SwRI engineers design, build and test a prototype wind turbine array

Going Green
SwRI engineers design,
build and test a prototype
wind turbine array
By David L. Ransom, P.E. and J. Jeffrey Moore, Ph.D.
An early move toward renewable,
wind-derived electric power
in the United States was made
in the 1930s when rural power
distribution was too expensive for energy
companies to pursue. Wind turbines of
yesteryear were low-power devices that
provided just enough power for radios
and lights. As part of the movement to
develop more renewable energy sources,
wind-derived energy is once again a topic
of growing interest. Today’s wind turbines
are built at the utility scale. Instead of
300 or 400 watts on the ranch, utility wind
turbines typically generate up to 3 megawatts,
enough to power entire communities.
This significant increase in power
brings significant technical challenges.
Issues such as mechanical reliability,
production costs, and energy storage
and distribution are of particular interest
in today’s push for renewable energy.
Southwest Research Institute has been
involved in this growing research field,
providing third-party engineering design
reviews of machinery reliability, modeling
torsional dynamics of drivetrain
components and testing sub-scale prototypes
of alternative concepts.
A team of SwRI engineers recently
assisted a client in testing an alternative
wind turbine concept that uses an array
of several small wind turbines. In this
concept, a single turbine of 200-meter
diameter can be replaced with an array
of 20 turbines, each having a diameter of
45 meters; thus each design has the same
swept area, the area swept by the revolving
wind turbine blades. The client’s needs
included validating the design concept,
demonstrating power performance equivalent
to that of a single turbine of the same
swept area and developing a validated
computer model for future design
studies. In support of the client’s development
program, SwRI was contracted to
validate the initial concept through experiment
and computer simulation, looking
for any possible blade-wake interactions
between adjacent rotors, which might
serve to either prevent some of the turbines
from reaching full power or lead to
adverse blade dynamic stresses caused by
the flow interaction at the turbine blade
tips. The intent of this study was to validate
the wind turbine array concept and gain
an initial understanding of the anticipated
2
Technology Today • Summer 2009

D016925-2097
David L. Ransom, P.E., (left) is a principal
engineer in SwRI’s Fluids Engineering
Department in the Mechanical and Materials
Engineering Division. Ransom specializes in
rotordynamics and structural dynamics for both
energy and space exploration applications.
Dr. J. Jeffrey Moore is a program manager in
the Fluids Engineering Department. Moore’s
areas of expertise include turbomachinery
rotordynamics and fluid dynamics research for
the natural gas, power generation and wind
power industries.
D017017
design challenges for a full-scale array.
The SwRI team proposed to arrange
multiple commercially available wind
turbines in an array pattern and perform
tests to evaluate the design for
performance as well as mechanical
integrity. SwRI researchers also used
computational fluid dynamics to study
the possible blade-wake or tip-vortex
interactions between adjacent rotors.
Wind tunnel testing
As a result of the initial evaluation,
the SwRI team designed, built and tested
a turbine array prototype to verify key
performance characteristics and to support
future design work
on larger turbine arrays.
The array consisted of
seven 400-watt, commercially
available wind
turbines constructed on
a 25-foot-tall platform.
The center turbine of the
array was erected at a
height of 15 feet to meet
the centerline of the
wind tunnel at NASA’s
Langley Research Center
in Hampton, Va. The
Langley Full Scale Tunnel
(LFST) is a National Historical
Landmark significant
to the advancement
of aeronautical research
dating from 1931 and
continuing today through model tests
of all current front-line U.S. jet fighter
aircraft. The tunnel is 50 feet in length, 60
feet wide and 30 feet high.
Performance test results
Researchers added instrumentation
to each turbine in SwRI’s seven-rotor array
to provide data on center turbine power,
blade strain and turbine speed. Several
flow visualization features were included
in the experimental rig to better understand
potential turbine interactions.
The seven-rotor array was tested in
a variety of conditions, including varying
the number of active turbines, spacing
between active turbines, wind speed and
The wind turbine array was evaluated in the Langley Full Scale Tunnel,
at NASA Langley Research Center, Hampton, Va. The center turbine is
15 feet from the tunnel floor. The wind tunnel is 30 feet high.
D017014
Technology Today • Summer 2009 3

D017028
Spacing study results for 9 meters per second wind
speed indicate no measurable difference in power
performance of the middle turbine over the full range of
spacing conditions.
array yaw relative to wind direction. The
results of all conditions were compared
to the baseline performance results of a
single turbine. The repeatability of the
results was found to be around 4 percent.
This demonstrates that even across a
wide range of test article configurations,
the maximum possible impact to turbine
performance is less than 4 percent, suggesting
that performance from operation
in an array configuration does not significantly
affect performance.
The SwRI team conducted a study in
which the outer six turbines were initially
spaced at 2 percent of the diameter relative
to the center turbine. Spacing was
then incrementally increased to a maximum
of 16 percent. Spacing study results
indicated that the influence of the neighboring
rotors in the full seven-rotor array
was not detectable within the repeatability
of the test results. This indicated that
any possible effects of rotor interaction
on array performance were limited to less
than 4 percent when compared to singlerotor
performance. Tests run in the yaw
condition did show a distinct drop in performance
as expected, but the spacing
influence in yaw was still negligible.
Flow visualization
Using a
smoke rake integrated
with the
LFST traversing
mechanism, SwRI
engineers injected
smoke streams
into the flow for
overall streamline
visualization. Initial
results showed
periodic fluctuations in the smoke streams
corresponding to the vortex shedding frequency
of the primary rake structure. This
is an unsteady flow phenomenon in
which vortices form behind an obstruction
in the flow (the rake structure)
and periodically detach, creating disturbances
in the smoke flow. Because
of this, additional smoke testing was
performed by rotating the rake so that
the outflow ports of the tines were not
directly downstream of this wake.
The team attempted several variations
in the smoke rake arrangements,
such as spacing and restricting the number
of tubes to obtain clearer images,
but inherent turbulence in the flow
made it difficult to generate a clear
image of the flow field surrounding the
A smoke generator and smoke rake were
installed in the Langley Full Scale Tunnel.
The traversing mechanism permitted remote
control of rake location to help visualize flow
at different locations on the array.
D016580
4
Technology Today • Summer 2009

D017011
Smoke streams illustrate periodic vortex shedding
from the smoke rake wake on the smoke
streams. The influence of vortex shedding is
minimized by rotating the rake.
turbines. In still images and video, no
clear structure of the flow field was evident
because of the turbines. Standard
and slow-motion video with stroboscopic
illumination synchronized to the
rotation of the center turbine showed
the flow field passing through the turbine
planes with no apparent effect.
This was evidenced by the fact that the
strobe light could not “freeze” images of
smoke patterns. Instead, the smoke continually
moved in all recorded images.
SwRI researchers used a mixture of
titanium dioxide (TiO 2
), kerosene and
oleic acid to record flow patterns on
the rotating blades. The mixture was
painted onto the front and back sides
of one of the center turbine blades, and
the tunnel was subsequently brought
up to operating condition. This allowed
the TiO 2
mixture to travel on the surface
under the aerodynamic forces while the
kerosene evaporated, leaving a trace of
the surface flow patterns.
The SwRI team installed a grid
of retro-reflective tufts on the array
structure downstream of the turbine
blade plane to
examine flow
structure in the
wake of the turbines.
During
operation, the
tufts reacted to
the local flow
field. Photographs
of the
tuft array showed the flow structure at
the tuft grid for the two-rotor case when
the center and upper rotors are separated
by 2 percent. Capturing images in
this configuration was difficult because
of the added wake of the photographer
and camera when filming from upstream
of the turbine array. Nonetheless, the
tufts showed some angularity in the wake
of the two active turbines and were very
well aligned with the mean flow direction
everywhere else. A more straight-on
view might have shown the dividing line
between the swirled flow at the bottom
of the top turbine (swirled flow to the
left) and the top of the middle turbine
(swirled flow to the right).
D017013
Strain gage measurements
Researchers installed strain gages
on a single blade of the array’s center
turbine. The gages were placed at three
locations: outboard, midboard and inboard
from the tip and data was transmitted
through a telemetry system. Measured
root mean square (RMS) strain distribution
at the 12 m/sec wind speed shows
typical cantilever beam shape with peak
strain at the inboard (IB) location and
minimum strain at the outboard (OB)
location. This is an indication of the blade
load distribution, which would demonstrate
more variation between various
Technology Today • Summer 2009 5

This photo shows grid tuft visualization for a
two-rotor configuration at 2 percent turbine
array spacing.
D017012
spacing configurations if there was a
real influence on turbine performance.
These results show no clear difference
in the RMS strain distribution, even with
closely spaced turbines.
Researchers compared data from
the frequency spectrum of the measured
dynamic strain between the two-rotor
data at 2 percent spacing and the full
array data at 2 percent spacing, all at 9
meters per second (m/sec) wind speed.
The synchronous response was elevated
for the two closely spaced rotor tests
compared to the single rotor results. This
indicates some amount of blade wake
interaction, despite the inability to detect
interaction with either performance measurement
or flow visualization.
Computational fluid dynamics (CFD)
simulation
To support future design studies,
the research team developed and analyzed
a CFD model of the test rotor for
D017015
various
wind speed
and turbine
spacing conditions.
With a
CFD simulation
that was validated
by experimental data,
SwRI engineers gained
more insight into the
structure of the flow
through the turbine array.
Similar to the wind tunnel tests,
a parametric spacing study was
performed in CFD simulation with
similar results. The CFD-predicted
power performance of the turbines
was not strongly influenced by the
array configuration. However, it was
clear that there was some interaction
between neighboring turbines, such as a
reduction in tangential velocity between
two co-rotating turbines.
The center turbine blade was instrumented with six strain gages using
a telemetry system to transmit data to the data acquisition system.
The freestream wind flowing into
the rotor blade usually generates a wake
that develops downstream of the blade
because of rotation. The wake can be
divided into the central vortex
generated by the hub and the
tip vortex developed from
the blade tip. The flow
field results in an
unusually complex
three-dimensional
vortex structure
that was captured accurately
with the
CFD simulation
for an array of
two co-rotating
turbines. The interaction
between the vortex structure
produced by two co-rotating turbines is
quantified and power performance for the
turbine array is predicted.
6
Technology Today • Summer 2009

The graph shows measured root mean square (RMS) strain
distribution along the turbine blade. Measured
results show no clear difference in RMS strain for varying
turbine array spacing.
Acknowledgments
The authors would like to acknowledge the contributions
of the other team members who made
this project a success: Program Manager Leigh
Griffith, Staff Scientist Dr. Jerome Helffrich, Research
Engineer Dr. Vishy Iyengar, Research Engineer
Jonathan Moody, Principal Technician Terry
Schiebel, Engineer Melissa Wilcox and
former SwRI staff member Dr. Daniel Scharpf.
D017029
Future studies
Based on the results of
this combined experimental
and analytical development
program, SwRI researchers
drew several conclusions
regarding the operation of
wind turbines in a closely
spaced array.
Any possible effects of rotor interaction
on array performance were limited
to less than 4 percent when compared to
single-rotor performance. Based on the
single session repeatability, it was more
likely that the influence of adjacent rotors
was less than one percent for nearoptimum
tip speed ratios. Operation in a
yawed orientation did not impact the array
any more than it did a single rotor. Computational
fluid dynamics tools can be
used to model the multi-rotor concept,
which can be very important in evaluating
future wind turbine designs.
The results of this activity were
encouraging, and the experimental program
provided a large amount of data
to support future wind turbine array
development work. The performance
data, flow visualization results and blade
strain data can all be used to validate
simulation tools as required in future
design cycles. The progress made on the
CFD modeling provided the knowledge
necessary to more accurately model
future designs with the ability to validate
new ideas and techniques against existing
experimental results. The effort
expended in the current phase will continue
to pay dividends as the wind turbine
array concept matures and reaches
field operation. v
Questions about this article
Contact Ransom at (210) 522-5281 or
david.ransom@swri.org or Moore at
(210) 522-5812 or jeff.moore@swri.org.
Computational fluid dynamic vector plot results demonstrate reduced tangential
velocity at the interface between two turbines. CFD simulation predicts only slight
influence of adjacent turbines on array performance.
D017018
Technology Today • Summer 2009 7