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## 2021 Vol. 53, No. 12

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In order to achieve the "3060" target of carbon peak and carbon neutrality, wind power will play an important role in our nation's energy system. The wake of a wind turbine is a key factor that affects the performance and levelized cost of wind power. It needs to be fully considered in the layout and control design of the wind turbine. This article first introduces the computational methods of wind turbine wakes, including analytical models, low-order models, large-eddy simulation, and methods for generating inflow turbulence. Analytical models and low-order models can compute wind turbine wakes fast, but they rely on model parameters and cannot or cannot accurately predict the turbulence characteristics of wind turbine wakes. Large-eddy simulation with parameterized models for wind turbines can accurately predict turbulence characteristics such as wake meandering. It is a powerful tool for investigating wake mechanism and can provide data and theoretical support for development of fast prediction models. Next, the article introduces the tip vortices, hub vortex and wake meandering and discusses their mechanism. For turbulent inflows, tip vortices mainly exist in the near wake. Meandering is the main feature of the far wake, which affects the characteristics of inflow for downstream wind turbines. There are two mechanisms for wake meandering: large-scale eddies of the incoming flow and shear layer instability. Numerical and observation results show that the two mechanisms coexist. The nacelle and the hub vortex have an important influence on wake meandering. Using the actuation surface model of the blade and the nacelle can accurately predict wake meandering. Research has shown that the turbulence characteristics for different designs of wind turbines are similar, which provides a theoretical basis for the development of a fast prediction model for wake turbulence. Current research efforts have been focused on wind turbine wakes on flat terrain. The mechanism of the atmospheric turbulence and wind turbine wake in complex terrain and marine environments are complex, which cannot be accurately predicted using the existing engineering models and needs further in-depth research.
2021, 53(12): 3169-3178. doi: 10.6052/0459-1879-21-493
2021, 53(12): 3179-3180. doi: 10.6052/0459-1879-21-629
2021, 53(12): 3181-3189. doi: 10.6052/0459-1879-21-396
2021, 53(12): 3190-3205. doi: 10.6052/0459-1879-21-472
Laser selective melting (SLM) is one of the most popular technologies in the field of metal additive manufacturing (3D printing), which can directly fabricate complex metal parts with nearly full density and performance similar to forged one. The thermal and kinetic behaviour of powder particles in SLM forming process is complex, which has significant influences on the fusion defects such as pores and cracks and hence on the mechanical properties of the final fabricated parts. This paper introduces the innovative application of the discrete element method (DEM) and computational fluid dynamics (CFD) combined model in the simulation of SLM, and explores the thermal and kinetic behaviour mechanisms of the powder particles the powder spreading and powder melting processes of SLM, combined with in-situ testing and online monitoring on the fabrication process. During the powder spreading process of SLM, it is found that the adhesion effect, wall effect and percolation effect compete with each other to control the dynamic behaviour of powder particles and ultimately determine the packing quality of the deposited thin powder bed. In the process of powder bed melting, the high-temperature metal vapour jetting from the molten pool drives the environmental protection gas to form an internal vortex flow. Then, the vortex flow drives the discrete particles in the powder bed to form complex fluid-solid coupling motion, resulting in spattering and denudation in the powder bed. The thermal buoyancy effect from the laser heating has no dominant effect on powder motions such as spattering and denudation. In this paper, the bi-directional dynamic coupling DEM-CFD model is proposed, which can fully consider the thermodynamic coupling forces between the discrete powder particles and the metal vapour form the molten pool, and provides a new way for the simulation of the thermal-kinetic behaviours of powder particles in SLM such as spattering and denudation.
2021, 53(12): 3206-3216. doi: 10.6052/0459-1879-21-403
The final quality of the printed products is greatly affected by the powder spreading in selective laser melting (SLM). However, little attention is paid on the influence of rough surface of deposited area on the quality of the powder spreading. Therefore, a rough deposited area is modeled as the new substrate for investigating the effect of surface morphology of the deposited substrate and processing parameters on the quality of powder bed during spreading using the discrete element method (DEM). The particle dynamic behavior and powder bed formation mechanism of metal powder on the surface of the deposited area during powder spreading are analyzed. The numerical results show that the quality of powder layer can be effectively improved by rotating the powder spreading direction to a certain angle along the laser scanning direction of the bottom layer, and the influence of the surface of deposited area can be significantly reduced by increasing the powder bed thickness. Reducing the hatch overlap rate can improve the retention capacity of particles in the deposited area, so that more particles are deposited in the deposited area, thereby increasing the packing density of the powder bed. But the powder collides with the rough surface of the deposited area causing more particles to splash. During spreading, the number of strong chains produced by the powder pile on the rough deposited area is more than that on the smooth deposited area due to the increase in roughness of the deposited area. Meanwhile, under the action of the roller, the force arch is destroyed and the particles are rearranged to form a dense powder layer. At the boundary of the deposited area, the generation of force arch will eventually lead to the appearance of vacancy defects in the powder layer at the boundary. The present study is helpful to improve the quality of powder bed during the powder spreading.
2021, 53(12): 3217-3227. doi: 10.6052/0459-1879-21-399
Compared to casting and other traditional manufacturing techniques, metallic powder-based additive manufacturing is manifesting its superiority in many fields like aerospace engineering, bio-medical engineering due to its short product cycle and feasibility. Among them, laser direct deposition, which has higher degree of freedom, has been widely employed in manufacturing and repairing complicated components. However, during its process, cross-scale multi-physics phenomena and phase change simultaneously happen under the laser spot, with extremely high temperature and pressure gradient, which makes experiments per se incompetent in investigations. In previous simulation frameworks, powders are inserted as Lagrangian points without consideration of ambient fluid, particle-particle interactions and phase change. The proposed framework here introduces volume of fluid technique into the recently-developed kernel approximation-based semi-resolved CFD-DEM, leading to a new semi-resolved VOF-DEM (or semi-resolved CFD-DEM-VOF) method which takes both thermodynamics, solid particles, phase change and free surfaces into consideration. Therefore, for the first time, the developed semi-resolved VOF-DEM model realizes the simulation of real physics involved in direct laser deposition. In this framework, shielding gas and metal, either melted or solidified, are two phases in VOF, and the interface between them is reconstructed by iso-Advector. DEM represents the unmelted solid particle, and CFD cells herein can resolve the metal particles thanks to the kernel approximation. Hence, the collision, adhesion, melting and coalescence of metallic particles, formation and evolution of molten pool and tracks are all reproduced. It is believed that this semi-resolved VOF-DEM modeling framework can provide a paradigm for simulation of direct laser deposition, along with other fields where particle system evolves with phase change.
2021, 53(12): 3228-3239. doi: 10.6052/0459-1879-21-361
Laser-directed energy deposition (L-DED), as a coaxial powder feeding metal additive manufacturing process, has a broad application prospect in the fields such as aerospace and transportation for its' advantages of high deposition rate and fabrication of large parts. However, the L-DED suffers from process defects in the resolution of metal part size and shape, such as significant size deviation and surface unevenness, which requires high efficiency and accurate numerical model to predict the shape and size of the cladding track. In this work, we proposed a high-fidelity multi-physics numerical model that considers the interaction between powders, laser beam, and melt pool. In this model, the laser beam is modeled as a Gauss surface heat source, a Lagrangian particle-based model is used for the powders-laser beam interaction, and then the Lagrangian particle-based model is integrated to finite volume method and volume of fluid to simulate the interaction between powders and melt pool and the corresponding melting and solidification process. The proposed model is validated by the experimental data of single-track TC17 alloy fabricated using L-DED. Based on the validated numerical model, a set of single tracks with different combinations of process parameters are predicted, followed by an in-depth analysis of process parameters' effect on the sizes and shapes of the cladding tracks and the corresponding underlying physical mechanism. It is identified that the process parameters dependent temperature distribution of the injected powders and the ratio of energy absorbed by powders to that by the substrate play an essential role in the velocity field of the melt pool and the size and shape of the cladding track. We expect that the proposed numerical model is a powerful tool to aid the process parameters optimization for the L-DED additive manufacturing process. At the same time, the results of this study can provide theoretical guidance on the shape and size resolution control of the fabricated parts.
2021, 53(12): 3240-3251. doi: 10.6052/0459-1879-21-420
Laser Additive Manufacturing (LAM) technology is very suitable for the near net forming of complex integral components and the rapid repair of high value-added damaged parts. However, the complex dynamic solidification process in the molten pool of LAM significantly affects the final microstructure of the formed parts, thereby restricting its service performance. A multi-scale mathematical model that integrates a macro heat and mass transfer and a multi-phase fields was established for the direct energy deposition by laser (DED-L) process of Inconel 718. The direct coupling of the macro-micro temperature field of the molten pool is solved. The two-dimensional global quantitative microstructure simulation of the molten pool is realized based on MPI parallel program design. The grain evolution process in the solidification of the molten pool is studied. The results show that the simulated molten pool size and solidification interface morphology are in good agreement with the experimental results. The morphology of solidification interface and the preferred orientation of crystal are important factors affecting the grain evolution. On the cross-section of the molten pool, the smaller the angle between the preferred orientation and the direction of temperature gradient, the more dominant the grain growth, because the solidification process is mainly driven by the direction of temperature gradient. On the longitudinal-section of the molten pool, the grain growth shows the characteristics of bending growth and "upper triangle". The gradual change of temperature gradient leads to the grain bending, and the competition behavior of adjacent grains determines the grain morphology. In this work, the mechanism of grain evolution in metal LAM is elucidated, which helps to clarify the thermophysical, chemical and metallurgical processes of additive manufacturing, and provides theoretical guidance for the prediction and control of microstructure. In addition, the multi-scale mathematical model can also be applied to the LAM process of other metal materials.
2021, 53(12): 3252-3262. doi: 10.6052/0459-1879-21-364
In order to predict the correlation among the processing parameters, microstructures, and mechanical properties for additive manufacturing, a computational framework integrating discrete element method, phase-field simulation, crystal plasticity finite element method, and extreme value statistics is proposed. The framework is applied to reveal the influence of laser scanning velocity on the microstructure evolution, yield stress, fatigue indicator parameter (FIP) distribution, and fatigue dispersity, in order to show its capability in simulating additive manufacturing process and the resultant mechanical properties. Firstly, discrete element method simulations are carried out to spread the powder bed layer by layer with the consideration of powder size distribution. The spreading is performed on the curved surface of the previously solidified layer. Secondly, the heat-melt-microstructure coupled non-isothermal phase-field simulations are performed to obtain the temporal and spatial evolution of melt, pore, grain boundary, grain distribution/orientation, etc., as well as the final polycrystal microstructure. Thirdly, crystal plasticity finite element method is utilized to attain the macroscopic mechanical response and stress/strain distribution of the additively manufactured polycrystal microstructure (AMPM) and FIP which is a surrogate measure for the driving force to form fatigue cracks. Fourthly, extreme value statistics are carried out to analyze the extreme value distribution of FIPs and the fatigue dispersity of the AMPM. Comprehensive simulations are put into practice for the selective laser melting based additive manufacturing of a typical metallic material 316L stainless steel. The simulation results indicate that the macroscopic yield stress of the AMPM is anisotropic and decreases with the increasing laser scanning velocity. The extreme value of FIPs from the AMPM with random distribution of grain orientations correlates well with the Gumbel extreme value distribution. The increase of laser scanning velocity could decrease the fatigue dispersity of the AMPM, but increase the FIP extremum and the associated driving force for fatigue crack initiation and thus notably decrease the fatigue life.
2021, 53(12): 3263-3273. doi: 10.6052/0459-1879-21-389
Severe aerothermal heating loads are commonly encountered on the V-shaped cowl lips of three-dimensional inward-turning inlets. To reveal the effects of the geometry parameters on the aerothermal heating loads, a simplified model called V-shaped blunt leading edge is employed, and numerical simulations and shock tunnel experiments are performed at a freestream Mach number of 6. The results indicate that with the combined effects of the R/r (i.e., the crotch rounding radius R to the blunt radius r) and the half-span angle β, three types of shock reflections are generated at the crotches, leading to obvious differences in the position and intensity of the heating peaks on the wall. In the geometric parameter space (R/r, β), the regular reflection occurs at the crotch when R/r and β are small. The supersonic jet impingement near the stagnation point causes the first type of central heating peak, which can reach up to 12 times stagnation heat flux of a cylinder with the same r. For larger parameter values of R/r and β, the Mach reflection occurs at the crotch. The collision of the robust jets and the shock wave/boundary layer interactions result in the second type of central heating peaks and outermost heating peaks, respectively, and these peaks are less than the first type of central heating peaks. An intensity transition criterion between the second type of central heating peaks and outermost heating peaks is established in terms of R/r and β. When R/r is large enough, the regular reﬂection from the same family occurs at the crotch. Correspondingly, both the second type of central heating peaks and outermost heating peaks decrease significantly.
2021, 53(12): 3274-3283. doi: 10.6052/0459-1879-21-448
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 jet in the type IV interference under different gas models are presented to obtain a quantitative assessment of the effect of high temperature gas effects on the transition criterion for the type of interference for thermal protection work. The comparison results of the radical interference structure and wall pressure and wall heat flux distribution for multiple sets of critical conditions on the transition criterion surface show that the interference types and flow field structures under different gas models differ significantly, and the obtained quantitative prediction model has certain reference value for the prediction of aerodynamic thermal environment in practical engineering applications. In the end, multiple sets of critical working conditions on the transition criterion surfaces are used to prove it, revealing the engineering significance of the criteria.
2021, 53(12): 3284-3297. doi: 10.6052/0459-1879-21-385
For underwater supercavitation vehicles powered by jet propulsion, the stability and morphological control for ventilated cavity are the key issues. In this paper, we use the VOF coupled level set interface tracking method, the compressibility of the gas is considered. By changing jet strength and model length, a series of numerical simulations is studied on the interaction between ventailated cavity and supersonic tail jets, and focused on the stability and closed position of the cavity. The numerical results show that: (1) under the action of the supersonic tail jet, the interface of the ventilated cavity will experience expansion, necking, fracture and retraction, and then begin to periodically oscillate and deflate. The morphological length of the ventilated cavity is greatly reduced compared with that under the condition of no jet. (2) Strong shear on both sides of the gas-liquid interface may induce cavity instability and collapse, and this cavity instability mechanism mainly depends on two dimensionless parameters $\overline J$(the ratio of jet thrust and cavitator resistance) and $\overline L$(the ratio of the model length to the diameter of the cavitator). The larger $\overline J$and the smaller $\overline L$, the more easily the cavity is destabilized. On this basis, the critical curves for the two states of stability and instability in the calculation examples are further summarized. (3) The more stable the cavity, the lower the amplitude and frequency of the pressure fluctuation at the nozzle outlet. At this time, the ventilated cavity could provide stable ambience for the rocket engine. (4) For the condition of instability cavity, the cavity is closed at the nozzle outlet; while for the stable cavity, the length from the nozzle outlet to the closed position is only related to the parameter$\overline J$, but not to the model length.
2021, 53(12): 3298-3309. doi: 10.6052/0459-1879-21-346
The double swept waverider has advantageous performances in subsonic characteristics and nonlinear vortex lift, while maintaining the high lift-to-drag ratio in hypersonic state, overcoming some deficiencies of the traditional waverider. However, it still has some defects such as poor low-speed stability. The waverider design given a 3D leading edge is developed from the osculating-cone treatment, and the double swept waveriders with wing dihedral and anhedral were generated by customizing the leading edge curves, which shared the same planform shape. Using CFD techniques, the low speed performances of the waveriders were evaluated, the lift drag characteristic and vortex structure were analyzed, and the effect of the wing with positive and negative dihedral angles, namely wing dihedral and wing anhedral, on stability was also studied. Results show that compared with the configuration whose projection in the front-view direction was a horizontal line, the configurations with wing dihedral and anhedral had nearly the identical lift-to-drag ratio. Different configurations were all unstable in longitudinal static stability, and the pitching moment of these configurations were similar. With wing anhedral, aerodynamic center moved backwards, increasing longitudinal stability; the wing dihedral improved the directional stability, while wing anhedral decreased it; wing dihedral improved the lateral stability, and the effect was stronger with higher wingtips. The dynamic directional stability of the waveriders can be improved by the wing dihedral obviously, while wing anhedral decreased it, and the effects was positive correlation to the distance of wingtips went up or down. According to the results, it was feasible to improve the low-speed stability of waverider by wing dihedral and anhedral, and the method provides a novel way to design the wide-velocity-range hypersonic vehicles.
2021, 53(12): 3310-3320. doi: 10.6052/0459-1879-21-234
Plasma flow control technology is an active flow control technology with plasma aerodynamic actuation as the control means. In order to further improve the controllable wing scales of plasma actuator, experimental investigations on characteristics of symmetrical dielectric barrier discharge (DBD) plasma actuator and flow separation control over a supercritical wing SC(2) - 0714 at high angle of attack using symmetrical DBD plasma actuator have been carried out by the force measurement and particle image velocimetry (PIV). The influence of wing scaling effect on plasma control is deeply studied and the energy consumption ratio coefficient suitable for separated flow control is proposed. In addition, the separated flow control mechanism of the plasma actuator is explored, and the influence law of Wing scale on separated flow control is mastered. The results show thats (1) with the increase of wing size, the length of copper foil electrode arranged on the wing is increased accordingly. The average power consumption of the actuator does not increase linearly with the increase of the electrode length within the scope of parameters in the present manuscript. When the electrode length reaches a certain threshold, the average power consumption of the actuator tends to a fixed value. (2) In the case of fixed Reynolds number, with the increase of wing scale, the control effect of plasma is not decreased, and the energy consumption ratio coefficient of plasma actuator increases. (3) The large-scale spanwise vortices and a series of coherent structures which are generated by the symmetrical DBD plasma actuator in the mainstream flow area and in the vicinity of the wall respectively become the key to the control of separated flow. The research results provide technical support for realizing the separated flow control of real aircraft using plasma actuators and promoting the engineering application of plasma flow control technology.
2021, 53(12): 3321-3330. doi: 10.6052/0459-1879-21-379
Laser shock processing (LSP) can effectively improve the fatigue life of materials, which is widely used in the aerospace field. CoCrFeMnNi high-entropy alloy is a classic high-entropy alloy system, so the studies on microstructure evolutions and shock wave responses after LSP play an important role in the application of this material in the aerospace field. The molecular dynamics method is used to simulate the shock of CoCrFeMnNi high-entropy alloy, and it is obtained that the elastoplastic two-wave separation phenomenon is related to the shock direction, showing obvious orientation-dependence. It is found that there is no two-wave separation structure when shocking along the [100] direction, and an intermediate phase will be produced in the process of plastic deformation. But, when shocking along the [110] and [111] directions, a two-wave separation structure is produced, and there are a large number of stacking faults and disordered structures in the impacted area, the high dislocation density is an important reason for the disordered structure. The phenomenon of two-wave separation is related to the number of active slip systems, the Hugoniot elastic limit and the critical impact velocity for plastic deformation when impacted along different orientations are related to the Schmid factor of the active slip systems. In addition, a gradient dislocation density structure is induced due to the shocking loading, the dislocation density first increases and then decreases along with the shock depth, and a greater dislocation density is produced when shocked in the close-packed direction. After the shock, there is residual compressive stress at the both ends of the model, the residual tensile stress is at the core of the model, and the magnitude of residual stress has obvious orientation dependence. Finally, compared with pure Ni with the same size and orientation, it is found that there are more disordered structures in CoCrFeMnNi high-entropy alloy than pure Ni during the impact process due to the lattice distortion effect.
2021, 53(12): 3331-3340. doi: 10.6052/0459-1879-21-468
The metal magnetic memory micro-magnetic non-destructive testing method can detect and evaluate the damage location and degree using the local change of magnetic state caused by the stress concentration or plastic zone of ferromagnetic materials. The quantitative theoretical analysis for micro-magnetic signals can provide important guidance for its engineering application. This paper reports on the research development of the magnetic-elastoplastic constitutive relationship of ferromagnetic materials under weak environmental magnetic field and its application in micro-magnetic signal analysis for the metal magnetic memory micro-magnetic non-destructive testing method. Regarding the research on the magneto-mechanical constitutive relationship, under the weak magnetization conditions of micro-magnetic testing, an analytical expression for the ideal magnetization constitutive relationship of ferromagnetic materials subjected to an elastoplastic load is established based on the effective field theory. Then, combined with the approaching principle between magnetization and ideal magnetization, the influence of the historical process of stress and strain loading on the magnetization under a constant and applied weak magnetic field is analyzed to consider the magnetization hysteresis effect. For the micro-magnetic non-destructive testing signal analysis, based on the elasticity theory, magnetostatics theory and the newly established magnetic-elastoplastic constitutive relationship, a two-dimensional model of the surface magnetic signal induced by the elastic stress or plastic zone in the ferromagnetic specimen under a weak magnetic field is established and solved by the finite element method. Combing with the exsting experiment results, the quantitative ability of theoretical model to describe the influence of elastoplastic factors on micro-magnetic signals is confirmed, and the correlation between the characteristic parameters of micro-magnetic signals and the size of local elastic stress or plastic zone is analyzed in detail. Compared with the existing magneto-mechanical constitutive relationship, the analytical expression of the ideal magnetization established in this paper is more concise, which helps to improve the understanding and application of the magneto-mechanical coupling effect.
2021, 53(12): 3341-3353. doi: 10.6052/0459-1879-21-325
The field of robotics involves many disciplines such as mechanics, mechanics, materials, control, electronics and computers. Among them, creeping robots can work in extreme environments, which in turn can effectively reduce the risk of manual work and improve work efficiency. Therefore, creeping robots have always been the focus of research in the field of robots. Piezoelectric ceramic is a new functional ceramic material that can convert mechanical and electrical energy into each other. The inverse piezoelectric effect refers to when an electric field is applied in the polarized direction of the dielectric, these dielectrics produce mechanical deformation or mechanical pressure in a certain direction, and when the applied electric field is removed, these deformations or stresses disappear. Based on the reverse piezoelectric effect of piezoelectric ceramics, an integrated three-legged crawling robot supported by three bending cross-sectional beams is designed. We use method of theoretical mechanics to establish the overall force analysis equation for the three-legged crawling robot. Then, we use Hamilton’s principle to establish the dynamic equation of the beams (with variable cross-sections and variable angles) of the piezoelectric-driven leg, and finally obtain the complete equations which can be used to calculate resonant frequencies of the piezoelectric-driven leg of the three-legged crawling robot. More importantly, the three-legged crawling robot is designed and produced, and the influence of the different bending angle, the different driving frequency, the different load and the different driving voltage waveform on the direction and speed of motion is explored in experiments. Finally, the asymmetric driving voltage is used to make the three-legged crawling robot realize the approximate straight motion without rail and movement of left turn and right turn, realize the designed movement in three directions, and finally analyze the energy consumption of the robot. This study can provide reference for the design and test of micro-crawling robots.
2021, 53(12): 3354-3365. doi: 10.6052/0459-1879-21-430
Fractional calculus has many excellent characteristics and is mainly used to improve the research accuracy for vibration characteristics of nonlinear systems in the field of dynamics. In this paper, the fractional-order derivative is introduced into the quasi-periodic Mathieu equation and the influences of fractional-order term on the stability of the equation are studied. Firstly, the conditions of the periodic solutions are obtained by the perturbation method, and the approximate expressions of the transition curves are also gotten. The accuracy of the approximate analytical solution is verified by comparing with the numerical solution, and they are in good agreement with each other. Moreover, approximate expressions of transition curves under different conditions are summarized. By analyzing their formal characteristics, it is found that the fractional-order term exists in the form of equivalent linear stiffness and equivalent linear damping in the equation, the general forms of equivalent linear damping and equivalent linear stiffness are obtained, and the thickness of unstable region is defined. Finally, the effects of fractional-order parameters on the size of stability region and the position of transition curves are analyzed intuitively by numerical method. It is found that the fractional-order term has both damping and stiffness characteristics, and the fractional coefficient and fractional order affect the transition curves of the equation in the form of equivalent linear stiffness and equivalent linear damping. Even in some cases, the effect of fractional-order term is almost equal to linear damping or linear stiffness. Reasonable selection of fractional-order parameters can make it show different degrees of stiffness or damping characteristics, and have different degrees of influence on the stability region of the equation and the position of the transition curve, thus affecting the value range of the stability parameters of the equation. These results are of great significance for the study of dynamic characteristics of such systems.
2021, 53(12): 3366-3375. doi: 10.6052/0459-1879-21-455
Identification of mechanical connections plays a significant role in predicting the dynamic behavior of an assembled structure. Due to the noise affection and the difficulty to directly use the measurable data to identify the dynamic properties of the joint with traditional methods based on substructure decoupling, a new method is proposed. Firstly, the components of the basic equation of substructure decoupling on the measurement degree of freedoms (DOFs) are extracted, and the form containing the joint dynamic stiffness matrix is obtained by matrix transformation. Then, the real joint dynamic stiffness matrix is decomposed into a known initial matrix and an incremental matrix to be solved, and the incremental iterative equation with convergence property is derived to enhance the numerical stability of identification when the number of interface DOFs is extremely large. Polynomial fitted dynamic stiffness is used to form a frequency domain estimation equation of fitting coefficients representing joint properties. By selecting appropriate frequency points to simultaneous equations according to the given criteria, an iterative formula for identifying joint properties is obtained, in which only the measurement frequency response functions (FRFs) of the assembly are needed. Finally, a numerical example and an experimental example are provided to verify the method and to describe the identification procedure. Numerical simulation of a 10-DOFs spring-mass system verifies the correctness and anti-noise of the proposed method. Tests and identifications are also carried out on a T-shaped beam structure with one adhesive connection and a L-shaped beam structure with two bolted connections. The result shows that the FRFs calculated by the finite element model (FEM) of the assembly recombination by the residual structure and the identified joint is in good agreement with the measured values in a wide frequency band, which indicates the effectiveness of this method in identifying the joint properties in the actual assembly structure.
2021, 53(12): 3376-3388. doi: 10.6052/0459-1879-21-280
In recent years, the exploitation and utilization of marine resources have risen to a social research highlight. In this context, domestic and overseas scholars work to promote the study on many aspects of underwater vehicles. As an important part of the research, the hull shape design of the underwater vehicles has direct relation to their underwater hydrodynamic characteristics. A variety of fishes in nature have attracted extensive attention from researchers for their good hydraulic resistance performance. In order to design a hull shape of underwater vehicle with good hydrodynamic characteristics, this paper focuses on the study of Sphyrna with a winglike head which enables their nimble swimming in oceans. This paper took Sphyrna as the bionic object, analyzed the water resistance reduction effects of three species of Sphyrna by establishing models, selected the biological shape characteristics of hammerhead shark as the reference to design the characteristic outline curve of the hull. Furthermore, a kind of hull shape of underwater vehicle has been designed according to engineering practice. Ansys Mosaic Technology was applied to establish a 3D structured flow field mesh model, and a Fluent simulation of the model followed. Apart from that, the author compared it with the shapes of common wing-shaped hulls and rotational underwater vehicles, focusing on its resistance performance. The result of a simulation analysis shows that the Sphyrna bionic model demonstrates better hydrodynamic characteristics in a stationary flow field than those of the two kinds of common underwater vehicles. Additionally, this paper also designed a comparative experiment to explore the characteristics of the flow field around the Sphyrna bionic model, which is of great guiding significance for the study on reducing the interference of the vehicles to the surrounding flow field and improving the stealthiness of the vehicles, and also provides a new direction for the design of underwater vehicles.
2021, 53(12): 3389-3398. doi: 10.6052/0459-1879-21-160
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.
2021, 53(12): 3399-3412. doi: 10.6052/0459-1879-21-200
The characterization and simulation of discrete fracture networks is a hot topic at home and abroad. In the development process of unconventional oil or gas reservoir, the in-situ stress field will have a significant impact on the flow properties of fractures. If fractures are regarded as static objects, there will be a great deviation from the field data. Therefore, more in-depth research should be done based on dynamic fractures. In this paper, an efficient hybrid numerical discretization method is proposed to solve the coupled mechanical problems of coupling geomechanics and fluids flow in tight oil reservoirs. The extended finite element method (XFEM) is used to solve the elastic deformation of rock, and the mixed boundary element method (MBEM) is adopted to accurately calculate the unsteady flux between matrix and fracture. The two numerical schemes are fully-coupled and the time-terms in overall calculation scheme is solved by the fully-implicit method, which can accurately and efficiently simulate the mechanism of fracture deformation and fluids flow in the development of tight oil reservoirs. In addition, the embedded pre-treatment is used to characterize the large-scale hydraulic fracture, and the effect of proppant is considered. The dynamic information of matrix and small-scale natural fracture can be captured by using the double-porosity effective stress principle and the characterization method of implicit fracture in dual-media. Therefore, the hybrid model proposed in this paper comprehensively characterizes the complex system composed of matrix, natural fractures and hydraulic fractures. The accuracy of proposed model is demonstrated by several examples in this paper. The study shows that the influence of the flow parameters change and the fracture aperture reduction caused by stress-field can not be ignored when evaluating the productivity of fractured horizontal well in tight oil reservoirs. This work can provide theoretical guidance for the development of unconventional oil and gas resources.
2021, 53(12): 3413-3424. doi: 10.6052/0459-1879-21-300
2021, 53(12): 3425-3426. doi: 10.6052/0459-1879-21-662