[Raymond_1981]

Hello!

I would like to receive some help with understanding which integration time step is used in the unsteady dpm model in Fluent.



I have the following case. I’m modeling Stairmands High-Efficiency cyclone. I start with a steady state k-epsilon RNG simulation after which I change the turbulence model to RSM. One converged I switch to a transient simulation and let the solution converge for a period of time. I use a fluid time step of 1e-05s and a CFL of approximately 0.5.

The next phase is to inject particles in the transient simulation. These particles are tracked unsteady and I use one-way coupling. To account for turbulent dispersion, I use the random walk model with a random Eddy lifetime. The number of steps are set to 1000 and the step length is set to 5. The accuracy settings are default and I use the trapezoidal rule for integration. The simulation runs fine and the results are as expected.

Now I have some trouble understanding the time step used in the integration of the dpm equations. I wasn't able to find an answer in the theory guide unfortunately. Is the time step automatically set to the random Eddy lifetime? And is this integration (with a random Eddy lifetime) repeated for a number of steps until the continuous phase time step of 1e-05s is reached, after which it solves for the continuous phase again and repeats the process? Or is the time step based on the step length? But how is then the random Eddy lifetime incorporated?

Who can help me understanding the procedure used in Fluent? Any help is appreciated. Thanks!

Best regards, Raymond