It is assumed that the solution is a combination of the convection-free solution and a homogenous convection-diffusion solution as given by: ??({circumflex over (x)},??)=???({circumflex over (x)},??(+{circumflex over (v)}({circumflex over (x)},??).???(6) The convection-free solution, ???({circumflex over (x)},??), is a polynomial expression that satisfies Eq. (1) and the boundary conditions.