Copyright © A. Filippone (1999-2003). All Rights Reserved.

Wind Tunnels


Low-Speed Wind Tunnels

Low Speed wind tunnels are used for operations at very low Mach number, with speeds in the test section up to 400 Km/h (M=0.3). They are both of open-return type (Fig. 2), or return-flow (Fig. 3). The air is moved with a propulsion system made of a large axial fan that increases the dynamic pressure to overcome the viscous losses.

Open Return Wind Tunnel

Figure 2: open-return wind tunnel (Eiffel type)

Closed Return Wind Tunnel

Figure 3: closed-return low speed wind tunnel

In a return-flow wind tunnel the return duct must be properly designed to reduce the pressure losses and to insure smooth flow in the test section.

Transonic Wind Tunnels

Transonic wind tunnels are able to achieve speeds close to the speed of sound. The highest speed is reached in the test section. Testing at transonic speeds presents additional problems, mainly due to the reflection of the shock waves from the walls of the test section.

Supersonic Wind Tunnels

The first problem with a supersonic wind tunnel is to produce supersonic speeds (Mach numbers up to 5). This can be achieved with an appropriate design of a convergent-divergent nozzle.

When the sonic speed is reached in the test section, the flow accelerates in a nozzle slower, than it expands.

The final speed is determined by the ratio between the areas in the outlet section and the throat. For a supersonic speed of M=5, this ratio is of the order of 30.

Power Requirements

The power required to run such a wind tunnel is enormous (up to 50 MW per square meter of test section.) For this reason most tunnels operate intermittently using energy stored in high-pressure tanks (intermittent supersonic blowdown) as shown in Fig. 4, or vacuum storage tanks (indraft supersonic wind tunnels).

Other problems include high pressure ratio at the start, enough supply of dry air, wall interference effects, and the need to use fast instrumentation for intermittent measurements.

Supersonic Blow Down Wind Tunnel

Figure 4: supersonic blowdown wind tunnel

Hypersonic Wind Tunnels

For hypersonic speeds it is intended Mach numbers between 5 and 15. As for the supersonic wind tunnel, these type of tunnels must run intermittently with very high pressure ratios at the start.

Since the temperature drop with the expanding flow is so high that the air might undergo liquefaction, a pre-heating is necessary (while the nozzle may require cooling). High pressure and temperature ratios can be produced with a shock tube.

Technological Problems

There are several technological problems is designing and constructing a hyper-velocity wind tunnel: supply of high temperatures and pressures for times long enough to perform a measurement, reproduction of equilibrium conditions, structural damage produced by over-heating, fast instrumentation, power requirements to run the tunnel, etc.

Simulation of a flow at 5.5 km/s, 45 km altitude would require tunnel temperatures of 9000 K, and a pressure of 3 GPa. For a review of hypersonic wind tunnels see Owen, 1989.

Hotshot Wind Tunnel

Hotshot tunnels are designed to operate at the highest speeds (up to M=27), for analysis of flows past ballistic missiles, space vehicles in atmospheric entry and plasma physics, heat transfer at high temperatures. It runs intermittently, like other high speed tunnels, but it has a very low running time (one second or less).

The mechanism of operation is based on a high temperature/ pressure gas (air or nitrogen) produced in an arc-chamber, and a near-vacuum in the remaining part of the tunnel.

Pressure in the arc-chamber can reach several MPa, while pressures in the vacuum chamber can be as low as 0.1 Pa (pressure ratios of the order of 10 million); temperatures of the hot gas are up 5,000 K.

The high pressure gas is separated by the vacuum chamber by a diaphragm that breaks down as its resistance is exceeded.

Pressurized Wind Tunnels

In a pressurized wind tunnel experiments can be performed at flow densities different (generally higher) from the atmospheric pressure (the invention of the variable density wind tunnel is attributed to M. Munk).

A model on scale 1:4 should be tested at four times the operational speed in a atmospheric wind tunnel. By increasing the density to four times the atmospheric pressure keeps the Reynolds number constant at the operational speed. This type of tunnel has its own peculiar problems.

Meteorological Wind Tunnels

These are tunnels used to study effects on suspension bridges, high-rise buildings, towers, dispersals of pollutants from factories, etc. They often require a special sizing, but they are characterized by the long test sections.

Unlike most wind tunnels, it is important to simulate the effects of the boundary layers (atmospheric boundary layer). The test section is generally very large. Testing of elastic structures (ex. bridges) requires lengthy preparation of the model.

Tunnels with Moving Ground

Wind tunnels with moving ground are used by the automobile industry, by racing cars teams, by the truck industry and for high-speed trains. A moving belt is used to simulate the road conditions.

The belt moves with autonomous motor synchronized with the tunnel speed. A boundary layer removal system is generally present upstream the test section.

Simulation of a Road Vehicle

Simulating a road vehicle in a wind tunnel presents peculiar problems related to the presence of the ground boundary layer, the ground effect, and the rotating wheels.

There are several approximations of the road conditions, some of which are sketched in Fig. 5. The most accurate approach consists in having a moving belt on the wind tunnel floor (Other methods include vertical suction, tangential blowing, boundary layer removal ahead of the model, etc.)

Car in Wind Tunnel

Figure 5: Car in a wind tunnel

Moving Belt

The moving belt approach is mandatory for vehicles with very low ground clearance (Formula 1 racing cars) or low drag coefficients (sports cars).

The moving belt is generally inadequate to support the vehicle in the test section. The model is therefore suspended to a vertical strut, streamlined to reduce the aerodynamic interference, although the wheels are in contact with the belt and are driven by it.

Fig. 6 below shows the requirements for moving belt or blowing jet, as a function of the maximum CL and the trailing-edge ground clearance of a high-lift system (data elaborated from NASA Ames studies).

Moving Belt Limits

Figure 6: Moving Belts Limits

Effect of Rotating Wheels

Having rotating or non-rotating wheels in the test facility makes a considerable difference in the resulting drag coefficient. This is of the order of 0.02 on an ordinary passenger car, much larger for open-wheel vehicles. Considerable experience in this field has been gained by racing cars teams (Formula 1, Indy CART and others).

Measuring aerodynamic lift and drag is quite elaborate, because of the struts interference, and because of the rolling resistance of the tires, which depend on the vertical load.

Tunnels for Aerodynamic Noise

Aerodynamic noise can be measured only if there is a large difference in sound pressure between the noise produced by the model and the background noise of the tunnel and the external environment.

Description of Some Wind Tunnels

Here are some examples:
  • The DNW German-Dutch tunnel in the Netherlands. It is an atmospheric low speed wind tunnel, with a test section of 8m x 6m, which allows investigations on relatively large models, wings with up to 5 m span, full aircraft configurations with under-carriage, control surfaces, etc. There is a possibility to use a moving ground facility.

  • The NASA Langley 30 ft x 60 ft tunnel, Langley Field, Virginia

  • The Pressurized wind tunnel F1 at ONERA, Le Fauga- Mauzac, France, with a test section 11m x 4.5m x 3.5m (length, width, height). The powerplant is a 16 blade variable-pitch, constant-speed fan with 9.5 Mw motor. The maximum pressure is estimated a 3.85 bar. The tunnel is able to create realistic flight Reynolds and Mach numbers.

Selected References

  • Baals DD, Corliss WR. Wind Tunnels of NASA, NASA SP-440, 1981.

  • Owen FK. Measurements of Hypersonic Flowfields, in Special Course of Aerothermodynamics of Hypersonic Vehicles, in AGARD R-761, June 1989.

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Copyright © A. Filippone (1999-2003). All Rights Reserved.