FLIP emitter stops emitting when underwater

[AlphaJoe]

I had a quick question, or not so quick maybe.

 

 If I have a FLIP simulation where the emitter goes underwater, why does it stop emitting?  I assume it is because it really isn't an emitter but a fluid source, and if there are already water particles in the source area it doesn't emit more.  If this is the case, how can I force an emitter to push more water into the scene if the emission point is inside the water volume?  I was trying to make a fountain where the water starts at the top and pours down the structure, but when I put the emitter at the top, it would stop emitting when the top bowl got full and covered the emission volume.

 

   I can post a sample file as soon as this render I am running finishes.  But it is easy to recreate, just create a default FLIP tank, create a sphere and make it a particle fluid emitter, translate the sphere below the water line.  When you hit play it will not do anything.

[Diego A Grimaldi]

Hey Joe,

 

Took me and my friend a little try to figure out why that happened but we did.

 

Here's the secret: In the Source Volume of the DOP network, in the Particle Operation Tab, the KILL INSIDE parameter is ON by default. Turn that OFF!

 

That should do the trick.

 

Good luck!

[icarus]

if you want to 'emit' fluid in an area that is already saturated with fluid you need to source divergence order to get the expansion behavior you are looking for.

[AlphaJoe]

I am really sorry, I didn't have notifications turned on on this post and I forgot about it.  I am back on this issue again and I have some samples to show what is happening.

 

@Diego - Turning off the Kill Inside parameter definitely adds particles to the scene, however, for some reason it does not add volume to the water.  I can see there are more particles in the scene where the volume exists, but it seems like that area stops obeying particle separation.  

 

@icarus - I am not sure what you mean.  How do you source divergence?

 

I made a simple scene demonstrating my issue.  I also did a FlipBook showing what happens.  I created a narrow FLIP volume and added a sphere as an emission source.  I turned off Kill Inside as Diego mentioned, and I added Curl Noise to try and get the extra particles to move around and maybe mix better to create more volume.

 

Here is the flipbook: https://youtu.be/Ml-FOIZGkEc

 

The hip file is attached.  It is the Apprentice version, just a heads up.

 

 

FlipEmissionIssue.hipnc

Edited September 10, 2015 by AlphaJoe

[danw]

The reasoning behind the divergence thing is, in simple terms - you don't exactly "emit" water... wherever you place an emitter just converts that space from not-water to water, so if it's already water, nothing happens.  If you want to emit water into an area that already contains it, you need to "expand" the fluid by using divergence instead.  Divergence basically tells the solver to break the zero-divergence equation that maintains fluid volume, but only in a controlled area.

 

Simplest way to emit divergence is by using the Source Volume node, and selecting the "Expand" preset.  Then source a SOP volume called "divergence", and it'll add it into the divergence field of the sim... positive numbers should mean expansion, negative numbers will shrink the fluid instead.

 

You'll likely need to either also emit with "kill inside", or run your solver with reseeding turned on, otherwise the expansion will cause the fluid to... "run out" of particles and collapse in on itself.