  We study compressible turbulent flow in a circular pipe, at computationally
high Reynolds number. Classical related issues are addressed and discussed in
light of the DNS data, including validity of compressibility transformations,
velocity/temperature relations, passive scalar statistics, and size of
turbulent eddies.Regarding velocity statistics, we find that Huang's
transformation yields excellent universality of the scaled Reynolds stresses
distributions, whereas the transformation proposed by Trettel and Larsson
(2016) yields better representation of the effects of strong variation of
density and viscosity occurring in the buffer layer on the mean velocity
distribution. A clear logarithmic layer is recovered in terms of transformed
velocity and wall distance coordinates at the higher Reynolds number under
scrutiny ($\Rey_{\tau} \approx 1000$), whereas the core part of the flow is
found to be characterized by a universal parabolic velocity profile. Based on
formal similarity between the streamwise velocity and the passive scalar
transport equations, we further propose an extension of the above
compressibility transformations to also achieve universality of passive scalar
statistics. Analysis of the velocity/temperature relationship provides evidence
for quadratic dependence which is very well approximated by the thermal analogy
proposed by Zhang et Al.(2014). The azimuthal velocity and scalar spectra show
an organization very similar to canonical incompressible flow, with a
bump-shaped distribution across the flow scales, whose peak increases with the
wall distance. We find that the size growth effect is well accounted for
through an effective length scale accounting for the local friction velocity
and for the local mean shear.
