  We investigate hydrodynamic fluctuations in a 2D granular fluid excited by a
vibrating base and in the presence of gravity, focusing on the transverse
velocity modes. Since the system is inhomogeneous, we measure fluctuations in
horizontal layers whose width is smaller than the characteristic hydrodynamic
lengths: they can be considered as almost-homogeneous subsystems. The large
time decay of autocorrelations of modes is exponential and compatible with
vorticity diffusion due to shear viscosity, as in equilibrium fluids. The
velocity structure factor, which strongly deviates from the equilibrium
constant behavior, is well reproduced by an effective fluctuating hydrodynamics
described by two noise terms: the first associated with vorticity diffusion and
the second with the local energy exchange, which have internal and external
character, respectively.
