# Non-linear equation system (DEFINE_source)

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 February 24, 2014, 11:05 #2 New Member   Alexey Join Date: Jan 2014 Posts: 18 Rep Power: 5 Pls, any ideas?

 February 11, 2015, 07:39 #3 Senior Member   B_Kia Join Date: May 2014 Location: Ir Posts: 121 Rep Power: 5 hi , please forgive because i don have any idea , i just have a question, what does dS[eqn] = 0; do in code ? i want to introduce a source to epsilon equation for my model. thank you very much

 February 11, 2015, 09:16 #4 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 540 Rep Power: 10 Hi @ Hypernova; that's a source term linearization function which you can add for stability of the solver. Basically, the source term is outputted as Code: `S = A + B*X` where A is the constant part which you typically calculate as a source, and B = dS/dX. The FLUENT UDF manual gives an example where S is a function of Vx, so you can add the derivative dS/dVx. @ Laexzz; It seems to me there is no choice but to linearize your equation set. This happens automatically during iteration, for example S(2) is calculated while holding C(2),(3),(4) constant (alternatively, you can link the derivative as outlined above). Of course, your set of equations is still coupled and eventually the result should converge. I don't think the problem with your set of equations is the linearization, but I'm afraid it's a stiff set of equations. The time constant of the first equation is much higher than all the other ones, and I am guessing this causes problems. What happens if you run your model for a simple configuration (so excluding any flow) in say, MATLAB? Do you get good, smooth results? Does ODE45 interation work at all or do you need a special stiff routine. FLUENT should have some options to deal with stiff chemistry (I have no experience with that however), alternatively you may want to use a quasi-equilibrium assumption or so to deal with the fast reaction step. HyperNova likes this.

 February 11, 2015, 17:11 #5 Senior Member   B_Kia Join Date: May 2014 Location: Ir Posts: 121 Rep Power: 5 thank you very much for spending time to explain, i don know any thing on species topic otherwise i help you , but in near future i have to learn because i need it in my work , thank you very much

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