CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   Some quiestions on multiphase flow (http://www.cfd-online.com/Forums/cfx/65490-some-quiestions-multiphase-flow.html)

fjalil June 16, 2009 14:38

Some quiestions on multiphase flow
 
Hi, I'm working on a simulation on multiphase flow. My problem is a launder that carries fluid and particles, and above air, so there are 3 fluids: particles, air, and water. I have some questions on the modeling and I wanted to see if anyone could help. My questions are:

1. For the reference buoyancy density, I don't know if I should use the air density or water density. In the manual it says "For a flow containing a continuous phase and a dilute dispersed phase, you should set the buoyancy reference density to that of the continuous phase", that is, water. On the other hand, it says that for free surface flows, you should use "You should choose the density of the lighter fluid", that is, air. So which one should I use?

2. a) Regarding the fluid details, in the Multiphase Examples section, there is an example of water and sand, where it says to use water as continuous fluid, and sand as dispersed fluid. Why should I use dispersed fluid instead of dispersed solid?

b) In the same section it says that the solid phase should be assigned free slip boundary conditions at walls. To do this and give free slip only to the solid phase (fluid dependent wall influence on flow), I think I must inactivate the homogeneous turbulence option, but on the other hand the manual advices that homogeneous turbulence "is recommended for free surface flow using the inhomogeneous model, separated flow, stratified flow and any situation where the phases tend to separate out". Should I just omit this recommendation?


Thank you very much.

Francisca Jalil V.

ckleanth June 16, 2009 15:58

to model particles in a flow you have two choices. if you chose to use an Eulerian-Eulerian setup then :

1) if you have an Eulerian-Eulerian setup with disperced you should set the buoyancy reference density to that of the continuous phase. if you have two fludis and one disperced use the density of the lighter continious fluid.

2) if you have solid particles use solid disperced but basicaly it depends on your aplication. in order to be able to setup such a model you need to use two fluids. if in your case you have solid particles make a new material and assign solid phase a small insignificant molecular viscosity to it so you can chose the material. The solid phase should be assigned with free slip at walls becasue basicaly its a solid not a fluid and shouldnt "stick" to the walls as fluids do.
use homogenious models unless you have a lot of stirring between phases

fjalil June 16, 2009 16:29

wall influence on flow
 
Thank you very much for your reply. Only one question remains: żis it possible to put fluid dependent wall influence on flow when using homogeneous turbulence models? This option disappears when I mark homogeneous turbulence...so I must put free slip for all the phases and the wall roughness disappears, what can I do if I need wall roughness for the continuous phases (at least for water)?

ckleanth June 16, 2009 18:06

just a tip: when creating the the solid disperced phase, the material thermodynamic state needs to be solid but add the dynamic viscosity value by using cel


it seems that in this case you will need to use fluid dependant turbulence

fjalil June 17, 2009 12:32

Thanks!
 
Thank you!!

Francisca


All times are GMT -4. The time now is 18:35.