I understand what Neumann BC means physically and how to imply them. However, I am not able to perfectly understand it the way it is represented mathematically as $\partial u / \partial n$ where $n$ is the outward normal vector to the boundary and $u$ is the solution.
I understand that the normal vector is essential for calculation over surface integral. But what are we doing here? Traction is outcome of partial derivative of the solution vector w.r.t to the normal vector? What does this mean?
Can any one try to simplify this briefly?
Thanks a lot!