Hello all,
I have a question about the way OpenFAST handles the Reynolds Number.
Some information about my case:
I want to compare dynamic stall through a BEM model at a Re number of 10e6 with CFD results at the same Re number. For the BEM modelling I use the static polar information for Re=10e6. I use AerodynDriver to generate the dynamic stall loops.
The issue:
By changing the Reynolds number in OpenFAST input files, there is no
change in the polar information (aoa;cl;cd). The same applies to the
kinematic viscosity. However, if I change the inflow velocity or the chord length, then the polar information and the output of the Re-number change.
The question:
So if I can change the velocity and chord length, I also change the Re number, but I still have the polars that were calculated at a different Re number. Are the results then incorrect? Would I have to keep the correct ratio between velocity, chord lenth and kinematic viscosity to get the polar at the correct Re number? Is this a bug in the BEM modelling or maybe in my head? 
How does OpenFAST handles the Reynolds Number?
Hopefully I have been able to express my concern in a comprehensible way.
I appreciate any tips and help!
Kind regards
Jasson
Dear @Jason.Printezis,
I’m not sure I fully understand when you say:
The issue:
By changing the Reynolds number in OpenFAST input files, there is no
change in the polar information (aoa;cl;cd). The same applies to the
kinematic viscosity. However, if I change the inflow velocity or the chord length, then the polar information and the output of the Re-number change.
Regardless, there is an option in AeroDyn to interpolate airfoil data based on Reynolds number or not. That is, when AFTabMod = 2, AeroDyn will interpolate airfoil data based on angle of attack and Reynolds number, assuming your airfoil data files contain airfoil data at multiple Reynolds number. If AFTabMod /= 2, or if your airfoil data files do not contain airfoil data at multiple Reynolds number, AeroDyn can still calculate the Reynolds number, but it will not really effect the simulation results.
Best regards,
Dear @Jason.Jonkman,
thanks a lot for the reply.
Maybe with some charts my question will be more understandable.
Case 1 is here not important.
Case 2: Static polar information calculated at Re=10e6. Vrel in OpenFAST at 62,7m/s
Case 3: Static polar information calculated at Re=15e6. Vrel in OpenFAST at 80m/s
Due to the chosen windspeed and chord length, case 2 has a different Re number Output than the Re number where the statis polars were calculated.
In comparison case 3 has the same Re number output as the Re number input.
Are the results of case 2 wrong? Should we keep the Re input the same as the Re output for good results?
Kind regards
Jasson
Dear @Jason.Printezis,
I’m still not sure I fully understand your question. It sounds like you have static polars at two different Reynolds numbers (10e6 and 15e6). Are you using AFTabMod = 2 and specifying airfoil data input at both Reynolds numbers? If so, AeroDyn will interpolate between the two polars to apply the airfoil data most appropriate to the actual Reynolds number calculated within the simulation.
Best regards,
Dear @Jason.Jonkman,
no I am not using AFTabMod = 2. I am not mixing the airfoil data at both Reynolds numbers. Case 2 and Case 3 are two independent simulations. All parameters are the same, except the airfoil input data and the inflow velocity.
Kind regards
Jasson
Dear @Jason.Printezis,
If AFTabMod /= 2 and you are specifying airfoil data at a Reynolds number that is different than the Reynolds number calculated within AeroDyn for the case you are simulating, than you’d expect some amount of error in the simulation results, depending on how sensitive the real airfoil data is to Reynolds number.
Best regards,
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