Engineering art - the design of a fan
The design of the fan, which is situated on top of the heatsink, always bears a conflict. On the one hand the user wants a high air flow, on the other the cooler should not make too much noise due to friction or whirling. Unfortunately, the air flow depends on the RPM rate and the size of the fins.
The perfect solution would be a clean path for the air with little whirls at the entry area and outflow area. An example for a badly designed solution regarding airflow are the radial coolers made by Thermaltake and Titan. At first glance they look pretty "cool", but in reality they both have a mediocre cooling effect.
Bad example -the Thermaltake Aircooler looks interesting but does not reach any high cooling effect. There is too much distance between the fins and the processor.