This Letter describes, to the best of our knowledge, a new approach to flow tagging, nitric oxide (NO) Ionization Induced Flow Tagging and Imaging (NiiFTI), and presents the first experimental demonstration for single-shot velocimetry in a near Mach 6 hypersonic flow at 250 kHz. The mean velocity of 860 m/s was measured with a single-shot standard deviation of as low as 3.4 m/s and mean velocity uncertainty of 5.5 m/s. NiiFTI is characterized by a long fluorescence lifetime of nitrogen with 1e decay of approximately 50 μs measured in air. The method relies on a single nanosecond laser combined with a high-speed camera, creating an opportunity for the utilization of a typical nitric oxide (NO) laser-induced fluorescence (LIF) experimental setup with minor modifications as well as pulse-burst lasers (PBLs) for ultrahigh repetition rates.

In this article, the authors present an experimental investigation of the thermochemical relaxation regions downstream of symmetric Mach reflections generated in Mach 8.5 flows with stagnation enthalpies from 7 to 10 MJ/kg. The experiment was performed in the Hypervelocity Expansion Tunnel at the National Aerothermochemistry and Hypersonics Laboratory of Texas A&M University. The investigation focused on studying the nitric oxide morphology in the vicinity of a strong normal shock wave and shear layers. Experimental planar laser-induced fluorescence measurements at 250 kHz repetition rate with a quasi-simultaneous natural emission photography demonstrate a sufficient rejection of natural emission and reveal intricate flow structures in the shock vicinity. Steady computational fluid dynamics (CFD) flow solutions obtained with the US3D software were used to simulate laser-induced fluorescence signal levels for quantitative comparison against the experimentally acquired data. The investigation accomplished four objectives: complex flowfield visualization in the vicinity of the Mach stem with 4 μs temporal resolution, evaluation of the level of high-temperature chemistry effects introduced to the freestream by the expansion tunnel, both qualitative and quantitative CFD simulation validation, and cross-checking or introduction of fundamental properties of nitric oxide laser-induced fluorescence including absorption cross section for vibrationally excited states of nitric oxide and a collisional quenching cross section of nitric oxide by atomic nitrogen.

This Letter describes the first, to the best of our knowledge, demonstration of a velocity measurement by nitric oxide ionization induced flow tagging and imaging (NiiFTI) of a high-enthalpy hypersonic flow utilizing naturally formed nitric oxide. The measurements were conducted in the hypervelocity expansion tunnel (HXT) at Texas A&M University in Mach 8.5 and Mach 10 flows near an ogive test article. The region of interest contained boundary layers (BL) and the hot flow region behind the oblique shock front. The BL profiles were recorded with the velocity ranging between 2516 m/s and 985 m/s measured as close as 0.5 mm to the model surface. Single-shot measurement in the core flow showed an average velocity of 2.5 km/s with a slight decelerating trend. Overall, NiiFTI demonstrated a good performance in high-enthalpy hypersonic flow, extending its usability to reacting environments.

This work presents the first, to the best of our knowledge, experimental demonstration of slow light imaging spectroscopy for thermometry of liquid water. This novel technique for measuring temperature relies on detecting the spectral shift of Brillouin peaks in water using the temporal delay through a cell containing an atomic vapor. Stand-off sensing capabilities are achieved by time-domain measurements of Brillouin scattering tuned to be near a rubidium atomic resonance and passed through a cell filled with rubidium vapor. An injection seeded optical parametric oscillator (OPO) is demonstrated to be a versatile light source for slow light imaging spectroscopy applications. The narrow OPO pulse spectrum allows for a precise profiling of slow light features of rubidium and accurate tracking of the temperature dependence of Brillouin scattering spectral shift. A comparison between the experimental data and numerical simulation over a temperature range of 20 to 99 degrees Celsius shows a good agreement for both qualitative and quantitative results.

Spontaneous Raman scattering, also known as common old ordinary Raman scattering (COORS), is revisited to evaluate its applicability for hypersonic flow characterization. Due to its very low cross section, Raman scattering is often considered unsuitable for measuring low-pressure gas properties that are found in ground test simulations of high-altitude hypersonic flights. Utilizing a recently developed one-dimensional (1D) light scattering technique with a volume Bragg grating filter and Stokes sideband windowing, we demonstrate 1D rotational Raman measurements of temperature and neutral gas density across a bow shock in front of a blunt wedge model under Mach 6 hypersonic flow. The experiment was conducted in the Hypervelocity Expansion Tunnel at Texas A&M University. The measurements were successfully obtained during a single run of the tunnel operation, capturing the temperature and density distributions with dynamic ranges of 200–2000 K and 5×10²³−4×10²⁴/m³, respectively, over both the free-stream and post-shock regions, covering approximately 10 mm in length with a spatial resolution of <0.5mm. Time-resolved high-speed measurement capability at 100 kHz was also demonstrated, showcasing the robustness of 1D COORS for gas diagnostics.

This article presents, to the best of our knowledge, the first demonstration of burst-mode nitric oxide (NO) planar laser-induced fluorescence measurement of a reacting environment via Nd:YAG 5th harmonic excitation. A tunable pulse-burst laser generated 1 ms bursts of resonant 213 nm radiation at 200 kHz, 500 kHz, and 1 MHz repetition rates with 16 mJ, 3.5 mJ, and 0.35 mJ average pulse energies, respectively. Nitric oxide, naturally formed in the vicinity of an oxy-acetylene flame, was targeted via three excitation schemes: γ(1, 0), γ(1, 0) + β(5, 1), and β(4, 1). The generated fluorescence was collected in two separate spectral windows to demonstrate the selectivity of the electronic excitation and explore new filtering approaches for high background luminosity environments: 240 nm–260 nm and 275 nm–375 nm. Overall, this new method of UV-light generation for NO excitation performed very well with a substantial amount of collected signal even with modest energy levels accessible at a 1 MHz rate. The primary advantage of utilizing the 5th harmonic to access NO resonances was its simplicity and robustness through direct burst mode laser up-conversion and simplified master oscillator, power amplified (MOPA) frequency control, obviating the need for an optical parametric oscillator.