October 2000 Newsletter - Naval Postgraduate School

FEATURED PROJECT
MICRO-AIR VEHICLE AERODYNAMICS, continued from page 8
and computational studies of flapping-wing propulsion. It is
the purpose of this article to provide a review of the current
status of our work. We start by first describing the two test
facilities which are used for the experimental studies, namely a
low-speed wind tunnel and a water tunnel.
The Aeronautics Low-Speed (Smoke) Tunnel
The low-speed wind tunnel, sometimes referred to as the
smoke tunnel, is a continuous, flow-through facility, with a
five-foot by five-foot test section. The tunnel, depicted in
Figure 1, has a speed range from zero to about 10 meters/
second (~22.5 mph). The relatively low speed makes it
particularly suitable for flow visualization investigation where
smoke, fog or other visualization techniques are required, and
this is also the speed range of interest for micro-air vehicles.
The tunnel is driven by a constant-speed motor with a
variable pitch fan, changing the pitch of the fan to control the
wind-speed.
The tunnel is equipped with a positionable pitot-static tube
and several high-accuracy differential pressure transducers for
wind-speed measurement, as well as a 2-component Laser-
Doppler Velocimetry (LDV) probe for measuring unsteady,
local flow speeds with high accuracy. LDV is a non-intrusive
method of measuring fluid velocity by optical means. Our
system uses a water-cooled 5 Watt Coherent Innova 70C CW
Argon-Ion Laser for a light source. The Coherent laser emits
Figure 1. Schematic of the low-speed wind tunnel.
several discrete colors (green - 514.5nm, blue - 488nm and
purple 476nm), and a TSI Model 9201 ColorBurst
multicolor beam separator is used to split the beams and
frequency-shift one beam in each color. The green and blue
beams, and their frequency shifted counterparts, are passed to
a 2-component, 4-beam TSI Model 9832 probe through a
fiber-optic cord. Probe beam spacing is 50mm, and lenses
with focal lengths of 350mm and 750mm are available.
Several methods are available for flow visualization. A Roscoe
4500 fog machine is attached to a smoke rake which is placed
outside the tunnel intake, and this provides sufficient flowseeding
for the LDV measurements as well as flow visualization
at higher tunnel speeds. A smoke-wire may be used to
generate a sheet of smoke inside the test-section, a useful
technique at lower tunnel speeds, as shown in Figure 3.
Additionally, a laser light-sheet may be used with the Coherent
laser, providing a bright, two-dimensional slice through
the seeded flowfield.
The Eidetics Water Tunnel
We also have an Eidetics water tunnel, shown schematically
in Figure 2, an ideal tool for flow visualization. The tunnel
has a speed range between 0 and about 0.5 m/s, with a usable
test section that is about 38cm square. It is a continuous flow
tunnel with a 6:1 contraction ratio.
Flow visualization is performed by injecting dye into the
water. Multiple colors of dye can be used to identify different
--continued on page 42
Figure 2. Schematic of the Eidetics water tunnel.
NPS Research page 9
October 2000