Set I Short Questions

1. What do you mean by a transonic wind tunnel?

            High subsonic wind tunnels (0.4 < M < 0.75) or transonic wind tunnels (0.75 < M < 1.2) are designed on the same principles as the subsonic wind tunnels. Transonic wind tunnels are able to achieve speeds close to the speeds of sound. The highest speed is reached in the test section. The Mach number is approximately one with combined subsonic and supersonic flow regions.

2. What is 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 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).

3. What is hypersonic wind tunnel?

            A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section. The speeds of these tunnels vary from Mach5 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.

4. What is a blow down wind tunnel?

            Blow down tunnels are normally used from high subsonic to high supersonic flow conditions. The Mach number in the test section is determined by pressure and temperature in the plenum and the area ratio between the test sections on the nozzle throat. As the flow expands in the nozzle, the pressure decreases and any moisture in the tunnel may condense and liquify in the test section.

5. What are in draft wind tunnels?

            Some blow down tunnels, called indraft tunnels, do not use a high pressure chamber, but open the plenum chamber to the atmosphere. The indraft tunnel uses the low pressure (vacuum) chamber downstream of the test section to produce flow. The advantage of this configuration is that the conditions in the plenum remain constant and there is no need for a pressure regulator. The disadvantage is that the pressure ratio across the test section is usually lower than a closed configuration and therefore the maximum Mach number is lower.

6. What are the advantages of Blow down type wind tunnels?

1.      High Mach capability. Easy tunnel "starting".

2.      Lower construction and operating costs.

3.      Superior design for propulsion and smoke visualization. There is no accumulation of exhaust products in an open tunnel.

4.      Smaller loads on model during startup because of faster starts.

7. What is a shock tube?

            The shock tube is a device to produce high speed with high temperatures by traversing normal shock waves that are generated by the rupture of a diaphragm which separates high pressure gas from a low pressure gas.

8. What is a shock tunnel?

            Shock tunnels are wind tunnels that operate at mach numbers of the order 25 or higher for time intervals up to a few milliseconds by using air heated and compressed in a shock tube.

9. What are the disadvantages of Blow down type wind tunnels?

1.      Shorter test times require faster (often more expensive) instrumentation.

2.      Need for pressure regulator valves.

3.      Noisy operation.

10. What are gun tunnels?

            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.