Introduction to Fluid Simulation

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Introduction to Fluid Simulation

• Jacob Hicks
• 4/25/06
• UNC-CH

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Different Kind of Problem

• Can be particles, but lots of them
• Solve instead on a uniform grid

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No Particles gt New State

• Particle
• Mass
• Velocity
• Position

• Fluid
• Density
• Velocity Field
• Pressure
• Viscosity

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No Particles gt New Equations

• Navier-Stokes equations for viscous,
  incompressible liquids.

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What goes in must come out

• Gradient of the velocity field 0
• Conservation of Mass

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Time derivative

• Time derivative of velocity field
• Think acceleration

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Advection term

• Field is advected through itself
• Velocity goes with the flow

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Diffusion term

• Kinematic Viscosity times Laplacian of u
• Differences in Velocity damp out

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Pressure term

• Fluid moves from high pressure to low pressure
• Inversely proportional to fluid density, ?
  

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External Force Term

• Can be or represent anythying
• Used for gravity or to let animator stir

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Navier-Stokes

• How do we solve these equations?

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Discretizing in space and time

• We have differential equations
• We need to put them in a form we can compute
• Discetization Finite Difference Method

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Discretize in Space
Staggered Grid vs Regular
X Velocity Y Velocity Pressure
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Discretize the operators

• Just look them up or derive them with
  multidimensional Taylor Expansion
• Be careful if you used a staggered grid

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Example 2D Discetizations
Divergence Operator
Laplacian Operator
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Make a linear system

• It all boils down to Axb.

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Simple Linear System

• Exact solution takes O(n3) time where n is number
  of cells
• In 3D k3 cells where k is discretization on each
  axis
• Way too slow O(n9)

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Need faster solver

• Our matrix is symmetric and positive
  definite.This means we can use
• Conjugate Gradient
• Multigrid also an option better asymptotic, but
  slower in practice.

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Time Integration

• Solver gives us time derivative
• Use it to update the system state

Ut
U(t?t)
U(t)
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Discetize in Time

• Use some system such as forward Euler.
• RK methods are bad because derivatives are
  expensive
• Be careful of timestep

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Time/Space relation?

• Courant-Friedrichs-Lewy (CFL) condition
• Comes from the advection term

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Now we have a CFD simulator

• We can simulate fluid using only the
  aforementioned parts so far
• This would be like Foster Metaxas first full 3D
  simulator
• What if we want it real-time?

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Time for Graphics Hacks

• Unconditionally stable advection
• Kills the CFL condition
• Split the operators
• Lets us run simpler solvers
• Impose divergence free field
• Do as post process

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Semi-lagrangian Advection
CFL Condition limits speed of information travel
forward in time
Like backward Euler, what if instead we trace
back in time? p(x,t) back-trace
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Divergence Free Field

• Helmholtz-Hodge Decomposition
• Every field can be written as
• w is any vector field
• u is a divergence free field
• q is a scalar field

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Helmholtz-Hodge
STAM 2003
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Divergence Free Field

• We have w and we want u
• Projection step solves this equation

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Ensures Mass Conservation

• Applied to field before advection
• Applied at the end of a step
• Takes the place of first equation in Navier-Stokes

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Operator Splitting

• We cant use semi-lagrangian advection with a
  Poisson solver
• We have to solve the problem in phases
• Introduces another source of error, first order
  approximation

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Operator Splitting
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Operator Splitting

• Add External Forces
• Semi-lagrangian advection
• Diffusion solve
• Project field

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Operator Splitting
u(x,t)
W0
W1
W2
W3
W4
u(x,t?t)
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Various Extensions

• Free surface tracking
• Inviscid Navier-Stokes
• Solid Fluid interaction

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Free Surfaces

• Level sets
• Loses volume
• Poor surface detail
• Particle-level sets
• Still loses volume
• Osher, Stanley, Fedkiw, 2002
• MAC grid
• Harlow, F.H. and Welch, J.E., "Numerical
  Calculation of Time-Dependent Viscous
  Incompressible Flow of Fluid with a Free
  Surface", The Physics of Fluids 8, 2182-2189
  (1965).

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Free Surfaces
MAC Grid
Level Set








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Inviscid Navier-Stokes

• Can be run faster
• Only 1 Poisson Solve needed
• Useful to model smoke and fire
• Fedkiw, Stam, Jensen 2001

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Solid Fluid Interaction

• Long history in CFD
• Graphics has many papers on 1 way coupling
• Way back to Foster Metaxas, 1996
• Two way coupling is a new area in past 3-4 years
• Carlson 2004

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Where to get more info

• Simplest way to working fluid simulator (Even has
  code)
• STAM 2003
• Best way to learn enough to be dangerous
• CARLSON 2004

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References
CARLSON, M., Rigid, Melting, and Flowing Fluid,
PhD Thesis, Georgia Institute of Technology, Jul.
2004. FEDKIW, R., STAM, J., and JENSEN, H. W.,
Visual simulation of smoke, in Proceedings of
ACM SIGGRAPH 2001, Computer Graphics Proceedings,
Annual Conference Series, pp. 1522, Aug.
2001. FOSTER, N. and METAXAS, D., Realistic
animation of liquids, Graphical Models and Image
Processing, vol. 58, no. 5, pp. 471483,
1996. HARLOW, F.H. and WELCH, J.E., "Numerical
Calculation of Time-Dependent Viscous
Incompressible Flow of Fluid with a Free
Surface", The Physics of Fluids 8, 2182-2189
(1965). LOSASSO, F., GIBOU, F., and FEDKIW, R.,
Simulating water and smoke with an octree data
structure, ACM Transactions on Graphics, vol.
23, pp. 457462, Aug. 2004. OSHER, STANLEY J.
FEDKIW, R. (2002). Level Set Methods and Dynamic
Implicit Surfaces. Springer-Verlag. STAM, J.,
Real-time fluid dynamics for games, in
Proceedings of the Game Developer Conference,
Mar. 2003.