To bolster the tube, fashion three tight-fitting rectangular frames from wood slats and glue them to the outside of the tunnel. Horizontal slats help to make the structure more rigid and support the measuring equipment. Some household fans can drive wind as quickly as five meters per second. Decreasing the cross-sectional area in the test region by two thirds will, for example, nearly triple the airspeed you can attain. If you need still stiffer winds, check your local industrial liquidators for more powerful fans.
Most commercial fans use a four-position switch to control their speed.
For finer adjustment, replace this unit with a household light dimmer. Use duct tape to attach the fiaps of the box to the fan. Plastic trash bags, slit at the bottom, make excellent airtight sleeves. Slip one over the fan and the box and tape it into place. Flair the entrance to the wind tunnel by bending the fiaps on the opposite side outward and attaching them to a square fashioned from two wire coat hangers.