The boundary between a fluid and a solid doesnt occupy a voxel, so the calculations needed here are done on the obstacle voxel. The properties of this voxel must be adjusted to prevent acceleration towards the obstacle voxel. This is done by copying the pressure properties of adjacent non-obstacle voxel, copying the velocity to the incident velocity and the velocity of the faces perpendicular to the incident face to the velocity of the adjacent voxel.

Other conditions include in-flows, and out flows which are the sources and sinks of the fluids respectively. There is also a periodic boundary condition if the fluid is to be looped. This is achieved by copying the last row of voxels to the first row.

A boundary model showing the boundary conditions.