Hi,
I am running a model in FLAC3D with the power law creep. I have large strain on and I am running it over a fairly large time total.
I configured the maximum and minimum timestep to have automatic adaptation of timestep before starting my solver. Everything it fine and the timestep progressively increase until it reaches the maximum time step. But about a few 10th of 000 cycles where I get an error about my timestep being too large.
I could of course reduce my maximum timestep but that would significantly slow down my model. Are there options for the solver to not only increase the timestep but also decrease it when needed when it automatically adapt the timestep?
I have the feeling that the servo on option would do the trick but I did manage to make it work.
Is there a way to do it without having to manage the solver time stepping by hand?
Below is how I configured my solver:
model restore “step_0”
[runtime=2] ; years
; config solver
model creep active on
model creep timestep automatic on
model creep timestep servo on
model creep time-total = 0.0
model creep timestep starting 1.0e-6
model creep timestep minimum 1.0e-6
model creep timestep maximum 1.0e-2
; run
model solve time-total [365]
model save “step_1yr”
As a general comment on creep modeling in FLAC3D: The creep should be a very small stress correction at each given timestep, so that - at least in principle - each step of your creep modeling is in itself a mechanically equilibrated state. When the creep timestep is too high, you don’t have enough mechanical cycling steps to transport the information of e.g. converging caverns through the model. Since increasing the timestep is very tempting to “get the job done quickly”, I often see mistakes of that nature. A simple sanity check is to just turn off creep or set a fixed very small timestep at any given time in your model and continue the calculation. If your creep timestep was good, you should see only very little movement or stress changes aftwards. If however you then still see significant movement, your creep timestep was to high and the model never really in equilibrium.
To your question: The automatic timestepping in FLAC3D increases the timestep whenever the solve-ratio drops below a certain value and vice versa. This is controlled by the lower-bound and upper-bound variables, which you have not defined.
If the manual is right, the default solve ratio for increasing is 1e-3, which is way too high. (Remember that the default solve target for simple solving 1e-5!). So I would suggest setting something like
model creep timestep lower-bound 1e-5
model creep timestep upper-bound 2e-5
I don’t know which creep model you have chosen, but some do have this additional safeguard against too large timesteps, which you should take seriously. And yes, this probably means your model will take longer to run, but (assuming there is no other error in your model causing higher solve-ratios) then there is no way around it, if you want the “right” results.
Thank you for the reply.
I am not trying to get a timestep as large as possible but rather get the right timestep to get a correct results. In my application deformation is not constant through time. In the earlier stage, deformation is small and I can afford a large timestep while in a later phase deformation get bigger and the timestep need to be smaller for the solution to remain correct.
I could do some stop-and-go and hand tweak the timestep by repeatedly check that convergence is reach like you suggested in your first paragraph (I actually do it at some key times to cross check the results) but that is very tedious. I would rather be adapted automatically and I seem to fail to configure the solver properly to do so.
To get back to my question:
From what you wrote I think my issue is that I did not understood the documentation.
I used model creep timestep maximum and model creep timestep minimum to place bounds on the timestep and have adapt between these bounds based on the default convergence ratio of 1e-5. Now that I reread the documentation after your comment it is clear that I misunderstood the documentation completely.
It seems that I should have been using model creep timestep lower-bound and model creep timestep upper-bound instead.
Still, even if my convergence ratio was fairly large it does not explain why the solver stopped instead of adapting the timestep accordingly.
Not instead, but you need to set these as well as the max and min timesteps.
I don’t know the values of the convergence rate in your model, but as I said 1e-3 is too high and probably your ratio was still below that, so the servo simply continued at a high timestep before eventually the constitutive model itself complained. Solve ratio is only one measure of system equilibrium, that does not mean there can’t be other reasons for the constitutive model to complain about too high timesteps.
Keep your max/min timesteps for now and set different lower/upper bounds and see if it improves. Place histories for timestep and solve ratio, so you know what is going on