Eigenfrequencies of NREL 5MW Blade & full system

Discuss theory and modeling of wind-turbine structures.

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Zimo.Zhu
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Organization: THE HONG KONG POLYTECHNIC UNIVERISTY
Location: Hong Kong

Eigenfrequencies of NREL 5MW Blade & full system

Postby Zimo.Zhu » Wed Jul 01, 2020 5:08 am

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 https://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 :D

Best regards
Zimo

Jason.Jonkman
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Re: Eigenfrequencies of NREL 5MW Blade & full system

Postby Jason.Jonkman » Wed Jul 01, 2020 7:15 am

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Zimo.Zhu
Posts: 4
Joined: Wed Apr 29, 2020 1:07 am
Organization: THE HONG KONG POLYTECHNIC UNIVERISTY
Location: Hong Kong

Re: Eigenfrequencies of NREL 5MW Blade & full system

Postby Zimo.Zhu » Thu Jul 02, 2020 4:36 am

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 viewtopic.php?f=3&t=1264&p=11526&hilit=blade+bmodes#p11526, 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
fig 1. sensitivity analysis.png (22.92 KiB) Viewed 261 times


I am not able to find the reference value for BModes analysis results, but I compare the results with several previous posts like viewtopic.php?f=4&t=1657&p=7725&hilit=bmodes+bmodes+blade+5mw#p7725 and viewtopic.php?f=4&t=1485&p=6616&hilit=bmodes+bmodes+blade+bmodes+blade+5mw#p6616. 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) https://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: Select all

======================   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: Select all

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

Jason.Jonkman
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Location: Boulder, CO
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Re: Eigenfrequencies of NREL 5MW Blade & full system

Postby Jason.Jonkman » Thu Jul 02, 2020 7:05 am

Dear Zimo,

Here are my responses:
As mentioned in post viewtopic.php?f=3&t=1264&p=11526&hilit=blade+bmodes#p11526, I set very small number for flp_iner & edge_iner, and very high number for tor_stff & axial_stff.

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.

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) https://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.

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Zimo.Zhu
Posts: 4
Joined: Wed Apr 29, 2020 1:07 am
Organization: THE HONG KONG POLYTECHNIC UNIVERISTY
Location: Hong Kong

Re: Eigenfrequencies of NREL 5MW Blade & full system

Postby Zimo.Zhu » Thu Jul 02, 2020 10:35 pm

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


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