Transition Measurements at High Reynolds Numbers using Phenol-Formaldehyde Resin

When excited with light in an appropriate wavelength range, certain Phenol-Formaldehyde Resins (PFRs) also emit light, which is at a wavelength larger than the excitation wavelength, and which intensity depends on the temperature. These properties are analogous to those of luminophores used in temperature-sensitive paints, but with the obvious difference that the material itself is temperature-sensitive. As a first step towards transition measurements using wind-tunnel models fully made of PFR, the current study presents an application of PFR as temperature-sensitive material for a component of a two-dimensional flat-plate model, which was previously used for the examination of the impact of surface bumps on transition. A PFR insert was machined to reproduce one of the bump configurations studied in earlier work and mounted on the model in the place of the corresponding insert. The model with the PFR insert was investigated in the Cryogenic Ludwieg-Tube Göttingen at freestream Mach numbers up to 0.77, Reynolds numbers (based on the model chord length c = 0.2 m) up to 10 million, and various streamwise pressure gradients, implemented by varying the model angle-of-attack. The focus of these investigations was on the detection of bump-induced transition on the PFR insert.