Blender Fluid Simulation Outflow Setting Guide

In Blender's Mantaflow physics engine, the Outflow setting designates an object as a drain or fluid sink that permanently deletes fluid or gas particles upon contact. This article covers what the Outflow setting is, how it operates within a fluid domain, its primary configuration options, and best practices for using it effectively in simulations.

What is an Outflow Object?

An Outflow object acts as the opposite of an Inflow object. While an Inflow continuously adds volume to a scene, an Outflow removes any fluid (liquid, smoke, or fire) that intersects with its geometry. It is an essential tool for controlling fluid volume, managing computational performance, and preventing simulations from overflowing within the bounded Domain.

How the Outflow Setting Works

In Blender, fluid simulations require a Domain to define the simulation bounds and at least one Flow object. Setting a Flow object's behavior to Outflow instructs the solver to destroy any fluid cells or particles that enter the volume or surface area of that specific mesh.

When fluid enters the Outflow boundary:

  1. The solver detects the intersection during the simulation step.
  2. The mass, velocity, and particle data associated with the intersecting fluid are deleted.
  3. The remaining fluid continues its trajectory without being influenced by the deleted volume.

Setting Up an Outflow Object

To configure an object as an Outflow:

  1. Select the mesh intended to act as the drain.
  2. Navigate to the Physics Properties tab.
  3. Click on Fluid and set the Type to Flow.
  4. Set the Flow Type to match your simulation (Liquid or Gas).
  5. Set the Flow Behavior dropdown to Outflow.

Important Settings to Consider

  • Is Planar: Enable this checkbox if using a flat, 2D plane (single-sided mesh) as your drain. Leave it unchecked if the object is a closed, 3D manifold mesh (such as a cube or cylinder).
  • Sampling Substeps: Found in the Domain settings, increasing subframes ensures fast-moving fluid does not pass entirely through a thin Outflow object between simulation frames.

Common Use Cases

  • Drains and Pipes: Simulating sinks, bathtubs, or drainage systems where liquid exits the visible scene.
  • Infinite Rivers and Streams: Placing an Inflow at one end of a channel and an Outflow at the other to create continuous water movement without domain flooding.
  • Open Ocean Boundaries: Placing Outflow boundaries around the edges of a domain to prevent fluid from splashing or reflecting off the domain walls.
  • Smoke and Fire Vents: Removing smoke accumulation in enclosed interior scenes through chimneys, exhaust fans, or open windows.

Best Practices and Troubleshooting

  • Mesh Normals: Ensure the mesh normals of the Outflow object are facing correctly. Flipped normals can prevent the solver from accurately registering fluid intersections.
  • Domain Resolution: If liquid appears to pass through an Outflow object without disappearing, either thicken the Outflow mesh geometry or increase the Domain's base resolution so the grid cells properly detect the boundary.
  • Placement: Keep the Outflow geometry fully inside the Domain boundaries. Any portion of an Outflow mesh extending outside the Domain will not function.