  We investigate a granular gas in a shaken quasi-two-dimensional box in
molecular dynamics computer simulations. After a sudden change (quench) of the
shaking amplitude, transient density correlations are observed orders of
magnitude beyond the steady-state correlation length scale. Propagation of the
correlations is ballistic, in contrast to recently investigated quenches of
Brownian particles that show diffusive propagation [Rohwer et al., Phys. Rev.
Lett., 118, 015702 (2017), Rohwer et al., Phys. Rev. E, 97, 032125 (2018)]. At
sufficiently strong cooling of the fluid the effect is overlaid by clustering
instability of the homogeneous cooling state with different scaling behavior.
We are able to identify different quench regimes. In each regime correlations
exhibit remarkably universal position dependence. In simulations performed with
side walls we find confinement effects for temperature and pressure in
steady-state simulations, and an additional transient wall pressure
contribution upon changing the shaking amplitude. The transient contribution is
ascribed to enhanced relaxation of the fluid in the presence of walls. From
incompatible scaling behavior we conclude that the observed effects with and
without side walls constitute distinct phenomena.
