Near-wall measurements with high-spatial resolution Hydroxyl Tagging Velocimetry
Open Access DepositedFlow measurement techniques have been pivotal in advancing our understanding of fluid-structure interactions, wall-bounded flows, and the evolution of boundary layers, among many other crucial topics. A plethora of techniques were developed in the 20th century, particularly after World War II. Non-intrusive, time-of-flight optics-based techniques have earned a strong reputation due to the reliability of the data they provide. While Particle Image Velocimetry (PIV) has proven suitable and trustworthy for many applications, it has not been preferred for investigations of large test models in anechoic flow wind tunnels, especially under high-Reynolds-number (over 500,000) flow conditions, for compelling reasons. To address the limitations that PIV demonstrates in these settings, Molecular Tagging Velocimetry (MTV) presents promising features. Hydroxyl Tagging Velocimetry (HTV), a subset of MTV, is a seedless technique that relies on hydroxyl (OH) radicals generated from water molecules present in the air. Although setting up an HTV test bench requires significant precision, HTV overcomes all the shortcomings of PIV for investigating near-wall high-Reynolds-number flow over large test objects in anechoic flow facilities. The current work was initiated to perform aerodynamic measurements of near-wall turbulence around a NACA 0019-94 airfoil placed inside the Anechoic Flow Facility (AFF) at the Naval Surface Warfare Center, Carderock Division (NSWCCD), Bethesda, MD, USA. Data was recorded at 0.263c location for 10, 20 and 30 m/s flow speed (1.5*10^6 ≤ Re ≤ 4.6*10^6; M < 0.1). The campaign was successful and yielded some of the first-of-a-kind single-line near-wall HTV datasets of high-Reynolds-number flow. However, the OH fluorescence signal in HTV is weak, introducing uncertainty in the interpretation of near-wall data. A rigorous analytical review of the overall approach revealed significant opportunities for improvement. Subsequently, several optical upgrades enhanced the overall performance of the scheme: (a) changing the delivery window material reduced unwanted wall fluorescence near and at the wall, (b) using a microlens array produced thinner tracing beamlets and allowed multiline data acquisition, (c) incorporating microscopy improved the signal-to-noise ratio (SNR) and spatial resolution, (d) replacing the microscopic attachment by a tandem lens system further improved the SNR significantly. These advancements ultimately enabled data processing near and/or, at wall, providing valuable turbulence statistics such as velocity profiles, velocity fluctuations, boundary layer properties and wall shear stress.
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