[mtaskin]

Hi,
I have been asked to perform Gauss-Seidel methods(explicit and implicit) and Line Successive Over-relaxation method on a linear 2-D Elliptic Equation. So i wrote a fortran code and now i have to compare my results with exact results in a reference textbook. The problem is the absolute errors are quite high(sometimes 20%).


So i checked my code multiple times but there is no error. Than i realized, my indicated step size was 0.02 ft where the textbook uses 0.05 ft. The funny thing, when i run the code with textbook step size(larger step size) i get perfect match with the result. So i am confused and started looking for an answer. Here is what i did so far;


- I discretized BS(Backward space) rather than CS(central space) but errors are getting larger which is not surprising(first order and all stuff considered)
- I derived modified equation and found out that artificial viscosity is dominating error. But dont you think 20% absolute error is quite high for artificial viscosity alone?
- I thought i did something wrong in modified equation and decided to run my code with double precision, in case round-off error is dominating. The same phenomena happens.
- I theorized that textbook authors made a mistake by publishing unconverged solution but relative error control sequence shows it is converged result(it was a low possibility anyways)
- I know i can produce lower accuracy with lower step sizes for non linear equations but this one is linear so this is not the case, i eliminated this reason pretty quickly.



So i ran out of ideas. I know there are lots of experienced people here. Can you think of any possible reason? Even a little hint would be appreciated. Thank you in advance


Best regards,
mtaskin