Unified Dark Fluid with Constant Adiabatic Sound Speed: Including
  Entropic Perturbations
  In this paper, we continue to study a unified dark fluid model with a
constant adiabatic sound speed but with the entropic perturbations. When the
entropic perturbations are included, an effective sound speed, which reduces to
the adiabatic sound speed when the entropic perturbations are zero, has to be
specified as an additional free model parameter. Due to the relations between
the adiabatic sound speed and equations of state (EoS)
$c^2_{s,ad}(a)=w(a)-d\ln(1+w(a))/3 d\ln a$, the equation of state can be
determined up to an integration constant in principle when an adiabatic sound
speed is given. Then there are two degrees of freedom to describe the linear
perturbations for a fluid. Its micro-scale properties are characterized by its
EoS or adiabatic sound speed and an effective sound speed. We take the
effective sound speed and adiabatic sound speed as free model parameters and
then use the currently available cosmic observational data sets, which include
type Ia supernova Union 2.1, baryon acoustic oscillation and WMAP 7-year data
of cosmic background radiation, to constrain the possible entropic
perturbations and the adiabatic sound speed via the Markov Chain Monte Carlo
method. The results show that the cosmic observations favor a small effective
sound speed $c^2_{s,eff}=0.00155_{- 0.00155}^{+ 0.000319}$ in $1\sigma$ region.
