Hello,
My question regards BModes vs. Modes. I understand that BModes is the currently recommended pre-processor for obtaining FAST-input mode shapes. However, when I used Table 6-1 from the Definition of a 5MW Reference Turbine report for the inputs to BModes and Modes, my results indicated Modes was closer to the NREL distributed mode shapes/frequencies than the results from BModes. As an example, the fore-aft frequency result for Mode 2 from BModes was 2.2416 Hz vs. Modes 2.9405 Hz vs. NREL (from Table 9-1 in “Definition”) 2.8590.
Perhaps I’ve entered something incorrectly in my BModes input file to cause this discrepancy?
Here are my tower sections:
[code][size=85]Tower section properties (NREL 5MW Onshore Reference Turbine)
11 n_secs: number of blade or 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 5590.87 24866.3 24866.3 6.14E+11 6.14E+11 4.73E+11 1.38E+11 0 0 0
0.1 0 0 5232.43 21647.5 21647.5 5.35E+11 5.35E+11 4.12E+11 1.29E+11 0 0 0
0.2 0 0 4885.76 18751.3 18751.3 4.63E+11 4.63E+11 3.56E+11 1.21E+11 0 0 0
0.3 0 0 4550.87 16155.3 16155.3 3.99E+11 3.99E+11 3.07E+11 1.12E+11 0 0 0
0.4 0 0 4227.75 13838.1 13838.1 3.42E+11 3.42E+11 2.63E+11 1.04E+11 0 0 0
0.5 0 0 3916.41 11779 11779 2.91E+11 2.91E+11 2.24E+11 9.68E+10 0 0 0
0.6 0 0 3616.83 9958.2 9958.2 2.46E+11 2.46E+11 1.89E+11 8.94E+10 0 0 0
0.7 0 0 3329.03 8356.6 8356.6 2.06E+11 2.06E+11 1.59E+11 8.22E+10 0 0 0
0.8 0 0 3053.01 6955.9 6955.9 1.72E+11 1.72E+11 1.32E+11 7.54E+10 0 0 0
0.9 0 0 2788.75 5738.6 5738.6 1.42E+11 1.42E+11 1.09E+11 6.89E+10 0 0 0
1 0 0 2536.27 4688 4688 1.16E+11 1.16E+11 8.91E+10 6.27E+10 0 0 0
**Note: If this file is for a TOWER, the following section properties are read but overwritten as follows:
str_tw is set to zero
tw_iner is set to zero
cg_offst is set to zero
sc_offst is set to zero
tc_offst is set to zero
edge_iner is set equal to flp_iner
edge_stff is set equal to flp_stff[/size][/code]
And here is my .bmi input file (I copied the tower top mass properties from the CS_Monopile .dat-file).
[code][size=85]====================== BModes v1.03 Main Input File ==================
Modes of ORT tower (89.6 m) with tip mass 350e3 kg
--------- General parameters ---------------------------------------------------------------------
False Echo Echo input file contents to *.echo file if true.
2 beam_type 1: blade, 2: tower (-)
0. romg: rotor speed (rpm), automatically set to zero for tower modal analysis
1.0 romg_mult: rotor speed muliplicative factor (-)
87.6 radius: rotor tip radius measured along coned blade axis OR tower height (m)
0. hub_rad: hub radius measured along coned blade axis OR tower rigid-base height (m)
0. precone: built-in precone angle (deg), automatically set to zero for a tower
0. bl_thp: blade pitch setting (deg), automatically set to zero for a tower
1 hub_conn: hub-to-blade connection [1: cantilevered; other options not yet available]
7 modepr: number of modes to be printed (-)
f 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 --------------------------------------------
3.500003109E+005 tip_mass blade-tip or tower-top mass (kg)
-0.4137754432 cm_loc tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.9669893542 cm_axial tip-mass c.m. offset tower tip measures axially along the z axis (m)
4.370E7 ixx_tip blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.353E7 iyy_tip blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.542E7 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)
1.169E6 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]
‘ORT_props.dat’ sec_props_file name of beam section properties file (-)
Property scaling factors…
1.0 sec_mass_mult: mass density multiplier (-)
1.0 flp_iner_mult: blade flap or tower f-a inertia multiplier (-)
1.0 lag_iner_mult: blade lag or tower s-s inertia multiplier (-)
1.0 flp_stff_mult: blade flap or tower f-a bending stiffness multiplier (-)
1.0 edge_stff_mult: blade lag or tower s-s bending stiffness multiplier (-)
1.0 tor_stff_mult: torsion stiffness multiplier (-)
1.0 axial_stff_mult: axial stiffness multiplier (-)
1.0 cg_offst_mult: cg offset multiplier (-)
1.0 sc_offst_mult: shear center multiplier (-)
1.0 tc_offst_mult: tension center multiplier (-)
--------- Finite element discretization --------------------------------------------------
61 nselt: no of blade or tower elements (-)
Distance of element boundary nodes from blade or flexible-tower root (normalized wrt blade or tower length), el_loc()
0 0.003481894 0.010445682 0.017409471 0.024373259 0.031337047 0.038300836 0.045264624 0.052228412 0.059192201 0.066155989 0.073119777 0.080083565 0.087047354 0.094011142 0.10097493 0.107938719 0.114902507 0.121866295 0.128830084 0.135793872 0.13990 0.149721448 0.156685237 0.163649025 0.170612813 0.177576602 0.18454039 0.191504178 0.198467967 0.205431755 0.212395543 0.219359331 0.22632312 0.233286908 0.240250696 0.247214485 0.250696379 0.320334262 0.37971 0.424791072 0.45961 0.486635 0.51366 0.54068 0.5677 0.594715 0.62173 0.64875 0.67577 0.70279 0.72981 0.75683 0.78385 0.81087 0.83789 0.864905 0.89192 0.91894 0.94596 0.97298 1.0
--------- Properties of additional tower support subsystem (read only if beam_type is 2) ------------
0 tow_support: : additional tower support [0: no additional support; 1: Tension guy wires for land-based tower; 2: offshore turbine support: floating platform or monopile] (-)
Tension-wires data
0 n_attachments: no of wire-attachment locations on tower, maxm allowable is 2; 0: no tension-wire support (-)
3 3 n_wires: no of wires attached at each location (must be 3 or higher) (-)
6 9 node_attach: node numbers of attacments location (node number must be more than 1 and less than nselt+2) (-)
1.e8 1.e8 wire_stfness: wire sifnness in each set (see users’ manual) (N/m)
45. 45. th_wire: angle of tension wires (wrt a horizontal plane) at each attachment point (deg)
[/size][/code]
Kind Regards,
Wystan Carswell