2.6 The Main Sections of a Wind Tunnel
43
The wind tunnels can be classified according to the flow velocity they achieve.
– Subsonic wind tunnel can operate up to 100 m/s (incompressible flow): suitable
for ground vehicles, low-speed aircraft and drones, aircraft during take-off or
landing phase, civil engineering applications and energy production to quote a few.
There are specialised subsonic wind tunnels dedicated to study severe weather
conditions such as rain, snow, sand ingestion, ice accretion and vertical wind
tunnels for the study of extreme flight manoeuvres. Anechoic wind tunnels are
intended for aeroacoustic studies. There are also wind tunnels dedicated to study
the aerodynamics of athletes and for other sports applications.
– Transonic wind tunnels operate at 0.7 < Mach < 1.3: they are mainly used for civil
transport aircraft, combat aircraft, projectiles, etc. The transonic regime consists of
both a subsonic and supersonic flow, which make them particularly complex. This
regime is commercially sensitive because it is the usual flight regime of commercial
and business aircraft which has the biggest share of the aviation industry. It also
concerns propulsion systems: flow in turbomachines, engine intakes and propulsive
nozzles, etc.
– Supersonic wind tunnel, 1.6 < Mach < 5: nowadays are mainly used for combat
aircraft, missiles, space launchers in the atmospheric flight phase and ammunitions. For commercial applications it was previously used during the design of the
Concorde but renewed interest in supersonic commercial flight might lead to an
increase of their usage.
– Hypersonic wind tunnels cover several flight regimes, 5 < Mach < 10 for
hypersonic vehicles such as those of the X-series, missiles and space launchers;
10 < Mach < 25 highly hypersonic speed where thermal control is the predominant design consideration; Mach > 25, regime of operation of re-entry vehicles for
which an ablative heat shield is required.
There is a clear boundary between the subsonic and supersonic regimes, characterised by the radical change of the flow behaviour when the local speed of sound
is attained and this normally results in the appearance of phenomena such as expansion and compression waves, and shock waves. On the other hand, the demarcation
between supersonic and hypersonic is vaguer, the Mach number being in both cases
supersonic. The truly distinctive character of the hypersonic regime is the heating
of the body due to the high-speed flow where the kinetic energy is transformed
into heat which is more severe around the stagnation region on the body. This heating also affects the chemical behaviour of the air with appearance of so-called real
gas effects resulting from the non-equilibrium between the degrees of freedom of
the molecules (translational, rotational, vibrational) and chemical decomposition at
very high temperature.
A subsonic or supersonic wind tunnel is made of the following parts, from
upstream to downstream.
– The settling chamber where the freestream air is stabilised before entering the
test section. This chamber is equipped with honeycomb cells to straighten the
flow and turbulence screens or meshes (see Fig. 3.7) to reduce the size of the
43
The wind tunnels can be classified according to the flow velocity they achieve.
– Subsonic wind tunnel can operate up to 100 m/s (incompressible flow): suitable
for ground vehicles, low-speed aircraft and drones, aircraft during take-off or
landing phase, civil engineering applications and energy production to quote a few.
There are specialised subsonic wind tunnels dedicated to study severe weather
conditions such as rain, snow, sand ingestion, ice accretion and vertical wind
tunnels for the study of extreme flight manoeuvres. Anechoic wind tunnels are
intended for aeroacoustic studies. There are also wind tunnels dedicated to study
the aerodynamics of athletes and for other sports applications.
– Transonic wind tunnels operate at 0.7 < Mach < 1.3: they are mainly used for civil
transport aircraft, combat aircraft, projectiles, etc. The transonic regime consists of
both a subsonic and supersonic flow, which make them particularly complex. This
regime is commercially sensitive because it is the usual flight regime of commercial
and business aircraft which has the biggest share of the aviation industry. It also
concerns propulsion systems: flow in turbomachines, engine intakes and propulsive
nozzles, etc.
– Supersonic wind tunnel, 1.6 < Mach < 5: nowadays are mainly used for combat
aircraft, missiles, space launchers in the atmospheric flight phase and ammunitions. For commercial applications it was previously used during the design of the
Concorde but renewed interest in supersonic commercial flight might lead to an
increase of their usage.
– Hypersonic wind tunnels cover several flight regimes, 5 < Mach < 10 for
hypersonic vehicles such as those of the X-series, missiles and space launchers;
10 < Mach < 25 highly hypersonic speed where thermal control is the predominant design consideration; Mach > 25, regime of operation of re-entry vehicles for
which an ablative heat shield is required.
There is a clear boundary between the subsonic and supersonic regimes, characterised by the radical change of the flow behaviour when the local speed of sound
is attained and this normally results in the appearance of phenomena such as expansion and compression waves, and shock waves. On the other hand, the demarcation
between supersonic and hypersonic is vaguer, the Mach number being in both cases
supersonic. The truly distinctive character of the hypersonic regime is the heating
of the body due to the high-speed flow where the kinetic energy is transformed
into heat which is more severe around the stagnation region on the body. This heating also affects the chemical behaviour of the air with appearance of so-called real
gas effects resulting from the non-equilibrium between the degrees of freedom of
the molecules (translational, rotational, vibrational) and chemical decomposition at
very high temperature.
A subsonic or supersonic wind tunnel is made of the following parts, from
upstream to downstream.
– The settling chamber where the freestream air is stabilised before entering the
test section. This chamber is equipped with honeycomb cells to straighten the
flow and turbulence screens or meshes (see Fig. 3.7) to reduce the size of the