  We propose a natural stretching function for DNS of wall-bounded flows, which
blends uniform near-wall spacing with uniform resolution in terms of Kolmogorov
units in the outer wall layer. Numerical simulations of pipe flow are used to
educe optimal value of the blending parameter and of the wall grid spacing
which guarantee accuracy and computational efficiency as a results of
maximization of the allowed time step. Conclusions are supported by DNS carried
out at sufficiently high Reynolds number that a near logarithmic layer is the
mean velocity profile is present. Given a target Reynolds number, we provide a
definite prescription for the number of grid points and grid clustering needed
to achieve accurate results with optimal exploitation of resources.
