I have two problems. first about the normal derivative. if I have a vector, u=(ux,uy) whose normal derivative is defined as du/dn. is it as : du/dn=( (dux/dx)N.x+(dux/dy)N.y , (duy/dx)N.x+(duy/dy)N.y )?
or on the other hand, du/dn=(dux/dn, duy/dn)?
2- to find (du/dn).n=(dux/dn N.x+ duy/dn N.y)?