# FLUENT 6 - Time step size for Geo-Reconstruct VOF simulations

 Selecting the appropriate physical time step is important for transient VOF simulations using the Geo-Reconstruct formulation. A good time step will make the run: 1) Robust - relative to convergence and stability issues 2) Efficient - relative to CPU time spent on the entire time interval to be modeled How can we estimate this time step? Proir to FLUENT 6.1, the answer was difficult and it involved a quick estimation of things like the maximum velocity in the cells near the interface between the phases the volume of these cells. This information was difficult to estimate before running the solution. FLUENT 6.1 and 6.2 offer an answer to this question - and the answer is given by the code itself, without the user having to guess the velocity field and find cell volumes. The resolution to this problems is described below. This method allows you to exactly pinpoint the size of the VOF sub-timestep based on an existing velocity field. The size of the VOF sub-timestep can be determined after the velocity field has been solved at least for 1 time step. This new feature was introduced in FLUENT 6.1. An error message is sent to the console window by the code signaling the fact that the physical time step needs A CERTAIN NUMBER of VOF sub-timesteps to converge. You can use this information to quickly find the exact magnitude of the VOF sub-timestep. The procedure has the following steps and it is recommended to be done (if possible) in the serial FLUENT version (the parallel FLUENT session will give the error message with the information, but it will hang in 6.1.22): 1) Solve the flow at least for 1 time step - if you are just starting out the simulation, specify a time step that is very small (4-5 orders of magnitude) versus the time scale of the problem to avoid any convergence issues in the beginning. 2) Check the velocity field of this solution. If the values are physical, proceed to step 3. If you get unphysically large velocities, find the cause (mesh quality ? setup ? etc.) and resolve it. 3) Save the data file of a converged time step before proceeding to the next step of the procedure. 4) For the very next time step, specify max iter per time step of 1 and a much larger time step - several orders of magnitude larger than the time step that you used in step 1 above. FLUENT will respond with an error message of this sort: Updating solution at time level N... Error: Too many (31346) VOF sub-timesteps. The velocity field is probably diverging. Please check the solution, and reduce the time step if necessary. Error Object: #f The key piece of information in this error message is the number between parenthesis. In this example, 31346 VOF sub-timesteps are needed to complete 1 physical time step. You are now able to compute the VOF sub-timestep required by the current velocity field and the existing mesh, by simply dividing the large physical time step which you used in step 4 with the number (31346 in this example) of VOF sub-timesteps. This will yield the size of the VOF sub-timestep. Visit the Solve>Iterate panel and specify a time step within the range: VOF sub-timestep