vortex formation

Hi Dear All,
I find some thing for flows around a single circular cylinder. I set the computational domain the length of the domain is fixed like 500 lattice nodes, but the y-direction varies like 40, 60, 80,120 and 140, the Reynolds numbers are fixed. When i check this for these different conditions for Re=50, so for 40, 60 i find the vortex shedding for Re=50 but when i increased the value of y-direction like 120 and 140 so the same code i used but i not find the vortex shedding and just find two steady bubbles, even i get the drag and lift. (60x500,re=50 and 120x500,re=50)
Any comments if some friends know about this.

are you using no-slip on top and bottom wall? in that case, the instabilities (vortexes) are probably generated by the wall. as the wall gets farther from the cylinder, the flow becomes steady again. when walls are very far from the cylinder, i think instabilities start around re=100.