hi,

i'm modiling 3-D hydrofoil. can nebody describe me why solutions are unstable at an small angle of attack while there is no separation.

>can nebody describe me why...

No.

(that's the best answer I could come up with, based on the (lack of) information provided)

In fact the word 'unstable' in the question is also confusing, whether he wanted to say that solver is unstable that it does not converge or blows up etc etc, or unstable that it does not show steady profile but produces changing velocity profile. Personally I also did not get the question.

May be he did not set underrelaxation or overrelaxation and caused the code to diverge.