axisymmetric problem
 

I want to simulate a cylinder with a small pipe injecting a constant mass flow rate of methane gas into the cylinder. The pipe is located in the center line of the cylinder and the methane is injected upward. It is a compressible turbulent isothermal flow. In real experimental conditions, I have a mass flow rate value. My question is this: If I take advantage of the cylindrical symmetry, then I have a plane with an axisymmetric axis along the center line. The mass flow rate boundary condition is also along the centerline and it is not asurface anymore. What should be the mass flow rate value in this case? Is it still similar the three dimensional case value? or shoud I use another bounday condition. Thanx in advance