hi,

may i know what's the formula for convective BC when the exit is not vertical, like in an o-grid.

when i'm using c-grid, the exit is vertical hence,

du/dt+U*du/dx=0.

Is it the same for o-grid as well?

or should it be du/dt+U*du/dn=0.?

Thank you.

No, if du/dt+U*du/dx=0 is your governing equation, then this equation is valid everywhere, in the interior domain as well as on all boundaries, regardless of their shape. The other equation, i.e. du/dt+U*du/dn, is not equivalent to that. The difference between a straight vertical and curved boundary is that in the former case the implementation of du/dx is easier. You'll have to think how to express du/dx across the curved boundary, and that will indeed involve du/dn, but it's not equal.