| On this page |
The FLIP Solver DOP evolves an object dynamically as a particle fluid.
A FLIP solver is a type of hybrid between a particle based and volume
based fluid simulations. All fluid data is stored in the particles and
only particles need to persist frame to frame, ensuring no unwanted
mixing or loss of fluid can occur. However, the pressure projection
step is done on a volume that is created just in time. The fluid is
represented by particles with their pscale, and can be moved around
with POP and DOP forces.
When FLIP fluids are solved, a temporary velocity field is made. The particle velocities are transferred to this grid and the grid is used to perform the fluid projection. This is what prevents the particles from all going on top of each other and start moving in similar directions.
Note
Advanced users can also add volume velocity forces at this stage as well.
FLIP fluids are faster than SPH fluids, if you don’t need to substep the FLIP fluid. If you need to substep the FLIP fluid because of fast moving colliders, you may find SPH just as a fast or faster. FLIP fluids are also useful because particles can be placed on top of each other without destabilizing the system. SPH tends to blow up if you move particles too close.
The advantage of the FLIP Solver is that you run with only a few time steps per frame while SPH requires anywhere from 7 to 20 time steps or more per frame to stabilize. FLIP utilizes a few grids (volume fields) to help tame the instantaneous impulses that can arise in a fluid sim.
Various fields are used to tame the FLIP Solver so that you can run far fewer points at far fewer time steps and the inter-spacing between particles can be random. You can introduce new particles at any time with little to no consequence. This opens up so many new work flows in POPs that were simply not possible with SPH. For example, introducing splash particles with their own property attributes is now possible.
The FLIP Solver contains an embedded POP Solver, enabling the use of
POP forces by plugging them into the Particle Velocity input or
inline below any 
Source Volume emitters. Any
POP node that updates the v, targetv, or force attribute can
be used, as well as others that affect attributes like particle color
or age. The POP Solver also supports other particle behavior such
as aging and reaping particles.
Splashy vs Swirly Kernel
The Velocity Transfer parameter allows you to choose between the default Splashy Kernel and a Swirly Kernel. The Splashy Kernel is typically used for high energy, noisy large FLIP simulations like rivers or oceans, where the surface from FLIP noise isn’t a problem. The Swirly Kernel is typically used for high vorticity simulations where you need to reduce surface noise as much as possible while still retaining the swirling nature of the simulation. For example, for small scale fluids where surface noise can be really distracting or for lava type simulations.
Note
The Splashy Kernel corresponds to the FLIP/PIC method of velocity transfer, while the Swirly Kernel is implemented with the newer APIC method. All of the Viscous shelf tools use the Swirly Kernel.
Tips
For emitting large numbers of particles, the Source Volume
DOP can be much faster than the 
Particle Fluid
Emitter DOP. You can set up surface
emission using the 
Source From Surface shelf
tool or volumetric emission using 
Source From
Volume from the Populate Containers shelf.
If your scene is modeled in units besides meters, be sure to set Spatial Scale correctly. Otherwise the FLIP Solver will adjust particle positions with more accuracy than required for the scene scale. The resulting performance degradation mostly affects Reseeding and the Move Outside Collision collision detection method.
Turning off Use Preconditioner might improve solve performance if simulating on a system with multiple CPU cores.
Use the Performance Monitor to detect bottlenecks in your FLIP
simulations. In general the FLIP Solver should consume most of the
solve time for a typical DOP Network. If emitting particles is taking a
large amount of time, consider using Source
Volume emission as described above and possibly
even caching the points created by the corresponding 
Fluid Source
SOP.
If writing out large FLIP simulations to disk using the 
Dop I/O
SOP, enable Save In Background to allow the
simulation to continue while the disk writes occur in the background.
Deleting any unneeded attributes using the Compression tab will also
save disk space and write time.
Enable Reseeding to help preserve surface detail, possibly increasing Surface Oversampling to 2 or more.
The Velocity fields are heavily blurred and thresholded for large changes in velocity, which is completely tunable if the FLIP solver appears to be exploding. The Pressure field is used to monitor any areas of very high and very low pressure and then the fluid sim can compensate the Velocity fields which then push the points around. This is where the controls for cavitation, entrapped air, and fluid escape are managed to a great extent. The Surface field can be used to test the past if some particles have flown far too far off the surface and then constrained back.
Increasing the Particle Separation will lower the resolution, which will make your simulation faster to process.
Usually you will not need to increase Substeps with FLIP. However, you may want to increase this value when simulating fast moving fluid that collides with other objects. You can also increase Substeps to improve the look of slow moving viscous fluid. Surface tension is an exception where substepping is likely required to maintain stability.
You can vary amounts of stickiness to different objects, by building your own sticky field. The Fluid Source can be used to take a point attribute like sticky and make it a varying volume where you want stickiness to occur. Then, in the solver, you can use this field as the control field for the Stick On Collision function on the Collisions tab of the solver.
A good debugging rule of thumb for FLIP when seeing unexpected collision behavior is to immediately enable the Collision and Collision Velocity visualization on the 
FLIP Object, so you can see exactly what the solver is seeing for collisions.
Turning Reseed Particles off and having the Jitter Scale set to 0 gives a uniform-spaced grid of particles. This is generally not recommended, because you won’t end up filling the voxels as effectively as with jittered points, and as they move around you’ll get an uneven distribution due to numerical error, especially with Reseed Particles turned off.
Collision Tips
Accurate velocities for moving collision objects are extremely important for
fluid simulations in general, but particularly for FLIP. Incorrect collision
velocities can lead to less dynamic splashes or fluid leaking through
geometry. For DOPs to calculate accurate collision velocities with moving or
deforming DOP objects, the 
DOP network must have Cache
Simulation enabled. However, with very large FLIP simulations, the FLIP
object data for a single timestep might not fit in cache memory. In this
case, disable Allow Caching on the FLIP Object to
remove the FLIP data from the cache, while still caching the collision
objects.
Many FLIP simulations use 2 or more substeps. For deforming collision
objects, make sure that the input geometry properly interpolate to subframes.
The 
Collision Source SOP handles interpolating the
geometry and creating VDB Signed Distance volumes that can be used with FLIP.
Use the 
Deforming Object shelf tool to set up deforming
geometry as a FLIP collision object.
Another option for adding collision objects to fluid simulations was the
Collide With Object shelf tool, which adds
collision volumes directly to the simulation using the
Source Volume DOP. However, as of Houdini 14, this
option is deprecated and not recommended. Instead use the the
Deforming Object shelf tool.
Note
If using Source Volume based collisions, you must use the Move Outside Collision method for particle collisions, because the Particle method only works when a collision is represented by an actual DOP object.
Note
The default velocity scale for Source Volume is 1.5, which will cause larger splashes by default, but for moving containers this should be set to 1.
In some cases, especially with thin collision objects, the FLIP Solver
may not resolve the collision properly when simulating at relatively low
resolutions. Enabling Collision Separation on the FLIP
Object and setting this value to the Particle
Separation or smaller will create a higher-resolution collision field
that will help the solver resolve the collision better. With very thin
objects you may even need to create proxy collision geometry that
generates a thicker SDF.
Move Outside Collision is the fastest collision handling method and provides the smoothest splashes, however it is not as accurate with fast-moving collision geometry. It is also the only collision method that works with Source Volume based collisions.
Parameters
Substeps
Timescale
A scaling factor for time inside this solver. 1 is normal speed, greater than 1 makes the FLIP simulation appear speeded up, less than 1 makes the simulation appear to be in slow motion.
You can use expression functions such as doptime, dopframe,dopsttot, and dopttost to convert between global times and simulation times.
Note
Changing the Timescale only affects the timestep of the simulation. If adding velocities calculated in SOPs to the simulation for collisions or pumps with the Source Volume DOP, scale the incoming velocities by 1 / Timescale to match the timestep of the simulation.
Min Substeps
The FLIP solver will always enforce this minimum number of substeps.
This should only rarely need to be changed.
Max Substeps
The FLIP solver will not break the simulation down in to more substeps than this value.
CFL Condition
The CFL condition is a factor used to automatically determine what size substep the scene requires. The idea is to control the distance that a particle in the particle fluid object can travel in a given substep.
For example, when this parameter is set to 0.5, the solver will set the length of each substep such that no particle travels more than 50% of its particle separation in a given substep.
Particle Advection CFL
The CFL condition for particle advection. This can be set lower than the CFL condition for the entire solver, as it only affects the accuracy of particle advection in the velocity field.
Quantize to Max Substeps
Alway use substeps that divide up the frame by Max Substeps. For example, if Max Substeps is set to 4, but the CFL Condition only requires 3 substeps, the solver will take frame steps of 0.25, 0.5, and 0.25. This option can be useful for re-using input geometry that has been cached to file at increments of 1/Max Substeps.
Particle Motion
Apply External Forces
Standard DOP forces, such as gravity from a 
Gravity
DOP, will be applied to the particles.
This would only be turned off if you are combining a FLIP solver as part of a larger solver system.
Force Override
If enabled, this attribute will disable the volume forces wherever it is 1 on the particles, and enable it when where it is 0. Ie, particles with this attribute at 1 will fly ballistically with no reference to other particles. However, their motion will still affect other particles which have volume forces.
Under-Resolved Particles
Depending on the values of the Particle Radius Scale and the
volume Grid Scale on the FLIP Object DOP,
individual particles can become under-resolved on the grid and
"disappear" from the pressure solve. This parameter controls how
the solver treats such particles:
No Detection
Make no attempt to detect under-resolved particles.
Detect Only
Set a float-valued underresolved attribute on each particle
that indicates the amount it is under-resolved.
Treat as Ballistic
Set the underresolved attribute and combine it with any
specified Force Override attribute to determine whether the
particle should be treated ballistically or as a fluid.
Use Extrapolated Velocity
Use the extrapolated fluid velocity unless a particle is outside the range of valid extrapolated values as specified by Max Cells to Extrapolate. For particles outside the valid range, this setting is the same as Treat as Ballistic. This will reduce stringiness in splashes.
Kill
Kill any fully under-resolved particles.
Note
If the Particle Radius Scale / Grid Scale >= sqrt(3) / 2, then particles will never be under-resolved.
Collision Detection
Controls handling of collisions between particles and other objects.
None
Do not perform particle collision detection. Particles will only avoid collisions as a result of the pressure solve, but due to numerical error can possibly end up inside collision objects.
Particle
Perform particle collision detection, including Feedback between particles and other simulated objects. This collision method also supports friction and bounce. It is the most accurate, but slowest collision method.
Move Outside Collision
Perform collision detection by moving particles outside any collision objects. This is faster than the Particle method, but not as accurate for thin or fast-moving objects.
Kill Unmoveable Particles
Kill any particles that become stuck inside the collision SDF (in local minima) when using Move Outside Collision collision handling.
Behavior
Collide with Volume Limits
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 Outside Volume Limits
Kill any particles outside the volume limits.
Use Friction and Bounce
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 ID Attribute
Add a unique ID attribute to each particle.
Age Particles
Age the particles over time.
Reap Particles
Kill any particles whose age attribute is greater than its life attribute.
Delete Attributes
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.
Reseeding
Reseed Particles
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.
Particles Per Voxel
The goal number of particles per voxel.
Surface Oversampling
The goal number of particles will be scaled by this amount when within Oversampling Bandwidth of the surface.
Oversampling Bandwidth
Oversample within this number of voxels from the surface or any Surface volume boundaries, if Oversample at Boundaries is enabled.
Oversample At Boundaries
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.
Birth Threshold
Particles will be added to a voxel once the current number drops below the product of this parameter and the goal number of particles.
Death Threshold
Particles will be deleted from a voxel once the current number rises above the product of this parameter and the goal number of particles.
Random Seed
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.
Interpolate Attributes
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.
Note
Setting the various Reseeding values too high can lead to fluid gaining volume over time in very splashy fluid simulations. Decreasing the Particle Radius Scale or the Death Threshold can reduce the volume by causing overcrowded voxels to be pruned earlier. Decreasing the amount and/or bandwidth of Surface Oversampling can help alleviate this problem as well.
Separation
Apply Particle Separation
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.
Separation Iterations
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.
Separation Rate
How far to move the particles towards their desired separation locations. This can be reduced to get the effect of a fractional iteration.
Separation Scale
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.
Droplets
Detect Droplets
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.
Min Particle Density
A particle has a droplet value of 1 (droplet) when the surrounding fluid particle density is below this value.
Max Particle Density
A particle has a droplet value of 0 (fluid) when the surrounding fluid particle density is above this value.
Behavior
Controls handling of droplet particles.
Blend With Fluid
When a fully droplet particle rejoins the fluid, make its velocity a blend between the the droplet’s velocity and the existing fluid velocity, as specified in the Velocity Blend parameter.
Kill On Detection
Immediately kill any fully droplet particles.
Kill At Fluid
Kill any fully droplet particles that rejoin the fluid.
Velocity Blend
Controls the blend between droplet particle velocity and the fluid velocity when Behavior is set to Blend With Fluid.
Vorticity
Add Vorticity Attribute
Compute the vorticity of the fluid velocity field and mix it into the vorticity
attribute on the particles.
Preservation Rate
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.
Mix Method
How to blend the computed vorticity with the existing vorticity attribute value.
Vorticity Scale
A scale applied to the computed vorticity.
Rest
Add Rest Attribute
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.
Dual Rest Attributes
Creates a rest2 attribute that is one back from the the main rest attribute, allowing you to run longer simulations without popping by blending between the two attributes.
Frames Between Reset
Number of frames before resetting the rest attributes.
Frame Offset
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.
Volume Motion
Velocity Transfer
Specifies the method used for transferring velocity from the particles to the underlying grid before solving, then back to the particles.
Splashy Kernel
Transfers the change in velocity from solving back to the particles, along with a small portion of the current velocity as specified by the Velocity Smoothing parameter. This method gives splashy, high-energy simulations, but can also introduce noise at the surface which can be distracting for small scale or viscous fluids.
Swirly Kernel
Transfers the velocity field directly to the particles using additional attributes to preserve the angular momentum of the fluid. This method retains vortices well with reduced noise at the surface, yielding swirling simulations with fewer splashes that can be effective at small scale or high viscosity.
In this case, the transfer radius is based on the voxel size, so using smaller voxels by
setting the Grid Scale on the FLIP Object to 1.5 can help retain
small fluid details. Because of the larger radius and extra attributes, this method is slightly
slower and uses more memory than the Splashy Kernel. Also, the Under-Resolved Particles
options are disabled for this method, as particles will always be fully resolved on the grid.
Note
The Splashy Kernel corresponds to the FLIP/PIC method of velocity transfer, while the Swirly Kernel is implemented with the newer APIC method.
Force Scale
The result of the velocity projection is a suggested change to the particles velocity. The force scale allows you to scale this change. A value of 1 will update the particles to the new desired velocity, 0 will leave them unchanged.
Velocity Smoothing
Particles can either update themselves with the change in velocity determined by the projection, or update themselves with the actual new velocity from the projection. The latter causes particle velocities to smooth out as they share more of their velocities with their neighbors.
Smooth Surface
Use a more accurate algorithm for tracking the surface SDF over time, providing smoother surfaces for stationary or slow moving liquids. This improvement may not be noticeable in very turbulent simulations such as rivers or waterfalls and can be disabled for a small performance gain.
Update Surface
Controls how the surface SDF is updated at the end of each substep to match the particle positions.
None
Do not update the surface SDF.
Advect
Advect the surface SDF through the velocity field. This is the faster than Rebuild, but not as accurate.
Rebuild
Rebuild the entire surface SDF from the particles. This the most accurate method, but also the slowest.
Note
Because particle positions are updated last in the solve order, the surface SDF will lag slightly behind the particle positions if Update Surface is set to None.
Update Velocity
Controls how the velocity field is updated at the end of each timestep to match the particle positions.
None
Do not update the velocity field.
Advect
Advect the velocity field through itself. This is the faster than Rebuild, but not as accurate.
Rebuild
Rebuild the entire velocity field from the particles. This the most accurate method, but also the slowest.
Note
Because particle positions are updated last in the solve order, the velocity field will lag slightly behind the particle positions if Update Velocity is set to None.
Volume Limits
If stray particles fly off too far, the resulting volume grid can become unwieldy even with constant tile compression. This tab allows you to set maximum bounds.
Visualize Limits
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.
Box Size
The size of the maximum volume.
Box Origin
The origin of the maximum volume.
Dynamically Resize Fields
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.
Fill New Volume
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 controls how the new particles are created.
Use Waterline
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.
Visualize Waterline
Display the waterline plane where fluid above the plane can freely flow out of an otherwise closed boundary.
Waterline
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.
Waterline Direction
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.
Use Boundary Layer
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.
Apply Boundary Velocities
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 neighbouring particles.
Lower Padding
The Boundary Layer will be offset into the simulation volume by the specified amount from the lower boundary of the simulation volume.
Upper Padding
The Boundary Layer will be offset into the simulation volume by the specified amount from the upper boundary of the simulation volume.
Surface Volume
The surface in boundary layer can be maintained by a provided SDF. Particles above the surface will be removed. The volume below the surface will re-seeded. The particles seeded in a new volume region will be created based on this surface.
Velocity Volume
The velocity of the particles in the boundary layer and new volume will be assigned a velocity sampled from this velocity volume.
Collisions
Velocity Scale
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.
Note
Use Transparency to reduce the overall effect of collisions on fluid.
Velocity Type
The method used to calculate collision velocities for colliding DOP
Objects. This setting does not affect collision velocities
introduced by a Source Volume DOP.
Rigid Velocity
The collision velocity of the objects is calculated by treating it as a rigid body. Only the angular velocity and linear velocity will be used, no local deformation will be taken into account.
Point Velocity
The collision velocity of the objects tries to take into account deformation by using point history. This only works if the topology doesn’t change.
Volume Velocity
Uses the SDF representation of the object to detect deformation. Does not require a fixed topology over time, but cannot detect tangential velocities.
Surface Extrapolation
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.
Note
Setting this to 0 is not recommended.
Volume Fraction Method
The solver uses fractional estimates of collision volumes to increase the accuracy of the pressure solve around curved or sloped surfaces.
Voxel Face Area
Estimate the fractional collision weight by determining how much of each voxel face is occluded by the collision volume. This method provides the smoothest handling of sloped objects, but can treat square objects as excessively smooth.
Collision Supersampling
Estimate the fractional collision weight by sampling the collision volume multiple times for each voxel. This method handles square-shaped collision objects better, but can introduce some false friction-like forces on sloped surfaces.
None
Do not calculate collision weights. This should only be used by
advanced users for a high degree of control over the collision
weights by calculating them manually, usually with a 
Gas SDF To
Fog DOP. attached to the Volume
Velocity input.
Samples Per Axis
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.
Transparency
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.
Note
If using Transparency, the Collision Detection method of the Particle Motion tab should usually be set to None, otherwise the particle collision detection will not allow any particles to intrude into collision objects, no matter what the Transparency value.
Stick on Collision
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.
Stick Scale
The amount of collision velocity to blend into the fluid’s velocity, where a value of 1 indicates fully matching the collision velocity.
Max Distance
Specifies the world-space distance within which to apply the effect.
Max Cells
Specifies the maximum number of voxels within which to apply the effect.
Stick Bias
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.
Normal Scale
Scale the amount of velocity adjustment in the direction normal to the collision surface.
Tangent Scale
Scale the amount of velocity adjustment in the direction tangent to the collision surface.
Tip
Keeping a high Normal Scale and decreasing the Tangent Scale can create a "sticktion" effect, where the fluid flows quickly around the collision but remains stuck to the surface.
Control Field
Scale the effect by this spatially varying field, which should match the collision field in resolution.
Viscosity
Note
See the discussion of viscosity in the user guide.
Enable Viscosity
Enable the viscosity solver.
Viscosity by Attribute
Use the specified attribute to overwrite the viscosity field.
Attribute Name
The float attribute on the particles to use for the viscosity amount.
Mix Method
How to blend the point attribute with the existing viscosity field.
Viscosity Scale
A scale applied to the Viscosity field after applying the attribute effect. This is useful for quickly adjusting the global amount of viscosity.
Float Precision
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.
Slip on Collision
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.
Slip Scale
The amount of fluid velocity to blend into the collision’s tangential velocity, where a value of 1 indicates fully matching the fluid velocity.
Control Field
Scale the effect by this spatially varying field, which should match the collision field in resolution.
Density
By default, the fluid has uniform density as set on the Physical tab
of the FLIP Object. To simulate fluids of varying
density, you can use a per-particle density attribute to control the
density throughout the fluid.
If you override the Density field through the Volume Velocity input to the flip solver, you can arbitrarily edit the density field.
Density by Attribute
Use the specified attribute to manipulate the density field.
Attribute Name
The float attribute on the particles to use for the density amount.
Mix Method
How to blend the point attribute with the existing density field.
Density Scale
A scale applied to the Density field after applying the attribute effect. This is useful for quickly adjusting the global amount of density.
Tip
By setting the Mix Method to Multiply, you can use relative density values that scale the default fluid density. A density attribute of 1, for example, will have the density of water by default, while a density attribute of 0.5 would indicate fluid of half that density. Relative density is also known as specific gravity; values for typical liquids can be found here. Note that because the FLIP Solver does not explicitly track the boundary between fluids of different densities, it might be necessary to exaggerate density ratios to avoid excess mixing.
Air
By default, the air volume is not simulated and is treated entirely as a void. This assumption causes air regions to collapse or expand based on the motion of the liquid. Enforcing air incompressibility restricts the liquid motion to treat each air region as incompressible.
Enforce Air Incompressibility
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.
Apply Moving Collision
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.
Min Air Region Voxels
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.
Tip
Turn off Dynamically Resize Fields when enforcing air incompressibility inside collision containers. Otherwise the enforcement might not detect the enclosed air region and the liquid could pour through unaffected.
Note
The Enforce Air Incompressibility feature does not solve for the velocity in the air volume. It simply constrains the velocity of the liquid from compressing or expanding the air volume. This approach makes the simulation much more efficient than fully simulating the entire air-volume.
Note
Bubbles submerged in the fluid could dissapear during the render stage when using the default settings in 
Particle Fluid Surface. Activate the Preserve Bubbles in Particle Fluid Surface to maintain the bubbles inside the fluid.
Divergence
By default, the volume motion is divergence free. These enforces neighboring particles to neither separate nor converge. However, you may want to adjust the particle spacing. Doing this with forces is difficult because the volume projection will undo your forces.
Instead, you can add a per-particle float attribute to store how much the particles should diverge. Positive values cause particles to spread out, and negative values cause them to clump together.
If you override the Divergence field through the Volume Velocity input to the flip solver, you can arbitrarily edit the divergence field.
You must turn on Add Divergence Field in the FLIP
Object for this to work.
Diverge by Attribute
Use the specified attribute to overwrite the divergence field.
Attribute Name
The float attribute on the particles to use for the divergence amount.
Mix Method
How to blend the point attribute with any existing divergence field. The divergence field is not reset frame-to-frame.
Divergence Scale
A scale applied to the divergence field after applying the attribute effect. This is useful for quickly adjusting the global amount of divergence.
Surface Tension
Surface tension flattens irregular features and pulls water into droplets. It is useful for small-scale simulations where the effect of surface tension is often more important than gravity in controlling the dynamics.
For large values, additional substeps are often required to ensure stability.
Enabling surface tension will create the surfacepressure field.
This field can be used for other effects than just surface tension,
such as suction or avoidance.
Enable Surface Tension
Enable the surfacepressure field.
Surface Tension
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 (ie, 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.
Solver
Spatial Scale
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.
Tip
Choose the correct Unit Length in Edit > Preferences > Hip File Options before you create your DOP network. That way, the FLIP solver node will automatically get the correct Spatial Scale when it’s created.
Mass Scale
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.
Tip
Choose the correct Unit Mass in Edit > Preferences > Hip File Options before you create your DOP network. That way, the FLIP solver node will automatically get the correct Mass Scale when it’s created.
Feedback Scale
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.
Extrapolation Mode
Controls the method used for extrapolating velocity in the solver.
Normal
A fast method that properly handles curved collision containers. This is the recommended extrapolation mode.
Fast-moving Colliders
A slower method that can be more accurate with fast moving colliders and a low number of substeps.
Automatic Extrapolation By Speed
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.
Max Cells to Extrapolate
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.
Use Preconditioner
During the pressure projection and viscosity solves, the matrices involved can be preconditioned to speed up the solution. However, this is a single threaded process. On machines with 4+ sockets it may be faster to disable this preconditioning and use a simpler Jacobi preconditioner which multithreads well, but can take more iterations to converge.
Use OpenCL
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
The following collision fields are used mainly for creating guiding simulations.
They are only connected to the collision fields in both the pressure and
viscosity solvers. All other nodes that apply collision information use the
internal fields. Please refer to Gas Guiding Volume DOP
for more information about creating guiding fields.
Collision Surface
The collision surface that is connected to pressure and viscosity solvers.
Collision Weights
The collision weights improve the accuracy of the pressure and viscosity solvers.
Collision Velocity
The collision velocity is applied to the fluid through the solvers at the collision surface.
Note
The following fields are used mainly to maintain FLIP particles for guiding
simulations. These fields are connected to a Gas Seed Markers DOP.
Source Surface
FLIP particles are seeded into the source surface. This is useful for allowing the fluid to flow out of a guiding simulation surface.
Sink 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.
Narrow Band
When this tab is enabled, FLIP particles are only maintained near the fluid surface. The rest of the fluid is represented by volumes (e.g. surface or velocity).
Removing particles deep inside the fluid reduces the memory footprint and can often improve the performance of the fluid simulation.
Note
Because the fluid is largely represented by volumes, Density by Attribute and Viscosity by Attribute are not supported.
Enable Particle Narrow Band
A narrow band of FLIP particles will be used to update the fluid’s surface and velocity volumes within the Bandwidth of the fluid surface. Any particles deeper into the fluid than the Bandwidth will be removed from the simulation.
Bandwidth
The narrow band of FLIP particles will be maintained at this voxel thickness. Very fast moving simulations might require a larger bandwidth to maintain stability.
Distribution
FLIP simulations are distributed in the same manner as particle simulations; by first specifying a series of Slice planes that divide space up. Each machine then is responsible for the fluid that lies within its slice.
Each substep all particles along the slice boundaries are exchanged. The bandwidth specified should thus be large enough to cover how far particles are expected to move in a substep.
All machines synchronize their substep size automatically, so variable substeps can be used.
Tracker Address
What machine will run the simtracker.py process for synchronization. If this is blank, there will be no attempt at synchronization or data transfer.
Tracker Port
The port specified when starting the simtracker.py process for communication.
Job Name
The job name to describe this synchronization or data exchange event. By using different job names one can have machines part of separate data-exchange and synchronization events.
Slice/Peer
The slice number that this machine should report itself as. Each machine connecting under the job name should have its own unique slice number. Sometimes this can be inferred from the operation so this parameter will be absent.
Number of Slice/Number of Peers
Total number of machines that have to synchronize. Sometimes this can be determined from the operation, so this parameter will be absent.
Distributed Pressure Solve
The pressure projection may be distributed across machines. This avoids any loss of fluid and is required for tank-style simulations, but not needed for flowing rivers. It requires considerably more network bandwidth, so becomes advantageous only with very large simulations.
Parameter Operations
Each data option parameter has an associated menu which specifies how that parameter operates.
Use Default
Use the value from the Default Operation menu.
Set Initial
Set the value of this parameter only when this data is created. On all subsequent timesteps, the value of this parameter is not altered. This is useful for setting up initial conditions like position and velocity.
Set Always
Always set the value of this parameter. This is useful when specific keyframed values are required over time. This could be used to keyframe the position of an object over time, or to cause the geometry from a SOP to be refetched at each timestep if the geometry is deforming.
You can also use this setting in
conjunction with the local variables for a parameter value to
modify a value over time. For example, in the X Position, an
expression like $tx + 0.1 would cause the object to
move 0.1 units to the right on each timestep.
Set Never
Do not ever set the value of this parameter. This option is most useful when using this node to modify an existing piece of data connected through the first input.
For example, an 
RBD State
DOP may want to animate just the mass of an
object, and nothing else. The Set Never option could be used
on all parameters except for Mass, which would use Set
Always.
Default Operation
For any parameters with their Operation menu set to Use Default, this parameter controls what operation is used.
This parameter has the same menu options and meanings as the Parameter Operations menus, but without the Use Default choice.
Make Objects Mutual Affectors
All objects connected to the first input of this node become mutual affectors.
This is equivalent to using an 
Affector
DOP to create an affector relationship between
* and * before connecting it to this node. This option makes it
convenient to have all objects feeding into a solver node affect
each other.
Group
When an object connector is attached to the first input of this node, this parameter can be used to choose a subset of those objects to be affected by this node.
Data Name
Indicates the name that should be used to attach the data to an object or other piece of data. If the Data Name contains a "/" (or several), that indicates traversing inside subdata.
For example, if the 
Fan Force DOP has the default Data Name
"Forces/Fan". This attaches the data with the name "Fan" to an
existing piece of data named "Forces". If no data named "Forces"
exists, a simple piece of container data is created to hold the
"Fan" subdata.
Different pieces of data have different requirements on what names should be used for them. Except in very rare situations, the default value should be used. Some exceptions are described with particular pieces of data or with solvers that make use of some particular type of data.
Unique Data Name
Turning on this parameter modifies the Data Name parameter value to ensure that the data created by this node is attached with a unique name so it will not overwrite any existing data.
With this parameter turned off, attaching two pieces of data with the same name will cause the second one to replace the first. There are situations where each type of behavior is desirable.
If an object
needs to have several Fan Forces blowing on it, it is
much easier to use the Unique Data Name feature to ensure that
each fan does not overwrite a previous fan rather than trying to
change the Data Name of each fan individually to avoid
conflicts.
On the other hand, if an object is known to have RBD
State data already attached to it, leaving this
option turned off will allow some new RBD State
data to overwrite the existing data.
Solver Per Object
The default behavior for solvers is to attach the exact same solver to all
of the objects specified in the group. This allows the objects to be
processed in a single pass by the solver, since the parameters are identical
for each object. However, some objects operate more logically on a single
object at a time. In these cases, one may want to use $OBJID expressions
to vary the solver parameters across the objects. Setting this toggle will
create a separate solver per object, allowing $OBJID to vary as expected.
Inputs
Fluid to Solve
The simulation object to evolve as a particle fluid.
Particle Velocity
This input injects microsolvers after the particle velocity has been updated but before it has been transferred to the volume fields. It is thus the ideal place to add solvers that manipulate the particle velocity or position still farther.
Volume Velocity
Solvers in the volume velocity input are added after the volume velocity field has been created but before it has been made non-divergent. This is the best place for field-affecting microsolvers to be attached, and the only place for microsolvers that want to add forces to the velocity field.
Note
Solvers attached to this input should not create or delete particles.
Sourcing (post-solve)
The main use for this input is to add or remove particles using the
Source Volume node that imports particles
and volumes from a geometry network. See fluid
sourcing for more information.
Nodes attached to this input can also edit the simulation network’s velocity fields, for example to apply custom forces, after the main simulation steps.
Outputs
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
Examples
DensityViscosity Example for FLIP Solver dynamics node
This example demonstrates two fluids with different densities and viscosities interacting with a solid object.
FlipColorMix Example for FLIP Solver dynamics node
This example demonstrates the use of the Flip Solver to mix the colors of a red fluid with a blue fluid to form a purple fluid.
FlipColumn Example for FLIP Solver dynamics node
This example demonstrates how a mixture of fluid colours can have their colour changed by a collision with a static object.
FlipFluidWire Example for FLIP Solver dynamics node
This example demonstrates the use of the Flip Solver and the Fluid Force DOP. The Fluid Force DOP is used to apply a drag force on a wire object according to the motions of a flip fluid. The drag force is only applied at locations where fluid exists in the fluid object.
SpinningFlipCollision Example for FLIP Solver dynamics node
This scene shows how to create FLIP fluids based on the velocity of geometry by generating new particles from points scattered on the original geometry based on the velocity vectors. It also shows how to set up the original geometry to act as a collision object for the fluid.
VariableViscosity Example for FLIP Solver dynamics node
This example demonstrates interaction between three fluids of varying viscosity and a moving collision object.
| See also |
