Can Air Be Compressed Without a Container?

[Becourbin]

I have read in different theorem's (Bernoulli's for example), that free air is considered incompressible below a speed of 0.4 Mach.
If I were to place air in a jug and reuce the amount of space it occupies, then I am compressing it, but is this possible under certain conditions in free air?

 

[rcgldr]

That's a generalization about how air reacts to solid objects moving through the air. Somewhere between mach .3 and mach .4, there's about 5% compression in most common situations. It's less at lower speeds, enough that simplified mathematical models can ignore it.