The Physics of blowing a puff of air

[jhan]

If we noticed, when we released wind from our mouth with a constant velocity ,Why is the speed of the close part of our mouth is maximum speed and while gradually moving away is also gradually weakened until it is zero?
How it can be resolved using in mathematical expression?

 

[PeroK]

You mean why is a puff of air subject to air resistance?

 

[Drakkith]

Why is the speed of the close part of our mouth is maximum speed and while gradually moving away is also gradually weakened until it is zero?

In the confined space of the small tunnel made by your lips, all of the air is moving rapidly. But when it exits your lips it runs into stationary air and pushes it out of the way. But then this air runs into even more stationary air and pushes it away. Which runs into even more stationary air...etc etc etc. On top of this, the air that's pushed out of the way is replaced by slow moving air coming in from behind, which itself then has to be pushed out of the way by your breath. The end result is that your breath only has a small distance in which it retains any significant velocity.

A similar thing happens with rocket engines. Standing directly underneath a large rocket engine would blow you away immediately (and cook you like a turkey under a blowtorch), but you can stand underneath a rocket flying high overhead and not feel even the slightest breeze.

How it can be resolved using in mathematical expression?

I throw in the towel when it comes to anything beyond basic calculus, of which fluid mechanics definitely is beyond.

 

[PeroK]

I throw in the towel when it comes to anything beyond basic calculus, of which fluid mechanics definitely is beyond.

The physics of a towel thrown through the air as it gradually opens up must be quite complicated, I imagine.