Static pressure at outlet - Stupid question???

Miry

Hi,

i think i have a stupid question for you all: i need to simulate a pipe with outflow in atmosphere. i know the total pressure at inlet.

Is it correct to impose a static pressure of 1 atm at the output section of the tube or should i have to model a big external plenum where to impose the pressure of 1 atm?

i make this question because in my opinion it is not physically correct to impose the atmospheric pressure at the output section of the tube, because the flux expand outside the tube and reach the pressure of 1 atm faraway the output section of the tube.

Am i correct? What is the correct boundary condition?

Another thing: can you bring to me an example where to use a static pressure at the inlet?

Thank you very much and excuse for my bad english.