Small boundary mass flux of scalar leads to null values of concentrations in CS v6.0 and v7.0

[burkhart68]

Putting a small mass flux Qm of the order of microgram/m2/s ( rcodcl (ifac, isca(iscal), 3) = -Qm in cs_user_boundary_conditions.f90 ) leads to zero mass fraction values, when larger values of Qm give finite values. I tried different turbulence models (k-eps, k-omega-sst) with different wall functions (2 scales + scalable with/without log laws). My y+ are quite small (<1).
Following one advice from Y.Fournier, I rewrote the routine as a volumic source term close to the emiting surface, but the problem persists.

[YvanFournier]

Hello,

This might be due to truncation error/numerical sensitivity aspects. If you have a small and simple test case, I could try to test this "in depth" to try to understand the behavior and provide a fix or workaround.

as a workaround, if the mass flux is very small relative to the "main" mass flux through a cell's faces, the mass fraction addition could be ignored, and a scalar concentration could be injected directly as a Neumann condition.