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

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.

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.

One-dimensional air temperature measurements by air resonance enhanced multiphoton Ionization thermometry (ART)

In this work, a detailed calibration study is performed to establish non-intrusive one-dimensional (1D) rovibrational temperature measurements in unseeded air, based on air resonance enhanced multiphoton ionization thermometry (ART). ART is generated by REMPI (resonance enhanced multi-photon ionization) of molecular oxygen and subsequent avalanche ionization of molecular nitrogen in a single laser pulse. ART signal, the fluorescence from the first negative band of molecular nitrogen, is directly proportional to the 2-photon transition of molecular oxygen C³Π (v = 2) ← X³Σ (v'=0), which is used to determine temperature. Experimentally, hyperfine structures of the O₂ rotational branches with high temperature sensitivity are selectively excited through a frequency-doubled dye laser. Electron-avalanche ionization of N₂ results in the fluorescence emissions from the first negative bands of N₂ ⁺ near 390, 425, and 430nm, which are captured as a 1D line by a gated intensified camera. Post processing of the N₂ ⁺ fluorescence yields a 1D thermometry line that is representative of the air temperature. It is demonstrated that the technique provides ART fluorescence of ∼5cm in length in the unseeded air, presenting an attractive thermometry solution for high-speed wind tunnels and other ground test facilities.

Resonance-enhanced, rare-gas-assisted femtosecond-laser electronic-excitation tagging in argon/nitrogen mixtures

Multiphoton-resonance enhancement of a rare-gas-assisted nitrogen femtosecond-laser electronic-excitation-tagging (FLEET) signal is demonstrated. The FLEET signal is ideal for velocimetric tracking of nitrogen gas in flow environments by virtue of its long-lived nature. By tuning to three-photon-resonant transitions of argon, energy can be more efficiently deposited into the mixture, thereby producing a stronger and longer-lived FLEET signal following subsequent efficient energy transfer from excited-state argon to the C (³Π_u) excited state of nitrogen. Such resonant excitation exhibits as much as an order of magnitude increase in this rare-gas-assisted FLEET signal, compared to near-resonance excitation of seeded argon demonstrated in previous work, while reducing the required input excitation-pulse energies by two orders of magnitude compared to traditional FLEET.

Three-component flow velocity measurements with stereoscopic picosecond laser electronic excitation tagging

Nonintrusive three-component (3C) velocity measurements of free jet flows were conducted by stereoscopic picosecond laser electronic excitation tagging (S-PLEET) at 100 kHz. The fundamental frequency of the burst-mode laser at 1064 nm was focused to generate the PLEET signal in a free jet flow. A stereoscopic imaging system was used to capture the PLEET signals. The 3C centroids of the PLEET signal were determined by utilizing simultaneous images from two cameras placed at an angle. The temporal evolutions of the centroids were obtained and used to determine the instantaneous, time-resolved 3C velocities of the flows. The free jets with various inlet pressures of 10-40 bars exhausting into atmospheric pressure air (i.e., underexpanded free jet with large pressure ratios; Reynolds numbers from the jet ranged from 39,000 to 145,000) were measured by S-PLEET. Key 3C turbulent properties of the free jets, including instantaneous and mean velocities, were obtained with an instantaneous measurement uncertainty of about ${\rm{\pm 10}}\;{\rm{m/s}}$, which is about 2% of the highest velocities measured. Computation of higher-order statistics including covariances related to turbulent kinetic energy and the Reynolds stress component was demonstrated. The 3C nonintrusive and unseeded velocimetry technique could provide a new tool for flow property measurements in ground test facilities; the measured high-frequency turbulence properties of free jet flows could be useful for turbulence modeling and validations.

Temporal characterization of heating in femtosecond laser filamentation with planar Rayleigh scattering

Temporal and spatial evolution of temperature in femtosecond laser filamentation is investigated using planar Rayleigh scattering combined with optical flow algorithm, the corresponding mechanism is analyzed. The temperature increases sharply with a characteristic time of 4.53μs and reach a maximum value of 418 K within 1∼10μs, then decreases slowly to around 300 K with a characteristic time of 136μs. While the temperature first diffuses rapidly in the radial direction and then diffuses very slowly, an obvious step is observed around 2μs. The mechanism of heat transfer is the result of energy exchange between electron and heavy particles and heat conduction. Within 1 ns to 10μs, molecules obtain energy continuously due to collision with electrons, which is much larger than the energy loss due to thermal conduction, leading to rise of gas temperature and the high-speed movement of the filament edges. After 10μs, thermal conduction becomes the dominant factor, resulting gas temperature decreasing and slower movement of the filament edges.

Freestream velocity-profile measurement in a large-scale, high-enthalpy reflected-shock tunnel

ABSTRACT

We apply Krypton Tagging Velocimetry (KTV) to measure velocity profiles in the freestream of a large, national-scale high-enthalpy facility, the T5 Reflected-Shock Tunnel at Caltech. The KTV scheme utilizes two-photon excitation at 216.67 nm with a pulsed dye laser, followed by re-excitation at 769.45 nm with a continuous laser diode. Results from a nine-shot experimental campaign are presented where N 2 and air gas mixtures are doped with krypton, denoted as 99% N 2 /1% Kr, and 75% N 2 /20% O 2 /5% Kr, respectively. Flow conditions were varied through much of the T5 parameter space (reservoir enthalpy h R ≈ 5 - 16  MJ/kg). We compare our experimental freestream velocity-profile measurements to reacting, Navier-Stokes nozzle calculations with success, to within the uncertainty of the experiment. Then, we discuss some of the limitations of the present measurement technique, including quenching effects and flow luminosity; and, we present an uncertainty estimate in the freestream velocity computations that arise from the experimentally derived inputs to the code.

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

Krypton (Kr)-based tagging velocimetry is demonstrated in a Kr/N₂ jet at 100 kHz repetition rate using a custom-built burst-mode laser and optical parametric oscillator (OPO) system. At this repetition rate, the wavelength-tunable, narrow linewidth laser platform can generate up to 7 mJ/pulse at resonant Kr two-photon-excitation wavelengths. Following a comprehensive study, we have identified the 212.56 nm two-photon-excitation transition as ideal for efficient Kr-based velocimetry, producing a long-lived (∼40µs) fluorescence signal from single-laser-pulse tagging that is readily amenable to velocity tracking without the need for a second "read" laser pulse. This long-lived fluorescence signal is found to emanate from N₂-rather than from Kr-following efficient energy transfer. Successful flow velocity tracking is demonstrated at multiple locations in a high-speed Kr/N₂ jet flow. The 100 kHz repetition rate provides the ability to perform time-resolved velocimetry measurements in high-speed and even hypersonic flow environments, where standard velocimetry approaches are insufficient to capture the relevant dynamics.

Multi-line FLEET by imaging periodic masks

A simple linear configuration for multi-line femtosecond laser electronic excitation tagging (FLEET) velocimetry is used for the first time, to the best of our knowledge, to image an overexpanded unsteady supersonic jet. The FLEET lines are spaced 0.5-1.0 mm apart, and up to six lines can be used simultaneously to visualize the flowfield. These lines are created using periodic masks, despite the mask blocking 25%-30% of the 10 mJ incident beam. Maps of mean single-component velocity in the direction along the principal flow axis, and turbulence intensity in that same direction, are created using multi-line FLEET, and computed velocities agree well with those obtained from single-line (traditional) FLEET. Compared to traditional FLEET, multi-line FLEET offers increased simultaneous spatial coverage and the ability to produce spatial correlations in the streamwise direction. This FLEET permutation is especially well suited for short-duration test facilities.

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

The applications of femtosecond lasers to the diagnostics of combustion and flow field have recently attracted increasing interest. Many novel spectroscopic methods have been developed in obtaining non-intrusive measurements of temperature, velocity, and species concentrations with unprecedented possibilities. In this paper, several applications of femtosecond-laser-based incoherent techniques in the field of combustion diagnostics were reviewed, including two-photon femtosecond laser-induced fluorescence (fs-TPLIF), femtosecond laser-induced breakdown spectroscopy (fs-LIBS), filament-induced nonlinear spectroscopy (FINS), femtosecond laser-induced plasma spectroscopy (FLIPS), femtosecond laser electronic excitation tagging velocimetry (FLEET), femtosecond laser-induced cyano chemiluminescence (FLICC), and filamentary anemometry using femtosecond laser-extended electric discharge (FALED). Furthermore, prospects of the femtosecond-laser-based combustion diagnostic techniques in the future were analyzed and discussed to provide a reference for the relevant researchers.

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.

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.

Temperature perturbation related to the invisible ink vibrationally excited nitric oxide monitoring (VENOM) technique: a simulation study

The limits of applicability of the invisible ink variant of the vibrationally excited nitric oxide monitoring (VENOM) technique for three distinct flow fields is reported in this work. This technique involves the generation of a grid of vibrationally excited NO (X,Π2) by exciting the NO A-X electronic transition at 226 nm, which subsequently relaxes via fluorescence and collisional quenching to produce vibrationally excited NO (X,Π2). This grid is then probed by two laser sheets tuned to distinct rotational states. The resulting images allow for the simultaneous measurement of temperature and velocity. The flow fields presented in this work provide a range of NO concentrations, vibrational lifetimes, pressures, temperatures, and collisional quenching, which explore the applicability of the invisible ink variant to a wide range of conditions. We have modelled the initial NO, O₂, and N₂ vibrational and rotational energy distribution resulting from the combination of fluorescence and quenching of electronically excited NO. The subsequent rethermalization of the sample, in particular the long-time vibrational relaxation, has been modelled using a forced harmonic oscillator model. The time-dependent temperature perturbation due to the invisible ink technique is evaluated for two distinct timescales: a short-timescale temperature rise resulting from collisional quenching and rotational/translational thermalization and a long-timescale temperature rise caused by vibrational thermalization. Under low pressures where fluorescence dominates quenching, there is minimal temperature perturbation of the flow field on the timescale of a VENOM measurement, and the short-timescale temperature perturbation only becomes significant at high NO seed concentrations. The predicted signal-to-noise ratio of the invisible ink method is unaffected for low-pressure, low-temperature flow fields. However, preserving signal-to-noise ratio for a high-temperature, high-pressure flow field could prove challenging due to the impact of quenching and self-absorption. Overall, we find that the invisible ink method is predicted to be a viable laser-based diagnostic for velocimetry and thermometry over a wide range of experimental conditions.

Unseeded Velocimetry in Nitrogen for High-Pressure, Cryogenic Wind Tunnels, Part 1: Femtosecond-Laser Tagging

Femtosecond laser electronic excitation tagging (FLEET) velocimetry is characterized for the first time at high-pressure, low-temperature conditions. FLEET signal intensity and signal lifetime data are examined for their thermodynamic dependences; temperatures range from 89 K to 275 K while pressures are varied from 85 kPa to 400 kPa. The FLEET signal intensity is found to scale linearly with the flow density. An inverse density dependence is observed in the FLEET signal lifetime data, with little independent sensitivity to the other thermodynamic conditions apparent. FLEET velocimetry is demonstrated in the NASA Langley 0.3-m Transonic Cryogenic Tunnel. Velocity measurements are made over the entire operational envelope: Mach numbers from 0.2 to 0.75, total (stagnation) temperatures from 100 K to 280 K, and total pressures from 100 kPa to 400 kPa. The velocity measurement accuracy is assessed over this domain of conditions. Measurement errors below 1.15 percent are typical, with slightly decreasing accuracy as temperatures are decreased. Assessment of the measurement precision finds a zero-velocity precision of 0.4 m/s. The precision is observed to have a weak temperature dependence as well, likely a result of the shorter lifetimes experienced at higher densities. The velocity dynamic range is found to have a nominal value of 650. Finally the spatial resolution of the measurements is found to be a dominated by the physical size of the FLEET signal and advective motion. The transverse spatial resolution is found to be 1 mm, while the streamwise spatial resolution is dependent on velocity with a minimum of 2 mm and a maximum of 3.3 mm.

Filamentary anemometry using femtosecond laser-extended electric discharge - FALED

We demonstrate a non-contact spatiotemporally resolved comprehensive method for gas flow velocity field measurement: Filamentary Anemometry using femtosecond Laser-extended Electric Discharge (FALED). A faint thin plasma channel was generated in ambient air by focusing an 800-nm laser beam of 45 fs, which was used to ignite a pulsed electric discharge between two electrodes separated over 10 mm. The power supplier provided a maximum voltage up to 5 kV and was operated at a burst mode with a current duration of less than 20 ns and a pulse-to-pulse separation of 40 μs. The laser-guided thin filamentary discharge plasma column was blowing up perpendicularly by an air jet placed beneath in-between the two electrodes. Although the discharge pulse was short, the conductivity of the plasma channel was observed to sustain much longer, so that a sequence of discharge filaments was generated as the plasma channel being blown up by the jet flow. The sequential bright thin discharge filaments can be photographed using a household camera to calculate the flow velocity distribution of the jet flow. For a direct comparison, a flow field measurement using FLEET [Appl. Opt. 50, 5158 (2011)] was also performed. The results indicate that the FALED technique can provide instantaneous nonintrusive flow field velocity measurement with good accuracy.

Simplified read schemes for krypton tagging velocimetry in N2 and air

The background and results for two simplified read schemes for krypton tagging velocimetry (KTV) are presented. The first scheme utilizes the excitation/re-excitation approach found in the literature but replaces the pulsed dye laser used for the re-excitation step with a continuous wave, narrowband laser diode. The second scheme is a single-laser setup with no read laser where the fluorescence of the tagged Kr is imaged at successive times. Results are presented and compared to historical data for experiments performed in 99%N₂/1% Kr and 95% air/5% Kr underexpanded jets. The approach with the laser diode has a higher signal, while the single-laser approach yields more consistent results. Both schemes maintain an SNR comparable to that in the literature, but with a simpler setup that enables future high-repetition rate KTV experiments.

Femtosecond laser tagging for velocimetry in argon and nitrogen gas mixtures

Tagging is demonstrated in argon and nitrogen gases using a femtosecond laser with pulse energies of approximately 70 μJ through a nonresonant ionization process at 267 nm. The signal fluorescence lifetime in pure argon and nitrogen-argon mixtures are measured and found to be long enough to make mean velocity and turbulence measurements in a subsonic flow. In pure argon, the dominating processes involve atomic transitions between 700 and 900 nm. In argon-nitrogen mixtures, nitrogen quenches atomic argon species and the dominant radiating processes are transitions in the nitrogen second positive system. In pure nitrogen, emission on the microsecond time scale comes from the nitrogen first positive system. Lower energy density is needed for tagging and narrower tagged lines are produced using 267 nm as compared to femtosecond laser tagging in argon and nitrogen using 400 nm or 800 nm. Velocimetry using the 267 nm line is demonstrated in a turbulent argon pipe flow and the Taylor microscale of the flow is determined.

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.

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.

FLEET velocimetry for combustion and flow diagnostics

We report the use of femtosecond laser electronic excitation tagging (FLEET) for velocimetry at a 100-kHz imaging rate. Sequential, single-shot, quantitative velocity profiles of an underexpanded supersonic nitrogen jet were captured at a 100-kHz rate. The signal and lifetime characteristics of the FLEET emission were investigated in a methane flame above a Hencken burner at varying equivalence ratios, and room temperature gas mixtures involving air, methane, and nitrogen. In the post-flame region of the Hencken burner, the emission lifetime was measured as two orders of magnitude lower than lab air conditions. Increasing the equivalence ratio above 1.1 leads to a change in behavior, with a doubled lifetime. By measuring the emission in a cold methane flow, a short-lived signal was measured that decayed after the first microsecond. As a proof of concept for velocimetry in a reacting environment, the exhaust of a pulsed detonator was measured by FLEET. Quantitative velocity information was obtained that corresponded to a maximum centerline velocity of 1800 m/s for the detonation wave. Extension of FLEET to larger scale, complex flow environments is now a viable option.

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.

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.