This is my first time to use the ANSYS Fluent package, and I hope I can get some help from you.
Here is my background: I am trying to model a transient pipe flow into vacuum thru a nozzle as shown below. (transient, 2d planar, density based, k-epsilon model, energy equation with ON). The working gas is set to be helium with "ideal gas" For this kind of model (compressible flow), the boundary conditions used are the pressure inlet and outlet conditions, as several other tutorials have shown( e.g., a YouTube tutorial:
Here is my problem: as I am trying to model a propagation of a gas along the pipe (it is a short pipe currently with a length of 2mm), I am interested in transient situation in order to compare the pipe flow evolution along the pipe and in the vacuum chamber. My expectation was that at the zeroth step in the transient calculation, there is a step function like pressure drop (as indicated by light blue) because there is no gas down the pipe except at the inlet location. However, ANSYS fluent result show that, even at t=0, the pressure profile shows a linear pressure decrease along the tube (shown below), and there is a finite density and velocity along the tube (not shown). So clearly, Fluent have propagated the flow down the pipe already at the t=0 step, which is not realistic.
I do not think that I correctly modeled the situation. I have tried different boundary conditions (such as mass flow inlet), but the solver did not like it.
Is there another approach (or boundary conditions) to properly model this kind of compressible flow propagation?
Thanks for your time in advance.