Hi
I was trying to play around with FLIP fluids, but after five minutes I already run into my first problem, which I cannot solve by myself.

I created a sphere, moved it up and rotated it slightly, so that it's local Y-axis is pointing 45° to the right. I then converted it into a “Emit Particle fluids”, by using the shelf tools. When asked I chose Flip fluids (but it also didn't work with SPH).
In the AutoDOP network, in the particleFluidObject1 node, under the Initial Data Tab, I tried to modify the direction the fluids get emitted. But no matter what I enter into x, y and z, the fluids always come out the same way.

As my Input type I used the default “Surface SOP”. In the manual it says “…Use this option to generate particles inside of the specified surface…”.
But in my scene only at the point positions particles get generated, not from the volume.

So my questions are:
- How can I control the initial direction in which fluids get emitted?
- How can I emit FLIP fluids from a volume instead point positions?

cheers,
Daniel

p.s. A bonus question to avoid multiple threads
- Why are Viscosity and Surface Tension disabled in the FLIP fluids?

schyzomaniac
In the AutoDOP network, in the particleFluidObject1 node, under the Initial Data Tab, I tried to modify the direction the fluids get emitted. But no matter what I enter into x, y and z, the fluids always come out the same way.

The source controls in the Particle Fluid Object are only for sources that give you emission at the initialization frame. You are not doing this since you selected an object as a continuous source emission.

So my questions are:

schyzomaniac
- How can I control the initial direction in which fluids get emitted?

Use the Particle Fluid Emitter DOP to control the continuous stream emission of particles.
You will see that in the Velocity tab, there is an initial velocity of 1 in the X direction. Please change to suit.

schyzomaniac
- How can I emit FLIP fluids from a volume instead point positions?

Again use the Particle Fluid Emitter DOP built for you. First you have to disable stream emission which by default births from points.
Then select the Emission Type to Volume.
In the Birth tab, increase the Continuous Emission rate to something greater than 0.

Please be careful with moving away from stream emission. The FLIP solver builds a vel vector field to project the particles forward. The resolution of this vel grid is determined by the particle separation. Once you move away from Stream Emission which tries to birth particles that respects the particle separation which you set in the Particle Fluid Object, you need to be aware of the particle separation.

Particle Separation is tied to the vel vector field voxel size via a simple channel reference in to the Flip Configure Object DOP right under it. The fields by default are twice the particle separation as this gives a good starting point for FLIP fluid simulations.

Please turn on the Velocity visualization data as you progress forward tweaking your flip simulation. You want to have a nice balance between the number of particles and the voxel size always aiming for a nice balance of about 8 particles per grid cell.

Certainly you can birth far more particles and create higher densities per grid voxel size in your flip sim but you may find you need to decrease the particle separation to decrease the voxel size (which will increase the resolution of your fields).

schyzomaniac
- Why are Viscosity and Surface Tension disabled in the FLIP fluids?

Because they are supported by the SPH (Smooth Particle Hydrodynamics) solver and not the FLIP solver at this point in time.

FLIP (FLuid Implicit Particle) solver is an entirely different beast than SPH. SPH does not use fields to project the velocity in to the particles. The particles themselves behave like small ball bearings. FLIP uses fields to project the velocity back in to the particles. The particles themselves hold on to the velocity between time steps and the velocity fields are not recycled for FLIP but reset every frame reading the velocity attributes on the particles.

You can also modify the advection stage of the vel vector fields to modify the flow rate.

You can initialize the pressure and divergence fields to deal with the biggest issue with FLIP: particles collapsing in to themselves and never separating apart after that.

These are known requests so let's see if there are solutions for this in the future. For now you can work directly with the particles themselves in the POP Solver DOP built for you from the shelf and modify the velocities directly. Remember with FLIP, the velocities present on the particles are used to build the vel vector fields. This vel vector field is then processed with fluid micro-solvers and then projected back on to the particles.

Please watch the FLIP Fluid masterclass by Jeff Lait who implemented the FLIP solver in Houdini from our web site. He discusses the pros and cons of flip, how to work with divergence and how to use the pressure field to do a couple interesting things.

Hello Jeff
Thank you so much for answering my questions in such detail. I really appreciate it. And since you did such a great job telling me what I did wrong or missed, everything works fine now and I can continue to dive into the FLIP fluid solver.

Next I will definetly watch the masterclass.

Thanks again,
Daniel