Supersonic Turbulence
Turbulent motions are detected in interstellar
gas. The average speed often exceeds the speed of sound. How is it possible
for such turbulence to survive? How does supersonic turbulence dissipate?
The answers will help us understand the distribution of matter on many
scales in our Universe.
Here are some simulations - mpeg movies - which demonstrate not
only how supersonic turbulence dissipates but also how molecules will be
destroyed by high speed turbulence, yet the molecules reform once the turbulence
has died away (keep watching to see the orange foliage return). Here is the
Movie of Supersonic Molecular Turbulence.
Details can be found in two papers in Montly Notices of the Royal Astronomical Society:
Pavlovski G., Smith M.D., Rosen A., Mac Low M.-M., 2002, ``Hydrodynamical simulations of the decay of high-speed molecular turbulence. I. Dense molecular regions'', MNRAS, 337, 477, and
Pavlovski, G.; Smith, M. D.; Mac Low, M. -M.., 2006, ``Hydrodynamical simulations of the decay of high-speed molecular turbulence. II. Divergence from isothermality'', MNRAS, 2006MNRAS.368..943P.
We previously investigating Supersonic Turbulence as observed
in the environments of young stars, such as DR 21, and as generated
on supercomputers. Mordecai Mac Low produced a superb page on
our simulations of the Decay
of Supersonic Turbulence back in 1998, complete with movies of the decaying shocks....
I am now investigating how to recognise different
types of turbulence, given that we are not shaken by the turbulence (as
on an aircraft) but observe from a safe distance how molecules and atoms
are shaken. We have found some fascinating differences.
Supersonic turbulence produces shock waves: no
advance warning is given of their arrival. We can recognise the type of
turbulence by counting the number of each type of shock. We can measure
the shock strength by the jump in speed across the shock wave. If
the turbulence is simply left to DECAY, then the shock distribution
is exponential and decays exponentially.
Here are four snapshots
of the strong shocks in one simulation which began with an
average Mach number of 50 (average speed 50 times the speed of sound).
So how does this compare to DRIVEN turbulence?
We find that uniformly driven supersonic turbulence is recognisable by
a power law distribution in shocks with a steep high-speed tail. The
power law is: the number of shocks goes as the inverse square root of
the shock jump speed. How do the shocks look? Here is an image of the
driven shocks and an image with self-gravity
driven shocks included.
Michael D. Smith
.
CAPS, Kent