negative values for diffusion scalar

cmv

Hi

The situation is very simple; I have defined a passive scalar and define it to be solved by diffusion. If you take a slab of cells and inject this passive scalar at one end and extract it at the opposite side, normally you would expect to have a zero value somewhere near the middle of the slab.

The problem is that I have not been able to get this 0 point at the middle of the slab but almost near the extraction region.

I have plot the value by monitoring the scalar at the injection & extraction regions but I get a correct value only on the injection region and I usually get a slightly negative value on the extraction region. I would expect these to be the same size but with opposite signs.

Do you know why it is not the same magnitude?