With the development of computer technology, numerical simulation has been more and more widely used in many fields of our society. Simulation techniques not only play very important roles in scientific study, but also occupy very important places in education, military, entertainment and almost any fields that we can imagine. Before we start to discuss the details of the numerical simulation of processes in space, it is necessary for us to understand the basic concept of simulation. The Oxford English Dictionary gives the following definition of simulation:
The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, especially for the purpose of study or personnel training.
And Glossary of Geology(Third Edition, AGU) gives a definition of simulation especially appropriate for scientific simulations:
The representation of a physical system by a device such as a computer or model that imitates the behavior of the system.
These definitions provide a basic idea of simulation.