Shield Behavior Research Articles

A novel method to model the effect of environmental constraint of geometry in the mechanism design

Few studies have considered the effect of environmental constraints throughout the entire design process of mechanical systems. Neglecting such effect might lead to misunderstanding the kinematic performance of the mechanical systems. In our work, an environment-based design concept was proposed by using passive limbs to model and analyze geometrical constraints in each step of the theoretical design process. Redesigning of a thrust system in a shield machine is taken as an example for better demonstration of this design approach. A new equivalent model of the thrust system was synthesized by using passive limbs to express environmental constraints of geometry based on the shield behavior. The uniform kinematic model was developed for the integrated equivalent mechanism of the thrust system. The effectiveness of the passive limbs in expressing environmental constraints was then evaluated by kinematical analysis. A singularity analysis was performed to assess the consistency between the equivalent model and a real thrust system. The reachable angular workspace was also analyzed to demonstrate the influence of the passive limbs as an example of environment-based parameter design. A motion simulation was performed to demonstrate the improvement of the forward kinematics by using the passive limbs to solve the singularity problem. A dexterity performance index based on the proposed model was conducted to demonstrate the kinematical property of the thrust system.

Numerical studies of the ignition characteristics of a high-pressure gas jet in compression-ignition engines with glow plug ignition assist: Part 1—Operating condition study

This article presents the results of computational studies investigating the ignition of high-pressure natural gas jets in a compression-ignition engine with glow plug ignition assist. The simulation was conducted using a KIVA-3V-based three-dimensional engine model, along with an improved fuel injector model, a detailed cut-off glow plug shield model and a modified two-step methane reaction mechanism, to simulate the natural gas injection and ignition. The simulated results demonstrate the significance of using a shield for the glow plug. Compared to an unshielded (bare) glow plug, the shield not only reduces the heat loss from the hot glow plug surface to the cold intake air charge and the cold injected gas jet but also traps the fuel mixture to increase its residence time adjacent to the hot surface. Over a representative range of heavy-duty diesel engine operating conditions, a shielded glow plug greatly improves the natural gas engine performance and provides reliable ignition, while an unshielded glow plug can only be optimized for specific conditions. The understanding of glow plug shield behavior gained from the simulations suggests avenues for improved shield designs that would yield further reduced ignition delays.

Advanced finite element modeling of excavation and advancement processes in mechanized tunneling

Mechanized tunneling is characterized by complex interactions between the shield machine and the surrounding ground during the TBM advancement process. In this paper, a new computational framework is developed to enable an efficient and realistic three-dimensional modeling of the tunneling process for arbitrary alignments using the finite element method. A new steering algorithm for the advancement of the Tunnel Boring Machine (TBM) for arbitrary alignments during shield tunneling is incorporated in the proposed model. This algorithm simulates the shield behavior and accordingly provides the numerical model with the required information to keep the TBM on track during the simulation. However, the utilization of this algorithm is only possible using a finite element discretization which adapts to the actual motion path of the shield machine. For this purpose, a re-meshing technique is proposed in order to automate the process of mesh generation in the vicinity of the tunnel face, denoted as the region of interest, within the advancing process. The combination of a computational steering algorithm and a 3D automatic adaptive mesh generation procedure form a novel framework for process oriented finite element simulations of the mechanized tunnel construction process. The applicability of the proposed modeling technique for predicting the shield behavior and the soil-tunnel interactions during tunneling along curved alignments is demonstrated by means of selected examples.

Optimization-based model of tunneling-induced distributed loads acting on the shield periphery

This paper presents a new approach to predict the loads acting on the shield periphery during excavation. This approach not only represents the dynamic variation of loads, but also can be used to calculate the rectification loads for moving shield backward to the planned alignment. Firstly, the ground reaction curves are adopted to calculate the earth pressure. Particularly, the ground displacement is considered to be induced by two kinds of motion: tunneling-induced ground loss and snake-like motion of shield movement. Then, optimization criteria for load balance, minimum potential energy, shield behavior, and ground settlement have been considered. Finally, basic example is carried out in a straight alignment for the single layer of homogeneous ground. The validity is discussed by comparing the optimization model with numerical simulation, and the results confirm that the proposed approach is reasonable enough to predict the distributed loads acting on the shield periphery.

Study of Shield Behavior Measured by a New Survey System

Study of Shield Behavior Measured by a New Survey System

Whipple shield performance in the shatter regime

A series of hypervelocity impact tests have been performed on aluminum alloy Whipple shields to investigate failure mechanisms and performance limits in the shatter regime. Test results demonstrated a more rapid increase in performance than predicted by the latest iteration of the JSC Whipple shield ballistic limit equation (BLE) following the onset of projectile fragmentation. This increase in performance was found to level out between 4.0 and 5.0 km/s, with a subsequent decrease in performance for velocities up to 5.6 km/s. For a detached spall failure criterion, the failure limit was found to continually decrease up to a velocity of 7.0 km/s, substantially varying from the BLE, while for perforation-based failure an increase in performance was observed. An existing phenomenological ballistic limit curve was found to provide a more accurate reproduction of shield behavior that the BLE, prompting an investigation of appropriate models to replace linear interpolation in shatter regime. A largest fragment relationship was shown to provide accurate predictions up to 4.3 km/s, which was extended to the incipient melt limit (5.6 km/s) based on an assumption of no additional fragmentation. Alternate models, including a shock enhancement approach and debris cloud cratering model are discussed as feasible alternatives to the proposed curve in the shatter regime, due to conflicting assumptions and difficulties in extrapolating the current approach to oblique impact. These alternate models require further investigation.

Simulation of Shield Tunneling Behavior along a Curved Alignment in a Multilayered Ground

The kinematic shield model has been proposed to simulate shield behavior during excavation based on equilibrium condition, taking into account ground displacement around the shield. The model has been validated by the simulation of an earth pressure balanced shield behavior in a straight alignment for a single ground layer. To verify the model performance for shield tunnel excavation at curved alignment in multilayered ground, the slurry shield behavior is simulated in this study using the in situ data, and the immediate ground movements around the shield are computed by three-dimensional FEM employing the enforced displacement, which is obtained from the shield behavior simulation. As a result, it is found that the simulated shield behavior and the computed immediate ground movements around the shield during shield tunneling are in good agreement with the observations. Furthermore, the results reveal that the excavated area including the area generated by copy cutter is a predominant factor affecting the shield behavior, and the ground displacement, at the excavated surface plays an important role in the surrounding ground movements during shield tunneling.

An ATP-EMTP-Based Model for Analysis of Shielding Properties of Ferromagnetic Cable Sheaths

Ferromagnetic materials, such as steel, are used for shielding of power and telecommunication cables. These shields often provide path for currents during faults or lightning and are important for the integrity and safety of electrical systems and devices. Since such currents might be with low- or high-frequency content, and of low or high intensity, ferromagnetic shield behavior might be quite different due to hysteresis. This paper describes a method for analyzing the dynamic behavior of ferromagnetic cable shields based on the Jiles model of hysteresis. The solution is implemented in ATP-EMTP, which allows one to analyze the complex behavior of ferromagnetic cable shields in conjunction with that of the other elements of the power system. Due to limitations of the model of hysteresis, application of the proposed model is limited to frequencies up to about 10 kHz. The model is illustrated for a practical 100-m-long cable.

Theoretical Model of Shield Behavior During Excavation. I: Theory

Closed-type shield tunneling methods were developed together with computer-aided automatic control systems. However, automatic control systems are based on empirical relationships and do not have a precise theoretical background. In this paper, a model of the theoretical dynamic load acting on the shield during excavation is developed, taking into account shield tunnel engineering practices; i.e., the excavated area, the tail clearance, the rotation direction of the cutter face, sliding of the shield, ground loosening at the shield crown, and the dynamic equilibrium condition. To evaluate the validity of the model qualitatively, the simulation of shield behavior and the sensitivity analysis of the model parameters on the shield behavior are carried out in both straight and curve alignments for both sandy and clayey ground. The results for the shield behavior are examined, comparing them with the empirical and the theoretical one, and it is confirmed that the proposed model represents the shield behavior reasonably well.

Theoretical Model of Shield Behavior During Excavation. II: Application

Simulation of the shield behavior is carried out by applying the model of the loads acting on the shield to the shield tunneling work. The shield tunneling was carried out using an earth pressure balanced shield in sandy gravel layer entirely and the observed data were obtained by the real time measuring data system. To escape the errors of data, the filtering process is applied to the measured data prior to processing the simulation. The input ground properties are carefully determined from the two-dimensional finite element analysis, reverse analysis, and empirical values. The simulation results are compared with the observed data to verify the model performance. The results indicate that; (1) the ground loosening due to local collapse of the ground at the shield face and at the shield crown are necessary to take into consideration in order to simulate the actual shield behavior; (2) the force due to the deformation of wire brush affects the shield postures; and (3) the simulation results are in good agreement with the observed data.

A 2-D Numerical Simulation of ITER-FEAT Disruptive Hot Plasma-Wall Interaction and Model Validation against Disruption Simulation Experiments

The two-dimensional (2-D) radiation-magnetohydrodynamic (R-MHD) code FOREV-2 was developed for modeling of disruptive hot plasma-wall interaction, for calculation of erosion by evaporation of the ITER-FEAT vertical divertor target, and for calculation of the impurity transport in the divertor. FOREV-2 uses a 2 1/2-D MHD model, a 2-D scheme for anisotropic radiation transport, and a solution of the magnetic field equations in the plasma shield for all three components of the magnetic field. Details of FOREV-2 with emphasis on the MHD equations, the equations for the magnetic field, the vaporization model, the angular dependent multigroup radiation transport, and the optical properties of plasmas, and validation of FOREV-2 against analytical results are discussed.Moreover, disruption simulation experiments, performed at the plasma gun facilities at TRINITI Troitsk were used for validation of FOREV-2 and for investigations of the MHD of typical plasma shields. From the results of the numerical analysis of the simulation experiments, it is concluded that turbulence in the experimental plasma shields is absent and that the stability of the cold and dense part of the plasma shields, which determines the target erosion, can be adequately modeled by FOREV-2 by use of the classical magnetic field diffusion coefficient. The experimentally observed downstream drift of plasma shields along the surface of the vertical targets is due to the lateral motion of a cold and dense plasma layer close to the target. The observed upstream shift of the erosion profiles of vertical targets is due to reradiation from the expanding plasma shield. Line radiation and an appropriate model for anisotropic radiation transport are necessary for a realistic calculation of the reradiation from carbon plasma shields. Moreover, inclusion of the line radiation allows one to get a realistic radiation cooling after switching off the heating. Target erosion in the simulation experiments is caused by radiation. The agreement between calculated and measured erosion for graphite and quartz demonstrates the adequacy of the calculated 2-D radiative target heat loads.The 2-D numerical analysis of the disruption simulation experiments allows one to conclude that such experiments adequately simulate the tokamak plasma shield properties and its dynamics. The extensive validation exercise of FOREV-2 against disruption simulation experiments gives confidence that the numerical analysis of erosion for the ITER-FEAT vertical targets during the thermal quench phase of a disruption and the impurity production during ELMs and its transport toward the x-point to be performed with FOREV-2 is based on sound principles and covers all important aspects of plasma shield behavior and plasma shield stability.

Non-schmid yield behavior in single crystals

A parently, certain single crystals do not obey Schmid's law for slip on individual systems. For example, many intermetallic compounds such as Ni 3Al with the LI 2 structure display yield behaviors that are called “anomalous” in the sense that the critical resolved shear stress on the primary slip system at yield is a function of the orientation of the loading axis and the sense of load. To accommodate such non-Schmid behaviors, a generalization of Schmid's law is proposed where stress components other than the Schmid stress also enter the criterion. The general effects of “cross slip” in FCC single crystals as well as the detailed shield behavior of Ni 3Al are well described. Finally, we demonstrate that non-Schmid effects can make it more difficult to activate many slip systems simultaneously.

Shield tunneling performance and behavior of soft ground, osaka, japan : Kitamura, M; Ito, S; Fujiwara, T Proc 5th Rapid Excavation and Tunneling Conference, San Francisco, 3–7 May 1981, V1, P201–220. Publ New York: AIME, 1981

Shield tunneling performance and behavior of soft ground, osaka, japan : Kitamura, M; Ito, S; Fujiwara, T Proc 5th Rapid Excavation and Tunneling Conference, San Francisco, 3–7 May 1981, V1, P201–220. Publ New York: AIME, 1981

Concrete Properties Relevant to Reactor Shield Behavior

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