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    Experimental Investigation on Motion Hydrodynamics of Water Entry for Inclined Cylinders
    XIA Weixue, WANG Cong, LI Jiachuan, HOU Dongbo
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 67-76.   DOI: 10.16183/j.cnki.jsjtu.2019.116
    Abstract855)   HTML1)    PDF(pc) (2031KB)(529)       Save

    Experiments of vertical water entry for a cylinder with different inclined angles are performed at a low Froude number to investigate the cavity evolution and hydrodynamics. Unique phenomena of double cavities and separated cavity are observed. The accurate trajectories and attitude angles for the inclined cylinders are proposed, which are extracted by utilizing the method of digital image correlation from the image sequence recorded by a high-speed camera. The raw data of trajectory and attitude angle are fitted using the method of quintic smoothing spline, with which, the velocity and acceleration of the cylinders during water entry are estimated, and the cylinder characteristics of motion and hydrodynamic force are studied. The experimental results demonstrate that the unique cavity phenomena are observed for the inclined cylinder during water entry, such as cavity separation and double cavities. The acceleration increases quickly after the cylinder penetrates into water and achieves the maximum value after cavity separation occurs. After that, the acceleration decreases quickly and tends to zero. The vertical velocity of the cylinder with a large initial inclined angle decreases faster than that with a small inclined angle, while the corresponding horizontal velocity increases rapidly. The trajectories of the cylinder with different initial inclined angles generally present the characteristics of first moving in the upstream direction and then in the downstream direction. With respect to the properties of inclined angle, the angular acceleration responds very quickly to the hydrodynamic force, and it generally first increases and then decreases. In addition, the angular speed of cylinder for a large initial inclined angle increases faster than that for a small inclined angle. The inclined angle shows the same trend as well. The cylinder drag and lift coefficients rapidly increase after the cylinder enters the water, and then slowly increase after the cavity pinches off. Moreover, the force coefficients increase more quickly for the cylinder with a large initial inclined angle.

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    Entrainment and Turbulence Characteristics of Continuous-Flux Release Gravity Current over Rough Beds
    HAN Dongrui, YU Junyang, YUAN Yeping, HE Zhiguo, WANG Yuhang, LIN Yingdian
    Journal of Shanghai Jiao Tong University    2021, 55 (1): 77-87.   DOI: 10.16183/j.cnki.jsjtu.2019.110
    Abstract688)   HTML0)    PDF(pc) (4226KB)(440)       Save

    In both the natural environment and hydraulic engineering, there widely exists the phenomenon of gravity current. In practical conditions, most beds are covered with gravel and sediment particles of different sizes, which can be regarded as rough bed conditions. Therefore, it is of practical scientific significance and engineering application value to study the dynamic characteristics of gravity current flowing over rough beds. By conducting flume experiments for continuous-flux gravity current, and considering the bed roughness and the current initial mass fraction comprehensively, the propagation characteristics such as head position, head velocity, and entrainment coefficient of gravity current are analyzed, the turbulence characteristics such as the turbulence intensity and Reynolds stress at different cross-sections are studied, and the bed shear stress is calculated by using the Reynolds stress method and the turbulent kinetic energy method. The results show that the velocity of the gravity current head is negatively correlated with the bed roughness, but positively correlated with the current initial mass fraction. In the experimental runs of conspicuous roughness and high initial mass fraction, the former is the dominant factor for the kinematic characteristics of gravity currents. When the bed roughness increases to a certain extent, the "lifting phenomenon" of the averaged longitudinal velocity profiles occurs in the gravity current body. One minimum and two maximun values are presented in the turbulence intensity profile of gravity current, and the longitudinal turbulence intensity is the main controlling factor for the current turbulent structure. Besides, the vertical turbulence intensity over rough beds increases significantly compared with that of smooth beds. Near the bed, the Reynolds shear stress is positive, whereas far away from the bed, the Reynolds shear stress is negative. The bed shear stress calculated by using the Reynolds stress method is higher than that by using the turbulent kinetic energy method at identical bed roughness. At the same bulk Richardson number, the entrainment coefficient of gravity current is positively correlated with the bed roughness. It can be concluded that the influence of rough beds on gravity current is mainly as follows: increasing friction resistance, reinforcing mixing effect, redistributing current density near the bed, thickening turbulent boundary layer, and "lifting phenomenon" of the averaged longitudinal velocity profiles.

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    Numerical Simulation and Uncertainty Analysis of Wave-Co-Current Interaction with Irregular Waves
    YAO Shun, MA Ning, DING Junjie, GU Xiechong
    Journal of Shanghai Jiao Tong University    2021, 55 (3): 337-346.   DOI: 10.16183/j.cnki.jsjtu.2019.375
    Abstract707)   HTML2)    PDF(pc) (2881KB)(358)       Save

    Uniform current effects on the characteristic of irregular waves along with its uncertainty are presented considering wave-current interaction in the actual ocean environment. First, relative tests of interaction of irregular wave and uniform current are conducted in the circulating water channel at Shanghai Jiao Tong University. Then, the wave elevations are measured to validate the numerical results obtained from the numerical simulation of wave-current interaction based on Reynolds-Averaged Navier-Stokes (RANS) equations. Finally, significant wave height and average period of irregular wave are selected to conduct the uncertainty analysis including both grid-convergence and time-step-size convergence studies. The results show that wave height probability distribution agrees well with Rayleigh distribution in the co-current and no-current cases. The spectral peak of irregular wave moves to the low frequency in the co-current conditions. Besides, significant wave height of irregular wave is more sensitive to grid size, while the average period is more affected by time step size. Moreover, uniform co-current can reduce the degree of dependence of significant wave height on time step size, while the influence on the average period is on the contrary.

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    Effect of Linearly Stratified Environment and Submerged Vegetation on Hydrodynamic Characteristics of Downslope Gravity Currents
    LIU Yayu, LIN Yingtien, YUAN Yeping, HE Zhiguo
    Journal of Shanghai Jiao Tong University    2021, 55 (4): 412-420.   DOI: 10.16183/j.cnki.jsjtu.2019.228
    Abstract644)   HTML4)    PDF(pc) (3512KB)(268)       Save

    A series of lock-exchange experiments of slope gravity currents were conducted to analyze the development and evolution characteristics of rigid vegetations in uniform and linear stratified environments. The development of gravity currents was recorded by using a digital camera, and the local flow field structure was obtained by particle image velocimetry (PIV). The results show that the head velocity of gravity currents with submerge vegetation experience the processes of acceleration, deceleration, second acceleration, and second deceleration. As the stratification degree increases, the transition points among the four stages move ahead, but the vegetation density does not significantly impact that point. In the stratified environments with the submerged vegetation patches, the phenomena of “first separated then advanced and finally separated from the slope” is observed. Vegetation can restrain the development of vorticity fields of gravity currents. When the vegetation densens, the vorticity of gravity currents will decrease more significantly. Both stratified environments and submerged vegetation patches can inhibit the development of vorticity fields of gravity currents, and the stratified water environment plays a more important role in this inhibition.

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    Element-Free Galerkin Scaled Boundary Method Based on Moving Kriging Interpolation for Steady Heat Conduction Analysis with Temperatures on Side-Faces
    WANG Feng, CHEN Jiali, CHEN Denghong, FAN Yong, LI Zhiyuan, HE Weiping
    Journal of Shanghai Jiao Tong University    2021, 55 (11): 1483-1492.   DOI: 10.16183/j.cnki.jsjtu.2020.215
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    The element-free Galerkin scaled boundary method (EFG-SBM) based on moving Kriging (MK) interpolation is used to solve steady heat conduction problems with temperature loads on side-faces, in which the circumferential boundary is discretized based on MK interpolation and the element-free Galerkin (EFG) method. As the shape functions constructed from the MK interpolation possess the Kronecker delta interpolation property, the MK shape functions overcome the shortcomings of moving least squares (MLS) approximation which is difficult to impose essential boundary conditions directly and accurately. As a new boundary-type meshless method, EFG-SBM has advantages of the EFG and scaled boundary finite element method (SBFEM). This method inherits the semi-analytical property of SBFEM by introducing the scaled boundary coordinate system, in which the governing differential equations are weakened in the circumferential direction and can be solved analytically in the radial direction. Unlike the traditional SBFEM, the preprocessing and postprocessing processes of EFG-SBM are simplified since only the nodal data structure is required in the circumferential direction. Numerical examples show that the EFG-SBM based on MK interpolation can obtain a higher accuracy than the SBFEM based on Lagrange polynomials. Compared with the finite element method (FEM), this method can better characterize the thermal singularity at the sharp corner and the temperature distribution of the infinite region.

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    Intelligent Monitoring Micro-Electro-Mechanical-System Sensor of Herringbone Gate of Dateng Gorge
    XIE Ziyi, DUAN Li, WENG Haotian
    Journal of Shanghai Jiao Tong University    2021, 55 (11): 1401-1407.   DOI: 10.16183/j.cnki.jsjtu.2020.102
    Abstract519)   HTML13)    PDF(pc) (10442KB)(283)       Save

    Bottom pivot bearing acts as the supporting and rotating component of the important water conservancy structure. The wear in the turning and closing operation is directly related to the normal operation and reliability of the gate. To directly monitor the wear of the bearing under severe deep water working conditions, a novel thin film resistive wear sensor was designed and constructed by using the micro-electro-mechanical-system (MEMS) micro-manufacturing technology. The wear measurement and characterization experiments were conducted. Besides, a wear test was simulated by computer simulation modeling. The relationship between the measured resistance and the wear parameters under different working conditions was specifically analyzed. The results show that the production and installation process of the sensor is feasible, and the experimental results are basically consistent with the simulation results. In the allowable range of working conditions, as the resistance increases, the accuracy of wear measurement increases. The sensor is expected to be applied in the intelligent monitoring of the herringbone gate of Dateng Gorge, and realize the Internet of things (IoT) and intelligent monitoring of the water conservancy projects in the 21st century.

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