VSPaero: some questions (and issues)

[docV]

Dear Rob,
I'm very happy to finally see a 3D panel method with OpenVSP: it' what I ever wanted, but never dared to ask. By the way propeller model is great.
Anyway I have some questions about it:
1) what kind of singularities? (I mean vortex, source/sink, doublet? Continous or concentrated? Where are the control points on panels?) Which Green identity is the panel method based on?
2) it's very hard to get the max pressure coefficient close to 1 (e.g. .995) at Mach number equal to 0. What's the right tassellation for wings and bodies to get correctly the stagnation line/point on them?
3) I performed some convergence tests about single components, to get the right min and max pressure coefficients. The convergence depends on dimensions of wings/bodies. If I use the same tassellation on a scaled wing (scale factor 0.1) the convergence fails. If the mesh is too much fine, I get numerical garbage (minimum pressure coefficient equal to -27 where it was .92 with a coarse mesh). It seems to depend on panels dimensions (so I can't study a mini UAV). Why?
Thanks in advance.
 

[Nick Brake]

docV,

1. Have you watched the theory presentation from David Kinney (author of VSPAERO)?  If not it it provides a really good background on VSPAERO and what is used under the hood.

2. The right tessellation will depend on your application.  The LE/TE and Root/Tip clustering can help adjust the tessellation to group in high gradient areas and help refine your results.

3. I did some quick testing with a default wing and was able to replicate your observations.  There may be a precision or significant figures limitation in the degengeom geometry file or *.vspaero setup file.  I'll look into this further.  In the meantime you can still model small/micro UAVs by scaling your vehicle larger, for instance use millimeters as your length unit instead of centimeters.  If you want to utilize the CDo calculation in VSPAERO you will also need to adjust Rho to the appropriate units.  If you do not use the CDo values from VSPAERO then there is no need to adjust the Rho value after scaling your airplane.

n

[Nick Brake]

I created a github issue for the scaling issue

https://github.com/OpenVSP/OpenVSP/issues/65

n

On Sunday, September 18, 2016 at 2:13:39 PM UTC-7, docV wrote:

[Hugh Blackburn]

Hi Nick

I had a question that is related to your response on the CDo calc.  I'm not sure I understand why one would need to reset Rho, so I just want to check my understanding of how CDo is calculated.  In the setup for vspaero we supply

Vinf

Rho 

ReCref

Cref

Together, ReCref, Vinf and Cref set the kinematic viscosity to be used in skin friction/CDo correlations/calculations, right?  Actually, I'm not sure why there are 4 parameters here - could you explain what Rho is used for? Actuator disk calculations?

thanks

Hugh

[Rob McDonald]

Yes, the rho is used in the actuator disk -- not in the CD0.

I'm a little fuzzy on it, but Dave needs it to be able to convert the
CT given in terms of normal propeller thrust coefficient into a
dimensional thrust force that he can then non-dimensionalize to an
airplane thrust/drag coefficient.

When I think through that process in my head, I think the rho should
cancel out, but I know there was some reason Dave needed it. Possibly
to feed his swirl model... The last time I chased it through the
source, the rho was not used if you don't use an actuator disk.

Rob

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