Conference Paper

NREL/CP-500-47953

April 2010

Verification of BModes: 

Rotary Beam and Tower Modal 

Analysis Code

Preprint

Gunjit Bir

To be presented at 51st AIAA/ASME/ASCE/AHS/ASC Structures, 

Structural Dynamics, and Materials Conference

Orlando, Florida

April 12-15, 2010
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1

Verification of BModes: Rotary Beam and Tower Modal 

Analysis Code

Gunjit Bir

National Renewable Energy Laboratory, Golden, Colorado

Abstract

This paper describes verification of BModes, a finite-element code developed to provide coupled 

modes for the blades and tower of a wind turbine. The blades, which may be rotating or nonrotating, and the towers, whether onshore or offshore, are modeled using specialized 15-dof 

beam finite elements. Both blade and tower models allow a tip attachment, which is assumed to 

be rigid body with six moments of inertia, and a mass centroid that may be offset from the blade 

or tower axis. Examples of tip attachments are aerodynamic brakes for blades and nacelle-rotor 

subassembly for towers. BModes modeling allows for tower supports including tension wires, 

floating platforms, and monopiles on elastic foundations. Coupled modes (implying coupling of 

flap, lag, axial, and torsional motions) are required for modeling major flexible components in a 

modal-based, aeroelastic code such as FAST1

. These are also required for validation of turbine

models using experimental data, modal-based fatigue analysis, controls design, and 

understanding aeroelastic-stability behavior of turbines. Verification studies began with uniform 

tower models, with and without tip inertia, and progressed to realistic towers. For the floating 

turbine, we accounted for the effects of hydrodynamic inertia, hydrostatic restoring, and mooring 

lines stiffness. For the monopole-supported tower, we accounted for distributed hydrodynamic 

mass on the submerged part of the tower and for distributed foundation stiffness. Finally, we 

verified a model of a blade carrying tip mass and rotating at different speeds (verifications of 

other blade models, rotating or non-rotating, have been reported in another paper2). Verifications 

were performed by comparing BModes-generated modes with analytical results, if available, or

with MSC.ADAMS® results. All results in general show excellent agreement. 

1 Introduction

BModes is a finite-element code developed for high-fidelity modal analysis of a wind turbine’s

blade or tower, whether offshore or onshore. Both the blade, rotating or non-rotating, and the 

tower may have an arbitrary distribution of structural properties. Both blade and tower models 

allow a tip attachment, which is assumed to be rigid body with six moments of inertia, and a 

mass centroid that may be offset from the blade or tower axis. Examples of tip attachments are 

aerodynamic brakes for blades and nacelle-rotor subassemblies for towers. In addition, the blade 

may have a precone and an arbitrary pitch-control setting. For the tower, BModes allows five 

configurations: a land-based tower, an offshore monopile-supported tower, an offshore floatingbarge-supported tower, an offshore tension-leg-supported tower, and an offshore spar-buoysupported tower. Figure 1 shows the three floating turbine configurations. Optionally, the landbased tower may have tension support wires. 

The modes of a blade or a tower are generally coupled, which implies the presence of flexural, 

axial, and torsional motions in a natural mode of vibration. For the blade, flexural motion means 
2

flap and lag bending, whereas for the tower it means fore-aft and side-to-side bending. Coupled 

modes are crucial to several applications:

• modeling of major flexible components for modal-based aeroelastic codes

• dynamic characterization

• validation of turbine models using experimental data

• modal-based fatigue analysis

• in-depth interpretation of turbine aeroelastic stability. 

The cause of coupling can be geometric, dynamic, or material anisotropy.3 Obtaining rotatingblade coupled modes involves solving a complex boundary-value problem, and only a few codes 

rigorously handle this. BModes handles it accurately because it uses a finite-element approach in 

conjunction with analytical linearization and a unique finite-element assembly that properly 

captures Coriolis and centrifugal effects. The finite-element method (FEM) is based on a 15-dof 

element with three internal and two boundary nodes. A salient feature of BModes is its potential 

to handle a complex range of boundary conditions.

The structural properties along the blade length are specified in terms of the flap bending, lag 

bending, torsion, axial stiffness, mass, section moments of inertia, and chordwise offsets of 

section shear center, tension center, and center-of-mass. These properties may be obtained from

experimental tests, the blade manufacturer, or a properties-extractor code, such as VABS4

, 

PreComp5 or BPE.

6 Currently, BModes allows only metallic or isotropic composite blades. With 

the aforementioned inputs, the code uses the FEM followed by an eigenanalysis to compute 

mode shapes and frequencies.

Figure 1: Floating turbine concepts
3

Since its initial release, BModes3 has undergone several upgrades specifically related to offshore 

modeling. Examples of the upgrades include: 

• extension of the FEM model to include distributed hydrodynamic added mass and elastic foundation stiffness

• inclusion of floating-platform and monopile-support DOF

• inclusion of gravity effects. 

In addition, a rigid-body finite element was introduced to model floating platforms subject to 

added hydrodynamic inertia, hydrostatic stiffness (buoyancy matrix), and mooring-line stiffness.

The influence of gravity on the modes of an offshore turbine can be significant.

This paper verifies BModes modal analysis capability for towers and blades with tip inertias 

(verification of other blade models has been covered in another paper). Section II provides a 

brief description of BModes and its modeling and analysis approach. Section III presents 

verification of its tower modal analysis capability. We compare results from BModes with those 

from MSC.ADAMS®. Section IV presents verification results for a spinning blade carrying a tip 

mass. The paper concludes with a summary of the results and an outline of future plans. 

2 A Brief Description of BModes

BModes computes coupled modes for blades and towers using a finite-element approach. Both 

the blade and the tower are modeled as Euler-Bernoulli beams, which can have an arbitrary 

distribution of geometry and structural properties along their length. The properties are specified 

in terms of stiffness (flexural, torsional, and axial), section mass, section moments of inertia, and 

chordwise offsets of section shear center, tension center, and center-of-mass variation along the 

blade. The stiffness properties are defined with respect to principal axes centered at the shear 

center. The inertia properties are defined with respect to the principal inertia axes centered at the 

mass centroid. The orientation of each set of principal axes and the built-in blade twist may also 

vary along the blade. Rotary inertia and certain nonlinear stiffness effects are accounted for in 

the code. Currently, the code allows only metallic or isotropic composite blades. Both the blade 

and the tower may have end attachments. An end attachment is modeled as a rigid body with 

mass, six moments of inertia, and a mass centroid that may be offset from the blade or tower 

axis.

As mentioned earlier, BModes has recently been upgraded to handle offshore turbines. The 

tower may be supported on a sea-floating platform or on a monopile embedded in the seabed. 

The floating platform may be subjected to hydrodynamic-added mass, hydrostatic restoring, and 

mooring line stiffness. The added mass and stiffness are generally distinct from physical inertias 

and springs and are expressed as 6X6 matrices associated with the six DOF (three translational 

and three rotational). For a monopile support, the soil effect is modeled as a distributed stiffness 

along the soil-buried part of the tower. Also, hydrodynamic-added mass may be specified along 

the submerged part of the tower, i.e., the part of the tower between the mean sea level (MSL) and 

the mudline.

Accurately obtaining rotating-blade or tower coupled modes is a challenging boundary-value 

problem, and only a few codes handle it rigorously. BModes modeling is high fidelity, using a 

finite-element approach in conjunction with analytical linearization and a special finite-element 

assembly that accurately captures Coriolis and centrifugal effects. Its finite-element approach, 
4

built on UMARC7, uses a 15- DOF element with three internal and two boundary nodes. The 15 

DOFs comprise 3 DOFs for torsion deflection and 4 DOFs each for axial, flap, and lag 

deflections (Figure 2). In the figure, u, v, w, and φ represent the axial, tower fore-aft, tower sideside, and twist DOF, respectively.

Figure 2. The 15-DOF finite element

The new finite element accounts for distributed stiffness and added mass, which may arise for 

platform- and monopile-supported towers. The distributed hydrodynamic added mass is effective 

only in the lateral directions (v, w) and not in the torsional and axial directions. The distributed 

stiffness, like the distributed added mass, may vary along the element length and it also may be 

different in the two lateral directions (normal to the element axis). The blade tip inertia, the tower 

head (nacelle-rotor subsystem) and the tower base attachment (e.g., floating barge) are modeled 

using rigid-body elements.

A full discussion of the BModes theory basis is outside the scope of this paper. However, Figure 

3 outlines the technical approach. First, we idealize the blade or the tower as an Euler-Bernoulli 

beam that undergoes bending, elastic twist, and axial deflections. Next, we formulate energy 

expressions and use Hamilton’s principle to derive coupled integro-partial differential equations 

(PDEs) governing the dynamics of the elastic beam attached to rigid end inertias.

Distributed stiffness

Distributed hydrodynamic mass
5

Figure 3. Technical approach: computation of tower coupled modes

These PDEs are nonlinear and include spatial integral terms, which represent Coriolis and 

centrifugal effects. We use finite element analysis (FEA) to discretize the spatial variables in the 

PDEs. This FEA is compatible with Hamilton’s principle. The beam is divided into 15-DOF 

elements. Continuity of displacement and slope for bending deflections and continuity of 

displacement for elastic twist and axial deflections are maintained between elements. The elastic 

twist is represented by a quasi-coordinate (non-Euler); this eliminates axial integral terms that 

otherwise would appear in orientation matrices for an elastic beam. Finite elements assembly, 

specialized for integro-partial differential equations, followed by application of appropriate 

boundary conditions, yields nonlinear ordinary differential equations (ODEs) in nodal 

coordinates. Analytical linearization (distinct from finite-differencing) yields inertia and stiffness 

matrices. Finally, we perform an eigenanalysis to obtain coupled modes and frequencies.

3 Verification of Tower Modal Analysis Capability

Because accurate modes play a crucial role in several applications, we performed extensive 

verification studies on BModes—those related to blades have already been reported2

, though in 

part only. This paper presents the remaining result for the blades. We also present verification 

results for the tower, which is the focus of this section. As mentioned in the previous section, 

tower modal analysis capability necessitated several extensions to BModes. The geometric and 

structural properties of the tower configurations used in the verification studies are available in 

Ref. 11. The tower head (rotor-nacelle subsystem) is the same for all configurations and its 

properties are also listed in the same reference.

We first used BModes to build simple tower models (e.g. uniform and tapered beams with end 

masses) and compared the modal results with those from analytical models. Excellent agreement 

was obtained for all models (results are not shown here to keep the paper length short, but will be 

Beam Structural

Idealization

Nonlinear

Coupled PDEs

Spatial Discretization

using FEA

ODEs in Time

Analytical

Linearization

Coupled Mode Shapes

& Frequencies

Eigenanalysis

Hamilton’s Principle

Beam Structural

Idealization

Nonlinear

Coupled PDEs

Spatial Discretization

using FEA

ODEs in Time

Analytical

Linearization

Coupled Mode Shapes

& Frequencies

Eigenanalysis

Hamilton’s Principle
6

included later in a publically available report). For a more realistic verification, we built several 

tower models using BModes and MSC.ADAMS®. Sample verification results are presented

below.

3.1 Land-Based Tower (with and without rotor-nacelle head mass)

We modeled the land-based tower as a beam cantilevered to the ground and computed its natural 

frequencies and mode shapes. Table 1 lists the modal frequencies of the tower (with and without 

the rotor-nacelle head mass) and also compares these with frequencies obtained from the 

ADAMS models. We used 50 finite elements in BModes and 99 tower parts in ADAMS. The 

head mass was idealized as a rigid body in both BModes and ADAMS. 

The modal frequencies are shown to four decimal points to aid verification. All frequencies, 

except very high ones, show good agreement. Also, for the tower with no head mass, note that 

BModes predicts i

th F-A and ith S-A modal frequencies to be identical, as should be the case. 

Similar results from ADAMS differ, though by a very small margin. We believe BModescomputed results, particularly for higher modes, are more accurate because it uses a consistentmass approach, whereas ADAMS effectively uses a lumped mass approach. Note that the head 

mass substantially lowers all the modal frequencies. As mentioned earlier, the head mass (rotornacelle subsystem) is considered rigid. Flexibility of the rotor would alter the system frequencies

and the mode shapes somewhat; the torsion frequencies, in particular, would be higher.

Figures 4 and 5 show the BModes-computed mode shapes for the land-based tower without and 

with the head mass (such modes are quantitatively difficult to extract from ADAMS). All mode 

shapes presented in this paper are mass-normalized. We do not normalize modes for unity tip 

deflection, as is typically done. This will be explained later. Also, the tower section height has 

been normalized with respect to H, the height of the flexible tower, which is 87.6 m. Thus, zero

represents the flexible tower base and one represents the tower top. Without the head mass 

(Figure 4), the tower fore-aft (F-A) and side-to-side (S-S) modes are identical as expected of an 

axi-symmetric tower. The presence of the tower head mass introduces coupling between the sideto-side and torsion motions (Figure 5b). The fore-aft modes, however, remain uncoupled (Figure 

5a). Also note that the head mass substantially alters the mode shapes. The rotary inertia of the 

head mass results in 2nd fore-aft and 2nd side-to-side mode shapes with near-zero deflections at 

the tower tip. An attempt to normalize such modes for unit tip deflections may lead to errorprone mode shapes. In fact, if the tip deflection were exactly zero (possible in reality), it would 

be impossible to normalize modes for unity deflections. That is why BModes uses massnormalization.

Table 1. Modal Frequencies of Land-Based Tower with and without Head Mass.

Mode

Number

Mode

Type

Without Head Mass With Head Mass

Frequency (Hz) Frequency(Hz)

BModes ADAMS Diff BModes ADAMS Diff

1 1st SS 0.8913 0.8904 0.001 0.3291 0.3188 0.010

2 1st FA 0.8913 0.8904 0.001 0.3324 0.3218 0.011

3 2ndSS 4.3743 4.3437 0.031 1.8805 1.8820 0.002

4 2nd FA 4.3743 4.3435 0.031 2.2432 2.2391 0.004

5 3rd SS 11.3911 11.1856 0.205 4.6526 4.7244 0.072

6 3rd FA 11.3911 11.1843 0.207 4.9865 5.1833 0.197

7 1st Torsion 11.9656 11.4448 0.521 1.4703 1.4763 0.006

8 1st Axial 16.5217 16.5222 0.001 8.1311 7.9375 0.194
7

9 4th SS 21.8655 21.1146 0.751 11.3142 11.2678 0.046

10 4th FA 21.8655 21.1093 0.756 11.4591 11.4719 0.013

11 2nd Torsion 27.7783 26.1221 1.656 17.9632 17.9535 0.010

12 5th SS 35.8273 33.8392 1.988 21.7054 21.3291 0.376

13 5th FA 35.8273 33.8236 2.004 21.7625 21.4419 0.321

14 2nd Axial 43.4596 42.1715 1.288 30.2109 30.1182 0.093

15 3rd Torsion 44.8623 43.4578 1.405 35.3975 34.5078 0.890

16 6th SS 53.2770 48.9445 4.332 35.6336 34.5830 1.051

17 6th FA 53.2770 48.9071 4.370 35.6636 35.3740 0.290

18 4th Torsion 62.2312 58.6564 3.575 52.9449 50.5171 2.428

19 3rd Axial 71.4741 65.8776 5.596 53.0811 52.8981 0.183

20 7th SS 74.2155 65.9541 8.261 53.0673 50.5720 2.495

Figure 4. Modes of land-based turbine tower without head mass

Figure 5a. Fore-aft modes of land-based tower with head mass

0

0.2

0.4

0.6

0.8

1

-0.04 0 0.04 0.08 0.12

Modal displacement

Tower section height / h

1st S-S and 1st F-A

2nd S-S and 2nd F-A

3rd S-S and 3rd F-A

1st Twist

h=87.6 m

Undeformed tower 

position

0

0.2

0.4

0.6

0.8

1

-0.06 -0.02 0.02 0.06 0.1

Modal displacement

Tower section height / h

1st F-A

 2nd F-A

3rd F-A

h=87.6 m
8

Figure 5b. Coupled side-to-side and twist modes for the land-based tower

3.2 Floating-Barge-Supported Tower

Using BModes, we model the floating-barge-supported turbine as a beam with all the six DOF 

(three translational and three rotational) unconstrained at both ends. The tower-top end of the 

beam carries head mass and the tower-base end carries the platform modeled as a rigid body. The 

barge is a rectangular box and is attached to eight mooring lines (two at each end). In the static 

equilibrium position, the top surface of the platform (barge) coincides with the mean sea surface. 

To account for the hydrodynamic and hydrostatic effects on the platform, 6x6 inertia (added 

mass) and 6x6 restoring (stiffness) matrices, obtained from WAMIT8

, are input to BModes. The 

added mass values were chosen to be the values in the added mass matrix at the infinitefrequency limit of the frequency-dependent solution to the wave-radiation problem. A 6x6 

matrix accounting for the mooring system stiffness is also included within BModes. This matrix 

was found by numerically linearizing the nonlinear mooring system module of FAST.

1 Because 

this particular configuration has no sea-submerged tower part, no distributed added mass 

(hydrodynamic) effect was included. Table 2 lists modal frequencies for the three platform 

models: platform inertia only (platform physical inertia without hydrodynamic added mass or 

stiffness), platform with all effects except the added mass, and the full platform model.

In Table 2, the surge mode is the fore-aft motion of the platform, sway is the side-to-side motion, 

heave is the up-down motion, yaw is the rotational motion about the tower axis, roll is the 

rotational motion about the platform longitudinal axis, and pitch is the rotational motion about 

the platform lateral axis. In the absence of any added mass and spring forces on the platform 

(platform-inertia-only model), the turbine behaves as a free-free flexible beam with two end 

inertias. Consequently, six zero-frequency, rigid-body modes were observed. Note that the added 

0

0.2

0.4

0.6

0.8

1

-0.1 0 0.1 0.2 0.3

Modal displacement

Tower section height / h

1st S-S

2nd S-S

3rd S-S

1st S-S (twist part)

2nd S-S (twist part)

3rd S-S (twist part)

1st Twist

1st Twist (S-S part)
9

hydrodynamic mass lowers all frequencies somewhat. The heave mode is substantially lower

(from 0.2576 Hz to 0.1283 Hz). The last column in the table lists ADAMS-computed frequencies 

for the full platform model. All frequencies agree well, except the heave frequency (identified in 

blue in the table). This result is because the ADAMS linearized model admits only a physical 

mass and inertia, whereas BModes permits the use of both physical and added masses. To 

account for the additional (hydrodynamically added) mass and inertia in the ADAMS model, we 

augmented the physical mass and inertia and altered the c.m. location of the barge. This could 

not be done perfectly because the actual hydrodynamic-added mass of the barge in heave is quite 

different than the added mass in surge and sway (unlike the physical mass, which is identical in 

all directions). Augmentation of the barge body mass properties ensured that the surge, sway, 

pitch, roll, and yaw elements (including off-diagonal elements)—but not the heave element—

accounted for the added mass matrix in the ADAMS model.

Table 2. Modal Frequencies of Floating-Platform-Supported Turbine

Frequency (Hz)

Mode

Number

Mode

Type

BModes ADAMS

Platform 

Inertia 

Only

Platform Inertia + 

Hydrostatic & 

Mooring Stiffness

Full

Platform 

Model

Full 

Platform 

Model

1 Surge 0.0000 0.0081 0.0076 0.0076

2 Sway 0.0000 0.0081 0.0076 0.0076

3 Yaw 0.0000 0.0206 0.0198 0.0198

4 Roll 0.0000 0.1106 0.0978 0.0966

5 Pitch 0.0000 0.1109 0.0980 0.0968

6 Heave 0.0000 0.2576 0.1283 0.2463

7 1st SS 0.7349 0.7671 0.5489 0.5374

8 1st FA 0.7494 0.7820 0.5556 0.5440

9 1st Torsion 1.4836 1.4836 1.4826 1.4890

10 2nd SS 1.9943 1.9962 1.9270 1.9327

11 2nd FA 2.3652 2.3666 2.2942 2.2950

12 3rd SS 4.7559 4.7562 4.7011 4.7742

13 3rd FA 5.0799 5.0801 5.0293 5.2260

14 1st Axial 8.5014 8.5017 8.2186 8.2759

15 4th SS 11.3835 11.3835 11.3542 11.3138

16 4th FA 11.5280 11.5280 11.4983 11.5167

17 2nd Torsion 17.9683 17.9683 17.9679 17.9639

18 5th SS 21.7584 21.7584 21.7406 21.3760

19 5th FA 21.8153 21.8153 21.7975 21.4885

20 2nd Axial 30.4368 30.4368 30.2625 30.3275
10

Figure 6a. Modes of platform-supported turbine

Figures 6a–6c show BModes-computed mode shapes for the full platform-supported turbine 

model. Note that all modes in the longitudinal plane of symmetry of the turbine are uncoupled, 

whereas other modes are coupled. Note also that the low-frequency platform modes (surge, etc) 

are close to rigid-body modes, as expected.

Figure 6b. Modes of platform-supported turbine

0

0.2

0.4

0.6

0.8

1

-0.1 -0.05 0 0.05 0.1 0.15

Modal displacement

Tower section height / h

Surge

Pitch

1st F-A

2nd F-A

3rd F-A

h=87.6 

0

0.2

0.4

0.6

0.8

1

-0.1 -0.05 0 0.05 0.1 0.15

Modal displacement

Tower section height / h

Sway

Roll

1st S-S

1st S-S (twist)

2nd S-S

2nd S-S (twist)

3rd S-S

3rd S-S (twist)
11

Figure 6c. Modes of platform-supported turbine

3.3 Monopile-Supported Tower

Figure 7 shows the monopile-supported tower and its equivalent model. The soil is modeled as a distributed spring 

supporting the tower base. The effect of sea water surrounding the submerged part of the tower is modeled as 

distributed hydrodynamic-added mass.

Figure 7. Monopile foundation and its equivalent model

Table 3 compares BModes- and ADAMS-computed modal frequencies. All frequencies agree 

well, thereby verifying BModes modeling of the elastic foundation. Figure 8 shows the first few

0

0.2

0.4

0.6

0.8

1

-0.05 0 0.05 0.1 0.15 0.2

Modal displacement

Tower section height / h

Yaw

1st Twist

2ndTwist

Mud Line

MSL

87.6 m

20 m

36 m

H =

143.6 m

distributed

added mass

distributed

stiffness
12

modes; the tower section height has been normalized with respect to H, the height of the flexible 

tower, which is 143.6 m.

Table 3. Comparison of BModes- and ADAMS-Predicted Frequencies for the DS Model

Mode

Number

Mode

Type

Frequency (Hz) Difference

BModes ADAMS (Hz)

1 1st SS 0.2513 0.2457 0.01

2 1st FA 0.2530 0.2472 0.01

3 1st Torsion 1.2752 1.2777 0.00

4 2nd SS 1.3680 1.3549 0.01

5 2nd FA 1.5316 1.5056 0.03

6 3rd SS 2.7425 2.7810 0.04

7 3rd FA 3.0874 3.1788 0.09

8 4th SS 5.9778 6.0090 0.03

9 4th FA 6.0506 6.2269 0.18

10 1st Axial 6.7671 6.5515 0.22

11 2nd Torsion 10.3875 10.3448 0.04

12 5th SS 11.4049 11.2947 0.11

13 5th FA 11.4758 11.4022 0.07

14 6th SS 17.9595 17.6518 0.31

15 6th FA 17.9704 17.7208 0.25

16 2nd Axial 18.1887 18.0404 0.15

17 3rd Torsion 21.1075 21.1944 0.09

18 7th SS 26.3980 25.7959 0.60

19 7th FA 26.4215 25.8452 0.58

20 4th Torsion 32.9711 32.8559 0.12

Figure 8. Mode shapes of the monopile-supported turbine

0

0.2

0.4

0.6

0.8

1

-0.08 -0.04 0 0.04 0.08 0.12

Tower section height / H

Modal displacement

1st S-S 

3rd S-S

2nd S-S

Mud 

Mean Sea 

0

0.2

0.4

0.6

0.8

1

-0.08 -0.04 0 0.04 0.08 0.12

Modal displacement

1st F-A 

3rd F-A

2nd F-A

Mean Sea 

Level

Mud 

Line

0

0.2

0.4

0.6

0.8

1

0 0.05 0.1 0.15 0.2 0.25 0.3

Modal displacement

1st 

Twist 

2nd 

Twist

Mud 

Line

Mean Sea 

Level
13

3.4 Spar-Buoy-Supported Tower

Table 4 compares BModes- and ADAMS-computed modal frequencies for the spar-buoysupported turbine. The table also shows how gravity influences the modal frequencies, in 

particular the pitch and roll frequencies. Again, good agreement was observed between the 

BModes and ADAMS results except for a few modal frequencies identified in blue. As explained 

earlier in section III-B, this is because the ADAMS linearized model admits only a physical mass 

and inertia, whereas BModes permits the use of both physical and added masses. The 

discrepancy in the side-to-side second and third modes is perhaps due to erroneous interpretation 

of modes in ADAMS (investigationis underway).

Table 4. BModes- and ADAMS-Predicted Frequencies for the Spar-Buoy-Supported Tower

Mode

Number

Mode

Type

Frequency (Hz)

Difference

(Hz) BModes

(no gravity)

Bmodes

(with 

gravity)

ADAMS

1 Surge 0.0085 0.0085 0.0084 0.0001

2 Sway 0.0089 0.0089 0.0089 0.0000

3 Heave 0.0324 0.0325 0.0180 0.0145

4 Roll 0.0711 0.0376 0.0377 0.0001

5 Pitch 0.0712 0.0377 0.0378 0.0001

6 Yaw 0.1649 0.1649 0.1649 0.0000

7 SS-1 0.5000 0.4893 0.4861 0.0032

8 FA-1 0.5104 0.5103 0.5103 0.0000

9 Twist 1.693 1.695 1.700 0.0051

10 SS-2 2.067 2.065 0.7647 1.3003

11 FA-2 2.569 2.568 2.562 0.0062

12 SS-3 6.029 6.026 3.720 2.3057

13 FA-3 6.360 6.329 6.326 0.0031

14 Axial 8.033 7.678 7.669 0.0087

4 Verification of Modal Analysis of Rotating Blade with Tip Mass

We use this model to verify that the BModes correctly captures the interaction among rotational,

inertial, and elastic effects. We assume uniform structural properties along the blade with no 

twist and no offsets of the elastic and center-of-mass axes. The blade length, L, is 31.623 m and 

its mass m per unit length is 100 kg/m. Its cross-sectional flexural rigidities for flap, lag, and 

torsion are respectively, 108 N-m2, 109 N-m2, and 105 N-m2. The tip mass is 3162.3 kg. This 

rather high mass is selected to allow comparison with published results. We built a BModes

model for a uniform blade using 15 beam elements and computed its flap, lag, and torsion 

modes. Analytical results are available for the flap modes9 of this model and we use these for 

comparison (see Table 5). Note that the first flap frequency variation with rotor speed, computed 

by BModes, shows an exact agreement with the analytical results, at least up to the significant 

digits shown. The higher frequencies also show excellent agreement. The largest discrepancy is 

about 0.02% for the second flap frequency when the blade is spinning at 12 rad/sec.
14

Table 5: Comparison of Flap Frequencies: Rotating Uniform Blade with Tip Mass

Blade Spin 

Rate, 

(rad/sec)

First Flap Frequency 

(rad/sec)

Second Flap Frequency 

(rad/sec)

Analytical BModes Analytical BModes

0 1.557 1.557 16.25 16.25

1 1.902 1.902 16.76 16.76

2 2.670 2.670 18.19 18.19

3 3.582 3.582 20.35 20.35

4 4.543 4.543 23.03 23.03

5 5.522 5.522 26.04 26.04

6 6.509 6.509 29.29 29.29

7 7.501 7.501 32.70 32.70

8 8.495 8.495 36.21 36.21

9 9.490 9.490 39.80 39.80

10 10.49 10.49 43.45 43.45

11 11.48 11.48 47.14 47.15

12 12.48 12.48 50.86 50.87

Conclusions and Future Plan

All results indicate that BModes is capable of modeling and analyzing rotating blades and a wide 

range of tower support structures with a high degree of fidelity. Plans are underway to use it for a 

wide range of applications ranging from system identification to coupling with CFD software. 

BModes has been successfully used for finite element model updating of a turbine blade and a 

tower10. An important future plan is to introduce a Timoshenko beam element, specialized to 

include rotational and hydrodynamic effects, and extend BModes to handle composite beams

with non-isotropic material properties.

Acknowledgment

Thanks to Jason Jonkman from NREL for providing the ADAMS models of the towers used in this paper. This 

work was performed in support of the U.S. Department of Energy under contract number DE-AC36-83CH10093.

References

1. Jonkman, J.M.; Buhl Jr., M.L. FAST User's Guide. NREL/EL-500-29798. Golden, Colorado: National 

Renewable Energy Laboratory, 2005. 

2. Bir, G. S., “Blades and Towers Modal Analysis Code (BModes): Verification of Blade Modal Analysis,” 

Proceedings of the 47th AIAA Aerospace Sciences Conference and Wind Energy Symposium, Orlando, 

Florida, January 5-8, 2009.

3. Bir, G.S. User’s Guide to BModes: Software for Computing Rotating Beam Coupled Modes. NREL TP-500-

38976. Golden, Colorado: National Renewable Energy Laboratory, 2005.

4. Yu, W.; Volovoi, V. V.; and Hodges, D. H.: “Validation of the Variational Asymptotic Beam Sectional 

Analysis,” AIAA Journal, vol. 40, no. 10, 2002, pp. 2105-2113.

5. Bir, G.S. (2005). User’s Guide to PreComp (Pre-Processor for Computing Composite Blade Properties). 

NREL/TP-500-38926. Golden, CO: National Renewable Energy Laboratory.

6. Malcolm, D.J., Laird D.L. “Identification and Use of Blade Physical Properties.” Proceedings of the 

ASME/AIAA Wind Energy Symposium, Reno, Nevada, January 2005.
15

7. Bir, G. S., and Chopra, I. “Status of the University of Maryland Advanced Rotorcraft Code (UMARC).” 

Presented at the American Helicopter Society Aeromechanics Specialist Conference. San Francisco, California, 

January 19-21, 1994.

8. Lee, C. H. and Newman, J. N. WAMIT® User Manual, Versions 6.3, 6.3PC, 6.3S, 6.3S-PC. Chestnut Hill, MA: 

WAMIT, Inc., 2006.

9. Wright, A. D., Smith C. E., Thresher R. W., and Wang, J. L. C., “ Vibration Modes of Centrifugally Stiffened 

Beams,” Journal of Applied Mechanics, Vol. 104, March 1982.

10. Bir, G.S.; Oyague, F. (2007). Estimation of Blade and Tower Properties for the Gearbox Research Collaborative 

Wind Turbine. 30 pp.; NREL Report No. TP-500-42250. 

11. Jonkman, J. and Matha, D., “A Quantitative Comparison of the Responses of Three Floating Platform 

Concepts.” European Offshore Wind 2009 Conference & Exhibition, 14–16 September 2009, Stockholm; 

NREL/CP-500-46726. Golden, CO: National Renewable Energy Laboratory.
F1147-E(10/2008)

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Verification of BModes: Rotary Beam and Tower Modal Analysis 

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14. ABSTRACT (Maximum 200 Words)

This paper describes verification of BModes, a finite-element code developed to provide coupled modes for the blades and tower 

of a wind turbine. The blades, which may be rotating or non-rotating, and the towers, whether onshore or offshore, are modeled 

using specialized 15-dof beam finite elements. Both blade and tower models allow a tip attachment, which is assumed to be rigid 

body with six moments of inertia, and a mass centroid that may be offset from the blade or tower axis. Examples of tip 

attachments are aerodynamic brakes for blades and nacelle-rotor subassembly for towers. BModes modeling allows for tower 

supports including tension wires, floating platforms, and monopiles on elastic foundations. Coupled modes (implying coupling of 

flap, lag, axial, and torsional motions) are required for modeling major flexible components in a modal-based, aeroelastic code 

such as FAST1. These are also required for validation of turbine models using experimental data, modal-based fatigue analysis, 

controls design, and understanding aeroelastic-stability behavior of turbines. Verification studies began with uniform tower models, 

with and without tip inertia, and progressed to realistic towers. For the floating turbine, we accounted for the effects of 

hydrodynamic inertia, hydrostatic restoring, and mooring lines stiffness. For the monopole-supported tower, we accounted for 

distributed hydrodynamic mass on the submerged part of the tower and for distributed foundation stiffness. Finally, we verified a 

model of a blade carrying tip mass and rotating at different speeds (verifications).

15. SUBJECT TERMS

Bmodes; wind turbine modal analysis code; floating platforms; modeling

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