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.

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.

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.

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.

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.

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.)

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).

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).

• 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.