Dear Andre,
Here are my answers to your questions:
- The high Mach number warning has been discussed on our forum before, e.g. see: Mach Number exceeds 0.3 for large wind turbines (15MW) - #4 by Garrett.Barter.
- I’m not sure I can comment based on only on the information you’ve provided me. How does the system response (platform motion, blade deflection, loads, etc.) compare when you change the water depth? Could you redesign the mooring system in the shallower water condition so that the water depth is not impacting the results as much as it is now?
- AeroDyn always uses the inflow local to each aerodynamic analysis node, so, the aeordynamic loads will always be effected by the skewed inflow. SkewMod = 2 adds the Pitt/Peters skewed wake correction to the local induction. SkewMod = 2 is generally more accurate than SkewMod = 1, but SkewMod = 2 is invalid for very large yaw errors (much greater than 45 degrees). It seems odd to me that you’d get high yaw errors with SkewMod = 2, not seen at the original water depth, by changing the water depth / mooring design.
- I would expect Can and Cat in MoorDyn to change when changing the line type (e.g., changing from synthetic line to chain), but not by simply changing the diameter or length of a given line type.
A linearization analysis and free-decay simulations can both be used to identify system natural frequencies and dampings, but they are not the same thing. By “free-decay”, I mean provide a nonzero initial condition (e.g., initial platform-surge displacement) and run a (nonlinear) time-domain simulation without incident waves (likely also without wind); this can be used to identify the the platform natural frequencies, dampings, and quadratic drag. It is common to run six separate free-decay simulations (perturbing each platform DOF one at a time) to identify the natural frequencies and damping of the six floating platform modes (surge, sway, heave, roll, pitch, yaw).
Best regards,