  Well-resolved direct numerical simulations (DNSs) have been performed of the
flow in a smooth circular pipe of radius $R$ and axial length $10\pi R$ at
friction Reynolds numbers up to $Re_\tau=5200$. Various turbulence statistics
are documented and compared with other DNS and experimental data in pipes as
well as channels.Small but distinct differences between various datasets are
identified. The friction factor $\lambda$ overshoots by $2\%$ and undershoots
by $0.6\%$ of the Prandtl friction law at low and high $Re$ ranges,
respectively. In addition, $\lambda$ in our results is slightly higher than
that in Pirozzoli et al. (J. Fluid. Mech., 926, A28, 2021), but matches well
with the experiments in Furuichi et al. (Phys. Fluids, 27, 095108, 2015). The
log-law indicator function, which is nearly indistinguishable between the pipe
and channel flows up to $y^+=250$, has not yet developed a plateau further away
from the wall in the pipes even for the $Re_\tau=5200$ cases. The wall shear
stress fluctuations and the inner peak of the axial velocity intensity -- which
grow monotonically with $Re_\tau$ -- are lower in the pipe than in the channel,
but the difference decreases with increasing $Re_\tau$. While the wall values
are slightly lower in channel than pipe flows at the same $Re_\tau$, the inner
peaks of the pressure fluctuations show negligible differences between them.
The Reynolds number scaling of all these quantities agrees with both the
logarithmic and defect power laws if the coefficients are properly chosen. The
one-dimensional spectrum of the axial velocity fluctuation exhibits a $k^{-1}$
dependence at an intermediate distance from the wall -- as also seen in the
channel flow. In summary, this high-fidelity data enable us to provide better
insights into the flow physics in the pipes and the similarity/difference among
different types of wall turbulence.
