topic Re: Lift and Drag Calcution on Aerofoil Body in CFD Forum

Lift and Drag Calcution on Aerofoil Body

Anonymous — Mon, 26 Sep 2016 18:46:04 GMT

Hi all, I have been running simulations on various aerofoil to test their efficiency of lift to drag (lift to drag ratio), and have used the wall forces tool to calculate the force magnitude for the wind tunnel (with "surfaces" selected) in the x and y directions to get drag and lift, respectively. As I understand it, however, is that if I select either volume or surface, the force (or selected parameter) is not calculated as that acting directly on the CAD model, however on the surface(s) or in the volume as a whole. Although I understand that I could probably make a really small wind tunnel (which would mess up changes in lift due to wingtip vortices; I do not want that) or alternatively I could count the high force in the x direction as a result of the airflow velocity. Neither of these would, to my knowledge, give the results I need. There does seem to be the option of cutting the aerofoil out of the wind tunnel and measuring the forces on that cut out, then using “symmetry” as a boundary condition if I feel it is conducive to do so; I came across this method on an Autodesk CFD tutorial page (https://knowledge.autodesk.com/support/cfd/learn-explore/caas/CloudHelp/cloudhelp/2014/ENU/SimCFD/files/GUID-13079C5D-8F6F-4601-8DEE-F9AAA6E00BA7-htm.html ), however they did not specify how to make the cut out and I cannot figure out how to do so myself. Does anybody know how to do this? Or possibly is able to suggest another viable workaround which doesn’t involve me having to learn an entirely new aspect of the UI – I’m relatively short on time. Thanks. P.S. I am using Autodesk CFD Motion 2017

Re: Lift and Drag Calcution on Aerofoil Body

Jon.Wilde — Tue, 27 Sep 2016 07:32:19 GMT

Selecting the surfaces, or just the body of the aerofoil should give you what you need I think? I am not sure what the other method you are using might be? 🙂

Re: Lift and Drag Calcution on Aerofoil Body

Anonymous — Tue, 27 Sep 2016 10:20:22 GMT

That's good advice, however unfortunately I have tried to do that and oddly I am not able to do so. I even tried hiding the wind tunnel and selecting the body, which resulted in an error and force close - twice.

Re: Lift and Drag Calcution on Aerofoil Body

Jon.Wilde — Tue, 27 Sep 2016 11:48:37 GMT

What might be faster is putting all of the surfaces you wish to capture into a group before you run the analysis (select them all and right click to do this). Then once complete, you can group select them in the wall calculator 🙂

Re: Lift and Drag Calcution on Aerofoil Body

Anonymous — Tue, 27 Sep 2016 14:33:32 GMT

Okay, I will try this...is there any way that I will be able to do this directly on the wing body (i.e. measure the lift on the wing body and not on the tunnel walls)?

Re: Lift and Drag Calcution on Aerofoil Body

Jon.Wilde — Tue, 27 Sep 2016 19:41:39 GMT

Yes, only select the surfaces of the wing and not the tunnel 🙂

(There isn't a way to write that without sounding condescending, apologies 😉 )

Re: Lift and Drag Calcution on Aerofoil Body

Anonymous — Wed, 28 Sep 2016 06:56:25 GMT

Sorry I don't mean to be a bother 😞 however that is exactly my problem; I have not been able to select the wing surfaces - only those of the wind tunnel. My wings are not cut out from the wind tunnels - if I could do so, then they would be surfaces on which I could measure wall forces. Nowhere does it specify how to make this type of cutout - do you know how to do so?

Re: Lift and Drag Calcution on Aerofoil Body

Anonymous — Wed, 28 Sep 2016 06:58:24 GMT

...I probably should have specified that earlier. lol.

Re: Lift and Drag Calcution on Aerofoil Body

Anonymous — Thu, 29 Sep 2016 12:54:30 GMT

Okay, I have just tried that method again, to make sure it is not something which I am doing wrong. I hid the wind tunnel walls however once I selected wall forces and hovered over the wing the software gave me an assertion failure.

Re: Lift and Drag Calcution on Aerofoil Body

Anonymous — Thu, 29 Sep 2016 13:13:09 GMT

Also, the error goes as such:
Assertion failed

File:

j:\work_simcfd\simcfd\cfdesign\production\2017\release\libruaries\glview3\include\libcore\cvArray.inl
Expression: size > 0
(Press retry to Debug application)

This also happens when I select all surfaces and has done so on two computers, so I assume that this could be an issue with the model?