What is the better way to create less turbulent flow with a blower?

In summary, the tunnel has a higher entrance so the blower can be fitted inside, but the flow created by the blower is pushed inside the tunnel.
  • #1
T C
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TL;DR Summary
Blowers are being used to create airflow but in most of the cases those are turbulent. But if a blower is placed at the end of a tunnel then whether the flow created inside the tunnel will be less turbulent than the flow created by a blower placed at the entrance of the tunnel.
I want to start with two scenarios and both contains a blower and a tunnel fitting for the blower and both have almost the same diameter. The tunnel has s little higher so that the blower can be fitted inside. In the first scenario, the blower is fitted at the end of the tunnel i.e. the flow created by it is directed to open air but the air it sucks comes from the tunnel. In the second scenario, the blower is placed at the entrance of the tunnel and the flow created by it is being pushed inside the tunnel.
I want to know in which case the flow created inside the tunnel will be more laminar, the first or the second. Recently I have studied wind tunnels and found that the blowers used in is placed in such a way that the turbine/machinery that will be tested are being placed behind the blower instead of the front position.
 
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  • #2
T C said:
I want to know in which case the flow created inside the tunnel will be more laminar, the first or the second.
The rotation of a fan will cause a little upstream turbulence, but significant downstream turbulence. For that reason, it is better to pull air from the tunnel. At the tunnel entrance, there should be an impedance matching horn, (shaped like a trumpet), that will reduce turbulence caused by the step where air flows into the tunnel opening.
https://en.wikipedia.org/wiki/Velocity_stack

Maybe consider some form of honeycomb flow collimator in the tunnel, near the fan.
https://en.wikipedia.org/wiki/Flow_conditioning
 
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  • #3
Baluncore said:
Maybe consider some form of honeycomb flow collimator in the tunnel, near the fan
At the entrance of the tunnel or just before the blower?
 
  • #4
If I understand the described setup correctly, the diameter of the tunnel is greater than the inlet and outlets diameters of the blower.
In that case, if the blower is placed at the end of the tunnel, some air will be sucked from that end (due to by-pass effect) rather than from the tunnel.

Eliminating that by-pass by ducting tunnel end to inlet will increase the amount of air forced to flow through the tunnel.
As mentioned by @Baluncore above, a formed straight entrance of the tunnel and a set of vanes will improve the flow to be analyzed.

Please, see:
https://basementscience.scienceblog.com/225/wind-tunnel-honeycomb/
 
  • #5
Lnewqban said:
In that case, if the blower is placed at the end of the tunnel, some air will be sucked from that end (due to by-pass effect) rather than from the tunnel.
For the time being, let's consider that the blower is fitted in such a way that no bypassing has occurred.
 
  • #6
T C said:
For the time being, let's consider that the blower is fitted in such a way that no bypassing has occurred.
Install it at the end of the tunnel then.
 
  • #7
Lnewqban said:
Install it at the end of the tunnel then.
That is correct.

To reduce upstream turbulence, the effect of the blower rotation swirl going back up the tunnel, install a honeycomb between the tunnel and the blower. Where the air enters the tunnel, install a horn to eliminate the vena contracta and edge turbulence.

You might be concerned with resonance in the tunnel, that could be induced by the blower blades, like an organ pipe. Resonance would probably be reduced by the horn, which reduces reflection from the impedance mismatch step at one end of the tunnel.
 
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  • #8
Baluncore said:
install a horn
By "horn", do you want to mean a velocity stack?
 
  • #9
A velocity stack', 'trumpet, or air horn, ...
https://en.wikipedia.org/wiki/Velocity_stack

Horn (acoustic), a conical or bell shaped aperture used to guide sound.
https://en.wikipedia.org/wiki/Horn_(acoustic)
As an impedance matching device.
https://en.wikipedia.org/wiki/Horn_loudspeaker#Exponential

If you stand in a tunnel and shout, you will hear your echo from the open end. If you fit a well-designed horn to the open end of the tunnel, the sound should not reflect, so low-frequency resonance should not be possible. That horn will also prevent formation of the vena contracta and increase the laminar flow.
 
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  • #10
For turbulence, I decided to research Wind Tunnels. I figured those designers knew what worked best.

The largest wind tunnel in the world is at the NASA Ames Research Center, Moffet Air Field, California. The big test section is 80ft. tall and 120ft. wide, big enough to hold a Boeing 737 plane.
The air flow is intake (about the size of a football field), fans (6 of them about 22,000HP each), then to the test sections.

The air flow direction is mentioned at 6:50 in the video:


Cheers,
Tom

p.s. I toured that facility many years ago and got to enter the large Test Section. Looking at it from the entry doorway, it looked BIG... I started walking to the model mounted in there and it seemed like I wasn't moving!

The inside of the tunnel is pretty much featureless except for the hardwood floor, and the model was far enough away that visual depth perception didn't work. Quite a first-time experience.
 
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