Weak interaction between particle and fluid in DPM model within crossflow?

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Thomas Frank

If you use DPM, than the "water particles" are modeled as individual, volume-less mass points being injected and traversed into/by the air stream. Of course by modeling a massive water jet with this kind of (even two-way coupled) DPM you will not be able to obtain a wake behind the jet. The volume occupied by the jet is essentially not obstructed, only mass and momentum source terms are exchanged between the DPM particles and the fluid flow.

If you are interested in the phenomenon which you describe, you need to use the newly developed VOF2DPM approach, which was exposed in the recently released ANSYS Fluent 2019.R1 in January this year (formerly already available as very early beta model). This should allow to solve your problem accordingly, but for the expense of massive computational time and some further pre-requirements to mesh sizing and time step.

Regards,

Dr. Th. Frank.

Yiheng Tong

Thomas Frank Thank you for your answer to my question. I have tried using VOF to DPM in high speed crossflow. The air flow was treated as ideal gas and the solver is density based solver. But I could not get any good results about the breakup of the liquid jet or any good results for the high speed flow either. So do you have any comments or suggestions for the VOF to DPM model? Thank you very much for your help.

Dr. Yiheng Tong

VOF2DPM is a very specific and new model in ANSYS Fluent 19.x and 2019.R1. I have great doubt, that it is compatible or even available with the density based solver. I have seen only applications with the pressure based solver. So most likely we are not speaking about the same model. Up to now I do not have any personal experience with the VOF2DPM model. I can only guess, that you need a quite specifically tailored mesh in order to trigger the VOF --> DPM transfer of liquid massflow rate at the desired ligament size, which makes sense in both VOF (from the perspective of still valid free surface resolution on the given mesh) and in DPM (from the perspective of treating all ligaments of this and smaller size as DPM particles).

Thomas Frank Thank you for your reply to my question. I will try to use VOF to DPM model with the pressure based solver. I will shear my experience after having some good results about simulation of spray breakup and atomization in high speed crossflow. Thank you again.

Tong

Your remark about high-speed crossflows was keeping my brain a bit busy. What do you understand under "high-speed", i.e. what is the Mach number in your gaseous flow? Why do I ask? Essentially because with the VOF model you like to resolve the movement of a free surface flow (the liquid jet and the forming liquid ligaments of the breaking-up liquid jet) in both space and time. If you now have a high-speed gas flow, than this in my opinion could potentially lead to quite very small timesteps in order to resolve the breakup of a jet in crossflow. A good idea would be to look at the local Courant numbers in your simulation and to make sure, that approx. CFL

Thomas Frank The Ma number for the high speed crossflow is 1.95 or 2.1 in my case. Some colleague suggest to use density based solver due to its good performance in simulating high Ma number flow. But the density based solver could not be used with VOF model. So now I will try pressure based model + VOF to DPM model + small time steps + very small mesh scale (in order to have a small CFL). Then the numerical would be time consuming. But I will try and see. Thank you for your comments.