Investigation of boundary layer transition at high Reynolds numbers using time-resolved temperature-sensitive paint

Kurzfassung

Understanding the fundamental principles of the processes that lead to laminar-turbulent transition on aerodynamic surfaces is of significant importance to correctly describe, model and perhaps ultimately influence the transition region. The understanding is still limited for many relevant flow conditions due to the challenges involved in both experimental and numerical studies. This thesis studies the phenomena and processes (i.e. specifically turbulent spots, intermittency distribution and turbulent wedges) of laminar-turbulent transition of a quasi-two-dimensional boundary layer over a flat plate in a compressible subsonic flow at high Reynolds numbers for which experimental data is scarce, and uses the specially developed time-resolved temperature-sensitive paint (iTSP) measurement technique. Experiments were conducted in the Cryogenic Ludwieg-Tube Göttingen at ambient temperatures with various streamwise pressure gradients, freestream Mach numbers ranging 0.3 ≤ M ≤ 0.8, and local Reynolds numbers 1 Million ≤ Rex ≤ 6.5 Million with high temporal (20-100 kHz) and spatial (17 px/mm) resolution. A Ru(phen)-based iTSP was investigated for its suitability, characterised to enable a quantitative surface temperature and heat flux determination, and applied for the first time under these investigated flow conditions. The emphasis was laid on systematically investigating turbulent spots for independently varied test conditions. The characteristic streamwise pointing triangle shape of the turbulent spot with its rounded wing tip region could be identified for all flow conditions. Additionally, the leading and trailing edge celerities, spreading and opening angles, and resulting turbulent spot propagation parameters were determined. While turbulent spots were observed to grow unaffected when merging laterally with other turbulent spots or turbulent wedges, the leading edge celerities of trailing spots were found to be considerably slower if trailing within about one spot length of another spot. A direct intermittency determination was possible when considering the surface heat flux computed from the time-resolved iTSP data. There was good agreement between transition location, defined at 50% intermittency, with commonly used definitions for stationary surface temperature measurements and the intermittency distribution in the transition region with models from literature. Turbulent wedges were found to consist of a fully turbulent core and intermittent outer region, although the latter was smaller than those found in other investigations. Intermittent turbulent wedges could be resolved and were shown to be a series of individual turbulent spots. For the first time, unsteady laminar-turbulent transition phenomena could be temporally resolved and characterised with an image based thermographic surface measurement technique under the investigated flow conditions. These findings are an essential aid to understanding and modelling the transition region for compressible high Reynolds number flow conditions and to demonstrate the capabilities of this new time-resolved iTSP technique to help gain a new understanding of instationary boundary layer transition phenomena.