There are many different kinds of wind tunnels; an overview is given in the list below:
Low-speed wind tunnels are used for operations at very low mach number, with speeds in the test section up to 480 km/h (~ 134 m/s, M = 0.4)(Barlow, Rae, Pope; 1999). They may be of open-return type (see figure below), or closed-return flow (see figure below) with air moved by a propulsion system usually consisting of large axial fans that increase the dynamic pressure to overcome the viscous losses.
Fig: Schematic of an open wind tunnel with a closed test section
The working principle is based on the continuity and Bernoulli’s equation:
The continuity equation is given by:
The Bernoulli equation states:
Putting Bernoulli into the continuity equation gives:
The contraction ratio of a wind tunnel can now be calculated by:
Closed wind tunnel:
Fig: Schematic of a closed (return-flow) wind tunnel
In a return-flow wind tunnel the return duct must be properly designed to reduce the pressure losses and to ensure smooth flow in the test section. The compressible flow regime: Again with the continuity law, but now for isentropic flow gives:
The 1-D area-velocity is known as:
The minimal area A where M=1, also known as the sonic throat area is than given for a perfect gas:
High subsonic wind tunnels (0.4 < M < 0.75) and transonic wind tunnels (0.75 < M < 1.2) are designed on the same principles as the subsonic wind tunnels. The highest speed is reached in the test section. The Mach number is approximately 1 with combined subsonic and supersonic flow regions. Testing at transonic speeds presents additional problems, mainly due to the reflection of the shock waves from the walls of the test section (see figure below or enlarge the thumb picture at the right). Therefore, perforated or slotted walls are required to reduce shock reflection from the walls. Since important viscous or inviscid interactions occur (such as shock waves or boundary layer interaction) both Mach and Reynolds number are important and must be properly simulated. Large-scale facilities and/or pressurized or cryogenic wind tunnels are used.
De Laval nozzle:
With a sonic throat, the flow can be accelerated or slowed down. This follows from the 1D area–velocity equation. If acceleration to supersonic flow is required, a convergent-divergent nozzle is required. Otherwise:
Conclusion: The Mach number is controlled by the expansion ratio
Supersonic Wind Tunnel:
Fig: Supersonic Wind tunnel
A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2<M<5) The Mach number and flow are determined by the nozzle geometry. The Reynolds number is varied by changing the density level (pressure in the settling chamber). Therefore a high pressure ratio is required (for a supersonic regime at M=4, this ratio is of the order of 10). Apart from that, condensation of moisture or even gas liquefaction can occur if the static temperature becomes cold enough. This means that a supersonic wind tunnel usually needs a drying or a pre-heating facility. A supersonic wind tunnel has a large power demand, so that most are designed for intermittent instead of continuous operation.
Hypersonic wind tunnel:
A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section. The speed of these tunnels varies from Mach 5 to 15. As with supersonic wind tunnels, these types of tunnels must run intermittently with very high pressure ratios when initializing. Since the temperature drops with the expanding flow, the air inside has the chance of becoming liquefied. For that reason, preheating is particularly critical (the nozzle may require cooling). High pressure and temperature ratios can be produced with a shock tube.
Hot shot wind tunnel:
One form of HWT is known as a Gun Tunnel or hot shot tunnel (up to M=27), which can be used for analysis of flows past ballistic missiles, space vehicles in atmospheric entry, and plasma physics or heat transfer at high temperatures. It runs intermittently, like other high speed tunnels, but has a very low running time (less than a second). The method of operation is based on a high temperature and pressurized gas (air or nitrogen) produced in an arc-chamber, and a near-vacuum in the remaining part of the tunnel. The arc-chamber can reach several MPa, while pressures in the vacuum chamber can be as low as 0.1 Pa. This means that the pressure ratios of these tunnels are in the order of 10 million. Also, the temperatures of the hot gas are up to 5000 K. The arc chamber is mounted in the gun barrel. The high pressure gas is separated by the vacuum by a diaphragm that breaks down as its resistance is exceeded.
Prior to a test run commencing, a membrane separates the compressed air from the gun barrel breech. A rifle (or similar) is used to rupture the membrane. Compressed air rushes into the breech of the gun barrel, forcing a small projectile to accelerate rapidly down the barrel. Although the projectile is prevented from leaving the barrel, the air in front of the projectile emerges at hypersonic velocity into the working section. Naturally the duration of the test is extremely brief, so high speed instrumentation is required to get any meaningful data.