### Mach number

(ratio of a flow with the speed of sound)

A **Mach number** is a ratio of the relative velocity of
a flow of a fluid by an obstruction and the speed of sound
in that fluid.

An **Alfvén Mach number** is such a ratio specifically
with the speed of Alfvén waves.
The term **sonic Mach number** is sometimes used for the
usual Mach numbers to distinguish the two.

Both these Mach numbers are used in the study of the dynamics
of molecular clouds and dense cores, to characterize
turbulence according to the relative velocities of adjacent
fluids. They are of interest in determining the affect of
turbulence, jets, and winds on the stability of parcels of
high-density gas, essentially to study the particulars of star formation feedback.

(*physics,fluid dynamics,measure,turbulence*)
http://en.wikipedia.org/wiki/Mach_number

**Referenced by:**

Alfvén wave

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