Air in bus

In a moving bus air enters inside through its windows, but from where does this moving air go outside?


New Delhi

Yes, air enters inside a moving bus through open windows. But it has to go out since the total mass of air inside the bus has to be constant. It amounts to saying volume flow rate of air moving inside should exactly balance the outflow rate (in cubic metres per second or similar units).

Why is it that we do not usually feel the outflow which leads us to this interesting question? It is because of the air velocity distribution inside a bus. Imagine a bus with only two windows one at the front and an identical window at the rear. The average velocities must be equal at these two windows but the front one letting the air in and the other one out. The air that streams through these windows has slightly different characteristics. The air that is gushing in moves like a jet, spreading sideways only a little and hence travels a larger distance inside the vehicle; we feel the air flow. The exhaust air flow, on the other hand, has a wider spread inside. Hence the same amount of exhaust airflow moving out though an identical window is contributed from a larger cross-sectional area inside the bus. Consequently it has smaller velocity and we do not feel it.

Most vehicles especially the cars have exhaust vents. These are carefully designed for better ventilation of the compartment and also to make sure engine fumes and dust do not enter the vehicle. There is also leakage through the gaps.

This opens up another question: What decides whether the air should enter or leak out from an opening? It is the ram air pressure at the opening. Hence air enters from a front window and exits from a rear window. This ram pressure distribution on the vehicle surface is decided by its shape and velocity including flow through the compartment itself.

These ideas are useful in dust, smoke and fire spreading in a building. Interesting studies in this regard are made by keeping models in a wind tunnel or through numerical simulation on a computer.


M.S. Ramaiah School of Advanced Studies


More In: Science | Sci-Tech