Continuative or outflow boundaries always pose a problem for low-speed calculations, because whatever prescription is chosen it can affect the entire flow upstream.

What is needed is a prescription that permits fluid to flow out of the mesh with a minimum of upstream influence. Commonly used conditions for such a boundary is where is the unit normal vector.

The boundary condition that has more generality at the outflow is described by Orlanski (1971). This condition allows changes inside the flow field to be transmitted outward, but not vice-versa:

where is the average velocity at the outflow plane and represents or any dependent variable.

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