CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   FLUENT (https://www.cfd-online.com/Forums/fluent/)

 cshsgy November 17, 2018 03:14

Adding a source term in k-epsilon model

Hi all,
I am testing out a model raised by S.B. Pope, which modified the epsilon equation with a source term. I tried to implement it with udf in the FLUENT, but the convergence seems to be a problem. The modified epsilon-equation looks like the following: The added source term is related to the term, which is a measure of the vortex stretching. Here, .
The case I am simulating is basically a turbulent round jet. I firstly calculated the case until convergence, and then implemented the source term to the epsilon equation as the following code:
Code:

#include "udf.h" DEFINE_SOURCE(epsilon_Pope_source, cell, thread, dS, eqn) { real source; real time_scale = C_K(cell,thread) / C_D(cell,thread); real omega = 0.5 * time_scale * (C_DUDY(cell,thread)-C_DVDX(cell,thread)); real x_c[ND_ND]; real kai; C_CENTROID(x_c,cell,thread); kai = time_scale * omega * omega * C_V(cell,thread)/x_c;// Use DVDY instead of V/r? source = 0.79 * kai / time_scale * C_D(cell,thread); dS[eqn] = source * (-1) / C_D(cell,thread); return source; }
However, the convergence is not possible. The plot of residual is as follows:
https://image.ibb.co/f30FH0/1234.png
Clearly the white line (continuity) is driven to a very high level. Also other variables would not converge. Interestingly if I do not initiate with the calculation of the standard model, the residual stabilizes at a even higher value. Cannot figure out what is going wrong.
I tried to observe the flow field, and find out that the v,k,epsilon fields are all making no sense. I think the problem comes with the udf and I might have not done it properly. What I am doing is simply to add the term as the source term directly. What is the issue with that?
Thanks so much for your help!

 All times are GMT -4. The time now is 05:19.