Inquiry on CFL Number Sensitivity and Robust Parameter Settings in IBLevelSet Simulations

[LIMING CHAO]

Dear all,

I am currently using IBLevelSet to evaluate the propulsion performance of oscillating foils with different amplitudes under incoming flow, all at the same oscillation frequency. The foil motion follows a fixed sinusoidal pattern.

During my tests, I observed an unexpected behavior: when using the same grid resolution and time step, foils with larger amplitudes sometimes result in a smaller CFL number, while those with smaller amplitudes yield a larger CFL number, which in some cases causes the simulation to diverge.

I suspect this issue may be related to the parameters LS_TAG_VALUE or LS_TAG_ABS_THRESH, as they may influence how the level set field evolves during motion. I would like to ask whether there are robust values or recommended strategies for these parameters that can ensure stable simulations across different frequencies and amplitudes.

Best,

Li-Ming

[Boyce Griffith]

Sorry for the delay on this.

Can you post a screenshot of the dynamics just before the simulation diverges? Do you have “strong” vortices hitting outflow boundaries, which can result in “back flow divergence”?

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[LIMING CHAO]

Dear Prof. Boyce,

I did not save the visualization files, so unfortunately I cannot provide you with screenshots. However, I agree with your point. Do you think it would be possible to add a forced boundary outlet in order to avoid backflow?

Best

Li-Ming  

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