Hello everyone and @itascan @msepu @itascaYang
I am currently facing an issue with my model. The model converges when I use model solve 1e-2. I have defined several history points to monitor parameters at different locations. However, when I try to plot the history charts, no curves appear.
Additionally, when I export the results to a CSV file, it contains only a single value corresponding to one step.
To investigate further, I ran the model using model cycle 5000. In this case, the history curves appear, and the exported file contains values over multiple steps. However, the results differ significantly from my theoretical calculations.
This leads me to believe that 5000 cycles may not be sufficient for the model to reach equilibrium. I would like to ask if there are any guidelines or criteria to determine the appropriate number of cycles required for a model to reach equilibrium.
Any suggestions or insights would be greatly appreciated.
Thank you for your time and support.
While 1e-2 might sound like a small number, in the context of the unbalanced force ratio; 1e-3 is generally considered a preliminary convergence criterion and for most engineering applications, a ratio on the order of 1e-5 is the standard for equilibrium (although some models may require something even smaller than this).
Also note that the default history increment is every 10 steps (this can be changed with history interval <i_step> command). It may be that your initial model hasn’t reached 20 steps. You can see number of steps in the console after a solve or cycle or step command. This can also be displayed in any plot by enabling step in the plot legend settings, or by checking the object tree.
To ensure you have reached equilibrium and minimize solve times, instead of relying solely on a standard number, it’s always good practice to check these three “safety signs”:
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The History Plot: Plot the “Max Unbalanced Force Ratio” vs. “Step.” The curve should be flat (asymptotic). Note that if this curve flattens out at a value above your target threshold then the model may have reached a state of continuous plastic flow: your slope is sliding, your tunnel is collapsing, or your foundation has failed.
Velocity: Check the maximum velocity in the model. It should be approaching zero (e.g., 10^-6 or less).
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Displacement: Monitor a “target” node in the zone of interest. If the displacement is still climbing linearly when the model stops, you haven’t reached equilibrium. If it becomes stable for a long period of time, you can reduce the target to solve faster.
Here’s a summary of things to consider:
Is the default maximum unbalanced force ratio for model solving 1e-5? When the maximum unbalanced force ratio is 1e-2, the model may reach the “requirement” after only a very few steps. Additionally, based on my experience in FLAC, the plastic zones stabilize when the maximum unbalanced force ratio is below 1e-3. In my simulations, if I don’t want to spend too much time on in-situ stress balance, I usually set the maximum unbalanced force ratio to 1e-4.
Hello @ddegagne
Thank you for your reply.
Basically, the issue I am facing is related to solving my model. Initially, I used a convergence criterion of 1×10−41×10−4, but the model kept running continuously for 2–3 hours (even longer) without achieving convergence. After that, I tried 1×10−31×10−3, but the model still did not converge. Finally, when I used 1×10−21×10−2, the model completed the run successfully.
However, another issue appeared in this case. I used the history command to monitor some parameters. After the model run was completed, I attempted to plot the results using the history chart, but no curve was displayed. When I exported the data as a CSV file from the empty plot, it showed only a single step of data, and in some cases, no data was recorded at all.
At this point, I am quite confused about how to resolve this issue.
Hello @itascaYang
Thank you for your reply.
Basically, the issue I am facing is related to solving my model. Initially, I used a convergence criterion of 1×10−41×10−4, but the model kept running continuously for 2–3 hours (even longer) without achieving convergence. After that, I tried 1×10−31×10−3, but the model still did not converge. Finally, when I used 1×10−21×10−2, the model completed the run successfully.
However, another issue appeared in this case. I used the history command to monitor some parameters. After the model run was completed, I attempted to plot the results using the history chart, but no curve was displayed. When I exported the data as a CSV file from the empty plot, it showed only a single step of data, and in some cases, no data was recorded at all.
At this point, I am quite confused about how to resolve this issue.
