The difference between steady and unsteady DPM when the flow is steady

    • reza121

      Hello. I hope you are doing well.

      I am simulating the trap of particles on a screen which is located in a duct. The flow is steady. I have a surface injection at the inlet. In one case, I run steady DPM as a post-process and activate DRW (discrete random walk) with a number of tries=10 (the results won't change when I increase that). In the other case, while "interaction with the flow" and "unsteady particle tracking" is activated, I run the simulation for the required iterations. DRW is activated in this case too but there is no option for the number of tries input (I guess the turbulence fluctuation is applied every time step, right?). The ultimate result is the percentage of trapped particles on the screen and the two cases are compared. As the flow is steady and the same for both of the cases, I was expecting to get similar results for both cases while they are a few percent different. What do you think is the reason for this observation?

      It is noteworthy that the mass flow rate of the discrete phase in comparison to the airflow is insignificant, so, there is a one-way interaction between the phases.

    • Rob
      Ansys Employee

      With steady particles the whole trajectory is computed in one go. So each track is seen from inlet (injection) to wherever it exits the domain. The plot looks like coloured string. 

      Unsteady particles are release every (particle) time step from the injection and their position is updated by the distance they can travel in one time step. If you plot this you'll see many dots/spheres in your domain. 

      The result should be much the same provided you give the transient particle case time for the particles to reach the exit. A few percent different could be down to insufficient time, or lack of calculated trajectories: if one stream misses the surface how does that alter the %? 

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