Fluid Solver dynamics node

Sets and configures a Fluid solver.

All

Parameters

Inputs

Outputs

Local variables

The Fluid Solver DOP sets objects to use the Fluid Dynamics solver.

If an object has this DOP as its “Solver” subdata, it will evolve itself as a Fluid object.

This solver makes use of various field subdata on the object. The object should have a scalar field “surface” to track the surface of the fluid in space. The object should have a vector field called “vel” to track the velocity of each point in space.

To setup an object with the requisite subdata, use the Fluid Configure Object DOP.

Parameters

Substeps

Minimum Substeps

While the Fluid Solver tries to estimate the correct substep size for a stable simulation, if unusual forces are present it may take too large of a step.

By setting the minimum substep one can enforce stability.

Maximum Substeps

The Fluid Solver will not break the simulation down into more substeps than this.

It is a very good idea to always have a maximum to ensure frames will be finished regardless of their complexity. Lowering this ceiling can ensure a minimum computation time at the expense of accuracy.

CFL Condition

The CFL Condition is a factor used for automatically determining what size substep a scene requires.

The idea is that any substep should not allow any objects to interpenetrate by more than one voxel cell. This condition is met when this parameter is at 1. A value of 10 would allow a substep to move the fluid by as much as 10 voxel cells, possibly tunneling through objects rather than properly deflecting.

Internal Forces

Viscosity

Viscosity is a force which tries to ensure that neighboring voxels have the same velocity.

A zero value allows fluid in adjacent voxels to move any direction without resistance. Higher values introduce a penalty effect when a voxel’s velocity varies from that of its neighbors. This is currently implemented by applying a diffusive term to the velocity field.

Use Strain

An additional Strain Field will be added to track the amount of deformation that any part of the fluid has undergone.

This can be used to provide a restorative force to create a visco-elastic fluid.

Plastic Flow Rate

This is the rate of plastic flow. The current strain is dissipated at this rate per second. A value greater than the timestep per second can lead to overshoot.

Plastic Flow Threshold

This is the threshold for plastic flow. When the norm of the strain exceeds this threshold, the strain is dissipated according to the plastic flow term.

This causes the fluid to lose its history and permanently enter its new configuration.

Strain Elastic Modulus

Similar to a drag coefficient, the elastic modulus provides the scale factor for how to translate a certain amount of distortion into a restorative force.

A larger value will cause the fluid to spring back faster. However, large values may require more timesteps to keep the simulation stable.

Confinement Strength

An overall scale adjustment for the vorticle forces. Vorticle forces are small pin-wheel like forces that add turbulence to fluids. NOTE

Vorticle forces are only present if data named vorticles is attached to the object.

Feedback Scale

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

The value can be thought of as a density, so to have a default RBD object to balance a value of 1000 should be used.

Speed Limit

If set, each axis of the velocity field will be clamped to this maximum speed. This is done after the force computation and before the divergence or advection, so it is possible for the speed to end up slightly larger than this value.

Note

As the clamping is done per-axis, the velocity along the diagonal can be up to sqrt(3) higher.

Use Friction and Bounce

Determines if the fluid’s physical parameters will be used to apply friction and bounce to collisions. If not set, it will be treated as zero friction and zero bounce.

Scale by Surface

External forces will be scaled by an estimate of how much of the cell contains liquid. This allows for sub-voxel waves to be evened out under gravity.

However, it also means that the edges of containers will have less gravity and hence will creep upwards in an undesirable manner.

Advanced

Reinitialize Rate	The rebuilding and reinitialization of the SDF will only be performed every every N frames where N is the reinitialization rate. The rebuilding introduces artifacts into the simulation so ideally this should be as large as possible without the resulting distortions in the signed distance field introducing their own problems.
Rebuild SDF	The voxels that border the zero isocontour are used to determine the initial zero isocontour. The rest of the field is then rebuilt from these values directly.
Reinitialize SDF	If the field is already almost a signed distance field, a less drastic error correction can be performed. The reinitialization equation is applied which will converge on a signed distance field without affecting the boundary.
Reinitialize Iterations	The number of times to apply the reinitialization equation.
Build Inside Markers	Construct marker surfels on the inside of the surface. Inside surfels increase volume as they correct the surface by pushing out the boundary.
Build Outside Markers	Construct marker surfels on the outside of the surface. Outside surfels decrease volume as they correct the surface by pushing in the boundary.
Minimum Marker Radius	The minimum radius of marker surfels, measured in terms of voxel size.
Maximum Marker Radius	The maximum radius of marker surfels, measured in terms of voxel size.
Marker Escape Threshold	The distance the marker must be on the wrong side of the surface, as measured in terms of the marker’s radius, to qualify as escaped. Only escaped markers will update the surface value.
Correct Surface	The motion of the collision objects may cause the fluid to interpenetrate the fluid objects. The Correct Surface step will remove any fluid that is penetrating a collision object.
Limit Velocity Extrapolation	When extrapolating the velocity field into non-fluid cells, the extrapolation is stopped when a certain number of cells has been filled along the border.
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.
Preserve Bubbles	Trapped air will be given a pressure value independent of the boundary pressure conditions. This prevents the bubbles from collapsing in on themselves. If not set, fluid will be free to flow into bubbles from all sides ensuring their swift dissipation.
Sticky Fluid	Determine if collision objects should prevent fluid from leaving their surface instead of just preventing the fluid from entering their surface. This should be left on.
Ghost Fluid	Pressures are extrapolated into air cells rather than just set to zero. This causes sub-voxel ripples in the air side to be suppressed.
Variational	Contribution to the pressure equation is proportional to the amount of the cell in the liquid. This attempts to avoid stairstep boundaries on solid boundaries. Currently, only at most one of Ghost Fluid and Variational should be set.

Collisions

Restrict Mask to Bandwidth

The collision mask is an SDF built storing the distance to the collision geometry for each node. By default, it is only calculated up to a certain distance from the collision geometry.

However, if one wishes to use this field for special effects (such as having things react before they reach the object), one might want to compute the full range of the mask.

Velocity Type

Rigid Velocity	The collision velocity of the object 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 object 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.

Sources

Velocity Merge

Controls how the source object’s velocity will affect the liquid’s velocity field.

None	The source’s velocity will be ignored.
Net New Source	Velocity will be set in those areas where fluid will be added this frame from the source.
New Source	Velocity will be set in those areas where the source is present.

Velocity Type

Rigid Velocity	The velocity of the object 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 velocity of the object attempting 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.

Pumps

Velocity Type

Controls how the pump object’s per point velocity is computed.

Rigid Velocity	The velocity of the object 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 velocity of the object attempting 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.

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 `$positionx + 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.

Inputs

First Input

This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.

If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.

All Other Inputs

If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.

The specific types of subdata that are meaningful vary from node to node. Click an input connector to see a list of available data nodes that can be meaningfully attached.

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.

Local variables

channelname	This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname). Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter. This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like: dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname) If the data does not already exist, then a value of zero or an empty string will be returned.
DATACT	This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.
DATACF	This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.
RELNAME	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to the name of the relationship the data to which the data is being attached.
RELOBJIDS	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.
RELOBJNAMES	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.
RELAFFOBJIDS	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.
RELAFFOBJNAMES	This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP). In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.
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:

$positionx + 0.1

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

Usages in other examples

Example name	Example for
FluidWireInteraction	Fluid Force dynamics node	Load \| Launch
TeapotUnderTension	Gas Surface Tension dynamics node	Load \| Launch
ReferenceFrameForce	Reference Frame Force dynamics node	Load \| Launch
RiverBed	Fluid Object dynamics node	Load \| Launch
RestartFluid	Fluid Object dynamics node	Load \| Launch
SourceAndSink	Fluid Object dynamics node	Load \| Launch
VariableDrag	Fluid Object dynamics node	Load \| Launch
PaintedGrog	Fluid Object dynamics node	Load \| Launch
FillGlass	Fluid Object dynamics node	Load \| Launch
BallInTank	Fluid Object dynamics node	Load \| Launch
FluidFeedback	Fluid Object dynamics node	Load \| Launch