Fragmentation, Merging, and Internal Dynamics for PIC Simulation with Finite Size Particles
In plasma or rarified gas physics, collisions are rare but non-ignorable events. To model systems with arbitrary collisonality, it is necessary to start with a model that is fully capable of capturing collisionless, kinetic behavior. It is also necessary to build strategies to provide the essential economies into the scheme as collisions become more frequent. The desired model should progress smoothly and continuously from collisionless particle dynamics to collision-dominated fluid. We are developing a new approach [Hewett, JCP 2003; Larson, JCP 2003] to recover the physics of this partially collisional regime. Our approach, called CPK (for Collisional Particle Kinetics) is basically a ''smart particle'' PIC scheme with particles that have internal parameters representing internal ''fluid'' behavior as the CPK particles become large via merging. In the collisionless limit, the individual macro particles become numerous, small and cold through fragmentation--leading naturally to the traditional PIC limit. The new ''smart'' particle is a Gaussian distribution in all phase space directions. An arbitrary distribution of real particles can be made as a superposition of these ''particles''. One of the key capabilities is the ability to fragment each particle in a way that will not introduce new physics. With this procedure we can replace each particle with a set of particles that, when reassembled, give the original particle to arbitrary precision. Further, with some time-dependent parameters built into each particle, fragmentation is not required to preserve existing details during time evolution. Collisionless, free expansion of an isolated puff of gas can be followed by a single macro-particle with no fragmentation, if necessary. The redundancy introduced by the fragmentation provides the freedom to represent new features emerging from the nonlinear time evolution. In addition to collisionless internal particle dynamics, we have also developed internal dynamics consistent with a {gamma}-law gas within each particle. This ability, coupled with a very aggressive strategy for merging particles with disparate velocities, puts full fluid behavior within reach. The force between CPK particles is, presently, only through (perhaps aggressive) merging of CPK particles. A merger of two CPK particles is equivalent to an inelastic collision. The gradient of pressure is not computed and the ''mesh'' enters only in a nonfundamental way as a means to facilitate evaluating particle ''overlap'' in phase space in preparation for merging. The mesh never carries any essential part of the physics; the mesh could be discarded every time step if, for some reason, a new one offered advantages.
- Research Organization:
- Lawrence Livermore National Lab., CA (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15004657
- Report Number(s):
- UCRL-JC-148321-REV-1
- Country of Publication:
- United States
- Language:
- English
Similar Records
Grid and particle hydrodynamics: Beyond hydrodynamics via fluid element particle-in-cell
Computational Methods for Collisional Plasma Physics