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SOP FLIP fluids provide three methods for sourcing particles through the FLIP Source SOP node’s Boundary drop-down menu: None, Velocity, and Pressure. Find out more about their fundamental differences and how they work in the subchapters below.
Source objects have to be closed and solid 3D or volumetric objects. Open or 2D geometry such as planes or disks can’t be used.
To illustrate the differences between the various FLIP Boundary nodes and their sourcing methods, imagine you fill a glass with water. It’s important to place the source object, e.g. a Sphere SOP node, at the bottom of the glass. The videos below show the results.
FLIP Boundary: None
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.
FLIP Boundary: Velocity
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.
At least one velocity parameter has to be greater than
0 to create particles at all - or the object is moving/deforming.
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.
FLIP Boundary: Pressure
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:
Go to the start frame and set Uniform Pressure to
Alt + LMB click Uniform Pressure to create an animation key.
Go to the frame where full pressure should be applied and enter
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
Simulations with Hydrostatic Pressure only work correctly, if the source object is positioned below the adjusted Water Level and the pressure target value is
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 sometimes leads to flickering particles. You can improve results by turning off the FLIP Solver’s Advanced ▸ Solve Pressure With Adaptivity option.
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.
Create a FLIP Container SOP to specify the simulation domain’s dimensions.
Add a FLIP Solver and connect its first three inputs with the outputs of the container.
In the solver, enable Waterline to create a water surface.
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 4th input. Detailed tutorials on how to work with this method can be found under