Dear all,

Theses days I simulated mode shape for blade and tower respectively using BModes. And I compared my results with OpenFAST’s default mode shape of blade and Tower ( adopted by Elastodyn in r-test folder). And I found something interesting. In the following, I will separately demonstrate results of blade and tower with different configurations (i.e. foundation condition, torsional/axial stiffness).

**1. Blade mode shape + very large torsional/axial stiffness**

input file:

```
--------- General parameters ---------------------------------------------------------------------
true Echo Echo input file contents to *.echo file if true.
1 beam_type 1: blade, 2: tower (-)
0. romg: rotor speed, automatically set to zero for tower modal analysis (rpm)
1. romg_mult: rotor speed muliplicative factor (-)
63 radius: rotor tip radius measured along coned blade axis, OR tower height above ground level [onshore] or MSL [offshore](m)
1.5 hub_rad: hub radius measured along coned blade axis OR tower rigid-base height (m)
-2.5 precone: built-in precone angle, automatically set to zero for a tower (deg)
0. bl_thp: blade pitch setting, automatically set to zero for a tower (deg)
1 hub_conn: hub-to-blade or tower-base boundary condition [1: cantilevered; 2: free-free; 3: only axial and torsion constraints] (-)
20 modepr: number of modes to be printed (-)
t TabDelim (true: tab-delimited output tables; false: space-delimited tables)
f mid_node_tw (true: output twist at mid-node of elements; false: no mid-node outputs)
--------- Blade-tip or tower-top mass properties --------------------------------------------
0 tip_mass blade-tip or tower-top mass (kg)
0 cm_loc tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
0 cm_axial tip-mass c.m. offset tower tip measures axially along the z axis (m)
0 ixx_tip blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
0 iyy_tip blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
0 izz_tip torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0. ixy_tip cross product of inertia about x and y reference axes(kg-m^2)
0 izx_tip cross product of inertia about z and x reference axes(kg-m^2)
0. iyz_tip cross product of inertia about y and z reference axes(kg-m^2)
--------- Distributed-property identifiers --------------------------------------------------------
1 id_mat: material_type [1: isotropic; non-isotropic composites option not yet available]
'blade_secs_simpack.dat' : sec_props_file name of beam section properties file (-)
```

blade property file (which is identical to blade data in NRELOffshrBsline5MW_Blade.dat):

```
Tower section properties
26 n_secs: number of tower sections at which properties are specified (-)
sec_loc str_tw tw_iner mass_den flp_iner edge_iner flp_stff edge_stff tor_stff axial_stff cg_offst sc_offst tc_offst
(-) (deg) (deg) (kg/m) (kg-m) (kg-m) (Nm^2) (Nm^2) (Nm^2) (N) (m) (m) (m)
0 0 0 7.15E+02 1.00E-10 1.00E-10 1.81E+10 1.81E+10 1.00E+20 1.00E+20 0 0 0
0.003252033 0 0 7.15E+02 1.00E-10 1.00E-10 1.81E+10 1.81E+10 1.00E+20 1.00E+20 0 0 0
0.035772358 0 0 7.80E+02 1.00E-10 1.00E-10 1.73E+10 1.97E+10 1.00E+20 1.00E+20 0 0 0
0.068292683 0 0 6.24E+02 1.00E-10 1.00E-10 1.05E+10 1.52E+10 1.00E+20 1.00E+20 0 0 0
0.100813008 0 0 4.47E+02 1.00E-10 1.00E-10 6.09E+09 9.37E+09 1.00E+20 1.00E+20 0 0 0
0.133349593 0 0 4.02E+02 1.00E-10 1.00E-10 4.82E+09 7.05E+09 1.00E+20 1.00E+20 0 0 0
0.165853659 0 0 4.49E+02 1.00E-10 1.00E-10 4.49E+09 7.37E+09 1.00E+20 1.00E+20 0 0 0
0.198373984 0 0 4.28E+02 1.00E-10 1.00E-10 3.13E+09 7.33E+09 1.00E+20 1.00E+20 0 0 0
0.230894309 0 0 3.72E+02 1.00E-10 1.00E-10 2.42E+09 5.19E+09 1.00E+20 1.00E+20 0 0 0
0.263414634 0 0 3.65E+02 1.00E-10 1.00E-10 2.14E+09 4.94E+09 1.00E+20 1.00E+20 0 0 0
0.328455285 0 0 3.48E+02 1.00E-10 1.00E-10 1.71E+09 4.37E+09 1.00E+20 1.00E+20 0 0 0
0.393495935 0 0 3.30E+02 1.00E-10 1.00E-10 1.26E+09 3.85E+09 1.00E+20 1.00E+20 0 0 0
0.458552846 0 0 3.02E+02 1.00E-10 1.00E-10 8.16E+08 3.20E+09 1.00E+20 1.00E+20 0 0 0
0.523577236 0 0 2.67E+02 1.00E-10 1.00E-10 5.04E+08 2.59E+09 1.00E+20 1.00E+20 0 0 0
0.588617886 0 0 2.32E+02 1.00E-10 1.00E-10 3.01E+08 1.84E+09 1.00E+20 1.00E+20 0 0 0
6.54E-01 0 0 1.89E+02 1.00E-10 1.00E-10 1.71E+08 1.33E+09 1.00E+20 1.00E+20 0 0 0
7.19E-01 0 0 1.63E+02 1.00E-10 1.00E-10 1.06E+08 1.02E+09 1.00E+20 1.00E+20 0 0 0
0.783756098 0 0 1.36E+02 1.00E-10 1.00E-10 7.59E+07 7.10E+08 1.00E+20 1.00E+20 0 0 0
0.848780488 0 0 1.04E+02 1.00E-10 1.00E-10 4.94E+07 4.55E+08 1.00E+20 1.00E+20 0 0 0
0.897560976 0 0 8.74E+01 1.00E-10 1.00E-10 3.47E+07 3.54E+08 1.00E+20 1.00E+20 0 0 0
0.930081301 0 0 7.24E+01 1.00E-10 1.00E-10 2.65E+07 2.81E+08 1.00E+20 1.00E+20 0 0 0
0.946357724 0 0 6.25E+01 1.00E-10 1.00E-10 1.96E+07 1.59E+08 1.00E+20 1.00E+20 0 0 0
0.962601626 0 0 5.53E+01 1.00E-10 1.00E-10 1.28E+07 1.19E+08 1.00E+20 1.00E+20 0 0 0
0.978861789 0 0 4.83E+01 1.00E-10 1.00E-10 7.54E+06 8.51E+07 1.00E+20 1.00E+20 0 0 0
0.995121951 0 0 1.21E+01 1.00E-10 1.00E-10 2.44E+05 6.61E+06 1.00E+20 1.00E+20 0 0 0
1 0 0 1.09E+01 1.00E-10 1.00E-10 1.73E+05 5.01E+06 1.00E+20 1.00E+20 0 0 0
```

As you can see, the blade has zeroed twist angle, very small flp_iner and edge_iner, very large tor_stff and axial_stff (as suggested by Jason, please see this post https://forums.nrel.gov/t/blade-mode-shapes-with-bmodes/1441?u=ran.tu ). So result will not contain any torsional mode shape. The fist three mode shapes are 1st flap-wise bending mode, 1st edge-wise bending mode, and 2nd flap-wise bending mode.

After fitting the mode coefficients, I plot the curve and compare it with default mode shape coefficients in NRELOffshrBsline5MW_Blade.dat:

I may come into it that:

when simulating blade mode shape in BModes, we can ignore flap/edge-wise inertia, and assign very large value to the torsional/axial stiffness. The results will be adequately accurate to be used in OpenFAST because OpenFAST do not use torsional mode shape.