Help with CompGeom: theoretical vs wetted volume

[Josh Camp]

Hi all!

I'm using OpenVSP to get wetted area and volumes for weight and volume buildup calcs, but I'm getting results that aren't intuitive and I'm wondering if the setup is wrong or my expectations simply aren't correct.

The attached vsp file is representative of what I'm working with.  It's a simple fuselage + wing combo.  What I'm trying to pull out are the Wet_Area and Wet_Vol for the fuselage and wing separately (my assumption is those represent the component areas and volumes after they've been intersected).  

Since the geometry is relatively simple, I'm able to do some hand calcs and the calcs for the component wetted areas look right.  However, I'd expect that the fuselage Theo_Vol and Wet_Vol to be the same, with only the wing volume decreasing to account for the part of the wing inside the fuselage being removed.  Instead, the wetted wing volume is about ~180 units higher than what my hand calcs would say, which is roughly the same as the decrease in fuselage wetted volume--in other words, it appears as if the wing is "stealing" volume from the fuselage.

Is my interpretation of the wetted volume not correct? Or is there something wrong in my setup?  The CompGeom output is copied below.  Thanks!

Comp Geom Output:

...Comp Geom...
2 Num Comps
3 Total Num Meshes
6896 Total Num Tris

Theo_Area   Wet_Area   Theo_Vol    Wet_Vol  Name
 2568.523   2328.423   5349.808   5138.631  Fuselage      
 1421.965    560.189   1440.600    601.339  WingGeom      
-------------------------------------------------
 3990.489   2888.612   6790.408   5739.970  Totals        

Tag_Theo_Area   Tag_Wet_Area    Tag_Name
     2568.523       2328.423    Fuselage_S_Surf0
      710.983        280.094    WingGeom_S_Surf0
      710.983        280.094    WingGeom_S_Surf1
WARNING: 1 open meshes merged
     Merged: WingGeom

Josh Camp Director of Vehicle Engineering jo...@venusaero.com C: 281.381.8354

[Rob McDonald]

You're understanding things mostly correct.

The wetted volume is a bit of a mess -- while we can clearly attribute the wetted area of the wing and fuselage after trimming, the volume where the wing and fuselage overlap is claimed by both.  Which one should it be allocated to?

Consequently, OpenVSP splits the wetted volume up between components.  The total wetted volume is correct, but the partition between components is rather arbitrary.  I don't recommend you use it.

On the other hand, if you use the Mass Properties tool, you can use the priority setting to determine which component gets any overlapping region.  You could set the density to 1.0 and use the priority to achieve what you want.

Rob

[Josh Camp]

Yes, that does indeed work, thanks Rob!  

I used the tutorial here as a guide:  https://vspu.larc.nasa.gov/training-content/chapter-1-vspfundamentals/general-component-information/mass-property-priority-and-intersections/

Just in case anyone else runs into this issue, this is what I did...

I set the priority of the fuselage to 1 and the wing to 0.

density_fuselage = 1, density_wing = 0 results in the fuselage volume when using the mass properties tool

density_fuselage = 0, density_wing = 1 results in the wing volume

Thanks!

Josh Camp Director of Vehicle Engineering jo...@venusaero.com C: 281.381.8354

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