problem in turbulent flow over flat plate with stagnation point
 

I am trying to simulate turbulent flow over the flat plate with stagnation point and symmetry plane upstream of the plate. after few iteration it diverges quickly and velocity near stagnation point increase too much. I set symmetry boundary condition for velocity and for turbulence model's parameter (SST k-omega turbulence model was applied) in symmetry plane (du/dn=0). I speculate that this is because of symmetry boundary condition but I say it tentatively. does any one have any idea about this problem? I should mention I evaluate my code with flat plate without stagnation point (without leading edge and one-dimentional plate).