Consistency between Bmodes and MBC

Dear Feizbakhshi,

For the same input data, BModes should give quite similar results as FAST when computing tower natural frequencies. For a fixed-base tower (not flexible foundation or floating offshore) Here are few things to keep in mind:

  • You should ensure that the tower-top mass, center of mass, and inertias match between BModes and FAST, which is not trivial. To do this, what we’ve done before is linearize a FAST model with zero platform and tower mass and only the platform DOFs enabled to derive the 6x6 rigid-body mass matrix of the rotor-nacelle assembly, from which the tower-top mass, center of mass, and inertias can be derived for input to BModes.
  • Ensure that the same distributed tower properties are specified in both BModes and FAST. Here’s how you would set up the BModes distributed data file to match FAST:
    sec_loc = HtFract
    str_tw = 0
    tw_iner = 0
    mass_den = TMassDen
    flp_iner = Very small number (you can’t specify exactly zero for this input in BModes)
    edge_iner = Very small number "
    flp_stff = TwFAStif
    edge_stff = TwSSStif
    tor_stff = Very large number (FAST’s tower model neglects torsion)
    axial_stff = Very large number (FAST’s tower model neglects stretching)
    cg_offst = 0
    sc_offst = 0
    tc_offst = 0
  • Derive the tower mode shapes for FAST by selecting the appropriate mode shapes output by BModes and fitting them with the polynomial needed by FAST. The ModeShapePolyFitting.xls spreadsheet provided in the FAST archive is useful for this.
  • Linearize the FAST model for use in MBC3 with only the tower DOFs enabled.

I hope that helps.

Best regards,