# Numerical Instability and Convergence

22/07/2017 12:16

The software appears to be not running if the solution algorithm becomes numerically unstable. Numerical instability during the program run is unfortunately a problem with most software packages of this nature. There are however many measures that can be taken to fix the problem from the software input windows. Here is a short summary of steps to follow to obtain solutions (numerical convergence):

- Try different initial conditions for the design parameters in the Solution Control Window. Select “Auto” initialization so that the initial parameters are calculated by the software.
- If the problem still persist then change the values of the relaxation parameters in the Solution Control Window. The values should be less than 1 and greater than zero. If it is too small then the software might show that the solution converged when it actually hadn’t. If the solution converged then use the applicable results in the “output.txt” file as initial values in the Solution Control Window. Use larger relaxation parameters when re-running the software with the new initial conditions.
- Make sure that the black command window that open when the software is running is closed before attempting a new run. If the old command window is open the software can't start a new run.
- If a cooling tower is designed from scrath it is important to select cooling tower dimentions that are realistic. It is important to update the fan diameter dimentions as well. The fan diameter is not enetered in the cooling tower specification window, but in the fan specification window.
- If the software runs successfully for the Merkel model but not the Poppe model (and you have already tried the steps above with no success) then try the following:

- Run the software with the Merkel method. Use the applicable outputs as initial values in the Solution Control Window. Rerun the software with the Poppe model selected.
- If the program still not runs then select a different “Solution algorithm” from the Model Settings Window. Change the settings on the window from “Secant” to “Energy eq.” or vice versa.