Do wind tunnels suck?

[LegendRider]

Yes.

Was listening to this podcast and learned that wind tunnels used for bikes suck the air from the rear and not blow from the front. Fan blades create "messy" air so it's better to pull "clean" air.

https://www.velonews.com/2019/01/bikes-and-tech/tech-podcast-how-a-wind-tunnel-works_483213

[tsarpepe]

Nice thread title!

[dbnm]

they blow.

[Mark McM]

The turbulence from fan blades can be mitigated by using vanes and plenums, but that adds extra complexity to the tunnel design. For low speed wind tunnels, it is just simpler and less expensive to use a suction design. For high speed testing (like for aircraft, especially supersonic aircraft), suction tunnels can't generate air speeds high enough (remember, suction relies on atmospheric air pressure). So for high speed testing blowdown wind tunnels (with vanes and plenums) are used.

[unterhausen]

I think almost any wind tunnel that can use a fan has them behind the test section. I was in a giant U.S. air force wind tunnel with fans at the outlet. There are closed return wind tunnels where the same fans are behind and in front of the test section, it's magic.

[wallymann]

Yes.

Was listening to this podcast and learned that wind tunnels used for bikes suck the air from the rear and not blow from the front. Fan blades create "messy" air so it's better to pull "clean" air.

https://www.velonews.com/2019/01/bikes-and-tech/tech-podcast-how-a-wind-tunnel-works_483213

further, once you get to a certain airspeed wind-tunnels use a feed-back loop...so they dont have to constantly accelerate a new mass of air to the desired velocity...its both a push and pull!

https://www.grc.nasa.gov/www/k-12/airplane/tuncret.html

https://www.grc.nasa.gov/www/k-12/airplane/Images/tuncret.jpg

[unterhausen]

bicycle testing is probably always done in an "open return" wind tunnel, i.e. there is no return.

[Mark McM]

I think almost any wind tunnel that can use a fan has them behind the test section. I was in a giant U.S. air force wind tunnel with fans at the outlet. There are closed return wind tunnels where the same fans are behind and in front of the test section, it's magic.

Sure - but even though the fan is physically behind the test subject, it is still pushing the air toward the subject. It just happens that in this case, it is pushing the air through the circular circuit back to the front of the test subject (which gives the air path more distance to mitigate the turbulence). It must be this way, because suction is a fictitious force, not a real force.