SimScale CAE Forum

Too High Stall angle for biplane

Hi, I’m trying to obtain Clmax values for an Aerodesign SAE biplane. Despite I’m reaching nearby values comparing to results given by other programs like tornado and AVL, the stall angle where I reach the maximum lift coefficient is too high. Tornado predicted stall angle of 17.7 degrees and here in simscale it reached about 45 degrees. Already checked almost all the simulation parameters. Here is a link to the project if anyone wnat to check it, help would be apreciated

https://www.simscale.com/projects/aerousb/tx4_completo_clmax/

Hi there!

I am quite sure that @DaleKramer would be interested in that and can give you some advice on the setup (@CFD-SQUAD).

Cheers,

Jousef

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In your ‘coefficients’ results items, for lift and drag directions you must rotate the unitized vectors (the vector length must be 1) so that the lift vector is perpendicular to the inlet airflow and the drag unitized vector is parallel to the inlet airflow. You may want to also check out my ‘rotation of forces’ discussion topics here…

Also, I see that you have some residuals bouncing off of a lower limit of 1e-6. This causes output errors, so in your numerics you should allow them to get smaller (I use 1e-30 wich effectively never limits the lowest value they can reach.

Sorry I can’t spend much more time explaining things right now, I am spare time limited these days :wink:

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Awesome ¡ thanks a lot! I’m running one simulation with the rotated vectors and the numeric correction. Just one more thing. I decided to calculate the lift and drag vectors by using the same excel sheet with my velocity input values ( first obtaining the unitary vector of the velocity for using it at drag and then using a normal vector of the last one for lift) But I notice that for certain values of alpha, the coordinates of the unitary vectors are rejected because “ the must be unitary” ( although they already are) I suspect it has to do something with rounding errors, any suggestion for avoiding this kind of issues?

This is the first I have heard that the interface is now checking for vector length of 1 so I can not comment on that.

Perhaps @Ricardopg can shed some light here ???

Thanks for the tag @DaleKramer

And yes, @aerousb, it’s indeed rounding errors. If I recall correctly, from my last tests, I had to input 8 or 9 significant figures, otherwise my vector was getting flagged as non unitary.

Cheers

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