Vortex Detection on Unstructured Grids

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Vortex Detection on Unstructured Grids

• Monika Jankun-Kelly
• Mississippi State University ERC
• Simcenter

2
Motivation

• Vortical fluid flow applications
• Airplane wing design
• Naval propeller design
• Bronchial airflow modelling
• Goal understand vortex behavior
• Detect and verify
• Characterize
• Track

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What is a Vortex?

• Swirling flow
• Velocity vectors are tangent to a helical path
  around a vortex core

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Detection Algorithm

• Ming Jiangs combinatorial vortex detection for
  structured grids adapted for unstructured grids
• For each cell
• Calculate swirl vector at cell center
• Project velocity vectors at nodes onto swirl
  plane
• Check for direction spanning

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False Positives

• Sources and sinks
• Non-vortical direction spanning flow

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Swirl Vector Computation

• Two possible swirl vectors
• Vorticity vector, curl of velocity field
• Real eigenvector of velocity gradient tensor

vorticity
real eigenvector
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Velocity Gradient Computation

• Structured grids have explicit connectivity and
  permit use of simple central difference at nodes
• Unstructured grids have implicit connectivity
  (node-node neighbors not specified)
• Unstructured grid gradient computed at cell
  center using coordinate transform method

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Results

• Vortices detected
• False positives

utv propeller dataset
p5168_rfrm propeller dataset
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False Positive Elimination

• Surface cells not considered (ineffective)
• Stronger spanning requirement
• Velocity vectors rotate in same direction
• Cell size thresholding
• Aggregate size thresholding (unimplemented)

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Stronger Spanning Requirement
3 out of 4 quadrants
4 out of 4 quadrants

• 3 out of 4 quadrants is just right in most cases
• 4 out of 4 is too strict
• 2 out of 4 is not strict enough

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Same Rotation Direction
all vectors have same direction
most vectors have same direction

• Same direction for all, too strict
• Same direction for most, can be helpful

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Cell Size Thresholding
50th percentile
5th percentile

• Cells in regions of interest tend to be small
• Effectiveness of depends on parameter
• Unscientific hack, but makes pretty pictures

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Aggregate Size Thresholding

• Neighboring vortex core cells aggregate
• Cells in aggregates of size lt N eliminated
• Used in structured code
• Currently unimplemented in unstructured

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Future Work

• Near future
• Compare results with structured algorithm
• Verification
• Characterization
• When time permits
• Different gradient computation methods
• Other file formats (CGNS, Cobalt)
• Long term goal
• tracking