力学学报

PIEZOELECTRIC METAMATERIALS AND WAVE CONTROL: STATUS QUO AND PROSPECTS

Yuan Yi, You Zhenyu, Chen Weiqiu

Elastic wave metamaterial is a kind of artificially designed periodic structure. It has received extensive attention due to its special mechanical properties and has shown valuable and unique application prospects in both military and civilian fields. Actively or passively controlling the characteristics of elastic wave metamaterials according to the needs can endow them with stronger applicability. There are lots of tuning ways, among which using piezoelectric materials is a convenient, fast, high-precision, small-sized and low-cost method. In this article, we first briefly introduced the basic aspects of elastic wave metamaterials, tunable metamaterials, piezoelectric materials and several commonly used shunt circuits. Then, according to the different application forms of piezoelectric materials in elastic wave metamaterials, they are divided into two categories: in the first category, the piezoelectric material constitutes the major structure or acts as a part of the major structure; in the second category, the piezoelectric material is used in the form of a spring or a patch attached to the surface of the major structure or embedded in the structure, acting as an actuator or/and a sensor. We elaborated on the research topics and the development history of the two categories of piezoelectric elastic wave metamaterials, related to band gap regulation, waveguide, negative refraction, super transmission, topological state, cloak as well as shunt circuits. Finally, we summarized the research deficiencies of piezoelectric elastic wave metamaterials and outlined corresponding future research prospects.

2021, 53(8): 2101-2116. doi: 10.6052/0459-1879-21-198

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Preface of Theme Articles on Recent Advances of Porous Flow in Unconventional Oil and Gas Resources

Cai Jianchao, Liu Yuewu

2021, 53(8): 2117-2118. doi: 10.6052/0459-1879-21-408

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ADVANCES IN MICROMECHANICAL PROPERTIES OF HYDRATE-BEARING SOILS

Zhao Yapeng, Liu Lele, Kong Liang, Liu Changling, Wu Nengyou

As one kind of clean and unconventional energy resources, natural gas hydrates have drew enormous interests worldwide due to their high energy density, large reserves, and wide distribution in nature, and lots of countries have tried their best to develop suitable methods for gas hydrate production with acceptable safety, efficiency, continuity, and controllability. Industrialized production of gas hydrates basically needs to deeply understand mechanical properties of hydrate-bearing soils and fully clarify how these mechanical properties evolve during gas hydrate production. Mechanical properties of hydrate-bearing soils are inherently governed by their micro structures inside, and great efforts have been made to study macro mechanical properties of hydrate-bearing soils from the micro perspective, which is of great significance to deep understandings of how mechanical properties of hydrate-bearing soils evolve during gas hydrate production. In this study, advances in mechanical properties of gas hydrate crystal, interface cementation between gas hydrate and soil particles, and bulk hydrate-bearing soils are summarized. Gas hydrate crystal structures and pore-scale hydrate morphologies in hydrate-bearing soils are briefly introduced. Then, fundamental principles and advantages of microscopic testing techniques such as computed tomography (CT), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and atomic force microscopy (AFM) applied to mechanical property characterizations are emphasized. Then, up-to-date researches performed by using triaxial shearing tests combined with CT, particle flow code (PFC) and molecular dynamics (MD) numerical simulations are reviewed, and the shearing mechanism as well as constitutive models of hydrate-bearing soils are analyzed. At last, challenges in current studies on micromechanical properties of hydrate-bearing soils are discussed, and corresponding suggestions are subsequently proposed to further studies on mechanical properties of hydrate-bearing sediments.

2021, 53(8): 2119-2140. doi: 10.6052/0459-1879-21-138

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DYNAMIC FRACTURE PROPAGATION MECHANISM AND APPLICATIONIN TIGHT OIL RESERVOIR

Di Shiying, Cheng Shiqing, Bai Wenpeng, Wei Cao, Wang Yang, Qin Jiazheng

Tight oil reservoirs have achieved certain oil increase effect by supplementing formation energy with water-injection huff and puff. However, formation pressure and production decrease rapidly after multiple rounds of water injection. In order to improve the oil enhancement effect of tight oil reservoirs, changing the development method quickly became hotspot research. This paper analysis the stress field distribution near the tip of type I fracture considered the complex fracture morphology of tight oil reservoirs based on Irwin theory and elastic mechanics. A multi-fracture cross-fracture propagation model is established based on seepage mechanics, fractured tight reservoir characteristics and dynamic fracture seepage characteristics. The fracture propagation length is obtained based on the fracture propagation mechanism and the energy conservation principle. It is proposed to turn water-injection huff and puff into unstable pulse water injection according to the principle of reverse imbibition in tight oil reservoirs. Comparative analysis of two energy supplementary generation methods, water-injection huff and puff and pulse water injection, predicting cumulative oil production, pressure and remaining oil distribution in 10 years. The results show that the net internal pressure of the fracture increases with the increase of water injection, and the stress field intensity factor also increases. When the stress field intensity factor reaches the fracture toughness, it will expand at the fracture tip. The expanded and extended natural fractures communicate with each other, presenting irregular and complex fracture networks. Reverse imbibition mainly occurs in the complex fracture networks. Pulse water injection has a high cumulative oil production, a wide area of water injection and strong reverse imbibition. The findings of this study can help for better understanding of the transformation of water-injection huff and puff into pulsed water injection from horizontal wells in fractured tight oil reservoirs. It can give full play to the effects of reverse imbibition and linear displacement. This research provides guidance for it can achieve the purpose of effective oil displacement of the dynamic fracture network.

2021, 53(8): 2141-2155. doi: 10.6052/0459-1879-21-154

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FAST AND ACCURATE PHASE EQUILIBRIUM CALCULATIONS FOR CONDENSATE SHALE GAS RESERVOIRS

Zhang Tao, Bai Hua, Sun Shuyu

Phase equilibrium calculations of complex fluids in shale gas reservoirs require the establishment of advanced numerical models that consider capillary effects, and the design of fast and reliable algorithms to handle the various components in the reservoir fluids in practical working conditions. In this study, we develop a thermodynamically consistent VT-type pore-scale flash calculation scheme based on realistic equations of state suitable for oil/gas reservoirs, e.g. the Peng-Robinson equation of state. The effect of capillarity has been incorporated in the scheme for a more accurate description of the thermodynamic properties of shale gas, and the diffuse interface model is applied to establish a dynamic evolution scheme in the phase equilibrium process, and a convex splitting method is used to model the evolution of compositional moles and volume. In order to accelerate the iterative flash calculations for realistic reservoir fluids containing a large number of components, a self-adaptive deep learning algorithm is developed in this paper with a novel structure to achieve wider applicability to various components in different fluids. The input and output features of the neural network are selected as the key thermodynamic features on the basis of thermodynamic analysis, and the network hyper-parameters have been carefully tuned to achieve a better performance on both accuracy and efficiency. Advanced deep learning technics resolving overfitting problems have been applied in our algorithm. The trained model significantly accelerates the conventional flash calculation based on iterative methods, while a good prediction accuracy has been preserved. Phase stability test and phase splitting calculations are automatically incorporated in our prediction, and we can significantly capture the effect of capillarity on phase equilibrium behaviors. Such a fast, accurate and reliable shale gas phase equilibrium calculation scheme using deep learning algorithms can provide an initial phase distribution field with physical meanings for subsequent multiphase flow simulations, while the number of phases can be also determined. The thermodynamic information and analysis can also be used as a thermodynamic basis for a multiphase numerical model with built-in physical conservation.

2021, 53(8): 2156-2167. doi: 10.6052/0459-1879-21-229

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A NOVEL METHOD FOR EVALUATING SHALE LOST GAS AMOUNT AND DESORPTION GAS AMOUNT BASED ON SEGMENTED VARIABLE BOUNDARY MODEL

Zeng Kecheng, Xie Haipeng, Jiang Peixue, Zhou Shangwen, Xu Ruina

The accurate evaluation of gas content is significant for the efficient exploration of unconventional natural gas reservoir, direct method adopts lost gas model and combines with desorption curve to evaluate reservoir gas content. However, the classical lost gas model is derived from constant pressure condition and spherical particle assumption in the estimation of coal bed methane, such as USBM method proposed by US Bureau of Mine, it brings a lot of errors for the deep-depth shale gas reservoir in which core sample is cylindrical shape. Based on diffusion theory, this work used time-varying pressure condition and cylindrical coordinate to solve one-dimensional diffusion equation and obtained analytical solution, then proposed a novel lost gas model (segmented variable boundary model). This model is enabled to describe two processes, i.e., drifting process and desorption process, with different gas-diffusion features. The result of segmented variable boundary model shows that the pressure drop between core sample and boundary increases in the drifting process when core sample is drifted from bottom hole to ground, due to the decreases of boundary pressure, lead to that the diffusion rate is accelerating when gas is escaping from core sample to environment, thus the diffusion curve of drifting process is concave. In the desorption process, core sample is placed in desorption canister and boundary pressure is constant, pressure drop between core sample and boundary is decreasing along the gas escaping from core sample, the diffusion rate is moderative and thus the curve of desorption process is convex. For further validating this model, a lost gas-desorption gas simulating experiment system was set up in lab based on the principle of similitude, and we conducted simulating experiment using cylindrical shale samples to obtain the diffusion curve in drifting process and desorption process, through comparing the experimental data with segmented variable boundary model and USBM model, demonstrated the validation of segmented variable boundary model. Moreover, the segmented variable boundary model is applied to fit the experiment data from the Y151 well in South Sichuan Basin, the fitting results have good consistency with experimental data, which indicates that the segmented variable boundary model is suitable for practical engineering condition.

2021, 53(8): 2168-2178. doi: 10.6052/0459-1879-21-187

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STUDY ON GAS ADSORPTION AND TRANSPORT BEHAVIOR IN SHALE ORGANIC NANOPORE

Song Wenhui, Yao Jun, Zhang Kai

The characteristics of shale gas reservoir are expressed in terms of complex pore structure and multiple gas occurrence pattern. Adsorbed gas and bulk gas coexist in nano-scale organic pores. The influence of organic pore shape on confined gas adsorption and flow behavior is not clear, which consequently causes the inaccuracy to understand gas transport mechanisms in shale gas reservoir. To solve this problem, different shapes of organic pore (circular pore, slit pore, triangle pore, square pore) are constructed in this study and Grand canonical Monte Carlo method is first applied to simulate gas adsorption process in different shapes of organic pore across different pore pressure. We found that gas adsorption behavior in different shapes of pores conforms well with the Langmuir adsorption pattern. The absolute adsorption, excess adsorption and gas adsorption parameter change with pore size and pore pressure is analyzed and the influence of pore shape on gas adsorption is studied on this basis. The mathematical model of adsorbed gas surface diffusion and free gas slip flow in different shapes of organic pore is established based on the understanding of thermodynamic gas adsorption pattern in different shapes of organic pore. The respective contribution of adsorbed gas flow and free gas flow on total gas permeability is studied in organic pores with different pore shape and pore size combining molecular simulation results with established gas transport models. The results indicate that the slit pore exhibits the largest value of maximum gas concentration, Langmuir pressure and the weakest adsorbed gas surface diffusion capacity compared with the other three pore shapes. The adsorbed gas surface diffusion influence on gas permeability can be neglected at pore radius larger than 5 nm for different pore shapes. This study reveals the influencing mechanism of organic pore shape on gas adsorption and flow ability during practical shale gas reservoir production.

2021, 53(8): 2179-2192. doi: 10.6052/0459-1879-21-224

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EXPERIMENT OF ADSORPTION-DESORPTION HYSTERSIS OF GAS IN COAL SHALE BY USING NUCLEAR MAGNETIC RESONANCE SPECTRUMS

Tang Jupeng, Tian Hunan, Pan Yishan

Coal shale gas was mainly composed of adsorbed and porous medium-confined gas. The desorption of coal shale gas generally lagged behind the adsorption, therefore, it was significance to quantitatively study the adsorption and desorption hysteresis law from the micro perspective for stable production and stimulation of shale gas wells in the later stage. Based on a large number of fruitful previous studies and the theory of nuclear magnetic resonance spectrums, an improved micro evaluation model for the accurate characterization of the adsorption and desorption hysteresis effect of coal shale gas was deduced. In order to simulate the whole process of adsorption and desorption hysteresis of gas in coal shale, the nuclear magnetic resonance spectrum technique was applied for the experimental research on adsorption and desorption hysteresis of gas in coal shale from coal floor of No. 36 coal seam in No. 3 mining area of Dongbaowei Mine in Shuangyashan Basin. And on such bases, the micro quantitative research on adsorption and desorption hysteresis law of the adsorbed gas, porous medium-confined gas, macro gas measurement by micro method in coal shale was carried out, the mechanism of adsorption and desorption hysteresis and its influence on shale gas mining in deep coal measures were preliminarily explored. Results indicated that in the process of adsorption and desorption under stress state, both adsorbed and porous medium-confined gas had obvious hysteresis; the relationship between macroscopic hysteresis coefficient of gas and the average effective stress was power function; while the relationship between hysteresis coefficient and average effective stress caused by adsorbed gas could be fitted by quadratic polynomial, and it was the same with porous medium-confined gas. The coupling of stress damage and gas diffusion in the pore fracture structure of coal series shale might be one of the fundamental reasons for the gas adsorption and desorption hysteresis of coal shale.

2021, 53(8): 2193-2204. doi: 10.6052/0459-1879-21-264

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EVALUATION METHOD AND APPLICATION OF FOAM DYNAMIC STABILITY IN HETEROGENEOUS CORES BASED ON NUCLEAR MAGNETIC RESONANCE TECHNOLOGY

Li Yuan, Di Qinfeng, Wang Wenchang, Hua Shuai

Based on the conservation of effective pore volume and nuclear magnetic resonance technology, an evaluation method for the dynamic stability of foam in the cores was established. The oil and water calibration method was used to measure the volume of the oil phase and foam liquid in the cores, and the dynamic stability factor of the foam during the core displacement process was calculated. The transverse relaxation spectrum and nuclear magnetic resonance image of the double-layer heterogeneous core were tested. The oil displacement effect and dynamic stability factor of the nanoparticles-enhanced foam and the surfactant foam were compared. The results showed that the water phase volume in the core rose rapidly before 2.0 PV of foam was injected and then was basically stable; while the gas volume increased gradually, and the rising rate decreased after 5.0 PV of foam was injected. The dynamic stability factor of the foam had experienced three stages which was sharp decreasing, progressive increasing and stabilization. The oil displacement effect in the early stage of the foam mainly depended on the water phase. As the water phase volume was basically stable, the oil production rate of the cores had an obvious positive correlation with the growth rate of the foam dynamic stability factor, that was, the displacement of the remaining oil depended on the foam gas during middle and late stages. Compared with surfactant foam, nanoparticles-enhanced foam improved the sweeping capacity and oil displacement efficiency in the low permeability layer, inhibited the unstable stage of foam development and improved the final equilibrium value of the dynamic stability factor. The stability evaluation method could be used to reflect the characteristics of foam seepage and to screen stable foam systems suitable for reservoir characteristics.

2021, 53(8): 2205-2213. doi: 10.6052/0459-1879-21-278

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FLOW MODEL OF IONIC LIQUIDS IN POROUS MEDIA UNDER COUPLED ELECTROMAGNETIC AND SEEPAGE FIELDS

Jia Hu, Zhang Rui, Li Pengwu

Ionic liquids (ILs), as a class of green and environment-friendly materials, are adjustable and multifunctional. ILs have excellent electromagnetic field response, which hold a great promise for the adjustment of waterflooding pathway. In this paper, the electromagnetic response mechanism of ILs in capillary is firstly analyzed. Then a flow model of ILs in porous media under coupled electromagnetic and seepage fields is established. Finally, the theoretical derivation and numerical analysis results show that the capillary flow rate under coupled electromagnetic and seepage fields is mainly determined by the ratio of ILs conductivity to viscosity (internal factor), electromagnetic field strength and pressure gradient (external factors). The electromagnetic field generates an electromagnetic drive pressure on the ILs by Lorentz force, forming an electromagnetic drive equivalent pressure gradient analogous to the pressure gradient, thereby changing the flow rate of ILs. When the electromagnetic field strength is 2.0 × 10⁴ V/m·T, the electromagnetic field can form a 10 kPa/m electromagnetic drive equivalent pressure gradient on an ILs with a conductivity of 0.5 S/m. Meantime, the flow direction of ILs in porous media can be controlled by adjusting the direction of electromagnetic field, which can solve the difficult problem of using pressure difference to control flow paths, and provides a theoretical basis for intelligent oil displacement of ILs. Furthermore, the thermal effect generated by the electromagnetic field will affect the flowing capacity of ILs and the oil displacement efficiency.

2021, 53(8): 2214-2224. doi: 10.6052/0459-1879-21-156

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PORE-SCALE SIMULATION OF MULTIPHASE FLOW CONSIDERING THERMO-HYDRO-MECHANICAL COUPLING EFFECT IN POROUS MEDIA

Cai Shaobin, Yang Yongfei, Liu Jie

Researches on the thermo-hydro-mechanical (THM) coupling effect in porous media from the perspective of pore-scale is of great significance for the study of enhanced oil recovery, nuclear storage, and geological sequestration of CO₂. In order to study the migration of oil resources in porous media in the deep oil reservoir, we used a combined method to realize the THM process occurring in porous media. The Darcy-Brinkman-Biot method was applied to simulate the THM process and the calculation of thermal stress was achieved by adding the Duhamel-Neumann thermoelastic stress to the model. A water-oil two-phase flow process considering THM coupling effect in porous media was then realized. The model has simulated flow of multiphase fluid in the pore space by solving the Navier-Stokes equations and calculated flow of the fluid in the rock matrix by solving the Darcy equation. The two processes were coupled with a series of momentum exchange equations to obtain the displacement of solid particles, thus realizing the fluid-solid coupling effect. On this basis, a heat transfer model was added to numerical model to consider the influence of temperature field on the two-phase flow process. Temperature field acts on the matrix in the form of thermoelastic stress to realize the THM coupling process. Based on the model, we simulated the flow process of water-oil two-phase fluid in a two-dimensional porous media model. The results have shown that: (1) the direction of thermal stress was opposite to that generated by fluid-structure coupling effect, which made the total stress smaller than that under the fluid-structure coupling effect; (2) the porosity of the model increased with the increase of temperature, however, when the injection temperature difference reached 150 K, the porosity no longer increased significantly; (3) with the increase of temperature, the relative permeability of the water phase increased and the equal-permeability point shifted to the left.

2021, 53(8): 2225-2234. doi: 10.6052/0459-1879-21-294

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DISCONTINUOUS GALERKIN FEM METHOD FOR THE COUPLING OF COMPRESSIBLE TWO-PHASE FLOW AND POROMECHANICS

Ma Tianran, Shen Weijun, Liu Weiqun, Xu Hao

Understanding the coupled multiphase flow and solid deformation processes in porous media is a significant issue in the area of developing and utilizing underground resources. This study first established the coupled modeling of compressible two-phase flow and deformation of porous media, which considers capillarity and gravity. Meanwhile, the strong form and the corresponding weak form of coupled multiphase flow and solid deformation model were presented. Then, the capacity of the proposed DG method for the coupled hydromechanical model was veriﬁed by comparison with analytical and numerical results of the one-dimensional Terzaghi consolidation problem. Subsequently, the two- and three-dimensional cases were performed to study the flow behaviors and deformation characteristics, respectively. In addition, the effects of the penalty factors $ {\delta }_{\rm{s}} $ and $ {\delta }_{\rm{f}} $ on the stability of the numerical results were analyzed. The simulation results show that gas saturation and pore pressure continually increase with the injection of gas. The increment of pore pressure reduces the effective stress, which results in deformation and expansion of the porous medium. The gas floats up and gathers at the top boundary due to gravity. The decrease of the penalty factors $ {\delta }_{\rm{s}} $ and $ {\delta }_{\rm{f}} $ trends to cause the fluctuation of saturation, pressure, effective stress, and displacement. The increases in penalty factors are beneficial to suppress the discontinuity of the finite element function crossing the elements.

2021, 53(8): 2235-2245. doi: 10.6052/0459-1879-21-177

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SEMI-ANALYTICAL METHOD FOR UNSTABLE PRESSURE OF 3D FRACTURE NETWORK

Wang Zhikai, Cheng Linsong, Cao Renyi, Wang Jin, Jia Pin, Wang Xuanru

Due to the influence of in-situ stress and fracturing technology, the distribution of hydraulic fracture network in highly deviated wells is complex with different inclined directions, different distribution forms and different penetration degrees. In this paper, the fracture surface is discretized into several rectangular micro elements to realize the effective characterization of fracture morphology. The seepage process is divided into two stages: matrix flow to fracture and fracture flow to wellbore. The numerical solution of unsteady seepage in discrete fracture surface is constructed by using finite difference method, and the analytical solution of unsteady seepage in matrix is constructed by combining closed boundary source function and superposition principle. The solution of the unstable pressure of the 3D fracture network is obtained by coupling the numerical solution of flow in fracture and the analytical solution of flow in matrix. Based on the integral mean value theorem, a solution method of point source and special line source instead of surface source is proposed to solve the seepage in matrix. The feasibility and applicable conditions of this method are analyzed, which can ensure the accuracy of the model and improve the calculation efficiency. The research shows that the point source function area fraction method can accurately solve the bottom hole pressure dynamic of 3D pressure fracture network in the matrix, but the calculation is quite inefficient. The point source and special line source approximate surface source method based on the integral mean value theorem can greatly improve the calculation efficiency, and can achieve higher accuracy when the fracture micro element division is more precise (the dimensionless side length of micro element is less than 0.15). Based on this model, the flow regimes and related sensitivity analysis are analyzed. The results show that the fracture conductivity, fracture dip angle, fracture height and fracture interval have great influence on the typical well test curves of highly deviated wells. Fracture dip angle and fracture interval mainly affect the pressure and pressure derivative curve from linear flow to early radial flow, especially when the fracture height is small, the pressure derivative has backflow phenomenon.

2021, 53(8): 2246-2256. doi: 10.6052/0459-1879-21-183

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AN UNSTABLE POROUS FLOW MODEL OF MARINE-CONTINENTAL TRANSITIONAL SHALE GAS RESERVOIR

Chen Zhiming, Wang Jianan, Liao Xinwei, Zeng Lianbo, Zhao Pengfei, Yu Wei

Marine-continent transitional shale is often interbedded with coal seams and sandstone. The reservoir has poor continuity, rapid lateral change, and high heterogeneity. Hydraulic fracturing technology is the key method to obtain economic production. However, there is currently a lack of effective unstable porous flow model of Marine-continent shale gas reservoirs. In response to this problem, considering the characteristics of shale gas adsorption/desorption, diffusion, fractures, and shale heterogeneity, a heterogeneous shale gas mathematical model was established and solved by using boundary element method. First, a vertically fractured well model in MCT shale-gas reservoir is built. Second, Radial composite model is used to reflect the high heterogeneity, and Langmuir isothermal adsorption curve is applied to describe the gas adsorption/desorption and diffusion. Dual-porosity model, boundary element model, and Pedrosa’s substitution are respectively used to simulate the natural fractures, hydraulic fractures, and effect of stress-sensitive permeability The analysis results show that the unstable flow characteristics in Marine-continent transitional shale gas reservoir include flow early stage, bilinear flow, linear flow, inner radial flow, shale gas desorption, transition section, outer radial flow and boundary-dominated flow stage. This model has been applied in the process of well testing of Marine and continental transitional shale gas in Ordos Basin, and the actual effect is good. The research results can provide some theoretical support for the fracturing evaluation of similar shale gas reservoirs, and it has a good application prospect.

2021, 53(8): 2257-2266. doi: 10.6052/0459-1879-21-271

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STUDY ON INSTABILITY MECHANISM AND EVOLUTION MODEL OF PROPELLER TIP VORTICES

Wang Lianzhou, Wu Tiecheng, Guo Chunyu

The propeller wake dynamics is a fundamental but very complicated fluid mechanics problem. Its complexity comes from its sophisticated vortex system, which keeps evolving in high-speed shear layer flow. The mechanism of propeller wake behaviors such as the evolution from stable regime to unstable regime and the flow phenomenon in a complex operating environment have always been difficult and hot topics in the field of fluid mechanics. From the perspective of engineering applications, propeller wakes are directly related to the macroscopic characteristics of marine structures, a better understanding of the dynamic characteristic of the propeller wake under multiple operating conditions helps to improve the propulsion performance related to vibration, noise, and structure problems and has important practical significance for the design and optimization of next-generation propellers with good comprehensive performance. In this paper, the propeller wake dynamics are analyzed numerically using DDES, LES and NTM methods and experimentally based on PIV flow measurements, and the triggering mechanism of the instability of the propeller wake is revealed. Based on the evolution mechanism of the tip vortex in the uniform inflow, an evolution model of the tip vortices is proposed. The proposed model can accurately reproduce the evolution process of propeller tip vortex, predict the instant and position of tip vortex merging, which is of great significance to the prediction and control of propeller flow noise and the design of propellers with excellent performance.

2021, 53(8): 2267-2278. doi: 10.6052/0459-1879-21-151

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EXPERIMENTAL STUDY ON THE EFFECT OF SOLID PARTICLES ON RIBLET-PLATE TURBULENT BOUNDARY LAYER

Yan Dong, Sun Jiao, Gao Tianda, Chen Pi, Cheng Yuting, Chen Wenyi

The particle image velocimetry (PIV) is used to conduct experimental research in the solid-liquid two-phase wall turbulent boundary layer in smooth and riblet surface. The streamwise-normal two-dimensional velocity field information of clean water (as single phase) and water with polystyrene particles which diameter is 155 μm was collected, and the turbulence statistics such as the average velocity profile, turbulence intensity and Reynolds shear stress of the smooth and riblet surface are compared in particle phase and clean water to analyze the behavior of fluid in different wall boundary layers. Coherent structures were detected by quadrant splitting method and the concept of local average velocity structure functions of the streamwise is utilized to extract the sweep and eject motions under different operating conditions. Under the different wall conditions, the dimensionless fluid velocity with particles was greater than that of clean water, the Reynolds stress in the logarithmic law region is decreased and the turbulence intensity is receded. The addition of particles reduces the drag reduction near the riblet surface with different velocities, but the drag reduction effect of particles acting on smooth wall surface is not obvious. The number of coherent structures is increased with the addition of particles and the normal fluctuating velocity is decreased. The number of coherent structures is increased near the riblet surface, the normal fluctuating velocity is increased in a higher free flow velocity and the normal fluctuating velocity is decreased in a lower free flow velocity. This indicates that the large vortices can be broken into more vortices under different drag reduction conditions, and this effect is increased by the addition of particles.

2021, 53(8): 2279-2288. doi: 10.6052/0459-1879-21-149

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POISSON’S RATIO AND TRIAXIAL COMPRESSION DEFORMATION PATTERN OF CLOSED-CELL ALUMINUM FOAM

Jia Ran, Zhao Guiping

Poisson’s ratio is a key parameter in the phenomenological constitutive models of closed-cell aluminum foam. In order to resolve the divergence in understanding the change law of closed-cell aluminum foam Poisson’s ratio and understand the physical meaning of the characteristic points in the closed-cell aluminum foam Poisson’s ratio change rule, the numerical simulation method is used. The 3D-Voronoi model and 2D-Voronoi model of closed-cell aluminum foam are established and simulated under the boundary condition of lateral displacement coupled uniaxial compression. Based on the phenomenological characteristics of the closed-cell aluminum foam constitutive model, it’s also very important to study the deformation patterns of the closed-cell aluminum foam. In order to clarify its deformation patterns under triaxial compression, the 3D-Voronoi model of the closed-cell aluminum foam is simulated under the boundary condition of lateral displacement limited axial compression. The results show that the thickness reduction characteristic in the contact of conventional shell elements is the reason for the divergence of the Poisson’s ratio of closed-cell aluminum foam, however, the thickness reduction does not affect the deformation mode of the cell structure of the aluminum foam model before densification; the accurate change law of the Poisson’s ratio of closed-cell aluminum foam is an “S” curve of “increasing-decreasing-increasing again”, and the maximum value of the curve corresponds to the deceleration point of the energy absorption efficiency growth; in the state of proportional axisymmetric loading paths, closed-cell aluminum foam has four lateral deformation patterns, namely, “(short-term) compression→expansion”, “compression→expansion→compression→expansion”, “compression→(short-term) expansion” and “compression”.

2021, 53(8): 2289-2297. doi: 10.6052/0459-1879-21-173

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CONTACT MECHANICAL RESPONSE ANALYSIS OF SOIL-FOUNDATION SYSTEM UNDER THE INFLUENCE OF TUNNEL EXCAVATION

Hong Xuefei, Zhang Dingli, Fang Huangcheng, Fang Qian, Zhou Mozhen, Hou Yanjuan, Sun Zhenyu

To realize the theoretical prediction for mechanical response of soil-foundation system due to new tunneling below, an analytical method for the mechanical response of the soil-foundation system under the disturbance of tunnel construction is established, considering the multi-body contact effect. In this method, the stratum is regarded as a homogeneous isotropic medium with linear-elastic property. Next, the contact theory is introduced into the derivation process to consider the contact effect between stratum and foundation, and a new analytical strategy, “exchanging the sequence between tunnel excavation and foundation action”, is proposed to determine the analytical expression of contact pressure in final state. Then, the difficulties of determining the contact pressure under the coupling action of tunnel excavation and multi-body contact are overcame. Finally, the proposed solution is obtained using the superposition of elasticity solution. The analytical results are in good agreement with the numerical results, which verifies the correctness of the analytical solution. Based on the proposed solution, the effects of stratum parameters, tunnel depth, radial displacement of tunnel boundary and external load concentration on the vertical displacement increment, contact pressure and internal force distribution of foundation are analyzed. The results show that the analytical method can accurately predict the mechanical response of the soil-foundation system, and it can realize the quantitative description of the complex contact mechanical behavior between the stratum and foundation. The effects of Young’s modulus and Poisson’s ratio of formation focus on deformation and stress respectively, while the variation of tunnel buried depth and radial displacements along the tunnel boundary lead to significant changes in deformation and stress. The ground displacement is the result of the coupling effect of tunnel excavation and multi-body contact, and the significant impact region is limited near the contact area. Under the influence of tunnel construction, the contact pressure will redistribute, which is characterized by “release in the middle and concentration at the end”. As a result, the internal force of foundation is found to increase greatly, and when the disturbance of tunnel construction is severe, it may even lead to incomplete contact phenomenon with discontinuous vertical displacement. The research has important theoretical significances and application values for the prediction of mechanical response of soil-foundation system caused by the construction of urban shallow tunnel.

2021, 53(8): 2298-2311. doi: 10.6052/0459-1879-21-213

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STUDY ON GLOBAL MODE DYNAMIC MODELING AND EXPERIMENT FOR A SOLAR ARRAY OF THE FLEXIBLE SPACECRAFT

He Guiqin, Cao Dengqing, Chen Shuai, Huang Wenhu

Modern flexible spacecraft are usually installed with large solar arrays to provide the power needed for the operation of the spacecraft in orbit. The solar arrays are expanded in orbit and locked into hinged multi-panel structures, and the characteristics such as light weight, large span, and low stiffness make the issues on the study of low frequency and nonlinear vibration become more and more important. The key process of dealing with the vibration for such a kind of structure is to establish the exact nonlinear dynamic model of the system. In this paper, an analytic extraction method of the global mode of the multi-panel structure is presented, and the natural frequencies and the global analytic mode functions of the solar array are obtained. Considering the nonlinear stiffness and friction moments of the hinges, and the nonlinear dynamic model of the system can be obtained by the global mode discretization. The nonlinear characteristics of the solar array under periodic excitation are studied. The experimental researches on the solar array are carried out, and the mode shapes are obtained by hammering tests, and the sinusoidal sweep excitation is applied by the vibration table. The physical experimental results are compared with the theoretical results, so as to verify the rationality and accuracy of the global mode dynamic modeling method. The experimental results show that the structural parameters such as hinge stiffness have a great influence on the inherent characteristics of the system, and the existence of hinges will make the dynamic response of the solar array appear nonlinear phenomena such as jump. Global mode dynamical modeling method can well solve the difficult problem of obtaining the analytical global mode for the multi-panel structure with the non-classical boundaries. The global mode of the system can reflect the real elastic vibration mode of the parts in the system, and the established dynamic model is low-dimensional and high-precision. It has important reference value for nonlinear dynamic modeling of complex composite structures.

2021, 53(8): 2312-2322. doi: 10.6052/0459-1879-21-170

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DYNAMIC ANALYSIS OF VARIABLE MESH ISOGEOMETRIC THIN PLATE BASED ON T-SPLINE

Wang Yue, Cui Yaqi, Yu Zuqing, Lan Peng, Lu Nianli

The local strain of a thin plate with large displacement and large deformation will change dramatically under contact and collision working conditions. In order to ensure the accuracy and computational efficiency of flexible thin plate system’s dynamic analysis, this investigation integrates computer aided design (CAD) and computer aided engineering (CAE) systems, and proposes an isogeometric analysis (IGA) method for variable mesh flexible multibody system based on T-spline surface elements. Firstly, the kinematic description of a Kirchhoff thin plate based on T-spline surface elements is modeled, and the elastic model of a thin plate discretized by T-spline surface elements is established according to the nonlinear Green−Lagrange strain. Secondly, the goal of updating mesh of T-spline surface locally is achieved by inserting knots into the local region of the corresponding T-mesh. The transformation matrix, which is used to calculate the new generalized coordinates, generalized velocities and generalized accelerations of the refined system, is obtained by using the T-spline blending function refinement algorithm. The calculating solution algorithm for the dynamic equation of the system with variable degrees of freedom is created by combining the generalized α method with geometry update routine, and thus the local mesh refinement algorithm for the surface which is modeled by T-spline is formed. Finally, statics examples and flexible pendulum model verify the correctness for the elastic model of Kirchhoff thin plate based on T-spline surface, as well as computation precision and convergence for the proposed method in the dynamics analysis respectively. The dynamic analysis of the impacted flexible thin plate shows that the T-spline element and local refinement algorithm proposed in this paper can realize local mesh update only in the area where the strain changes violently, such as contact and collision, so as to control the degree of freedom of the system and improve the computational efficiency.

2021, 53(8): 2323-2335. doi: 10.6052/0459-1879-21-199

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APPLICATION OF RECURRENT NEURAL NETWORK IN RESEARCH OF INTELLIGENT WIND TUNNEL BALANCE

Nie Shaojun, Wang Yue, Wang Yunpeng, Zhao Min, Sui Jing

The shock tunnel ground test is vitally important to the research of the high-enthalpy aerodynamic characteristics of hypersonic vehicles, and the high-accuracy aerodynamic measurement is the key technology. When a force measurement test is conducted in an impulse shock tunnel, the flow field is established instantly after the starting process of shock tunnel, at this time, the great impact loads are acting on the force measurement system. The force measurement system is excited under the action of instantaneous impact, and the inertial vibration signal of the system cannot be rapidly attenuated during the short test time. The output signal of the balance will contain the interference due to the inertial vibration, which leads to a bottleneck in the further improvement of the accuracy of the transient force test. In order to improve the force measurement accuracy in the short-duration shock tunnel, the development of high-accuracy dynamic calibration technology is the key method to improve the performance of balance affected by inertial interference. Therefore, in this paper, recurrent neural network is used to train and intelligently process the balance dynamic calibration data, aiming to eliminate the vibration interference signals in the output dynamic signals. The error analysis of the current method is carried out, and the reliability of the current method is verified. The method is applied to the data processing of force test obtained in shock tunnel, and the effect of inertial vibration on the output signal of the balance is effectively reduced. According to the sample verification analysis of the intelligent model, the relative error of each component load is relatively small, where the case of high-frequency axial force component is about 1%. In the verification of wind tunnel force test data, the good results are also obtained, which are compared with those processed by the convolutional neural network model.

2021, 53(8): 2336-2344. doi: 10.6052/0459-1879-21-168

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SEEPAGE PROXY MODEL AND PRODUCTION FORECAST METHOD BASED ON MULTIVARIATE AND SMALL SAMPLE

Cao Chong, Cheng Linsong, Zhang Xiangyang, Jia Pin, Shi Junjie

There are so many multi-scale, multi-variable, and multi-physics coupling nonlinear seepage problems in the process of porous media seepage, which presents a huge challenge for the characterization of complex mechanism of flow behavior in the porous media and the analytical solution of mathematical models. The complex mathematical model considers the key mechanical problems of fluid flow in porous media, and its solution is a trade-off between computational cost and calculation accuracy. In recent years, the seepage proxy model based on various types of oilfield data has provided some possible alternatives for efficiently solving multi-variable nonlinear fluid flow problems. However, the application of seepage proxy model in oilfields is limited by the small sample data due to incomplete records and improper operation. A data-driven proxy model is proposed in this paper to predict the cumulative oil production based multi-variable and small sample oilfield data. Through a series of data preprocessing methods such as filling in missing values, one-hot encoding of classified data, data standardization etc., the database to forecast oil production can be built; In this paper, the random split techniques can be used to divided the whole database into train data and test data. Besides, ten-fold cross validation can be applied to test the error and accuracy of three data-driven models, which include Random Forest, extreme Gradient Boosting and Artificial neural networks. The results show that the determination coefficients of the three data modes all exceed 0.8, and the prediction results are more consistent with the actual data; In addition, for the small sample of multivariate oilfield data, data preprocessing methods have a significant impact on the accuracy of the cumulative oil production prediction; Moreover, after data standardization, the Random Forest algorithm performs best (mean square error of 0.12, coefficient of determination 0.87), which is more suitable for small samples of multivariate production forecast problem.

2021, 53(8): 2345-2354. doi: 10.6052/0459-1879-21-155

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2021 Vol. 53, No. 8

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