  The impact of viscosity in the Intracluster Medium (ICM) is still an open
question in astrophysics. To address this problem, we have run a set of
cosmological simulations of three galaxy clusters with a mass larger than
$M_{\mathrm{Vir}} > 10^{15} $M$_{\odot}$ at $z=0$ using the SPMHD-code
OpenGadget3. We aim to quantify the influence of viscosity and constrain its
value in the ICM. Our results show significant morphological differences at
small scales, temperature variations, and density fluctuations induced by
viscosity. We observe a suppression of instabilities at small scales, resulting
in a more filamentary structure and a larger amount of small structures due to
the lack of mixing with the medium. The conversion from kinetic to internal
energy leads to an increase of the virial temperature of the cluster of
$\sim$5% - 10%, while the denser regions remain cold. The amplitude of density
fluctuations is found to increase with viscosity, as well as the velocity
fluctuations. However, comparison with observational data indicates consistency
with observed density fluctuations, challenging the direct constraint of
viscosity solely through density fluctuations. Furthermore, the ratio of
density to velocity fluctuations remains close to 1 regardless of the amount of
viscosity, in agreement with the theoretical expectations. Our results show for
the first time in a cosmological simulation of a galaxy cluster the effect of
viscosity in the ICM, a study that is currently missing in the literature.
