Decoupling the Stationary Navier-Stokes-Darcy System with the Beavers-Joseph-Saffman Interface Condition

This paper proposes a domain decomposition method for the coupled stationary Navier-Stokes and Darcy equations with the Beavers-Joseph-Saffman interface condition in order to improve the efficiency of the finite element method. The physical interface conditions are directly utilized to construct the boundary conditions on the interface and then decouple the Navier-Stokes and Darcy equations. Newton iteration will be used to deal with the nonlinear systems. Numerical results are presented to illustrate the features of the proposed method

Asymptotic Boundary Conditions with Immersed Finite Elements For Interface Magnetostatic/electrostatic Field Problems with Open Boundary

Many of the magnetostatic/electrostatic field problems encountered in aerospace engineering, such as plasma sheath simulation and ion neutralization process in space, are not confined to finite domain and non-interface problems, but characterized as open boundary and interface problems. Asymptotic boundary conditions (ABC) and immersed finite elements (IFE) are relatively new tools to handle open boundaries and interface problems respectively. Compared with the traditional truncation approach, asymptotic boundary conditions need a much smaller domain to achieve the same accuracy. when regular finite element methods are applied to an interface problem, it is necessary to use a body-fitting mesh in order to obtain the optimal convergence rate. However, immersed finite elements possess the same optimal convergence rate on a Cartesian mesh, which is critical to many applications. This paper applies immersed finite element methods and asymptotic boundary conditions to solve an interface problem arising from electric field simulation in composite materials with open boundary. Numerical examples are provided to demonstrate the high global accuracy of the IFE method with ABC based on Cartesian meshes, especially around both interface and boundary. This algorithm uses a much smaller domain than the truncation approach in order to achieve the same accuracy

An Iterative Immersed Finite Element Method for an Electric Potential Interface Problem Based on Given Surface Electric Quantity

Interface problems involving the non-homogeneous flux jump condition are critical for engineering designs in the magnetostatic/electrostatic field. In applications, such as plasma simulation, we often only know the total electric quantity on the surface of the object, not the charge density distribution on the surface which appears as the non-homogeneous flux jump condition in the usual interface problems considered in the literature for the magnetostatic/electrostatic field. Based on structured meshes independent of the interface, this article proposes an iterative method that employs both the immersed finite element (IFE) method with non-homogeneous flux jump conditions and the regular finite element method with ghost nodes introduced in the object to solve the 2D interface problem for the potential field according to the given total electric quantity on the surface of the object. Numerical experiments are provided to illustrate the accuracy and efficiency of the proposed method

Decoupling the Stationary Navier-Stokes-Darcy System with the Beavers-Joseph-Saffman Interface Condition

This paper proposes a domain decomposition method for the coupled stationary Navier-Stokes and Darcy equations with the Beavers-Joseph-Saffman interface condition in order to improve the efficiency of the finite element method. The physical interface conditions are directly utilized to construct the boundary conditions on the interface and then decouple the Navier-Stokes and Darcy equations. Newton iteration will be used to deal with the nonlinear systems. Numerical results are presented to illustrate the features of the proposed method

Numerical Simulation of Interaction between Hall Thruster CEX Ions and SMART-1 Spacecraft

The interaction between the plume of Hall thruster and the surface of the SMART-1 spacecraft is investigated by developing a three-dimensional IFE-PIC-MCC code, with the emphasis on the effect of the disturbance force and thermal loading caused by charge exchange ions (CEX) impingement on the surface of the spacecraft. The parameters such as heat flux and forces of CEX ions which impinge on SMART-1 and solar arrays are obtained. The disturbance force of CEX ions to the spacecraft is calculated for different divergence angles and different solar array rotation cases. The simulation results show that the disturbance force and heat flux on spacecraft change very little as the divergence angle changes. The effect of maximum disturbance force can be neglected since it is so small comparing with the nominal value of the main thrust. Solar arrays receive the least thermal heating from the CEX ions when the beam ions flow is perpendicular to the solar array plane

An Immersed-Finite-Element Particle-in-Cell Simulation Tool for Plasma Surface Interaction

A novel Immersed-Finite-Element Particle-in-Cell (IFE-PIC) simulation tool is presented in this paper for plasma surface interaction where charged plasma particles are represented by a number of simulation particles. The Particle-in-Cell (PIC) method is one of the major particle models for plasma simulation, which utilizes a huge number of simulation particles and hence provides a first-principle-based kinetic description of particle trajectories and field quantities. The immersed finite element method provides an accurate approach with convenient implementations to solve interface problems based on structured interface-independent meshes on which the PIC method works most efficiently. In the presented IFE-PIC simulation tool, different geometries can be treated automatically for both PIC and IFE through the geometric information specified in an input file. The set of parameters for plasma properties is also assembled into a single input file which can be easily modified for a variety of plasma environments in different applications. Collisions between particles are also incorporated in this tool and can be switched on/off with one parameter in the input file. Efficient modules are adopted to integrate PIC and IFE together into the final simulation tool. Hence our IFE-PIC simulation package offers a convenient and efficient tool to study the microcosmic plasma features for a wide range of applications. Numerical experiments are provided to demonstrate the capability of this tool

Three-Dimensional IFE-PIC Numerical Simulation of Background Pressure\u27s Effect on Accelerator Grid Impingement Current for Ion Optics

A three-dimensional numerical simulation modeling is developed to investigate the background pressure\u27s effect on the characteristics of ion impingement on the accelerator grid for the ion optical system. The immersed-finite-element particle-in-cell (IFE-PIC) method is combined with Monte Carlo method to compute the electric field, track the ions, and describe the charge-exchange collision process while direct simulation Monte Carlo (DSMC) method is adopted to simulate the motion of neutral atoms. Results show that the residual neutral atoms in the vacuum chamber play an important role in the neutral atoms distribution when the background pressure is higher than a specific magnitude, and the accelerator grid impingement current increases with the increase of the background pressure. To improve the reliability of ion thruster service lifetime prediction in the ground life tests, the background pressure in the vacuum chamber should be below 10-3 Pa

Influence of the Decelerator Grid on the Optical Performance of the Ion Thruster

In order to reduce the erosion of the ion thruster accelerator grid, which is caused by charge-exchange (CEX) ions, the 2-grid optical system is added to a decelerator grid to block the reflux CEX ions. The previous experiment and simulation results have proven that the decelerator grid can effectively reduce the Pit and Groove erosion. However, the influence of the decelerator grid on the optical performance has not yet been studied well. In this paper, a three-dimensional Immersed Finite Element Method-Particle in Cell-Monte Carlo Collision (IFE-PIC-MCC) algorithm was adopted to investigate the effect of the decelerator grid on the optical performance under crossover and normal circumstances. Results show that the decelerator grid has no effect on the focusing state and the distribution of beam ions. It also has little effect on the CEX ions from the upstream and extraction (center) regions. However, it has great influence on the downstream CEX ions. When the upstream plasma number density is small, the decelerator grid will cause most of the downstream reflux CEX ions to impinge on the accelerator grid aperture barrel, resulting in the significant increase of the Barrel erosion of the accelerator grid. With the increase of the upstream plasma number density, the downstream reflux CEX ions tend to impact the downstream surface of the decelerator grid, which means the decelerator grid begins to block the downstream backflow of CEX ions

A novel silica nanowire-silica composite aerogels dried at ambient pressure

A novel silica nanowire-silica composite aerogels with excellent thermal insulation and mechanical properties was prepared by adding SiO2 nanowires as a secondary phase into the silica matrix and drying at ambient pressure. The SiO2 nanowires and silica aerogel matrix have outstanding compatibility and dispersibility because the components of nanowire and aerogel matrix all are silica, which contributes to the improvement of the mechanical property of the silica composite aerogels. The physical, mechanical and thermal properties of the SiO2 nanowires based composite silica aerogel are investigated and discussed in detail. The results indicate that the monolithic aerogels with high specific surface area, porosity and large pore volume. The thermal conductivity of the composite silica aerogels only increases gently (0.006 W.m(-1) K-1) as the addition of SiO2 nanowires rises from 0 wt% to 14 wt%, while their mechanical properties have been improved greatly. The SiO2 nanowires based composite silica aerogel would be widely used in the thermal insulation application because of its outstanding thermal insulation property. (C) 2016 Elsevier Ltd. All rights reserved