I have some questions regarding the generation of a Mass Flux into a domain to obtain a predefined amount of component within that domain.
The rough set-up is rather simplistic: I need the Mass Flux as a Pulse from the inflow boundary which allows me to have a later concentration free water pulse, or more descriptive: a single Pulse application of a compound to the domain surface and a later precipitation event that transports the compound into the domain. Domain is a 2D XZ plane and inflow is from the top boundary.
So far I understood that the time dependent Mass Flux can be implemented by assigning a time variable source term for the water flux and a time variable Dirichilet BC for the compound concentration which satisfies m = SUM (C * q * A) * delta t (in the discrete form, where m is the final amount of compound wanted in the domain, C the Dirichilet BC for the concentration, q the Darcy flux resp. Darcy velocity and A the perpendicular boundary area) during the initial pulse and is zero thereafter to allow a concentration free subsequent water pulse. I hope this is correct to this end.
Now for my question:
My domain is unsaturated, thus Richards_Flow is used. In the unsaturated case the velocity OGS computes at the inflow boundary (VELOCITY_Z1 in my case) is much lower than the Darcy velocity I assigned for the water flux in the *.ST file (as $CONSTANT_NEUMANN). Does OGS use this velocity V_Z1 to compute the Mass Flux over the boundary or will it still compute Mass Flux with the assigned Darcy velocity? For the latter I’d be good, but if it uses V_Z1 also the Mass Flux will be smaller and I won’t get the full amount of compound I want in the domain. Does anyone know how this is handled and if OGS indeed calculates Mass Flux from V_Z1 how I can correctly assign the Dirichilet BC?
The subsequent water pulse. As this pulse has to be free of concentration I changed the Dirichilet BC for the compound concentration to zero one this pulse is active. This works, but will affect my mass balance as from one time step to the other all concentration values at the entire inflow boundary are changed from a rather high value to zero. Does anyone know of a work around for this issue?
I would very much appreciate any help!