Unified dark fluid with fast transition: including entropic
  perturbations
  In this paper, we investigate a unified dark fluid model with fast transition
and entropic perturbations. An effective sound speed is designated as an
additional free model parameter when the entropic perturbations are included,
and if the entropic perturbations are zero, the effective sound speed will
decrease to the adiabatic sound speed. In order to analyze the viability of the
unified model, we calculate the squared Jeans wave number with the entropic
perturbations. Furthermore, by using the Markov Chain Monte Carlo method, we
perform a global fitting for the unified dark fluid model from the type Ia
supernova Union 2.1, baryon acoustic oscillation and the full information of
cosmic microwave background measurement given by the WMAP 7-yr data points. The
constrained results favor a small effective sound speed. Compared to the
$\Lambda$CDM, it is found that the cosmic observations do not favor the
phenomenon of fast transition for the unified dark fluid model.
