BModes : Input parameters about tower support subsystem
Moderators: Bonnie.Jonkman, Jason.Jonkman
BModes : Input parameters about tower support subsystem
Dear NREL Team:
I'm using New version(Not Publicly released) of BModes code which could consider the flexibility of the foundation. The code is downloaded from http://wind.nrel.gov/public/jjonkman/BModes/ released by Dr. Jonkman.
In the "OC3Hywind.bmi" input file, there are 3 matrix representing the foundation and mooring's parameters:
Platformreferencepointreferred hydrodynamic 6X6 matrix (hydro_M);
Platformreferencepointreferred hydrodynamic 6X6 stiffness matrix (hydro_K);
Mooringsystem 6X6 stiffness matrix (mooring_K).
I compared the parameters with the paper "Definition of the Floating System for Phase IV of OC3". The "hydro_K" seems to be the linearized restoring matrix from all mooring lines, and the "hydro_K" is more like the linear hydrostaticrestoring matrix, but both have small differences with the value in the paper.
Could you tell me what's the meaning of those matrix, and how to get them?
Thanks a lot,
Lei
I'm using New version(Not Publicly released) of BModes code which could consider the flexibility of the foundation. The code is downloaded from http://wind.nrel.gov/public/jjonkman/BModes/ released by Dr. Jonkman.
In the "OC3Hywind.bmi" input file, there are 3 matrix representing the foundation and mooring's parameters:
Platformreferencepointreferred hydrodynamic 6X6 matrix (hydro_M);
Platformreferencepointreferred hydrodynamic 6X6 stiffness matrix (hydro_K);
Mooringsystem 6X6 stiffness matrix (mooring_K).
I compared the parameters with the paper "Definition of the Floating System for Phase IV of OC3". The "hydro_K" seems to be the linearized restoring matrix from all mooring lines, and the "hydro_K" is more like the linear hydrostaticrestoring matrix, but both have small differences with the value in the paper.
Could you tell me what's the meaning of those matrix, and how to get them?
Thanks a lot,
Lei

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 Location: Boulder, CO
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Re: BModes : Input parameters about tower support subsystem
Dear Lei,
The "hydro_K" matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (43) of the OC3Hywind report (perhaps with slight numerical rounding) except that the (6,6) element has been augmented with the "additional yaw spring" documented in Table 51 of the OC3Hywind specifications report.
The "mooring_K" matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (53) of the OC3Hywind report except that the (4,4) and (5,5) elements have been augmented with the contribution of system weight/gravity, which is not otherwise accounted for in BModes as explained in the following forum topic: viewtopic.php?f=4&t=541. System weight is important for the pitch and roll restoring of deepdrafted floating platforms, such as spar buoys. The augmentation equals m*g*z, where m is the total system mass (platform + ballast + tower + nacelle + rotor), g is gravity, and z is the vertical location of the center of gravity of the total system mass (z is negative in value for this case).
I hope that helps.
Best regards,
The "hydro_K" matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (43) of the OC3Hywind report (perhaps with slight numerical rounding) except that the (6,6) element has been augmented with the "additional yaw spring" documented in Table 51 of the OC3Hywind specifications report.
The "mooring_K" matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (53) of the OC3Hywind report except that the (4,4) and (5,5) elements have been augmented with the contribution of system weight/gravity, which is not otherwise accounted for in BModes as explained in the following forum topic: viewtopic.php?f=4&t=541. System weight is important for the pitch and roll restoring of deepdrafted floating platforms, such as spar buoys. The augmentation equals m*g*z, where m is the total system mass (platform + ballast + tower + nacelle + rotor), g is gravity, and z is the vertical location of the center of gravity of the total system mass (z is negative in value for this case).
I hope that helps.
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
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
Re: BModes : Input parameters about tower support subsystem
Dear Jason,
thank you for your quick response, that helps a lot!
Lei
thank you for your quick response, that helps a lot!
Lei
Re: BModes : Input parameters about tower support subsystem
Dear Jason,
I have another question about the input file for BModes.
If the added mass matrix varies with the frequency, then how to define the "Hydro_M"?
also,the following lines in the input file, i think they are talking about the same value for tower, the different is just the +/ symbol, am i right?
hub_rad: hub radius measured along coned blade axis OR tower rigidbase height (m)
draft : depth of tower base from the ground or the MSL (mean sea level) (m)
best regards,
Lei
I have another question about the input file for BModes.
If the added mass matrix varies with the frequency, then how to define the "Hydro_M"?
also,the following lines in the input file, i think they are talking about the same value for tower, the different is just the +/ symbol, am i right?
hub_rad: hub radius measured along coned blade axis OR tower rigidbase height (m)
draft : depth of tower base from the ground or the MSL (mean sea level) (m)
best regards,
Lei

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 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: BModes : Input parameters about tower support subsystem
Dear Lei,
You cannot define a frequencydependent hydrodynamic added mass in BModes, so, you, must pick a frequency at which to characterize the added mass. Typically, the zero or infinitefrequency limit of added mass is chosen, but you could also choose the added mass closest to the frequency of most interest to your structural response.
I'm not sure I understand your second question. Please clarify.
Best regards,
You cannot define a frequencydependent hydrodynamic added mass in BModes, so, you, must pick a frequency at which to characterize the added mass. Typically, the zero or infinitefrequency limit of added mass is chosen, but you could also choose the added mass closest to the frequency of most interest to your structural response.
I'm not sure I understand your second question. Please clarify.
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
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
Re: BModes : Input parameters about tower support subsystem
Dear Jason,
My second question means:
The following lines colored in red in the input file for OC3Hywind (http://wind.nrel.gov/public/jjonkman/BM ... Hywind.bmi), i think they are referring to the same distance  from the MSL to Tower base, but the direction is opposite. That means the "tower rigidbase height" should be 10, and the "draft" should be 10.
But in the input file, the "tower rigidbase height" value is 0. So, I’m a little confused, am I misunderstanding the meaning of the two parameters?
Best regards,
Lei
====================== BModes v3.00 Main Input File ==================
NREL 5MW Tower
 General parameters 
true Echo Echo input file contents to *.echo file if true.
2 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 ()
87.6 radius: rotor tip radius measured along coned blade axis, OR tower height above ground level [onshore] or MSL [offshore](m)
0.0 hub_rad: hub radius measured along coned blade axis OR tower rigidbase height (m)
0. precone: builtin precone angle, automatically set to zero for a tower (deg)
0. bl_thp: blade pitch setting, automatically set to zero for a tower (deg)
2 hub_conn: hubtoblade or towerbase boundary condition [1: cantilevered; 2: freefree; 3: only axial and torsion constraints] ()
20 modepr: number of modes to be printed ()
t TabDelim (true: tabdelimited output tables; false: spacedelimited tables)
f mid_node_tw (true: output twist at midnode of elements; false: no midnode outputs)
 Bladetip or towertop mass properties 
3.500003109E+005 tip_mass bladetip or towertop mass (kg)
0.4137754432 cm_loc tipmass c.m. offset from the tower axis measured along xtower axis (m)
1.9669893542 cm_axial tipmass c.m. offset tower tip measures axially along the z axis (m)
4.370E7 ixx_tip blade lag mass moment of inertia about the tipsection x reference axis (kgm^2)
2.353E7 iyy_tip blade flap mass moment of inertia about the tipsection y reference axis (kgm^2)
2.542E7 izz_tip torsion mass moment of inertia about the tipsection z reference axis (kgm^2)
0. ixy_tip cross product of inertia about x and y reference axes(kgm^2)
1.169E6 izx_tip cross product of inertia about z and x reference axes(kgm^2)
0. iyz_tip cross product of inertia about y and z reference axes(kgm^2)
 Distributedproperty identifiers 
1 id_mat: material_type [1: isotropic; nonisotropic composites option not yet available]
'OC3Hywind_tower_secs.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 fa inertia multiplier ()
1.0 lag_iner_mult: blade lag or tower ss inertia multiplier ()
1.0 flp_stff_mult: blade flap or tower fa bending stiffness multiplier ()
1.0 edge_stff_mult: blade lag or tower ss 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 
50 nselt: no of blade or tower elements ()
Distance of element boundary nodes from blade or flexibletower root (normalized wrt blade or tower length), el_loc()
0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400 0.1600 0.1800 0.2000 0.2200 0.2400 0.2600 0.2800 0.3000 0.3200 0.3400 0.3600 0.3800 0.4000 0.4200 0.4400 0.4600 0.4800 0.5000 0.5200 0.5400 0.5600 0.5800 0.6000 0.6200 0.6400 0.6600 0.6800 0.7000 0.7200 0.7400 0.7600 0.7800 0.8000 0.8200 0.8400 0.8600 0.8800 0.9000 0.9200 0.9400 0.9600 0.9800 1.0000
 Properties of tower support subsystem (read only if beam_type is 2) 
1 tow_support: : aditional tower support [0: no additional support; 1: floatingplatform or monopile with or without tension wires] ()
10.0 draft : depth of tower base from the ground or the MSL (mean sea level) (m)
89.9155 cm_pform : distance of platform c.m. below the MSL (m)
7466.33E3 mass_pform : platform mass (kg)
Platform mass inertia 3X3 matrix (i_matrix_pform):
4229.23E6 0. 0.
0. 4229.23E6 0.
0. 0. 164.23E6
...
My second question means:
The following lines colored in red in the input file for OC3Hywind (http://wind.nrel.gov/public/jjonkman/BM ... Hywind.bmi), i think they are referring to the same distance  from the MSL to Tower base, but the direction is opposite. That means the "tower rigidbase height" should be 10, and the "draft" should be 10.
But in the input file, the "tower rigidbase height" value is 0. So, I’m a little confused, am I misunderstanding the meaning of the two parameters?
Best regards,
Lei
====================== BModes v3.00 Main Input File ==================
NREL 5MW Tower
 General parameters 
true Echo Echo input file contents to *.echo file if true.
2 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 ()
87.6 radius: rotor tip radius measured along coned blade axis, OR tower height above ground level [onshore] or MSL [offshore](m)
0.0 hub_rad: hub radius measured along coned blade axis OR tower rigidbase height (m)
0. precone: builtin precone angle, automatically set to zero for a tower (deg)
0. bl_thp: blade pitch setting, automatically set to zero for a tower (deg)
2 hub_conn: hubtoblade or towerbase boundary condition [1: cantilevered; 2: freefree; 3: only axial and torsion constraints] ()
20 modepr: number of modes to be printed ()
t TabDelim (true: tabdelimited output tables; false: spacedelimited tables)
f mid_node_tw (true: output twist at midnode of elements; false: no midnode outputs)
 Bladetip or towertop mass properties 
3.500003109E+005 tip_mass bladetip or towertop mass (kg)
0.4137754432 cm_loc tipmass c.m. offset from the tower axis measured along xtower axis (m)
1.9669893542 cm_axial tipmass c.m. offset tower tip measures axially along the z axis (m)
4.370E7 ixx_tip blade lag mass moment of inertia about the tipsection x reference axis (kgm^2)
2.353E7 iyy_tip blade flap mass moment of inertia about the tipsection y reference axis (kgm^2)
2.542E7 izz_tip torsion mass moment of inertia about the tipsection z reference axis (kgm^2)
0. ixy_tip cross product of inertia about x and y reference axes(kgm^2)
1.169E6 izx_tip cross product of inertia about z and x reference axes(kgm^2)
0. iyz_tip cross product of inertia about y and z reference axes(kgm^2)
 Distributedproperty identifiers 
1 id_mat: material_type [1: isotropic; nonisotropic composites option not yet available]
'OC3Hywind_tower_secs.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 fa inertia multiplier ()
1.0 lag_iner_mult: blade lag or tower ss inertia multiplier ()
1.0 flp_stff_mult: blade flap or tower fa bending stiffness multiplier ()
1.0 edge_stff_mult: blade lag or tower ss 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 
50 nselt: no of blade or tower elements ()
Distance of element boundary nodes from blade or flexibletower root (normalized wrt blade or tower length), el_loc()
0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400 0.1600 0.1800 0.2000 0.2200 0.2400 0.2600 0.2800 0.3000 0.3200 0.3400 0.3600 0.3800 0.4000 0.4200 0.4400 0.4600 0.4800 0.5000 0.5200 0.5400 0.5600 0.5800 0.6000 0.6200 0.6400 0.6600 0.6800 0.7000 0.7200 0.7400 0.7600 0.7800 0.8000 0.8200 0.8400 0.8600 0.8800 0.9000 0.9200 0.9400 0.9600 0.9800 1.0000
 Properties of tower support subsystem (read only if beam_type is 2) 
1 tow_support: : aditional tower support [0: no additional support; 1: floatingplatform or monopile with or without tension wires] ()
10.0 draft : depth of tower base from the ground or the MSL (mean sea level) (m)
89.9155 cm_pform : distance of platform c.m. below the MSL (m)
7466.33E3 mass_pform : platform mass (kg)
Platform mass inertia 3X3 matrix (i_matrix_pform):
4229.23E6 0. 0.
0. 4229.23E6 0.
0. 0. 164.23E6
...

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 Location: Boulder, CO
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Re: BModes : Input parameters about tower support subsystem
Dear Lei,
"hub_rad" and "draft" are similar, but not identical (although they do have the opposite sign as you noticed). "hub_rad" in BModes is the distance from the top of the platform along the tower to the beginning of the flexible part of the tower. "draft" is the distance from the MSL to the top of the platform, positive downwards. So, the base of the flexible tower is located a distance of "hub_rad  draft" above the MSL. The tower top is located a distance of "radius" above the MSL. Thus, the flexible length of the tower equals "radius  hub_rad + draft".
In the OC3Hywind system, the rigid platform extends 10m above the MSL and the tower is modeled flexibly along the entire tower  from 10m to 87.6m above the MSL. So, draft = 10 m, hub_rad = 0 m, radius = 87.6 m and flexible length of the tower equals 77.6 m.
I hope that helps.
Best regards,
"hub_rad" and "draft" are similar, but not identical (although they do have the opposite sign as you noticed). "hub_rad" in BModes is the distance from the top of the platform along the tower to the beginning of the flexible part of the tower. "draft" is the distance from the MSL to the top of the platform, positive downwards. So, the base of the flexible tower is located a distance of "hub_rad  draft" above the MSL. The tower top is located a distance of "radius" above the MSL. Thus, the flexible length of the tower equals "radius  hub_rad + draft".
In the OC3Hywind system, the rigid platform extends 10m above the MSL and the tower is modeled flexibly along the entire tower  from 10m to 87.6m above the MSL. So, draft = 10 m, hub_rad = 0 m, radius = 87.6 m and flexible length of the tower equals 77.6 m.
I hope that helps.
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
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
Re: BModes : Input parameters about tower support subsystem
Dear Dr. Jonkman,
Thank you for your help.
In order to confirm if i'm using BModes right, i do a calculation of Tower Mode of OC3Hywind with the input file and Code in http://wind.nrel.gov/public/jjonkman/BModes/. After that, a polynominal fitting is performed using the Excel table(ModeShapePolyFitting.xls) in FAST archive.
Then, i compared the results with the tower input file "NRELOffshrBsline5MW_Tower_OC3Hywind.dat". The first order mode shape agree with each other well, but the second order mode shape have remarkable difference. The coefficient and figure is shown below.
Could you give me some hints where I do wrong?
the second order ss mode shape:
! ~~~~~~~~ FAST_Tower_Input # Bmodes calcultion
coefficient of x^2 term 60.2285 8.396705988
coefficient of x^3 term 27.5868 1.440145029
coefficient of x^4 term 30.3887 5.071404927
coefficient of x^5 term 33.6738 0.201117338
coefficient of x^6 term 32.4208 1.08627337
Thank you for your help.
In order to confirm if i'm using BModes right, i do a calculation of Tower Mode of OC3Hywind with the input file and Code in http://wind.nrel.gov/public/jjonkman/BModes/. After that, a polynominal fitting is performed using the Excel table(ModeShapePolyFitting.xls) in FAST archive.
Then, i compared the results with the tower input file "NRELOffshrBsline5MW_Tower_OC3Hywind.dat". The first order mode shape agree with each other well, but the second order mode shape have remarkable difference. The coefficient and figure is shown below.
Could you give me some hints where I do wrong?
the second order ss mode shape:
! ~~~~~~~~ FAST_Tower_Input # Bmodes calcultion
coefficient of x^2 term 60.2285 8.396705988
coefficient of x^3 term 27.5868 1.440145029
coefficient of x^4 term 30.3887 5.071404927
coefficient of x^5 term 33.6738 0.201117338
coefficient of x^6 term 32.4208 1.08627337

 Posts: 5164
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: BModes : Input parameters about tower support subsystem
Dear Lei,
The approach you describe to derive mode shapes for FAST from BModes sounds correct.
I'm sure that the reason for the differences you are seeing between the BModesgenerated mode shapes and the mode shapes derived by NREL for the OC3Hywind tower contained in "NRELOffshrBsline5MW_Tower_OC3Hywind.dat" is because NREL did not use BModes to derive the tower mode shapes for the OC3Hywind tower contained in "NRELOffshrBsline5MW_Tower_OC3Hywind.dat". When NREL derived the tower mode shapes for the OC3Hywind tower, the ability to derive tower mode shapes for systems with floating platforms was not available in BModes. Instead of BModes, we used the linearization functionality of a fullsystem ADAMS model to obtain the tower modes for the NREL 5MW turbine models (both landbased and floating). That is, we built an ADAMS model of the wind turbine (using the FASTtoADAMS preprocessor), enabled all system DOFs, and linearized the model. Then we passed a bestfit polynomial through the resulting tower mode shapes to get the equivalent polynomial representations of the tower mode shapes needed by FAST. For the tower modes, there is not only an influence from the floating platform DOFs on the tower mode shapes, but there can also be a great deal of coupling with the drivetrain and rotor DOFs. While BModes can now account for the floating platform DOFs, it still treats the drivetrain and rotor rigidly, so, the BModes and ADAMSgenerated mode shapes will differ.
Best regards,
The approach you describe to derive mode shapes for FAST from BModes sounds correct.
I'm sure that the reason for the differences you are seeing between the BModesgenerated mode shapes and the mode shapes derived by NREL for the OC3Hywind tower contained in "NRELOffshrBsline5MW_Tower_OC3Hywind.dat" is because NREL did not use BModes to derive the tower mode shapes for the OC3Hywind tower contained in "NRELOffshrBsline5MW_Tower_OC3Hywind.dat". When NREL derived the tower mode shapes for the OC3Hywind tower, the ability to derive tower mode shapes for systems with floating platforms was not available in BModes. Instead of BModes, we used the linearization functionality of a fullsystem ADAMS model to obtain the tower modes for the NREL 5MW turbine models (both landbased and floating). That is, we built an ADAMS model of the wind turbine (using the FASTtoADAMS preprocessor), enabled all system DOFs, and linearized the model. Then we passed a bestfit polynomial through the resulting tower mode shapes to get the equivalent polynomial representations of the tower mode shapes needed by FAST. For the tower modes, there is not only an influence from the floating platform DOFs on the tower mode shapes, but there can also be a great deal of coupling with the drivetrain and rotor DOFs. While BModes can now account for the floating platform DOFs, it still treats the drivetrain and rotor rigidly, so, the BModes and ADAMSgenerated mode shapes will differ.
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
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
Re: BModes : Input parameters about tower support subsystem
Dear Jason,
Thank you for your patience, I really appreciate your kind help.
Best regards,
Lei
Thank you for your patience, I really appreciate your kind help.
Best regards,
Lei

 Posts: 11
 Joined: Sun May 29, 2016 5:42 pm
 Organization: The university of Auckland
 Location: New Zealand
Re: BModes : Input parameters about tower support subsystem
Hi Jason,
I am using BModes for the purpose of gaining the coefficients of the polynomial equation used to model the foreaft and sidetoside mode shape of the tower in FAST.
I have 2 questions for modelling the tower in FAST and BModes.
Firstly, in BModes, I am required to place the value of towertip mass c.m. I am wondering that the tower is usually designed symmetrically, so its centre of mass will be on the tower axis, and therefore, tipmass c.m in the table should be 0. However, it is a nonzero value in the example file as well as in the descriptive figure. Can you explain me this parameter?
Secondly, the model I am using is put in a small lab. So, the tower I define here is a cylinder containing 2 ends. One end is located on the surface of nacelle, while another one is emerged out of the surface, instead of touching to the ground. Now, I wonder that as I model this tower in FAST and BModes, the results I acquire are accurate enough, isn't it? Should I model it as a tower?
Thank you very much.
I am looking forward to hearing from you.
Kind regards,
Ngoc Ha Tran.
I am using BModes for the purpose of gaining the coefficients of the polynomial equation used to model the foreaft and sidetoside mode shape of the tower in FAST.
I have 2 questions for modelling the tower in FAST and BModes.
Firstly, in BModes, I am required to place the value of towertip mass c.m. I am wondering that the tower is usually designed symmetrically, so its centre of mass will be on the tower axis, and therefore, tipmass c.m in the table should be 0. However, it is a nonzero value in the example file as well as in the descriptive figure. Can you explain me this parameter?
Secondly, the model I am using is put in a small lab. So, the tower I define here is a cylinder containing 2 ends. One end is located on the surface of nacelle, while another one is emerged out of the surface, instead of touching to the ground. Now, I wonder that as I model this tower in FAST and BModes, the results I acquire are accurate enough, isn't it? Should I model it as a tower?
Thank you very much.
I am looking forward to hearing from you.
Kind regards,
Ngoc Ha Tran.

 Posts: 5164
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: BModes : Input parameters about tower support subsystem
Dear Ngoc Ha Tran,
The towertop mass, center of mass, and inertias in BModes refer to the lumpedmass representation of the rotornacelle assembly (RNA). Normally, the mass and inertia of the RNA are large enough to have a sizable affect on the natural frequencies and mode shapes of the tower.
I'm not sure I understand your second question.
Best regards,
The towertop mass, center of mass, and inertias in BModes refer to the lumpedmass representation of the rotornacelle assembly (RNA). Normally, the mass and inertia of the RNA are large enough to have a sizable affect on the natural frequencies and mode shapes of the tower.
I'm not sure I understand your second question.
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
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

 Posts: 11
 Joined: Sun May 29, 2016 5:42 pm
 Organization: The university of Auckland
 Location: New Zealand
Re: BModes : Input parameters about tower support subsystem
Hi Jason,
I solved that problem due to your answer. Thank you very much.
Now, one more thing is as I run BModes with your example case "Test01_nonunif_blade.bmi", I always encounter the following error:
"Running BModes (v1.03.01, 25Sept2007, compiled using double precision).
Linked with the NWTC Subroutine Library (v1.01.08, 26Sept2007).
The input file, "blade_sec_props.dat", was not found.
Aborting Bmodes."
So, can you guide me how to solve this problem to run Bmodes successfully?
Thank you very much.
Kind regards,
Ngoc Ha Tran.
I solved that problem due to your answer. Thank you very much.
Now, one more thing is as I run BModes with your example case "Test01_nonunif_blade.bmi", I always encounter the following error:
"Running BModes (v1.03.01, 25Sept2007, compiled using double precision).
Linked with the NWTC Subroutine Library (v1.01.08, 26Sept2007).
The input file, "blade_sec_props.dat", was not found.
Aborting Bmodes."
So, can you guide me how to solve this problem to run Bmodes successfully?
Thank you very much.
Kind regards,
Ngoc Ha Tran.

 Posts: 5164
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: BModes : Input parameters about tower support subsystem
Dear Ngoc Ha Tran,
Do you have such a file?
Best regards,
Do you have such a file?
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
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

 Posts: 11
 Joined: Sun May 29, 2016 5:42 pm
 Organization: The university of Auckland
 Location: New Zealand
Re: BModes : Input parameters about tower support subsystem
Dear Jason,
Now, I can know how to run Bmodes.
However, in the blade section property file, I need to supply the geometrical properties of the blade. I can obtain most parameters, except "shear center".
I am trying to use PreComp to obtain this parameter. However, the blade I use to experiment in the lab is solid. It means that it doesn't have any webs and laminates. But in PreComp, I need to complete an auxiliary input file to describe the internal shape of the blade. I try to put "unused" under the column requiring the name of this auxiliary file in the main input file. However, immediately, the software can not run.
What should I do in my case?
Thank you very much.
Kind regards,
Ngoc Ha Tran.
Now, I can know how to run Bmodes.
However, in the blade section property file, I need to supply the geometrical properties of the blade. I can obtain most parameters, except "shear center".
I am trying to use PreComp to obtain this parameter. However, the blade I use to experiment in the lab is solid. It means that it doesn't have any webs and laminates. But in PreComp, I need to complete an auxiliary input file to describe the internal shape of the blade. I try to put "unused" under the column requiring the name of this auxiliary file in the main input file. However, immediately, the software can not run.
What should I do in my case?
Thank you very much.
Kind regards,
Ngoc Ha Tran.
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