Eigenfrequencies of NREL 5MW Blade & full system

Wind & Water Structural Analysis
1

Hi everyone

I am now doing the finite element modeling for NREL 5MW wind turbine. I have built a cantilever shell blade in ANSYS and the eigenfrequencies match well with the values provided by Sandia report prod-ng.sandia.gov/techlib-noau … 132569.pdf. The eigenfrequencies from Sandia report are listed below:
mode Description Reference value
1 1st flapwise bending 0.87
2 1st edgewise bending 1.06
3 2nd flapwise bending 2.68
4 2nd edgewise bending 3.91
5 3rd flapwise bending 5.57
6 1st torsion 6.45

However, when I calculate the blade eigenfrequencies by BModes, the results are quite different. I have set the pretwist to 0, is it the reason for the differences? Seems the flapwise eigenfrequencies will be affected a lot by the pretwist angle.
mode Description Reference value
1 1st flapwise bending 0.6958
2 1st edgewise bending 1.1755
3 2nd flapwise bending 1.9596
4 2nd edgewise bending 4.0471
5 3rd flapwise bending 4.5657

To compare the full eigenfrequencies of the whole wind turbine system including tower and blade, I have build finite element model for the full system. Results shows that the modes reagrding blade modes are not accurate when compared to value provided in Table 9-1 in the report, while the values of tower modes are quite accurate. I wonder if pre-twist angle is considered in the FAST eigenanalysis? Should I also set pretwist angle = 0 in my model to obtain correct results?

I would be appreciated it if you could reply. Thank you :smiley:

Best regards
Zimo

2

Dear Zimo,

I’m not sure how sensitive the blade natural frequencies of the NREL 5-MW baseline turbine are to the structural pretwist, but you could certainly run BModes with and without the structural pretwist to find out.

Yes, the FAST results reported in that table include the effect of structural pretwist.

Given the differences in natural frequencies between your ANSYS 3D shell model and BModes 1D beam model, can you comment on how close the equivalent 1D beam properties are between the two models? The Blade Property Extractor (BPE) post-processing tool can be used to derive equivalent beam properties from an 3D ANSYS model of the blade, assuming that you created the ANSYS model from NuMAD.

Best regards,

3

Dear Jason

Thank you very much for your quick reply! It definitely gives me some hints of the modeling.

I am not very familiar with the Blade Property Extractor (BPE) post-processing tool, so it may take me some time to learn it before I can derive equivalent beam properties from 3D ANSYS model.

I do carry out the analysis to check how sensitive the blade natural frequencies of the NREL 5-MW baseline turbine are to the structural pretwist. As mentioned in post NREL 5MW Blade Mode Shape - #5 by Paul.Feja, I set very small number for flp_iner & edge_iner, and very high number for tor_stff & axial_stff. I have tested three senarios, str_tw=0, str_tw=13.308 and str_tw=StrcTwst in Table 2-1. The results are summarized as below:

fig 1. sensitivity analysis.png

I am not able to find the reference value for BModes analysis results, but I compare the results with several previous posts like NREL 5MW Blade Eigenfrequencies with Bmodes - #2 by Ivano.Quarta. Seems we have got similar results. The 1st flapwise bending frequency is around 0.69Hz. This value also matches the full system frequencies in Table 9-1. However, as can be seen from the table above, the results obatined when considering the structural pretwist are still quite different from the reference values provided by Sandia report (Table 15) prod-ng.sandia.gov/techlib-noau … 132569.pdf. In my understanding, both the 3D ANSYS model in Sandia report and the BModes model consider the structural pretwist, and the boundary conditions are same (both cantilever), similar results are expected. The differences are around 20% bewtween the two models. I am now quite confusing about these results. I wonder if there are anything wrong in my understanding or calculation? I would be appreciate it if you could shed some light on this.

Best regards
Zimo

[code]====================== BModes v1.03 Main Input File ==================
Sample non-uniform blade (output is space-delimited)

--------- General parameters ---------------------------------------------------------------------
False 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)
1.0 romg_mult: rotor speed muliplicative factor (-)
63.0 radius: rotor tip radius measured along coned blade axis OR tower height (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 connection [1: cantilevered; other options not yet available] (-)
20 modepr: number of modes to be printed (-)
f TabDelim (true: tab-delimited output tables; false: space-delimited tables)
t 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 blade axis measured along the tip section y reference 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]
‘test03_blade_sec_props.dat’ sec_props_file name of beam section properties file (-)

Property scaling factors…
1.04536 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 --------------------------------------------------
48 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 0.00325 0.01951 0.03577 0.05203 0.06829 0.08455 0.10081 0.11707 0.13335 0.14959 0.16585 0.18211 0.19837 0.21465 0.23089 0.24715 0.26341 0.29595
0.32846 0.36098 0.3935 0.42602 0.45855 0.49106 0.52358 0.5561 0.58862 0.62115 0.65366 0.68618 0.7187 0.75122 0.78376 0.81626 0.84878 0.8813
0.89756 0.91382 0.93008 0.93821 0.94636 0.95447 0.9626 0.97073 0.97886 0.98699 0.99512 1

--------- Properties of tension wires suporting the tower --------------------------------
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) (-)
0.e0 0.e0 wire_stfness: wire spring constant in each set (see users’ manual) (N/m)
0. 0. th_wire: angle of tension wires wrt the tower axis at each attachment point (deg)
[/code]

[code]Blade section properties
49 n_secs: number of blade 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)
0.00000 13.308 13.308 678.94 1.39402795E-17 1.39454166E-17 1.811000E+10 1.811360E+10 5.56440000E+29 1.394285E+30 0.0 0.0 0.0
0.00325 13.308 13.308 678.94 1.39402795E-17 1.39454166E-17 1.811000E+10 1.811360E+10 5.56440000E+29 1.394285E+30 0.0 0.0 0.0
0.01951 13.308 13.308 773.36 1.54056463E-17 1.47041778E-17 1.942490E+10 1.955860E+10 5.43159000E+29 1.505491E+30 0.0 0.0 0.0
0.03577 13.308 13.308 740.55 1.26030747E-17 1.48128106E-17 1.745590E+10 1.949780E+10 4.99398000E+29 1.370794E+30 0.0 0.0 0.0
0.05203 13.308 13.308 740.04 1.03174827E-17 1.63429855E-17 1.528740E+10 1.978880E+10 4.66659000E+29 1.333023E+30 0.0 0.0 0.0
0.06829 13.308 13.308 592.50 7.09915447E-18 1.28635531E-17 1.078240E+10 1.485850E+10 3.47471000E+29 9.981354E+29 0.0 0.0 0.0
0.08455 13.308 13.308 450.28 4.63334462E-18 9.13918803E-18 7.229720E+09 1.022060E+10 2.32354000E+29 6.886266E+29 0.0 0.0 0.0
0.10081 13.308 13.308 424.05 3.78312628E-18 8.31293078E-18 6.309540E+09 9.144700E+09 1.90787000E+29 6.048029E+29 0.0 0.0 0.0
0.11707 13.308 13.308 400.64 3.08588692E-18 7.47312401E-18 5.528360E+09 8.063160E+09 1.57036000E+29 5.279505E+29 0.0 0.0 0.0
0.13335 13.308 13.308 382.06 2.61556115E-18 6.30591497E-18 4.980060E+09 6.884440E+09 1.15826000E+29 4.460738E+29 0.0 0.0 0.0
0.14959 13.308 13.308 399.66 2.30631196E-18 6.35229712E-18 4.936840E+09 7.009180E+09 1.00212000E+29 4.329305E+29 0.0 0.0 0.0
0.16585 13.308 13.308 426.32 1.96237798E-18 6.95957884E-18 4.691660E+09 7.167680E+09 8.55899000E+28 4.460978E+29 0.0 0.0 0.0
0.18211 13.181 13.181 416.82 1.45862352E-18 7.79188452E-18 3.949460E+09 7.271660E+09 6.72268000E+28 4.625254E+29 0.0 0.0 0.0
0.19837 12.848 12.848 406.19 1.14769873E-18 8.90082570E-18 3.386520E+09 7.081700E+09 5.47485000E+28 5.024262E+29 0.0 0.0 0.0
0.21465 12.192 12.192 381.42 9.17898818E-19 7.83188211E-18 2.933740E+09 6.244530E+09 4.48844000E+28 4.374890E+29 0.0 0.0 0.0
0.23089 11.561 11.561 352.82 7.33254659E-19 6.22654873E-18 2.568960E+09 5.048960E+09 3.35918000E+28 3.479902E+29 0.0 0.0 0.0
0.24715 11.072 11.072 349.48 6.31492382E-19 5.89160478E-18 2.388650E+09 4.948490E+09 3.11345000E+28 3.261549E+29 0.0 0.0 0.0
0.26341 10.792 10.792 346.54 5.68060945E-19 5.49111062E-18 2.271990E+09 4.808020E+09 2.91939000E+28 3.029586E+29 0.0 0.0 0.0
0.29595 10.232 10.232 339.33 4.57599820E-19 4.67238898E-18 2.050050E+09 4.501400E+09 2.60995000E+28 2.564994E+29 0.0 0.0 0.0
0.32846 9.672 9.672 330.00 3.62773543E-19 3.97273878E-18 1.828250E+09 4.244070E+09 2.28820000E+28 2.167756E+29 0.0 0.0 0.0
0.36098 9.110 9.110 321.99 2.76513885E-19 3.48562482E-18 1.588710E+09 3.995280E+09 2.00750000E+28 1.881069E+29 0.0 0.0 0.0
0.39350 8.534 8.534 313.82 2.05186540E-19 3.03415442E-18 1.361930E+09 3.750760E+09 1.74378000E+28 1.619670E+29 0.0 0.0 0.0
0.42602 7.932 7.932 294.73 1.33275021E-19 2.36229130E-18 1.102380E+09 3.447140E+09 1.44469000E+28 1.247783E+29 0.0 0.0 0.0
0.45855 7.321 7.321 287.12 8.50710178E-20 1.95722944E-18 8.758000E+08 3.139070E+09 1.19982000E+28 1.021150E+29 0.0 0.0 0.0
0.49106 6.711 6.711 263.34 5.29502622E-20 1.46490556E-18 6.813000E+08 2.734240E+09 8.11924000E+27 7.589279E+28 0.0 0.0 0.0
0.52358 6.122 6.122 253.21 3.35460545E-20 1.28443992E-18 5.347200E+08 2.554870E+09 6.90910000E+27 6.589930E+28 0.0 0.0 0.0
0.55610 5.546 5.546 241.67 2.03280769E-20 1.09167905E-18 4.089000E+08 2.334030E+09 5.74541000E+27 5.560036E+28 0.0 0.0 0.0
0.58862 4.971 4.971 220.64 1.36170572E-20 8.23933360E-19 3.145400E+08 1.828730E+09 4.59151000E+27 4.187752E+28 0.0 0.0 0.0
0.62115 4.401 4.401 200.29 8.83615884E-21 6.75317642E-19 2.386300E+08 1.584100E+09 3.59767000E+27 3.420769E+28 0.0 0.0 0.0
0.65366 3.834 3.834 179.40 5.53226673E-21 5.35084781E-19 1.758800E+08 1.323360E+09 2.74412000E+27 2.703085E+28 0.0 0.0 0.0
0.68618 3.332 3.332 165.09 3.23001429E-21 5.92782253E-19 1.260100E+08 1.183680E+09 2.09031000E+27 2.980061E+28 0.0 0.0 0.0
0.71870 2.890 2.890 154.41 2.50478757E-21 4.76571060E-19 1.072600E+08 1.020160E+09 1.85425000E+27 2.395379E+28 0.0 0.0 0.0
0.75122 2.503 2.503 138.94 1.93547705E-21 3.52327780E-19 9.088000E+07 7.978100E+08 1.62761000E+27 1.771316E+28 0.0 0.0 0.0
0.78376 2.116 2.116 129.56 1.46951003E-21 2.91133034E-19 7.631000E+07 7.096100E+08 1.45346000E+27 1.463013E+28 0.0 0.0 0.0
0.81626 1.730 1.730 107.26 1.05502194E-21 1.92533969E-19 6.105000E+07 5.181900E+08 9.07286000E+26 9.679450E+27 0.0 0.0 0.0
0.84878 1.342 1.342 98.78 7.65632246E-22 1.58521299E-19 4.948000E+07 4.548700E+08 8.05730000E+26 7.964347E+27 0.0 0.0 0.0
0.88130 0.954 0.954 90.25 5.42684525E-22 1.28783642E-19 3.936000E+07 3.951200E+08 7.07829000E+26 6.466316E+27 0.0 0.0 0.0
0.89756 0.760 0.760 83.00 4.50412928E-22 1.09274324E-19 3.467000E+07 3.537200E+08 6.09268000E+26 5.486237E+27 0.0 0.0 0.0
0.91382 0.574 0.574 72.91 3.94069136E-22 5.56944169E-20 3.041000E+07 3.047300E+08 5.75387000E+26 2.804424E+27 0.0 0.0 0.0
0.93008 0.404 0.404 68.77 3.22735664E-22 4.99038450E-20 2.652000E+07 2.814200E+08 5.33059000E+26 2.511329E+27 0.0 0.0 0.0
0.93821 0.319 0.319 66.26 2.71616888E-22 4.41651551E-20 2.384000E+07 2.617100E+08 4.93516000E+26 2.221839E+27 0.0 0.0 0.0
0.94636 0.253 0.253 59.34 2.04409601E-22 2.24633634E-20 1.963000E+07 1.588100E+08 4.23806000E+26 1.133389E+27 0.0 0.0 0.0
0.95447 0.216 0.216 55.91 1.42995713E-22 1.70703516E-20 1.600000E+07 1.378800E+08 3.65886000E+26 8.606674E+26 0.0 0.0 0.0
0.96260 0.178 0.178 52.48 9.70456723E-23 1.27896814E-20 1.283000E+07 1.187900E+08 3.12686000E+26 6.443364E+26 0.0 0.0 0.0
0.97073 0.140 0.140 49.11 6.36489939E-23 9.47217935E-21 1.008000E+07 1.016300E+08 2.64323000E+26 4.767914E+26 0.0 0.0 0.0
0.97886 0.101 0.101 45.82 3.80421843E-23 6.76168197E-21 7.550000E+06 8.507000E+07 2.17187000E+26 3.399862E+26 0.0 0.0 0.0
0.98699 0.062 0.062 41.67 1.53378029E-23 3.85801509E-21 4.600000E+06 6.426000E+07 1.58095000E+26 1.936676E+26 0.0 0.0 0.0
0.99512 0.023 0.023 11.45 1.07587362E-24 7.64479204E-22 2.500000E+05 6.610000E+06 2.53690000E+25 3.827775E+25 0.0 0.0 0.0
1.00000 0.000 0.000 10.32 5.48167599E-25 4.50813826E-22 1.700000E+05 5.010000E+06 1.86546400E+25 2.256810E+25 0.0 0.0 0.0

**Note: If the above data represents TOWER properties, the following are overwritten:
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
[/code]

4

Dear Zimo,

Here are my responses:

These recommendations are suggested when using BModes to derive mode shapes for FAST / OpenFAST’s ElastoDyn module (to ensure the blade model representations are similar between BModes and ElastoDyn). I’m not sure you need to apply that recommendation for your analysis. Regardless, I would make sure that the very high tor_stff and axial_stff you’ve defined are not causing numerical problems in your results. Settings these to be a few orders of magnitude higher than the bending stiffness is likely sufficient for the purposes of stiffening these modes without causing numerical problems.

I believe you are running BModes correctly (again, I would verify that the high tor_stff and axial_stff are not causing numerical problems). I would guess the blade developed by Sandia in NuMAD/ANSYS has properties that are a bit different than the original definition of the NREL 5-MW blade from NREL.

Best regards,

5

Dear Jason

Thanks again for your quick reply!

I have revised the tor_stff and axial_stff value to avoid possible errors caused by the “extremely” large number. I have tried the value that are 100 times and 1000 times larger than the bending stiffness, same results are obatined. Seems no errors are caused by the value of large number.

I reviewed both the NREL report and Sandia blade report again in case there are any missing details. In the report prepared by Sandia National Laboratories, “Match the blade mode shapes and frequencies represented by the NREL 5MW distributed blade properties” mentioned in Page 18 is classified as “Desired Criteria”. I guess this may possibly explain the differences between BModes results and 3D shell model reference value. Model updating or further refinement is needed in order to match the eigenfrequencies.

Thank you again for your help!

Best regards
Zimo