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Why do wind tunnel not have oval turns to reduce energy which is lost in sharp 90 degree turns and decrease turbulence?

Wind tunnel with oval turns:

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Wind tunnel with 90 degree turns:

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  • $\begingroup$ Is that a top down view, or a cross section? If top down, how does air enter the tunnel? What tunnel has 90 degree "turns"? $\endgroup$
    – Ron Beyer
    Commented Dec 29, 2020 at 13:31
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    $\begingroup$ @RonBeyer This is closed wind tunnel,no inlet.Bottom picture has four 90deg. turns. $\endgroup$
    – user53913
    Commented Dec 29, 2020 at 14:28
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    $\begingroup$ Just a guess really but I'd say it's because the square corners with turning vanes are cheaper to build and corners/vanes aren't that big a deal because the flow straighteners do the work of preparing the air for the nozzle, so how turbulent it is upstream is not critical. $\endgroup$
    – John K
    Commented Dec 29, 2020 at 22:05
  • $\begingroup$ I'm guessing too, but I think the square corners have a smaller footprint. Important when space is limited. $\endgroup$
    – Eric S
    Commented Dec 31, 2020 at 17:33

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Some closed wind tunnels do have rounded corners, for instance the inner circuit ("thermal test chamber") of the Jules Verne climatic wind tunnel in Nantes.
This plan view is from fig. 1 of "Numerical modeling of the flow conditions in a closed-circuit low-speed wind tunnel" by Moonen et al, J. Wind Eng. & Industrial Aero. 94(10):699-723.
plan view

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

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Here is one for the Bernoulli fans. If one can picture the pressure and flow rates in the tunnel, understanding the design becomes easier.

Notice the nozzles of both designs are identical. Before the nozzle we have an area of higher pressure and much slower flow rate. Shape is not as important here.

After the test section, the diffuser would again slow the flow rate.

One may also imagine square before the test section, and rounded before the fan (straighteners also being important), but it is the flow in the test section that is critical.

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