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 gundul August 21, 2008 10:02

Multiphase flow for flow around ship

Hi everyone,

Now I'm trying to simulate the flow around ship and I use multiphase flow for analize 2 fluid flow around ship (air and water) . Water under draft and air upper draft ( under air there is water) . I need help in the following points : 1.What a physical desciption for the location which separate of the two fluids and what boundary condition for it ? 2.How about with inlet and outlet ? should I separate inlet for air and water and outlet for air and water? 3.What about with top from water and air ? Is it a wall or opening?

If someone have tutorial about modelling flow around ship, if you don't mind please send me at my email : c.mocca@yahoo.co.uk.

regards, gundul

 MAB August 21, 2008 10:12

Re: Multiphase flow for flow around ship

Hi, You could consult tutorial 7 for the boundary conditions. the interface between air and water can be defined using CEL EXPRESSIONS by using step() function. at outlet there's no need for separate BC whereas at inlet you can specify water velocity again by using step() function. the velocity would be zero for air region . other BCs should be same as in tutorial 7. have a look. MAB

 gundul August 22, 2008 11:37

Re: Multiphase flow for flow around ship

thanks MAB, I'm beginner for this software so I need more information from you or everyone in this forum. I' realy don't understand with your command "the interface between air and water can be defined using CEL EXPRESSIONS by using step() function" and " at inlet you can specify water velocity again by using step() function. the velocity would be zero for air region". Please give me some information about that.

regards, gundul

 MAB August 22, 2008 11:44

Re: Multiphase flow for flow around ship

when modelling multiphase flow. you have to specify volume fraction for each phase in the domain. now the volume fraction is either 1 ot 0. Let's say you modelling water and air. Now in the region where there is only air the volume fraction for air in that region would be 1 and for water 0. similarly in the region of water , the volume fraction for water would be 1 and for air 0. You use ' step ' function in CFX to control this. interface is the sharp line that separates water and air. at interface volume fraction for water and air is 0.5. Try to understand tutorial 7. step function works as follows. step(x) =1 for +ve x,step(x) =0 for -ve x, and step(x)=0.5 for x=0,

MAB

 Sulfi August 28, 2008 02:39

Re: Multiphase flow for flow around ship

Fluent is the best package as far i know for simulating multi-phase flow around ship. Next to Fluent is Star-CCM+..

 Chebeba September 2, 2008 16:06

Re: Multiphase flow for flow around ship

Read the section on Free Surface Flow in the CFX Modeling Guide, starting on page 196 in the PDF version which you will find in C:\Program Files\ANSYS Inc\v110\CFX\help\pdf or similar.

It has all the info.

Also: for good results you need very fine mesh (10mm resolution) around the surface. And your simulation domain should be AT LEAST 5 hull lengths long and 2 wide.

Usually only possible usng fully structured mesh.

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