PRODUCTS AND SERVICES
dynamic stability assessment
in a high speed wind tunnel
The Clean Space initiatives provide a challenge
for engineers at INCAS, Romania’s National
Institute for Aerospace Research “Elie Carafoli”,
in Bucharest
INCAS operates a 1.2 x 1.2m (4 x 4ft)
supersonic wind tunnel. The tunnel is a
blowdown type, with a high level of
performance and a good flow quality.
Supporting the operation of the wind tunnel
is a team of experts who have carried out
nearly 150 test programs and over 9,300 test
runs for national and international clients
over the past 42 years.
The Clean Space initiatives require the
aerothermal characterization of reentry
debris, especially those surviving the reentry
process. Some of the debris, such as titanium
fuel tanks are recovered in a good condition.
First stage elements also come back on Earth
in good condition because the breakup
occurs at less intensive flow regimes. The
footprint of the falling debris is of importance
and the main challenge is to have good
aerodynamic assessment. When performing
6DOF dynamics, it is important not only to
know the forces coefficients, but also the
damping ones, typically determined in the
wind tunnel.
Dynamic stability assessment in high
speed wind tunnels requires very well
designed/characterized instruments and
their numerical procedures. The free pitching
movement and the damping
coefficient are highly
dependent on the initial
conditions. Forced
oscillation rigs provide good
results for low amplitude
motions, namely for quite
stable shapes. For shapes
that could be unstable or
bi-stable such as reentry capsules, complete
movement rigs can provide more
information. This is the case for space debris
like interstage rings, cone or cylinder shaped.
A free to tumble rig can produce a lot of
information, while being intrusive with the
flow field by the presence of the rod creates a
detached shock and has a significant impact
on the leeside flow, as well as on the lateral
model surfaces. The free to tumble procedure
can be particularly useful for flow-through
models, such as the interstage segments of
the space launchers. The back-flow
interaction with the pitching rod produced
flow structures is very much reduced versus
flow opaque models like reentry capsules/
bodies. An uncertainty factor is the startup
procedure of the high speed wind tunnel, that
in case of blow down facilities induces a level
104 SEPTEMBER 2019 \\ AEROSPACETESTINGINTERNATIONAL.COM
of random motion. This may be considered a
positive factor bringing robustness and a
larger range of results compared to the forced
oscillations rig.
Besides the movement recording, flow
visualization is only possible with the
Schlieren system in the supersonic regime
due to some strength restrictions. Two
additional standard cameras provide top and
side views in real time.
Considering the simplicity of the free to
tumble rigs it is convenient to produce them
customized for almost all models to be
tested, although a high-quality potentiometer
is installed, requiring the carrying out of a
basic calibration procedure.
A wind tunnel campaign has been
dedicated to assess the damping of a
troncone interstage ring, covering regimes
from transonic (Mach 1.1) to supersonic (Mach
3.5). Although stability is expected to be
weaker in the transonic regime, it was found
to be very strong for this shape, as well as for
supersonic regime. Another interesting point
is that in transonic regimes, the shape
stabilizes at an angle of attack of about 15˚,
regardless of the initial conditions, as was
revealed by repeating the tests. At Mach 2.5
and 3.5 stability is significant and the vehicle
stops oscillations at AoA=0˚. \\
FREE READER INQUIRY SERVICE
incas
1 // Free to tumble rig with
troncone interstage ring
shape
2 // Schlieren visualization,
Mach 2.5, shape stabilized
at AoA=0 deg
3 // Schlieren visualization,
Mach 3.5, shape stabilized
at AoA=0 deg
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