# How to make Fluent realizes the particle mass?

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January 5, 2017, 15:19
How to make Fluent realizes the particle mass?
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Join Date: Jun 2015
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Hi all,
I'm simulating collision of a particle and a droplet in mid-air. I use dynamic mesh in a 2D domain and a particle defined as a wall. I assign the particle mass and moment of inertia through DEFINE_SDOF_PROPERTIES. However, the particle velocity does not change due to the impact, and Fluent considers it as a wall with a constant velocity, while we know that velocity should change both due to having a non-ideal collision and also interfacial forces while particle penetrates into the droplet.
How can I make Fluent consider both 'mass' and 'change in velocity during the impact' for the particle (wall)?
I've attached a picture from my domain and one of solution results.
Attached Images
 domain and solution phase diagram.png (158.1 KB, 15 views)
Attached Files
 udf.c (475 Bytes, 9 views)

Last edited by Hossein1; January 5, 2017 at 20:57. Reason: edit picture

 January 6, 2017, 06:45 #2 Senior Member   Kevin Join Date: Dec 2016 Posts: 138 Rep Power: 7 Do you include gravity? If not, then I don't think it matters necessarily that the particle's velocity doesn't change, as velocities are only relative. So what matters more is the relative velocity of the particle and droplet. Just a thought.

 January 6, 2017, 06:59 #3 Member   Join Date: Jun 2015 Posts: 46 Rep Power: 9 I think it's not only relative velocity which matters. Consider two cases in which a particle with 6 m/s collides into a droplet moving with 1 m/s, but in case#1 the particle is made from plastic and case#2 has the same diameter but is made from steel. Relative velocity is the same but the momentum that particle in case#2 exchanges with the droplet due to impact is much higher than case#1, and therefore mass of the particle also matters as well as the relative velocity.

 January 6, 2017, 07:21 #4 Senior Member   Kevin Join Date: Dec 2016 Posts: 138 Rep Power: 7 Yes, I agree. But what I meant to say is you need to look at the relative velocity between the particle and droplet, not one part of the system's absolute velocity. I mean, the effect of the particle's material doesn't necessarily need to be reflected in the particle's velocity, but could also be reflected in the droplet's velocity. Oh, and I mean the relative velocity during and after the collision, not before the collision. I agree that the total momentum before the collision changes its outcome. Basically, what I'm trying to say is you should conserve momentum, but what reference frame you're using is not important. So maybe Fluent uses the particle as reference frame and keeps the particle's velocity constant, while changing the droplet's velocity. So change the particle's mass/momentum, and see if it changes the droplet's velocity and deformation. If it does, then the particle's mass does have an effect.

 January 8, 2017, 09:29 #5 Member   Join Date: Jun 2015 Posts: 46 Rep Power: 9 Thanks Kevin, I tried what you mentioned, but with the falling box tutorial case and udf (to make sure that the case file has no problem). The box size and mass are 1(m)*1(m) and 666kg. I gave an initial velocity to the box (via C.G. Velocity in Dyn. Mesh window) and simulation works very well and velocity of box increases slowly in the air, and then starts reducing when the box is going into the water pool (I believe that up to here, it means that Fluent realizes the box mass for momentum exchange). However, when I scale-down the domain by a factor of 500 to have a box of 2(mm)*2(mm) and also update the box mass and moments in the udf, the box velocity reduces from 5 to 1 m/s while it is traveling in the air (which does not seem reasonable as I expect velocity to increase while the box has a free fall). Do you know what might be the reason of such inconsistency by scaling the problem? Thanks PS. I think the velocity that we see in the C.G. field, is the absolute vel. of the particle in each time. 1) I keep the k-eps model for the small box as well. 2) Could it be because of the very tiny values of mass and moment in the second udf? or something related to 'Reference Values' or 'Solution Control'? Last edited by Hossein1; January 8, 2017 at 20:04.

 January 9, 2017, 07:26 #6 Senior Member   Kevin Join Date: Dec 2016 Posts: 138 Rep Power: 7 I replied to the tutorial case in the other thread you created. For this case, are you including gravity? And is the collision taking place in the gravitational direction or not? Also, are you using a surrounding fluid, and if so, what is it? Air?

 January 9, 2017, 08:46 #7 Member   Join Date: Jun 2015 Posts: 46 Rep Power: 9 The gravity is included and collision happens in the gravity direction (in my experiments, droplet is falling down and I shoot the particle upward), and the surrounding fluid is air.

 Tags collisions, dynamic mesh, two phase flow, velocity calculation