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July 20, 2010, 05:31 
Convergence problem

#1 
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lalupp
Join Date: Jul 2010
Location: India
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I am developing a 3d code with SST turbulence model (incompressible,unsteady) which is built based on OPENFVM. My problem is that my k and omega equations are giving convergence for the first time step but diverges in the next iteration onwards (segregated approach).
Is it because of lack of proper updation procedure or if some modification in the linear equation especially in the source term is necessary for the implementation of wall boundary condition I rechecked my k and omega equation several times but not giving My problem is flow past a sphere but please help Thanks in advance 

July 21, 2010, 17:01 

#2 
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Join Date: Apr 2009
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Dear friend your statement is general.
It can occur because of several reasons. What is is your initialization for k and omega? What is flow Reynolds number? Probably your simulation has vortex shedding process. The SST turbulence model uses kw near the wall and ke far from the wall.what is your methodology for matching this two turbulence models? be more specific for your problem. QUOTE=lalupp;268153]I am developing a 3d code with SST turbulence model (incompressible,unsteady) which is built based on OPENFVM. My problem is that my k and omega equations are giving convergence for the first time step but diverges in the next iteration onwards (segregated approach). Is it because of lack of proper updation procedure or if some modification in the linear equation especially in the source term is necessary for the implementation of wall boundary condition I rechecked my k and omega equation several times but not giving My problem is flow past a sphere but please help Thanks in advance[/QUOTE] 

July 21, 2010, 17:02 

#3 
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Join Date: Apr 2009
Posts: 35
Rep Power: 8 
Dear friend your statement is general.
It can occurs because of several reasons. What is is your initialization for k and omega? What is flow Reynolds number? Probably your simulation has vortex shedding process. The SST turbulence model uses kw near the wall and ke far from the wall.what is your methodology for matching this two turbulence models? tell more detail about your problem. QUOTE=lalupp;268153]I am developing a 3d code with SST turbulence model (incompressible,unsteady) which is built based on OPENFVM. My problem is that my k and omega equations are giving convergence for the first time step but diverges in the next iteration onwards (segregated approach). Is it because of lack of proper updation procedure or if some modification in the linear equation especially in the source term is necessary for the implementation of wall boundary condition I rechecked my k and omega equation several times but not giving My problem is flow past a sphere but please help Thanks in advance[/QUOTE] 

July 23, 2010, 00:33 
Re Convergence problem

#4 
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lalupp
Join Date: Jul 2010
Location: India
Posts: 37
Rep Power: 7 
Thanks
I got my k and omega equation from the following link http://turbmodels.larc.nasa.gov/sst.html For near wall , omega was assigned a large value (equation below) and k was set to zero. I used a blending function to blend kw and kepsilon equation as per Nasa site (which depends on the distance to wall) . Other boundary conditions are inlet , outlet etc I checked my code several times but not giving convergence Re ~= 1e6 , vortex shedding present Thanks 

July 23, 2010, 06:33 

#5 
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Join Date: Apr 2009
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Dear friend.
my first suggestion is about your numerical simulation if your code is well prepared. 1.check the CFL condition (the relation between time step and grid cell sizes) 2. what is the properties of fluid? at given Re number use of air with density of 1kg/m3 help to better convergence rather than water with density of 1000 kg/m3. 3.if you are using iterative solution check for the under relaxation values.Don't select high values like usual commercial software(s).I doubt the reality of this great values in commercial software(s). 

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