Increasing of Courant Number
 

In my simulation, i use explicit scheme and the Courant number is default 0.25.
Because of the explicit scheme, the time step used is 0.00001 and the iteration is around 30000.
Before i change the under-relaxation factor, the Courant number increase
until 1000+.Thus, i have to minimized the under-relaxation factor to 0.01, to make it more stable. but the problem of Courant number still happen at iteration around 20000.

Should i use smaller under-relaxation factor or else??

Anyone please give me suggestion, thank you.