HOUDINI 9 ADVANCED FLUIDSBuilding a Fluid SolverSIGGRAPH 2007 Master Class TrainingJeff Lait

http://www.sidefx.com/papers/advfluid07.html

Fluid Solvers in Houdini 9

The Shelf quickly sets up a fluid simulation

Note the auto created Fluid Solver node

Inside Fluid Solver DOPs

The inside of the node is complex

We will rebuild this

Volume Primitives

New Primitive type in Houdini 9

A box divided into a grid with a value stored at each node

Manipulated by SOPs

Single point stores center

Transform stores size/rotation, like Spheres and Metaballs

Mantra can render it directly

IsoOffset, IsoSurface, VolumeMix SOPs

IsoSurface

IsoSurface allowed the expression of implicit functions

Outputs polygons at the zero-crossing of the function

Volume from IsoSurface

Build Volume option stores implicit function value at
every grid point

IsoSurface from Volume

The $V local variable in IsoSurface provides the incoming volume primitive value

Building Velocity Field

snoise is builds three different noise fields

Three fields merged together to provide x, y, and z components of velocity

Import to DOPs

SOP Vector Field imports three volume primitives as
vector field

Velocity Visualization

An abundant number of ways to visualize the velocity field with guide options

Vector Guide Options

Point Geometry

Begin with a lattice of points...

Add Geometry to DOPs

SOP Geometry DOP imports the geometry

Geometry Copy DOP makes it writeable

Advect Geometry

Gas Advect Microsolver moves the points through the velocity field

Advect Geometry

Motion of points follows the streamers

Note the clumping of points at stable points of field

Stamp Tubes

Primitive Tubes are stamped onto the points

Tube Advection

Tubes stay upright in playback as
only points
are moved

Velocity Stretch

Gas Velocity Stretch DOP uses local velocity values to twist the tubes

Tube Rotation

Tubes now rotate as the velocity field twists

Velocity Stretch

Gas Velocity Stretch DOP can also stretch the tubes according to shear/scale in velocity field

Tube Scaling

Tubes now fully distort according to field, swiftly becoming planes/points

(This is not
a nice field!)

Scalar Fields

IsoOffset allows you to build a Signed Distance field from geometry and store it in a Volume Primitive

Scalar Fields 2

Volume Mix manipulates values in volume primitives

expression builds a shell

Scalar Fields in DOPs

Import volume to DOPs with SOP Scalar Field

View as Smoke guide option

Scalar Field Guides

Inspect a slice with a plane guide

Scalar Field Advection

Gas Advection node can also advect a field

Smoke Advection

Smoke moves through the velocity field

Plane Advection

Single slice of the field as the field moves through the velocity field

Scalar Field Scaling

Respect the expansion/contraction in the field

Smoke Scaling

Note that the smoke stays sharper and we don't get a net gain of smoke

Plane Scaling

Compression effects are clear in single cross section view

Signed Distance Fields

Voxels store 1 for fluid,
0 for none

SDFs store negative for fluid, positive for none

Zero crossing not restricted to voxel boundaries, allows
sub-voxel detail

Iso Guide Options

Guide options for scalar fields allows one to track a specific IsoSurface

SDF Advection

One can advect distance fields in the same way as density fields

What is Divergence?

Measure of imbalance of velocity field

Divergence at center can be measured by comparing in going and outgoing velocties of boundary of cell

Incompressible fluids should have zero divergence

Consists of swirl, shear, translate factors. Does not have scale factor

Removing Divergence

Scale components can be represented as a
pressure field

Find a pressure field that removes our divergence

Changing one cell, however, affects neighbouring cells

Entire fluid is coupled and needs to be solved at once

Use preconditioned conjugate gradient method with an incomplete Cholesky preconditioner.

Or just use a Gas Project Non-Divergent DOP

Project Non-Divergent

Add a Gas Project Non-Divergent DOP to our non-nice velocity field

Pressure Computation

Pressure field created to remove divergence

Pressure Application

Final non-divergent field

2d Divergence

Same process in 2d for clarity

Note the central rarification becomes a low pressure
zone and is removed

Divergent Free Advection

Smoke no longer bunches up

Changing Velocity Field

Add a DOP Fan Force

Gas External Forces updates the velocity field

Fan Motion

Velocity accelerates inside fan

Exterior velocity only adjusted
by pressure

Loss of momentum!

Velocity Advection

Conserve momentum by moving the velocity

Do this by advecting the velocity with itself

Velocity Advection

Fan-accelerated air carries its velocity outside of the fan

Entire system starts moving

Combustion Inputs

Inputs to combustion model

Combustion Flow

Temperature
that exceeds
Ignition Temperature determines
potential burn

Burn clamped by fuel

Actual burn rate determines
feedback effects

Add Fields

We add to our fluid object the required fields,
such as Fuel

Add Field Temperature

Likewise, temperature

Note the initial hotspot to trigger the combustion

Add Burn Field

The output will be this burn field

Add Combustion Model

We use the Gas Combustion DOP to express this model

Inside Combustion Model

Flow chart implemented by a series of Gas
Calculate DOPs

Buoyancy Force

To get hot air to rise, we add a Gas Buoyancy DOP

Simple Combustion

Flame doesn't spread

No expansion at flame front

Intentional Non-Zero Divergence

To get the gas to expand at the flame front, set the burn field to be our desired divergence

With Gas Expansion

Temperature field stays sharp

Flame front doesn't spread against wind direction

Temperature Diffusion

A Gas Diffuse DOP lets us smear out the
temperature field

With Temperature Diffusion

Fire chases
the fuel against the updraft

Only Burn Field

Examining the burn field alone

Shows which voxels burn on this timestep

Soot Field

Turn on the display of the density field we use to store the burnt fuel

Smoke and Fire

Both displayed at once

Augmenting Houdini Solvers

Add a Colour field to Houdini's Fluid Solver

Build three volume primitives for Red/Green/Blue

Add to DOPs

SOP Vector Field adds the colour data

Gas Advect moves it by the velocity

Apply in SOPs

Import extra colour field

apply_colour is copying the colour field onto Cd

VOP SOP for Field Application

red, green, blue, are Volume Sample VOPs

Colour Scalar

Colour tracks fluid

Boundary blends the two colours together

Could add
Gas Diffuse
to increase blending

Sharp Boundaries

Build an SDF to store which colour we want.

Add Colour Key to DOPs

Replace the SOP Vector Field with SOP Scalar Field.

Use Colour Key to set Colour

VOP SOP converts negative/positive to either red or green

Colour Key

Colour tracks fluid

Sharp boundary between colours is maintained

Summary

Arbitrary data fields can be added to extend a fluid simulation

Small atomic microsolvers can be used to build a complicated simulation out of manageable pieces

Packaged solvers are built in this manner a good
source to see how micro solvers can be used

Packaged solvers have extra inputs to take your own added layers of simulation

Click to edit the title text format

Click to edit the outline text format

Second Outline Level

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