Accurate Viscous Free Surfaces for Buckling, Coiling, and Rotating Liquids

Christopher Batty and Robert Bridson

University of British Columbia

• Many common liquids exhibit viscosity…

Viscous Liquids

Viscous Buckling and Coiling

• Characteristic of highly viscous liquids

• Dependent on correct forces at the surface

Viscous Buckling

Goals

• Accurate free surface behavior

• Fully implicit, for large stable time steps

• Handle variable viscosity

• Easy implementation & efficient solution

Eulerian Fluid Simulation

Pressure Projection

Viscosity

Advection

External Forces

Related Work

• Carlson et al. 2002, Roble et al. 2003– Viscous liquids with a simplified

implicit solve

• Rasmussen et al. 2004– Variable viscosity liquids with

IMEX integration

• Goktekin et al. 2004, Zhu & Bridson 2005– Non-Newtonian liquids

(viscoelastic, granular)

Fundamentals

• Viscosity is analogous to a fluid friction

• Nearby elements of fluid exchange velocity, affecting their flow

• Shear stress tensor, , is:– a measure of the resulting force per unit area– dependent on the gradient of velocity

Complete Form

• Shear stress is expressed as:

• To apply the resulting forces to the fluid:

• This is the full PDE form for viscosity

The Usual Simplification

(Full form)

The Usual Simplification

(Full form)

(Constant viscosity)

The Usual Simplification

(Full form)

(Constant viscosity)

(Expand)

The Usual Simplification

(Full form)

(Constant viscosity)

(Expand)

(Calculus identity)

The Usual Simplification

(Full form)

(Constant viscosity)

(Expand)

(Calculus identity)

(Incompressibility, )

The Simplified Form

• Looks like diffusion/smoothing of velocity – Velocity components are decoupled– 3 implicit Poisson-like systems, solved with PCG

• Eg. [Carlson et al, 2002]

– What about the free surface?

• Air applies zero force on the liquid surface

Free Surface Condition

Free Surface Condition

• Air applies zero force on the liquid surface

• The term is needed to enforce the constraint - it can’t simplify!– Free surfaces require the full stress expression

even for constant viscosity

Incorrect Free Surfaces

• What are the side effects?– Neumann BC:

• Adds erroneous “ghost” forces • halts rotation

– Dirichlet BC: • prevents viscosity from acting at the surface• liquid seems less viscous

• Buckling fails to arise in either case.

Correct Free Surfaces

…are very difficult to discretize directly. • GENSMAC method (Tomé, McKee, et al.) is the only other MAC-

based approach

• Velocity gradients aren’t naturally co-located• The constraint should be applied only at the surface• Difficult to avoid special cases• Can it be solved implicitly?• How is the linear system affected? (symmetry,

definiteness, etc.)

Key Idea

• The free surface is actually a natural boundary condition in this setting– Using the proper variational form, it will fall out

automatically

• Idea: Replace the viscosity solve with minimization of a variational principle.

Characterizing Viscous Flow

• Minimum Dissipation Theorem– The solution to a Stokes problem minimizes

viscous dissipation [Helmholtz, 1868]

Characterizing Viscous Flow

• Minimum Dissipation Theorem– The solution to a Stokes problem minimizes

viscous dissipation [Helmholtz, 1868]

• Viscous dissipation:– Kinetic energy dissipated by viscosity

Variational Form

• Minimize dissipation while perturbing velocity as little as possible

• This is equivalent to the full PDE form

Variational Form

• Benefits:– No need to enforce the free surface discretely

• Just estimate integrals and minimize

– Fully implicit, SPD system• Take large timesteps, solve with CG

– Supports variable viscosity– Exhibits the correct behaviour

• Caveat…– Velocity components are no longer decoupled

• Get a single 3x larger linear system

Discretization

• Use the classic (MAC) staggered grid– Velocities at cell faces– Stress at cell centres and edges

• See [Goktekin et al, 2004]

• syncs up naturally with positions of velocity gradients

Discretization

• Compute terms at each sample point– Faces for 1st integral, edges/centres for 2nd integral– Use centred differencing for velocity gradients

• Scale by the liquid fraction

in the surrounding cube

Linear System

• Identical to a MAC-based discretization of the full viscosity PDE…– but with new volume weights added!

Before:

After:

Results

• Artifact-free rotation and bending

• Viscous buckling and coiling

• Efficient, stable, highly variable viscosity

Future Work

• The linear system is no longer an M-matrix– Incomplete Cholesky may be less effective– Can we find better preconditioners?

• Full free surface condition involves pressure, viscosity & surface tension– Can we solve all three simultaneously?– Should we? (speed vs. accuracy tradeoff)

• Accuracy– Further analytical and ground truth comparisons

Conclusions

• Don’t solve the PDE – minimize the variational principle!

• For viscosity, this approach…– drastically simplifies complex boundary

conditions– yields efficient, straightforward, robust code– produces convincing simulations of purely

viscous liquids

Thanks!

• I’ll be happy to take any questions…