3 research outputs found
Numerical study of dynamo action at low magnetic Prandtl numbers
We present a three--pronged numerical approach to the dynamo problem at low
magnetic Prandtl numbers . The difficulty of resolving a large range of
scales is circumvented by combining Direct Numerical Simulations, a
Lagrangian-averaged model, and Large-Eddy Simulations (LES). The flow is
generated by the Taylor-Green forcing; it combines a well defined structure at
large scales and turbulent fluctuations at small scales. Our main findings are:
(i) dynamos are observed from down to ; (ii) the critical
magnetic Reynolds number increases sharply with as turbulence sets
in and then saturates; (iii) in the linear growth phase, the most unstable
magnetic modes move to small scales as is decreased and a Kazantsev
spectrum develops; then the dynamo grows at large scales and modifies
the turbulent velocity fluctuations.Comment: 4 pages, 4 figure
Dynamo action at low magnetic Prandtl numbers: mean flow vs. fully turbulent motion
We compute numerically the threshold for dynamo action in Taylor-Green
swirling flows. Kinematic calculations, for which the flow field is fixed to
its time averaged profile, are compared to dynamical runs for which both the
Navier-Stokes and the induction equations are jointly solved. The kinematic
instability is found to have two branches, for all explored Reynolds numbers.
The dynamical dynamo threshold follows these branches: at low Reynolds number
it lies within the low branch while at high kinetic Reynolds number it is close
to the high branch.Comment: 4 pages, 4 figure