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The Gas Adaptive Viscosity DOP is a microsolver used in building larger fluid simulations. Advanced users could try to build an entire new solver out of microsolvers.
This node applies a viscosity effect to a velocity field. A viscosity force causes nearby parts of the liquid to have similar velocities. It can be thought of as a localized drag, where each part of the fluid is slowed to the speed of neighboring fluid. The effect of viscosity is solved on an adaptive (octree) background grid to significantly improve performance over solving with a uniform grid (as done in the 
Gas Viscosity DOP).
For small amounts of viscosity, a simple Gas Blur DOP can be used on
a velocity field to achieve this effect. However, this does not scale to large viscosities. It also will dampen rotational motion incorrectly.
Note
For the most accurate results, the velocity field provided should already be divergence-free, usually as a result of applying the Gas Project Non Divergent Variational DOP . However, the velocity field produced by the viscosity solve may no longer be divergence-free, so a typical viscous fluid solve will solve for non-divergence before and after the viscosity solve.
Parameters ¶
Surface Field
An SDF that specifies which voxels are to be considered in the viscosity calculations. Voxels are either inside (less than 0) or outside. Viscosity is only applied on the interior voxels. This avoids 'frame dragging' from the surrounding empty liquid.
Surface Weights Field
This field stores what percentage of each voxel is inside the surface field. It is usually computed with a Gas SDF To Fog DOP.
Velocity Field
The velocity field to apply viscosity to.
Collision Field
The viscosity equations assume a no-slip boundary condition for fluid along collisions, meaning the fluid’s velocity will match the collision’s velocity along their boundary.
Apply Collision Weights
Compute collision weights for the solver in a similar method as surface weights. Using collision weights in the solver helps alleviate grid artifacts that can appear at fluid-solid collisions.
Collision Velocity Field
The velocity of each voxel in the collision object. If not provided, the collision velocities will be assumed to be zero.
Viscosity Field
A scalar field allowing the viscosity to vary across the fluid. The value of the field is multiplied with the Scale.
Scale
The strength of the viscosity effect, multiplied with the viscosity field if present.
Note
Either the viscosity field or the scale parameter need to be scaled to reflect the units of the simulation, but not both. The standard 
Flip Fluid Solver setup uses a dimensionless scale and has the units applied to the viscosity field.
Min Viscosity
The applied viscosity will revert to zero if less than this minimum. This helps avoid ill-conditioned matrices that may fail to converge. This value is in units of kinematic viscosity (meters-squared / second), and should be equal to the minimum valid dynamic viscosity divided by the maximum valid fluid density.
Density Field
A field that gives the relative inertia of each voxel, allowing for viscosity to take different density fluids into consideration. Often the massdensity scalar field is used to avoid confusion with the traditional density field of smoke simulations. If this solver is used in conjunction with the Gas Project Non Divergent Variational DOP solver, this should be the same field provided to the Density parameter there.
Custom Fine-cell Field
By default, the adaptive solver will construct fine-cells along the fluid boundary that match the simulation resolution and build coarse cells in the fluid interior. By specifying a custom field, fine cells will also be generated in the fluid interior for voxels in the custom field with values 1.
Error Tolerance
How much error to allow in solving the viscosity equations. Smaller errors will take longer to converge but produce more accurate results.
Max Solver Iterations
The linear solver will terminate after this specified number of iterations and the state of the linear system will be written to the velocity field. This parameter should be treated as a fail-safe upper limit for the solver. It should not be used to increase performance, it’s preferable to modify Error Tolerance instead.
Surface Extrapolation Cells
Extrapolate the surface field into any collision objects if they are within this fraction of a voxel. This can provide more stable collisions along curved surfaces, although it can also introduce a small amount of stickiness into the fluid-solid collision.
Samples Per Axis
The number of samples per axis for supersampling when computing internal surface weights. Total samples is the cube of this, so two samples per axis is actually eight samples.
Float Precision
This determines the floating point precision that is 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.
Octree Levels
When solving for viscosity, the interior of the fluid volume is filled with coarse cells. This sets the largest allowable coarse cell, where the width of the coarsest cell is larger than the simulation’s voxel size by a factor of 2xOctree Levels.
Fine-cell Bandwidth
In order to capture highly detailed fluid behavior, cells within a thin layer along the boundary of the fluid are kept at the finest resolution and interior cells are progressively coarsened until reaching the specified Octree Levels limit of coarsening.
Output Octree Visualization Geometry
For visualization and debugging purposes, enabling this option generates the underlying octree geometry. It does not produce the actual octree data structure, but rather a geometry of active cell center points to be used as a visual aid.
Only Generate Octree Geometry
For special circumstances where only the underlying octree geometry is required, the solver will terminate early, only generating the visualization geometry.
Octree Visualization Geometry
The geometry data to generate the octree visualization geometry with.
Use Waterline
See FLIP Solver.
Waterline
See FLIP Solver.
Waterline Direction
See FLIP Solver.
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 $OBJID
to vary as expected.
Setting this is also required if stamping the parameters with a Copy Data DOP.
Inputs ¶
All Inputs
Any microsolvers wired into these inputs will be executed prior to this node executing. The chain of nodes will thus be processed in a top-down manner.
Outputs ¶
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
Locals ¶
channelname
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($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 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
The simulation time for which the node is being evaluated.
Depending on the settings of the 
DOP Network
Offset Time and Scale Time parameters,
this value may not be equal to the current Houdini time
represented by the variable T.
ST 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 $FF == 1.
SF
The simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.
Depending on the settings of the DOP Network parameters,
this value may not be equal to the current Houdini frame number
represented by the variable F. Instead, it is equal to
the simulation time (ST) divided by the simulation timestep size
(TIMESTEP).
TIMESTEP
The size of a simulation timestep. This value is useful for scaling values that are expressed in units per second, but are applied on each timestep.
SFPS
The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
SNOBJ
The number of objects in the simulation. For nodes that
create objects such as the 
Empty Object DOP,
SNOBJ increases for each object that is evaluated.
A good way to guarantee unique object names is to use an expression
like object_$SNOBJ.
NOBJ
The number of objects that are evaluated by the current node during this timestep. This value is often different from SNOBJ, as many nodes do not process all the objects in a simulation.
NOBJ may return 0 if the node does not
process each object sequentially (such as the 
Group
DOP).
OBJ
The index of the specific object being processed by the node. This value always runs from zero to NOBJ-1 in a given timestep. It does not identify the current object within the simulation like OBJID or OBJNAME; it only identifies 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
is -1 if the node does not process objects sequentially (such
as the Group DOP).
OBJID
The unique 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. This is very useful in situations where each object needs to be treated differently, for example, to produce a unique random number for each object.
This value is also the best way to look up information on an object using the dopfield expression function.
OBJID is -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
The simulation time (see variable ST) at which the current object was created.
To check if an object was created
on the current timestep, the expression $ST == $OBJCT should
always be used.
This value is zero if the node does not process
objects sequentially (such as the Group DOP).
OBJCF
The simulation frame (see variable SF) at which the current object was created. It is equivalent to using the dopsttoframe expression on the OBJCT variable.
This value is zero if the node does not process objects
sequentially (such as the Group DOP).
OBJNAME
A string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named “myobject”,
specifying strcmp($OBJNAME, "myobject") == 0 in the activation field
of a DOP will cause that DOP to operate on only those 20 objects.
This value is the empty string if the node does not process objects
sequentially (such as the Group DOP).
DOPNET
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 DOP,
you could write the expression:
$tx + 0.1
…to make the object move 0.1 units along the X axis at each timestep.
