I'm making a model of a square box where water comes in and the water level rises. I want it to be a transient, turbulent, VOF-model. The velocity of water entering changes in time ($-0.2$ to $0.2$ m/s).
For now I use a steady, laminar inflow of $0.2$ m/s, a grid of $10,000$ cells (the geometry is roughly $100\times 100\times 6$ m) and solution method PISO. To run the model I've used time step size $0.1$ s and $3600$ time steps ($10$ minutes); bigger time steps led to divergence. It took $2.5$ hours to calculate this and it wasn't even turbulent yet. In the end I've to model a period of $12$ hours, with this speed of calculation that will take more than $5$ days.
What are things I can adapt in Fluent which will (extremely) reduce this calculation time?
Thanks in advance.