# calculation time in Fluent

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?

• I know that FLuent has licenses for parallel computing (distributed and shared memory). Have you tried using these? – Paul May 28 '13 at 14:11
• Now didn't try it yet. I wanted first to try and make the model setup less complicated or time consuming to calculate. – Elyse May 28 '13 at 15:04
• How would you quantify extremely? Transient, turbulent problems are difficult to compute, so 5 days does not strike me as dramatic for a problem like this. – Schorsch May 30 '13 at 17:47
• Oke thank you. I don't have a lot time left to do this research (2 weeks), that's why I was wondering if there was a quicker way. – Elyse Jun 7 '13 at 9:41

The reduction of calculation time in CFD is a general problem. Although I have never worked with FLuent, I don't think that there are features that magically speed up your simulation. That's why I can only give some VERY general hints. For further insight, I suggest you browse the internet, e.g. cfd-online for the highlighted keywords.
• Use mesh adaptation to reduce the scale of your problem. In the regions away from the walls, you maybe can reduce the number of cells. For the near-wall regions you have to take care of the boundary layer. Monitoring the y+ value helps you estimate how coarse you can go. Also the use of wall functions for your turbulence model can be considered.
• Do some try and error with the relaxation parameters. This is eventually balancing stability and convergence speed.