# initial value for k and ε

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 September 1, 2014, 17:12 initial value for k and ε #1 New Member   Anastasios Stampoultzoglou Join Date: May 2014 Posts: 18 Rep Power: 4 Hi all, I have some questions about the start values of k and ε for internal and boundary field. a)Why do we need start-values for k and ε in a k-ε model? For example if i put 0 in the start values i have an error, why is that happening? b) How can i find the start values for k and ε for my simulation? Is there any equations for that reason or do i have to took the decision by comparing the results with the experiment data? For example i made some tests for k and ε values (0.000001 , 0.0002 and 0.2), and the best results were for the 0.2 value. I wanna know if the correct way to take the start values for k-ε is the way that i took or there is another way? To be more specific, while i was doing my tests i used the same start values both for the internal field and boundary field. P.S. i used interFoam solver and i have a dam break problem, so i don't have start value for velocity. Thank you very much i would appreciate any help. Regards, Tasos.

 September 2, 2014, 10:31 #2 Senior Member   Jens Höpken Join Date: Apr 2009 Location: Duisburg, Germany Posts: 156 Rep Power: 9 Have you checked this site: http://www.cfd-online.com/Wiki/Turbu...ary_conditions ? 6863523 likes this. __________________ Blog: sourceflux.de/blog "The OpenFOAM Technology Primer": sourceflux.de/book Twitter: @sourceflux_de Interested in courses on OpenFOAM?

 September 2, 2014, 10:48 #3 New Member   Anastasios Stampoultzoglou Join Date: May 2014 Posts: 18 Rep Power: 4 First of all, thank you for your reply. Yes i saw this yesterday, but the thing is that i have a dam break problem. So i don't have a start value for velocity (inlet). The boundary conditions that i have are : leftWall, downWall, atmosphere and at the right side outlet. Any ideas?

 September 2, 2014, 11:48 #4 Senior Member   Pablo Higuera Join Date: Jan 2011 Location: Singapore Posts: 339 Rep Power: 8 Hi Anastasios, since the case starts from rest I would initialize k and epsilon to very small values (but not zero to avoid zero division error). No velocity means 'zero' turbulent kinetic energy and dissipation rate, and will yield 'zero' turbulent viscosity. If your turbulence is not producing the results you expect there are a number of elements to take a look at: interface compression, mesh, turbulence model constants... Best, Pablo

 September 2, 2014, 12:17 #5 New Member   Anastasios Stampoultzoglou Join Date: May 2014 Posts: 18 Rep Power: 4 Hello Pablo, Thank you for your answer. As i said before, i had put 3 start values for both k and ε. (k and ε 0.2, 0.0002, 0.000001). One would expect that closer to the experimental results would be the simulation with the start value 0.000001 but this didn't happen. The best results was for the start value 0.2. Thats why i am concerned. What do you think? Best regards Tasos

 September 6, 2014, 16:01 #7 New Member   Anastasios Stampoultzoglou Join Date: May 2014 Posts: 18 Rep Power: 4 Thank you very much Mr Bruno and all of you, i appreciate your answers. Helped me a lot

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