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| Courtesy of the Università degli
Studi di Napoli, Italy |
Researchers at the Dipartimento di Scienza e Ingegneria dello Spazio
(DISIS) at the Università degli Studi di Napoli in Italy have recently
used FLUENT software to simulate the entry of a planetary module into
the atmosphere surrounding Mars. The recent interest in Mars missions
has encouraged engineers to use shock tunnels, plasmatrons, or high enthalpy
arc facilities to carry out research studies on Earth in similar conditions.
The Small Planetary Entry Simulator (SPES) located at DISIS has been improved
and upgraded for this purpose. It can now simulate the relevant thermal
and fluid dynamic conditions over planetary entry vehicles. The facility
is a 40 KW blow-down plasma wind tunnel able to reproduce the temperatures
and gas mixtures of different compositions that are characteristic of
planetary entry conditions. In particular, mixtures composed of carbon,
oxygen, and nitrogen in a variety of molecular forms can be used to simulate
the Martian atmosphere with specific enthalpies close to free flight values.
Temperatures in the SPES can reach 15,000 K and densities are on the order
of 0.005 kg/m3.
Temperature distribution around the Mars Pathfinder during
entry
at 6 km/sec
Through comparison with experimental data, FLUENT has been validated
for its prediction of nonequilibrium effects that occur at high temperatures
in hypersonic wind tunnels like the SPES. In normal free stream conditions,
the chemical composition of the Martian atmosphere consists primarily
of CO2 and N 2. For objects travelling at high speeds,
however, high temperatures found behind the shock wave in front of the
body cause dissociation and recombination, which lead to the formation
of six additional species. This phenomenon also happens when spacecrafts
reenter the Earth's environment. The Earth's atmosphere is primarily composed
of O2 and N2 , but during reentry, high temperatures
cause reactions to occur, which result in the formation of several other
molecules as well. In the FLUENT model of the SPES / Martian atmosphere,
eight species (CO 2 , CO, C, O, O 2 , N 2
, N, NO) are used with fourteen reactions to simulate this complex environment.
Most plasma wind tunnel facilities are not able to reproduce free flight
conditions to scale because it is not possible to duplicate all of the
dimensionless parameters at once (Mach number, Reynolds number, and Damköhler
numbers, for example). When wind tunnel tests are performed, the engineers
are usually interested in one specific feature of the flow. For example,
they might want to reproduce the same specific total enthalpy and the
same heat flux to test the thermal protection systems on the craft exterior.
Even so, differences still exist between free flight and wind tunnel conditions
operated at the same total enthalpy due to nonequilibrium effects. If
a CFD code can properly simulate these effects on the laboratory scale,
it can be extrapolated for use in modeling the real free flight conditions
encountered during atmospheric entry.
Species compositions along the stagnation line of the
Mars Pathfinder
(click image for enlarged view)
At high temperatures, the probe surface can act like a catalyst, favoring
the recombination of atoms. The boundary conditions in the FLUENT model
reflect experimental measurements on the catalytic properties of different
surface materials. This has allowed the researchers to respond to one
of the main objectives of the study, which was to evaluate the stagnation
point heat fluxes over probes built from materials with different catalytic
properties. Results suggest that the surface catalycity has a strong influence
on the near-body reactions in a Martian-like atmosphere.
Temperature and streamlines around a capsule model in
the SPES test chamber in a Martian-like atmosphere
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