  Clusters of galaxies, the largest collapsed structures in the Universe, are
located at the intersection of extended filaments of baryons and dark matter.
Cosmological accretion onto clusters through large scale filaments adds
material at cluster outskirts. Kinetic energy in the form of bulk motions and
turbulence due to this accretion provides a form of pressure support against
gravity, supplemental to thermal pressure. Significant amount of non-thermal
pressure support could bias cluster masses derived assuming hydrostatic
equilibrium, the primary proxy for cluster cosmology studies. Sensitive
measurements of Doppler broadening and shift of astrophysical lines, and the
relative fluctuations in thermodynamical quantities (e.g., density, pressure,
and entropy) are primary diagnostic tools. Forthcoming planned and proposed
X-ray (with large etendue, throughput, and high spectral resolution) and SZ
observatories will provide crucial information on the assembly and
virialisation processes of clusters, involving turbulent eddies cascading at
various spatial scales and larger gas bulk motions in their external regions to
the depth or their potential wells.
