  Forced turbulence simulations are used to determine the turbulent kinematic
viscosity, nu_t, from the decay rate of a large scale velocity field. Likewise,
the turbulent magnetic diffusivity, eta_t, is determined from the decay of a
large scale magnetic field. In the kinematic regime, when the field is weak,
the turbulent magnetic Prandtl number, nu_t/eta_t, is about unity. When the
field is nonhelical, eta_t is quenched when magnetic and kinetic energies
become comparable. For helical fields the quenching is stronger and can be
described by a dynamical quenching formula.
