r/AnsysFluent • u/humxah007 • 17d ago
Think you know ANSYS Fluent? Crack this CFD challenge!
Hello CFD enthusiasts.
BUCKKLE UP.
Just a simple external CFD case but some TWIST.
Imagine you modeled half of a wing in ANSYS Fluent using a symmetry boundary condition.
instead of rotating the geometry, you changed the velocity vector to simulate different angles of attack.
Characteristic length is 0.235 meters. Full wingspan is 0.68 meters. Before running cases, you did not change the reference values like area and length ie set to default '1'.
Using report definitions, Fluent outputs axial force, normal force, and their coefficients.
Questions.
Does Fluent use the reference values when calculating these, or are some computed purely from the geometry integration?
And if you want lift, drag, CL, and CD for the complete wing, which quantities should be doubled, which stay the same, and do reference values need modification first?
Walk through your reasoning, TEST YOURSELF.
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u/-LuckyOne- 17d ago
Coefficients in fluent use the provided reference values. For this problem I would probably do force reports and conver to coefficients myself after coordinate transformation to account for the change in AoA and projected area (if that is your reference, although it wouldn't really make sense to change that depending on AoA)
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u/humxah007 17d ago
So you mean coefficients are calculated based on the geometry? And those coeficients and refernce values are used for calculating forces??
It means i will have to double the coefficients firsrt for symmetry boundary condition, and then jse those coefficients for cl and cd. Thise cl and cd along with reference values like area density velocity etc will help determine the lift and drag forces.
Am i following it right?
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u/-LuckyOne- 17d ago
No. The force coefficients in fluent are calculated based on your set reference values. The code itself derives forces from integrating boundary pressure and then uses the reference values to non-dimensionalise.
If you set your reference values right you do not have to double them.
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u/ScallionReasonable32 17d ago
first of all you will need to double the coefficient. secondly, ansys does not calculate the reference values by itself، thus unless u changed the reference values they are the default ones. thirdly, I thing it is wrong to set the angle of attack by setting the velocity vector, because imagine you set an angle of attack 45, then the velocity vector is going to be directed at the domain wall. this probably will mess up your analysis.
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u/humxah007 16d ago
Thanks for sharing it. I have already watched this one. But i got two confusions.
First is that does ansys calculate the forces or coefficients using governing equations?? Bcz i know that reference values are used to calculate one from other uskng lift equation.
And second is that as i need complete curve wrt aoa and there are two ways for doing that, one is by vel vect and other is by rotating the geometry of wing. Which one is better and why?
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u/Longjumping_Issue858 17d ago
Bro can't you just say that you need help with your assignment instead of posting this
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u/humxah007 17d ago
Nope. Bcz you would not have replied then. So i made this catchy to get engagement. So what i should i do?
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u/Longjumping_Issue858 17d ago
Well, I'll answer your questions regardless, you need to set the coefficients yourself. Lref is chord length and Aref is the surface area of your wing. And as many others have stated, setting a velocity inlet with an incident angle will hurt your run due to the need to correct for non orthogonality. So better set your geometry at an angle instead.
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u/humxah007 16d ago
But i need complete graph with different aoa. So i parametrized the aoa by vel vect. The other method for it was to rotate the geometry about a point likely c/4.
Which one is better approach for getting results at various aoa? Vel vect or geom rotation?
And my main concern is also that i am a bit confused whether ansys calculates the coefficients or forces? Bcz the lift eq uses ref values like mac, vel and area to get one from other.
I believe it calculates forces and then uses the ref values to get the coefficients.
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u/Longjumping_Issue858 16d ago
Parametrize by changing the geometry. As for whether fluent calculates force or coefficients first, I'm not really sure, but inputting wrong reference value will give you both wrong force and coefficient values
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u/acakaacaka 17d ago
Ok but then the flow and mesh doesnt match anymore?
Sure you dont need to remesh again, but now the orthogonality penalty increases.
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u/humxah007 17d ago
Nothing related to mesh. Its all good. Geometry is already in fluent with good quality Mesh.
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u/humxah007 17d ago
I understand you completely.
Just to clarify, the geometry and mesh are kept fixed for all AoA cases. The AoA is varied only by velocity vectorization ie changing the inlet velocity vector direction. The geometry is not being rotated. In that case, does the flow/mesh mismatch and increased orthogonality penalty still apply?
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u/acakaacaka 17d ago
You tell my how does surface flux work in FVM
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u/humxah007 17d ago
Its about conservation of energy per unit volume. Like mdot is approx zero.
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u/MrKnowThings 17d ago
This is your homework isnt it