Multi-point FLEET velocimetry in a Mach 4 Ludwieg tube using a diffractive optical element.

A diffractive optical element was paired with femtosecond laser electronic excitation tagging (FLEET) velocimetry and used to probe multiple locations in a high-speed wind tunnel. Two configurations were explored, one that uses the traditional method of viewing from a perspective orthogonal to the beam axis and another that uses a perspective parallel to the beam axis. In the latter, the FLEET emissions are viewed as points that can allow for FLEET measurements in a wall normal fashion without the laser needing to impinge upon the surface. The configurations are demonstrated in a Mach 4 Ludwieg tube, highlighting their utility in high-speed flow measurements.

Multi-depth focused laser differential interferometer based on chromatic aberration.

A modified version of focused laser differential interferometry (FLDI) is demonstrated with adjacent beam pairs distributed along the optical axis. This feature is accomplished using two different wavelengths of light in the interferometer and accounting for the chromatic aberration of the lenses in the optical setup. It is demonstrated that ray trace calculations can be modified to predict the focal points of each of the two different colored beams, and experiments using a tube jet and a laser-induced blast wave show the instrument still has the expected features of an FLDI as well as continued capability for velocimetry. This modification is in effort to allow FLDI to be used for the analysis of three-dimensional flows, especially if combined with other multi-point variations and targeting high-frequency flow content.

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.

O2 based resonantly ionized photoemission thermometry analysis of supersonic flows.

Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O2 RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of the C3Π, (v = 2) ← X3Σ(v' = 0) absorption band of O2. The efficient ionization of O2 liberates electrons that induce electron avalanche ionization of local N2 molecules generating N2 +, which primarily deexcites via photoemissions of the first negative band of N2+(B 2 Σ u+-X 2 Σ g+). When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O2 RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique's fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.

Laser-induced schliere anemometry in a Mach 6 flow with collinear light entry.

In this work, we demonstrate velocity, Mach number, and static temperature measurements in a Mach 6 flow using the recently developed laser-induced schliere anemometry (LISA) technique. To our best knowledge, this represents the first application of LISA for characterizing flow in the hypersonic Mach number regime, and a comparison with known tunnel values is provided. The laser-induced schliere in this work are written from a distance of roughly 76 cm away from the final lens, much further than in previous work. Furthermore, the schliere are created from a laser beam introduced parallel to the collimated schlieren light, which is a new arrangement that could be useful for facilities with limited optical access. A discussion of setup limitations is provided. The mean core flow velocity determined from LISA is within 1% of the expected value from isentropic theory, while the mean Mach number measurements are within 1.6% of the M=5.85 value used in literature for the facility. Furthermore, the determined mean static temperature of the core flow is within 2.5% of the value measured simultaneously in the facility.

Linear array focused-laser differential interferometry for single-shot multi-point flow disturbance measurements.

In this Letter, a modification of the well-known focused-laser differential interferometer (FLDI) is demonstrated, with the primary focus being increasing the number of probed locations efficiently. To generate multiple beams in the FLDI system, a diffractive optical element is used. This approach is significantly more cost-effective and easier to implement than the current approach of generating multiple FLDI beam pairs using a series of Wollaston prisms. The measurements shown here utilize a 1D linear array of points, and the ability to generate a 2D array is demonstrated using two linear diffractive optical elements in tandem. Therefore, this technique, referred to as linear array FLDI (LA-FLDI), is able to provide measurements of fluid disturbances at multiple discrete locations while allowing for high data acquisition rates (>1MHz). This technique provides a much simpler approach to multipoint FLDI measurements and can increase the throughput of FLDI measurements in impulse aerospace testing facilities.

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.

Emissions in short-gated ns/ps/fs-LIBS for fuel-to-air ratio measurements in methane-air flames.

A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550-800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the ${{\rm{N}}_{\rm{II}}}({{565}}\;{\rm{nm}})$ and ${{\rm{N}}_{\rm{II}}}({{594}}\;{\rm{nm}})$ emissions is presented. In studying line emission ratios for FAR calculation, it is found that ${{\rm{H}}_\alpha}({{656}}\;{\rm{nm}})/{{\rm{N}}_{\rm{II}}}({{568}}\;{\rm{nm}})$ is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while ${{\rm{H}}_\alpha}({{656}}\;{\rm{nm}})/{{\rm{O}}_{\rm I}}({{777}}\;{\rm{nm}})$ is optimal for the ps-LIBS and fs-LIBS cases. Due to low continuum background and short time delay for spectra collection, fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS.

Measurement of supersonic jet screech with focused laser differential interferometry.

Focused laser differential interferometry (FLDI) is used to measure a well-characterized, 17 kHz screech tone emitted from an underexpanded Mach 1.5 jet. Measurements are made at numerous spatial locations in and around the jet flow-field, where intrusive diagnostics would otherwise influence the flow-field. Results from FLDI measurements are shown to agree with measurements from microphones and analyses of high-speed schlieren. The agreement is used to demonstrate FLDI is a valid and accurate technique for measuring screech tones in jet flow-fields, and furthermore that FLDI can be used to measure jet screech at various spatial locations around the jet, and notably inside of the jet, where microphones and other intrusive diagnostics cannot be used effectively.

Compressed single-shot hyperspectral imaging for combustion diagnostics.

This paper demonstrates a compressed sensing-based single-shot hyperspectral imaging system for combustion diagnostics. The hyperspectral system can capture well-resolved spectra in a 2D plane through a single shot, i.e., converting a 3D data cube of 2D spatial and 1D spectral information into a compressed 2D hyperspectral image. Experimentally, the light emissions are first coded by a random binary pattern to generate the hyperspectral content, which is then sent through a spectrometer. The resulting compressed hyperspectral image is computationally analyzed to recover original 2D spatial and 1D spectral information. C2∗ and CH∗ chemiluminescence emissions of a methane/air flame at various equivalence ratios are measured using the compressed hyperspectral imaging technique. Comparison to traditional measurements shows good agreement in the correlation of emission ratio to equivalence ratio. The technique can be further applied to other laser-based combustion diagnostics.

Measurement of electron density and temperature from laser-induced nitrogen plasma at elevated pressure (1-6 bar).

Laser-induced plasmas experience Stark broadening and shifts of spectral lines carrying spectral signatures of plasma properties. In this paper, we report time-resolved Stark broadening measurements of a nitrogen triplet emission line at 1-6 bar ambient pressure in a pure nitrogen cell. Electron densities are calculated using the Stark broadening for different pressure conditions, which are shown to linearly increase with pressure. Additionally, using a Boltzmann fit for the triplet, the electron temperature is calculated and shown to decrease with increasing pressure. The rate of plasma cooling is observed to increase with pressure. The reported Stark broadening based plasma diagnostics in nitrogen at high pressure conditions will be significantly useful for future studies on high-pressure combustion and detonation applications.

Quantitative fuel-to-air ratio determination for elevated-pressure methane/air flames using chemiluminescence emission.

The fuel/air ratio (FAR) in a methane-air Hencken flame at pressures of 1-5 bar is measured using the chemiluminescence-based method. Emission spectra are used to investigate the effects of pressure on the OH* (308 nm), CH* (430 nm), and C2* (500 nm) emissions and the effect on equivalence ratio determination from the ratios of these emission peaks. Both OH*/CH* and C2*/CH* ratios are linear to FAR at atmospheric pressure. At elevated pressures, C2*/CH* remains roughly linear to FAR, while OH*/CH* becomes highly nonlinear. There are significant spectral contributions from continuum radiation at higher pressures, likely due to increasing soot production. Therefore, while it is a truly passive and nonintrusive diagnostic method, the use of chemiluminescence for FAR determination at high pressures could be limited. Possible improvements to the measurement setup and future studies are discussed.

Single-shot nanosecond-resolution multiframe passive imaging by multiplexed structured image capture.

The Multiplexed Structured Image Capture (MUSIC) technique is used to demonstrate single-shot multiframe passive imaging, with a nanosecond difference between the resulting images. This technique uses modulation of light from a scene before imaging, in order to encode the target's temporal evolution into spatial frequency shifts, each of which corresponds to a unique time and results in individual and distinct snapshots. The resulting images correspond to different effective imaging gate times, because of the optical path delays. Computer processing of the multiplexed single-shot image recovers the nanosecond-resolution evolution. The MUSIC technique is used to demonstrate imaging of a laser-induced plasma. Simultaneous single-shot measurements of electron numbers by coherent microwave scattering were obtained and showed good agreement with MUSIC characterization. The MUSIC technique demonstrates spatial modulation of images used for passive imaging. This allows multiple frames to be stacked into a single image. This method could also pave the way for real-time imaging and characterization of ultrafast processes and visualization, as well as general tracking of fast objects.

Simultaneous LIBS signal and plasma density measurement for quantitative insight into signal instability at elevated pressure.

Laser-induced breakdown spectroscopy (LIBS) evaluates the emission spectra of ions, radicals, and atoms generated from the breakdown of molecules by the incident laser; however, the LIBS signal is unstable at elevated pressures. To understand the cause of the signal instability, we perform simultaneous time-resolved measurements of the electron density and LIBS emission signal for nitrogen (568 nm) and hydrogen (656 nm) at high pressure (up to 11 bars). From correlations between the LIBS signal and electron number density, we find that the uncontrollable generation of excess electrons at high pressure causes high instability in the high-pressure LIBS signal. A possible method using ultrafast lasers is proposed to circumvent the uncontrolled electron generation and improve signal stability at high pressure.

High-speed flame chemiluminescence imaging using time-multiplexed structured detection.

In this work, high-speed flame chemiluminescence has been obtained by using the time-multiplexed structured detection (TMSD) imaging method from a single snapshot. TMSD sheers the time lapse into the spatial frequency shifts, which allows multiple high-speed images to be frequency upshifted into distinct spatial frequency regions from the original image. A cumulative exposure captured in a single snapshot image contains distinct time evolution. Each distinct image is demultiplexed and recovered by hyperdyne mixing with the modulation frequency. TMSD is an optical frequency domain analog to carrier frequency modulation in radio and microwave detections. Specifically, a digital micromirror device (DMD) spatially modulates flame chemiluminescence just prior to the camera. Spatial frequency of each distinct image is mixed between the original spatial frequency components and DMD modulation pattern frequency, resulting in a coded snapshot. The high-speed flame chemiluminescence image is recovered by demodulation. TMSD is similar to structured illumination used in super-resolution microscopy, but offers more advantages, for it does not alternate incident illumination light. Since DMDs are available with speeds up to 40 kHz, this technique shows promise as a cost-effective means of high-speed imaging and diagnostics of combustion phenomena.

Acoustic detection of resonance-enhanced multiphoton ionization for spatially resolved temperature measurement.

In this Letter, acoustic detection of resonance-enhanced multiphoton ionization (A-REMPI) is characterized and used to measure spatially resolved O2 rotational temperature in air. The acoustic signal is generated using O2 REMPI in air and is detected by a single microphone operating within the audible range. Compared to electron number measurements by coherent microwave scattering, nonlinear light absorption and subsequent local pressure perturbation are captured by the microphone. A typical acoustic cycle of compression and rarefication of the acoustic wave is observed in the A-REMPI. Since the pressure perturbation can be regarded as close to thermodynamic equilibrium, the rotational temperature measured by A-REMPI is lower and closer to the realistic condition.

Spatially localized, see-through-wall temperature measurements in a flow reactor using radar REMPI.

See-through-wall coherent microwave scattering from resonance-enhanced multiphoton ionization (REMPI) for rotational temperature measurements of molecular oxygen has been developed and demonstrated in a flow reactor at atmospheric pressure. Through limited, single-ended optical access, a laser beam was focused to generate local ionization of molecular oxygen in a heated quartz flow reactor enclosed by ceramic heating elements. Coherent microwaves were transmitted, and the subsequent scattering off the laser-induced plasma was received, through the optically opaque ceramic heater walls and used to acquire rotational spectra of molecular oxygen and to determine temperature. Both axial and radial air-temperature profiles were obtained in the flow reactor with an accuracy of ±20 K⁢(±5%). The experimental results show good agreement with a steady-state computational heat transfer model. This technique shows great potential for non-invasive, high-fidelity measurement of spatially localized temperature and radical species concentration in combustion kinetic experiments and confined combustors constructed of advanced ceramic materials in which limited or non-existing optical access hinders usage of conventional optical diagnostic techniques to quantify thermal non-uniformity.

High-speed 2D Raman imaging at elevated pressures.

Two-dimensional (2D) Raman scattering at 10 kHz in non-reacting flow mixtures is demonstrated by employing a burst-mode laser with a long-duration pulse of about 70 ns and pulse energy of about 750 mJ at 532 nm. To avoid optical breakdown, the pulse width of the laser was varied in the range of 10-1000 ns. The effects of pulse shape, pulse energy, and harmonic conversion on 2D measurements are also studied. The applications of high-speed, single-shot, 2D imaging of CH4 and H2 jets in N2 at elevated pressures are demonstrated. In addition, the scalar dissipation rate of CH4 in N2 at 20 bar is determined, and multi-dimensional, multi-species, high-speed imaging of flows at elevated pressures is demonstrated.

Sensitivity, stability, and precision of quantitative Ns-LIBS-based fuel-air-ratio measurements for methane-air flames at 1-11 bar.

Nanosecond laser-induced breakdown spectroscopy (ns-LIBS) is employed for quantitative local fuel-air (F/A) ratio (i.e., ratio of actual fuel-to-oxidizer mass over ratio of fuel-to-oxidizer mass at stoichiometry, measurements in well-characterized methane-air flames at pressures of 1-11 bar). We selected nitrogen and hydrogen atomic-emission lines at 568 nm and 656 nm, respectively, to establish a correlation between the line intensities and the F/A ratio. We have investigated the effects of laser-pulse energy, camera gate delay, and pressure on the sensitivity, stability, and precision of the quantitative ns-LIBS F/A ratio measurements. We determined the optimal laser energy and camera gate delay for each pressure condition and found that measurement stability and precision are degraded with an increase in pressure. We have identified primary limitations of the F/A ratio measurement employing ns-LIBS at elevated pressures as instabilities caused by the higher density laser-induced plasma and the presence of the higher level of soot. Potential improvements are suggested.

Time-Gated Single-Shot Picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) for Equivalence-Ratio Measurements.

Time-gated picosecond laser-induced breakdown spectroscopy (ps-LIBS) for the determination of local equivalence ratios in atmospheric-pressure adiabatic methane-air flames is demonstrated. Traditional LIBS for equivalence-ratio measurements employ nanosecond (ns)-laser pulses, which generate excessive amounts of continuum, reducing measurement accuracy and precision. Shorter pulse durations reduce the continuum emission by limiting avalanche ionization. Furthermore, by contrast the use of femtosecond lasers, plasma emission using picosecond-laser excitation has a high signal-to-noise ratio (S/N), allowing single-shot measurements suitable for equivalence-ratio determination in turbulent reacting flows. We carried out an analysis of the dependence of the plasma emission ratio Hα (656 nm)/NII (568 nm) on laser energy and time-delay for optimization of S/N and minimization of measurement uncertainties in the equivalence ratios. Our finding shows that higher laser energy and shorter time delay reduces measurement uncertainty while maintaining high S/N. In addition to atmospheric-pressure flame studies, we also examine the stability of the ps-LIBS signal in a high-pressure nitrogen cell. The results indicate that the plasma emission and spatial position could be stable, shot-to-shot, at elevated pressure (up to 40 bar) using a lower excitation energy. Our work shows the potential of using ps-duration pulses to improve LIBS-based equivalence-ratio measurements, both in atmospheric and high-pressure combustion environments.