Vibrationally Excited Micro-Hydroxyl Tagging Velocimetry In Wall-Bounded Turbulent Flows*

In an attempt to improve the signal-to-noise ratio (SNR) and effective resolution in the Hydroxyl Tagging Velocimetry (HTV) technique previously deployed in a low-speed anechoic wind tunnel, several upgrades have been implemented. The scope of the current report is to discuss the relevant progress. A micro-lens array (MLA) is introduced into the tracer-creating beam path to create multiple beamlets, which provides several benefits: it helps achieve narrower beamlets, which is especially useful in the viscous sublayer; reduces the energy density in the beam transmitting through the delivery window; and, provides more flow information through multi-line tagging. A vibrationally mediated photodissociation (VMP) step, powered by an infrared laser, is introduced to increase the tracer number density, significantly increasing the signal strength. Finally, a low-noise qCMOS camera is added to the setup, replacing the existing intensified imaging system and thus removing the noise introduced by the image intensifier. The combined effect of the aforementioned upgrades results in high-resolution and high-SNR data within the viscous sublayer region.