Questions about inlet extension
 

It happens a lot that we extend the inlet or the outlet when we do a simulation.
Because of that we can get reasonable velocity profile.
But when i set pressure inlet condition,does the extension make sense?

What is the exact reason that we extend the inlet or outlet.

I hope i am clear.


Thank you

Leo

We extend the inlet or outlet because the boundary conditions are most accurate (and numerically stable) when the flow crosses the boundary perpendiculat and with zero (or low) normal gradient. When the boundary is too close it might cut recirculations and other complex flow features and the boundary will not accurately model a separation cut in two.

But you have raise an important point - if you extend the boundary and it is a pressure boundary then you may have additional viscous pressure loss, so you may have to offset the pressure to account for this.