N-Sim (Nested Grids Simulation) allows you to optimize memory usage and simulation speed where smoke flow has two (or more) distinctive regions.
For this tutorial please load Tut_09_start.max scene in your 3ds max session.
This scene contains a short animation; a simple source is emitting fire and smoke while rising towards a large deflecting ellipsoid. The first part of the animation (rising of the simple source) is mostly vertical and doesn't require wide simulation grid. On the other hand, the second part of the simulation (deflecting ellipsoid) does require a wide grid, but it occupies only part of the animation space. Normally you would be forced to use a grid tall enough for first part of the animation, and wide enough for the second part, resulting in a very large grid, most of which you don't really use. But with N-Sim you can respond to such challenge and create two smaller FumeFX Grids; a taller one and a wider one, and achieve gains in simulation speed and space usage.
Select "Master Grid" and open the FumeFX floater. Set Spacing to 0.6, Sensitivity type to Velocity and Fire expansion to 0.5. Turn on Fire Creates Smoke and set Smoke Density to 1. In the rendering parameters rollout set Step Size to 30 and Jittering to 30. Load Fire Color Gradient from file tut9_fire_color.agt. Load Fire Opacity AFC from file tut9_fire_opacity.afc. Enable Cast Shadows and Receive Shadows in Smoke rollout.
Switch to Illum tab and add Omni01 to the list of lights. Set Shadows Step Size to 50, Jittering to 30. Finally, add the simple source to the FumeFX Grid.
Now create a second FumeFX Grid so that it encapsulates deflecting motion from ellipsoid. Suggested values for width/length/height are 150/80/80 and for position 110/5/105. Rename the grid to "Slave Grid" and set Spacing to 0.6. Add simple source, ellipsoid and Omni01 to Slave Grid. Note that you don't need to add the ellipsoid to Master Grid as the ellipsoid is located outside its volume.
Now select Master Grid and open the simulation rollout. Turn on Use for N-Sim, Master Grid, and turn on all buttons for N-Sim Syncing. You can select all of them by holding the Shift key and left mouse clicking on any button.
This is practical in our setup because we don't need to repeat setting parameters for Slave Grid; any parameter change in the Master Grid will be changed in the Slave Grid as well. Please note that parameters are not propagated immediately, but when simulation starts. Select Slave Grid and enable Use for N-Sim (it is not necessary to enable N-Sim Syncing options for Slave Grid).
Identical grid spacing values are required for the best results (0.6 in this scene). Different values could result in visible artifacts in the overlapping area.
Smoke plumes with lots curling around the boundary area could result in the "bounced" smoke that will stay there for a long period of time.
Select Master Grid and simulate first 50 frames. While simulating open the preview window and select View>MultiView. This will allow us to see caches from all FumeFX Grids in one place.
Looking at the simulation result, it is almost impossible to tell that those are two different FumeFX grids merged into one larger simulation.