With this node it’s not necessary to define an initial velocity and you can start working with the default settings. When you simulate, the glass is filled until it reaches a certain level near the source object’s top. Then, sourcing stops and the water level remains constant. This is the behavior you can also see with the DOP-based FLIP Solver, where particles can’t be created under water.

By default, particles are emitted along the object’s normals with a Normal Velocity of 1. With Additional Velocity you can add more speed and give the particles a certain direction. The magnitude of this velocity vector is added to the Normal Velocity vector. With 0,-1,0, for example, the particles will fall down faster.

When you simulate, you can see the water level rising. With Boundary set to Velocity, water can overflow the source object and still create particles. If the simulation runs long enough, the glass will also overflow.

With the default settings, particles are emitted with a Uniform Pressure of 1 along the source object’s normals. Be careful with pressure changes, because the number of particles increases drastically. The particles can also become very fast, and this often causes leaking in conjunction with collision objects. You should therefore consider to animate the Uniform Pressure parameter:

1. Go to the start frame and set Uniform Pressure to 0.1.

2. Alt + LMB click Uniform Pressure to create an animation key.

3. Go to the frame where full pressure should be applied and enter 1.

4. Again, Alt + LMB click Uniform Pressure to create another key.

When you simulate, you can see that glass is filled quickly and the water overflows the source object. If the glass is high enough you will also observe that sourcing stops at a certain height. This is the moment, when the fluid has reached a pressure of 1: Uniform Pressure acts like a threshold.

With Hydrostatic Pressure you can define a maximum Water Level. When the fluid reaches this level, sourcing stops. The water level is indicated through a grid. To make it visible, turn on the source node’s Display/Render flag.

By default Water Level starts at 0. If you want to add an offset to this level, adjust the Y value of Water Origin. In the example scene with the glass you can see that the object’s hollow space starts approx. 0.15 units above the baseline. To get the correct filling height, change the Water Origin parameter’s Y value to 0.15.

You can combine pressure sourcing with an Additional Velocity to give the particles an initial direction. It’s also possible to use Scale Velocity and Normal Velocity, but be careful when you combine the various parameters: their values summarize and you might get extremely fast particles and leaking objects.

Pressure sourcing uses a Pressure Voxel Band parameter. This parameter shrinks the source volume internally and makes the fluid slower. The default value of 2 voxels works well in most situations.

The FLIP Solver can also create new particles at the domain’s boundaries. A FLIP Source node is not mandatory here. This is, for example, interesting for ships and boats on an ocean, or rivers. The solver creates a particle layer around the domain’s outside. Particles, interacting with the domain’s inside are deleted, new ones are sourced from the outside.

1. Create a FLIP Container SOP to specify the simulation domain’s dimensions.

2. Add a FLIP Solver and connect its first three inputs with the outputs of the container.

3. In the solver, enable Waterline to create a water surface.

4. You can also add an Additional Velocity to give the particles speed and direction.

Another way to create a particle layer is to use the solver’s fourth input. Detailed tutorials on how to work with this method can be found under

• Guided ocean waves

• Upres simulations