Particle Fluid Object dynamics node

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.

It is convention to relate this value to centimeters. A particle separation of 0.1 is referenced to as 10cm. Particle Separation values below 0.02 or 2 centimeters is at the threshold of what can currently be simulated on large scale set-ups, so don’t go below that value.

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.

When simulating a waterfall, large particle separations will result in water that overshoots the lip of the waterfall. Try decreasing the value to 0.04 for better results.

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.

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 particular object externally.

For the newly created objects, this parameter controls whether or not the solver for that object should solve for the object on the timestep in which it was created.

Usually this parameter will be 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 how 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.

The file to load fluid geometry from when Input Type is set to File.

This field expects a file containing particle fluid geometry; that is, geometry extracted from the Geometry field of a particle fluid object.

When running a long simulation, it is useful to save .bgeo files containing particle fluid geometry at each frame. The simulation can then be restarted from any frame by specifying one of these files in this field.

The object from which to fetch the data.

You can use group names, object names, object ids, and/or wildcard characters to match an object, and the negation character (^) to eliminate objects from consideration. If multiple objects match, Houdini uses the one with the lowest object ID as the source object.

Data matching the Source Data Name will be extracted from the source object and provided as the source geometry to control the initial locations of fluid particles.

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

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.

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. It’s 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.

Enables or disables particle visualization.

Selects between Spheres, Sprites, Grain, or Particles visualization of particles.

Sphere visualization stamps a scaled sphere at each point. Sprite will stamp a billboard image that looks like a sphere. Grain stamps arbitrary geometry on the points.

Controls the color of the visualization geometry.

This controls the size of the spheres in the guide geometry.

The sprite image to display when Visualization is set to Sprites.

Instead of a constant color, one of the particle attributes could be visualized.

If the attribute is a scalar attribute, or has been turned into a scalar attribute by the Speed visualization type, it can be remapped into a color spectrum.

Which point attribute to visualize as color.

Before mapping the visualization range, the attribute is multiplied by this scale.

The minimum and maximum values of the attribute are computed and used for the range. This allows for automatic bounding of the range. The detail attribute vis_range will be set to the computed range.

This range will be remapped into the 0..1 interval for setting the color or mapping by the Visualization Mode. Using a balanced interval, such as -1..1, is useful for detecting zero crossings of an attribute along with the Two-Tone visualization mode.

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 physical density of the particle fluid.

This quantity is used by the Particle Fluid Solver DOP to determine how to apply pressure forces to particles in the fluid. When the density of particles exceeds their rest density, they are pushed apart. Similarly, they are pulled together when their density is less than the fluid rest density.

This parameter is used by the Particle Fluid Solver to control the thickness and resistance to flow of the particle fluid. A fluid with higher viscosity tends to flow more slowly and appear thicker than one with low viscosity.

For this force to be applied by the particle fluid solvers, the Enable Viscosity Force toggle must be enabled.

Controls the magnitude of surface tension forces applied to particles in the fluid by the Particle Fluid Solver DOP. Surface tension forces attempt to pull surface particles more tightly in to the fluid, resulting in a more rounded fluid shape.

For this force to be applied by the Particle Fluid Solver, the Enable Surface Tension Force toggle must be enabled in the Internal Forces tab of the Particle Fluid Solver.

This only applies to SPH fluids.

Controls how far away from collision geometry particle collisions occur. If Volume Offset is set to 0, collisions occur directly at the boundary of the collision object. If it is set to 1.0, then collisions occur one particle radius away from the collision geometry.

The local velocity of an affector object is a combination of the angular and linear velocity. However, if the object is deforming and points match frame to frame, the local point velocity can be used as well to estimate the deformation effect.

If an affector object doesn’t have a stable point count, but does have a volume representation, the change in the volume representation can be used as an estimate of deformation velocity.

Stores the gradient of the fluid density field at each particle position. This may be useful for identifying particles close to the surface of the fluid, as the magnitude of this vector is larger for particles close to the fluid surface than it is for particles far from the surface.

Stores the last pressure force vector computed for each particle.

For each particle, this stores the average velocity of all neighbors of the particle. By comparing a particle’s velocity with its neighbor velocity, areas of particularly turbulent flow in the fluid may be identified.

Enables or disables the coordinate system on this object.

Since any coordinate system on a set of freely moving fluid particles is expected to gradually become incoherent, the coordinate system is designed to periodically reinitialize itself. This specifies the transition period.

During each transition period, the coordinate system remains constant for some period of time, and then transitions in to a reinitialized coordinate system over a specified transition length. Use this parameter to control that transition length.

By default, all fluid coordinates are in the range [0,1] and are defined with respect to the initial bounding box of the fluid.

Use this parameter to scale the range of each axis from [0,1] to [0,s] where s can be specified.

By default, all particle coordinates are determined with respect to the initial bounding box of the fluid. This bounding box is repeated spatially to accommodate particles that flow out of this bounding box.

Use this parameter to define coordinates with respect to a different bounding box.

The minimum boundaries of the user-defined coordinate bounding box.

The maximum boundaries of the user-defined coordinate bounding box.

Enables or disables use of a custom defined grain SOP.

When Custom Grain is enabled, the custom grain SOP path is defined here. Custom grains must be a set of rigidly-connected spheres.

This options lists a number of default grain shapes.

Controls the size of the grains.

Controls the size of the spheres that comprise the grain.