[Kondorfa]

Hi all, I run interFoam for simulating water running through a channel. Sometimes I get a solution computed in 6 sec. By e.g. changing the amount of input water to lower amounts, the solver does not get even a first time step calculated after an hour so I kill the job. The same problem occurs if I refine the cells to half size. The solver does not get anywhere and keeps on computing. How can one avoid this, predict this or check this from the log to find out what is wrong? Thx

[RobertHB]

Originally Posted by Kondorfa

Hi all, I run interFoam for simulating water running through a channel. [...] The same problem occurs if I refine the cells to half size. The solver does not get anywhere and keeps on computing.

Hi,

i haven't worked with interFoam so i cannot comment on the amout of water, but the issue with the cells will be the same for all solvers. Refine your cells by one level and you increase the number of cells by a factor of 8. Next refinement layer, another factor 8 increase and you end up with 64 cells instead of one.
Of course, this increases the resolution of your model, but it also increase your runtime (and by the same factor i'd guess).

How can one avoid this, predict this or check this from the log to find out what is wrong?

1. How can you avoid this?:

- Before creating your mesh, think about what you want to accomplish: Here, you are dealing with laminar flow through a pipe, so i might guess you want to sample a velocity profile. ~ 20 cells in width and height would be sufficient for that job. Dont create to many cells.
- Grab the fvSolution and fvSchemes files from an existing OF case similar to yours. Dont meddle with these if do not know what you are doing.
- Are you running a laminar or a transient case? IF laminar: Check your residual control settings. For a first test a value of 1e-4 would be sufficient. IF transient: Check your Courant number. You want to be below 1.0 but 0.99 is fine. You don't need to run your simulation with a Courant number off 0.001.
2. How can you predict this?: Keep in mind, more cells = more runtime. Co << 1 and you can increase your timestep.
3. What can you read from the log?: The courant number. Number of iterations. Residual values. The latter two should be decreasing, or at least, no increase over time. Doesn't tell you much in the way of what is wrong, more that something is amiss.