Learning Forum

[shisha3301]

I have a question about mass conservation in CFD and its relationship to real-world scenarios. Suppose I'm modeling an extract fan as a volume outflow boundary condition in a room, with an airflow rate of 3m³/s, and there’s a very small natural opening. In both real life and CFD, would I still expect to see 3m³/s inflow through this small opening, even if it's too small to realistically accommodate that flow rate?

Additionally, in a scenario where there's no natural opening and the extract fan is removing 3m³/s, how would mass conservation apply if air is being removed with no corresponding inflow? Would this imply that mass isn't conserved, even though we assume mass inflow equals outflow in any system?

[SRP]

Hi,

When modeling an extract fan in a room with a very small natural opening, if the fan is set to remove air at a rate of 3m³/s, in a real-world scenario, the small opening would not realistically accommodate a 3m³/s inflow due to physical limitations. In CFD, however, the simulation must adhere to the conservation of mass, so it would show an inflow of 3m³/s if no other inflow or outflow boundaries are defined. This is because the simulation assumes that the mass entering a system equals the mass leaving the system. In the case where there is no natural opening and the extract fan is removing 3m³/s, the conservation of mass would still apply. In reality, the room would develop a negative pressure.

[shisha3301]

Thanks for your response. Regarding the second case, how would the conservation of mass still apply if there is no inflow in the system. 

[SRP]

Hi,

For conservation of mass to hold true, there needs to be an equivalent mechanism allowing air to replace what is being extracted. Conservation of mass still applies even if there is no natural opening and an extract fan is removing 3 m³/s of air. In this situation, since mass cannot be created or destroyed, for the conservation of mass to hold, there must be an equivalent inflow or compensatory mechanism to replace the air being extracted.