Coupling Water and Smoke to Thin Deformable and Rigid Shells

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Coupling Water and Smoke to Thin Deformable and
Rigid Shells

• Eran Guendelman 1,2
• Andrew Selle 1,3
• Frank Losasso 1,2
• Ronald Fedkiw 1,2
• 1Stanford University, 2Industrial Light Magic,
  3Intel Corporation

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Motivation

• Fluid simulation becoming more common
• Engineering, biomedicine, entertainment
• Want interaction with thin solids
• Parachutes
• Cardiovascular simulation
• CG characters w/clothing

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Goal

• Two-way coupling between
• Smoke or free-surface water
• Thin rigid and deformable open shells
• Prevent leaks across solid

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256x256x192 effective octree 30k triangles
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Volumetric vs. Thin Solids
Volumetric
Thin shell
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Related Work Volumetric

• DLM / Rigid Fluid Glowinski et al. 94Carlson
  et al. 04
• Inter-particle forces Génevaux et al. 03
  Müller et al. 04
• Coupling solid velocity fluid pressure
• Incompressible Takahashi et al. 02
• Compressible Yngve et al. 00 Fedkiw 02

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Diffuse Interface Methods

• Smear solid onto fluid grid
• e.g. Immersed boundary method Peskin 72
• Parasitic currents

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Sharp Interface Methods

• Incorporate jump conditions into stencils
• Ghost fluid method Fedkiw et al. 99 Tam et al.
  05
• Immersed interface method LeVeque Li 94

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Our Approach

• Couple using
• Solid velocity fluid coupling pressure
• Sharp interface treatment
• Prevent leaks using robust ray intersections

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Talk Overview

• Fluid simulation
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying coupling force
• Summary and future work

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Talk Overview

• Fluid simulation (focus on water)
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying fluid coupling force
• Summary and future work

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Fluid Simulation

• Assume incompressible inviscid
• Use projection method Chorin 68

un
(u is fluid velocity)
Advect un and add gravity ! u
u violates incompressibility
Compute pressure to enforce incompressibility
Project u ! un1
un1
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Fluid Grid

• Uniform octree grids Losasso et al. 04
• Staggered grid configuration Harlow Welch 65

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Advection

• First order semi-Lagrangian Courant et al. 52
  Stam 99
• Advection on nodes

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Particle Level Set Method
Enright et al. 02

• Level set ? captures water-air interface
• Particles help correct interface

water
air
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Water Simulation Step (n!n1)
un,?n
Advance particle level set ! ?n1
Advect ? and particles
Advect un and add gravity ! u
Project u ! un1
un1,?n1
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Now Add Solids to the Mix

• Fluid simulation
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying fluid coupling force
• Summary and future work

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Now Add Solids to the Mix

• Black box
• Input external forces
• Output positions and velocities

Guendelman et al. 03
Bridson et al. 02,03
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Surface Quantities

• Rigid body
• Directly compute

• Deformable body
• Barycentric weights

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Talk Overview

• Fluid simulation
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying fluid coupling force
• Examples, summary, and future work

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Key Visibility
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Thin Shell Aware Interpolation

• Check visibility of interpolation nodes

• Use replacement ghost value when interpolating

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Replacement Ghost Values
Fluid velocity (u)
use solid velocity
Level set (?)
average from nearest valid nodes
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Thin Shell Aware Advection

• Clip semi-Lagrangian rays

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Crossed Over Nodes

• Represent information from opposite side
• Reassign valid values by averaging

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Thin Shell Aware Fluid Step
un,?n
Thin shell aware advection(? and particles)
Advance particle level set ! ?n1
Advect un and add gravity ! u
Thin shell aware advection (u)
Project u ! un1
un1,?n1
see paper for more details
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210x140x140 uniform 30k triangles
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210x140x140 uniform 30k triangles
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Talk Overview

• Fluid simulation
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying fluid coupling force
• Summary and future work

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Rasterizing Solid

• Rasterize onto faces of fluid grid

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Solid Affecting Fluid

• Solid prescribes velocity on rasterized faces
• Enforce as Neumann boundary conditions in
  projection step

Project u ! un1
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Which Solid Velocities?

• At time n1!n2, at solid-fluid interface
• Fluid moves with velocity enforced during un1
  projection
• Solid moves from to Xn1 to Xn2
• Want these motions to match (reduce mass loss)
• Solution
• Enforce effective solid velocity
  Veff(Xn2-Xn1)/?t

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One-Way Coupling Step
un,?n,Sn,Sn1
(S is the solids state)
Advance particle level set ! ?n1
Advect un and add gravity ! u
Enforce effective solid velocities (n1! n2) at
solid-fluid interface
Project u ! un1
un1,?n1,Sn1,Sn2
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160x192x160 effective octree
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192x192x192 effective octree 60k triangles
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Talk Overview

• Fluid simulation
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying fluid coupling force
• Summary and future work

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Fluid Coupling Force

• Want to use fluid pressure
• Incompressible pressure can be noisy
• Incompressibility hard constraint
• Enforcing solid velocity hard constraint
• Better for compressible fluids Yngve et al. 00
  Fedkiw 02

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Smoother Coupling Pressure

• Treat solid as fluid
• Solve variable density fluid for pc
• Similar to projection step, but
• Solid velocities not enforced
• Fluid velocities not modified!

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Two Pressure Solves!

• Incompressible pressure (projection)
• Enforce incompressibility solid velocity
• Essential for reducing mass loss
• Coupling pressure
• Does not modify fluid velocity
• Essential for smoother coupling force on solid

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Must Enforce Solid Velocity
Rigid Fluid Carlson et al. 04
Enforcing solid velocity
Enforced!
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Computing Force on Solid

• Fluid pressure pushes on both sides

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Computing Force on Solid

• Net force is proportional to pressure jump pc

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Computing Force on Solid
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Two-Way Coupling Step
un,?n,Sn,Sn1
Advance particle level set ! ?n1
Advect un and add gravity ! u
Compute coupling pressure and apply force to solid
Advance solid ! Sn2
Project u ! un1
un1,?n1,Sn1,Sn2
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148x148x111 uniform 2.5k triangles
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200x200x200 effective octree 30k triangles
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256x256x192 effective octree 30k triangles
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256x256x192 effective octree 30k triangles
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Talk Overview

• Fluid simulation
• Solid simulation
• Preventing leaks across solid
• Enforcing solid velocity on fluid
• Computing and applying fluid coupling force
• Summary and future work

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Summary

• Sharp interface treatment
• Prevent leaks using ray intersections
  (visibility)
• Solid prescribes velocity boundary conditions
• Use effective velocity to reduce mass loss
• Smooth coupling force applied to solid
• Treat solid as fluid to compute smoother pressure

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Future Work

• Absorption, adhesion, permeability
• Compare against experiments

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Acknowledgements

• Mike Houston, Christos Kozyrakis, Mark Horowitz,
  Bill Dally, Vijay Pande
• Stanford Graphics Lab
• ONR, ARO, NSF, PECASE, Sloan Foundation, Packard
  Foundation

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The End