[jojoFD]

Hello,

I have problems to calculate the flow losses on my CFD study and could use some help.

The case is a piston with holes on the sides, oscillating (sinus curve) inside a closed 8m-long domain pressurized at 200bars (cf simplified picture).

I want to estimate the flow losses (dampening force, drag force) through the holes. So far I calculate (using Fluent) the global force acting on the whole piston and I substract it to the force acting on the rod (as the end of the rod is going out of the domain --> no counteracting force).

The problem is that at the beginning of the simulation, I get a higher force acting on the rod than on the rest of the piston, so my initial drag force is negative (-2000N) which doesn't make any sense to me. And this offset continues for the rest of the simulation (cf drag force curve (blue) / yellow curve : velocity).

Does anyone have any tips for me on how to estimate the flow losses properly, please?
That would be a great help, thanks a lot in advance!