Physical Processes in a Simulation

Is there a way to determine the relative importance of different physical processes involved in a simulation?

One simple way to get a partial answer to this question is to look at the file "solver Summary" listed under the button "Diagnostics" located on the left side of the FLOW-3D solver window. The information to look at follows the comment, "short print of time step stability limits." The search feature could be used to find these comments or they can be easily located by simply scrolling through the file.

A time-step stability limit is the maximum time-step size that is computationally stable for the process associated with that limit, e.g., advection, viscous stress, surface tension, heat conduction, etc. The smallest time-step limit controls the simulation time advancement. If a process has a time-step limit smaller than another, it indicates that the process is more important in the simulation. For instance, if the viscous time-step size is smaller than that or advection, then viscosity is more dominant than inertia. In fact, the ratio of these time steps is a kind of computational Reynolds number. Depending on the problem, similar ratios could be formed for such quantities as a Froude number, a Weber number, or several other common non-dimensional numbers.

Some care must be exercised in using this information for two reasons. First, the locations where the individual time-step limits are recorded are typically not the same, so that comparisons are between processes at different locations. The second reason for caution is that the time-step limits vary with time. This limitation, however, can actually be useful because it offers a means for tracking what physical process is dominating a solution at any given time.

Finally, if an implicit option, such as heat conduction is used, then it has no time-step limit, and none is reported in the Solver Summary file. Strictly speaking, an implicit option should not be used unless its explicit time-step size is limiting solution advancement, in which case the process would be the dominant one. When more than one implicit option is used, there is no output information available to indicate which processes is most important.