Supersonic wind tunnel

Engineers review a model airplane before a test run in a supersonic wind tunnel ( Lewis Flight Propulsion Laboratory ).

A supersonic wind tunnel is a wind tunnel in which objects can be examined experimentally at flow speeds of Mach numbers between 1.2 and 5. The Mach number and the flow are determined by the nozzle geometry, the Reynolds number is determined by varying the density (depending on the pressure in front of the nozzle). A high pressure ratio between inlet and outlet is necessary for operation (a pressure ratio of around 10 is necessary for operation at Mach 4). The cooling of the gas as it accelerates through the nozzle can cause condensation , which causes malfunctions. Therefore, supersonic wind tunnels are often equipped with an air drying or preheating device. Because of their very high energy requirements (see below), they are normally not used continuously.

Limiting factors for the design

Minimal necessary pressure ratio

An optimistic estimate is that the pressure ratio in the wind tunnel is smaller than the total pressure ratio of a vertical shock wave with the same Mach number:

${\ displaystyle {\ frac {P_ {t}} {P_ {amb}}} \ leq \ left ({\ frac {P_ {t_ {1}}} {P_ {t_ {2}}}} \ right) _ {M_ {1} = M_ {m}}}$

Temperature in the measuring section and condensation

The temperature after the nozzle is calculated

${\ displaystyle {\ frac {T_ {m}} {T_ {t}}} = \ left (1 + {\ frac {\ kappa -1} {2}} M_ {m} ^ {2} \ right) ^ {-1}}$

where is the isentropic exponent . Example: = 330K: = 70K at = 4 ${\ displaystyle \ kappa}$ ${\ displaystyle T_ {t}}$ ${\ displaystyle T_ {m}}$ ${\ displaystyle M_ {m}}$

The Mach number that can reasonably be achieved is therefore given by the temperature in the pressurized gas reservoir.

Energy demand

The energy requirement of a supersonic wind tunnel is enormous, for example on the order of 50 MW per square meter of test area. That is why most of the channels are supplied with gas from high-pressure storage tanks, but this only allows for a limited test duration. This type of construction is referred to in English as "intermittent supersonic blowdown wind tunnels" (see illustration). Another way to increase the pressure ratio is to use vacuum tanks at the wind tunnel outlet; one then speaks of "indraft supersonic wind tunnels". The following factors can generally be viewed as critical:

Sufficient supply of dry compressed air
Interference of the flow with the wind tunnel walls
Sufficiently fast measuring instruments for the short experiment times

Construction of a supersonic wind tunnel

Wind tunnels like the Ludwieg tube have short experiment times (usually less than a second), a comparatively high Reynolds number, but a low energy requirement because they are not operated continuously.

history

The first supersonic wind tunnel working according to the vacuum principle for a cross-section of 100 × 100 mm² with a variable Mach number up to Ma = 3.3 was designed by Rudolf Hermann (1904–1991) at the aerodynamic system directed by the Prandtl student Carl Wieselsberger (1887–1941) Institute of the TH Aachen constructed and put into operation in 1935. Hermann carried out the first aerodynamic stability studies on A3 rocket models on this wind tunnel in 1936 on behalf of Wernher von Braun .

From April 1937, Hermann set up his own aerodynamic institute for a supersonic wind tunnel with a cross-section of 400 × 400 mm² and Mach numbers Ma> 4 in the Peenemünde Army Research Center . In this, the aerodynamic design of the A5 and A4 was optimized. In 1944 this wind tunnel was relocated to Kochel am See . At the end of 1945 it was dismantled by the US armed forces and brought to White Oak, Maryland , as spoils of war .

swell

Pope, A .; Goin, K .: High-speed Wind Tunnel Testing . Krieger, 1978, ISBN 088275727X .

Web links

UTA supersonic wind tunnel test (video) ( WMV ; 16.4 MB)

Individual evidence

↑ D. Eckardt: The 1x1 m hypersonic wind tunnel in Kochel / Tullahoma 1940-1960. (PDF; 1.54 MB) German Aerospace Congress, 2014, accessed on January 7, 2020 .

[1] D. Eckardt: The 1x1 m hypersonic wind tunnel in Kochel / Tullahoma 1940-1960. (PDF; 1.54 MB) German Aerospace Congress, 2014, accessed on January 7, 2020 .