Normal derivative

Dear all

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)?


the second case is wrong:

you can define a

macro dn(v) ( [dx(v),dy(v)]'*[N.x,N.y]) //

and so
du/dn = (dn(ux),dn(dy))

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