We present a unified dark fluid model to describe the possible evolutionary behavior of $��N_\mathrm{eff}$ 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 $��N_\mathrm{eff}$ can be nicely explained without some drawbacks, such as the blowup of $��N_\mathrm{eff}$ and the non-vanishing interaction at the late time.

12 pages, 4 figures, revised version accepted by PTEP