OpenFAST aborts after long simulation – Mach number exceeds 1 for a specific seed

Wind & Water Computer-Aided Engineering Software Tools
1

Hello everyone,

I am running OpenFAST simulations of a gust-induced fault followed by an emergency shutdown.
The simulations are stochastic and were performed for multiple seeds using the same model and settings.

Most of the simulations run successfully. However, after about 7 hours of simulation time, the run corresponding to seed number 72 aborts abruptly with an error indicating that the Mach number exceeds 1.

Since all other seeds complete without issues, I am trying to understand:

  • Is it normal that OpenFAST runs normally for 71 seeds and failed for only one ?

  • What typically causes OpenFAST to report a Mach number greater than 1?

  • Could this be related to extreme local velocities during the emergency shutdown (e.g., blade tip speed, inflow conditions, or numerical instability)?

  • Are there recommended diagnostics or outputs I should check to better identify the source of this issue?

Any guidance on how to troubleshoot or mitigate this problem would be greatly appreciated.

Thank you in advance for your help.

Best Regards

Riad

2

Dear @Riad.Elhamoud,

Given the severity of the speed (greater than Mach 1), I would guess this implies a physical or numerical instability. I would also guess that your model is near a stability margin and the uniqueness of one specific seed is enough to push it over the edge. In practice, if you need 72 seeds, you could simply pick another seed and skip the one that is unstable. But it would be more satisfying to dig into the results to determine what the cause of the instability is and whether it is physical or numerical.

Best regards,

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