  In the present study, we perform direct numerical simulations of compressible
turbulent boundary layers at the free stream Mach number of 2 ~ 6 laden with
dilute phase of spherical particles to investigate the Mach number effects on
particle transport and dynamics. Most of the phenomena observed and
well-recognized for inertia particles in incompressible wall-bounded turbulent
flows, such as the near-wall preferential accumulation and clustering beneath
the low-speed streaks, the flatter mean velocity profiles and the trend
variation of the particle velocity fluctuations, are identified in the
compressible turbulent boundary layer as well. However, we find that the
compressibility effects are significant for large inertia particles. As the
Mach number increases, the near-wall accumulation and the small-scale
clustering are alleviated, which is probably caused by the variation of the
fluid density and viscosity that are crucial to particle dynamics. This can be
affected by the fact that the forces acting on the particles with the viscous
Stokes number greater than 500 are modulated by the comparatively high particle
Mach numbers in the near-wall region. This is also the reason for the abatement
of the streamwise particle velocity fluctuation intensities with the Mach
numbers.
