# interFoam:different nu, different falling velocity

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March 21, 2013, 10:04
interFoam:different nu, different falling velocity
#1
Senior Member

Dongyue Li
Join Date: Jun 2012
Location: Torino, Italy
Posts: 750
Rep Power: 9
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.

The case is attached,just run interFoam ,and change the nu to be bigger as you like.check the result.
Attached Images
 nu 1.48.jpg (31.7 KB, 34 views) nu 1.48e-6.jpg (28.1 KB, 33 views)
Attached Files
 2Dliquiddrop.part01.zip (95.7 KB, 12 views) 2Dliquiddrop.part02.zip (95.7 KB, 9 views) 2Dliquiddrop.part03.zip (84.0 KB, 7 views)

 March 21, 2013, 11:43 #2 Member   Robert Join Date: Aug 2012 Location: Berlin Posts: 74 Rep Power: 6 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.

March 21, 2013, 19:37
#3
Senior Member

Dongyue Li
Join Date: Jun 2012
Location: Torino, Italy
Posts: 750
Rep Power: 9
Quote:
 Originally Posted by vainilreb 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,

 March 21, 2013, 22:24 #4 Senior Member     Kyle Mooney Join Date: Jul 2009 Location: Amherst, MA USA - San Diego, CA USA Posts: 320 Rep Power: 10 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.

April 3, 2013, 00:01
#5
Senior Member

Dongyue Li
Join Date: Jun 2012
Location: Torino, Italy
Posts: 750
Rep Power: 9
Quote:
 Originally Posted by kmooney 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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