1. Automotive Soiling Simulation Based On Massive Particle Tracing Stefan Röttger Martin Schulz Wolf Bartelheimer Thomas Ertl Visualization and Interactive Systems Group University of Stuttgart

2. Visualization and Interactive Systems Group, University of StuttgartPage 2 Introduction Where did we start? Lattice-Boltzmann CFD solver (PowerFlow) used at BMW Hierarchical cartesian grids Fast tri-linear interpolation

3. Visualization and Interactive Systems Group, University of StuttgartPage 3 Standard Flow Visualization Tools Interactive and immersive navigation in virtual windtunnel Stream lines Stream ribbons Glyphs Cutting planes

4. Visualization and Interactive Systems Group, University of StuttgartPage 4 Goals What did we want? Massive particles for simulation of dust particles Interactively animated particles for more intuitive flow visualization Automotive soiling simulation

5. Visualization and Interactive Systems Group, University of StuttgartPage 5 Massive Particles Each particle is treated as ideal sphere with specific mass, diameter and initial position and velocity Particle drag, gravity and electrostatic forces affect acceleration of particles Stokes approximation of the drag is not good enough Particle drag in the flow is approximated by the formula of O´Seen for low Reynolds numbers

6. Visualization and Interactive Systems Group, University of StuttgartPage 6 Massive Particle Tracing Second order differential equations: a(t)=>v(t)=>x(t) Adaptive, embedded Runge-Kutta tracer of order 4(3) Below 3 µm the differential equations are becoming stiff, but then massive and massless particle tracing is almost equivalent => no soiling Average dust particle diameter in real world evalutions is 5 to 500 µm

7. Visualization and Interactive Systems Group, University of StuttgartPage 7 Animated Massive Particles Define emitters that generate particles at a certain frequency Emitters can be sized and positioned interactively Initial parameters of particles are assigned stochastically Particle stream is traced and displayed step by step The stream is displayed in slow motion at a given target frame rate Camera exposure model => particles leave short traces

8. Visualization and Interactive Systems Group, University of StuttgartPage 8 Animated Massive Particles Intuitive visualization analogue to smoke probes Simultaneous display of multiple particles Particle velocity is visible implicitly Better three-dimensional impression due to animation Life time is color coded

9. Visualization and Interactive Systems Group, University of StuttgartPage 9 Automotive Soiling Simulation Stochastically generate and trace massive particles Check for collision with the 70,000 surface triangles by utilizing an octree Color the hit points on the car body (nearest mesh vertex) Color code the number of hits on the surface (blue = no hits)

10. Visualization and Interactive Systems Group, University of StuttgartPage 10 Simulation Performance Hit probability is low Large number of particles Simulation can be parallelized efficiently (e.g. 64 CPU SGI Onyx) On an SGI Octane with 2x250MHz MIPS R10K approximately 1000 particles can be traced simultaneously at 7 Hz Scales well with #CPUs 1 hour 3 hours

11. Visualization and Interactive Systems Group, University of StuttgartPage 11 Simulation Quality Now using stationary flow fields (120M per time step) Turbulences are smoothed away in time averaged flow fields and hit probability is reduced even further => Instationary flow fields Electrostatic forces influence dust aggregation as well O´Seen Stokes

12. Visualization and Interactive Systems Group, University of StuttgartPage 12 Conclusion Animated massive particle streams for intuitive data set exploration Massive particle tracing used to compute automotive soiling simulation by employing collision detection Good coincidence with real world soiling situation More accurate simulations require instationary flow fields and research about near surface effects Massive particle tracing can be applied to other regions of interest like smog or droplet distribution simulations

13. Visualization and Interactive Systems Group, University of StuttgartPage 13 Discussion Questions?