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Fluids

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Fluid Dynamics

What is a fluid? Anything that takes the shape of its container (liquids and gasses)

Fluid Dynamics – not just for fluids• air flow (smoke, fire) “aerodynamics”• fluid (liquid) “hydrodynamics”

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“one of the most significant and commonly requested ares of real work simulation is fluid simulation” from The Science of Fluid Sims

computationally expensive

used outside of vfx – CFD (computational fluid dynamics) existed since mid 60s, mathematically as early as the 50’s

Navier-Stokes Equation – around for a couple of hundred years

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Physical models, unlike key frame or procedural based techniques, (fluid sims) permit an animator to almost effortlessly create interesting, swirling fluid-like behaviors. Also, the interaction of flows with objects and virtual forces is handled elegantly. Jos Stam. (1999). Stable Fluids, SIGGRAPH 1999 Conference Proceedings: SIGGRAPH Annual Conference Series. pp. 121-128.

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volumes are the focus

Stanely Osher -> student Ron Fedkiw -> student Robert Bridson

level set methods -> Particle Level Set (PLS) -> PhysBAM, PIC/FLIP

also with Marcus Nordenstam -> Exotic Matter 2008 - Naiad

read over interesting history in fxguide article

NOTE: H12.5 Docs “Houdini’s FLIP-PIC solver is based on the Siggraph paper Animating Sand as a Fluid by Yongning Zhu and Robert Bridson.”

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Solving Navier-Stokes Equation

vfx industry takes a subset of the problem with some key assumptions

output of the equation is a velocity field (also called flow field) – it really is about how the particles move, not their position

differs from RBD where solutions are usually trajectories of position of a particle

practical implementations are required

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“A full solution is out of the question so as with much of computer graphics we end up with clever workarounds and insightful cheats”

-from fxguide The Science of Fluid Sims

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Fluid simulation

• CG tool for generating realistic animations of water, smoke, explosions etc.

• Given initial configuration of fluid the simulator evolves the motion forward in time

(often calculated using Navier-Stokes equations which describe the physics of fluids)

• competing techniques Euler grid-based, SPH (smoothed particle hydrodynamics), vorticity-based and Lattice Boltzmann methods

• for further discussion see http://en.wikipedia.org/wiki/Computational_fluid_dynamics

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Terminology

• Grids (3D map) = voxels• In SOPS they are called volumes• In DOPS they are called fields

SDF – stores distance from object (float)

Fog – is it there or not (pixelation – it is in a voxel or not)

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Eulerian vs. Lagrangian

How do you store the attributes• Eulerian – storing values on grid• Lagrangian – storing values on particles

(traditional)

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FLIP – Fluid Implicit Particles

FLIP solver is a hybrid between a particle based and volume based

fluid simulation

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www.sidefx.com/docs/houdini12.5/dyno/liquids

FLIP fluids – similar to SPH

Instead of using “outward” forces to prevent liquid from collapsing, at each step the solver copies the particle's velocities onto a grid, calculates a new velocity field, then copies them back onto the particles.

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SPH – smoothed-particle hydrodynamics

SPH stands for smoothed-particle hydrodynamics

-computational method for simulating fluid flows

-it is a mesh-free Lagrangian method (coordinates move with the fluid)

-developed in 1977 for use in astrophysical problems

-divides the fluid into a set of particles (SPH computes using weighted contributions of neighboring particles rather than solving linear systems of equations)

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So why not FLIP?

-drawback over grid-based techniques is the need for large numbers of particles to produce simulations of equivalent resolution

-SPH is used in real-time animation and games

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Particle-based fluid solver (SPH and FLIP)

these are methods that exist in other packages as well (not unique to houdini)

Houdini’s recommended solver is flip

Reminder: scale is important

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Fluid Dynamics in Houdini

For Fluids there are several techniques:

Particle-based fluid solver (SPH and FLIP) Volume-based simulations (voxel fluids/SDF) Ripple solver Ocean tools

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Fluid Techniques

Particle-based fluid solver - unrestricted, intricate fluid behavior

- particularly good for splashes- marriage of POPs and DOPs

Volume-based simulations (voxel fluids) - restricted to a box but can get very accurate fluid

behavior - useful for liquid, smoke, fire Ripple solver/Ocean tools -useful for open surface like ponds or oceans

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Particle Fluids (SPH and FLIP)

• Particle fluid simulations use particles to represent the fluid – the surface is created based on the particles – H12.5 has changed its tools (more about that in a moment)

• Previously FLIP fluids were primarily used to create fast moving and large body simulation like waterfalls. You can also use pops with them.

• SPH fluids good for slow-moving fluids that react to fast moving constraints (glass of water)

Since H12 – Flips are recommended for liquid sims

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Discussion

http://forums.odforce.net/index.php?/topic/15286-h12-flip-fluid-emission-problems/

In H12 the sourcing system for FLIP fluids has changed. Previously there was a source node that acted like a POP’s source. Then there was a sourcevolume. Now the points are converted into an openVDB volume.

Fun post on odforce about Viscosity Settings inspired by Igor Zanic – note that this post pre-dates the improvements to viscosity in 12.5

http://forums.odforce.net/index.php?/topic/15034-viscous-teapots/

So what happened to SPH? Improvements to FLIP

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For those of you familiar with “older versions” there are a couple of new features

If you create a FLIP Fluid from a sphere:

• AutoDopNetwork – usual suspects: flipobject, flipsolver and gravity

• sphere_object1_fluid – now using the vdb nodes (there is a point wrangle node here to tranfer velocities)

• sphere_object1_fluidinterior – imports the geometry of the fluid/different material ie. not basicliquid as above but uniformvolume

GO TO MY WEBSITE for a diagram or try this yourself in Houdini

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Fluid Dynamics Homework

Homework

on the particles page:

• simulating liquids in Houdini• fluids- liquid

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