2D piston in steady closed pipe flow BC query!
 

Hi,

I am trying to model a 2D creviced piston in a rapid compression machine. As the fluid stays within the cylinder, the flow is a closed system. My aim is to analyse the flow field around the piston, looking particularly at pressure/ temperature changes.

Due to platform issues, I am unable to use dynamic meshing or user-defined functions to simulate the movement of a piston. Therefore I am limited to steady flow and a moving wall boundary condition.

The wall is moving toward the end of the cylinder, thus "compressing" the fluid. I am struggling finding the correct boundary conditions and was wondering if anybody might be able to help.

Originally I had a velocity inlet of 10m/s at the piston seal, simulating the seal pushing fluid out into the main chamber as the piston moves towards the cylinder end (moving wall boundary condition also set to 10m/s). The visulations of the flow field contain vortices and what I might expect to happen in real life. However I was recommended that this wasn't particularly realistic to the practical boundary conditions.

I was advised that it would be more appropriate to not have an inlet, but to have a mass flow outlet at the seal of the piston (provided the seal leaks slightly), would a pressure-outlet be more suitable? If I have no inlet and a pressure outlet, should I be using target mass flow rate in the pressure outlet boundary condition? the flow field visulation I get from pressure pathlines shows a vortex in the main pipe and also in the crevices, but there is no pressure distribution in the main section.

I am confused which boundary conditions would be most appropriate for my application.

Any help would be extremely useful!

Thanks,
Lisa