What you need is negative gauge pressure, which is less than the ambient pressure, but you need to specify the absolute pressure here. So, you can set the reference pressure as the atmospheric pressure (or other ambient value) and the pressure at the suction boundary as ambient + gauge pressure (negative).

Another option could be to specify the mass flow rate at the suction boundary (in case you have an idea of the flow rate).

I hope this helps.

Dear, Misagh Rezapour Sarabi

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