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CFD Validation of S809 airfoil.

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Old   July 5, 2011, 03:39
Default CFD Validation of S809 airfoil.
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Raashid Baig
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I want to perform CFD validation of S809 experimental data using commercial CFD software CFD++ (Somers, D. M., 1989, “Design and Experimental Results for the S809 Airfoil,” Airfoils, Inc., State College,PA)
for the following two reasons -
1. CFD results have not be matched correctly till date. Please correct me if I am wrong here. I have read only two papers in this field -
i) Wolfe, W.P. and Ochs, S.S. “CFD Calculations of S809 Aerodynamic Characteristics”, AIAA Paper AIAA-97-0972, 1997
ii) Langtry, R. B., Gola, J., and Menter, F. R., “Predicting 2D Airfoil and 3D Wind Turbine Rotor Performance using a Transition Model for General CFD Codes,” 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2006, AIAA-2006-395

2. Since this is essentially a 2D problem, I can try different turbulence models and higher order methodologies like LNS, DES, DDES (by extruding the 2D geometry) without much computational overhead.
This will be a validation of various options available for turbulence modelling as well.

I need suggestions from fellow users of cfd-online, if I am thinking on the correct lines. Weather I have to take into account some other factors ?

One problem is that while referring to the original document from Somers about the experiment I couldn't find the the values of Pressure, Temperature and Density etc. which I can use as my boundary values at inlet and outlet in my CFD simulation. Can someone help me point out where I can get these quantities ?

Any suggestion from other users who have worked on S809 airfoil earlier will be very helpful.

Regards,
Raashid
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Old   July 6, 2011, 02:26
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Martin Hegedus
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In general the input isn't really dependent on the true values of pressure, temperature, etc. as long as you match Mach number and Reynolds number since you'll be comparing to CL and CD. So choose standard sea level pressure and density. Then pick a velocity to get the correct Mach number. Then pick a viscosity to get the correct Reynolds numbers. The true value of temperature becomes more relevant as the Mach number increases because Sutherland's formula is based on (T0+C)/(T+C) where C is Sutherland's constant.

Onto the S809. It's more involved than most seem to be aware of, or at least admit to. And this is based on my own opinion which is backed up by some CFD work I did on it.

The S809 is very thick. Normal airfoil data and intuition do not apply to it.

There are two regions, pre stall and post stall. I believe the first stall starts to occur around 9.5 degrees.

The TU Delft data shows that there is only a small difference between tripped and untripped data. So I'm skeptical about some of the papers I've seen which are playing with turbulence models to try to match the data post stall. Pre stall, fine. Post stall, no. If you want to check out different turbulence models you should only compare to pre stall, if you are modeling it without the WT walls. In my opinion.

When you look at the data, there appears to be TWO lift curve slopes, prestall and post stall. And the airfoil undergoes TWO stalls.

The reports only state the the flow is 2D prestall. One can not assume that the flow is 2D post stall. Again, in my opinion.

So this is what I discovered when I modeled the airfoil with the WT walls. Once the airfoil stalls, a vortex develops at the junction of the wing and wall. Once the vortex develops it knocks down the lift of the airfoil. This is why there is a dip at 12.5 degrees. As the angle of attack increases past this point, the lift increases until the wing stalls a second time.

Unfortunately, the vortex effect is dependent on the thickness of the boundary layer at the wall. You can have many different results by starting the WT boundary layer at different upstream locations. The thinner the boundary layer, the less likely the vortex will be occur. And it is somewhat discreet. It's either there, or not as a function of axial station. What a pain.

You must also include the top and bottom walls for pre stall values. The top and bottom does effect the lift curve slope.

Unfortunately I wasn't able to finish the test case. The 3D analysis with WT walls takes a lot of grid points and I didn't have the resources available.

Some information about the Delft WT: (This came from Nando Timmer, MSc, Wind Energy Research Group, Faculty of Aerospace Engineering, Delft University of Technology)
"1) The low-speed tunnel has a test section of 1.25 (height) x 1.80 (width)x 2.60 m (length) with solid walls.
2) The axial pressure gradient is virtually 0 as a result of a slightly bigger test section exit as entrance, so compensating for the t.s. boundary layer."

Good Luck!
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