Towards Hydroxyl Tagging Velocimetry Integrated With Fourier Integral Microscopy Of High-Speed Flow Around A Modified NACA Airfoil*

As a first-of-its-kind experimental campaign in a high-speed (up to Re = 4,600,000 based on the chord) anechoic wind tunnel over a modified NACA 0019-94 airfoil, Hydroxyl tagging velocimetry (HTV) was deployed in June 2022. In a nutshell, a two-laser system was used to create ('write' step) a continuous line of tracers and subsequently 'read' them. An excimer laser operating at 193 nm was used to 'write' the taglines, afterwards a tunable dye laser 'read' those lines at 281.905 nm and a sCMOS camera equipped with an image intensifier captured the flow phenomena at 308 nm. The 'write'/'read' time interval was optimized between 30 μs to 160 μs for the three different speeds studied: 10, 20 and 30 m/s. However, some improvement opportunities were identified to enhance the overall measurement technique. One of them is employing a low-glare laser delivery window for the betterment of post-processing of the images especially in the viscous sublayer region. Another major change is a better optical assembly to achieve better resolution. On top of this, integration of a Fourier integral microscopy (FIMic) approach with the HTV technique is underway which will allow us to extract 3D flow information in micro-scale from 2D data.