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 Arturo Ortiz September 19, 2000 14:56

Particle Fate

Dear all.

I am simulating a system in which a liquid with diluted particles flows in normal direction to a sphere. I am interested in knowing how many particles are impacted in the sphere's surface. I've set up several simple cases, but no matter what I change ( density of the fluid or particle, velocity, gravity, etc ), fluent reports that no particles are impacted on the sphere. I think that for dense fluids that is correct because the fluid will deviate the particles when they approach the surface, but I've used air and the same happens. Any sugestions?, am I missing some physics? Thanks in advance

 Jin-Wook LEE September 19, 2000 21:33

Re: Particle Fate

Dear Arturo Ortiz

As you may know, particle hits or does not hit the wall depending on the flow condition & particle properties. So, even though particle density is much higher than fluid density, particel MAY not reach the wall depending on the flow condition, because of the DRAG force of light gas.

The most important properties whcih have significant effect on the particle behavior are particle density and diameter. So, for your simple test, set very high value for particle diameter & particle density, e.g., 1.e4 micro-meter and 1.e5 kg/m3 or higher. I am sure, you can see that 'particle hits the wall(sphere)'. Please remember that this input is physically unrealistic for most case. I would like to say that flow condition is more important for particle behavior.

Sincerely, Jinwook

 Arturo Ortiz September 20, 2000 12:07

Re: Particle Fate

Thanks for your comments, I've tried what you suggested; I used steel spheres (density >2.5)of 1 mm on air, even using a flat surface as target, but still can't get the particles impacted on the solid. I've tried as boundary conditions for the particles: escape and/or trap. I've made some simulations on complicated geometries and I obtained the particles impacted but not in simple ones.

Still wonder why.

 Jin-Wook LEE September 21, 2000 05:55

Re: Particle Fate

Dear Arturo Ortiz

O.K. I myself am wondering.

But, density = 2.5 ? Maybe it's specific gravity. Density is, I think, at least 7000 kg/m3.

Sincerely, Jinwook

 dimitris September 29, 2000 08:06

Re: Particle Fate

In case of interacting phases (main phase and dispersed one) the resulting flow field is affected by the motion of both phases. However, the flow when facing an obstacle will deviate trying to avoid it. So do the particles. The solution is to turn on the stochastic model (I suppose your case is turbulent) and set the tries at a number >20 to get a statistically representative particle motion. In this case particles will follow "strange", zig-zag paths, due to the assumed turbulent fluctuations and a number of them will hit the solid. This number should stay approximately the same at each iteration for the particles. If not, increase the number of tries.

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