[maccheese]

Hi,

i am working on a simulation which contains the motion of a rigid wall zone defined by a profile.

The motion is divided into 3 phases:

1. Translation from A to B
2. Certain time span without motion
3. Translation from B back to A

I want to solve the simulation with a variable time step. At the beginning of my simulation, due to the movement of the wall zones a quite small initial time step is needed (to prevent mesh folding), which is 1e-6 in this case.

During phase 2, Fluent automatically increases the time step to sizes up to a few hundreths, which is fine and contributes to an acceptable calculation time.

The transition from phase 2 to 3 is where my problem is. Fluent would need to decrease the time step size to ~ 1e-6 for the wall zones to move back again. The variable time stepping can not cope with this and the simulation stops with negative cell volume error due to mesh folding. Although I defined a really small minimum time change factor, which would allow Fluent to reduce the time step size from one time step to the next to a size small enough for the mesh to not fold, Fluent fails with this.

So it looks like I need to use a user defined time step.

My question is if I can define a udf in the following way:

Fixed time step for certain amount of time at the beginning.

Use Variable Time stepping for Phase 2 of the simulation

Fixed time step for certain amount of time at the end.

So in other words, can I define the usage of the variable time stepping method in a user defined time step?

Thanks in advance!