Fluids 10202013 Deborah R Fowler 1 Fluid Dynamics

Fluids 10/20/2013 Deborah R. Fowler 1

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” 10/20/2013 Deborah R. Fowler 2

“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 60 s, mathematically as early as the 50’s Navier-Stokes Equation – around for a couple of hundred years 10/20/2013 Deborah R. Fowler 3

Physical models, unlike key frame or procedural based techniques, (fluid sims) permit an animator to almost effortlessly create interesting, swirling fluidlike 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. 10/20/2013 Deborah R. Fowler 4

volumes are the focus read over interesting history in fxguide article Stanely Osher -> student Ron Fedkiw -> student Robert Bridson level set methods -> Particle Level Set (PLS) -> Phys. BAM, PIC/FLIP also with Marcus Nordenstam -> Exotic Matter 2008 - Naiad NOTE: H 12. 5 Docs “Houdini’s FLIP-PIC solver is based on the Siggraph paper Animating Sand as a Fluid by Yongning Zhu and Robert Bridson. ” 10/20/2013 Deborah R. Fowler 5

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 10/20/2013 Deborah R. Fowler 6

“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 10/20/2013 Deborah R. Fowler 7

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 10/20/2013 Deborah R. Fowler 8

Terminology • Grids (3 D 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) 10/20/2013 Deborah R. Fowler 9

Eulerian vs. Lagrangian How do you store the attributes • Eulerian – storing values on grid • Lagrangian – storing values on particles (traditional) 10/20/2013 Deborah R. Fowler 10

FLIP – Fluid Implicit Particles FLIP solver is a hybrid between a particle based and volume based fluid simulation 10/20/2013 Deborah R. Fowler 11

www. sidefx. com/docs/houdini 12. 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. 10/20/2013 Deborah R. Fowler 12

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) 10/20/2013 Deborah R. Fowler 13

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 10/20/2013 Deborah R. Fowler 14

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 10/20/2013 Deborah R. Fowler 15

Fluid Dynamics in Houdini For Fluids there are several techniques: Particle-based fluid solver (SPH and FLIP) Ø Volume-based simulations (voxel fluids/SDF) Ø Ripple solver Ø Ocean tools Ø 10/20/2013 Deborah R. Fowler 16

Fluid Techniques Ø Ø Ø 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 10/20/2013 Deborah R. Fowler 17

ØParticle Fluids (SPH and FLIP) • Particle fluid simulations use particles to represent the fluid – the surface is created based on the particles – H 12. 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 H 12 – Flips are recommended for liquid sims 10/20/2013 Deborah R. Fowler 18

Discussion http: //forums. odforce. net/index. php? /topic/15286 -h 12 -flip-fluid-emission-problems/ In H 12 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 open. VDB 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 10/20/2013 Deborah R. Fowler 19

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For those of you familiar with “older versions” there a couple of new features If you create a FLIP Fluid from a sphere: • Auto. Dop. Network – usual suspects: flipobject, flipsolver and gravity • sphere_object 1_fluid – now using the vdb nodes (there is a point wrangle node here to tranfer velocities) • sphere_object 1_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 10/20/2013 Deborah R. Fowler 21

Fluid Dynamics Homework on the particles page: • simulating liquids in Houdini • fluids- liquid 10/20/2013 Deborah R. Fowler 22