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Wind Tunnel Project


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#1 Ben1445

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Posted 16 September 2014 - 09:27

Not sure if this is the best place for this but I thought I might try and some people might be interested. 

 

My A2 physics coursework requires a free choice 'open-ended' coursework investigation. For this I chose to design and build a small wind tunnel which I have completed myself. I have done a few test with balsa aerofoil shapes measuring the increase in mass on a small set of scales and it seems to work quite well. 

 

15253038771_0d2f09df98_z.jpg

 

Due to the open ended requirement, I really need to think about as many things to test and measure as possible. The working section is quite small at 100x100mm but the speed is decent (as of yet properly measured). I understand I am no expert and I have many limitations given the nature of the project but If anyone has any ideas or questions that would be great  :)

 

If not then hey, I built a wind tunnel and here's a picture...

 


Edited by Ben1445, 16 September 2014 - 09:41.


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#2 Magoo

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Posted 16 September 2014 - 23:27

Very nice, good luck with it. 

 

...Just for fun and if you haven't already, you might look into the Wright bros. 1901 wind tunnel to see what they accomplished -- with no resources and inventing everything as they went along.  



#3 desmo

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Posted 17 September 2014 - 02:02

I immediately thought of the wright brothers too. Measuring the forces accurately can't be easy.

#4 Greg Locock

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Posted 17 September 2014 - 03:37

http://www.amazon.co...illigram scales

 

is a cheap solution.

 

Handy if you want to go all breaking bad as well.



#5 Peter0Scandlyn

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Posted 17 September 2014 - 05:54

 Measuring the forces accurately can't be easy.

 

Ferrari can testify to this  :)



#6 Ben1445

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Posted 17 September 2014 - 06:12



 

Handy if you want to go all breaking bad as well.

 

I think I'm already on a watch list  :)

 

I bought these mini scales and this is the kind of set up I'm running but now with shorter rods. With endplates and running as close to the tunnel floor as possible I can get about 60 grams out of one of these wings. It's more like 10-20 in the centre of the tunnel which makes me wonder if this is worth doing something with...I ought to try that without endplates. 

 

15078228569_7a9cdfafb3_z.jpg

 

I am aware of the Wright brothers efforts but, for some odd reason, it hadn't occurred to me to read in detail. Thanks, I suppose any basic ideas on what they did with limited tech could be interesting if not helpful...



#7 Magoo

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Posted 17 September 2014 - 12:04

Ferrari can testify to this  :)

 

Ouch. Tough crowd in the Autosport Technical Forum. 


Edited by Magoo, 17 September 2014 - 12:05.


#8 desmo

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Posted 17 September 2014 - 14:23

How does one easily measure the drag force though? And, I don't know, it might be better to invert the airfoil section and have the struts connect to the flat side, as I'm told airfoils do all their Bernoullian magic on the curved side and calculate the lift by subtraction rather than addition. Assuming of course the airspeed isn't enough to create a lift force greater than the airfoil section's weight.

#9 Ben1445

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Posted 17 September 2014 - 15:21

How does one easily measure the drag force though? And, I don't know, it might be better to invert the airfoil section and have the struts connect to the flat side, as I'm told airfoils do all their Bernoullian magic on the curved side and calculate the lift by subtraction rather than addition. Assuming of course the airspeed isn't enough to create a lift force greater than the airfoil section's weight.

 

The drag force is a sticky issue. In short, I don't know...yet.

 

One of my (hand carved) balsa wings did in fact end up creating a large enough lift force to rip it backwards into the fan blades...it's now in about 15 pieces. I have added a catch net in case of similar episodes. But yeah, having them the other way up is certainly worth thinking more about...

 

I think my first proper goal now I've established it works is to work out exactly what speeds it is producing, that'll give me a good base to start with I think.



#10 condor

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Posted 17 September 2014 - 15:48

Have you considered using a pitot tubed drag rake inserted 2 chord lengths downstream of the aerofoil's trailing edge ?



#11 gruntguru

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Posted 17 September 2014 - 23:41

You can calculate speed within a few percent with a very simple pitot-static setup. Two pieces of tube oriented to measure static and dynamic pressures, connected to each end of a length of clear plastic hose arranged to form a water-filled U-Tube manometer. Incline it if you need more sensitivity.



#12 Greg Locock

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Posted 18 September 2014 - 01:24

I think that in some proper windtunnels they measure the velocity profile in the tunnel to back calculate the drag, but that implies a closed tunnel.The more I think about it the more difficult it seems!



#13 gruntguru

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Posted 18 September 2014 - 02:43

You could instrument the setup in your photo to measure drag as follows.

 

- Put a knife edge (triangular prism) between the bottom block of wood and each scale. Set the knife edges apart by a distance = 1/10th the height from knife edge to airfoil.

- Bias the knife edges towards the rear of the specimen

- Eliminate any possible contact (friction) where the struts pass through the floor of the tunnel.

- Zero the scales and run the tunnel

- Add the scale readings to measure lift

- Subtract the scale readings and divide by 10 to measure drag

- Run the test with bare struts (no airfoil) to determine drag on the struts.

- Alter the spacing of the knife edges to adjust sensitivity of the drag measurement.


Edited by gruntguru, 18 September 2014 - 02:44.


#14 condor

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Posted 18 September 2014 - 07:29

It would be a good idea to make a NACA 0012 aerofoil section to use as a datum. There's plenty of experimental data freely available to validate your results.  :)



#15 Greg Locock

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Posted 18 September 2014 - 10:59

yabbut that doesn't solve the drag issue. If you just want to calibrate the instruments a flat plate would be better.



#16 condor

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Posted 18 September 2014 - 14:25

I thought he might have taken my earlier suggestion to put a drag rake of pitot tubes and a static in :) 



#17 bigleagueslider

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Posted 19 September 2014 - 04:49

"My A2 physics coursework requires a free choice 'open-ended' coursework investigation. For this I chose to design and build a small wind tunnel which I have completed myself. I have done a few test with balsa aerofoil shapes measuring the increase in mass on a small set of scales and it seems to work quite well."

 

Being a physics course project, I hope you remember that mass and force are not the same thing.



#18 gruntguru

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Posted 19 September 2014 - 08:25

Kitchen scales measure force and display in units of mass so I would tend to forgive someone speaking in either language.   :)



#19 Ben1445

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Posted 19 September 2014 - 15:54

"My A2 physics coursework requires a free choice 'open-ended' coursework investigation. For this I chose to design and build a small wind tunnel which I have completed myself. I have done a few test with balsa aerofoil shapes measuring the increase in mass on a small set of scales and it seems to work quite well."

 

Being a physics course project, I hope you remember that mass and force are not the same thing.

I am very much aware, thanks  :lol:

 

Probably should have put 'measuring the force as an increase in mass on the scales'....or something...I knew what I meant, but I will definitely be double checking my final write up  :)



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#20 Ben1445

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Posted 07 March 2015 - 08:22

16714371686_cb0afa414d_c.jpg

 

Well, I started this whole project back in July, so it's been a long one. As a last go, I thought I'd try and get smoke trails to work (in this case water vapour) and actually...quite pleased with it.



#21 gruntguru

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Posted 07 March 2015 - 09:39

Nice! Thanks for following up. Did you manage to instrument for drag?



#22 Ben1445

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Posted 07 March 2015 - 10:16

Drag has eluded me, to my disappointment. I was really limited by time (as despite being a long time frame overall time actually testing anything was limited) and budget so for a lot of the measurement equipment I had to make my own using whatever I could find. To change the angle of my models, for example, I ended up using an old fine tuning mechanism of a violin. 
Trying to get something to measure drag was just proving too unreliable with anything I could use. 

Must say though, I've enjoyed it all which for coursework is really good. Looked great on my UCAS application too.
 


Edited by Ben1445, 07 March 2015 - 10:16.


#23 Ben1445

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Posted 07 March 2015 - 12:02

And just for fun here's a video 



 



#24 Kelpiecross

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Posted 08 March 2015 - 03:45


The video seems to show what wind tunnels are meant to show - the airflow seems to have separated from the aerofoil - it has stalled. The test aerofoil looks very unusual and probably is not really a useable shape - it would be interesting to see how a conventional aerofoil shape (like a Clark Y) would perform under the same conditions in the same wind tunnel.

#25 bigleagueslider

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Posted 11 March 2015 - 05:33

The test articles used in wind tunnels are usually mounted on a balance device that records all the moments/forces on the model during testing. The arrangement shown using two scales that can each only measure force in one direction does not seem like a technically sound approach. For the same amount of money, you probably could have built a strain gauge balance system that would have done a much better job. While the picture of your wind tunnel looks like you did a good job with that, I think the technical quality of the instrumentation system used was a bit lacking. Any type of testing, including wind tunnel tests, are all about validating analysis work or making qualitative assements between one design and another under the same test conditions.