Simulation Concepts Introduction

XSim allows one to compute the dynamics of a system that has electromagnetic fields, and material shapes that are advanced dynamically a time step at a time. A XSim simulation can contain some or all of the objects, with them interacting in various ways.

The fields are defined on a structured grid in either cartesian or cylindrical coordinates. One can study just field dynamics, e.g., the propagation of electromagnetic fields on a grid, or solving for electrostatic fields for given boundary conditions and charge density, or fluid dynamics.

Material shapes, geometries, modify the dynamics of fields. For example, a conducting shape introduces an irregular region where the electric field vanishes. Consequently electromagnetic fields will scatter off of such a shape, and in electrostatics, such a shape will become an equipotential. Dielectrics shapes will modify the electric and magnetic.

To bring the power of many CPUs to simulation, XSim makes use of distributed memory (MPI) parallelism. In this method, the simulation region is divided into domains (domain decomposition), with each process containing both its domain plus some grid cells beyond. The addition grid cells are known as guard cells. They are used in the communication between processes. Additionally they are used as the locations of particle sinks. Particles that leave a simulation must be removed or reflected back into the simulation to prevent crashes caused by out-of-range accesses of memory.

Simulation results are analyzed by looking at the generated data. In the regular course of a simulation, the simulation data is periodically dumped. As well, XSim allows the definition of Histories, which are time sequences of data. Examples include the Poynting flux through a surface or the number of particles absorbed by a shape.

In this section, we will begin by going over simulation concepts and properties, including:

• Grids

  • Decomposition and guard cells

  • Periodic boundary conditions

• Geometries

• Fields

  • Electromagnetic fields

  • Electrostatic fields

  • Planar boundary conditions

  • Conformal boundaries

• Histories