Suitcase wind tunnel 1

[insideman]

I think I need a wind tunnel to test modifictions on ducted fans for model propulsion. First thought was a tunnel with 10 inch diameter test section, 20 inch diameter upstream, giving a 4:1 area ratio. Velocity desired is 50 mph. Grinding through the design gave a beast over 50 inches high, 20 inches thick and over 90 inches long. Then, thinking of ways to reduce size and complexity, I tried a design with NO area ratio. Constant duct diameter of 10 inches.

This is FAR simpler! The four corners are all alike. No tapering tubes. Even with advanced Collar type turning vanes, complexity is within the reach of a reasonable home shop. Dimensions decreased to suitcase size: 10 wide X 26 high X 42 long. Estimated power required is quite reasonable.

QUESTION: What problems am I creating by not having a large diameter upstream of the test section?

 

[berkshire]

insideman (Mechanical)
The biggest problem I see, is that you are going to have a wide turbulent boundary layer in your test area, limiting the section you can actually use.
B.E.

To the optimist, the glass is half full. To the pessimist, the glass is half empty. To the engineer, the glass is twice as big as it needs to be.

 

[insideman]

Thanks Berkshire. Can you elaborate please?

 

[berkshire]

In a parallel duct, particularly downstream of an elbow, the duct has vortices's generated by the elbow, even with turning vanes which reduce the size of, but do not eliminate the vortices's. You want a parallel stream of air at the site of your test article I believe.
The purpose of the larger area up stream is to first slow the air down, then to re accelerate it in a compressive flow to reduce the thickness of that turbulent boundary layer. Some tunnels use a diffuser grid at the point of slowest speed to re distribute the air to eliminate this turbulence.
B.E.


To the optimist, the glass is half full. To the pessimist, the glass is half empty. To the engineer, the glass is twice as big as it needs to be.