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andy_r March 24, 2009 19:08

CL vs AOA plot low reynolds difficulties

I am analysing the eppler 216 airfoil to be used on an MAV. The airfoil is operating at a Re = 80,000 which corresponds to 7 m/s, the chord length is 0.16 m. I have created both a 2d and 3d mesh. The 2d mesh is a c shape, with standard velocity inlet and pressure outlet. As part of my 2d study I am creating a Cl vs AOA plot. I am testing using K-w SST and K-e realizable. Unfortunately, so far the Cl plot I produce from fluent has a gradient far lower than predicted from XFoil, specifically the CL at 0 angle of attack is higher than predicted (Fluent = 0.638 - 0.72, XFoil = 0.43) and the CL at 11 degrees is lower than predicted. Does anyone have any hints that may help me out with this problem? turbulence models i could try, solvers etc. I think the problem may be with how I have specificed the turbulence models, can anyone direct me to a set of rules that govern the values for near ground level flight. Also with respect to the reference values for 2d simulations is the following correct?

Area = chord
Length = chord
Depth = 1, no effect on CD or CL for 2d simulations?

Anyway help would be greatly appreciated,

Thanks in advance, Andy

Alagesanj March 25, 2009 07:36

Hi Andy,

Are you comparing your fluent results with any experimental results or only with Xfoil results. At low reynolds numbers Xfoil's CL will be higher than the actual CL. So it is very difficult to compare fluent results with xfoil.

Cheers, Alagesan

andy_r March 25, 2009 08:31

Hi, Thanks for getting back to me. There is no experimental data available for the e216 airfoil, or none that I can find, so I am comparing directly to XFoil. I have found some experimental data on the internet for another airfoil in the eppler series and compared that to XFOIL or XFLR5, and the results almost perfectly match. So thats why I was confused with my FLUENT results.

Using k-e RNG or k-w SST what would you set as your turbulence parmaters for this case (Re = 80,000, in a farfield but v. inlet and p. outlet bc)? Would you suggest using any other solvers or turbulence models?

Also, in regards to reference values are the ones I set above correct?

I have my results as an excel file, so if anyone would care to have a look, it would be greatly appreciated.

Regards, Andy

Alagesanj March 25, 2009 10:14

Hi Andrew,

Is your study involves only CL or both CL and CD ?

Using fluent is not as easy as Xfoil. There are many factors involving in fluent to get accurate results. Your mesh should be sufficiently good for this. So check your mesh first.

For turbulence parameters use turbulent intensity and length scale which is applicable for both k-e and k - w models. Xfoil's default freestream turbulence level (Turbulence intensity) is 0.07%. you can use this value to specify the turbulent intensity in fluent, if necessary you can vary it from 0.01% to 0.5%. For the case of turbulence length scale use between 0.001 to 0.1m. For pressure outlet bc the default values are ok, if necessary you can use the same values used for the V inlet bc.

i assume your reference values are correct.

If you have sufficient time then refine your airfoil coordinates using Xfoil (around 120 to 160 points) and use that refined coordinates to generate mesh for fluent simulation. It may help.

Cheers, Alagesan

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