MODES_SHAPES_INPUT_PARAMETERS

I am currently trying to create the mode shapes of both, tower and blades that FAST needs.
My problem is that I am confused with the parameters the user needs to introduce in order to define the tower:

  1. Total beam length (m) - I asume this is the total length of the tower itself, is it correct?
  2. Rigid beam length (m) - What is the meaning of this parameter? I asume it is related to the boundary conditions of the tower but I do not really know how to represent it, for example in the case that the tower is embedded.
  3. End mass (kg) - what is the meaning of this parameter?

Thank you very much,

Dear Aitor,

In Modes, The total beam length should be the height of the tower or the tip radius of the blade (TowerHt for the tower or TipRad for the blade in FAST v8’s ElastoDyn module) and the rigid beam length should be the rigid portion of the bottom of the tower (TowerBsHt, if any) or the hub radius of the blade (HubRad). End mass should be the total mass of the tower top / rotor-nacelle assembly for the tower (found in the ElastoDyn summary file) or the tip mass for the blade (TipMass).

Modes cannot be used to model an embedded tower with flexible foundation. For this, you should use BModes in place of Modes. In fact, as discussed in my May 31, 2012 post in the following forum topic: http://forums.nrel.gov/t/tower-eigenfrequencies-of-nrel-5mw-turbine/517/1, we now recommend that BModes be used in place of Modes in all cases (but especially for the tower).

Best regards,

Dear Jason,

The rotor RPM keeps changing with wind condition, meaning, in below rated wind speed, steady state angular velocity of rotor keeps changing depending on the wind speed. So, what should be given as input for “Modes” parameter, ‘Steady state angular velocity of rotor (rpm) [Ignored for towers]’ in the mode shape calculation of blades?

Regards,
Kumara

Dear Kumara,

The mode shapes of a spinning blade do depend on rotor speed, but ElastoDyn requires that you only enter one mode shape for each simulation. I generally recommend to derive mode shapes at the rated rotor speed, but you could always perform a sensitivity analysis on the impact of mode shapes derived from different rotational speeds on the dynamic response.

Best regards,