Editing Computer simulation (section)

== Types ==
Models used for computer simulations can be classified according to several independent pairs of attributes, including:
* [[stochastic process|Stochastic]] or [[Deterministic algorithm|deterministic]] (and as a special case of deterministic, chaotic) – see external links below for examples of stochastic vs. deterministic simulations
* Steady-state or dynamic
* [[Continuous function|Continuous]] or [[discrete mathematics|discrete]] (and as an important special case of discrete, [[Discrete event simulation|discrete event]] or DE models)
* [[Dynamic simulation|Dynamic system simulation]], e.g. electric systems, hydraulic systems or multi-body mechanical systems (described primarily by DAE:s) or dynamics simulation of field problems, e.g. CFD of FEM simulations (described by PDE:s).
* Local or [[distributed computing|distributed]].

Another way of categorizing models is to look at the underlying data structures. For time-stepped simulations, there are two main classes:
* Simulations which store their data in regular grids and require only next-neighbor access are called [[stencil code]]s. Many [[Computational fluid dynamics|CFD]] applications belong to this category.
* If the underlying graph is not a regular grid, the model may belong to the [[meshfree method]] class.

For steady-state simulations, equations define the relationships between elements of the modeled system and attempt to find a state in which the system is in equilibrium. Such models are often used in simulating physical systems, as a simpler modeling case before dynamic simulation is attempted.
* Dynamic simulations attempt to capture changes in a system in response to (usually changing) input signals.
* ''[[stochastic process|Stochastic]]'' models use ''[[random number generator]]s'' to model chance or random events;
* A ''[[discrete event simulation]]'' (DES) manages events in time. Most computer, logic-test and fault-tree simulations are of this type. In this type of simulation, the simulator maintains a queue of events sorted by the simulated time they should occur. The simulator reads the queue and triggers new events as each event is processed. It is not important to execute the simulation in real time. It is often more important to be able to access the data produced by the simulation and to discover logic defects in the design or the sequence of events.
* A ''continuous dynamic simulation'' performs numerical solution of [[Differential algebraic equation|differential-algebraic equations]] or [[differential equations]] (either [[partial differential equation|partial]] or [[ordinary differential equation|ordinary]]). Periodically, the simulation program solves all the equations and uses the numbers to change the state and output of the simulation. Applications include flight simulators, [[construction and management simulation games]], [[chemical process modeling]], and simulations of [[electrical circuit]]s. Originally, these kinds of simulations were actually implemented on [[analog computer]]s, where the differential equations could be represented directly by various electrical components such as [[op-amp]]s. By the late 1980s, however, most "analog" simulations were run on conventional [[digital computer]]s that [[emulator|emulate]] the behavior of an analog computer.
* A special type of discrete simulation that does not rely on a model with an underlying equation, but can nonetheless be represented formally, is [[agent-based model|agent-based simulation]]. In agent-based simulation, the individual entities (such as molecules, cells, trees or consumers) in the model are represented directly (rather than by their density or concentration) and possess an internal state and set of behaviors or rules that determine how the agent's state is updated from one time-step to the next.
* [[Distributed computing|Distributed]] models run on a network of interconnected computers, possibly through the [[Internet]]. Simulations dispersed across multiple host computers like this are often referred to as "distributed simulations". There are several standards for distributed simulation, including [[Aggregate Level Simulation Protocol]] (ALSP), [[Distributed Interactive Simulation]] (DIS), the [[High Level Architecture (simulation)]] (HLA) and the [[Test and Training Enabling Architecture]] (TENA).