Time step
 

Hi,

Is it reasonable to change the time step size to 5s or even bigger? At first i used time step size=0.01 but the time step seems to be too small, there nothing changing most of the time.

It's better to use adaptive form of timestep. Timestep size depends on your problem time gradiant. You can use a very big timestep if have a small time gradiant. and should use 1e-6 if have very high time gradiants.