A solver for Sign Distance Field (SDF) liquid simulations.
Use the 
Liquid Container shelf tool to create a liquid container and fluid solver automatically.
Setting up
If you are setting up a fluid network from scratch, you can use the Fluid Object node to create a DOP object with the data required by the Fluid solver already attached. If you already have a DOP object, you can use the the Fluid Configure Object node to add the necessary data to it.
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.
Tips
If your fluid keeps disappearing, try turning off Build Outside Markers. You can also set Marker Escape Threshold to
0to help keep fluid inside the box.Liquid streams smaller than a few voxels may disappear.
If your source object deforms or collides with a deforming object (using a DOP relationship), by default the solver will use the per-point deformation. To make the solver use per-point deformation, on the Fields > Sources subtab set Velocity Type to “Point Velocity”.
Parameters
Simulation
These parameters control how the simulation develops over time.
| Time Scale | A scaling factor for time inside this solver. |
| Confinement Strength | When the container has |
| 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. |
| Viscosity | Low values give more “watery”, “thin”, flowing fluid, while higher values give more “thick”, resistant fluid. Internally, viscosity is a force that tries to give neighboring voxels the same velocity. |
| Use Friction and Bounce | Use the fluid’s physical parameters for friction and bounce at collisions. Turn this off for a fluid with zero friction and zero bounce. |
Relationships
Prior to Houdini 12, the Pyro solver used DOP relationships to associate sources, pumps, sinks, and collision geometry with a fluid container, using the Merge DOP and/or Apply relationship DOP to create the relationship. The preferred method in Houdini 12 and later is to use SOP networks to create sources, pumps, sinks, and collision geometry and import them using the Source volume DOP.
If you want to use the old relationship method to set up sources, sinks, etc., you can enable relationships using the parameters on this tab. By default, relationships are turned off, and the solver ignores relationship data.
You can use both methods (import SOP geometry and attach it to the solver’s “sourcing” input, as well as set up DOP object relationships). When relationships are enabled, the solver will combine the sources, sinks, etc. from both methods.
| Enable Relationships | Use object relationship data to add sources, pumps, sinks, and collision geometry to the simulation (in addition to imported data connected to the sourcing input, if any). |
Sources
| Enable Source Relationship | Use DOP objects with a “source” relationship to the solver. | ||||||
| Velocity Merge | How the source object’s velocity will affect the container’s velocity field. Scale controls the amount to add.
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| Velocity Type | How to measure velocity on the source object. If the source geometry does not deform (change shape) over time, use “Rigid velocity”. If the source deforms but does not change topology over time, use “Point velocity”.
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Pumps
| Enable Pump Relationship | Use DOP objects with a “pump” relationship to the solver. | ||||||
| Velocity Merge | How the source object’s velocity will affect the container’s velocity field. Scale controls the amount of velocity to add. | ||||||
| Velocity Type | Controls how the pump object’s per point velocity is computed.
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Collisions
| Enable Collide Relationship | Use DOP objects with a “collision” relationship to the solver. | ||||||
| Restrict Mask to Bandwidth | Normally the collision mask SDF is only calculated up to a certain distance from the original collision geometry. Turn this off to compute the full range of the mask if you need it for some special effect, such as having things react before they reach the object. | ||||||
| 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. | ||||||
| Collide with Non-SDF | Allows the fluid to collide with objects that don’t have Geometry/SDF, such as other fluids. | ||||||
| Velocity Type | Controls how the collision object’s per point velocity is computed.
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Sink
| Enable Sink Relationship | Use DOP objects with a “sink” relationship to the solver. |
Advanced
You should generally not need to change these parameters.
| Minimum Substeps | Forces the solver to run a minimum number of substeps. Normally the pyro solver works best with no substeps. If you have smoke and unusual forces you may want to increase this parameter for better stability. Increasing this will usually make the simulation much slower. |
| Maximum Substeps | Forces the solver to not run more substeps than this maximum. Normally the pyro solver works best with no substeps. If you have smoke and unusual forces you may want to increase this parameter for better stability. Increasing this will usually make the simulation much slower. |
| CFL Condition | When Maximum substeps is greater than |
| Frames Before Solve | Specifies the number of frames to wait before a full solve cycle is computed. On those frames only operations applied to the last solver input (sources) are computed. This enables the sourcing and resizing of data without actually computing a full solve on the maximum grid. |
Surface
| 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. |
| 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 Rate | The rebuilding and reinitialization of the SDF will only be performed every every N frames where N is the reinitialization rate. |
| Reinitialize Iterations | The number of times to apply the reinitialization equation. |
| 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. |
Markers
| 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. |
| Markers Per Voxel | The desired number of markers to maintain inside each voxel. More markers allows for a better capturing of the surface behaviour, but can require a lot of memory and processing time for large complicated sims. |
| 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. |
Projection
| 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. Note: Uses the Gas Project Non Divergent Variational when this is set, so the ghost fluid and preserve bubble options are no longer available. |
| Scale by Surface | Scales external forces by an estimate of how much fluid is in each voxel. This allows sub-voxel waves to be evened out under gravity. |
| 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. |
| 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. |
| 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. |
Strain
| Use Strain | Adds a “strain” field to the object to track the amount of deformation the fluid has undergone at each voxel. |
| 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. |
| 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. |
Advection
| Advection Type | The algorithm to use for advecting the fields.
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| Clamp Values | The error correction of the BFECC and Modified MacCormack advection types can move voxel values outside the container, leading to strange effects such as negative density values. This parameter lets you choose a method to avoid this problem. The default is “Revert”.
Reverting can avoid checker artifacts where the error correction breaks down. | ||||||
| Vel Advection Type | The algorithm to use for advecting the velocity field. Higher types in the list will reduce the apparent viscosity of the field, but may add energy or cause chatter. | ||||||
| Advection CFL | When tracing the particles, this controls how many voxels the particles can move in a single iterations. Higher values give faster tracing and faster advection, but more errors. |
Collisions
| Feedback Scale | A scale factor for applying feedback forces to other objects. Setting this to |
Clear Fields
| Fields to Clear | Zeros out the specified types of fields after the solve step. This ensures the .sim files, which store the complete state of the simulation, do not have any information not needed, reducing their size and save time.
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| Additional | A space separated list of extra fields that should be cleared post-solve. |
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 |
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 |
| 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 |
| 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 |
| 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 |
| 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.
Examples that use this node
| Example for | Example name | |
|---|---|---|
| Fluid Force | FluidWireInteraction | Load | Launch |
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| Fluid Object | BallInTank | Load | Launch |
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| Fluid Object | FillGlass | Load | Launch |
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| Fluid Object | FluidFeedback | Load | Launch |
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| Fluid Object | PaintedGrog | Load | Launch |
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| Fluid Object | RestartFluid | Load | Launch |
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| Fluid Object | RiverBed | Load | Launch |
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| Fluid Object | SourceAndSink | Load | Launch |
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| Fluid Object | VariableDrag | Load | Launch |
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| Gas Surface Tension | TeapotUnderTension | Load | Launch |
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| Reference Frame Force | ReferenceFrameForce | Load | Launch |
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