Different output results for different max time step (DELTMX) values
~~I am running a Toughreact model. I run this model with the time step of 1200 seconds (DELTEN=1200 s) using different max time step values (DELTMX= Infinite, 15 years, and 35 years) for a period of 100,000 years (TIMAX=100,000). The output results for precipitated minerals (e.g., Uraninite) at a certain time (say 50,000 years) are different for these models.
I knew that there is RCOUR (courant coefficient) in TOUGHREACT that can limit max time step size. So, in the second try, I run the same model using RCOUR=0.9 and DELTMX=infinite and different time steps (DELTEN=1 s, DELTEN=10 s, DELTEN=100 s). Again, the results are different!
I would be thankful if someone could guide me why I am getting different results, and give me a hint for selecting appropriate value for DELTEN, DELTMX, and RCOUR in TOUGHREACT.
2 replies

A difference is expected especially if you are running in sequential mode since it will be a balance between the rate of transport and the rate of precipitation. The question is what level of difference you are seeing and whether it is acceptable. Is it converging as your timesteps get smaller?

Thanks George for reply. Since I am considering the coupling between chemistry and transport, the sequential mode is a must. There is a significant difference in volume fraction and location of precipitated Uraninite between the output results of these models. For example Uraninite precipitates with the volume fraction of 0.00457 when I use time step 10 seconds, and with the volume fraction of 0.00371 when I use time step of 100 seconds, and volume fraction of 0.000624 when I use time step of 1 seconds. Even Uraninite precipitates in different locations in these models.
No, they are not converging as I use smaller time steps.
Thanks for your help.