Natural frequency of NREL DS monotower in OC3.

Wind & Water Structural Analysis
6

Dear Lingling,

  1. The frequencies you are reporting are a bit low–I would expect the 1st two modes to be closer to 0.25 Hz. Are you using a heavily discretized tower in FAST? I used 99 TwrNodes.

  2. Your approach to running ModeShapePolyFitting.xlsx looks sound. Please note that you can obtain the “Slope” of cell F5 in worksheet “Input” from the BModes output. For the mode in question, this would be 0.001304, the s-s slope at the base of the first mode. The mode looks qualitatively close to the one I derived, but I would not expect a perfect match because I derived my mode shapes using ADAMS in place of BModes, as discussed in the following forum topics: Tower Eigenfrequencies of NREL 5MW Turbine (see my posted dated Tue Aug 28, 2012).

Best regards,