Hydroxyl tagging velocimetry (HTV) in experimental air flows

The new nonintrusive instantaneous molecular flow tagging method, hydroxyl tagging velocimetry (HTV), previously demonstrated only for high-temperature reacting flows, is now demonstrated in low-temperature (300 K) ambient air flowfields. Single-photon photodissociation of ground-state H₂O by a ∼ 193-nm ArF excimer laser 'writes' very long grid lines (> 50 mm) of superequilibrium OH and H photoproducts in a room air flowfield due to the presence of ambient H₂O vapor. After displacement, the positions of the OH tag lines are revealed through fluorescence caused by A² Σ⁺ (v′ = 0) ← X² Π_i (v″ = 0) OH excitation using a pulsed frequency-doubled dye laser with an operating output wavelength of ∼ 308 nm. The dye 'read' laser accesses the strong Q₁(1) line, compensating for the relatively weak 193-nm absorption of room-temperature H₂O. The weak absorption of ground vibrational state H₂O has previously precluded the use of HTV at low temperatures, since previous HTV systems relied on a KrF excimer 'read' laser that could only access a weak (3 ← 0) OH transition. The instantaneous velocity field is determined by time-of-flight analysis. HTV tag lifetime comparisons between experimental results and theoretical predictions are discussed. Multiple-line tag grids are shown displaced due to an experimental air flowfield, thus providing 2-D multipoint velocity information. Due to the instantaneous nature of the HTV tag formation, HTV is particularly suitable for, but not limited to, a variety of fast flowfield applications including nonreacting base flows for high-speed projectiles and low-temperature hypersonic external or internal flows.