Dark radiation from a unified dark fluid model

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