This page provides information on the Grid rollout for a LiquidSim object.

Overview


The Phoenix FD 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 camp fire, your container should be at most a couple of meters wide. Note that it doesn't 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 FD 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 (i.e. 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 Liquid Simulator | LiquidSim object|| > Modify panel > Grid rollout

 

Parameters


General


 

Scene Scale | scenescale – Specifies a multiplier for the original scene units of the scene. Phoenix FD 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 rolloutTime 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 on how changing the Scene Scale affects the simulation, see the Scene Scale example below.

Cell size | cellsz – The size of a single voxel, in scene units. For more information, see the Grid Resolution example below.

X, Y, Z | xc, yc, zc – The grid size in cells. The dimensions shown next to XYZ are the grid sizes 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.

Increase/Decrease resolution – Changes the resolution of the grid while maintaining its size. For more information, see the Grid Resolution example below.

 

 

Example: Scene Scale


The following video provides examples to show the differences of Scene Scale with values of 0.15.0 and 15.0.

Software used: Phoenix FD 4.30.01 Nightly (02 Oct 2020)

Download Example File

 

 

Example: Grid Resolution


The following video provides examples to show the differences when the Total cells from the Grid's Resolution is at 570,0004,000,000and 16,000,000.

Software used: Phoenix FD 4.30.01 Nightly (02 Oct 2020)

Download Example File

 

Container Walls



X, Y, Z | x_bound, y_bound, z_bound – 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. 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. When Adaptive is enabled, the grid will not expand in this direction.
Jammed(+) – The simulation behaves as if there is a solid boundary in the positive direction. When Adaptive is enabled, the grid will not expand in this direction.
Jammed Both – The simulation behaves as if there is a solid boundary in both directions. When Adaptive is enabled, the 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.

Confine Geom | usegridgizmo, gridgizmo – 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.

Cascade Source | usecascade, cascade  – Specifies the source LiquidSim to connect this Simulator to, forming a cascading simulation. 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. For more information, see the Connecting Two Simulators in a Cascade Setup section on the Tips and Tricks page.

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


Adaptive Grid



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

Adaptive Grid | adaptive – The grid will resize automatically during the simulation in order to prevent the liquid from leaving the bounds of the Simulator box. Note that only the Open Container Walls will expand and contract when Adaptive Grid is enabled.

Extra Margin | adapt_margin – 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 expansion | maxexp, expx/y/z/neg/pos – Specifies maximum growth sizes for each side of the grid, in cells. Using this, you can stop the expansion in certain directions.

Shrink to view | usegridfitcamera, grid_fit_camera – Species a camera whose frustum will be used to determine the maximum expansion. The Adaptive Grid will not resize beyond the frustum. 

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

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