Discrete Element Simulation of Macro and Micro Mechanical Properties of Round Gravel Material under Triaxial Stress

Shaokun Ma,Haijun Huang,Fapai Tian,Jian Gong,Jiabing Zhang,Zhibo Duan 대한토목학회 2024 KSCE Journal of Civil Engineering Vol.28 No.5

In this study, the effects of relative density and confining pressure on the shear characteristics of round gravel are investigated using a large-scale triaxial apparatus and the discrete element method. A simple and efficient numerical method for simulating flexible membranes is introduced. The results show that the stress-strain curves develop from hardened to softened type with increasing relative density, while the stress-strain curves develop from softened to hardened type with increasing confining pressure. As the axial strain increases, the strong contact force chains are vertically distributed, and the larger the relative density and confining pressure, the greater the number and thickness of the strong contact force chains. In the shear process, the distribution of average normal and tangential contact forces show “peanut-shaped” and “petal-shaped”, respectively. The increase in relative density increases the anisotropy of the specimen, while the increase in confining pressure results in a decrease. A linear relationship exists between the macroscopic stress ratio and the anisotropy coefficient. The anisotropy coefficient of the normal contact force provides the greatest contribution to the macroscopic shear strength (about 55%), followed by the anisotropy coefficient of the contact normal (about 26%) and that of the tangential contact force (about 19%).

Settlement and Load Transfer Mechanism of Pipeline Due to Twin Stacked Tunneling with Different Construction Sequences

Shaokun Ma,Ying Liu,Xilin Lv,Yu Shao,Ye Feng 대한토목학회 2018 KSCE JOURNAL OF CIVIL ENGINEERING Vol.22 No.10

The tunnel-pipeline-soil interaction is attracting more and more wide range of scholars and was treated as a plane strain problem, with little attention given to the responses of existing pipeline to twin-tunneling process with different construction sequences. Moreover, there are rare reports available on the influences of tunnel construction sequences on an existing pipeline. In this study, a three-dimensional centrifuge model test and numerical simulations using an advanced hypoplasticity constitutive model were conducted to investigate the influences of twin stacked tunneling with different construction sequences on an existing pipeline. The results shows that the maximum values of greenfield ground settlement, pipeline settlement and additional pipeline bending strain due to twin stacked tunneling when the upper tunnel was excavated first were 25%, 18% and 11% larger, respectively, than those when the lower tunnel was excavated first. The maximum values of greenfield ground settlement, pipeline settlement and additional pipeline bending strain due to twin stacked tunneling when the lower tunnel was excavated first were slightly smaller than those when the twin tunnels were excavated simultaneously. After the twin-tunneling process, the sagging regions were located within ± 1.5DT (tunnel diameter) in these three conditions. The maximum pipeline bending strain within the hogging regions was approximately half of that within the sagging regions. Because of shielding effects from the upper tunnel, the region where vertical stress of the soil reduced was transferred to both sides of the tunnel due to the second tunneling process when the upper tunnel was excavated first. As a result, the extended sagging regions due to the second tunneling process when the upper tunnel was excavated first were two times than that when the lower tunnel was excavated first.

Chaotic behavior of in-line bubbles rising with coalescences in non-Newtonian fluids: A multiscale analysis

Shaokun Jiang,Youguang Ma,Wenyuan Fan,Ke Yang,Huaizhi Li 한국화학공학회 2011 Korean Journal of Chemical Engineering Vol.28 No.1

The nonlinear dynamics of in-line bubbles rising with coalescence in non-Newtonian Carboxymethylcellulose sodium (CMC) fluids was studied through the techniques such as the multiresolution signal decomposition and the chaotic time series analysis. The temporary signals of bubble passages collected by an optical sensing device at different heights were investigated by a 12-level wavelet decomposition and the scalewise characteristics of bubble motion were extracted and analyzed. The chaotic time series analysis distinguished the periodicity or the deterministic chaos of bubble motion successsfully. The calculation of Kolmogorov entropy proves that in the ranges of experimental heights and gas flowrates, the bubble rising dynamics becomes more chaotic with the increase of height, and reaches the maximum chaotic extent in a certain height, while with the further increase of height, the chaotic extent decreases slowly. With the increase of gas flowrate, at the lower height, the bubble rising dynamics changes from periodicity to deterministic chaos, and at the higher heights it reaches the maximum chaotic extent in a certain gas flowrate; however,for both cases, it has little change in the higher gas flowrates. Moreover, with the increase of CMC concentration, the bubble rising dynamics becomes less chaotic when the height is beyond a certain value.

Comparison of formation of bubbles and droplets in step-emulsification microfluidic devices

Wei Zhang,Ziwei Liu,Shaokun Jiang,Chunying Zhu,Youguang Ma,Taotao Fu 한국공업화학회 2022 Journal of Industrial and Engineering Chemistry Vol.106 No.-

Monodispersed microbubbles and microdroplets are widely used as reaction carriers in microfluidics. Inthis study, the generation processes of bubbles and droplets in a step-emulsification microfluidic deviceare compared to show the similarities and differences in the emulsification process. By changing theplacement of the microdevice, the effects of buoyancy and gravity on the generation of bubbles and dropletsare introduced, and the feedback mechanism of the bubble layer and the effect of droplet accumulationon the emulsification process are clarified. Finally, based on the analysis of the difference of thepinch-off of the dispersed phase between the bubble and the droplet in this configuration, the Plateau-Rayleigh instability processes for the formation of bubble and droplet are revealed by using a highspeedcamera system, and the reasons for the difference of the operating ranges of the gas flow rateand liquid flow rate in the dripping flow regime are explained.

Microfluidic step emulsification techniques based on spontaneous transformation mechanism: A review

Ziwei Liu,Cong Duan,Shaokun Jiang,Chunying Zhu,Youguang Ma,Taotao Fu 한국공업화학회 2020 Journal of Industrial and Engineering Chemistry Vol.92 No.-

Rapid and robust generation of uniformly sized droplets is a research hotspot for emulsificationtechnology. Compared with conventional microfluidic devices such as cross-junction andflow-focusing,the size of the droplets generated in the step emulsifiers based on the spontaneous transformationmechanism is independent of theflowfluctuation, making this type of devices have many advantagesand important application value. To introduce the research progress of step emulsification technology inrecent years, this review discusses the device configurations, interfacial evolution for droplet formationmechanism,flow pattern classification, the influence of control variables on the emulsification processand the application advantages of such devices, and illustrates the design and selection principles of suchdevices, the control laws of manipulation variables, and the future applicationfields and directions.

Parameter Sensitivity Analysis of a New Fabricated Rectangular Tunnel Joint Using Numerical Method

Zhen Huang,Chenlong Zhang,Shaokun Ma,Hai Zhang,Zhang Zhou,Hongzhou Li 대한토목학회 2022 KSCE Journal of Civil Engineering Vol.26 No.2

An assembled rectangular tunnel has the advantages of high space utilization rate and strong urban applicability. This solution has been widely used in urban underground traffic construction. During their construction and operation, the joints are the weakest parts of the structure. The mechanical behavior of joints is closely related to the safety of the tunnel; thus, it is important to use a joint with good mechanical behavior. This study presents a new type of joints for shallow buried assembled rectangular tunnels. To evaluate the effect of design parameters (tenon depth and tenon angle) on the behavior of joints, the finite element method was used. At the same time, the mechanical behavior of rectangular tunnel joint was analyzed and compared with that of a traditional straight joint under the same loading conditions, showing better results. The performance-based engineering (PBE) concept was implemented to evaluate the robustness of rectangular tunnel joints. The results show that if the bolt is placed on the tensile side of the structure, it provides a strong restraining effect on the deformation of the joint. When the tenon depth and angle are 0.8 m and 3.6°, respectively, the overall behavior of the rectangular tunnel joint was improved. The new type of assembled rectangular tunnel joint designed in this study has a good application prospect in a shallow stratum and provides new ideas for the design and construction of such tunnels.

Deformation Response Induced by Surcharge Loading above Shallow Shield Tunnels in Soft Soil

Zhen Huang,Hai Zhang,Helin Fu,Shaokun Ma,Ying Liu 대한토목학회 2020 KSCE JOURNAL OF CIVIL ENGINEERING Vol.24 No.8

In the case of sudden surcharge loading, shallow shield tunnels in areas with soft soil experience substantial deformation responses. It is very important to understand the different loading modes and control measures above the shallow shield tunnels in soft soil for improving the safety of tunnel structure and reducing the influence of deformation. In this study, a three-dimensional numerical model of shallow shield tunnels in soft soil is established with FLAC3D. Ground and tunnel deformations are analyzed under different loading modes, and the effects of different deformation control measures are also studied. The numerical simulations in this paper show that the surface and tunnel deformation responses vary when induced by different loading modes above shallow shield tunnels in soft soil. After surface hardening with a 20-cm-thick layer of C20 concrete, the surface settlement is effectively controlled, and the uneven longitudinal settlement of the tunnel vault is improved. However, controlling the height of the surcharge is the most direct deformation control method. When the height of the surcharge is reduced from 6 m to 4 m and 2 m, the maximum ground settlement is reduced by 37.8% and 69.4%, respectively, and the maximum longitudinal settlement of the tunnel vault is reduced by 35.3% and 65.2%, respectively. During the operation of shallow shield tunnel in soft soil area, sudden surcharge loading should not be allowed. In the inevitable case, the surcharge loading on one side of the tunnel should be prevented and the surcharge loading height should be strictly limited.

Bubble formation in a step-emulsification microdevice: hydrodynamic effects in the cavity

Zhiwei Zhang,Zhongdong Wang,Fengrui Bao,Mengyu Fan,Shaokun Jiang,Chunying Zhu,Youguang Ma,Taotao Fu 한국공업화학회 2021 Journal of Industrial and Engineering Chemistry Vol.94 No.-

This study focuses on the generation of bubbles in a step-emulsification microdevice via a two-anglephotography method. It's found that the bubble generation mechanism is controlled by the interfacialtension, below a critical capillary number; while controlled by viscous force, inertial force and thedisturbance induced by the bubble swarm, above the critical capillary number. From the two-anglephotography method, a model is established for predicting the bubble size, by taking into account of thedynamic contact angle between gas-liquid interface and wall, and the hydrodynamic feedback of thecavity on bubble formation via the quantification of resistance by the volume fraction of gas in the cavity.