Let the concentration of tracer in the sand column experiment be c´ and the velocity with which it is being injected is v. So the influx of tracer in the column would be -vc´ (negative sign to denote influx)

My question is which inlet boundary condition should be applied? 1st type or 3rd type?

With 1st type condition at the inlet C= c´,

the same concentration as the experiment can be ensured, but the influx will be higher than actual experiment as

Model influx: D∇c´−vc´

Experimental influx: -vc´

But if 3rd trype boundary condition is applied i.e

(D∇C−vC)⋅n= -vc´

The same flux condition as the experiment can be ensured, but the inlet concentration will be lower, untill diffusive flux becomes zero.

In that case, which boundary condition should be applied?

Thanks in anticipation