FRIDAY, OCTOBER 29, 9 AM
JILA 10C
TITLE:
The zig-zag path of buoyant magnetic tubes and the generation of
vorticity along their periphery.
ABSTRACT:
When the Reynolds number is not small, the wake trailing a buoyant
magnetic flux tube sheds vortex rolls therefore producing a Von Karman
vortex street and an inbalance of vorticity in the tube which results
in a transverse oscillation of the tube as a whole (see e.g. Fan and
Zweibel 1998). The actual path followed by the tube is therefore
directly affected by the amount of vorticity being produced in the
boundary.
Fernando Moreno-Insertis, Mark Rast and myself have studied in some
details how vorticity is being produced along the boundary layer of
buoyant magnetic flux tubes and how this affect the rise. Scaling laws
for the magnetic and viscous generations of vorticity have been
obtained and checked numerically using a full compressible 2D MHD
code. I will show also how a simple analytical equation (which
includes buoyancy, drag and vortex forces) can satisfactorily
reproduce the observed trajectories. A curve for the Strouhal number
is obtained and compared with the classical measurements of the
hydrodynamical literature. This will give us a basis to discuss
extrapolation of numerical simulations to Reynolds-numbers higher than
those currently computationally achievable.