For simulating a droplet and particle collision, I assigned particle mass using 6DOF below and I defined particle velocity with 'Centre of Gravity Velocity' in 'dynamic mesh' window. As figure below shows, no difference is seen in the results even by increasing the mass 100 times. How should I define mass of a particle (moving wall) properly?
Thanks!
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#include "udf.h"
DEFINE_SDOF_PROPERTIES(particle, prop, dt, time, dtime)
{
prop[SDOF_MASS] = 4.4e-6;
}
The interface between the particle and droplet is 'wall'. So it can give only the effect of moving, and no momentum transfer between them. Mass may different, but the velocity is constant. So it displaces the same fluid in same manner.
As @Abubucker has told, It would not make any difference if you increase the mass. Since the droplet is coming in contact with the wall.
What you can try is to change the velocity of impact. This will make a difference.
Thanks all. I think it matters if I hit a plastic particle onto a droplet or hit a particle with the same diameter and velocity but made from steel. In fact, I assume that momentum of impact affects the impact products. My question is that how can I make Fluent to consider both 'mass' and 'change in velocity during the impact' for the particle (wall)?
PS. In reality, the particle velocity does not remain constant while it's getting into the droplet, and there is exchange of kinetic energy due to a non-ideal impact and also due to interfacial forces while particle penetrates into the droplet. Otherwise, the particle would have the same velocity after impact which is against the energy conservation.
Hi Sayed,
I think your problem is not still clear? What is the exact objective of your analysis?Is there any deformation happening on the surface of the particle?
Or is it just change in velocity of the particle due to the collision with the droplet?
Also, what is the particle material and the velocity with which it is traveling?
Hi Dhyan,
I'm simulating droplet impact with a particle in mid-air to see: 1) how the droplet deforms during the collision, and 2) how particle velocity changes as it penetrates into the droplet (I added a picture for more clarifications).
Particle velocity is 6 m/s and it's made from polystyrene and I assume there is no deformation on particle surface. The only important things to me from the particle are density (impact kinetic energy), and surface wettability (i.e. contact angle which affects how the droplet deforms around the particle).
In that case, how did you give velocity for the particle?
If you want to consider the momentum change in the particle, you will have to write a UDF for momentum source.
This UDF should calculate the change in momentum based on the pressure acting on the face of the particle when the droplet collides on it.
Thanks Dhyan,
Right now, I enter the velocity in the 'Dynamic Mesh Zone' window shown in the attached picture. Only when 'gravity' is activated, the particle velocity starts changing which means the value I've entered in the filed is considered as an initial velocity. But, without gravity, there is no change in velocity which I think means that Fluent ignores drag and interfacial forces here. Having said that, it seems that those forces are considered in the falling box tutorial [link] which is very similar to my case in that regard (unless the initial velocity is zero there), if it's similar to mine, how can I insert 'initial velocity' in that tutorial?
Regarding calculating the momentum change in UDF that you mentioned, it reminds me of DEFINE_CG_MOTION [link] which does something very similar. I tried to write a UDF using that, couple of days ago, but I was not sure which part of my UDF should be modified (it's attached and I really appreciate if you can have a look and give me even general hints how to revise that).
My doubt is that whether function F_P (f, t) inside the attached UDF is enough in my case to calculate the pressure accurately? (i.e. does it consider whole the effect of solid-liquid interfacial forces?)
Thank you so much for taking time to help me.
https://www.youtube.com/watch?v=ygvYygO_EyM
http://jullio.pe.kr/fluent6.1/help/html/udf/node86.htm
Hi,
As you told if the velocity which you specified is initial velocity, then the simulation should work normally. But, you should switch off the passive option. If you enable passive option, it does not consider the external forces.
Or what you can try is use the load property under 6DOF solver UDF and specify the force in the opposite direction of motion.This force should be calculated based on the pressure acting on the surface of the particle.
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