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6 Hypersonic Wind Tunnels
High Mach numbers are not the only specification of hypersonic flows encountered in space applications. Indeed, when a vehicle enters the atmosphere at a speed
of several kilometres per second, the friction from flow on the body leads to temperatures of several thousand degrees (adiabatic compression). This results in considerable wall heat fluxes and the triggering of chemical reactions within the air
due to non-equilibrium between internal energies, dissociation of molecules and
chemical reactions between molecules. The temperature is a measure of the energies
of the translational, vibrational and rotational motions of the constituents of a gas:
molecules, atoms and subatomic particles. In the so-called equilibrium state, energy
is distributed evenly among these various motions by energy exchanges. When the
gas is subjected to a rapid variation in conditions (expansion in a hypersonic nozzle,
or compression by a shock wave) the gas adjusts more or less quickly to the new
conditions: the translation occurs almost instantaneously, as well as the vibration but
the rotation being much slower. The gas is then said to be in non-equilibrium and no
longer corresponds to the model of the perfect gas: these effects are the so-called real
gas effects whose behaviour deviates from that of the perfect gas. A true hypersonic
wind tunnel must therefore reproduce not only large Mach numbers, but also very
high temperature levels; it is then called a hot hypersonic or hyper-enthalpic wind
tunnel.
6.2 Hypersonic “Cold” Wind Tunnels
Although the actual phenomena of hypersonic flight are not fully simulated, these
wind tunnels are a valuable tool for studying the aerodynamics at high Mach numbers.
In the most common case, and for generating stagnation temperatures not exceeding
1000 K, the heating system is made of a heater and a heat exchanger made of metal
plates or spheres. Before the test, the exchanger is heated by circulation of electrically
heated air, this rather long phase requiring only moderate power. In another technique,
the flow passes through a bed of alumina beads heated by a secondary stream of high
temperature air generated by the combustion of propane. In an instantaneous mode of
heating, the test gas passes into tubes heated by Joule effect, at the expense of higher
electrical power. The advantage of the latter method lies in attaining temperatures of
up to about 1200 K and the absence of dust generated in heat exchangers.
These facilities are of the long run type (from 10 s to several minutes) operating
on a reservoir of compressed air. The R1Ch, R2Ch and R3Ch wind tunnels at
ONERA Meudon centre, for instance operate on a reservoir at 250 bar and a vacuum
sphere of 500 m
3 shared between the three facilities. For the R1Ch and R2Ch wind
tunnels, before expansion the air in the nozzle is heated by passing through a heat
exchanger that can attain a temperature of 700 K at a flow rate of 40 kg/s. An auxiliary
high-pressure air supply (200 bar) is used to simulate propulsive jets. R1Ch and R2Ch
wind tunnels cover the range of supersonic to hypersonic Mach numbers, between
3 and 7 and unit Reynolds numbers ranging between 2.1 × 10
6 /m and 5.1 × 10
6 /m
using variations in the stagnation pressure from 0.5 to 80 bar. These wind tunnels use
6 Hypersonic Wind Tunnels
High Mach numbers are not the only specification of hypersonic flows encountered in space applications. Indeed, when a vehicle enters the atmosphere at a speed
of several kilometres per second, the friction from flow on the body leads to temperatures of several thousand degrees (adiabatic compression). This results in considerable wall heat fluxes and the triggering of chemical reactions within the air
due to non-equilibrium between internal energies, dissociation of molecules and
chemical reactions between molecules. The temperature is a measure of the energies
of the translational, vibrational and rotational motions of the constituents of a gas:
molecules, atoms and subatomic particles. In the so-called equilibrium state, energy
is distributed evenly among these various motions by energy exchanges. When the
gas is subjected to a rapid variation in conditions (expansion in a hypersonic nozzle,
or compression by a shock wave) the gas adjusts more or less quickly to the new
conditions: the translation occurs almost instantaneously, as well as the vibration but
the rotation being much slower. The gas is then said to be in non-equilibrium and no
longer corresponds to the model of the perfect gas: these effects are the so-called real
gas effects whose behaviour deviates from that of the perfect gas. A true hypersonic
wind tunnel must therefore reproduce not only large Mach numbers, but also very
high temperature levels; it is then called a hot hypersonic or hyper-enthalpic wind
tunnel.
6.2 Hypersonic “Cold” Wind Tunnels
Although the actual phenomena of hypersonic flight are not fully simulated, these
wind tunnels are a valuable tool for studying the aerodynamics at high Mach numbers.
In the most common case, and for generating stagnation temperatures not exceeding
1000 K, the heating system is made of a heater and a heat exchanger made of metal
plates or spheres. Before the test, the exchanger is heated by circulation of electrically
heated air, this rather long phase requiring only moderate power. In another technique,
the flow passes through a bed of alumina beads heated by a secondary stream of high
temperature air generated by the combustion of propane. In an instantaneous mode of
heating, the test gas passes into tubes heated by Joule effect, at the expense of higher
electrical power. The advantage of the latter method lies in attaining temperatures of
up to about 1200 K and the absence of dust generated in heat exchangers.
These facilities are of the long run type (from 10 s to several minutes) operating
on a reservoir of compressed air. The R1Ch, R2Ch and R3Ch wind tunnels at
ONERA Meudon centre, for instance operate on a reservoir at 250 bar and a vacuum
sphere of 500 m
3 shared between the three facilities. For the R1Ch and R2Ch wind
tunnels, before expansion the air in the nozzle is heated by passing through a heat
exchanger that can attain a temperature of 700 K at a flow rate of 40 kg/s. An auxiliary
high-pressure air supply (200 bar) is used to simulate propulsive jets. R1Ch and R2Ch
wind tunnels cover the range of supersonic to hypersonic Mach numbers, between
3 and 7 and unit Reynolds numbers ranging between 2.1 × 10
6 /m and 5.1 × 10
6 /m
using variations in the stagnation pressure from 0.5 to 80 bar. These wind tunnels use