For Closed Boundaries as specified on the FLIP Object DOP, particles will be constrained to stay inside the extents of the volumes as specified in the Volume Limits tab. This setting is useful for closed “tank” type fluid simulations.

Kill any particles outside the volume limits.

Enable support for particle Friction and Bounce as specified on the Physical tab of the FLIP Object DOP. Only enabled if Collision Detection is set to Particle.

Add a unique ID attribute to each particle.

Compute the relative density of each particle. This attribute represents how many particles are present in the neighborhood of a given particle. Low values correspond to isolated particles, and high values correspond to tightly packed neighborhoods.

Age the particles over time.

Kill any particles whose age attribute is greater than its life attribute.

Deletes internal attributes that are typically not required in the output of the simulation. Removing an attribute from this list will ensure it exists at the end of the timestep.

Perform both the guide-reseeding and the volume density reseeding in a single pass for more efficiency.

Turn off reseeding but keep seeding particles inside of the Source Surface. Constant reseeding of particles can cause noisy surface, this is particularly noticeable for slowly moving simulations. Turning this on can reduce this noise.

Create new particles in voxels where the particle count has dropped too low to properly represent the fluid surface, and delete particles in voxels that have become too crowded. Reseeding can help avoid collapsing pockets of air forming in the fluid near collisions, as well as provide a smoother surface from which to generate a polygonal mesh.

The goal number of particles per voxel.

The goal number of particles will be scaled by this amount when within Oversampling Bandwidth of the surface.

Oversample within this number of voxels from the surface or any Surface volume boundaries, if Oversample at Boundaries is enabled.

Oversample within Oversampling Bandwidth voxels of the fluid volume boundaries. This setting helps avoid collapsing pockets of fluid at the boundaries of a “Tank”-type simulation.

Particles will be added to a voxel once the current number drops below the product of this parameter and the goal number of particles.

Particles will be deleted from a voxel once the current number rises above the product of this parameter and the goal number of particles.

A seed that controls the time-varying random function used to generate new particle positions when reseeding. Because high-frequency splashes in a simulation are strongly influenced by particle positions, varying this value (e.g. with a Wedge ROP) can generate multiple simulations with similar bulk motion but different splash behavior from otherwise identical simulation parameters.

All attributes matching this pattern will be interpolated to newly seeded particles from the surrounding existing particles. This interpolation is more expensive than the default nearest neighbor copy, but yields a smoother particle sampling for attributes like velocity. For example, the viscosity attribute might be added to this list for fluids smoothly transitioning between melting or freezing, while it should be omitted for “multi-phase” fluids with distinct viscosity values.

Despite the velocity projection stage, particles can end up closer together than their pscale attribute. When this happens, internal forces can’t separate the particles because the velocity projection will remove those forces. This results in the fluid compressing over time.

The number of times to perform the separation relaxation step. This value can usually be set to 1 since successive frames of separation will have the same effect.

How far to move the particles towards their desired separation locations. This can be reduced to get the effect of a fractional iteration.

A fudge factor to account for the inability for the particles to actually pack at the pscale amount due to the sphere packing problem. To test different values, run a FLIP solve without any volume or particle forces and see which values cause your initial particles to retain the desired volume.

Identify particles that separate from the main body of fluid via a float droplet attribute that ranges from 0 to 1. As a particle approaches full droplet status, it will be less influenced by fluid forces. A fully droplet particle also does not contribute velocity back to the fluid simulation, which can alleviate the problem of individual falling particles causing unrealistically large disturbances on a smooth fluid surface. Enabling droplets can also break up tendrils on the leading edge of splashes, giving them a more diffuse look.

A particle has a droplet value of 1 (droplet) when the surrounding fluid particle density is below this value.

A particle has a droplet value of 0 (fluid) when the surrounding fluid particle density is above this value.

Controls handling of droplet particles.

Controls the blend between droplet particle velocity and the fluid velocity when Behavior is set to Blend With Fluid.

Compute the vorticity of the fluid velocity field and mix it into the vorticity attribute on the particles.

The amount of existing vorticity measurement on the particles to preserve per second before mixing in the current computed value. For example a value of 0.1 will retain 10% of the vorticity measurement over a second.

How to blend the computed vorticity with the existing vorticity attribute value.

A scale applied to the computed vorticity.

Creates a rest attribute, which can be used to track the position of the fluid over time. Turn this on to map noise or textures in the liquid shader.

Creates a rest2 attribute that is one back from the main rest attribute, allowing you to run longer simulations without popping by blending between the two attributes.

Number of frames before resetting the rest attributes.

Which frame the rest attributes will be reset on. If you are prerolling the simulation, delaying the rest attribute initialization until after the preroll will usually give a better result.

Display the guide showing the maximum limits of the simulation. The actual simulation size will most likely be smaller if Dynamically Resize Fields is enabled.

The size of the maximum volume.

The origin of the maximum volume.

Resize the volumes each timestep to contain the particle geometry, within the limits specified. The volumes will be padded by the number of voxels specified in Max Cells to Extrapolate.

If the Volume Limits change between frames, this option allows particles to be seeded into a new empty region in the volume. Use Waterline and Use Boundary Layer controls how the new particles are created.

If the FLIP Object uses Closed Boundaries, this option will set each corresponding boundary to be open above the waterline. This allows particles to elegantly leave the simulation and reduces reflections at the boundaries. This can also be used instead of the Surface Volume for Fill New Volume and Use Boundary Layer.

Display the waterline plane where fluid above the plane can freely flow out of an otherwise closed boundary.

The Waterline level and Waterline Direction define the waterline plane. Along a closed boundary, simulation voxels above this plane will be set as an open boundary. For Fill New Volume and Boundary Layer, the plane is defines the surface, where particles above the plane are considered to be outside of the surface.

The waterline direction specifies the “up” direction of the waterline plane. The border between the closed and open boundary region is a combination of the waterline level and the waterline direction.

Maintaining a thin region around the FLIP simulation can be used to imitate the behavior of an infinite tank. By deleting and reseeding particles according to a provided surface and velocity field, reflections at simulation boundaries can be reduced. Particles entering the boundary layer that are above the specified surface (given by the waterline or surface volume option) will be removed. Particles entering the boundary layer that are below the surface will be assigned a new velocity if the velocity volume option is used.

The Velocity Volume is automatically applied along the Closed Boundaries when Use Boundary Layer is activated. This option will also overwrite the velocity field inside the boundary layer according to the Velocity Volume (or the default of zero velocity when using the waterline). When this is turned off, newly created particles will interpolate velocities from neighboring particles.

The Boundary Layer will be offset into the simulation volume by the specified amount from the lower boundary of the simulation volume.

The Boundary Layer will be offset into the simulation volume by the specified amount from the upper boundary of the simulation volume.

The surface in the boundary layer can be maintained by a provided SDF. Particles above the surface will be removed. The volume below the surface will be re-seeded. The particles seeded in a new volume region will be created based on this surface.

The velocity of the particles in the boundary layer and new volume will be assigned a velocity sampled from this velocity volume.

Scale the input collision velocities, increasing the effect of moving objects on the fluid and creating bigger splashes, for example. This value should usually be 1 or greater.

The method used to calculate collision velocities for colliding DOP Objects. This setting does not affect collision velocities introduced by a Volume Source DOP.

When the fluid surface is within this voxel distance of a collision, the solver will consider it part of the collision object. This extrapolation helps fluid flow smoothly along curved surfaces, but can create a slight stickiness. Decreasing this value can create more dynamic splashes from collisions, especially as objects enter the fluid.

The solver uses fractional estimates of collision volumes to increase the accuracy of the pressure solve around curved or sloped surfaces.

How many samples are taken per axis when the Volume Fraction Method is set to Collision Supersampling. Increasing this value makes for a more accurate estimate, but note that the total number of samples taken is the cube of this number.

Make all collision objects “transparent” to the fluid by this amount, decreasing the effect of objects on the fluid and allowing some fluid to flow through the objects.

Causes the fluid’s velocity to match the collision velocity when close to collision objects. See the Gas Stick On Collision DOP for more information.

The amount of collision velocity to blend into the fluid’s velocity, where a value of 1 indicates fully matching the collision velocity.

Specifies the world-space distance within which to apply the effect.

Specifies the maximum number of voxels within which to apply the effect.

Controls how quickly within the stick distance the effect will reach the full Stick Scale. Values closer to 1 will have more effect throughout the stick distance.

Scale the amount of velocity adjustment in the direction normal to the collision surface.

Scale the amount of velocity adjustment in the direction tangent to the collision surface.

Scale the effect by this spatially varying field, which should match the collision field in resolution.

Enable the viscosity solver.

This option enables the adaptive viscosity solver. This replaces the pressure values for fluid far from the surface with coarser and coarser approximations. Usually the exact shape of deep vortices is not relevant, so this greatly reduces solve times of the pressure. This is most effective at high resolution and with deeper tanks. See the Gas Project Non Divergent Adaptive node for more details.

The main limitation is that adaptivity doesn’t preserve fine details in coarse regions. However, the resulting coarse pressure is up-sampled to the fine simulation resolution and progressively smoothed to closely match the fine details of a non-adaptive solver. Upsampling and smoothing introduces a small additional cost. Another limitation is that varying density is not currently supported in the adaptive solver, as they will not be coarsened properly in the adaptive region.

Use the specified attribute to overwrite the viscosity field.

The float attribute on the particles to use for the viscosity amount.

How to blend the point attribute with the existing viscosity field.

A scale applied to the Viscosity field after applying the attribute effect. This is useful for quickly adjusting the global amount of viscosity.

This parameter controls the floating point precision used internally by the viscosity solver. Float 32 bit uses less memory and is generally faster than Float 64 bit. However, the extra accuracy of 64-bit floating point numbers may be needed when simulating fluid with very high viscosity or large variations in viscosity.

Causes the collision velocity to match the fluid velocity when close to collision objects. This allows a viscous fluid to slide along the collision object.

The amount of fluid velocity to blend into the collision’s tangential velocity, where a value of 1 indicates fully matching the fluid velocity.

Scale the effect by this spatially varying field, which should match the collision field in resolution.

Use the specified attribute to manipulate the density field.

The float attribute on the particles to use for the density amount.

How to blend the point attribute with the existing density field.

A scale applied to the Density field after applying the attribute effect. This is useful for quickly adjusting the global amount of density.

The divergence free constraint will now also be applied to the air volume. This will prevent air volumes from collapsing and create rising bubbles. This is useful for capturing realistic air-liquid interaction like a glugging watercooler.

Moving collisions can interact indirectly with the liquid through an air pocket. Any compression applied to the air pocket by the collision is compensated for at the liquid surface of the air pocket.

Air regions containing fewer voxels than the minimum will not have their incompressibility enforced. This is useful for preventing small bubbles from emerging along collision surfaces.

Use the specified attribute to overwrite the divergence field.

The float attribute on the particles to use for the divergence amount.

How to blend the point attribute with any existing divergence field. The divergence field is not reset frame-to-frame.

A scale applied to the divergence field after applying the attribute effect. This is useful for quickly adjusting the global amount of divergence.

Enable the surfacepressure field.

The magnitude of the surface tension. This depends on the mass-density of the fluid, so must be changed if the mass-density is changed.

Given the same object, but reduced particle separation (i.e., a higher resolution simulation), you may need to reduce the magnitude of the surface tension to maintain the same qualitative result.

Negative values will cause the surface to rip itself apart in a spectacular fashion.

Cone:

Gaussian:

Indicates the scale of the simulation. This value controls the tolerance of various particle operations, particularly moving particles to match SDF iso surface values, and some solver defaults for density and viscosity. If your scene is modeled in meters, then the default value of 1 is sufficient. However, if your scene is modeled in centimeters, you should set Spatial Scale to 0.01.

Indicates the mass scale of the simulation. This value controls the solver tolerance for density and viscosity. If your scene is modeled in kilograms, then the default value of 1 is sufficient. However, if your scene is modeled in grams, you should set Spatial Scale to 0.001.

A scale factor used in applying feedback forces to other objects. A value of zero prevents any feedback from occurring.

The default RBD object has the same density as water, so to balance with the default fluid density a value of 1 should be used.

Controls the method used for extrapolating velocity in the solver.

Improve handling of fast-moving fluid by automatically increasing the extrapolation distance based on the fluid’s speed. In most cases, this avoids tuning the Max Cells To Extrapolate parameter. The final extrapolation distance is the greater of Max Cells To Extrapolate and the speed-calculated value.

The number of non-fluid cells that should be filled with velocity values on the non-fluid side of the velocity field. Increase this value for very fast-moving fluids and/or a low number of substeps.

When this checkbox is turned on, the pressure system is preconditioned with the Modified Incomplete Cholesky method to speed up the solution. However, this is a single-threaded process. On machines with 4+ sockets, it may be faster to use the simpler Jacobi preconditioner which multithreads well, but takes 6× more iterations to converge. Leaving this toggle off forces use of the Jacobi preconditioner (if Use Multigrid Preconditioner is also turned off).

When this checkbox is turned on, the pressure system is preconditioned using algebraic multigrid to speed up the solution. This option greatly reduces the number of iterations required to converge, at the expense of increasing the cost of each iteration. This typically greatly speeds up the solve, but does not support Air Incompressibility or distribution.

Uses pressure from the previous timestep as an initial guess for the solver. This will often provide faster solver convergence, resulting in faster simulation times.

Pressure in tanks of water increases from the top to the bottom. Frame to frame, this pressure gradient likely looks the same everywhere, so rather than starting from scratch, this option will try to re-use the previous pressure as a start condition. This causes it to find the final pressures faster, speeding up solve times. This is most effective in mostly static sims, especially deep tanks.

This option enables the adaptive pressure solver. This replaces the pressure values for fluid far from the surface with coarser and coarser approximations. Usually the exact shape of deep vortices is not relevant, so this greatly reduces solve times of the pressure. This is most effective at high resolution and with deeper tanks. See the Gas Project Non Divergent Adaptive node for more details.

The main limitation is that adaptivity doesn’t preserve fine details in coarse regions. However, the resulting coarse pressure is up-sampled to the fine simulation resolution and progressively smoothed to closely match the fine details of a non-adaptive solver. Upsampling and smoothing introduces a small additional cost. Another limitation is that varying density is not currently supported in the adaptive solver, as they will not be coarsened properly in the adaptive region.

Note that viscosity adaptivity is controlled separately, and for viscous sims it is more important to make viscosity adaptive than pressure.

Note this does not work with Enforce Air Incompressibility.

Solve the linear systems for viscosity and pressure using OpenCL. This setting is mostly beneficial for high resolution fluid simulations with viscosity, when run on a fast GPU. Note that Solve Pressure with Adaptivity should be turned off for the pressure solve to be performed using OpenCL.

The collision surface that is connected to pressure and viscosity solvers.

The collision weights improve the accuracy of the pressure and viscosity solvers.

The collision velocity is applied to the fluid through the solvers at the collision surface.

The SDF field used as a mask to limit the regions where incompressibility is enforced during the project non-divergent solve. It is also used for the same purpose during the viscosity solve. These two operations will only consider voxels where the field is negative (inside the SDF).

FLIP particles are seeded into the source surface. This is useful for allowing the fluid to flow out of a guiding simulation surface.

FLIP particles are removed when they enter the sink surface. This is useful for removing unneeded particles that flow deep into a guiding simulation surface.

The SDF field used as a mask to define where the Guiding Velocity field will replace the values of the velocity vel field. The velocity replacement will take place in regions where the Guiding Surface is negative (inside the SDF).

The vector field used to replace the values of the velocity vel field within the regions defined by the Guiding Surface field.