Progress Towards a Medical Image through CFD Analysis Toolkit for Respiratory Function Assessment on

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Progress Towards a Medical Image through CFD
Analysis Toolkit for Respiratory Function
Assessment on a Clinical Time Scale - PART 2 R.
F. Kunz, , D. C. Haworth, G. Dogan, A.
Kriete The Pennsylvania State University,
Applied Research Laboratory, Department of
Mechanical Engineering, Department of
Aerospace EngineeringDrexel University,
Department of Biomedical Engineering
Modular Software Infrastructure
Octree Based Grid Generation

• Goals are
• Automated medical image through CFD analysis
• Modular components facilitate sharing, updating
• Fast clinical time scale (hours)

• Open source drivers and some modules
• Robust to varying quality images
• Physics components to study sub-resolved scale
  effects of disease and aging

• HARPOON is used for grid generation of both lobe
  and airway trees. Grid generation is highly
  automated (batch execution, interactive views
  shown here) and very fast 1 million cells can be
  generated in approximately one minute on a single
  PC processor.
• Hexahedral dominant meshes with hanging nodes.
• Grid quality is increased by assigning different
  surface cell sizes to different generations ?
  streamed from partitioning attributes assigned
  above.
• Prism layers along boundaries for boundary layer
  resolution.

Lower Branch Modeling

• Lobe Volume Filling
• Another in-house code to reconstruct the
  tracheobronchial tree from each truncated airway
  down to the respiratory units of each lobe.
• A volume-filling algorithm has been devised such
  that the resulting spatial distribution of
  respiratory units is statistically uniform within
  each lobe.
• The branching algorithm reproduces geometric
  statistics (diameters, lengths, branching angles)
  of a human tracheobronchial tree.
• Typical tracheobronchial trees represent 18-22
  generations of branching, and contain between
  70,000 and 100,000 branches of which
  approximately half are terminal branches that end
  in a respiratory unit.

CFD Method (NPHASE-PSU)

• Approach
• Unstructured finite-volume CFD code arbitrary
  polyhedral element support
• Interphase coupling for Eulerian multiphase
  modeling (particle transport)
• Buoyancy included
• Deposition, drag forces, dispersion forces and
  eddy viscosity modeled
• Effect of turbulence on particle fields is
  accounted for following Aquino, Drew (2002)
• Assumes gas field drives particle turbulence for
  these disperse flows
• Turbulence dispersion force due to Carrica
  (1999)
• Impaction, sedimentation and diffusion
  deposition modeled
• Parallelized for rapid execution on cluster
  computers (2-8 hours)
• Volume, mass flow rates explicitly conserved
• Friction factors modeled
• Losses in lower branches due to pipe wall shear
  modeled using Q1D wall functions
• Losses in lower branches due to
  curvature/branching modeled using classical loss
  factors.

• Quasi-One-Dimensional Representation Using
  Arbitrary Polyhedra
• Multiscale approach taken is to fully resolve
  the upper branches (trachea through generation
  5-7) using 3D CFD, and the entire convective
  regime (generations 5-7 down through 17-20) with
  a Quasi-1D method.
• The octree grids that arise from the foregoing
  process yield between 105 ? 106 elements to
  capture the patient specific salient physics of
  the upper respiratory tract ? Secondary flows,
  turbulent boundary layers, flow splits, particle
  deposition, etc
• Another in-house code interfaces the truncated
  upper branch model and the volume filling
  tracheobronchial tree by constructing a closed
  volume pipe-and-branch model composed of O(105 ?
  106) arbitrary polyhedral sections.

Helicity contours and path lines at an inhalation
timstep
Two timesteps in an oxygen uptake simulation of a
breathing cycle