The Green-Gauss theorem is often stated as
∫Ωϕ[q⋅∇]dΩ=∫Γϕq⋅ndΓ−∫Ω[∇ϕ]⋅qdΩ where Ω is a closed region (e.g. a volume) and Γ its bounding surface (e.g. an area). n is the outward pointing normal on Γ. ϕ is a scalar and q a vector, both functions of the position x. If we write the corresponding expression in index notation, we have
∫Ωϕ∂xi∂qidΩ=∫ΓϕqinidΓ−∫Ω∂xi∂ϕqidΩ Let's set up this equation for a vector instead of the scalar ϕ. By using the index notation, each free index is actually just a scalar so we effectively just write three equations of the same as above, changing ϕ to uk with u=ukek.
∫Ωuk∂xi∂qidΩ∫Ωu⊗[q⋅∇]dΩ=∫ΓukqinidΓ−∫Ω∂xi∂ukqidΩ=∫Γu⊗q⋅ndΓ−∫Ω[u⊗∇]⋅qdΩ Similarly, we can write it exchanging the vector q=qiei with a 2nd order tensor, a=ajiej⊗ei
∫Ωuk∂xi∂ajidΩ∫Ωu⊗[a⋅∇]dΩ=∫ΓukajinidΓ−∫Ω∂xi∂ukajidΩ=∫Γu⊗a⋅ndΓ−∫Ω[u⊗∇]⋅aTdΩ This expression represents 9 equations (k and j). If we sum the equations where k=j, i.e. multiply by δkj, we get
∫Ωuj∂xi∂ajidΩ∫Ωu⋅[a⋅∇]dΩ=∫ΓujajinidΓ−∫Ω∂xi∂ujajidΩ=∫Γu⋅a⋅ndΓ−∫Ω[u⊗∇]:adΩ