Testing Flat Plate in Wind Tunnel: Uncertainty & Lift Analysis

[sid_galt]

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?

 

[FredGarvin]

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?

 

[sid_galt]

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.

 

[sid_galt]

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?

 

[FredGarvin]

I'm glad the smoke worked for you. Hopefully the onset of stall was well below the 45° point.

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?

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.

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.

 

[sid_galt]

Thank you for the reply.

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.

What sensor are you referring to? The lift force sensor?

Yes

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.

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.

 

[FredGarvin]

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?

 

[sid_galt]

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.

 

[jaap de vries]

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?