pressure
 

Hi
I understand that minus pressure in the boundary means a suction pressure, but what does minus pressure inside the domain mean?

A localised region having vaccuum or pressure less than atmospheric pressure

what does vacuum exactly mean?
if I consider my system in vacuum (there is no pressure from outside the system) and
regarding to the simulation based on the b.c. why should we care about the atmosphere, this is my issue getting confused of many times in simulation by Fluent,
what i want to simulate is a T shaped tube (with three openings), in two openings the pressure is known (there is a pressure from outside to inside the domain),
also in the third opening there is a suction implied by a vaccume system or a syringe, (so minus pressure),...
so i put 100 pa and 100 pa for the first two openings and -100 pa for the third opening......

A negative pressure means pressure less than atmospheric. You may have this at the suction side of fan and pump. This gauge pressure may be negative. but absolute pressure will be positive. The negativeness is relative to atmospheric pressure.