modeling best-practices for vspaero
TRANSCRIPT
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!
[email protected] Modeling Best-Practices for VSPAERO of 16
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!
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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?
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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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[email protected] Modeling Best-Practices for VSPAERO of 16
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)
[email protected] Modeling Best-Practices for VSPAERO of 16
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
[email protected] Modeling Best-Practices for VSPAERO of 16
• 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.
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Panel Mode• Try not to set subsurfaces exactly on interpolated sections.
• E.g, wing control subsurfaces, etc.
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[email protected] Modeling Best-Practices for VSPAERO of 16
• 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.
[email protected] Modeling Best-Practices for VSPAERO of 16
• 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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[email protected] Modeling Best-Practices for VSPAERO of 16
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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[email protected] Modeling Best-Practices for VSPAERO of 16
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
[email protected] Modeling Best-Practices for VSPAERO of 16
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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[email protected] Modeling Best-Practices for VSPAERO of 16
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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[email protected] Modeling Best-Practices for VSPAERO of 16
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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BRANDON LITHERLAND, ASTNASA LANGLEY RESEARCH CENTER
AERONAUTICS SYSTEMS ANALYSIS BRANCH
Thank you!Questions?