by
DrJonathan Pietarila Graham(Natl. Center for Atmospheric Research)
→
Europe/Stockholm
Nordita Building
Nordita Building
Description
Turbulence is an important process in astrophysical flows and is
linked to strong and impulsive events (intermittency) such as solar
flares, to magnetic reconnection in general, and to dynamos. Of
course, the range of scales encountered in MHD problems of
astrophysical interest is well beyond expected computer resolutions
for decades to come. For this reason, closure schemes are often
employed to model the effect of the unresolved scales. One such
closure is the MHD-alpha model. This model is an extension of the
alpha model in fluid dynamics which filters velocity fields locally
while leaving their associated vorticities unsmoothed, and has proven
useful for high Reynolds number turbulence computations. It differs
from large eddy simulations in that it preserves the physical
invariants of a given flow. We present MHD and MHD-alpha simulations
of forced and free decaying two-dimensional magnetohydrodynamic
turbulence. The model's ability to capture intermittency and sign
cancellations (rapid changes of field direction on very small scales
which is related to reconnection) are studied. MHD-alpha is found to
reproduce the intermittency of MHD as well as the statistics of sign
cancellations of the current (and vorticity) at small scales and the
fractal dimension of the dissipative structures. At large
Reynolds numbers, an independence of the cancellation exponent
with the Reynolds numbers is observed.
