DOP FLIP

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Solver

Fluids data tables

Fluid Implicit Particle - Based on Navier–Stokes equations, implemented in OpenCL (can combine with vellum) hybrid solver. Fluid data is stored in the particles and only particles need to persist frame to frame Pressure projection step is done on a volume.
Basic flips

  • Size sensitive: use units as meters.
  • Time sensitive
FLIP model
1 Transfers the particle velocities into a volume
2 Solves to maintain volume
3 Push results back to the particles
4 Deletes volume

Solver

FLIP Solver

  1. OBJECT - mandatory
    • Flip Object SurfaceSOP/ Particles /Flip Tank (narrowband)/
  2. PARTICLE VELOCITY
    • Pop VOP / Pop Wrangle - @viscosity=x;
    • Pop Force, Pop Curve force - (can merged)
    • Pop Solver / Sop Solver - Access any sop (cmi)
  3. VOLUME VELOCITY
    • Gas Field VOP - Volume from sim (Illume Jeff II )
    • Source Volume - Source as velocity field
  4. SOURCE
    • Volume Source - Initialize source (SOP link to: Flip Source SOP) default will use op1 Flip Object
    • Heat Volume - Can spread temp (ie. lava cool rate)
    • Gas Temp Update
    • Pop Color

Substeps

  • timestep - (divide 1 frame to few)
  • substep - (add steps in 1 frame) Mostly for fast moving fluid that collides with other objects and Surface tension. (no need in standard flips)

Particle Motion

Collision detection

  • None
  • Move Outside Collision fastest and provides the smoothest splashes, Not as accurate with fast-moving collision geometry. It is also the (only collision method that works with Volume Source based collisions).
  • Particle method only works when a collision is represented by an actual DOP object. (can use direction and bounce)

Behavior
Kill Collide Friction condition + ID, Age attribs

Reseed
Fix an uneven distribution due to numerical error. (Must be on for Narrow Band)

Separation
Additional separation that prevent volume compression overwritten by velocity.

Droplets
Particle below certain density turn to droplets more likely. (tend to combine)

  • After blend back to fluid: Blend / Merge / Kill

Vorticity
Chaos & White water

  • Preservation rate - Decay value (how much vorticity should reman at end of the second) Retain swirling motion.

Volume Motion

Velocity Transfer (Kernel)

  • Splashy FLIP - Implemented with the newer method
  • Swirly APIC - is not as noisy and turb. and keep vorticity better
    smoothing.

Volume Limits

  • Waterline - (semi open boundaries)(above open, below close) it should be on water level.
  • Use boundary layer - (adv waterline) (velocity volume - at volume boundaries) (surface volume control geo in boundary padding) if both not connected it will use warterline options. (Houdini 16 Masterclass)

Collisions

  • Default velocity scale - for Volume Source is 1.5, which will cause larger splashes by default, but for moving containers this should be set to 1.
  • Stick on collision - [viscosity free solutions!: ] ( simualte sth like (free slip condition) but inverse)

Viscosity
How much tend to be solid. Adhesive force which resist motion (lava). Measured in: (centi) Poise. Set value at [Flip Object] DOP.

  • Slip on collision
    • 0 - Collider velocity. Can’t slip - Sticky
    • 1 - Fluid velocity. Slip on surface - Slide (opposite to: stick on collision)
  • Viscosity by Attribute Control locally use attribute @viscosity on points.

Density
By default, the fluid has uniform density as set on the Physical tab of the FLIP Object. @density

Air
Enforce Air Incompressibility on the FLIP Solver. This feature prevents air pockets from collapsing.

Divergence
Measure of imbalance. Scale component is using pressure field. Set up in POP VOP

Surface Tension
Tendency of liquid surfaces to shrink into the min surface area. (Require small scale but, In small scale it can be unstable > substeps) Values: 0 - 50 000

  • Blurring shape.
    • Adhesion - menisk wklęsły
    • Cohesion - menisk wypukły

Narrow Band

Work only with reseed.

Object

[ FLIP Object ] DOP

  • Particle separation - Seed density. How far apart particles try and stay. Will change particles count.
  • Particle radius scale - actual radius of every particle
  • Grid scale - resolution of the volume voxels. Help preserve Volume. To low will lose informations.
Initialize:
  • Surface sop -
  • Particle field -
  • Narrow Band - for tanks
  • File - from file
Physical:
  • Bounce -
  • Ffd Bounce -
  • Friction - 0 - frictionless object
  • Dyamic Friction - how diferent static and dynamic object frictions are (relation)
  • Temperature -
  • Density - 1000 kg/m3 which is default
  • Viscosity - in range 1 - 10 000 000 (if viscosity by attribute in solver is checked and there is point attribute at sop viscosity will be multiplied)

[ Volume Source ] DOP

For sourcing fluids or vel fields

Initialize:
  • Source FLIP - default path to FLIP Source SOP
  • Sink FLIP - sink point (size depend speed of sink)
  • Pump - for vel from volume. Import vel file.

Input - path
Name source volumes - volume name

SOP

Source

[ FLIP Source ] SOP

Converts its input geometry into a volume that can be used to control simulations. i (for Volume Source DOP)

  • inject liquid into a FLIP
  • act as a sink in a smoke simulation.
  • volume operations set behaviour (add/overide velo)
  • velocity volumes source attributes - we can sample it from N or sth.
  • container settings - velocity

[ Flip Tank ] SOP

Points From Volume (for source volume DOP) // source from points (change[Initial Data] input to particle field).
Oceane Source (for flip object DOP) (particles(points)+volume for: sop path and surface volume) // (change [Initial Data] input to narrow band) Fluid Tank ?

Limit Refinment iteration - can remove glitches, use only in small scale (good for visc droplets ect…) (particle fluid surface)

Collisions

Deforming Object shelf tool

Volume Source based collisions: Work only with Move Outside Collision method for particle collisions, because the Particle method only works when a collision is represented by an actual DOP object.

Move Outside Collision is the fastest collision handling method and provides the smoothest splashes, however it is not as accurate with fast-moving collision geometry.

static object with collision mode to: volume sample {DeformingGeo shelf}

  • Enabling Collision Separation on the FLIP Object and setting this value to the Particle Separation or smaller will create a higher-resolution collision field
  • Accurate velocities for moving collision objects are extremely important.
  • {Deforming Object} shelf tool to set up deforming geometry as a FLIP collision object.
  • SOP Collision Source - interpolate deforming geometry, calculate point velocities, and create VDB. Usually used in conjunction with a Static Object DOP.

VEX

i@stopped - You can stop the particles in a flip sim with
v@v => vel
@viscosity -

Import simulation

[DOP import] SOP

Import Points

Particle Fluid Surface - limit refinement if unstable. / Change surfacing > surface output > from surface polygons to Surface VDB !! For emitting large numbers of particles, the Volume Source DOP can be much faster than the Particle Fluid Emitter DOP.

[DOP I/O] SOP

Points + 4volumes. For further processing.

DOP Network - path to DOP Node
DOP Node - path to Flip Object
Choose volumes from preset or import custom volumes

[Fluid Compress] SOP

before cache

[Fluid Surface] SOP

to get fluid surface

Setups

  • Viscosity - Lava
  • Surface Tension - Deep drops, Crown Splash
  • Suction - Avoidance
  • Divergence -

Micro

depend on velo, scale, grid size

  • On solver:
  • Sub steps up!! up 5+
  • On Surface tension
  • Velocity transfer: Splashy » Swirly
  • In self they reduce gravity. Gravity reduced in Drip and crown to have small scale

Crown Splash

  • Flip tank with boundary layer
  • Surface tension: 48 more motion / 200 high value less motion for
  • Grid scale // fat 2 / thin 1.5 (grid to create surface and measure curvature )
  • you can blur curvature

Droplet

  • Surface tension: high 48000 to prevent fall down

Suction Fluid

Extension of surface tension.

  • Pop wrangle: op2 antigravity (oppose gravity around object), inside have no gravity.
  • Gas field wrangle: op3 suction force. strength + gradient on distance sometimes you need to keyframe it
  • collision source on geometry (velocity, volume out) + interior exterior distance for gradient strength and ramp in world units meters (outside ) .
  • ! remember voxel size on collision source volume cretion. !!!!!!!!!
  • narrow band needed as well !!!!!

Values to animate:
outside distance: big: good to pull from ground, smaller give better shape.
strength: 120 000 anim > 480 000 keep inside

Multiple solvers

Floating RDB

DOP>flipsolver>Volume Motion>solver change Feedback Scale from 0 to 1.
https://vimeo.com/116176349#at=158
https://youtu.be/JzxXjHgkIIc

Merge: rigidbody solver + flip solver
Check density of object (mass)    
Bullet solver with rdb object is working as well
Pyro > Fluid

https://vimeo.com/157944287

Sim Pyro > Import > Points from volume
Flip Object > change sop path to points
Cration frame spec sim frame check
Pyro > Fluid

https://vimeo.com/296410457

…

Fluid Follow Curve

https://forums.odforce.net/topic/31596-fluid-follow-curve/

Mix Colors

https://youtu.be/DWxcfZMsZW8

www.sidefx.com/docs/houdini/nodes/dop/flipsolver.html
www.danenglesson.com/images/portfolio/FLIP/rapport.pdf



## Velocity1
you can create `velocity field` > `source volume`


## Populate containers  
Can work with flips   
`Pump from ` - create volume velocity (like)

http://igorfx.com/hou_adaptive_flip/
https://www.youtube.com/watch?v=3oikshVefJ4

Also use for collision in flips flips (FLIP Fluids https://vimeo.com/116176349)