[Sigurd]

Hi guys,

I've been trying to model unsteady flow (implicit unsteady) in a straight, rigid pipe using tabulated boundary conditions (imported as 'time' tables to StarCCM) of mass flow at inlet (A04_m_in.txt) and pressure at outlet (A04_p_out.txt). My time step is set to 0.001s and I'm letting the solution converge to a magnitude of e-5 before stopping the inner iterations and moving on to the next time step.

My problem is: AS THE solution is converging for the zeroth time step, the flow changes direction. It does this only once, gradually from positive to negative despite the flow inlet boundary conditions being positive in the first part of the cycle. Once it starts stepping through the time steps, the solution does not oscillate as expected, rather it seems to be converging towards a single solution.

Screen Shot 2016-03-18 at 10.01.54.jpg

Any help would be greatly appreciated! I am fairly new to CFD altogether, certainly new to transient simulations.

Thanks in advance,
Sigurd