One interesting thing to note is the P/Q fan curve. It plots how much air a fan, at a specific PWM frequency, moves against a variable restriction (measured in pressure drop). Here's a sample - https://noctua.at/en/nf-a12x25-performance-comparison-to-nf-...

Hypothetically, knowing the heatsink airflow curve, one can find the amount of air a fan will move across.

There are PQ curves (either reported or measured) for a bunch of fans and some radiators out there.