Using smoke, laser light, model airplane propellers
and a wind tunnel on the Homewood campus, a
team led by Johns Hopkins University researchers is trying
to solve the airflow mysteries that
surround wind turbines, an increasingly popular source of
"green" energy. The National Science
Foundation recently awarded the team a three-year $321,000
grant to support the project.
The rise in oil prices and a growing demand for energy
from nonpolluting sources have led to a
global boom in construction of tall wind turbines that
convert the power of moving air into electricity.
The technology of these devices has improved dramatically
in recent years, making wind energy more
attractive. For example, Denmark is able to produce about
20 percent of its electric energy through
wind turbines. But important questions remain: Could large
wind farms, whipping up the air with
massive whirling blades, alter local weather conditions?
Could changing the arrangement of these
turbines lead to even more efficient power production? The
researchers from Johns Hopkins and
Rensselaer Polytechnic Institute hope their work will help
answer such questions.
"With diameters spanning up to 100 meters across,
these wind turbines are the largest rotating
machines ever built," said research team leader Charles
Meneveau, a turbulence expert in Johns
Hopkins' Whiting School of Engineering. "There's been a
lot of research done on wind turbine blade
aerodynamics, but few people have looked at the way these
machines interact with the turbulent wind
conditions around them. By studying the airflow around
small, scale-model windmills in the lab, we can
develop computer models that tell us more about what's
happening in the atmosphere at full-size wind
farms."
To collect data for such models, Meneveau's team is
conducting experiments in a wind tunnel in
the basement of Maryland Hall. The tunnel uses a large fan
to generate a stream of air moving at
about 40 mph. Before it enters the testing area, the air
passes through an "active grid," a curtain of
perforated plates that rotate randomly and create
turbulence so that air currents in the tunnel more
closely resemble real-life wind conditions. The air
currents then pass through a series of small model
airplane propellers mounted atop posts, mimicking an array
of full-size wind turbines.
The researchers gather information on the interaction
of the air currents and the model
turbines by using a high-tech procedure called stereo
particle image velocimetry. First, they "seed"
the air in the tunnel with a form of smoke — tiny
particles that move with the prevailing airflow. Above
the model turbines, a laser generates two sheetlike pulses
of light in quick succession. A camera
captures the position of particles at the time of each
flash. "When the images are processed, we see
that there are two dots for every particle," said Meneveau,
who is the university's Louis M. Sardella
Professor of Mechanical
Engineering. "Because we know the time difference
between the two laser
shots, we can calculate the velocity. So we get an
instantaneous snapshot of the velocity vector at
each point. Having these vector maps allows us to calculate
how much kinetic energy is flowing from
one place to another, in much greater detail than what was
possible before."
Raul B. Cal, a Johns Hopkins postdoctoral fellow who
is working on the project with Meneveau,
said this data could lead to a better understanding of real
wind-farm conditions. "What happens when
you put these wind turbines too close together, or too far
apart? What if you align them staggered, or
in parallel?" he asked. "All of these are different effects
that we want to be able to comprehend and
quantify, rather than just go out there and build these
massive structures, implementing them and not
knowing what's going to happen."
Meneveau pointed out that dense clusters of wind
turbines could affect nearby temperatures
and humidity levels and cumulatively, perhaps, alter local
weather conditions. Highly accurate computer
models will be needed to unravel the various effects
involved. "Our research will provide the fluid
dynamical data necessary to improve the accuracy of such
computer models," Meneveau said. "We'd
better know what the effects are in order to implement wind
turbine technology in the most
sustainable and efficient fashion possible."
Meneveau and Cal are collaborating with Luciano
Castillo, associate professor in the Department
of Mechanical, Aerospace and Nuclear Engineering at
Rensselaer Polytechnic Institute, and Hyung S.
Kang, an associate research scientist in the Department of
Mechanical Engineering at Johns Hopkins.
The project's funding was provided through the
National Science Foundation's Energy for
Sustainability Program.
An online video about this research can be viewed at
www.jhu.edu/news/audio-video/wind.html.