Turbulent boundary layers: issues in Experiments

Turbulent boundary layers, appearing on solid surfaces of bodies submerged in fluids are among the canonical, wall-bounded, turbulent flows together with channel and pipe flows that have been the focus of experimental and analytical investigations for almost a century. The relevance of a thorough theoretical and practical understanding cannot be underestimated for industrial application in engineering and aeronautical application. Still there are several unresolved issues even related to fairly basic mechanisms, such as the variation of turbulent stresses with Reynolds number, scaling of spectra, what are the differences (if any) between confined flows (channel and pipe) and open boundary layer flows, the interaction between outer and inner region, the properties of small-scale turbulence near the dissipation range etc. Recent accurate experimental studies, but also direct numerical simulations (DNS) together with innovative theoretical studies have made it possible to further penetrate the details of such flows. At NORDITA classical experimental techniques will be presented and discussed together with new future developments.