Supersonic wind tunnel
A supersonic wind tunnel is a wind tunnel in which objects can be in flow velocities of Mach numbers from 1.2 to 5 investigate experimentally. The Mach number and the flow is determined by the nozzle geometry, the Reynolds number is defined by varying the density (depending on the pressure upstream of the nozzle). A high pressure ratio between the inlet and outlet is required for operation ( to operate at Mach 4, a pressure ratio of about 10 is necessary). The cooling of the gas in the acceleration through the nozzle to condensation, which causes malfunction. Therefore, supersonic wind tunnels are often equipped with air drying or preheating. Because of their very high energy demand (see below) they are not normally driven in continuous operation.
Limiting factors for the interpretation
Minimum necessary pressure ratio
An optimistic estimate is that the pressure ratio in the wind tunnel is smaller than the total pressure ratio of a normal shock wave with the same Mach number:
Temperature in the measuring section and condensation
The temperature after the nozzle is calculated as
Where the isentropic exponent is. Example: = 330K: 70K = at = 4
The sensible achievable Mach number is thus determined by the temperature in the pressurized gas reservoir.
Energy demand
The energy requirements of a supersonic wind tunnel is enormous, for example in the order of 50 MW per square meter test area. Therefore, most channels are supplied with gas from high-pressure reservoirs, which, however, only a limited test period allowed. This design is in English called "intermittent blowdown wind tunnels supersonic " ( see illustration). Another possibility to increase the pressure ratio, is the use of the vacuum tank Windkanalauslass; this is called " indraft supersonic wind tunnels ." The following factors may be generally viewed as critical:
- An adequate supply of dry compressed air
- Interference with the flow of the wind tunnel walls
- Sufficiently fast measurement instruments for short experiment times
Have wind tunnels such as the Ludwieg - tube short experiment times (usually less than one second), a relatively high Reynolds number, however, low energy consumption, since it does not run continuously.
Swell
- Pope, A.; Goin, K.: High-speed wind tunnel testing. Krieger, 1978, ISBN 088275727X.