This parameter controls the interaction distance between particles in the created Particle Fluid Object. Decreasing this value will decrease the distance between your particles making it more impressive, but may also slow down your simulation, since it will take longer to simulate. Decreasing particle separation means more particles that weigh less, but add up to the same mass per unit area.

If the Input Type for this object is set to Surface SOP, then this parameter also controls the number of particles spawned inside of the provided surface. That is, a smaller particle separation results in a greater number of particles and hence a particle-based fluid with higher resolution.

The radius of the particles is determined by scaling the Particle Separation by this parameter. Setting this value higher will result in more volume in the fluid but less surface detail as it gets smoothed out by the larger particle radius.

In versions prior to Houdini 12, this value was set internally at 2.

A scaling factor used to calculate the division size of the advection grid from the particle separation. The default should be good for most applications.

The division size for the collision-related fields.

It can be useful for the collision fields to be higher resolution than the other simulation fields, especially when protoyping simulations at low resolutions, since the FLIP Fluid Solver takes into account fractional collision voxels when solving.

If this value is set as a fraction of the overall Division Size, be careful to restore it when running high resolution simulations. Another option is to choose a division size that provides sufficient collision detail by visualizing the collision field, then leave it at that fixed value, independent from any changes to the overall division size.

Controls if the particles should be allowed to go ballistic or hit an invisible glass wall when the maximum bounds is reached, as specified in the Volume Limits tab of the FLIP Solver. Turning this checkbox on makes it easier to setup water tank style simulations.

Determines if the creation frame refers to global Houdini frames ($F) or to simulation specific frames ($SF). The latter is affected by the offset time and scale time at the DOP network level.

The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.

For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to 1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.

Instead of making a single object, you can create a number of identical objects. You can set each object’s parameters individually by using the $OBJID expression.

The name for the created object. This is the name that shows up in the details view, and is used to reference this object externally.

When turned on, newly created objects are solved by the solver on the timestep in which it was created.

This parameter is usually turned on if this node is creating objects in the middle of a simulation rather than creating objects for the initial state of the simulation.

By preventing a large object from being cached, you can ensure there is enough room in the cache for the previous frames of its collision geometry.

This option should only be set when you are working with a very large sim. It is much better just to use a larger memory cache if possible.

Determines how to interpret the SOP geometry specified in SOP Path.

This determines the initial configuration of fluid particles if Input Type is set to Surface SOP.

The geometry controlling the initial locations of fluid particles. How this is used depends on Input Type.

When Input Type is set to Surface SOP, a random jitter may be applied to the particles created.

This has the effect of making the initial fluid configuration less symmetrical. This parameter is a seed used in the random jitter application.

The magnitude of random jitter to apply to each particle.

If this toggle is enabled, the transform of the object containing the SOP geometry is applied to the geometry. This is useful if the initial location of the geometry is defined by an object transform.

This parameter is only meaningful if Input Type is set to Particle Field. In this case, when this parameter is enabled, the DOP will overwrite any existing attributes used by the Particle Fluid Solver DOP (mass, velocity, density, etc.) with new values when it initializes the fluid particles.

Leave this parameter disabled if you wish to initialize a particle fluid object from the particle geometry of an existing particle fluid simulation. This is the case when you are attempting to restart an old simulation, or combine two or more particle fluid objects in to the same object.

When sourcing from a grid of particles, they may already have a velocity. This option lets you override these velocities with your own constant velocity with the Initial velocity parameter below.

The initial velocity of the fluid particles created by this DOP.

A viscosity attribute is added, but not written to. Its default value is 1 to allow any new particles added to the sim to respect the global viscosity value.

Note the particle viscosity is usually treated as a multiplier, so 1 means to use the global viscosity value.

Instead of always making the local particle velocities divergent free, one can instead introduce artificial divergence or convergence using the divergence field. This field can also be manipulated by using the Divergence tab of the FLIP Solver.

The elasticity of the object.

If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill.

The coefficient of friction of the object. A value of 0 means the object is frictionless. This governs how much the tangential velocity is affected by collisions and resting contacts.

An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one.

A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome the object acts without friction.

The mass density of the fluid. This value is stored in the object’s density field and can be manipulated in the Volume Velocity input of the FLIP solver, or overridden with particle attributes by using its Density tab.

The dynamic viscosity of the fluid. The value of the Viscosity parameter depends on the scale of the particles. At the default scale, you will need values around 1000 for a thick fluid, and around 10000 for a doughy fluid. See the discussion of viscosity in the user guide.

This value is stored in the object’s viscosity field and can be manipulated in the Volume Velocity input of the FLIP solver, or overridden with particle attributes by using its Viscosity tab.

Enable Viscosity must be set on the Viscosity tab of the FLIP solver for this setting to have any effect.