100 kHz krypton-based flow tagging velocimetry in a high-speed flow

Long-lived nitric oxide molecular tagging velocimetry with 1 + 1 REMPI

The successful demonstration of long-lived nitric oxide (NO) fluorescence for molecular tagging velocimetry (MTV) measurements is described in this Letter. Using 1 + 1 resonance-enhanced multiphoton ionization (REMPI) of NO at a wavelength near 226 nm, targeting the overlapping Q1(7) and Q21(7) lines of the A-X (0, 0) electronic system, the lifetime of the NO MTV signal was observed to be approximately 8.6 µs within a 100-Torr cell containing 2% NO in nitrogen. This is in stark contrast to the commonly reported single photon NO fluorescence, which has a much shorter calculated lifetime of approximately 43 ns at this pressure and NO volume fraction. While the shorter lifetime fluorescence can be useful for molecular tagging velocimetry with single laser excitation within very high-speed flows at some thermodynamic conditions, the longer lived fluorescence shows the potential for an order of magnitude more accurate and precise velocimetry, particularly within lower speed regions of hypersonic flow fields such as wakes and boundary layers. The physical mechanism responsible for the generation of this long-lived signal is detailed. Furthermore, the effectiveness of this technique is showcased in a high-speed jet flow, where it is employed for precise flow velocity measurements.

Multiline molecular tagging velocimetry of nitric oxide at 100 kHz using an injection-seeded burst-mode OPO

An injection-seeded, burst-mode optical parametric oscillator (OPO) operating at a repetition rate of 100 kHz is used to demonstrate the multiline molecular tagging velocimetry of an underexpanded jet using nitric oxide fluorescence. The very narrow linewidth of the OPO system, along with the relatively high pulse energies of the burst-mode system, enables efficient single-photon excitation of nitric oxide along multiple laser beam lines at a high repetition rate. Simultaneous one-dimensional velocity profile measurements were obtained of an underexpanded jet system at six different locations using a reference initial image and single-shot delayed images. A methodology for calculating the uncertainty of single-shot velocity is also described. Mean and root-mean-square velocity profiles are obtained at multiple locations simultaneously over a sampling time of 1 ms. The high-repetition-rate velocity measurements also appear to capture the onset of velocity oscillations and has the potential to reveal velocity frequency content occurring in the tens of kHz. The demonstrated velocimetry technique could be paired with other emerging burst-mode laser capabilities for a quantitative multiparameter gas property or multicomponent gas velocity measurements for supersonic and hypersonic flows, especially within ground test facilities that are limited to very short run durations.

Burst-mode velocimetry of hypersonic flow by nitric oxide ionization induced flow tagging and imaging

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.

Quenching measurements of Kr 5p[3/2]2 ← ← 4p6 1S0 electronic transition using absorption spectra

Quenching rate is an important parameter to include in fluorescence measurements that are aimed at quantifying the thermochemical field of a reacting flow. Traditionally, the quenching measurements were obtained at low pressures using the direct measurements of quenching times followed by a linear scaling to the desired pressure. This approach, however, cannot account for the possible deviation from the linear pressure scaling at elevated pressures due to three and multi-body collisions. Furthermore, the best accuracy on the quenching rate is obtained with ultra-short pulse lasers that are typically not readily available. This study offsets these limitations by demonstrating a new approach for making direct quenching measurements at atmospheric conditions and using nanosecond lasers. The quenching measurements are demonstrated in a krypton-perturber system, and the 5p[32]₂←←4p ^{6 1} S ₀ two-photon electronic transition is accessed. A theoretical construct is presented that relates the absorption spectral parameters and the integrated fluorescence signal to the quenching rate, referenced to a given species and conditions. Using this formulation, the relative quenching rates for different perturber species, namely, air, C H ₄, C ₂ H ₄, and C O ₂, are reported as measured at 1 atm and 300 K. As such, the present technique is limited to the measurement of the relative quenching rate, unlike the previous studies where absolute quenching rates are measured. Nonetheless, when the reference quenching rate is independently measured, the relative quenching rates can be converted to absolute values.

Recent progress in high-speed laser diagnostics for hypersonic flows [Invited]

The recent progress in high-speed (≥100k H z) laser diagnostics for hypersonic flows is reviewed. Owing to the ultrahigh flow speed, a laser frequency of 100 kHz or higher is required for hypersonic diagnostics. Here, two main laser diagnostic techniques are discussed: focused laser differential interferometry (FLDI) and pulse-burst laser-based diagnostics. Single- and multiple-point FLDI measurements have been widely applied to hypersonic flows for flow velocity and density fluctuation measurements. The progress of pulse-burst laser-based hypersonic diagnostics, including flow velocity measurements and 2D flow visualization, is also discussed.

Mach 18 flow velocimetry with 100-kHz KTV and PLEET in AEDC Tunnel 9

Krypton Tagging Velocimetry (KTV) and Picosecond Laser Electronic Excitation Tagging (PLEET) velocimetry at a 100-kHz rate were demonstrated in Mach 18 flow conditions at the Arnold Engineering Development Center (AEDC) Tunnel 9 employing a burst-mode laser system and a custom optical parametric oscillator (OPO). The measured freestream flow velocities from both KTV and PLEET agreed well with the theoretical calculation. The increase in repetition rate provides better capability to perform time-resolved velocimetry measurements in hypersonic flow environments.

High-repetition-rate krypton tagging velocimetry in Mach-6 hypersonic flows

A 100 kHz krypton (Kr) tagging velocimetry (KTV) technique was demonstrated in a Mach-6 Ludwieg tube using a burst-mode laser-pumped optical parametric oscillator system. The single-beam KTV scheme at 212 nm produced an insufficient signal in this large hypersonic wind tunnel because of its low Kr seeding (≤5%), low static pressure (∼2.5torr), and long working distance (∼1m). To overcome these issues, a new scheme using two excitation beams was developed to enhance KTV performance. A 355 nm laser beam was combined with the 212 nm beam to promote efficient two-photon Kr excitation at 212 nm, and increase the probability of 2 + 1 resonant-enhanced multiphoton ionization by adding a 355 nm beam. A signal enhancement of approximately six times was obtained. Using this two-excitation beam approach, strong long-lasting KTV was successfully demonstrated at a 100 kHz repetition rate in a Mach-6 flow.

10 kHz laser-induced schliere anemometry for velocity, Mach number, and static temperature measurements in supersonic flows

A new, to the best of our knowledge, technique for measuring velocity and Mach number in freestream flow is discussed and demonstrated. The technique, laser-induced schliere anemometry, uses a laser to write a laser-induced schliere in the flow, which can then be imaged using high-speed schlieren imaging. Here, we use a laser-induced plasma from the focusing of nanosecond-duration laser pulses from a pulse burst laser to write the disturbance. The resulting localized index of refraction gradient left from the plasma is tracked well beyond the plasma emission lifetime using schlieren imaging, and velocity is found from tracking or through a simple correlation analysis. The blast wave is also used to independently determine the Mach number via the Mach cone effect, which provides information about the mean static temperature. This technique shows great potential for use in characterizing freestream flow in supersonic facilities and is demonstrated here in a Mach 2 blowdown facility and a Mach 4 Ludwieg tube.