id keyword eajbcs-667 equation eajbcs-667 diffusion eajbcs-667 j=1 eajbcs-667 solution eajbcs-667 method eajbcs-667 nāˆ’1āˆ‘ eajbcs-667 element eajbcs-667 advection eajbcs-667 finite eajbcs-667 matrix eajbcs-667 mass