Multiphase monitoring in fluent
 

Hello everyone,

I have a question regarding the appropriate monitoring method for my case,
My case is a multiphase sand-liquid slurry, initially a sand bed in places at the bottom of an annular conduit, then through mass flow inlet water is introduced into the domain, flow is turbulent
the goal is to see how the phases interact and see what flow rate of water erodes the bed,
I have used Eulerian-Euleian and also DDPM model for my simulation, so far the DDPM worked well, but I am confused by one thing, when I set a volume monitor of the secondary phase (i.e. sand) mass I see it always reduces as simulation goes on, I monitor the primary phase mass it is almost constant,
In the DDPM model form the fate report I see all the particles are still in the domain and have not left from the pressure outlet, now I am not sure if the mass monitor is a good indicator of the results for my case, my question is what else I can use for this case? this is especially important for Eulerian-Eulerian model where fate of single particles are not reported,

Best Regards
Majid