NASA Technical Reports Server (NTRS) 19770006591: Wind tunnel measurements of the tower shadow on models of the ERDA/NASA 100 KW wind turbine tower

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NASA TM X- 73548

NASA TM X- 73548

NASA TECHN ICAL

MEMORANDUM

(NASA-Ttt-X-7354o) wIAD aUNNAL MAASUfiriltNIS
Of TH£ lOliEE SHADOW ON HCDtLS Of THa
L cDA/AASA IOC KW mIND TUaAlNE TOWAB (Ut^Sk)

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B77- 13j34

Unclds

WIND TUNNEL MEASUREMENTS OF THE TOWER SHADOW
ON MODELS OF THE ER DA/NASA 100 KW
WIND TURBINE TOWER

by Joseph M. Savino and Lee H. Wagner
Lewis Research Center
Cleveland, Ohio
November 1976

1 Report No 2 Government Accession No 3 Recipient's Oitaiog No

NASA TM X-73548

1 — ' *

1 4 Title and Subtitle

WIND TUNNEL MEASUREMENTS OF THE TOWER SHADOW ON

Cl RciKirl Ddte

MODELS OF THE ERDA/NASA 100 KW WIND TURBINE TOWER

6 Perlomiing Organisation Code

7- Author(s!

8. Performing Organisation Report Nu

Joseph M. Savino and Lee H. Wagner

E-8984

10 Work Unit No

1 9. Performing Organisation Name and Address '

Lewis Research Center

11. Contract or Grant No.

National Aeronautics and Space Administration

Cleveland, Ohio 44135

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address

Technical Memorandum

National Aeronautics and Space Administration

14. Sponsoring Agency Code

Washington, D. C. 20546

15 Supplementary Notes

16- Abstract

Detailed wind speed profile measurements were made in the wake of a 1/25 scale and a
1/48 scale tower models to determine the magnitude of the speed reduction (the tower shadow).
The 1/25 scale tower modeled closely the actual wind turbine including the service stairway and
the equipment elevator rails on one face. The 1/48 scale model was made of all tubular mem-
bers. Measurements were made on the 1/25 scale model with and without the stairway and
elevator rails, and on the 1/48 all tube model without stairs and rails. The test results show
that the stairs and rails were a major source of wind flow blockage. The all tubular 1/48 scale
tower was found to offer less resistance to the wind than the 1/25 scale model that contained a
large number of square sections. Shadow photos are included to show the extent of t’.e blockage
offered to the wind from various directions.

17. Key Words (Suggested by Auihor(s)|

Energy Solar

Wind Electric power generation

18. Distribution Statement

1 Unclassified - \

inlimited

19 Security Classif (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

1 “ " ' 1

j 21, No, of Pages

22. Price*

' For sale by the National Technic.il Intoimatioii Service, Springfielrt Virginia 22161

NASA-C-108 (Rev. 10-75)

1 - 8981 *

WIND TUNNEL MEASUREMENTS OF THE TOWER SHADOW ON MODELS

OF THE ERDA/NASA 100 kW WIND TURBINE TOWER
by Joseph M. Savlno and Lee H. Wagner

w

INTRODUCTION

The ERDA/NASA 100 kW Wind Turbine (referred to as the Mod-0 ) Fig-
ure 1, is a tvo-bladed propeller-type in which the rotor is designed to
always operate on the downwind side of the support tower (Ref, l) . Dur-
ing operation, each blade must pass through the wake of the tower where
the wind speeds are always lower than the surrounding vinobstructed free
wind stream. Some of the early Mod-0 test results showed that the blade
root stresses were about 60 $ higher than the expected design values.

This finding led to an investigation to determine the magnitude of the
wind speed reduction caused by the tower. The purpose of this report is
to present the results of tests that were conducted on models of the
Mod-0 tower in a wind tunnel

DESCRIPTION OF APPARATUS

The Mod-0 wind turbine tower, shown in Figure 2, consisted of the
basic tower structure, with a service personnel stairway situated on the
inside, and a pair of I beams (for an equipment elevator) on one face.
The basic tower structure uses 8-inch pipe for the four legs, channels
for the horizontal members, back-to-back angles for the diagonals with
gusset-plate attachments .

Two tower models were tested, a 1/25 and a 1/^*8 scale model, fig-
ure 3* The l/U8-scale model was made of all tubular members without
gusset-plate Joints. In the 1/25 scale model, square bars were used to
simulate the horizontal channel members and the diagonal angle members
of the Mod-0 tower. The stairs were simulated by using small diameter
wires for the stair treads, and thin metal plates for the stair strings.
The hand rails were not modeled. The stairway model for the 1/25-scale
model was extended only up the lower four sections of the tower.

Figure 4 shows the 1/48 scale model (with the model stairway in-
stalled) and the wind speed measuring equipment as installed in the wind
tunnel. The pitot tube, used to measure the local wind speeds, was in-
stalled on a system of motor-driven carriages . One carriage was used to
position the pitot tube at any desired vertical. posit ion, and the other
slowly moved the probe horizontally across the tower wake to make a con-
tinuous measurement of the wind speed distribution. The total press\ire
sensed by the pitot tube was referenced to a static pressure sis measured
by a tap on the wind tunnel wall. The velocity head (total minus static
pressure) was sensed by a differential pressure gage of the strain gage
type. It was found by actual measurement that the static pressure dis-

STAR category 44

^ POOR

I

2

tribution was uniform in the plane behind the tower where the measurement a
were made.

All wind speed measurements were at the single free stream value of
100 nqph, ambient air temperatures, and neax atmospheric pressure. Pro-
files were taken at a v&uriety of elevations for wind approach angles to a
reference face of the tower (the face with the I-beams was used as the
reference face) of 0°, 10°, 25° » 35°, ^0°, and U5°, Figure 5. The major-
ity of profiles were made downwind of the third and fourth sections of the
tower above the ground because the tower shadow effect had the greatest
impact on the outer 50/? of the blade length. In addition, all four upper
sections were similar in construction except for their solidity so that
only a limited nxaaber of profiles would be needed behind the upper two
sections to determine their complete wake profile characteristics.

As in all model testing, the applicability of the tunnel test results
to the full-scale tower arose. When the Mod-0 is operating in wind speeds
from 10 to Uo mph, the Reynolds numbers based on the diameters and widths
of the tower members , wind speeds , and ambient air properties are in the
range where the drag coefficients of the members are constant, i.e., in-
dependent of the Reynolds Number. The same was true for Reynolds Number of
the tower model members. This means that when the drag coefficients of
the model members and the tower members are the same, then the wind speed
profiles in the wake are similar. On this basis, it was concluded that the
test resxilts measured with a model tower are applicable to the full-size
tower .

TEST RESULTS AND DISCUSSION

Some typical dimensionless wind speed profiles are shown in Figiare 6
that were measured in the wake of the models of the Mod-0 tower, with and
without the stairway and the equipment elevator rails on one face. These
profiles were selected to illustrate some of the most serious tower block-
age effects and some of the more favorable ones. The wind speeds and the
lateral position were made dimensionless by the free stream wind speed V
and the wake width d, respectively. In Figure 7 ore plotted the average^
wind speeds and the minimum wind speeds at various elevations behind sec-
tions 3, 5, and 6 of the tower models. The averages were calculated

by integrating the individual horizontal wind profiles. The minimums are
the lowest value measured in each profile.

When the profiles shown in Figure 6(a) through (f) are ccmipared
(a) with (b), (c) with (d), and (e) with (f) and when Figure 7(a) is com-
pared with 7(b) the effect of removing the stairs and rails from the
1/25 scale models is clearly evident. That effect is that there is a
sizeable increase in both the average and minimum wind speecte. This char-
acteristic was noted in all the wind speed profiles that were measured
after the stairs and rails were removed. One prominent feature of the
profiles behind the tower with the stairs and rails was the wide and deep
depression (Figure 7(a), (c), (e)) which existed over most of the tower
height above section 2. The effect of this depression waa that whenever

3

a blade passed behind the tower, it was exposed for a brief moment to winds
that were much lower than the average wind speed in the wake. These low
speeds caused a sudden reduction in the angle of attack on the blade, which
in turn caused a sudden reduction of both the thrust force and torq.ue on
the blade. Such load reduction impulses can be the source of blade and
tower vibrations if these components are not properly tuned.

Figures 6(i) through (m) are profiles measvired behind the bare
1/48 scale tower (without stairs and rails). When these are compared to
those profiles behind the bare 1/25 scale tower, it is seen that the pro-
files are quite similar, but, the wind flow through the all tubular 1/48
model is higher by about 5^ than through the 1/25 model. This latter fact
is also evident when Figure 7(c) is compared with Figure 7(b).

Figure 7 summarizes all the measured wind speed profile data.

The data in Figure 7(a) are for the 1/25 scale tower with the stairs
and rails. The greater blockage and the unsymmetrical natiire of the block-
age resulted in lower average and minimum wind speeds and greater scatter.
This scatter reflects the fact the wind flow resistance of the tower is
not uniform with elevation or the azimuthal position around it. A com-
parison of Figure 7(b) with 7(c) shows that both the average euid minimum
wind speeds are higher and less scattered for the 1 tubular 1/48 scale
model and that the average wind flow throxigh the 1/48 scale tower is
nearly constant within about a ±6^ for all heights and wind approach angles .
The largest decreases are seen to occur behind the horizontal members.

In F'’g\ire 8 is shown a select number of shadow photographs to give a
visual indication of the amount of blockage offered by the tower models
with and without the stairs and elevator rails and from various angles of
view. These shadow-photographs along with the profiles of Figure 6 show
that two primary factors contribute to a large local wind speed reduction:
the size of the obstruction(s ) directly upstream and the number of obstruc-
tions that are directly in line with the wind direction or nearly so. This
is most evident when comparing the shadow-photographs of the bare 1/25 model
with those of the model with stairs and rails for all wind approach angles v
In Figure 8(h) for example, the windward and leeward legs of the tower are
almost in line, thereby causing the greatest wind speed reduction directly
behind those legs where the horizontal and diagonals intersect to form a
joint.

CONCLUSIONS AND RECOMMENDATIONS
The results of these tests show that:

1. The presence of the stairs and elevator rails caused some very
large reductions (up to lOO;^) in the wind speed in the wake of the tower,
when compared to the basic bare tower without the stairs and rails . For
example, the local average wind speeds behind the 1/25 model with stairs
and rails ranged from about 6o^ to 77^ of free stream value whereas when
the stairs and rails are removed, the local averages increased to a range
between and 85^. The minimuins also increased from a range of 0 to
45^ to a range of 45 to 70^.

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4

2. Towers constructed frcm all tubuleu; members offer less resistance
to the wind than those made with some noncircular members. The average
speed thru the upper four sections of the all tubular 1/48-scale model
was about versus 805? for the 1/25-scale model. The minimum value
also increased for the all-tubular tower.

3. The average wind speed in the wake of the bare towers is very
nearly independent of the direction that the wind approaches the tower and
independent of the elevation. The exception is at those elevations behind
the horizonted. members where the average is about 55? lower.

4. The local wind speed reductions at any point or region in the wake
of a tower is largely determined by the size of the members and number of
members that are in line with the wind and directly upwind. In other words,
the wind speed reductions increase as the blockage (the solidity) upstream
increases . In the wakes of bare towers , the lowest wind speeds occur be-
hind the Joints where the vertical, horizontal, and diagonal members con-
verge, and when these joints are in line with the wind (a wind direction at
45® to a typical face).

From the above tests it ves learned that the following features should
be considered in tower design to increase the wind flow through the tower
(reduce tower shadow):

1. Use all tubular members.

2. Reduce the size and number of members to the minimum needed to
meet the other tower design requirement , that is , reduce the tower shadow
to a minimum.

3. Avoid the use of gussett plates at the joints.

4. Reduce the number of members that can line up with the wind to a
minimvm.

5. Reduce the number of members that meet to form a joint to a mini-
mum.

REFERENCE

1. Richard L. Puthoff : Fabrication and Assembly of the ERDA/NASA 100-

kilowatt Experimental Wind Turbine. NASA TM X-3390, 1976.

CS-73764

A/NASA 100 KV/ EXPERI-
^KDINE

Figure 2. - ORIGINAL HOD-0 TOWER WITH FFRVICF STAIRS AND FOUIPHFHT

ANGLES

DIAGONALS:!?

3V back ^

TO BACK

RAILS

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IGURE 4. - 1/48 SCALE MOHEL (WITH A flODEL STAIR) TONTED IN THE WIND TUNNEL AND WIND

r^'

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