Nonuniversality and finite dissipation in decaying magnetohydrodynamic
  turbulence
  A model equation for the Reynolds number dependence of the dimensionless
dissipation rate in freely decaying homogeneous magnetohydrodynamic turbulence
in the absence of a mean magnetic field is derived from the real-space energy
balance equation, leading to $C_{\varepsilon}=C_{\varepsilon, \infty}+C/R_-
+O(1/R_-^2))$, where $R_-$ is a generalized Reynolds number. The constant
$C_{\varepsilon, \infty}$ describes the total energy transfer flux. This flux
depends on magnetic and cross helicities, because these affect the nonlinear
transfer of energy, suggesting that the value of $C_{\varepsilon,\infty}$ is
not universal. Direct numerical simulations were conducted on up to $2048^3$
grid points, showing good agreement between data and the model. The model
suggests that the magnitude of cosmological-scale magnetic fields is controlled
by the values of the vector field correlations. The ideas introduced here can
be used to derive similar model equations for other turbulent systems.
