<em>Introduction to flute acoustics</em>

The air jet vibrates

In a flute, the air jet from the player's lips travels across the embouchure-hole opening and strikes against the sharp further edge of the hole.

The flute is an open pipe

The flute is open at both ends. It's obvious that it's open at the far end. If you look closely at someone playing a flute, you'll see that, although player's lower lip covers part of the embouchure hole, she leaves a large part of the hole open to the atmosphere.

More detailed information

There is more detailed discussions of flute acoustics in The Physics of Musical Instruments by N.H. Fletcher and T.D. Rossing (New York: Springer-Verlag, 1998).