Blowdown  Wind Tunnel

Wind tunnels are designed for a specific purpose and speed range. Therefore, there are many different types of wind tunnels and several different ways to classify wind tunnels. In this section of the website we shall present various types of wind tunnels and discuss some of the unique features of each type of tunnel.

On the figure, we show a schematic drawing of a blowdown wind tunnel. Blowdown tunnels are normally used from high subsonic to high supersonic flow conditions. There are several possible configurations for a blowdown tunnel. On the figure, we show completely closed supersonic configuration. The test section sits at the end of a supersonic nozzle. The Mach number in the test section is determined by pressure and temperature in the plenum and the area ratio between the test section 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. To prevent condensation, air is brought into the tunnel through a dryer bed. The air is pumped into a closed high pressure chamber upstream of the plenum. At the same time, air is pumped out of a closed low pressure chamber downstream of the test section.

Test times are limited in blowdown wind tunnels. At the beginning of the test run, valves are opened upstream and downstream of the test section. The pressure ratio establishes a supersonic flow in the test section and the air flows from the high pressure chamber to the low pressure chamber. As air leaves the high pressure chamber, the pressure in the chamber decreases. Likewise, as air enters the low pressure chamber, the pressure in that chamber increases. Eventually, the pressure in the two chambers equalize, the flow stops, and the test is finished. To provide constant conditions in the test section, a pressure regulator valve is normally installed in the plenum. A second throat is often employed downstream of the test section to shock down the supersonic flow to subsonic before entering the low pressure chamber.

A closed configuration with both high pressure and low pressure chambers is shown in the figure, but there are other configurations of blowdwon tunnels. Some blowdown 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 confifguration and therefore the maximum Mach number is lower. Another configuration retains the high pressure chamber, but exits to atmosphere instead of into a low pressure chamber. The advantage of this configuration is that it is cheaper than a closed configuration in both construction and operation. But the tunnel is very loud and normally requires some type of muffler downstream of the test section.

The blowdown tunnel has some advantages and some disadvantages relative to a closed continuous flow tunnel.

Advantages of the Blowdown Tunnel

• High Mach capability. Easy tunnel "starting".
• Lower construction and operating costs.
• Superior design for propulsion and smoke visualization. There is no accumulation of exhaust products in an open tunnel.
• Smaller loads on model during startup because of faster starts.

Disadvantages of the Blowdown Tunnel

• Shorter test times require faster (often more expensive) instrumentation.
• Need for pressure regulator valves.
• Noisy operation