Investigation of turbulent wedge intermittency with time-resolved temperature-sensitive paint

Kurzfassung

Turbulent wedges and turbulent spots play a significant role in the process of laminar-turbulent transition. In most cases, turbulent wedges are quasi-stationary in the sense that their core is fully turbulent, bounded by an intermittent region. For some flow conditions, however, intermittent turbulent wedges occur characterized by reduced intermittency values and heat transfer rates compared to classical, fully turbulent wedges. They were first described by Clark et al. in a low-speed flow and termed transitional wedge. In this study, turbulent wedges and turbulent spots were investigated experimentally at large Reynolds numbers (local Reynolds numbers up to Re_x=5.2*10^6), Mach numbers M from 0.5 to 0.8 and various stream-wise pressure gradients in the Cryogenic Ludwieg-Tube of DLR Göttingen. The examined wind-tunnel model was the two-dimensional flat plate "PaLASTra", which was designed to have a large area of uniform pressure gradient on the surface of interest - the upper side of the model. Time-resolved temperature-sensitive paint enabled the determination of global temperature measurements at camera frame rates of 20 kHz, which were further evaluated to obtain instantaneous heat-flux distributions and intermittency maps. These were used to quantitatively analyze turbulent wedges and turbulent spots. For the case of fully turbulent wedges, the intermittent region bounding the turbulent core was quantified for the first time using a thermographic measurement technique at these conditions. The spreading angle of the intermittent region (10.02 +- 0.09°) was found to be similar to those of individual turbulent spots (10.2+-0.4°), demonstrating the similarity of the mechanism of spanwise growth for turbulent wedges and turbulent spots. Additionally an intermittent turbulent wedge was identified. By determining the time-resolved heat flux distribution, it was possible to demonstrate that intermittent turbulent wedges comprise individual turbulent spots. Due to the growth of turbulent spots in both spanwise and streamwise direction, the intermittency values were found to increase in the downstream direction. Furthermore it was possible to detect the merging of individual turbulent spot. Detailed results of the time-resolved heat-flux and intermittency distribution of the turbulent wedges will be provided in the final contribution.