Deborah R. Fowler

Particle fluids (SPH and FLIP and now PBD). Particle fluid simulations use particles to represent fluid. For visualization and rendering, the particles are surfaced (surface is created based on the particles).

• SPH (particle-only) - not recommended since version 11, but good to be aware of technique. (good for slow-moving fluids that react to fast-moving constraints, such as a glass of water)
  
• FLIP (particle plus grid) - similar to SPH - solver copies the particles' velocities onto a grid and calculates a new velocity field that is copied back onto the particles. Recommended for large fluid sims.
  
• PBD (position-based dynamics) - similar to SPH, recommended for small fluid sims
  

You should have knowledge of other methods as well, however as of Houdini 12 and higher, FLIP is recommended. However in Houdini 17 pop fluid was introduced and is used for generating whitewater.

Houdini specific

To simulate most liquids use FLIP. As of H17, PBD (pop fluid) is available for small fluid sims. For fluids such as smoke and flame - use pyro.

Fountain tips

Documentation tips at http://www.sidefx.com/docs/houdini/fluid/tips.html

TIP: VOLUMES, VOLUMES, VOLUMES - collision objects can't have thin geometry - if they do, use a proxy - also, collision/emitter objects need volume, so a grid for example needs to be extruded.

TIP: OpenVDB can come in extremely handy with complex collision geometry (see tips and trips entry under Flips) - this is now default!

TIP: Animate your source to add some variety to your simulation (can be as simple as adding a mountain node with a time value in the offset parameter). Alternatively animate the velocity.

TIP: When creating a volume such as a pool at the bottom of a waterfall, you can use the sculpted particle fluid rather than waiting for the fluid to fill to a level.

TIP: Cache out your sim when rendering on the renderfarm. You can use the cache node or a rop output driver (two places to cache shown in class). If you are using whitewater be aware that it references the filecache node on the compressed cache in your base sim.

TIP: On the flat tank - increase the particle amount to get more spray in the whitewater_sim/waterwater_emiiter node (thanks Chao). Also there is a description in the documentation about the differences between the various flip tanks here.

TIP: Use feedback scale for dropped objects (Flip Solver/Volume Motion/Solver/Feedback Scale). Also adjust density of the object.
        Below is rubber toy with density 100, feedback 1 and particle separation .2 on the default beach tank. (To avoid rubber toy's body parts from self destructing be sure to use a hero RBD.)

TIP: Don't use bgeo.gz, now since H14 use bgeo.sc - there is also compress - see new default network shown below

15 TIP: If your fluid is "glitching" take a look at the tips for the particle surfacing fluid node. Turn reseed off, but you can also adjust the surfacing parameters: https://www.sidefx.com/docs/houdini/nodes/sop/particlefluidsurface.html

On linux, if you have multiple rops that would be useful to run at once, create fetch nodes in your out context (reference your rop driver in your fetch), attach them to a merge node and then command line rop out your cache bgeo.gz (or bgeo) files using the command line as follows (note this is also available on windows by running the Houdini Command Line Tools to bring up a ms-dos shell with the correct path settings. In ms-dos, dir is used instead of ls).

hscript filename.hipnc
cd out
render -Va -I merge1

Example file: fetchExample.hipnc
Here is a list of command line options from the documentation.
Keep in mind you could even read the bgeo information from another hipnc - all you need is the file node to read it in.

TIP: Don't use the default particle separation (values around say .06 (good values depend on the scene scale) will give you more detail)

TIP: Lumpy fluid? In H16.5 you can change values on the particle fluids surface node filters. For example, the type (below gaussian was selected, and level)
 
Are you in H12.5 or H13? In H12.5 - try a different operation type on your VDBSmoothSDF. Here is a diagram on a simple fluid showing the differences in the operations. But why use H12.5? Now in H13 it comes with a VDBfromParticleFluid which creates less lumpy fluid.

TIP: Just as with RBDs scale matters - 1 unit is 1 meter (roughly 3 feet). Time is seconds.

TIP:Interesting forum on making cloudy/murky liquid here. This entry includes a sample hip file as well.

TIP: WETMAPS excellent tutorial by Peter Quint. (A wetmap essentially is a shader applied to the surface of the object to appear darker in color and increase the specularity of where the fluid hits.) In his new tutorial, he uses the new solver available in 12 and point clouds. Make sure you write out the cache files before rendering. The shader references these files. UPDATE 3/9/2014 - there is a free otl download for this on orbolt - I have not used it myself but it has been recommended by Tyler. See tips and tricks student entry.

Flip Fluids

• AutoDopNetwork - inside are the usual suspects including gravity and the
  
• flip fluid object - particle fluid object
  
• particle separation - increasing lowers the resolution but is faster, measured in meters - low value of .02 give more details but it really depends on your scene scale. Default is set to .1
  
• particle radius scale - detail of the surface - higher values result in smoother surface
• grid scale - considered good for most applications
  
• unexpected collision behavior?
  
• tip - enable Collision and Collision Velocity visualization under the guides tab
• thin objects may need proxy collision geometry (volume proxy now default)
  
• flip solver - is a hybrid between a particle based and volume based fluid sim. Fluid data is stored in the particles however the pressure projection step is done on a volume that is created. A temporary velocity field is made. Particle velocities are transferred to a grid and the grid is used to perform the fluid projection.
• sphere_object1_fluid
•  (see image below)
  
• sphere_object1_fluidinterior - displays nothing, but imports the RENDER output from the fluid object. This object has a uniformvolume shader assigned to it (the other has basic liquid). A good example of its use is with the murky liquids mentioned above.

           
    From version H13 (vdb from particle fluid node replaces the previous 12.5 version of vdb from particles/vdb smooth nodes)