courant number
 

how will it be interpreted if the simulation pass the criteria of courant number 0.25,
how wrong will the result of the simulation will be if the courant number is abit higher than 0.25, or if it is 1, 1.5, 2 or very high,
I am asking this because i dont have very much sense of it,
i know the definition but i dont know how much it effects the results of the simulation
Thank u so much

just decrease the time step and see
decrease by 1/10 .
it will improve the results

If you dont get the required convergence then results may be inaccurate, otherwise using higher courant is recommended. This is general guideline, for your particular problem you must do the sensitivity analysis that how much convergence level is required and what is the effect of different courant no. on this convergence accuracy.