  Results of numerical simulations obtained by a staggered finite difference
scheme together with an efficient immersed boundary method are presented to
understand the effects of the shape of three-dimensional obstacles on the
transition of a boundary layer from a laminar to a turbulent regime. Fully
resolved Direct Numerical Simulations (DNS), highlight that the closer to the
obstacle the symmetry is disrupted the smaller is the transitional Reynolds
number. It has been also found that the transition can not be related to the
critical roughness Reynolds number used in the past. The simulations highlight
the differences between wake and inflectional instabilities, proving that
two-dimensional tripping devices are more efficient in promoting the
transition. Simulations at high Reynolds number demonstrate that the
reproduction of a real experiment with a solid obstacle at the inlet is an
efficient tool to generate numerical data bases for understanding the physics
of boundary layers. The quality of the numerical method to fully resolve the
small scales, that is one ingredient for a DNS was shown by a comparison of the
exponential range of the velocity spectra, in Kolmogorov units, with those for
isotropic turbulence. The good comparison reinforces the idea of local isotropy
at the smallest scales.
