[Janshi]

Hello,

Currently I analyse the pressure drop in a very simple pipe system consists of a pipe, a wind box, an outlet and an inlet extrusion.
The dimensions of the pipe, the wind box and the outlet extrusion are fix.
The length of the inlet extrusion will be varied.
Its a steady state calculation with air as an ideal gas. Turbulence model is the k-epsilon model, temperature of the air is 20 °C.

For the extrusion length 0-50 mm everything is fine, the computation reaches convergence after about 10000 iterations.
In the attachment there is a screenshot of the pressure for the length 50mm.

When I rise the length over 50mm, e.g. 60mm, things become weird after about 4000 iterations. The engineering variables are going to oscillate, the residuals are blowing up and then oscillates too.
In the attachment there are two screenshots of the pressure and the residuals.

So, does anybody have an idee what the reason for this very different behaviour could be and why the computation can´t reach convergence for a length higher than 50mm?

Thank you