Hello, I'm a senior year ME undergraduate and I'm building a low speed (velocity=30-45 m/s) wind tunnel for my college as a final year project. The initial size for the tunnel was 300mm x 300mm in the test section with an open circuit configuration. I've read some papers on the design of a wind tunnel's components such as Design Rules for Small Low Speed Wind Tunnels - Bradshaw and Mehta, among others and I have some questions: 1. What is the difference, besides the obvious, between a 2D and a 3D tunnel? If size is a factor, what is the minimum test section size for a 3D tunnel? 2. For the wall profile of the contraction, if a defining equation is used, why is it necessary to have its second derivative zero at the two ends? 3. For the square contraction (area ratio 8 -12), if the equation for the wall profile is known, what method could be used to fabricate it out of sheet metal? (I used NX sheet metal to develop one of the walls after i modeled it with a transformation of Bell and Mehta's curve, so that I could see its shape when unformed, if that could help, but I don't know what needs to be done after that) Also, papers that have a lot of information about the performance and design wind tunnel components mostly have to purchased from their publications. Is there a source where these materials can be obtained freely? If I have left out any relevant information, please let me know. Thank you.