model is established by the magnetic dipole method, the nonlinear restoring force of the linear-arch composed beam is measured experimentally, and the restoring force model is obtained by polynomial fitting. The dynamic model of the system is established based on Euler-Bernoulli beam theory and Lagrange’s equations. From the perspective of time domain, we analyse the influence of the horizontal distance, vertical distance of the magnets, and excitation acceleration on the dynamic characteristics of the system. A prototype of a linear-arch composed beam tri-stable piezoelectric energy harvester was fabricated, and an experimental platform was built for experimental research, by collecting the response speed data at the end of the composite beam after being excited, the speed-displacement data at the end of the composite beam was obtained, which verified the correctness of the theoretical simulation. The results show that the introduction of a nonlinear magnetic field can make the potential of the system have single potential well, double potential well or triple potential well. When we keep the excitation is constant, adjusting the horizontal and vertical spacing of the magnets can enable the system to achieve monostable, bi-stable or tri-stable motion, and the response displacement is relatively large during tri-stable motion. Increasing the excitation acceleration is beneficial for the system to across the barrier and achieve a large response. The research provides theoretical guidance for the design of linear-arch composed beam tri-stable piezoelectric energy harvester.
This paper established dynamic model of forced vibration of the curved piezoelectric energy harvesters with cracks. Green’s function of piezoelectric curved beam with cracks is obtained based on analytical solutions of vibration equation of the electromechanical coupled Prescott models and continuity conditions at crack sections. The system of the electromechanical coupled model is decoupled and the output voltage of the forced vibration of is acquired. The damage of the curved beam can be detected by inverse method, which is proposed in this paper and suitable for the structure in vibration．In the numerical simulations, the analytical solutions of damaged piezoelectric curved beam that have zero crack depth are compared with results in the previous references. The validity of solutions in this paper is verified. The influence of the crack depth, crack location, material geometric parameters and damping on frequency responses of the voltage is investigated. The results show that the response of damaged piezoelectric curved beam is decrease at the first order frequency that is the first order frequency of heathy curved beam. And the damaged piezoelectric curved beam is excited out the second-order frequency, which is the first-order frequency value of a healthy piezoelectric curved beam in a short-circuit. It is feasible and accurate to use the solution of vibration problem to detect the health condition of damaged piezoelectric curved beams.
The impact of granular flow such as debris flow and landslide, and how to design obstacles to deflect granular hazard, are becoming more and more important recently. In this study a bed-fitted depth-averaged model is established to simulate the interaction between granular flow and obstacles on steep terrains, which is able to simulate the birth and evolution of shock wave, reflection, bypass and runup during interaction between granular flow and obstacles on steep terrains. A series of numerical simulations concerning granular flows interacting with an array of tetrahedral obstacles of different distributions were conducted. A new dimensionless index called deflection efficiency was proposed, and the effects of tetrahedral obstacle arrays on the flow distance and lateral spreading characteristics of granular flow were quantitatively evaluated. A single tetrahedral obstacle plays a role of dissipation and deflection on granular flow, the latter of which even more obviously changes the granular flow pattern. An array of tetrahedral obstacles shows a comprehensive action of dissipation and deflection on granular flow, where multilevel actions dissipate energy in granular flow through bow shocks, and the splitting and changing actions on the flow path deflect granular flow. The obstacle system could control the final deposit to produce a protection region downstream.
Piezoelectric materials have good application prospects in the fields of vibration energy harvesting and structural vibration control due to their good electromechanical coupling characteristics. The piezoelectric interface control circuits based on synchronous switch and inductance can adjust the piezoelectric voltage amplitude and phase according to the working principle of oscillation circuit, optimizing the electromechanical energy conversion in piezoelectric vibration systems. The optimized synchronous electric charge extraction technique based on the interface control circuit mentioned above has realized the efficient piezoelectric energy conversion from vibration to electrical energy. This paper proposed a semi-active piezoelectric damping control circuit derived from the optimized synchronous electric charge extraction circuit. The energy conversion phenomenon between the primary and secondary sides of flyback transformer was unitized, the structure vibration suppression was then realized by transferring the electrical energy into the mechanical energy in piezoelectric vibration control systems. The new circuit which combines the piezoelectric electric charge energy extraction and the semi-active damping control approach realized the bidirectional control of piezoelectric vibration system, with a core of flyback transformer. The corresponding control circuit and its working principle were introduced, the piezoelectric vibration damping model under the new synchronized switch damping technology was also established. An experimental platform for the vibration control of a piezoelectric cantilever beam was built and the theoretical model is verified through experiments, the stability problem of the vibration control system was also solved through a simpler control approach.
2016, 48(4): 756-766.   doi: 10.6052/0459-1879-16-159

2017, 49(1): 3-21.   doi: 10.6052/0459-1879-16-348

2017, 49(1): 22-30.   doi: 10.6052/0459-1879-16-345

2016, 48(4): 767-783.   doi: 10.6052/0459-1879-16-161

2016, 48(4): 741-753.   doi: 10.6052/0459-1879-16-069

2019, 51(1): 1-13.   doi: 10.6052/0459-1879-18-054

2019, 51(3): 656-689.   doi: 10.6052/0459-1879-18-381

2016, 48(3): 519-535.   doi: 10.6052/0459-1879-15-436

2017, 49(2): 239-256.   doi: 10.6052/0459-1879-16-255

1978年，Barton提出的节理粗糙度系数（joint roughness coefficient，JRC）被国际岩石力学学会作为评估节理粗糙度的标准方法.然而该方法存在人为估值的主观性缺陷.就此，国内外学者围绕岩体结构面粗糙度定量化表征开展了大量的研究工作.首先，从二维节理轮廓线到三维岩体结构面，系统地阐述了其粗糙度定量化表征方法研究进展，并总结了各方法参数与JRC的关系；评价了各表征参数的本质特性及其适用性；指出了各方法参数获取过程中存在的问题，主要有：采样间隔的影响，三角形单元划分的影响，如何确定综合参数法中各参数的权重；针对这些问题，给出了笔者的一些想法、建议.与此同时，对结构面粗糙度表征的两个热点问题，即各向异性和尺寸效应的研究也进行了详细总结分析.最后，笔者认为：（1）分形维数因是描述自然界复杂几何体的一种简洁有力的工具，其仍是结构面粗糙度定量描述的有效方法；（2）3D打印技术的应用，有望在开展结构面各向异性、尺寸效应研究方面取得突破性进展.
2017, 49(3): 550-564.   doi: 10.6052/0459-1879-17-064

2012, 44(2): 269-277.   doi: 10.6052/0459-1879-2012-2-20120210
Abstract PDF(20)
Abstract:
In order to predict the cavitating flow characteristics in cryogenic fluids more exactly, a revised cavitation model considering the thermal effect with modified the evaporation and condensation source terms is established, which is based on Kubota cavitation model. The computations for cavitating flows in liquid nitrogen are conducted around an axisymmetric ogive by employing Kubota cavitation model and the revised cavitation model, respectively. The computational results are compared with the experimental data to evaluate the revised cavitation model. It is found that for the results of the revised cavitation model due to considering the thermal effects, the evaporation becomes smaller and the condensation becomes larger, the cavity length is shorter and the cavity interface becomes more porous compared with the results of original Kubota model. The results of the revised cavitation model are more accordant with the experimental data, and it dictates that the revised cavitation model can describe the process of mass transport more accurately in the cavitation process in cryogenic fluids and it is applicable for computations of cavitating flows in cryogenic fluids flow.
2012, 44(2): 252-258.   doi: 10.6052/0459-1879-2012-2-20120208
Abstract PDF(11)
Abstract:
This study focuses on the velocity-annular-effect (VAE) of compressible oscillatory flow inside parallel plate channel. By analyzing the mechanism of VAE, we conclude that VAE, which inevitably occurs in viscous oscillatory pipe flow, is most visible at the phase when the centerline velocity reaches zero. In order to quantitatively evaluate the VAE, coefficient of velocity annular effect (CVAE) was proposed as an index parameter, based on the slope of velocity profile when the centerline velocity reaches zero. Numerical computations with the index parameter CVAE were conducted to analyze the impacts of dimensionless parameters, i.e., Valensi number Va and maximum Reynolds number Remax, on the VAE of oscillatory flow inside parallel plate channel.

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