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Dear All,

I am trying to simulate the "Reoriented Potential Mixing" flow in the porous media with unsteady Darcy's law physic.
My model is a 2D circle and has four source-sink pairs which located at the circumference. The position between one pair to another pair differs 45 degrees. The expected scheme is below:

1. At time 0 - 4 day, Pair 1 active and remaining pairs not active.
2. Time 4 - 8 day, pair 2 active and remaining pairs not active.
3. Time 8 - 12 day, pair 3 active and remaining pairs not active.
4. Time 12 - 16 day, pair 4 active and remaining pairs not active.

The fluid only flows from the activated pair.

I have a problem on simulating the model as above scheme. My current result is when the Pair 1 active, the fluid flows from the Pair 1, Pair 2, and Pair 3. The expected result is the fluid only flowing from the Pair 1.

I used if function in the Mass Source domain to define the time function of each pair:
Pair 1: if(t<4[d],0.5,0)
Pair 2: if(4[d]<=t<8[d],0.5,0)
Pair 3: if(8[d]<=t<12[d],0.5,0)
Pair 4: if(t>=12[d],0.5,0)

I would like to know why the Pair in my model is not working as my expectation and how can I fix it? Please kindly let me know. I have been trying to work on this thing for more than two weeks. Please feel free to give suggestion if you think the model can be improved by doing it in any other way.

Thank you for your kind attention