flux calculation(urgent)

vid

hi

i am studying diffussion of one species in static fluid . the domain is 2d rectangular in which one wall is maintained at const( mass fraction=1) concentration and other 3 walls are maintained at zero concentration.(no inlet and outlet).

now i need to calculate mass flux from the wall.(i.e flux due to the term Dab*dc/dx).

can any one explain me how to calculate or write udf to obtain the flux in fluent .

thanks in advance