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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 collapsefor VUDF, baryons, and dark matter which are important to form the large scalestructure of our Universe. By varying the values of model parameters $\alpha$and $\zeta_{0}$, we discuss their effects on the nonlinear collapse of the VUDFmodel, and compare its result to $\Lambda CDM$ model. The analyzed results showthat, within the spherical top-hat collapse framework, larger values of$\alpha$ and smaller values of $\zeta_{0}$ make the structure formation earlierand faster, and the other collapse curves are almost distinguished with thecurve of $\Lambda  CDM$ model if the bulk viscosity coefficient $\zeta_{0}$ is less than$10^{-3}$.

Spherical top-hat Collapse of a Viscous Unified Dark Fluid