FLACS is a specialized computational fluid dynamics tool that simulates physical phenomena such as dispersion, fires and explosions in a given space and time range, and under given conditions. The spatial region of interest is called the *simulation domain*. The simulation domain has to be defined by the user such that it covers all fluid dynamic features and geometry that potentially are of interest. However, the larger the simulation domain, the longer a simulation takes, so there should be a compromise.

Computational fluid dynamics tools divide the domain in a finite number of *control volumes* (also called *cells*) and solve mathematical equations for the conservation of mass, momentum and energy for each of these volumes. The entirety of all control volumes (cells) is called a *grid* or *mesh*.

Consider, for example, the two-dimensional visualization of a fire event in an enclosure with one open door shown below. The domain of the event, and eventually the simulation domain, is the space containing the effects of interest, such as the fire and smoke flow inside the compartment and out of the door (indicated by grey shading). The domain is highlighted in blue in the picture below. In the right part of the figure, the domain is discretized by a grid (with a finite number of control volumes). Note that we have simplified the example in order to understand the concept of a simulation domain and a grid. The concept is the same for a three-dimensional domain.

In FLACS, the grid is Cartesian and consists of rectangular control volumes aligned with the coordinate directions. There are no cells with irregular shapes, such as inclined or curved cells, but you can modify the size of the cells by stretching the grid.