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

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