Reply To: non-physical negative uds concentrations with small diffusion coefficient

[grayg34]

The uds equation is being used to implement a drift-flux based aerosol model for particles ranging from 1-50 micrometer in diameter. I am applying the drift velocity with a udf flux function. The drift velocity adds the effects of gravitational settling and phoretic forces on the particle in addition to the convection from the background flow. I had deactivated this custom flux when trying to isolate the cause of the negative concentration. I am not removing the scalar with volumetric sinks corresponding to the background flow mass sinks. I am using the custom flux udf to convect the scalar out of the wall by applying a deposition velocity proportional to the drift velocity. This does cause the concentration to build up slightly beyond the nominal inlet value but this is fine and physically realistic. Again, I turned off the deposition when trying to isolate the negative concentration.