Hi all,

Does anyone know how to calculate a time step size? I am modelling a cylinder in water. I use a time step size of 0.005 and the solution takes more than 48 hours to iterate. Is this normal?

Thanks.

Sham.

Calculate characteristic time scales of processes (momentum, mass and heat transfer and reaction rate)involved in your problem. you should use a time step smaller that the smallest characteristic time you have. this method properly resolves the fatsest and slowest of the processes. however, if you are only interested in obtaining a steady state solution and are not bothered about temporal accuracy of solution, then you should take advantage of unconditional stability of implicit method and use larger time step. also, if you are not encountering high velocities, then using segregated solver will definitely improve speed of computation. another thing you may look at is whether your mesh is very fine. Hope some of these things will help in reducing time of computation.

Best of Luck,

Swarup.