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1)Why test section room is so so big,is this because to reduce interfrence between test object and walls?

2)Is test section room open to outside atmopsheric pressure or must be 100% air proof?

3)How they achieve clean airflow where test object stay if test section room is much wider than nozzle and collector,isnt air mixing with air jet?

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    $\begingroup$ "Aeroacoustic" wind tunnel - I'd venture to say that they need the space around the car to be able to measure the noise accurately. Not many cars drive in tunnels all the time. Note all the machinery and other non-areodynamicly smooth things in the background - they obviously don't matter to the measurements they're making in this tunnel. Of course, this is for (primarily) automotive purposes, so I'm not sure it's on-topic for Aviation $\endgroup$
    – FreeMan
    Commented Aug 17, 2020 at 18:49
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    $\begingroup$ You should ask only one question per post. $\endgroup$
    – Manu H
    Commented Aug 18, 2020 at 8:04
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    $\begingroup$ Maybe this question should be changed to "why does this wind tunnel have an unusually large test section?" $\endgroup$ Commented Aug 18, 2020 at 8:28

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An open jet wind tunnel has the advantage of giving a smaller effective blockage ratio than a closed jet wind tunnel. Because the jet is not constrained by walls around the jet in the test section, the airflow can distort more than a closed jet wind tunnel - giving the same flow results as a larger closed jet tunnel.

There are however disadvantages with the open jet tunnel, such as the effect of the collector on the flow (this creates a longitudinal pressure differential referred to as "buoyancy"), and the effect of the shear layer that forms at the jet nozzle (the Kelvin Helmholtz instability can create transient flow effects and can be a noise source). When standing in the test section of a running tunnel, there is a lot of secondary airflow outside of the jet because of the open section.

The test section does not need to have a pressure different to atmospheric pressure, and does not need to be sealed. There can be vents in the test or return section of the tunnel, but they may create flow disturbances as well as allowing noise into the test section.

The free shear flow around the edge of the jet can be a source of disturbance for flow in the test section, however tunnels are designed so the disturbance is still far away from the model. Of course there are pressure fluctuations that interact with flow over the model, which are difficult to control. Some wind tunnels are designed with adaptive and slotted walls, which combine the benefits of open/closed sections, however they need much more setup than just putting a model in and turning on the fans.

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  • $\begingroup$ whan accuracy is main focus,which type of test section is better open or closed?In closed test section,airlfow is bloked between test object and walls,which for sure change pressure around object in a way that dont exist in reality,how then they get correct results?? $\endgroup$
    – user50657
    Commented Aug 17, 2020 at 19:51
  • $\begingroup$ There are empirical corrections for blockage effects which is a whole research field in itself. The book Low-Speed Wind Tunnel Testing by Alan Pope describes all of these correction factors in great depth. $\endgroup$ Commented Aug 17, 2020 at 20:02
  • $\begingroup$ Also, what do you mean by "accuracy"? The tunnel above is an aeroacoustic tunnel, so is designed to measure noise rather than forces accurately. The lack of boundary layer control, belt system and constraint mechanism mean this tunnel will not give very accurate force measurements. $\endgroup$ Commented Aug 17, 2020 at 20:26
  • $\begingroup$ I mean on forces,lift ,drag etc $\endgroup$
    – user50657
    Commented Aug 17, 2020 at 20:28
  • $\begingroup$ @NoahPrandtl Yeah, this isn't the right tunnel for that. Of course you could measure them, but lots of compromises have been made in the design of the tunnel so it won't be as accurate as a dedicated force measurement tunnel like Windshear (windshearinc.com) $\endgroup$ Commented Aug 17, 2020 at 20:35