Spherical top-hat Collapse of a Viscous Unified Dark Fluid
  In this paper, we test the spherical collapse of a viscous unified dark fluid
(VUDF) which has constant adiabatic sound speed and show the nonlinear collapse
for VUDF, baryons, and dark matter which are important to form the large scale
structure of our Universe. By varying the values of model parameters $\alpha$
and $\zeta_{0}$, we discuss their effects on the nonlinear collapse of the VUDF
model, and compare its result to $\Lambda CDM$ model. The analyzed results show
that, within the spherical top-hat collapse framework, larger values of
$\alpha$ and smaller values of $\zeta_{0}$ make the structure formation earlier
and faster, and the other collapse curves are almost distinguished with the
curve of $\Lambda
  CDM$ model if the bulk viscosity coefficient $\zeta_{0}$ is less than
$10^{-3}$.
