Learning Forum

[grayg34]

I was surprised when setting the diffusiviuty to zero prevented the negative concentrations. I guess another option would be to limit the diffusion coefficient to stay above the 1e-4 threshold.. or to either 1e-4 or zero, whichever is closer to the actual calculated diffusion coefficient. 

The turbulent diffusion is equal to the turbulent viscosity (Schmidt number of 1) generated by the back ground flow. It is above 1e-4 for the majority of the cavity except for the lower stagnant region of the cavity. The brownian diffusion is dependent on the particle diameter being modeled by the uds equation. For the majority of particle sizes I am considering it is neglibibly small, <1e-12 m^/s. For a brownian diffusion coefficient on the order of 1e-4 I would need 0.1 nanometer particles which are far too small to be relavent to my study. 

The negative concentration does tend to initially arise near the walls of the cavity in the lower stagnant portion of the domain, however negative concentration will occasionaly develope in the center of the stagnant bottom region but not significant enough to cause problems. I have zero flux boundary conditions on all walls in the cavity. I will get some large positive concentrations near the background flow mass sinks which are located in the wall adjacentent cells on one wall. I do not have any scalar source terms. The scalar will be able to leave the domain (deposite on the wall) through a udf custom flux. I deactivated this for troubleshooting the negative concentration issues.