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[FAQ]

The treatment of quantities that are exactly zero is subject to the math processor of the machine, if a divide by zero is encountered. Warnings are issued to prevent this, or indicate that it is occurring. Diffusion (even with significantly non-zero coefficients) is a very slow process, so the effects of a low diffusivity on the scalar streamlines will be negligible (unless the residence time is huge). The best practice (to avoid the pitfalls of divide-by-zero calculations), is to set the diffusivity to something just above machine round-off, (ie: 1.0e-15), and solve with double precision.