modeling best-practices for vspaero

BRANDON LITHERLAND, ASTNASA LANGLEY RESEARCH CENTER

AERONAUTICS SYSTEMS ANALYSIS BRANCH

Modeling Best-Practices for VSPAERO

2021 OpenVSP Workshop, Virtual Broadcast

October 5-7, 2021

[email protected] Modeling Best-Practices for VSPAERO of 16

Outline• Build-Up Approach

• General Guidance

• VLM

• Panel

• Disks

• Rotating Blades

• Troubleshooting & Tips

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These guidelines are not comprehensive and should only be considered a good starting point. Refining your model is up to you!


Build-Up Approach• When in doubt, start simple… What are you trying to find out?

• Think of a free-body diagram. What matters?

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Lift

Drag

Momentvs.

Do NOT start with this!


Build-Up Approach

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Wing

Control Surfaces

SubsurfacesSeparate

Components

Tails

Horizontal

Vertical

Trim tabs

Bodies

NacellesFuselages

Propulsion

Rotating Blades

Actuator Disks

• Build trust in your model and methods.• Compare to experiment or real data if possible.

• Compare to similar models.

• Examine the trends in results.

• Think about the assumptions and limitations.

• Gradually increase complexity.• When you add something does it behave as

expected? If not, why?

• If VSPAERO isn’t working or gives oddresults:• Remove everything but the wing and try again.

• Add one thing at a time until things break.

• Why did that one thing break the model?


General Best-Practice• Use clustering to your advantage

• Group points around large expected gradients such as wingtips or behind props.

• Single Section Wing: Root 1.0-2.0 Tip 0.2-0.25.

• Multi-Section Wing: Set clustering near section breaks so neighbor cells are similar.

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General Best-Practice• More points doesn’t always mean a “better” solution.

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(A)NumW/NumU = 33 / 6

(B)NumW/NumU = 41 / 21

(C)NumW/NumU = 61 / 61

(D)NumW/NumU = 201 / 201

Avoid high aspect ratio “sliver” cells. Lead to numerical problems.

Overkill. Don’t do this.Good Better Even Better

(A)

(B)(C)


VLM-Mode• Avoid using ‘body’ components at first.

• Body components don’t have attached wakes and will not contribute to lift or drag.

• Body cruciform will change the total moments.

• Check your Degen surfaces and watch out for overlapping panels.

• Avoid coincident surfaces such as overlapping wings and body cruciforms.

• Avoid straight-sided sections• Rounded rectangle is the most common but General and Edit

Curve will also cause issues.

• Replace these sections with ellipses.

• This goes for body sections with vertical faces and closures as well. (dX = 0)

• Use LE/TE clustering of 0.25/0.25

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No wake

Overlap


• Do not use blunt trailing edges on wings or bodies.• Change native blunt edges to sharp using Closure or Cap.

• Sharpen aft-facing body surfaces by moving coincident aft sections slightly.

• LE/TE clustering at 0.25/1.0.• Can set to 0.25/0.25 later if needed.

Panel Mode

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Flat Cap- Will fail Panel mode

Edge Cap- Extends chord very slightly.

Closure > Skew Both > 0.0- Camber line maintained.- Upper/lower surface curves

altered slightly.Note: Closure > Extrapolate > 0.0 will also sharpen the edge but will also significantly extend the chord. Not recommended.

Airfoils are not all equal. Check which method works for you.


Panel Mode• Try not to set subsurfaces exactly on interpolated sections.

• E.g, wing control subsurfaces, etc.

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• Avoid surfaces and intersections that lead to bad triangulation.• Coincident/overlapping surfaces

• Mismatched tessellation

Panel Mode

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Coincident panels.Poor intersection.Slivers present.

Shortened tip chord, decreased thickness, increased root chord, adjusted sweep >> Much better intersection.


• For now, actuator disks in Panel mode need to be added to the *.vspaero file via text editor and run from command line.• Make this easy by creating a VLM version and then copy the rotor inputs.

• Run VSPAERO from the command line using “vspaero <filename>”

• GUI capability will be added back in upcoming release.

Panel Mode

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Actuator Disks• “Disk” vs “Prop”

• When using the GUI, you have to remember which direction the disk is pointing.• Check the *.vspaero input file to make sure the thrust direction is correct.

• Prop components have these markers built in. Simply switch Prop Mode to Disk for VSPAERO.• In Disk mode, the markers are not displayed.• Remember: Your model will update MUCH faster in Disk mode.• Hub diameter is automatically set by default for Prop based on r0.

• Can attach a Blank to Disk to indicate thrust direction (-X in Disk frame).

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Actuator DisksThrust and Power Coefficient Estimates

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Computing Thrust and Power Coefficients

• XROTOR

• Hamilton Standard “Red Book” maps

• Blade-Element Momentum (BEM) methods

• Known propeller performance from manufacturer


Rotating Blades

• Prop in “Blade” mode.

• Treat blades as just another VLM surface.• Same rules for clustering and tessellation.

• Try not to let them hit anything.

• Don’t include nacelles at first.

• Don’t set unrealistic RPMs for Props.• Your solution will ‘blow up’.

• Start with auto time stepping.

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Troubleshooting• Use the Viewer to examine the result.

• Does the pressure distribution make sense? Check the legend for large peaks.

• Do the wakes move as expected?

• Is something missing?

• Draw Control Surfaces Deflected is for visualization only. Panel breaks may not occur exactly on subsurf line.

• Check the results manager. Does the spanwise loading make sense?

• Check the convergence. Look at CL, CDi, and L/D. 10^-5 is the minimum for log residual.

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Other Tips• You can run Panel mode on an imported mesh!

• Same rules apply from Panel guidance. A CAD STL is unlikely to be good for aero.

• This also goes for CFDMesh grids from VSP.

• Use ‘-’ or ‘+‘ (keyboard -/+) to view agglomeration meshes.

• Copy the first line of the VSPAERO console output and paste in the command window to execute manually.

• Use Advanced Link to set CT and CP quickly. Lots of similar props? Use Link to set all at once.

• The OpenVSP Google Group is available for unique problems, questions, or bug reports at https://groups.google.com/g/openvsp

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https://groups.google.com/g/openvsp

BRANDON LITHERLAND, ASTNASA LANGLEY RESEARCH CENTER

AERONAUTICS SYSTEMS ANALYSIS BRANCH

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