# Pressure drop: Simple analysis not converging

Question asked by Shelon Marini on Dec 15, 2015
Latest reply on Dec 29, 2015 by Glenn Whyte

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!