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-   -   interFoam:different nu, different falling velocity (http://www.cfd-online.com/Forums/openfoam-solving/115002-interfoam-different-nu-different-falling-velocity.html)

sharonyue March 21, 2013 11:04

interFoam:different nu, different falling velocity
 
5 Attachment(s)
Hi,

I am sumilating a bulk of liquid droping,as you can see in the image.First I set the liquid is water,so nu is 1e-5,the I get the fine solution.and the velocity is about 5m/s.

But when I change nu to be 1,looks like this liquid sphere is falling slowly,and the falling velocity is about 0.05m/s.

I am confused about this ,What happend?Thanks in advance.

The case is attached,just run interFoam ,and change the nu to be bigger as you like.check the result.

vainilreb March 21, 2013 12:43

Are you sure you've changed the viscosity of the falling fluid? A much higher viscosity of the surrounding fluid would explain why the acceleration of the droplet is smaller and it moves slower at a certain point in time.

This is just quick guessing, I can't check for your files right now.

EDIT: Okay, I've checked them now. Your described change from 1e-5 to 1 indicates that you have changed the properties of phase 2 which indeed would mean you've changed the viscosity of the surrounding air.

sharonyue March 21, 2013 20:37

Quote:

Originally Posted by vainilreb (Post 415527)
Are you sure you've changed the viscosity of the falling fluid? A much higher viscosity of the surrounding fluid would explain why the acceleration of the droplet is smaller and it moves slower at a certain point in time.

This is just quick guessing, I can't check for your files right now.

EDIT: Okay, I've checked them now. Your described change from 1e-5 to 1 indicates that you have changed the properties of phase 2 which indeed would mean you've changed the viscosity of the surrounding air.

This is my transportDict:
Code:

phase1
{
   
    transportModel  Newtonian;
    nu              nu [ 0 2 -1 0 0 0 0 ] 1.48;
rho            rho [ 1 -3 0 0 0 0 0 ] 900;

}

phase2
{
    transportModel  Newtonian;
    nu              nu [ 0 2 -1 0 0 0 0 ] 1.48e-05;
    rho            rho [ 1 -3 0 0 0 0 0 ] 1;
}

sigma          sigma [ 1 0 -2 0 0 0 0 ] 0.07;

As you can see,phase 2's nu is still 1e-5,but why I changed the phase 1'nu to 1.48 which would lead to the increasing of air'nu? anyway, How can I prevent this?

Thanks very much for your help.

Reagrds,

kmooney March 21, 2013 23:24

While it is difficult to tell from your images, it appears that your interface is rather poorly resolved. With a poorly resolved interface and a high density ratio you should expect significant errors in your droplet's surface shear stress calculations.

Also, I don't see anything out of the ordinary in your simulation: As you increase the droplet viscosity, it yields to gravitational forces more slowly.

sharonyue April 3, 2013 01:01

Quote:

Originally Posted by kmooney (Post 415606)
While it is difficult to tell from your images, it appears that your interface is rather poorly resolved. With a poorly resolved interface and a high density ratio you should expect significant errors in your droplet's surface shear stress calculations.

Also, I don't see anything out of the ordinary in your simulation: As you increase the droplet viscosity, it yields to gravitational forces more slowly.

Hi,

Sorry,very late!
http://www.cfd-online.com/Forums/ope...-wierd-nu.html
This thread would make it clear.


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