How is air flow (Q) computed from cross-sectional area and velocity?

• A. Q = A × V
• B. Q = A ÷ V
• C. Q = V × A × 0
• D. Q = A + V

Answer: (A)

Volumetric air flow depends on how much area the air is moving through and how fast it moves. The amount of air crossing a cross section each second is the cross-sectional area times the velocity: Q = A × V. If the area increases or the speed increases, more air passes per second, so the flow rate rises accordingly. The units line up: area in square meters times velocity in meters per second gives cubic meters per second, a true measure of flow. If velocity is uniform across the section, this simple product is exact; with varying speeds across the area you’d add up or integrate the local contributions, but for most practice questions this product is the standard formula. The other expressions don’t represent a flow rate: dividing area by velocity would mix units to something like meters·seconds, multiplying by zero yields zero, and adding area and velocity doesn’t produce a rate.