Error: There is no valid value of Phi for these operating conditions


I’m working on a TLP Structure. Could someone share their knowledge why i face with the below mentioned error at times. What could be the possible reasons and how to mitigate this error?

Any help on this error will be highly appreciated.


Dear Jay,

Which version of AeroDyn v15 are you using? Does upgrading to the latest version of FAST v8 help?

This error means that the blade-element/momentum theory (BEMT) solution algorithm of AeroDyn v15 cannot find a valid solution. The error has been discussed several times on this forum. Please use “Search…” in the upper-right corner to see how other users have solved similar problems in the past.

From the following forum topic, we are aware of numerical problem in the BEMT convergence algorithm of AeroDyn v15 when Vx gets very close to zero, but your Vx is quite a bit lower than zero:

A negative Vx is a bit odd. Does the structural solution look satisfactory up until the point of failure, or is there perhaps a numerical instability whereby the structural motions become very large?

Best regards,

Dear Jason,

Thanks for your time and reply.
I’m using AeroDyn v15 and latest version of FAST. The structural solution does not look satisfactory and the motions are bit higher. There is no steadiness in the responses.

Could you please suggest what could be the possible reasons and how to approach this?


Dear Jay,

It is hard to offer specific advice without knowing more about your problem, but I always suggest that you simplify the model (e.g. disable modules, disable features, and/or eliminate DOFs) to isolate problems. Numerical instabilities and their fixes have been discussed a bit on this forum (search for “numerical” and “instability” in the upper-right corner). Some advice is also provided in the “Modeling Tips” section of the FAST v8 ReadMe file:

Best regards,