  We investigate numerically the decay of isotropic, rotating,
magnetohydrodynamic (MHD), and rotating MHD flows in a periodic box. The
Reynolds number $Re$ defined with the box size and the initial velocity is
$100$ at which the flows are in a weakly nonlinear regime, i.e. not laminar but
far away from the fully turbulent state. The decay of isotropic flow has two
stages, the first stage for the development of small scales and the second
stage for the viscous dissipation. In the rapidly rotating flow, fast rotation
induces the inertial wave and causes the large-scale structure to inhibit the
development of the first stage and retard the flow decay. In the MHD flow, the
imposed field also causes the large-scale structure but facilitates the flow
decay in the first stage because of the energy conversion from flow to magnetic
field. Magnetic Reynolds number $Rm$ is important for the dynamics of the MHD
flow, namely a high $Rm$ induces the Alfv\'en wave but a low $Rm$ cannot. In
the rotating MHD flow, slower rotation tends to convert more kinetic energy to
magnetic energy. The orientation between the rotational and magnetic axes is
important for the dynamics of the rotating MHD flow, namely the energy
conversion is more efficient and the stronger wave is induced when the two axes
are not parallel than when they are parallel.
