system
Given the following post, respond as if you're commenting on Reddit:
human
I have a thermos full of coffee and a fan. Is it more efficient for cooling the coffee to point the fan directly into the opening on the thermos or to have it blow perpendicularly across the opening?

I've done a bit of googling on the subject and was unable to find a good answer, although I may not be phrasing the question correctly. I'm not sure if the resulting low pressure zone from having the fan blow across the opening will result in better overall air circulation versus having the fan pointing directly at the opening where the internal air pressure could increase and then reach some sort of equilibrium against the increased flow from the fan.
gpt
Absolutely it’s more effective to blow across the top. 

This will create a suction effect - pulling air out of the thermos because the flow creates a pressure gradient from in-to-out.

The alternative - blowing in will create far less flow - it creates more pressure outside than inside and nowhere for the air to go.

For the academic types - this is basic Bernoulli effect. When there is high speed wind flow across the top of an airfoil (airplane wing) and less flow across the bottom, this is what causes the lift force. Higher speed fluid flow results in lower pressure.