Speaker
Fabio Del Sordo
(NORDITA)
Description
In the absence of rotation and shear, and under the
assumption of constant temperature or specific entropy,
purely potential forcing by localized expansion waves is
known to produce irrotational flows that have no vorticity.
Here we study the production of vorticity under idealized
conditions when there is rotation, shear, or baroclinicity,
to address the problem of vorticity generation in the
interstellar medium in a systematic fashion. We use
three-dimensional periodic box numerical simulations to
investigate the various effects in isolation. We find that
for slow rotation, vorticity production in an isothermal gas
is small in the sense that the ratio of the root-mean-square
values of vorticity and velocity is small compared with the
wavenumber of the energy carrying motions. For Coriolis
numbers above a certain level, vorticity production
saturates at a value where the aforementioned ratio becomes
comparable with the wavenumber of the energy carrying
motions. Shear also raises the vorticity production, but no
saturation is found. When the assumption of isothermality is
dropped, there is significant vorticity production by the
baroclinic term once the turbulence becomes supersonic. In
galaxies, shear and rotation are estimated to be
insufficient to produce significant amounts of vorticity,
leaving therefore only the baroclinic term as the most
favorable candidate. We also demonstrate vorticity
production visually as a result of colliding shock fronts.