nWhat meant be extending the domain , is that you would not care what happens in this extension. Let's say your domain is a 1 m pipe. By, having an extra 20 cm you will not care about this 20 cm and you care only about you 1 m pipe. Outflow BC will enforce velocity normal to the boundary. This will affect the flow field downstream and prevent the circulation. Now this will happen in the last 20 cm ... which is not part of your study.n