Wind tunnel size

[srauf]

I am trying to compare Coefficient of drag for pick-up trucks with and without a bed cover. I noticed that increasing the wind tunnel size to 4 or 5 times the model size reduces Cd (for both configurations). I am confused as to what wind tunnel size would work for a standard pick up truck for Autodesk Flow Design. Roughly 2 or 3 times or much greater than that?

[hartogj · Accepted solution]

Ideally a wind tunnel would be infinitely sized, avoiding the artificial effect of air being sequeezed between the obstruction (the truck in your case) and the outer walls. If the tunnel is too small, the air has to move faster around the object and can result in higher than expected forces. 

To minimize these effects, a typical goal is reducing the blockage to 5% of the total cross sectional area of the tunnel. This is something that comes up in both physical wind tunnels and CFD analysis. In Flow Design there is some logic in place to help correct for these blockage effects which helps a little bit but the tunnel still needs to be reasonably big. Here are some basic guidelines on wind tunnel sizing that are probably a decent starting point for a truck body:

http://knowledge.autodesk.com/support/flow-design/learn-explore/caas/CloudHelp/cloudhelp/ENU/FlowDes...

Lastly, regarding tunnel length, try to capture the entire wake within the tunnel. The streamlined, slow moving objects can get away with tunnels that are pretty short but a truck moving at relatievly high speed will likely have a long wake region.