The ICE3D module is based on an explicit time-marching scheme which requires the specification of only two parameters.
Ice accretion is inherently an unsteady process, therefore the governing equations are integrated in time. The process stops when the desired Total time of ice accretion (in seconds) is reached.
The time-marching procedure that governs the solution of the water film transport is subject to CFL stability constraints. Smaller grid elements on the surface and faster film speeds will require smaller time steps to avoid numerical instabilities. The Automatic time step option takes the guesswork out of defining a stable time step by computing the optimal time step for each grid/film speed combination, resulting in considerable computational cost savings.
If desired, the time step can also be set manually to a specific value. In this case film fluxes will be limited on nodes where CFL > 1 to maintain a stable run. This in turn will result in extra film being retained on such nodes, affecting the accuracy of the solution. The difference may be significant and you are advised to use the constant time stepping option with caution. Comparing the results of a short duration run done with and without is recommended before moving forward with the constant time step option.
Note: Automatic time step computes a stable and optimal uniform time step at each iteration, which may be higher or lower than 0.001 second, depending on the problem. For example, turbomachinery rotor blades typically have very thin leading edge elements and fast film speeds due to the rotation. The stable time step in this case may be in the range of 1e-4 to 1e-7 second, depending on the amount of runback. Film flows on aircraft components usually run with time steps of 0.01 to 0.1 second. Rime cases are not governed by film flow, therefore they will remain at the default 0.001 second time step value unless the Automatic time step option is disabled and a different time step is imposed.
Rime cases are not governed by film flow, therefore they are marched with the default 0.001 second time step value unless the Automatic time step option is disabled and a different time step is imposed.
If the automatic time step goes below 1e-5 and icing calculations appear to take too long, a constant time step of 1e-5 is likely to be a balanced compromise between the total run-time and solution accuracy.
If the Rime option is selected as the icing model, the entire duration of icing will be done in a single time step unless intermediate solution printout is requested.