Subtopic: Terminal velocity separation in pneumatic aspirator
Question Statement:
In a vertical pneumatic aspirator used for grain cleaning, separation between sound grain and chaff/straw is achieved when the upward air velocity $V_{air}$ satisfies:
For aerodynamic separation, the upward air velocity must exceed the terminal falling velocity of the light impurities (chaff/dust) so they are entrained upward, while remaining strictly lower than the terminal velocity of heavy sound grains so grains settle downward: $V_{t,\text{chaff}} < V_{air} < V_{t,\text{grain}}$.