Testing Flat Plate in Wind Tunnel: Uncertainty & Lift Analysis

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Discussion Overview

The discussion revolves around the testing of a flat plate in a wind tunnel, focusing on the measurement of lift and the associated uncertainties. Participants explore the behavior of lift at various angles of attack, particularly the unexpected increase in lift at higher angles, and the challenges of accurately measuring stall conditions and reducing measurement uncertainty due to vibrations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports that lift continues to increase up to 45-50 degrees before decreasing, raising questions about the measurement method.
  • Another participant suggests that the observed lift might be a reaction force due to the momentum of the airflow rather than true lift.
  • There is a discussion about the use of smoke to visualize airflow and identify stall, with one participant noting faint contrails indicating stall.
  • Concerns are raised about the measurement uncertainty, which is reported to be 20-25 g but increases to 45-50 g due to vibrations in the wind tunnel.
  • Participants inquire about the distance of the fan from the wing section, with suggestions that 3 ft may be too close and could contribute to turbulence.
  • One participant mentions the need for flow straighteners or honeycomb structures to reduce turbulence, discussing the challenges of creating an effective design without excessive pressure loss.
  • There is a mention of the possibility of exciting a natural frequency in the plate, although one participant dismisses this based on experiments with different plate shapes and materials.
  • Questions arise regarding the type of wind tunnel (blow down or suck through) and the potential effects of ground effect when the plate is too close to the wall.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the behavior of lift at high angles of attack and the effectiveness of measurement techniques. The discussion remains unresolved, with no consensus on the reasons for the observed phenomena or the best methods to mitigate measurement uncertainties.

Contextual Notes

Limitations include uncertainties in measurement due to vibrations, the effectiveness of airflow visualization techniques, and the influence of tunnel design on turbulence and lift measurements.

sid_galt
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I am trying to test a flat plate in a wind tunnel. The uncertainty in measurement is 50 g and the maximum lift comes to 250 g. The problem is that even after 25-30 degrees as the angle keeps on increasing, the lift too keeps on increasing instead of decreasing and tops at around 45-50 degrees and then decreases.
Any idea why this could be happening?
 
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It seems like you are not measuring lift any longer, but the reaction force due to the momentum of the oncoming air stream. Are you using a smoke wand or do you have tethers on the back face of the plate so you can see when separation is occurring?
 
I am not using smoke currently but will be doing so tomorrow. I was thinking of burning wood behind the fan but I doubt it would give fine streamlines. Would it? If not then I will go for the smoke wands (they are oil burning wires right?).But isn't this the way lift is measured in actual wind tunnels. The lift in professional wind tunnels must be dropping at stall. Otherwise how do they obtain their measurements for angles at or greater than the stall angle.
 
Last edited:
I tried smoke and although it is showing very faint contrails, it was enough to see that the wing was stalling.

On the other hand, how do I get the reading for stall on the sensor.

The balance I am using gives an uncertainty of about 20-25 g for measurement but the vibrations in the tunnel are increasing this to 45-50 g.
Any thoughts on reducing this.

The fan is a bit close to the wing section, approx. 3 ft. away. Is it too close? What should be the ideal distance?
 
I'm glad the smoke worked for you. Hopefully the onset of stall was well below the 45° point.

Sid said:
On the other hand, how do I get the reading for stall on the sensor.
What sensor are you referring to? The lift force sensor?

Sid said:
The balance I am using gives an uncertainty of about 20-25 g for measurement but the vibrations in the tunnel are increasing this to 45-50 g.
Any thoughts on reducing this.
Does your tunnes have any egg crates/flow straighteners in it? I am assuming that it is a straight section tunnel, not a closed circuit tunnel. Is it an old piece of equipment? It could be anything.

Sid said:
The fan is a bit close to the wing section, approx. 3 ft. away. Is it too close? What should be the ideal distance?
Since I don't know the size of your tunnel it is tough to say. It does sound a bit close to me. That, to me, further adds to the need for flow straighteners.
 
Thank you for the reply.

FredGarvin said:
I'm glad the smoke worked for you. Hopefully the onset of stall was well below the 45° point.
Yeah. It was around 20 degrees.

FredGarvin said:
What sensor are you referring to? The lift force sensor?
Yes

FredGarvin said:
Does your tunnes have any egg crates/flow straighteners in it? I am assuming that it is a straight section tunnel, not a closed circuit tunnel. Is it an old piece of equipment? It could be anything.

It is a straight section tunnel rather small in length.
It has gauzed wire between the fan and the wing section. By egg crates do you mean a honeycomb? I tried making one but failed because the pressure loss was getting too high.

FredGarvin said:
Since I don't know the size of your tunnel it is tough to say. It does sound a bit close to me. That, to me, further adds to the need for flow straighteners.

The size is 2 by 2 ft cross section and 4 ft length.
 
By egg crates I do mean honeycomb. I would think that if you are that close to the fan, you are getting way too much turbulence from the fan. I think you would benefit from getting some kind of flow straightener in there. Perrhaps if you increased the cell size and cut down on the length from what you tried before you could reduce the delta p across it.

There is also a possibility that you have, by pure luck, excited a mode (natural frequency) in the plate.

Is your sensor a mechanical or digital type?
 
It's a mechanical sensor.

As for natural frequency, I don't think so as I tried the experiment with two different sections of different shapes and materials and the trend was still the same - the lift was not decreasing at stall.
 
Is it a blow down or suck through tunnel. I am a TA using a open loop tunnel and the turbulence can be quite severe (and frustrating) when the plate is to close to the wall you might be creating 'ground effect' do you measure drag force too?
 

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