Recent developments in scaling of wall-bounded flows

Proper scaling of a fluid flow permits convenient, dimensionless representation of experimental data, prediction of one flow based on a similar one, and extrapolation of low-Reynolds-number, laboratory-scale experiments to field conditions. This is a particularly powerful technique for turbulent flows where analytical solutions derived from first principles are not possible. We extend in this presentation our recent work on scaling of turbulent wall-bounded flows ({\it Prog.\ Aerospace Sciences} {\bf 42}, p.\ 419--467, 2007) with respect to the most topical developments. The actual research tendencies in scaling go more and more toward investigating boundary layers under the influence of pressure gradient and/or of wall-roughness. Additionally, some new ideas employing local Kolmogorov scales arose. All together four main groups of questions are formulated that hopefully will be answered by future research.