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.

Methods to improve burst-mode laser spectral purity for high-speed gas-phase filtered Rayleigh scattering.

In the filtered Rayleigh scattering (FRS) technique, Doppler or homogeneously broadened light from weak molecular scattering is separated from orders-of-magnitude stronger elastic scattering from surfaces, windows, particles, and/or droplets using a narrowband filter. In this work, high-speed detection of such weak molecular scattering is enabled by a burst-mode laser system that can achieve a spectral purity of ∼0.999999. This allows for an additional two orders of magnitude of attenuation from a narrowband iodine molecular filter for high-speed detection of gas-phase FRS in the presence of direct surface scattering at 532 nm. The methodology, system characterization, and feasibility of single-shot gas-phase FRS at 100 kHz or higher are presented and discussed.

Hybrid time-frequency domain dual-probe coherent anti-Stokes Raman scattering for simultaneous temperature and pressure measurements in compressible flows via spectral fitting.

We demonstrate a hybrid time-frequency spectroscopic method for simultaneous temperature/pressure measurements in nonreacting compressible flows with known gas composition. Hybrid femtosecond-picosecond, pure-rotational coherent anti-Stokes Raman scattering (CARS), with two independent, time-delayed probe pulses, is deployed for single-laser-shot measurements of temperature and pressure profiles along an ∼5-mm line. The theory of dual-probe CARS is presented, along with a discussion of the iterative fitting of experimental spectra. Temperature is obtained from spectra acquired with an early, near-collision-free probe time delay (τ 1=0p s) and pressure from spectra obtained at probe delays of τ 2=150-1000p s, where collisions significantly impact the spectral profile. Unique solutions for temperature and pressure are obtained by iteratively fitting the two spectra to account for small collisional effects observed for the near zero probe delay spectrum. A dual-probe pure-rotational CARS system, in a 1D line-imaging configuration, is developed to demonstrate effectively the simultaneous temperature and pressure profiles recorded along the axial centerline of a highly underexpanded jet. The underexpanded air jet permits evaluation of this hybrid time-frequency domain approach for temperature and pressure measurements across a wide range of low-temperature-low-pressure conditions of interest in supersonic ground-test facilities. Single-laser-shot measurement precisions in both quantities and pressure measurement accuracy are systematically evaluated in the quiet zone upstream of the Mach disk. Precise thermometry approaching 1%-2% is observed in regions of high CARS signal-to-noise ratios. Pressure measurements are optimized at probe time delays where the ratio of the late probe delay to the Raman lifetime exceeds four (τ 2/τ R>4). The impact of low-temperature Raman linewidths on CARS pressure measurements is evaluated, and comparisons of CARS pressures obtained with our recent low-temperature pure-rotational Raman linewidth data and extrapolated high-temperature Q-branch linewidths are presented. Considering all measurements with τ 2/τ R≥4.0, measured pressures were on average 7.9% of the computed isentropic values with average shot-to-shot deviations representing a combination of instrument noise and fluid fluctuations of  5.0%.

Design of a multispectral plenoptic camera and its application for pyrometry.

A multispectral imaging system, based on a modified plenoptic camera, is presented. By adding a color filter in the aperture plane of the imaging system, it is possible to simultaneously image multiple discrete colors of light-seven in this design. To develop a measurement system that does not rely on in situ calibrations, each of the optical elements was characterized a priori. For the camera sensor, measurements of the exposure linearity, exposure duration, and quantum efficiency were measured. Additionally, the transmission of the optical filters, both spectral and neutral density, as well as the signal attenuation of the filter holder itself were measured. These measurements result in an instrument that can quantitatively image the emission of seven discrete spectral bands simultaneously. An example application of pyrometry is presented where the emission of a blackbody calibration source with known temperature was imaged. It was determined that by fitting the measured emission at seven wavelengths to Planck's law of radiation, the temperature could be determined to a mean difference of 0.65ºC across five temperatures from 600° to 1000ºC when compared to the set-point temperature.

10 kHz two-color OH PLIF thermometry using a single burst-mode OPO.

10-kHz hydroxyl radical (OH) two-color planar laser-induced fluorescence (TC-PLIF) thermometry was demonstrated with a single burst-mode optical parametric oscillator (OPO) and a single camera. A fast, dual-wavelength switched seed laser enabled a high-energy, high-repetition-rate burst-mode laser to generate two 10-kHz pulse trains at wavelengths of ${\sim}{354.8}\;{\rm nm}$. The two pulse trains are colinear with 3 µs time interval between the pulse pairs. The injection-seeded OPO efficiently converts the burst-mode laser output to 285.62 and 285.67 nm to excite the ${Q}_2({12})$ and ${P}_1({8})$ OH transitions. PLIF images were collected from each of the two excitation transitions, and intensity ratios from the images were used to determine local temperatures. The development of fast, dual-wavelength switching, burst-mode OPO technology significantly reduces the experimental complexity of the high-speed TC-PLIF thermometry and simplifies its implementation in harsh combustion and flow test facilities.

100-kHz Interferometric Rayleigh Scattering for multi-parameter flow measurements.

Simultaneous multi-point multi-parameter flow measurement using Interferometric Rayleigh scattering (IRS) at 100-kHz repetition rate is demonstrated. Using a burst-mode laser and an un-intensified high-speed camera, interferograms are obtained that contain spatial, temporal and scattered light frequency information. The method of analysis of these interferograms to obtain simultaneous multi-point flow velocity and temperature measurements is described. These methods are demonstrated in a 100-kHz-rate study of a choked, under-expanded jet flow discharged by a convergent nozzle. Measurement results and uncertainties are discussed. The 100-kHz IRS technique with un-intensified imaging is applicable in large-scale wind tunnels for the study of unsteady and turbulent flows.

Rayleigh-scattering-based two-dimensional temperature measurement at 100-kHz frequency in a reacting flow.

Two-dimensional, Rayleigh-scattering-based temperature measurements utilizing a turbulent jet flame were performed in this study at 100-kHz frequency. This tenfold increase in measurement speed-compared to the 10-kHz frequency considered previously-facilitated identification and tracking of several highly dynamic flow features. Findings of this study demonstrate that flow-feature dynamics become uncorrelated qualitatively and quantitatively prior to an elapse of 100 µs between successive measurements, thereby necessitating the temperature-measurement frequency to exceed 10 kHz. At the proposed 100-kHz measurement frequency, resolution of the Taylor microscale and integral scales have been demonstrated in both space and time for this flow.

Simultaneous measurements of mixture fraction and flow velocity using 100 kHz 2D Rayleigh scattering imaging.

Two-dimensional (2D) Rayleigh scattering (RS) imaging at an ultrahigh repetition rate of 100 kHz is demonstrated in non-reacting flows employing a high-energy burst-mode laser system. Image sequences of flow mixture fraction were directly derived from high-speed RS images. Additionally, a 2D instantaneous flow velocity field at 100 kHz was obtained through optical-flow-based analysis of the RS images. In further analysis of both the mixture fraction and flow velocity field, the result for the centerline mixture fraction agreed well with the scaling law. The demonstrated high-speed RS technique in conjunction with optical-flow-based analysis provides non-intrusive, simultaneous measurements of the flow mixing and velocity field, extending the measurement capability of the RS technique to high-speed non-reacting and reacting flows.

Femtosecond laser tagging in R134a with trace quantities of air.

Femtosecond laser tagging is demonstrated for the first time in R134a (1,1,1,2-Tetrafluoroethane) gas, and in mixtures of R134a with small quantities of air. A systematic study of this tagging method is explored through the adjustment of gas pressure, mixture ratio and laser properties. It is found that the signal strength and lifetime are greatest at low pressures for excitation at both the 400 nm and 800 nm laser wavelengths. The relative intensities of two spectral peaks in the near-UV emission change as a function of gas pressure and can potentially be used for local pressure measurements. Single shot precision in pure R134a and R134a with 5% air is demonstrated in quiescent gas and at the exit of a subsonic pipe flow. One standard deviation (68%) of the uncertainty lies within 5 m/s of the mean velocity in a low pressure quiescent flow using a delay time of 3µs, and 18 m/s in a 230 m/s flow using a delay of 5 µs. The parameter space of these results are chosen to mimic conditions used in the NASA Langley Research Center's Transonic Dynamics Tunnel. The precision and signal lifetime demonstrate the feasibility of using this technique for measuring flowfields that induce airfoil flutter.

High-repetition-rate interferometric Rayleigh scattering for flow-velocity measurements.

High-repetition-rate interferometric-Rayleigh-scattering (IRS) velocimetry is implemented and demonstrated for non-intrusive, high-speed flow-velocity measurements. High temporal resolution is obtained with a quasi-continuous burst-mode laser that is capable of providing bursts of 10-msec duration with pulse widths of 10-100 nsec, pulse energy > 100 mJ at 532 nm, and repetition rates of 10-100 kHz. Coupled with a high-speed camera system, the IRS method is based on imaging the flow field though an etalon with 8-GHz free spectral range and capturing the Doppler shift of the Rayleigh-scattered light from the flow at multiple points having constructive interference. The seed-laser linewidth permits delivery of a laser linewidth of < 150 MHz at 532 nm The technique is demonstrated in a high-speed jet, and high-repetition-rate image sequences are shown.

Fiber-coupled ultrashort-pulse-laser-based electronic-excitation tagging velocimetry.

Transmission of intense ultrashort laser pulses through hollow-core fibers (HCFs) is investigated for molecular-tagging velocimetry. A low-vacuumed HCF beam-delivery system is developed to transmit high-peak-power pulses. Vacuum pressure effects on transmission efficiency and nonlinear effects at the fiber output are studied for 100 ps and 100 fs laser beams. With a 0.1 bar vacuum in the fiber, transmission efficiency increases by ∼30%, while spectral broadening is reduced. A 1 m long, 1 mm core metal-dielectric-coated HCF can transmit ∼45 mJ/pulse and ∼2.9 mJ/pulse for 100 ps laser pulses (at 532 nm) and 100 fs laser pulses (at 810 nm), respectively. Proof-of-principle, single-laser-shot, fiber-coupled, ps and fs laser-based, nitrogen electronic-excitation tagging velocimetry is demonstrated in a free jet. Flow velocities are measured at 200 kHz to capture high-frequency flow events.

Development of a Premixed Combustion Capability for Dual-Mode Scramjet Experiments.

Hypersonic air-breathing engines rely on scramjet combustion processes, which involve high-speed, compressible, and highly turbulent reacting flows. The combustion environment and the turbulent flames at the heart of these engines are difficult to simulate and study in the laboratory under well controlled conditions. Typically, wind-tunnel testing is performed that more closely approximates engine development rather than a careful investigation of the underlying physics that drives the combustion process. The experiments described in this paper, along with companion data sets, aim to isolate the chemical kinetic effects and turbulence-chemistry interaction from the fuel-air mixing process in a dual-mode scramjet combustion environment. A unique fuel injection approach is adopted that produces a uniform fuel-air mixture at the entrance to the combustor and results in premixed combustion. This approach relies on the mixing enhancement of a precombustion shock train upstream of the dual-mode scramjet's combustor. For the first time a stable flame, anchored on a cavity flameholder, is reported for a scramjet combustor operating in premixed fuel-air mode. The new experimental capability has enabled numerous companion studies involving advanced diagnostics such as coherent anti-Stokes Raman scattering (CARS), particle image velocimetry (PIV), and planar laser induced fluorescence (PLIF).

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging.

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.

Mixture-fraction measurements with femtosecond-laser electronic-excitation tagging.

Tracer-free mixture-fraction measurements were demonstrated in a jet using femtosecond-laser electronic-excitation tagging. Measurements were conducted across a turbulent jet at several downstream locations both in a pure-nitrogen jet exiting into an air-nitrogen mixture and in a jet containing an air-nitrogen mixture exiting into pure nitrogen. The signal was calibrated with known concentrations of oxygen in nitrogen. The spatial resolution of the measurement was ∼180 µm. The measurement uncertainty ranged from 5% to 15%, depending on the mixture fraction and location within the beam, under constant temperature and pressure conditions. The measurements agree with a mixture fraction of unity within the potential core of the jet and transition to the self-similar region.

Unseeded Velocity Measurements Around a Transonic Airfoil Using Femtosecond-Laser Tagging.

Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used to study the flowfield around a symmetric, transonic airfoil in the NASA Langley 0.3-m TCT facility. A nominal Mach number of 0.85 was investigated with a total pressure of 125 kPa and total temperature of 280 K. Two-components of velocity were measured along vertical profiles at different locations above, below, and aft of the airfoil at angles of attack of 0°, 3.5°, and 7°. Velocity profiles within the wake showed sufficient accuracy, precision, and sensitivity to resolve both the mean and fluctuating velocities and general flow physics such as shear layer growth. Evidence of flow separation is found at high angles of attack. Velocity measurements were assessed for their accuracy, precision, dynamic range, spatial resolution, and overall measurement uncertainty as they relate to the present experiments. Measurement precisions as low as 1 m/s were observed, while the velocity dynamic range was found to be nearly a factor of 500. The spatial resolution of between 1 mm and 5 mm was found to be primarily limited by the FLEET spot size and advection of the flow. Overall measurement uncertainties ranged from 3 to 4 percent.

Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging velocimetry.

Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging (STARFLEET), a nonseeded ultrafast-laser-based velocimetry technique, is demonstrated in reactive and nonreactive flows. STARFLEET is pumped via a two-photon resonance in N2 using 202.25 nm 100 fs light. STARFLEET greatly reduces the per-pulse energy required (30 µJ/pulse) to generate the signature FLEET emission compared to the conventional FLEET technique (1.1 mJ/pulse). This reduction in laser energy results in less energy deposited in the flow, which allows for reduced flow perturbations (reactive and nonreactive), increased thermometric accuracy, and less severe damage to materials. Velocity measurements conducted in a free jet of N2 and in a premixed flame show good agreement with theoretical velocities, and further demonstrate the significantly less intrusive nature of STARFLEET.

Pressure measurements using hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering.

We investigate the feasibility of gas-phase pressure measurements using fs/ps rotational CARS. Femtosecond pump and Stokes pulses impulsively prepare a rotational Raman coherence, which is probed by a high-energy 5-ps pulse introduced at a time delay from the Raman preparation. These ultrafast laser pulses are shorter than collisional-dephasing time scales, enabling a new hybrid time- and frequency-domain detection scheme for pressure. Single-laser-shot rotational CARS spectra were recorded from N2 contained in a room-temperature gas cell for pressures from 0.4 to 3 atm and probe delays ranging from 16 to 298 ps. Sensitivity of the accuracy and precision of the pressure data to probe delay was investigated. The technique exhibits superior precision and comparable accuracy to previous laser-diagnostic pressure measurements.

Laser velocimetry with fluorescent dye-doped polystyrene microspheres.

Simultaneous Mie scattering and laser-induced fluorescence (LIF) signals are obtained from individual polystyrene latex microspheres dispersed in an air flow. Microspheres less than 1 µm mean diameter were doped with two organic fluorescent dyes, Rhodamine B (RhB) and dichlorofluorescein (DCF), intended either to provide improved particle-based flow velocimetry in the vicinity of surfaces or to provide scalar flow information (e.g., marking one of two fluid streams). Both dyes exhibit measureable fluorescence signals that are on the order of 10(-3) to 10(-4) times weaker than the simultaneously measured Mie signals. It is determined that at the conditions measured, 95.5% of RhB LIF signals and 32.2% of DCF signals provide valid laser-Doppler velocimetry measurements compared with the Mie scattering validation rate with 6.5 W of 532 nm excitation, while RhB excited with 1.0 W incident laser power still exhibits 95.4% valid velocimetry signals from the LIF channel. The results suggest that the method is applicable to wind tunnel measurements near walls where laser flare can be a limiting factor and monodisperse particles are essential.

Development of a dual-pump coherent anti-Stokes Raman spectroscopy system for measurements in supersonic combustion.

This work describes the development of a dual-pump coherent anti-Stokes Raman spectroscopy system for simultaneous measurements of the temperature and the absolute mole fraction of N2, O2, and H2 in supersonic combusting flows. Changes to the experimental setup and the data analysis to improve the quality of the measurements in this turbulent, high-temperature reacting flow are described. The accuracy and precision of the instrument have been determined using data collected in a Hencken burner flame. For temperatures above 800 K, errors in the absolute mole fraction are within 1.5%, 0.5%, and 1% of the total composition for N2, O2, and H2, respectively. Standard deviations based on 500 single shots are between 10 and 65 K for the temperature, between 0.5% and 1.7% of the total composition for O2, and between 1.5% and 3.4% for N2. The standard deviation of H2 is ~10% of the average measured mole fraction.