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Fluids 10/20/20131Deborah R. Fowler

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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” 10/20/20132Deborah R. Fowler

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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 SimsThe 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 10/20/20133Deborah R. Fowler

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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. 10/20/20134Deborah R. Fowler

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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.”Animating Sand as a Fluid 10/20/20135Deborah R. Fowler

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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 10/20/20136Deborah R. Fowler

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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 SimsThe Science of Fluid Sims 10/20/20137Deborah R. Fowler

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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 http://en.wikipedia.org/wiki/Computational_fluid_dynamics 10/20/20138Deborah R. Fowler

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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) 10/20/20139Deborah R. Fowler

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Eulerian vs. Lagrangian How do you store the attributes Eulerian – storing values on grid Lagrangian – storing values on particles (traditional) 10/20/201310Deborah R. Fowler

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FLIP – Fluid Implicit Particles FLIP solver is a hybrid between a particle based and volume based fluid simulation 10/20/201311Deborah R. Fowler

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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. 10/20/201312Deborah R. Fowler

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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) 10/20/201313Deborah R. Fowler

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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 10/20/201314Deborah R. Fowler

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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 10/20/201315Deborah R. Fowler

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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 10/20/201316Deborah R. Fowler

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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 10/20/201317Deborah R. Fowler

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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 10/20/201318Deborah R. Fowler

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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 10/20/201319Deborah R. Fowler

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10/20/201320Deborah R. Fowler

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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 HoudiniWEBSITE for a diagram 10/20/201321Deborah R. Fowler

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Fluid Dynamics Homework Homework on the particles page: simulating liquids in Houdini fluids- liquid 10/20/201322Deborah R. Fowler

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