Obstacle Inverse Workflow
This document will detail additional notes and applications of the Obstacle Inverse option.
By default (when this option is disabled), everything inside of the obstacle object volume will be converted into solid material within the simulator. This will result in liquid flowing around your obstacle object, as you would expect.
When the Inverse option is enabled, the inside and outside of the obstacle will be reversed within the simulator. This means that everything outside of the object volume will become solid material and everything inside of the object will be empty. By placing a Fluid or Inflow object inside of the inversed obstacle, the object now functions like a container that can hold fluid.
Using this option in your workflow can help simplify your simulation setup and speed up baking in certain cases. Let's start with a simple scenario to help explain how this option can be used in your projects: fluid flow through a pipe!
So you want to simulate a fluid flow within a closed pipe or network of pipes. Let's start by seeing how we can model this without using the Inverse option. Your first guess on how to simulate this scenario may be to create a simple pipe obstacle modeled just like a pipe in real life. Your pipe would be a hollow cylinder with thin walls and endcaps to create a closed container. One way to model this in Blender would be with a cylinder nested within a larger cylinder:
This method of simulating a pipe can work, but it is possible to run into problems when you start simulating your liquid. You might find that the liquid is leaking through the thin obstacle walls!
Using thin walled obstacles in a FLIP simulator can be tricky. Inside of the simulator, obstacle objects are converted into volumetric data on a grid. If the resolution is too low, there may not be enough grid detail to fully resolve the thin walls of the obstacle, leading to leakage. (Relevant article: Workflow Tip: Obstacle debugging in FLIP Fluids)
There are ways to resolve leaks when using thin obstacles, but the methods involve either increasing simulation detail which will take longer to simulate, or involve remodeling your object to use thicker walls which could be complicated. Both solutions can slow down your workflow.
Now let's take a look at how this simulation problem can be solved by using the obstacle Inverse option! Instead of modelling a pipe with walls, we'll instead use a simple cylinder or tube to model the interior of the pipe. With the Inverse option disabled, everything inside of the cylinder will be considered solid and fluid will flow around the object as expected:
Now let's enable the Inverse option and see what happens! Everything inside of the cylinder becomes empty and everything outside becomes solid. The obstacle now functions like a pipe that can hold fluid:
Using the Inverse option in this scenario is like creating a pipe with infinitely thick walls. You will not need to complicate your workflow by remodeling your pipe to have thicker walls in order to prevent leakage and you can perfectly contain fluid - even at low simulation resolutions!
Want to create a more complicated network of pipes? Multiple inverse obstacle objects are supported and the FLIP Fluids addon will automatically combine and merge them within the simulator:
