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.

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 Q(7) and Q(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.

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.

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.

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.

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.

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 ${\sim}{354.8}\;{\rm nm}$.

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.

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.

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.

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.

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.

Unseeded Velocimetry in Nitrogen for High-Pressure Cryogenic Wind Tunnels, Part 2: Picosecond-Laser Tagging.

Picosecond laser electronic excitation tagging (PLEET) is implemented in a large-scale wind tunnel for the first time. High-speed, unseeded velocimetry is performed in the NASA Langley 0.3-m Transonic Cryogenic Tunnel; repetition rates up to 25 kHz are tested.

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.

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.

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.

Development of NO-MTV for low-speed flow and in-situ deployment to an integral effect test facility.

A molecular tagging velocity (MTV) technique is developed to non-intrusively measure velocity in an integral effect test (IET) facility simulating a high temperature helium-cooled nuclear reactor in accident scenarios. In these scenarios, the velocities are expected to be low, on the order of 1 m/s or less, which forces special requirements on the MTV tracer selection. Nitrous oxide (NO) is identified as a suitable seed gas to generate NO tracers capable of probing the flow over a large range of pressure, temperature, and flow velocity.

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.

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.

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.

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 N using 202.25 nm 100 fs light.

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.

Synthesis of Fluorophore-Doped Polystyrene Microspheres: Seed Material for Airflow Sensing.

Kiton red 620 (KR620) doped polystyrene latex microspheres (PSLs) were synthesized via soap-free emulsion polymerization to be utilized as a relatively nontoxic, fluorescent seed material for airflow characterization experiments. Poly(styrene-co-styrenesulfonate) was used as the PSL matrix to promote KR620 incorporation. Additionally, a bicarbonate buffer and poly(diallyldimethylammonium chloride), polyD, cationic polymer were added to the reaction solution to stabilize the pH and potentially influence the electrostatic interactions between the PSLs and dye molecules.

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.

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.