What's the Difference between a Closed Circuit, and an Open Circuit Wind Tunnel ?

Closed Circuit Wind Tunnels:

In a Closed Circuit wind tunnel, the air that passes through the test section is drawn back into the fan and sent through the test section again and again.

Advantages:

1. In a well designed Closed Circuit wind tunnel, some energy is recovered. A smaller motor can be used.

2. Closed Circuit Wind Tunnels are completely enclosed, and so are shielded from rain, snow, and cold weather.

Disadvantages:

1. Closed Circuit Wind Tunnels are much more expensive to build due to the construction of the additional air return system.

2. The air in a closed Circuit Wind Tunnel is constantly heated due to friction with the fan. Most designs draw fresh air in to help, some even use air-conditioning, yet enclosed designs are always uncomfortably hot in the summer.

3. The air in a Closed Circuit Vertical Wind Tunnel is full of dust and debris from clothing, cushions, etc. This is constantly recirculated, and is very difficult to remove.

4. Due to the close proximity of the fan, the flight area of Closed Circuit designs is always very noisy.

Open Circuit Wind Tunnels:

In an Open Circuit Wind Tunnel, fresh air is drawn into the machine. The air that passes through the test section is discharged from the machine.

Advantages:

1. An Open Circuit Wind Tunnel is much less expensive to build.

2. In a scenic location, an outdoor Open Circuit Vertical Wind Tunnel is very spectacular.

Disadvantages:

Extreme cold or precipitation can make flying in an outdoor Open Circuit Vertical Wind Tunnel somewhat less comfortable.