Houdini 16.5 Nodes Dynamics nodes

FLIP fluid object dynamics node

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Creates a particle fluid object with the necessary data and parameters to work with a FLIP solver.

See particle fluid simulations for more information about FLIP fluid simulations.

Increasing the Particle Separation will lower the resolution, which will make your simulation faster to process.

Parameters

Properties

Particle Separation

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.

Particle Radius Scale

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.

Grid Scale

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.

Collision separation

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.

Closed Boundaries

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.

Creation

Creation Frame Specifies Simulation Frame

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.

Creation Frame

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.

Number of Objects

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.

Object Name

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.

Note

While it is possible to have many objects with the same name, this complicates writing references, so it is recommended to use something like $OBJID in the name.

Solve On Creation Frame

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.

Allow Caching

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.

Initial Data

Input Type

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

Surface SOP

Use this option to generate particles inside of the specified surface.

The initial separation between particles is determined by the Particle Separation parameter, and so the particle separation also determines the number of particles created.

Particle Field

Use this option to generate a fluid particle at each point in the specified geometry.

This can be used to specify a custom initial distribution for the fluid particles or to resume an existing particle fluid simulation. It can also be used to combine multiple fluids with different initial conditions.

When this option is selected, the particle separation is completely independent of the initial particle distribution. This means that changing the particle separation may substantially alter the results of a simulation.

However, if the Initialize Fluid Attributes toggle is disabled, then the Particle Fluid Object does not create or change any attributes on the imported fluid geometry, and expects those attributes to exist already.

File

Use this option to initialize a fluid simulation directly from a .bgeo file.

This can be used to easily reinitialize a simulation from saved geometry data. See the Fluid Geometry File parameter below.

Fetch Data

Use this option to generate particles inside of the specified DOP object geometry

The initial separation between particles is determined by the Particle Separation parameter, and so the particle separation also determines the number of particles created.

Narrow Band

Use this option to generate a Narrow Band FLIP simulation. This consists of a thin band of fluid particles near the fluid surface and volumes to represent the internals of the fluid (e.g. surface and velocity).

The SOP Path generates particles from the specified geometry (similar to Particle Field). The Surface Volume and Velocity Volume paths specify the SOPs to extract for the associated fluid volumes.

Initial Configuration

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

Grid

Particles are generated on an axis-aligned grid inside of the surface.

Tetrahedral

Particles are generated in a more tightly-packed tetrahedral arrangement inside of the surface.

This can be useful is the fluid needs to settle quickly inside of a container without losing too much of its initial height.

SOP Path

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

Jitter Seed

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.

Jitter Scale

The magnitude of random jitter to apply to each particle.

Use Object Transform

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.

Initialize Fluid Attributes

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.

Initialize Velocity

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.

Initial Velocity

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

Add Viscosity Attribute

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.

Add Divergence Field

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.

Guides

The parameters on the Visualization sub-tab let you turn various visualizations on or off quickly.

The parameters on the Particles sub-tab control the visualization of the fluid particles. These parameters are explained in the help for the Guides tab of the Particle Fluid Object DOP.

The other sub-tabs let you visualize various fields or attributes in the object. The help for the Scalar Field Visualization or Vector Field Visualization provides more details about how these work.

Physical

Bounce

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.

Friction

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.

Dynamic Friction Scale

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.

Density

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 overriden with particle attributes by using its Density tab.

Viscosity

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 overriden 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.

Collisions

Volume Offset

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.

Use Point Velocity for Collisions

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.

Use Volume Velocity for Collisions

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.

Locals

ST

This value is the simulation time for which the node is being evaluated.

This value may not be equal to the current Houdini time represented by the variable T, depending on the settings of the DOP Network Offset Time and Time Scale parameters.

This value is guaranteed to have a value of zero at the start of a simulation, so when testing for the first timestep of a simulation, it is best to use a test like $ST == 0 rather than $T == 0 or $FF == 1.

SF

This value is the simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.

This value may not be equal to the current Houdini frame number represented by the variable F, depending on the settings of the DOP Network parameters. Instead, this value is equal to the simulation time (ST) divided by the simulation timestep size (TIMESTEP).

TIMESTEP

This value is the size of a simulation timestep. This value is useful to scale values that are expressed in units per second, but are applied on each timestep.

SFPS

This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.

SNOBJ

This is the number of objects in the simulation. For nodes that create objects such as the Empty Object node, this value will increase for each object that is evaluated.

A good way to guarantee unique object names is to use an expression like object_$SNOBJ.

NOBJ

This value is the number of objects that will be evaluated by the current node during this timestep. This value will often be different from SNOBJ, as many nodes do not process all the objects in a simulation.

This value may return 0 if the node does not process each object sequentially (such as the Group DOP).

OBJ

This value is the index of the specific object being processed by the node. This value will always run from zero to NOBJ-1 in a given timestep. This value does not identify the current object within the simulation like OBJID or OBJNAME, just the object’s position in the current order of processing.

This value is useful for generating a random number for each object, or simply splitting the objects into two or more groups to be processed in different ways. This value will be -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

This is the unique object identifier for the object being processed. Every object is assigned an integer value that is unique among all objects in the simulation for all time. Even if an object is deleted, its identifier is never reused.

The object identifier can always be used to uniquely identify a given object. This makes this variable very useful in situations where each object needs to be treated differently. It can be used to produce a unique random number for each object, for example.

This value is also the best way to look up information on an object using the dopfield expression function. This value will be -1 if the node does not process objects sequentially (such as the Group DOP).

ALLOBJIDS

This string contains a space separated list of the unique object identifiers for every object being processed by the current node.

ALLOBJNAMES

This string contains a space separated list of the names of every object being processed by the current node.

OBJCT

This value is the simulation time (see variable ST) at which the current object was created.

Therefore, to check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

This value is the simulation frame (see variable SF) at which the current object was created.

This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

This is a string value containing the name of the object being processed.

Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.

The object name can also be used to treat a number of similar objects (with the same name) as a virtual group. If there are 20 objects named "myobject", specifying strcmp($OBJNAME, "myobject") == 0 in the activation field of a DOP will cause that DOP to operate only on those 20 objects. This value will be the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

This is a string value containing the full path of the current DOP Network. This value is most useful in DOP subnet digital assets where you want to know the path to the DOP Network that contains the node.

Note

Most dynamics nodes have local variables with the same names as the node’s parameters. For example, in a Position node, you could write the expression:

$tx + 0.1

…to make the object move 0.1 units along the X axis at each timestep.

See also

Dynamics nodes