The phase angle of a response is usually defined as positive for a phase lag because response tend to lag the force and this results in positive value for phase angles. Mathematically, a phase advance convention would be much better because a positive phase angle would rotate on the complex plane in the same sense as a rotation caused by a positive “wt”. A phase angle calculated directly from a complex algebra ( Ansys uses complex algebra in harmonic response analysis) dynamic solution will generally be a phase advance angle and will have to have sign reversed in order to convert to phase lag. This is explained well in Dynamics of Fixed Marine Structures Book by A. J. Adams and N. D. P. Barltrop.

Attachments:
1. 2041900.png