Cooling How does the Noctua NF A9x14 cool the CPU like this?

[dumplinknet]

I just saw this video on YT and notice that the Noctua NF A9x14 does not cool the component directly underneath it's fan (ie. The CPU). It blows around the fan only. So then, how exactly is this fan so efficient?
Maybe I don't understand aerodynamics well enough.

 

[blubblob]

The video is a bit of a misrepresentation of how the air moves through a fan and the heatsink of the L9i cooler specifically.

For one - there is no heatsink present which channels the air: If you take a look at the L9i heatsink you can see that two sides are completely and the other two sides mostly closed. So most of the air has to move through the heatsink.

Two - Every fan has a deadzone in the middle where the motor hub is located. So a running fan creates a ring of high pressure between the hub and the frame with the highest pressure being near the end of the blades. If you now put the fan on a bed of sand like in the video the ring of high pressure air tries to normalize and push the sand away.
To the sides that obviously works because the air has lower pressure there and the sand can move out of the way. Remember that the pressure has the form of a ring (3D - hollow cylinder) though, so the sand under the fan is actually pushed inward from the area of the highest pressure and can't escape because there is the motor hub on top, the metal bowl on bottom and a zone of high pressure air all around.

That there is so much sand left below the fan could also be interpreted as a sign of how focused the airflow is on the noctua fan. Another model creating stronger turbulence might move the sand away, which is not something you want for a heatsink fan.
Luckily your CPU is not sand-cooled but by the air being pushed through a heatsink. You want the air moving straight into the heatsink and become turbulent only once inside.

 

[Phuncz]

I also wouldn't consider a fan blowing some particulate in a pan as a realistic test for how airflow in a heatsink works. If it's dead zone was that huge, the Noctua NH-L12S wouldn't be able too cool the CPUs it does manage to cool in real life.

 

[tinyitx]

The fan does blow directly straight downward. It is just that a flat non-porous surface was underneath and the airflow is forced to spread out radially, blowing the sand away and giving the false impression that it is cooling the surrounding instead of the surface directly underneath.

When a fan is on top of a heatsink and is blowing down, airflow goes through the fins.
In fact, a fan never cools the CPU directly. Heat generated from the CPU is transferred by direct contact (ie conduction) to the base plate/heatpipes of the heatsink. Heatpipes transfer the heat to the fins which are then cooled by the airflow by convection.