Why do wind tunnels not have oval turns?
Some closed wind tunnels do have rounded corners, for instance the inner circuit (&quot;thermal test chamber&quot;) of the Jules Verne climatic wind tunnel in Nantes.
This plan view is from fig. 1 of &quot;Numerical modeling of the flow conditions in a closed-circuit low-speed wind tunnel&quot; by Moonen et al, J. Wind Eng. &amp; Industrial Aero. 94(10):699-723.

More diagrams of this wind tunnel are in &quot;Benefit of Wind Tunnels with Large Test Sections for Wind Engineering Applications&quot; by Flamand et al, Mathematical Modelling in Civil Engineering 15(2).