  We present a unified dark fluid model to describe the possible evolutionary
behavior of $\Delta N_\mathrm{eff}$ in dark radiation. This model can be viewed
as an interacting model for the dark sectors, in which dark matter interacts
with dark radiation. We show that the evolution of $\Delta N_\mathrm{eff}$ can
be nicely explained without some drawbacks, such as the blowup of $\Delta
N_\mathrm{eff}$ and the non-vanishing interaction at the late time.
