16 Uncertainty Treatment Applications: High-Enthalpy Flow Ground Testing
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16.2.2 Hypersonic Wind Tunnels
Hypersonic wind tunnels represent a large group of facilities that can operate with
high velocity flows ranging from Mach 5–20. A wide variety of solutions exist to
provide a source of gas at sufficiently large pressure and temperature to use it as
a working fluid for a hypersonic wind tunnel. Excellent reviews of the different
types of hypersonic wind tunnels together with a historical perspective are given in
[18, 19]. A detailed analysis of the limiting parameters of hypersonic facilities can
be found in [20]
The conventional design of closed-circuit supersonic wind tunnels can hardly be
extended to reach larger flow enthalpies/velocities. In order to reach the lower range
of hypersonic Mach numbers, numerous wind tunnels use instead the blowdown
principle: the test gas is stored in a reservoir at high pressure and high temperature
and subsequently released through a nozzle for a test duration of few seconds up to
few minutes depending on the amount of gas initially stored and the test conditions.
For Mach numbers larger than 6 or so, the blowdown technique is again rarely
applicable and other ways to achieve hypersonic flows have been sought, leading to
the impulse-type of wind tunnels. Typically, kinetic, thermal, electrical or chemical
energy is stored over a long period of time with a low input power and rapidly
released. In sum, hypersonic wind tunnels allow to produce high-enthalpy flows but
with a reduced run time, of the order of milliseconds.
16.2.2.1 Non-equilibrium Effects
Simulating the features of non-equilibrium post-shock environment in highenthalpy facilities for forebody stagnation regions is an important aspect of testing
in hypersonic wind tunnels. Considering a blunt body at a velocity of 7 km/s, the
temperature immediately after the shock is around 14,000 K, and around 8000 K
downstream the shock, where the flow may return to equilibrium. At such high
temperatures, thermo-chemical effects have a significant influence on the flow.
These remarks remind that the experimental hypersonic simulation for studying
non-equilibrium phenomena requires an accurate understanding of the ground
testing facility operation and a careful interpretation of the experimental data. To
give typical reference for air at a pressure of 1 atm, vibrational excitation begins at
800 K, O 2 begins to dissociate at 2500 K and is fully dissociated for 4000 K, point
for which N 2 begins to dissociate. At 9000 K, N 2 is fully dissociated and ionization
begins. In the aim at reproducing high temperature flow phenomena in ground
testing facilities one need to consider the conservation of mass when taking into
account thermo-chemical reactions that produce and/or destroy different chemical
species.
Different testing techniques are used in these facilities to gather data on several
aspects of the flow around the entry body where non-equilibrium effects can take
place [21]. The estimation of the free stream static pressure, for instance, is normally
515
16.2.2 Hypersonic Wind Tunnels
Hypersonic wind tunnels represent a large group of facilities that can operate with
high velocity flows ranging from Mach 5–20. A wide variety of solutions exist to
provide a source of gas at sufficiently large pressure and temperature to use it as
a working fluid for a hypersonic wind tunnel. Excellent reviews of the different
types of hypersonic wind tunnels together with a historical perspective are given in
[18, 19]. A detailed analysis of the limiting parameters of hypersonic facilities can
be found in [20]
The conventional design of closed-circuit supersonic wind tunnels can hardly be
extended to reach larger flow enthalpies/velocities. In order to reach the lower range
of hypersonic Mach numbers, numerous wind tunnels use instead the blowdown
principle: the test gas is stored in a reservoir at high pressure and high temperature
and subsequently released through a nozzle for a test duration of few seconds up to
few minutes depending on the amount of gas initially stored and the test conditions.
For Mach numbers larger than 6 or so, the blowdown technique is again rarely
applicable and other ways to achieve hypersonic flows have been sought, leading to
the impulse-type of wind tunnels. Typically, kinetic, thermal, electrical or chemical
energy is stored over a long period of time with a low input power and rapidly
released. In sum, hypersonic wind tunnels allow to produce high-enthalpy flows but
with a reduced run time, of the order of milliseconds.
16.2.2.1 Non-equilibrium Effects
Simulating the features of non-equilibrium post-shock environment in highenthalpy facilities for forebody stagnation regions is an important aspect of testing
in hypersonic wind tunnels. Considering a blunt body at a velocity of 7 km/s, the
temperature immediately after the shock is around 14,000 K, and around 8000 K
downstream the shock, where the flow may return to equilibrium. At such high
temperatures, thermo-chemical effects have a significant influence on the flow.
These remarks remind that the experimental hypersonic simulation for studying
non-equilibrium phenomena requires an accurate understanding of the ground
testing facility operation and a careful interpretation of the experimental data. To
give typical reference for air at a pressure of 1 atm, vibrational excitation begins at
800 K, O 2 begins to dissociate at 2500 K and is fully dissociated for 4000 K, point
for which N 2 begins to dissociate. At 9000 K, N 2 is fully dissociated and ionization
begins. In the aim at reproducing high temperature flow phenomena in ground
testing facilities one need to consider the conservation of mass when taking into
account thermo-chemical reactions that produce and/or destroy different chemical
species.
Different testing techniques are used in these facilities to gather data on several
aspects of the flow around the entry body where non-equilibrium effects can take
place [21]. The estimation of the free stream static pressure, for instance, is normally