Hello people,

Frequently I have to analyse the pressure drop through a piping, and many times I face the same problem: the goals do not converge rightly.

Here is a simple example: a linear pipe with diameter 250mm, and lenght 15m.

Boundary conditions:

Outlet velocity = 30 m/s

Static pressure at the inlet = 101325 Pa

Real wall at pipe faces

The fluid is air

Goal:

Static pressure at the outlet

So the pressure drop will be (static pressure at the inlet - static pressure at the outlet)

I've set the goals convergence as the unic stop criteria, and changed the delta to 10Pa.

Here are the results:

As you can see, the static pressure at the outlet is varing more than 5000Pa even in the last interactions, what is a hugeness.

Does someone know why is this happening, and how to solve the problem?

It is interesting to note that if I reduce the pipe's lenght, to 5m for example, the goal converge well.

Thanks for your attention!

The fact that changing your domain size alleviates this problem leads me to believe it may have something to do with your mesh. How many cells do you have along the diameter of the pipe in the problematic calculation?

One other thing I would try is setting an outlet mass flow rate or volumetric flow rate. By setting a constant velocity you are forcing a linear profile at the outlet which may be messing with your simulation.