The development of computer codes for calculating dynamic loads in horizontal axis wind turbines was examined, and a brief overview of each code was given. The performance of individual codes was compared against two sets of test data measured on a 100 KW Mod-0 wind turbine. All codes are aeroelastic and include loads which are gravitational, inertial and aerodynamic in origin

Spera, D. A.

English

NASA Technical Reports Server

COMPARISON OF COMPUTER CODES FOR CALCULATING DYNAMIC
LOADS IN WIND TURBINES
David A. Spera
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
INTRODUCTION
The development of computer codes for calculating dynamic loads in horizontal-
axis wind turbines has been part of the Federal Wind Energy Program for almost
four years.
During this period of time the Energy Research and Development Administration
(ERDA) has sponsored the development of at least seven codes by NASA and its
contractors. As might be expected in an area of new technology, these codes
differ considerably in approach and technique. Because of the generally com-
plicated nature of any structural dynamics analysis, a detailed comparison of
seven computer codes is extremely difficult. Therefore, the objectives of
this study have been limited to the following: (1) To present a brief over-
view of each code and identify sources for further detailed information, and
(2) to compare the performance of each code against two sets of test data
measured on the lO0 kW Mod-O wind turbine, an experimental machine in operation
at NASA's Plum Brook Station near Sandusky, Ohio. Comparison on the basis of
cyclic loads, peak loads, and harmonic contents was selected. The results of
this study are given in detail in Reference I.
DESCRIPTION OF CODES
The seven computer codes compared in this study are listed in Table I. All
codes are aeroelastic (i.e., air loads and blade deformations are coupled) and
include loads which are gravitational, inertial, and aerodynamic in origin.
Three of the codes (MOSTAB-WT, -WTE, and -HFW) analyze only rotor loads, while
the remaining four codes (REXOR-WT, GETSS, F-762, and MOSTAS) are complete
system codes which include rotor-tower interaction.
MOD-O DATA CASES
For purposes of establishing reference test data, two sets of blade load data
have been defined, which were measured on the Mod-O wind turbine (Ref. l).
These data sets have been designated as Mod-O Data Cases I and IV. The data
sets contain time histories and harmonic analyses of bending moment loads
measured in the Mod-O blades by means of strain-gage load cells. Moment loads
in the flatwise and edgewise directions at Station 40 (shank area, 5% span)
and Station 370 (midblade area, 49% span) were measured. Additional data are
also available for these two cases, including shaft bending and torque loads,
nacelle accelerations, and tower deflections. However, for purposes of
https://ntrs.nasa.gov/search.jsp?R=19780011674 2020-03-20T12:55:25+00:00Z
comparing computer codes, blade moment loads were judged to be critical, so
other measured data were not used in this study.
Operating Conditions
Data Case I, with single yaw drive and stairs in the tower, presents a high
level of rotor-tower interaction. These data were measured on December 18,
1975. On the other hand, Data Case IV with the yaw drive locked (relatively
rigid nacelle-to-tower connection) were measured on September II, 1976, after
the tower stairs were removed and therefore exhibit little rotor-tower inter-
action. Thus, these two cases represent relatively high and low levels of
blade loading sustained by the Mod-O wind turbine operating at nominal wind
speeds between 25 and 28 mph.
Typical Time-History Curves
A typical cycle of flatwise bending load during one rotor revolution is shown
in Figure I. A positive flatwise moment bends the blade toward the tower
causing tensile stresses on the low-pressure (downwind) surface. In Figure I,
flatwise moment My is plotted versus the blade azimuth _b, which is zero and
360 o when the blade points downward. As shown in the figure, the flatwise
time history for a rotor located downwind of the tower is dominated by the
impulse applied to the blade each time it passes through the tower's wake or
"shadow." For purposes of stress and fatigue analysis it is convenient to
define cyclic load 6M and steady load _, as shown in Figure I.
Figure 2 illustrates a typical time history of edgewise load, Mz, measured
during one revolution. A positive edgewise load on the blade tends to stop
the rotor, causing tensile stresses on the blade's leading edge. An edgewise
moment time history is usually composed of three components, as shown in Fig-
ure 2: (I) A relatively steady bending moment which produces shaft torque and
power, (2) a sinusoidal moment caused by the blade's own weight plus a small
wind shear effect (90 ° out of phase) and (3) high frequency dynamic loads
attributable to motions of the nacelle and tower.
RESULTS AND DISCUSSION
Cyclic Moment Load Comparisons
Figures 3(a) and (b) illustrate how cyclic moments calculated using the seven
codes compare not only with the specific data cases defined but also with the
trend of data measured over a period of time on the Mod-O wind turbine. This
trend is represented by a nominal variation of load with wind speed plus a
band of variation which is estimated to be -+ Io in width, thereby containing
loads for about 70% of the machine's revolutions. This band is approximately
equal to -+ 20% of the nominal loads with the exception of Case IV edgewise
loads. Variations are caused by changes in wind direction and velocity,
control changes, and unsteady factors not yet identified.
The empirical constants used in the MOSTAB-WTE code were selected to place its
results at the top of the variation band, as shown in Figures 3(a) and (b).
Data Cases I and IV do not necessarily represent the nominal loads. Other gen-
eral observations concerning the results shownin Figures 3 are as follows:
• Loads calculated by all codes fall within the data variation band, with the
exception of edgewise loads for Case I which were calculated using the
MOSTAB-WT and MOSTAB-HFW codes. MOSTAB codes are able to predict only the
gravity component of cyclic edgewise load because shaft motion is absent in
these codes.
2. REXOR-WT results generally coincide with nominal loads.
3. Results for the remaining codes tend to be mixed, falling both above and
below nominal load values.
Table 2 presents data for a more complete comparison of measured and calculated
cyclic loads.
Peak Moment Load Comparisons
A second comparison of calculated and measured moment loads will be made on the
basis of their peak values, defined as the maximum absolute value occurring
during one revolution. Measured and calculated peak moment loads are listed in
Table 3. Inspection of the normalized results shows a wide range of values,
from 0.57 to 1.30. With the exception of the empirical code MOSTAB-WTE, all
the codes exhibit load ratios both larger and smaller than unity, without any
significant trends being apparent.
Summary of Load Ratios
All the load ratios listed in Tables 2 and 3 for cyclic and peak loads, respec-
tively, were averaged and appear in Table 4. These average ratios signify the
general level of loading calculated using the various codes in comparison with
a blend of high and low nominal loads. MOSTAB-WTE, with its empirical con-
stants selected to place calculated loads well above nominal, was found to have
an average load ratio of 1.15, slightly lower than expected. Of the remaining
codes, all produced average load ratios within 97% + 3% of nominal. Load
ratios obtained with the REXOR-WT code showed significantly less deviation than
ratios for all the other codes.
Load Predictions
The MOSTAB-WTE and REXOR-WT were used to predict the effect on blade loads of a
new dual yaw drive system for the Mod-O wind turbine (Ref. 2). The results are
shown in Figures 4(a) and (b). The MOSTAB-WTE code provided an estimate of
nominal + I_ cyclic flatwise moment (Fig. 4(a)) in good agreement with data
obtained later. Prediction of edgewise loading using MOSTAB-WTE (Fig. 4(b))
appears to be somewhat conservative, at least in comparison with load bank
data. Additional synchronized operation data are required before the level of
conservation can be judged•
l •
.
•
.
•
l •
2.
.
l .
•
Q•
A.
SUMMARY OF RESULTS
Six of the seven codes studied (MOSTAB-WT and -HFW, MOSTAS, REXOR-WT,
GETSS, and F-762) calculated loads which on the average were within 4% of
nominal loads measured on the Mod-O wind turbine•
Loads calculated using an empirical code (MOSTAB-WTE) were 18% above nomi-
nal levels, in accordance with the objective of this code to provide load
margin.
Among the four system codes evaluated (MOSTAS, REXOR-WT, GETSS, and F-762),
the REXOR-WT code appeared to be the most consistent in producing calcu-
lated loads close to nominal loads•
All codes except MOSTAB-WT and -HFW satisfactorily calculated the general
pattern of both flatwise and edgewise loads for the two cases studied•
These two codes contain the assumption of rigid rotor support which elimi-
nates some edgewise load harmonics.
The empirical code MOSTAB-WTE was verified on the basis of comparisons with
the results of the system codes and test data obtained from the Mod-O wind
turbine with dual yaw drive.
CONCLUSIONS
WTG load prediction codes are in an advanced state of development.
Present system codes tend to predict nominal loads, so load margins must be
explicitly introduced into input or output data.
System codes have been validated for "stiff" WTG systems, but are not yet
validated for "flexible" systems.
REFERENCES
Spera, D. A.: Comparison of Computer Codes for Calculating Dynamic Loads
in Wind Turbines• NASA TM (to be published)•
Spera, D. A.; Janetzke, D. C.; and Richards, T. R.: Dynamic Blade Loading
in the ERDA-NASA lO0 kW and 200 kW Wind Turbines• NASA TM-73711, 1977.
DISCUSSION
How broadly applicable are the empirical corrections in MOSTAB-WTE?
The empirical equations apply to two-bladed hingeless rotors. However,
the two empirical constants have been evaluated only for stiff or semi-
stiff towers, to date.
Q. What accuracy is associated with the experimental bending moment data?
a.
e.
a.
Q.
a.
Qo
A.
e.
A.
Q.
a.
e.
a.
Q.
a.
About 5% accuracy for cyclic bending moments. However, zero errors can be
significant (see Ref. I, Table 7).
Has the yaw brake dramatically improved rotor/tower coupling effects in the
Mod-O WTG?
Yes. Blade loads are definitely lower when the yaw brake is on.
What causes the significant variation in blade loads for the same nominal
wind speed?
Variations in wind direction, control changes, yaw maneuvers, and other
factors we don't understand.
What is the frequency distribution of load variations?
Both normal and log normal distributions have been observed. More data
are definitely needed.
Have these codes been used to calculate stress levels?
No. The output of these codes is loads. Auxiliary stress codes are used
subsequent to these codes.
Please comment on costs per case and input/output format for the various
codes.
I don't have specific information for all the codes but the contact per-
sons listed in Table 1 do. For MOSTAB-WT and -WTE, typical input would
consist of about 50 cards, running time would be two minutes (IBM III0
computer) and typical output would be tabular listings of three moments
and three shear forces at up to a dozen blade stations at each of 24 azi-
muthal blade positions. Parametric studies are conveniently and economi-
cally performed by non-specialists. My general comments about the
remaining codes are that personnel must be well trained, running times
of 30 to 60 minutes (or more) are to be expected, and your organization
must be ready to provide dedicated support.
During preliminary" design, predicting load frequencies may be more impor-
tant tha'n predicting amplitudes. Do you plan to compare these programs on
the basis of predicted frequencies?
Yes. The harmonic contents of the predicted loads are compared in
Reference I.
Your experience has shown that softness in the Mod-O yaw drive induced
undesirable cyclic loads. How then can flexibility in future systems
reduce costs?
Flexibility can lead to lighter, less costly structures only if resonances
can be avoided. Unfortunately, softness in the Mod-O yaw drive placed it
at a resonance, leading to the undesired cyclic loads.
5
TABLE I. - COMPUTER CODES PRESENTLY USED FOR AERO-ELASTIC ANALYSIS OF DYNAMIC
LOADS AND DEFORMATIONS IN HORIZONTAL-AXIS WIND TURBINES
Code Type (domain) Source for Information
MOSTAB-WT
MOSTAB-WTE
MOSTAB-HFW
REXOR-WT
GETSS
F-762
MOSTAS
Single
blade; a
1 DOF
(time)
Same,
plus empirical
constants
Rotor;
4 DOF plus
imballing
time)
System;
multi-DOF
(time)
System;
mul ti-DOF
(freq.)
System;
multi-DOF,
(time)
System;
multi-DOF,
(time/freq.)
Barry Holchin
Mechanics Research Incorporated
9841 Airport Boulevard
Los Angeles, CA 90045
David A. Spera
NASA-Lewis 49-6
21000 Brookpark Road
Cleveland, Ohio 44135
John A. Hoffman
Paragon Pacific Incorporated
1601 E. El Segundo Boulevard
El Segundo, CA 90245
Robert E. Donham
Dept 75-21, Bldg. 360, Plant B-6
Lockheed-California Company
Burbank, CA 91520
Clyde Stahle
General Electric Space Division
Box 8661
Philadelphia, PA 19101
Richard Bielawa
United Technologies Research Center
East Hartford, CT 06108
John A. Hoffman
Paragon Pacific Incorporated
1601 E. El Segundo Boulevard
El Segundo, CA 90245
aDegrees of freedom
TABLE 2. - COMPARISON OF RELATIVE CYCLIC MOMENT LOADS,
NORMALIZED WITH RESPECT TO NOMINAL CYCLIC LOADS
Source
A
Relative cyclic moment loads
Flatwise a Edgewise b
Sta 40 Sta 370 Sta 40 Sta 370
(a) Data Case I
i m
Nominal 1.00 1.00 1.00 1.00
Test =
data Actual 1.02 0.97 0.87 0.95
-WT I. O0 1. O0 O. 60 O. 50
MOSTAB
rotor
codes
-WTE
-HFW
1.20
1.00
1.20
0.99
1.20
0.66
1.20
O. 58
REXOR-WT
GETsS
0.96 1.00 0.94 0.87
1.19 1,16 0.80 1.05
F-762 1.08 1.03 0.78 0.77
0.970.92 0.82MOSTAS 0.78
(b) Data Case IV
Test
data
Nominal 1.00 1.00 1.00 1.00
Actual 0.83 0.96 1.05 0.93
MOSTAB -WT 1.20 1.20 0.94 0.93
rotor -WTE 1.20 1.20 1.04 i 1.07
codes -HFW 1.17 1.17 l.O0 I 1.04
REXOR-WT 0.97 0.92 0.98 0.96
GETSS
F-762
MOSTAS
0.92 0.86 0.98 1.01
0.87 0.91 1.05 1.11
1.22 ' 1.23 0.97 i.01
a 6My  _My,nom
b _Mzl 6Mz,no m
TABLE3. -
• ill
Source
RELATIVE
RESPECT
Relative
Flatwise a
Sta 40 Sta 370
a) Data Case I
PEAK MOMENT LOADS, NORMALIZED WITH
TO NOMINAL PEAK LOADS
k
peak moment loads
Edgewise b
Sta 40 Sta 370
Test !Nominal
data Actual
MOSTAB -WT
rotor -WTE
codes -HFW
REXOR-WT
GETSS
1.oo 1.oo 1.oo 1.oo
1.01 0.98 0.89 0.96
1.01 1.12 0.66 0.57
1.12 1.25 1.13 1.17
0.78 0.73
MOSTAS
l.02
0.96 0.89
1.13
1.05
0.96
1.19
0.98
0.98 0.72
F-762 l.ll 0.84 0.95
0.88 0.86 0.94
0.85
1.1i
(b) Data
Test
data
MOSTAB
rotor
codes
Case IV
Nominal
Actual
1.00
O. 76
I. O0
0.97
i.30
1.30
l.00
I .03
0.94
0.94
-WT 1.23
-WTE 1.23
-HFW 1.22
REXOR-WT 1.02
GETSS
I. 30
0.99
i.06
0.96
1.00
0.94
0.96
0.96
1.13
0.93
F-762
MOSTAS
0.76
0.92
1.03
im
.IMyl x/IMy,noml
blM,Imax/IM,.oooloax
0
.67
.92
.05
0.77
1.01
0.99
1.17
J
1.27
1.07
8
TABLE 4. - SUMMARY OF LOAD RATIOS OBTAINED USING VARIOUS COMPUTER CODES AND MOD-O
WIND TURBINE TEST DATA
Code Type and Name Goal Blade Load Ratio a
of
Cal c. Average RMS
Load Dev. b
Rotor
Codes I MOSTAB-WTE
MOSTAB-HFW
MOSTAB-WT
F-762
System MOSTAS
Codes REXOR-WT
GETSS
Nom. + Io 1.15 + 0.I0
Nom. 1.00 + 0.20
" 0.95 +_ 0.24
" 0.99 _+0.14
" 0.98 -+ 0.12
" 0.95 + 0.05
" 0.94 _+ 0.16
a Calculated to nominal measured; based on 16 ratios combining 2 data cases,
2 blade stations, flatwise and edgewise directions, and cyclic and peak
bending moments.
b Root-mean-square deviation; includes approximately II of 16 ratios.
FLAI_ISE
SHANK
IV_OI_ENT,
My,
Ib-M
EDGEWISE
SHANK MOMENT,
Mz
Ib-_t
U
160xlO3
I FMAX LOAD, My,max
120 RESPONS
/I | TOTOwERI ,,rCYCLIC LOAD, 6My
SHADOw ,, i
___
I '- MIN LOAD, My. rain
-4oI i _ I J
O gO lBO 270 _0
BLADE AZIMUTH, @b' (leg
Figure I. - Typical cycle of blade flatwise moment meas-
ured on ERDA-NASA lO0-kilowatt Mod-O wind turbine.
1
My "_ (My.max * My.rain I
_t,,Ay • "*_ (My.max " M_j,min )
4O
15_RESPONSE TO NACELLE MOTION
i i
i /-C4_VITY, WIND SHEAR LOADS
_" "_'/_/ r STEADY LOAD
- I l
__. ....... _L .... -"
I I I ]
"800 90 180 Z'/O $60
BLADE AZIMUTH. _b' (leg
Figure 2. - Typical cycle of blade edgewise
moment measured on Mod-O wind turbine.
lO
O
E)
---{D---
A
13
D
0
4
NOMINAL*-1[7- l
(EST.) I TEST3 CYCLE AVG. DATA
MOSTAB-WT I
MOSTAB-WTE ROTOR
CODES
MOSTAB-HFW
REXOR-WT 1
GETSS SYSTEM
F-762 CODES
MOSTAS
CYCLIC -_
FLATWISE
SHANK MOMENT, I-
(_M),,
LB-FT i 4
%
_'DATA CASE I
I I I
ZO 30 4O
NOMINAL WIND SPEED, Vo, MPH
(a) Flatwise moment load.
Figure 3. - Comparison of measured and calculated
blade moment loads for Mod-O wind turbine at
various wind speeds with stairs and single yaw
drive. Station 40; 5-percent span.
II
CYCLIC
EDGEW]SE
SHAIIKMOMENT,
&Mz
±I0
_8
_6
±4
LB-FT
__2
0
4
_x x
- i
_-- DATA CASE
$
_-GRAVITY
-- ONLY
I0
I I I
20 _ 40
NOMINAL WIND SPEED, Vo, MPH
(b) Edgewise moment load.
Figure 3. - Concluded.
12
CYCLIC "_G
FLATWISE
MOMENTLOAD,± 4
4
=8 - ,I0
_ LOAD BANK I TESTDATA,
_SYNCHRONIZED MEAN * liT
PREDICTED
MOSTAB-WTE-,
- (NOMINAL* 1{_)
.t
i
REXOR-_T
(NOMINAL)
I I I
20 30 40
NOMINALWIND SPEED,Vo, MPH
(a) Flatwise moment load.
I
STA 40
CYCLIC
EDGEWISE
MOMENT LOAD,
_Mz,
LB-FT
_tG
z4
MOSTAB-WFTE"_
_-REXOR-_T
(NO_.INAL)
0 I I I I
i0 ZO 30 4O 5O
NOM,INAL WIND SPEED,Vo, MPH
(b) Edgewise moment load.
Figure 4. - Comparison of measured and calculated
cyclic blade moment loads for Mod-O wind turbine
at various wind speeds with stairs removed and
dual yaw drive installed. Station 40; 5-percent
span.
13


Comparison of computer codes for calculating dynamic loads in wind turbines

Seven computer codes for analyzing performance and loads in large, horizontal axis wind turbines were used to calculate blade bending moment loads for two operational conditions of the 100 kW Mod-0 wind turbine. Results were compared with test data on the basis of cyclic loads, peak loads, and harmonic contents. Four of the seven codes include rotor-tower interaction and three were limited to rotor analysis. With a few exceptions, all calculated loads were within 25 percent of nominal test data

Comparison of computer codes for calculating dynamic loads in wind turbines

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