UDF for 3D Dissolved Oxygen Model
 

Hello everyone,

I'm currently trying to do a 3D transient simulation to determine dissolved oxygen concentration in a pond. I'm using an impeller to create circulation in the pond. There are two cell zone, one is a rotating body containing the impeller and the other one is a stationary body containing the rest of the pond.

https://i.postimg.cc/Z5R2ZWfx/Screen...-30-130231.png

I'm using UDF to model the dissolved oxygen. The UDF is as follows:


#include "udf.h"

#define Cs 0.0085
#define C_initial 0.003
#define a 22.34

DEFINE_SOURCE(mass_source1, c, t, dS, eqn)
{
real S, C;
real time = CURRENT_TIME;

S = a * (Cs - C_initial);

S *= exp(-0.1 * time);

C = C_initial + (Cs - C_initial) * (1 - exp(-a * time));

if (C >= Cs) {
C = Cs;
S = 0.0;
}

C_UDSI(c, t, 0) = C;

dS[eqn] = 0.0;
return S;
}


I've added this to the Mass source term in both fluid zones
https://i.postimg.cc/VN0hr3PS/2.png https://i.postimg.cc/NjzV0QSq/3.png

After running the simulation, the results I obtained indicated that the dissolved oxygen model did not follow the flow of water in the pond. Instead, it radiated from the rotating body very slowly.

Velocity contour:
https://i.postimg.cc/pTD1kzTR/Whats-...t-13-16-21.jpg

Dissolved oxygen contour:
https://i.postimg.cc/tT0Mh29B/Whats-...t-13-16-18.jpg


I've looked into the mass source term documentation and it says the mass will be accelerated by the flow. What I want is for the dissolved oxygen to follow the flow of the water so I can find out the effects of different types of impellers to the dissolved oxygen concentration in the pond. What am I doing wrong here? Is my UDF wrong or do I need to add something else? As I am new to Ansys and UDF, any pointer or help would be highly appreciated. Thank you