Questions for simple algorithm programming
 

My fortran 'simple' program diverges at the 15th step with all tentative relaxation factor
,and at the break point,a wave form appears besides the at inflow boundary.I will appreciate it if someone tell me why and the image at the breakpoint is showed below.
And how comes the oblique line?The exit is twice the width of that of the entrance.(Vector velocity is set for zero ear all the walls.)