Hello I want to simulate pressure changes and mass flow in a system with the passage of time. When compressed natural gas (CNG) from a large volume storage tank filled in to the low volume cylinder simply by connecting these two. What type of B.C we have to apply on cylinders and connecting tube so that I can simulate the behavior with time. Please guide me in this regard.

I am using cfx5.5

I would suggest to place a subdomain between both cilinders. In that subdomain I would define a linear resistance factor which is a (step-)function of time. Then at, say time = 1 [s], release the resistance and of you go.....

Pascale

Hi, I'd build my model using 2 or 3 different solids for the different pipes and pressure vessels. I'd set my reference pressure to be atmospheric, then using a step function I'd define my high pressure region in the one pressure vessel. Then just run it transiently to observe the pressure changes through the system. You shouldn't need any more BC's than that. You could define some no slip walls for Post proc purposes ? Bob