This page provides information on the Grid rollout.

Overview

The Grid rollout lets you set the boundaries, size and resolution quality of the sim. Regarding size, the Phoenix Simulator works best when the scale of the container matches the real-world size of the simulated effect. For example, if you are simulating a campfire, your container should be at most a couple of meters wide.

Note that it does not matter if this is two meters or two thousand millimeters - the way you view the units is irrelevant. Phoenix always converts the units to a common world-size length, so the only important thing is the size of the container. If you are simulating a volcano, for example, the container should be several hundred meters wide, or several hundred thousand millimeters.

If your scene is structured in a way that makes it hard for you to scale the objects to their real-world size, you can use the Scene Scale parameter to tell Phoenix to treat the container as larger / smaller than it actually is when measured in Scene Units. This will influence the dynamics of the simulation, allowing to you achieve the correct behavior for your simulation without the need to tweak the size of the objects in your scene.

Using the parameters on this rollout you can:

  • Specify the Size and Resolution of the Simulator
  • Enable / Disable Adaptive Grid which is a performance optimization allowing you to keep the size of the Simulator as small as possible thus reducing RAM usage
  • Specify which Walls of the Simulator will be considered Open (infinite) or Jammed (ie. as solid obstacles, closed)
  • Link multiple Simulators in a Cascade setup - for more information, please check the following article: Transferring fluid between Simulators using a Cascade Connection.
  • Specify a Confine Geometry to limit the fluid calculations only to the volume of the specified object

UI Path: ||Select PhoenixFDSim|| > Attribute Editor > Grid rollout


Parameters

Lock Voxel Count | lockGridRes – When enabled, changing the Voxel Size will adjust the simulator size and preserve the original grid resolution. When disabled, changing the Cell Size will adjust the grid resolution and preserve the simulator size.

Units | metricUnits – Set the scale of the units used by the Phoenix simulation. By default the metric units are taken from Maya, but can be overridden to use certain units.

Scene Scale | unitsScale – Specifies a multiplier for the original units of the scene. Phoenix works best when the container size is close to the real-world size of the desired effect. You can use this parameter to make the fluid solver see the container as bigger or smaller than it actually is in the scene, in case you cannot change the general scene units of Maya. Check the sizes shown in the Total Cells field - this is the size the fluid solver will use. These sizes will change as you change the Scene Scale and should be close to the real world size of the effect you are simulating. It does not matter how you view the units - in meters, centimeters, inches, etc. For example, a candle simulation can be 20 cm tall, or 0.2 m tall - it's the same.

Larger scale would make the fluid move more slowly because it needs to travel a greater distance, while smaller scale makes the fluid move faster and more chaotic. Compared to the effect of the Time Scale option in the Dynamics rollout, Time Scale makes everything slower and things continue to work in the same way (except that more simulation steps will inevitable make the fluid dissipate some velocity and detail). Scene Scale will affect some simulation parameters like gravity, cooling, burning, surface tension, the rate of birth of splashes and mist, and make them behave like this is a much larger or smaller effect. For more information, see the Units (Scene) Scale example.

Voxel Size | cellSize – The size of a single voxel, in Units. When Lock Voxel Count is disabled, these values change the number of cells. When Lock Voxel Count is enabled, these values change the size of the grid without changing the number of cells. For more information on the impact of the Cell Size on your simulations, see the Grid Resolution example below.

X Size, Y Size, Z Size | xSize, ySize, zSize – The grid size in cells. The dimensions shown in the Total Cells info box are the grid size in the scene, multiplied by the Scene Scale parameter - these sizes show how the solver will see the grid box and you can use the Scene Scale to cheat the solver into simulating as if the grid box was larger or smaller. In case you want to see how big the container for the loaded cache is in the scene without accounting for the Scene Scale, see the Container Dimensions in the Simulation rollout. Changing one of the sizes to 1 allows the simulator to perform a 2D Simulation. For more information on how to do this, please see the Setting up a 2D Simulation example below.

Increase/Decrease Resolution – Changes the resolution of the grid while maintaining its size.


Example: Scene Scale

The following video provides examples to show the differences of Scene Scale with values of 0.1, 5.0 and 15.0.

<iframe width="720" height="405" src="https://www.youtube.com/embed/PrH8bezNChU?version=3&loop=1&playlist=PrH8bezNChU" frameborder="0" allowfullscreen></iframe>



Example: Grid Resolution

The following video provides examples to show the differences when the Total cells from the Grid's Resolution is at 570,000, 4,000,000and 16,000,000.

<iframe width="720" height="405" src="https://www.youtube.com/embed/L3TtSXu_upw?version=3&loop=1&playlist=L3TtSXu_upw" frameborder="0" allowfullscreen></iframe>


Container Walls

X,Y,Z | gridBCX, gridBCY, gridBCZ – Select between different container wall conditions for the simulation grid.

Open – The fluid is allowed to leave the bounding box of the Simulator through this wall. When simulating Liquids, if Fill Up for Ocean is enabled, the Wall is treated as if there is infinite liquid below the Initial Fill Up level.
Jammed(-) – The simulation behaves as if there is a solid boundary in the negative direction. The Adaptive Grid will not expand in this direction.
Jammed(+) – The simulation behaves as if there is a solid boundary in the positive direction. The Adaptive Grid will not expand in this direction.
Jammed Both – The simulation behaves as if there is a solid boundary in both directions. The Adaptive Grid will not expand in this direction.
Wrap – The left and right boundaries are connected (toroidal topology). E.g. Fluid leaving the Simulator from the +X wall will enter it again from the -X wall.


Geometry Connections

Use Confine Geometry | gridUseGizmo – When enabled, constrains the simulation only to the volume of a shape.

Set Selected Object as Confine Geometry – You can specify a closed geometry object with normals pointing outwards, and the simulation will run only inside this object. The rest of the cells will be frozen as if a solid body was covering them. This way you can fill irregular shapes with liquid, or generally speed up your simulation by chopping off empty cells when you have an irregular fluid shape, e.g. a rocket launch.

While using a Confine Geometry can speed up a simulation, it will not reduce RAM usage.

Confine Geometry field | gridGizmo – Shows the currently selected Confine Geometry.

Use Cascade Simulator | gridUseCascade – When enabled, turns on the Cascade Connection Simulator. The liquid will be transferred from another simulator to this one in the region where both simulators intersect.

Set Selected Object as Cascade Simulator – Specifies another Phoenix Simulator which will transfer fluid to this simulator. This allows you to join several simulators into a structure with a complex shape. This can help you reduce memory usage by using many smaller simulators in place of a single large simulator. To set the Cascade simulator, select the main simulator first, then select the simulator to be used for the cascade. Then from the top Phoenix FD Menu choose Set Simulator Cascade Connection. For more information, see the Cascade tutorial for liquid simulations and the Connecting Two Simulators in a Cascade Setup section on the Tips and Tricks page.

  • In order to use a Cascade Setup you must have a fluid which flows from one Simulator into another. For more information, see the Transferring fluid between Simulators using a Cascade Connection section on the Interactions Between Simulators page.

  • The simulators must be run sequentially and each one should be started only after the previous one has finished simulating. The Cascade Simulator parameter points to the previous simulator in the sequence.

  • For the Liquid Simulations to function correctly, you need to have the Velocity Grid Channel and all Particle Groups that are simulated in the Source Simulator exported to its cache files - otherwise the connection will not work properly.

  • If you intend to use any additional channels such as RGB, particle IDs or Ages, etc, they also need to be exported from the Source simulator's Output rollout before running the current simulator

Cascade Simulator field | gridCascade – Shows the currently selected Cascade Simulator.


Grid Adaptation

Adaptive Grid | adaptive – Enables the adaptive grid option and determines which channel to use. The grid will then resize to fit the volume occupied by the selected channel. Note that only the Open Container Walls will expand and contract using the Adaptive Grid.

Either keep Adaptive Grid disabled or set the Container Walls: Y to Jammed Both when simulating Oceans. The Ocean Level parameter in the Liquid Rendering rollout depends on the vertical size of your simulator.

Adapt By Particles | adaptby – Specifies the particle system that will be used for the grid adaptation. More than one particle system can be used by using a comma to separate the different systems. For example Liquid, Foam will resize the grid based on the movement of both the liquid and the foam particle systems. Available systems are Liquid, Foam, Splash, Mist, Wetmap. Make sure to enable Particle Velocity from the Liquid Output rollout for all particle systems you want to use for the adaptive grid.

Extra Margin | adaptMargin – Specifies the number of cells between the end of the grid and the active zone. You can use this to give the fluid a bit more room if the adaptive grid can't keep up with the simulation.

No Smaller Than Initial Grid | nbiGrid – When enabled, the Adaptive Grid can't contract to a smaller size than what is given as the initial X,Y,Z size for the Simulator. Note that this way the initial grid box is always included, even if the fluid has moved farther from it. If this option is disabled, the grid will always encompass only the active fluid and will move together with it if needed.

Expand and Don't Shrink | onlyExpand - When enabled the Adaptive Grid will expand without shrinking.

Maximum Adaptation Limits

By default, the adaptation algorithm uses the Container Walls to determine if the grid can be extended in a particular direction. If the wall is open, this means that cells in that direction can be added. The maximum grid expansion is only limited by total cells or memory used. This section allows fine control over the grid expansion in each direction.


Limit Maximum Expansion | adaptLimitEnbl – Enables using manual limits along each axis when adapting the grid.

-/+ X | adaptLimitXp, adaptLimitXn – Specifies the limits along X axis.

-/+ Y | adaptLimitYp, adaptLimitYn – Specifies the limits along Y axis.

-/+Z | adaptLimitZp, adaptLimitZn – Specifies the limits along Z axis.

Shrink to View | adaptCamera – Species a camera whose frustum will be used to determine the maximum expansion. The Adaptive Grid will not resize beyond the frustum. Note that the algorithm might not handle complicated cases properly. For such cases, the limits must be animated manually.

Set Selected Camera - When a camera and a Phoenix Simulator are selected, the selected camera's frustum will be used to determine the maximum expansion.

 When a Shrink to View Camera is provided, the Adaptive Grid will expand no further than the already specified Adaptation Limits.