Sanity check of NACA 0012 gives wrong coefficient of lift

[BobHope]

I confess, a couple of things are still unclear to me.

In the linked http://www.ae.metu.edu.tr/tuncer/ae443/ ... All-Re.pdf

They give the velocity as 68ft/sec which is 21 m/s

You have adjusted this to match the reynolds number to 33.4m/s ( due to the linked example using a pressurised wind tunnel ? )

How did you get from Re 3.12*10^6 to V=33.4 ?


If I want to solve a 3d Shape, do I calculate the reference area values based on the surface area of the shape ?

For example if I create wing with a 1.4m chord, and a 5m depth, is the surface calculation then 140cm x 500cm = 70000cm2 ?



I solved some values using your equations for pressure - I solved for 55m/s and 52m/s and 21m/s

Example solved by Thomas for 33.4 m/s

q = (1.2 kg/m^3) / 2 * (33.4 m/s)^2
= (1.2 / 2) * (33.4^2)
= 0.6 * 1115
= 669 N/m^2

Solve for 55 M/s

q = (1.2 kg/m^3) / 2 * (55 m/s)^2
= (1.2 / 2) * (55^2)
= 0.6 * 3025
= 1815 N/m^2

Solve for 21 M/S

q = (1.2 kg/m^3) / 2 * (21 m/s)^2
= (1.2 / 2) * (21^2)
= 0.6 * 441
= 264.6 N/m^2

Solve for 52 M/S

q = (1.2 kg/m^3) / 2 * (21 m/s)^2
= (1.2 / 2) * (21^2)
= 0.6 * 2704
= 1622 N/m^2


I did an example solve for a 40mm mesh

Solve for 40mm mesh

140cm x 4cm = 560cm2

I note the reference area rounds up the values.

[BobHope]

2412 and box mesh cylinder inets and outlets mesh done fast CoL point 4.FCStd

(47.1 KiB) Downloaded 39 times

Tried to get a full scale 3D 1mx5m wing of NACA 2412 working in an improved wind tunnel and mesh to match the test results ......getting close at 0.42ish

Cl from Naca 2412 5m wing.png (25.18 KiB) Viewed 1493 times

[thschrader]

...
You have adjusted this to match the reynolds number to 33.4m/s ( due to the linked example using a pressurised wind tunnel ? )
How did you get from Re 3.12*10^6 to V=33.4 ?

I used the chord-length as reference:

ReNumber.JPG (48.93 KiB) Viewed 1455 times

...
For example if I create wing with a 1.4m chord, and a 5m depth, is the surface calculation then 140cm x 500cm = 70000cm2 ?
...

Thats the way I would do it.

[BobHope]

Thank you Thomas, that is a great explanation.

I tried to get the NACA0012 aero foil simulation working to see if it would give me different results to the 2412 test. I used some some the techniques from your 2D 2412 file like the mesh volume, it was very useful to work from that file, thank you.

Values of 10 degree angle of attack

I recalculated pressures for 52m/s as 1622 PA

Reference area of 5m2 ( 1m chord and 5m length )

NACA0012 Big Tunnel working zero point seven CoL.FCStd

(98.65 KiB) Downloaded 40 times

It solves out to Cl of just over 0.7 ( Should be around 1 )

Cl from Naca 0012 5m wing.png (26.06 KiB) Viewed 1404 times

We are definitely getting somewhere... the results are in the right area, just off by around 30% each time which is promising.

I wondered if 30% was a reasonable loss from it not being a " perfect " aerofoil of infinite aspect ratio, I searched and I found this publication :

https://zenodo.org/record/4068002#.Y7899JBws6M

They test NACA 0012 with very short aspect ratios at the same 52 m/s inlet velocity.

This graph shows three wings with the Aspect Ratio ( AR ) 0.5 of the chord, 1x of the chord and 2x of the cord. My test wing has an aspect ratio 5 times that of the chord

Interestingly, the efficiency drop is surprisingly low, less than 10%
.

Cl from real Naca 0012 test.png (36.81 KiB) Viewed 1404 times

[BobHope]

I re-did the NACA 0012 as a 2D planar simulation with the same 52 ms/s calculations.

It got closer, around 0.9 Cof L , so the maths looks plausible

naca 0012 2d Planar.FCStd

(440.01 KiB) Downloaded 47 times

I wonder if it is the reference area value that we do not have full understanding of......

[BobHope]

I tried the full scale 3D sim of the NACA 0012 at a much higher mesh resolution and with more boundaries........very slow, took 15 hours.

It *is* better, CoL increased to 7.5

I estimate that a target of around 8.5 would be ideal, if the 2D gets a result of CoL 0.9, that would account for the efficiency loss of the aspect ratio.

NACA0012 Big Tunnel working fine mesh very slow.FCStd

(102.1 KiB) Downloaded 40 times

[thschrader]

...
I wonder if it is the reference area value that we do not have full understanding of......

Below you can see the definition of the variables in the NACA-report.
They differentiate between area "S" an area of wing "Sw". The definition of drag-/lift-coefficient
uses area "S". Unfortunately there is no further explanation in the following text.
My best guess is that area S=chord-length * wing-length
and area Sw is the area of the total wing-hull.
However, you can define your reference area as you like. Important point is, that everybody uses the
same definition for comparison.
At the moment I am running your files...

definition_table.JPG (139.06 KiB) Viewed 1221 times

[thschrader]

I re-did the NACA 0012 as a 2D planar simulation with the same 52 ms/s calculations.
...

Here is my result on intel i7 running all 12 threads.
Cut-off residual was set to 0,0001. Convergence after 740 iterations.
In my opinion the result is nice, compared to NACA.
I edit the control-dict after case writing and used the correct 1622 N/m^2 as pRef-value.
Maybe this gives the difference to your result.
Next step: running your 3D-case (on sunday, mate...)
Have a nice weekend!
Greetings Thomas

BTW: there where updates on cfdof-wb, cfmesh and Hisa last week.

naca_0012_2dplanar_result.JPG (127.14 KiB) Viewed 1212 times

[BobHope]

Thanks Thomas, I am really keen to get a working wind tunnel figured out, there are just so many practical uses for one, I am running a test now on the NACA 0012 as a 2D planar simulation - I changed the physics model to Transient and the results are near perfect in the report :

Physics changed to transient resutlts PERFECT.png (33.89 KiB) Viewed 1147 times

Running the 3D model now with a fairly efficient mesh and an improved volume mesh in the wake.

NACA0012 Efficent Mesh Transient Physics.FCStd

(102.21 KiB) Downloaded 45 times

As an aside, I tried the Spalmart physics model and the results was a CoL of 4.2 so something is off there, probably not Freecads fault though.

[BobHope]

Regarding Wing area, I wondered if it was wetted area rather than projected area, but this reference states it is projected.

https://wright.grc.nasa.gov/airplane/geom.html

States "
The wing area is the projected area of the planform and is bounded by the leading and trailing edges and the wing tips. Note: The wing area is NOT the total surface area of the wing. The total surface area includes both upper and lower surfaces. The wing area is a projected area and is almost half of the total surface area.

"