[Hossein1]

I'm simulating collision of a particle onto a droplet in mid-air. The particle is launched with 6 m/s towards the droplet while the drop is moving in opposite direction (pls. see Fig. 1, solution is symmetric).
For solving this problem, I want to use the basics of moving mesh tutorial for "falling box into water". However, I have no idea how to assign an initial velocity to my particle (wall).
It's different than "velocity of centre of gravity" which is adjustable in dynamic mesh window. It's just initial vel. of particle and all velocities for t>0 will be calculated step by step based on air drag, interfacial forces with droplet, etc.

Can anyone give me some hints how to do that?
I appreciate your answers.

[shashank312]

1. Make sure your particle has a different boundary ID on FLUENT. If not, please differentiate it by using "named selections" in the meshing suite under Ansys Workbench.

2. Particle can be assigned a velocity by using a "moving wall". One can assign x- or a y- velocity or a user-defined profile. Essentially, as the water droplet hits the wall or vice-versa, the wall exchanges momentum with the droplet.

[Hossein1]

My particle is defined as a wall and with a different boundary ID. Also, in 'Phases' => 'Interaction' I have activated 'wall adhesion' with 'surface tension'.

I am using moving mesh right now and with the "Centre of Gravity Velocity" in the Dynamic Mesh window, the simulation runs well. However, the particle velocity remains constant (the same value that I've defined for C.G), particle velocity doesn't change even after impacting onto the droplet. That's why I need to define just an "initial" velocity for particle.

As you mentioned in your point#2, I need a user defined velocity and I think I should use DEFINE_INIT. But, in all examples in manuals, they first determine those fluid 'cells' in which the velocity should be applied (e.g. with a FOR loop), and then they assign the initial velocity to those cells. My problem is that I don't know how to use this command for giving an initial vel to a solid object.

[shashank312]

To be honest with you, an easier way of solving this problem is to move the fluid in the opposite direction of the particle and keep the particle stationary. You can specify that same initial velocity to the fluid region (including the droplet), and see what happens as the particle collides with the droplet.

Use DEFINE_INIT or patch the fluid region with a user-defined velocity. If it is a constant value, using patch is an easier way. If the initial velocity is a function of time or space, you can also create a custom-field-function and then patch that onto the fluid domain.

Hope this helps.

[Hossein1]

I think, for using moving reference frame, I need to calculate the particle momentum and assign the equivalent velocity to the droplet. However, I prefer not to use this method.

[shashank312]

Who said anything about a moving reference frame? You cannot use moving reference frame in your problem because it would again fix that velocity to the particle.

What I was trying to convey is if the particle had to be initialized to the left at a velocity of U m/s, then instead of that you can simply initialize the entire fluid domain (barring the particle) at a velocity of U m/s to the right.

Hope you understand that. Forces like drag and shear can then be computed on the particle surface via post-processing.

[mmj_blackforce]

Originally Posted by shashank312

If the initial velocity is a function of time or space, you can also create a custom-field-function and then patch that onto the fluid domain.

Dear Shashank
how can we patch velocity as a function of time using a custom-field-function can u please explain?

[mehulm]

Hi Hossein1, actually I have to perform a similar simulation of the collision of particle and drop. Can you please tell me which model did you use VOF/DPM/DDPM in ANSYS Fluent.

Thanks.