Search

The transition of the boundary-layer state from laminar to turbulent on a 7° sharp cone in Mach 10 flow is measured using several high-speed optical diagnostics capable of time-resolved measurements of the second-mode instability with high spatial resolution. Specifically, megahertz-rate nitric oxide planar laser-induced fluorescence (NO PLIF) imaging is done at 1 MHz to provide a non-path-integrated visualization of second-mode associated structures in the transitioning boundary layer. A recent variation of focused laser differential interferometry (FLDI), namely, linear array FLDI (LA-FLDI), is also applied for multipoint measurements of frequency content in the transitioning boundary layer. From each data set, the frequency of the most unstable second-mode instability is obtained along with the phase velocity of the second-mode waves. Measurements show good agreement relative to each other and also with surface pressure sensors and high-speed schlieren imaging. Furthermore, the 1 MHz NO PLIF imaging demonstrated here shows great potential for visualization of transitional and turbulent boundary layers when path-integrated diagnostics cannot be used.

The detrended fluctuation analysis, a technique for detecting long-range correlations in fractal time series, is used to characterize thermoacoustic oscillations of impending combustion instabiliti...

This work demonstrates single-shot, two-dimensional temperature measurements in a premixed linear detonation channel using two-color OH planar laser-induced fluorescence thermometry. Detonation environments result in extreme thermodynamic conditions which create challenges in the resulting spectroscopic behavior. However, thermometry based on the ratio of the P1(9)/Q1(14) transitions within the A2Σ+←X2Π(1,0) band of OH was determined to be well-suited for detonation environments based on the spectral characteristics over a wide range of conditions. The technique is demonstrated in the post-induction region, focusing on the nominal equilibrium state of the Chapman-Jouguet conditions, on mixtures of stoichiometric H2 and O2 diluted with either N2 or Ar. High-speed chemiluminescence imaging at 2 MHz was used to assess the qualitative dynamics of the detonation structure. The measured temperature fields for the Ar dilution case are relatively spatially uniform, with the distributions mean agreeing well with the theoretical calculation of the Chapman-Jouguet temperature, consistent with preliminary work [Grib et al., AIAA SciTech Forum, (2021). 2021-0421]. Conversely, the temperature fields for a 50% N2 dilution case are highly irregular, showing a larger dynamic range in temperature as well as pockets of unburned reactants, which emphasized the significance of this measurement by highlighting various reaction scenarios dictated by detonation waves. The irregularity of the N2 dilution cases is explained in terms of the scale similarity between the channel size and the detonation cell sizes for the N2 dilution mixtures, leading to weak or failing detonation modes. Overall, the demonstrated thermometry technique produced a precision (1-σ) of approximately 4% in atmospheric conditions and approximately 7.7% in strong detonation environments. In addition, the accuracy, defined as the percent difference between the mean and CJ temperature, was approximately 1% in the Ar diluted case. The ability to distinguish and quantify detonation burning behavior is promising for employing this approach for application in pressure-gain combustion facilities such as rotating detonation combustors, however care needs to be taken when interpreting the resulting temperature field, particularly near the wave front, due to the precision and validation range of the present line pair.

The recent progress in high-speed ( ≥ 100 k 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.

Reported is a three-step procedure (also referred to as the observer) for fast and unbiased determination of the dominant frequencies and amplitude of thermoacoustic oscillations. The first step is...

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.

10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate ∼ 5 m J / p u l s e at 282 nm and 286 nm to excite the Q 1 ( 5 ) and Q 1 ( 14 ) transitions of the A 2 Σ + ← X 2 Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.

This paper analyzes the time averaged flow structure of a reacting jet in cross flow (RJICF), emphasizing the structure of the counter-rotating vortex pair (CVP) by using simultaneous tomographic particle image velocimetry (TPIV) and hydroxyl radical planar laser induced fluorescence (OH-PLIF). It was performed to determine the extent to which heat release, and the associated effects of gas expansion and baroclinic vorticity production, impact the structure of the CVP. These results show the clear presence of a CVP in the time averaged flow field, whose trajectory lies below the jet centerline on either side of the centerplane. Consistent with other measurements of high momentum flux ratio JICF in nonreacting flows, there is significant asymmetry in strength of the two vortex cores. The strength and structure of the CVP was quantified with vorticity and swirling strength (λci), showing that some regions of the flow with high shear are not necessarily accompanied by large scale bulk flow rotation and vice-versa. The OH PLIF measurement allows for correlation of the flame position with the dominant vortical structures, showing that the leeward flame branch lies slightly above, as well as, in the region between the CVP cores.

Quantitative, one-dimensional (1D), single-laser-shot, fuel–air ratio (FAR) measurements in both laminar and turbulent methane–air flames were conducted using time-gated nanosecond-laser-induced breakdown spectroscopy (ns-LIBS) line imaging. In the laminar methane–air flames at a pressure of 1–11 bar, hydrogen (Hα) and nitrogen (NII) atomic emission lines at 568 and 656 nm, respectively, were selected to establish a correlation between the line intensities and the local FAR. The spatial calibration profiles of the N/H ratios in the flames at various pressures were obtained in one dimension. The effects of the laser energy and pressure on the stability and precision of the 1D FAR measurements were investigated. It was observed that the N/H correlation is significantly reduced at ∼11 bar, which sets the limits of the 1D LIBS-based FAR measurements. Single-laser-shot 1D FAR measurements were conducted in a turbulent flame at atmospheric pressure, and multiline LIBS was performed to extend the measurement area of interest. Spatially and spectrally resolved line LIBS can provide the local FAR with a spatial resolution of ∼0.1 mm. These results hold promise for the utilization of ns-LIBS for spatially resolved 1D FAR measurements in turbulent flames at elevated pressures.

Krypton (Kr)-based tagging velocimetry is demonstrated in a K r / N 2 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 ( ∼ 40 µ s ) 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 N 2 —rather than from Kr—following efficient energy transfer. Successful flow velocity tracking is demonstrated at multiple locations in a high-speed K r / N 2 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.

In this work, a variable-pulse-oscillator is developed and coupled with a burst-mode amplifier for generation of high-energy laser pulses with width of 100 ps to 1 ms and near-Gaussian temporal pulse shape. Pulse energy as high as 600 mJ is demonstrated at 1064 nm, with a super-Gaussian spatial profile and beam quality as good as 1.6 times the diffraction limit. A time-dependent pulse amplification model is developed and is in general agreement with experimentally measured values of output pulse energy and temporal pulse shape of the amplified pulses. Key performance parameters (pulse energy, temporal pulse shape, and spatial beam profile and quality) are analyzed as a function of pulse width across seven orders of magnitude. Additionally, the model is used to elucidate deviations between the simulated and experimental data, showing that the relationship between pulse width and output pulse energy is dominated by the variable-pulse-width oscillator performance, not the burst-mode amplifier.

A burst-mode laser system is developed for hybrid femtosecond/picosecond (fs/ps) rotational coherent anti-Stokes Raman scattering (RCARS) at megahertz rates. Using a common fs oscillator, the system simultaneously generates time synchronized 1061 nm, 274 fs and 1064 nm, 15.5 ps pulses with peak powers of 350 MW and 2.5 MW, respectively. The system is demonstrated for two-beam fs/ps RCARS in N 2 at 1 MHz with a signal-to-noise ratio of 176 at room temperature. This repetition rate is an order of magnitude higher than previous CARS using burst-mode ps laser systems and two to three orders of magnitude faster than previous continuously pulsed fs or fs/ps laser systems.

Tomographic Rayleigh scattering (RS) imaging at a repetition rate of 10 kHz was demonstrated in non-reacting flows employing the second harmonic of a high-energy Nd: YAG burst-mode laser. Sequences of 100 images of the flow mixture fraction were directly derived from high-speed four-dimensional (4D) RS images. The tomographic reconstruction algorithm, measurement resolution, uncertainties, and jet flow mixing characteristics are discussed. Successful tomographic RS imaging using a high-energy burst-mode laser source lays the foundation for spatiotemporal, multidimensional analyses of density, mixture fraction, and temperature measurements in reacting and non-reacting flows of practical interest.

The legendary difficulties in studying turbulent flows stem, in part, from the lack of high-frequency, high-resolution measurements to interrogate small-scale structures and their rapid evolution. Our experiments, employing a burst-mode laser system, capture both spatially resolved velocity fields and their dynamics using high-resolution particle image velocimetry measurements at 100 kHz. We show directly that fluctuations of flow velocity in an axisymmetric jet flow are inhomogeneous and anisotropic. The velocity of the peak of the time-delayed cross correlation function C(r, r 0 ; τ ) is smaller than the convection velocity; thus Taylor’s frozen hypothesis [Taylor GI, Proc. R. Soc. London, Ser A 164, 476 (1938)] fails to generalize for inhomogeneous jet flows, consistent with prior studies. Its peak decays exponentially in time. Second, the structure functions are found to be isotropic at small distances, but not at large distances. Extended self-similarity is found to hold [Benzi R et al., Physics Review E 48, R29 (1993)], but no inertial range is found where the Kolmogorov 2 3 -law [Kolmogorov AN, Dokl. Akad. Naud. SSSR 30, 299 (1941)] holds. Spectral-energy density of the jet flow, although anisotropic, is consistent with the Kolmogorov-Obukhov 5 3 -law [Obukhov AM, Izvestiya Akad. Nauk. SSSR 32, 19 (1941)] in the flow direction.

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.

Conventional residual stress mitigation techniques involve long processing times at high temperatures and/or mechanical loading to build plastic compressive stress below the surface. In this study, we present a new residual stress mitigation methodology at near ambient temperature in less than a minute. This is demonstrated on a welded joint of 316 L stainless steel, where low-frequency DC current pulses are shown to recrystallize the specimen and reduce residual stress. We present experimental evidence of ∼30 % reduction in electrical resistance, which corresponded to ∼40 % decrease in both microhardness and residual stress, measured by the X-ray diffraction tests. Similar improvement was qualitatively observed through significant decrease in the low-angle grain boundary density, which also reflects the decrease of the residual stress. The technique can be applied to relieve residual stress in conditions difficult for the conventional processing, such as locations with extreme space constraints or objects that cannot be heat treated.

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.

Broadband femtosecond (fs) two-photon laser-induced fluorescence (TP-LIF) of the B1Σ+←X1Σ+, Hopfield-Birge system of carbon monoxide (CO) is believed to have two major advantages compared to narrowband nanosecond excitation. It should (i) minimize the effects of pressure-dependent absorption line broadening and shifting, and (ii) produce pressure-independent TP-LIF signals as the effect of increased quenching due to molecular collisions is offset by the increase in number density. However, there is an observed nonlinear drop in the CO TP-LIF signal with increasing pressure. In this work, we systematically investigate the relative impact of potential deexcitation mechanisms, including collisional quenching, forward lasing, attenuation of the source laser by the test cell windows or by the gas media, and a 2+1 photoionization process. As expected, line broadening and collisional quenching play minor roles in the pressure-scaling behavior, but the CO fs TP-LIF signals deviate from theory primarily because of two major reasons. First, attenuation of the excitation laser at high pressures significantly reduces the laser irradiance available at the probe volume. Second, a 2+1 photoionization process becomes significant as the number density increases with pressure and acts as a major deexcitation pathway. This work summarizes the phenomena and strategies that need to be considered for performing CO fs TP-LIF at high pressures.

Femtosecond, Two-photon Absorption Laser Induced Fluorescence (fs-TALIF) corrected for collisional quenching with Raman scattering is used to capture spatially resolved atomic oxygen profiles in lean premixed, hydrogen cellular tubular flames. This method has allowed comparisons of number density and O-atom concentration distributions in flames of variable stretch rates in a manner similar to that previously performed on the minor flame species H and OH. As stretch rate increases, the radii of peak O-atom in the cells decrease while O-atom concentrations remain relatively unaffected. This differs from non-cellular flame data where increasing stretch rate increases minor species number densities. Three chemical mechanisms are employed to perform direct numerical simulations of the O-atom profiles in the tubular flames and are found to be in close agreement with one another. For N2-diluted flames, the simulations predict O-atom number densities within the uncertainty of the data for the cellular region but over-predict the O-atom number densities in the dearth region of the 2D flames. Additionally, simulated O-atom concentrations contradict the trend of the data and increase with stretch rate. Changing the diluent from N2 to CO2 lowers the peak concentrations of atomic oxygen as CO2 becomes reactive at flame temperatures. This allows the CO + O ( + M ) ⇌ C O 2 ( + M ) reaction to consume atomic oxygen. Flames diluted with carbon dioxide caused the model to over-predict the O-atom concentration in these flames. This discrepancy is similar to past minor species measurements in cellular tubular flames though it does not occur in minor species profiles of non-cellular (1D), CO2-diluted tubular flames. The discrepancy could be caused by the simplifying relationships employed to convert the 3D geometry to 2D in the simulations.

This paper describes implementation of simultaneous, high speed (5 kHz) stereo PIV, OH and fuel-PLIF in a pressurized, liquid fueled, swirl stabilized flame. The experiments were performed to characterize the flow field, qualitative heat release and fuel spray distributions, and flame dynamics. Acquiring high speed OH-PLIF in pressurized, liquid fuel systems is difficult due to the strong overlap of the fuel's absorption and emission spectra with the OH fluorescence spectrum. To overcome difficulties associated with the overlap, the OH and fuel fluorescence signals were partially separated by using two cameras with differing spectral filters and data acquisition timing. Upon data reduction, regions containing fuel, OH and a mixture of fuel and OH are identified. Instantaneous and time-averaged results are discussed showing the flow field, flame position and dynamics, and spray distribution from the fuel signal for two multi-component liquid fuels, at two inlet temperatures and three pressures. These results are used to infer several important observations on coupled flow and flame physics. Specifically, the flame is “M-shaped” at higher preheat temperature and higher fuel/air ratio, as opposed to no visible reaction on the inside of the annular fuel/air jet at low temperature and fuel/air ratio conditions. While such fundamentally different flame topologies in gaseous, premixed flames are well known, these results show that there are also different families of flame shapes and heat release distributions in spray flames. In addition, the flame position with respect to the flow is different for the liquid-fueled flame than for gaseous, premixed flames—in premixed flames with this geometry, the flame lies in the low velocity shear layer separating the reactants and the recirculating products. In contrast, the flame location is controlled by the spray location in this spray flame, as opposed to the shear layer. For example, reactions are observed near the nozzle outlet in the core of the high velocity annular jet, something which would not be observed in the premixed flame configuration. Also of interest is the near invariance of the key flow features—such as jet core trajectory or shear layer locations—to the operating condition changes for this study, even as the spray penetration and distribution, and flame position change appreciably.

Spatio-temporally resolved measurements of temperature using hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering (fs/ps RCARS) are evaluated for characterizing the highly dynamic exhaust flow of a non-premixed hydrogen-air rotating detonation combustor (RDC). The RCARS system utilizes a recently developed kHz-rate probe-pulse amplification system that enables high probe-pulse energies and sufficient sensitivity to track RDC exhaust gas temperatures during the short ~1.5 s run time with a precision of ~2%. Because of the potential for high spatial gradients in temperature and pressure in the RDC exhaust, estimation of bias errors due to spatial averaging in the 700-µm-long RCARS probe volume is conducted by employing the results of a reactive three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) model with a structured grid of 48.5 million cells. This results in a potential bias error of ~1.5% due to exhaust temperature gradients and underscores the need for high spatial resolution. The experimental and predicted exhaust temperature histograms show good correspondence with a statistically similar skew-normal distribution relevant to the flow's local dynamical features. By utilizing a high-speed camera synchronized with the RCARS system, it was possible to compare the numerical simulation results with the measured exhaust temperature profile obtained from knowledge of the instantaneous detonation-wave azimuth position. Similar azimuthal spatial variations of ~300 K were observed in the experimental and computed temperatures, indicating a relatively well-mixed exhaust flow. The temperature pattern factors of 0.19 and 0.20 obtained from the experimental and numerical data, respectively, are relatively close to isobaric combustors in modern gas turbine engines. These results illustrate the ability of the fs/ps RCARS and numerical modeling approaches to evaluate characteristics of the RDC exhaust flow for future development in propulsion and power generation systems.

Rotationally resolved, broadband absorption spectra of the fundamental vibrational transition of the asymmetric C–H stretch mode of methane are measured under single-laser-shot conditions using time-resolved optically gated absorption (TOGA). The TOGA approach exploits the difference in timescales between a broadband, fs-duration excitation source and the ps-duration absorption features induced by molecular absorption to allow effective suppression of the broadband background spectrum, thereby allowing for sensitive detection of multi-transition molecular spectra. This work extends the TOGA approach into the mid-infrared (mid-IR) spectral regime, allowing access to fundamental vibrational transitions while providing broadband access to multiple mid-IR transitions spanning ∼150 cm−1 (∼160 nm) near 3.3 μm, thereby highlighting the robustness of this technique beyond previously demonstrated electronic spectroscopy. Measurements are conducted in a heated gas cell to determine the accuracy of the simultaneous temperature and species-concentration measurements afforded by this single-shot approach in a well-characterized environment. Application of this approach toward fuel-rich methane–nitrogen–oxygen flames is also demonstrated.

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 N I I ( 565 n m ) and N I I ( 594 n m ) emissions is presented. In studying line emission ratios for FAR calculation, it is found that H α ( 656 n m ) / N I I ( 568 n m ) is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while H α ( 656 n m ) / O I ( 777 n m ) 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.

Flash x-ray radiography is used to capture quantitative, two-dimensional line-of-sight averaged, single-shot liquid distribution measurements in impinging jet sprays. The accuracy of utilizing broadband x-ray radiation from compact flash tube sources is investigated for a range of conditions by comparing the data with radiographic high-speed measurements from a narrowband, high-intensity synchrotron x-ray facility at the Advanced Photon Source (APS) of Argonne National Laboratory. The path length of the liquid jets is varied to evaluate the effects of energy dependent x-ray attenuation, also known as spectral beam hardening. The spatial liquid distributions from flash x-ray and synchrotron-based radiography are compared, along with spectral characteristics using Taylor's hypothesis. The results indicate that quantitative, single-shot imaging of liquid distributions can be achieved using broadband x-ray sources with nanosecond temporal resolution. Practical considerations for optimizing the imaging system performance are discussed, including the coupled effects of x-ray bandwidth, contrast, sensitivity, spatial resolution, temporal resolution, and spectral beam hardening.

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.

Recent developments of burst-mode lasers and imaging systems have opened new realms of simultaneous diagnostics for velocity and density fields at a rate of 1 kHz–1 MHz. These enable the exploration of previously unimaginable shock-driven turbulent flow fields that are of significant importance to problems in high-energy density physics. The current work presents novel measurements using simultaneous measurements of velocity and scalar fields at 60 kHz to investigate Richtmyer-Meshkov instability (RMI) in a spatio-temporal approach. The evolution of scalar fields and the vorticity dynamics responsible for the same are shown, including the interaction of shock with the interface. This temporal information is used to validate two vorticity-deposition models commonly used for initiation of large scale simulations, and have been previously validated only via simulations or integral measures of circulation. Additionally, these measurements also enable tracking the evolution and mode merging of individual flow structures that were previously not possible owing to inherently random variations in the interface at the smallest scales. A temporal evolution of symmetric vortex merging and the induced mixing prevalent in these problems is presented, with implications for the vortex paradigms in accelerated inhomogenous flows.

We report the high-speed imaging of multi-species and multi-parameter combustion diagnostics for turbulent non-premixed jet flames using a three-legged burst-mode laser system. Simultaneous OH/CH2O planar laser-induced fluorescence and Rayleigh-scattering imaging measurements at a 10-kHz rate are obtained. OH and CH2O concentrations, flame temperatures, and heat-release rates are simultaneously acquired in two-dimensions at 10 kHz.

We report on the development of a robust fiber-coupled long-wavelength infrared (LWIR) hyperspectral sensor suite for accurate and reliable non-contact surface temperature measurements in propulsion systems with limited optical access. We first experimentally investigate various state-of-the-art LWIR optical fibers and identify the ideal fiber for efficient coupling and transmission of LWIR signals. The effects of the fiber material, structure, bending, and thermal heating on LWIR fiber transmission are characterized. Subsequently, we discuss the development of a fiber-coupled LWIR hyperspectral sensor using a multi-mode polycrystalline fiber. The temperature measurement accuracy and precision of the sensor are determined using a well-calibrated blackbody radiation source and heated thermal barrier coating. The sensor is integrated into a homemade water-cooled probe housing and environmental protection box and subsequently used for reliable combustor liner temperature measurements in a high-pressure, liquid-fueled combustor rig with no built-in optical access. We also discuss the measurement challenges associated with flame interference and potential solutions. The LWIR sensor shows significant promise in its application to surface temperature measurements, and our findings can aid propulsion system engineers and researchers in system design and operation optimization.

Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used in the boundary layer of an ogive-cylinder model in a Mach-6 Ludwieg tube. One-dimensional velocity profiles were extracted from the FLEET signal in laminar boundary layers from pure N 2 flows at unit Reynolds numbers ranging from 3.4 × 10 6 / m to 3.9 × 10 6 / m . The effects of model tip bluntness and the unit Reynolds number on the velocity profiles were investigated. The challenges and strategies of applying FLEET for direct boundary layer velocity measurement are discussed. The potential of utilizing FLEET velocimetry for understanding the dynamics of laminar and turbulent boundary layers in hypersonic flows is demonstrated.

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 ( 3 Π 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.

Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde ( C H 2 O ) concentrations in reacting and nonreacting conditions. The three-color scheme utilizes a 532 nm pump beam and a scanned Stokes beam near 624 nm for Raman excitation of the C–H symmetric stretch ( ν 1 ) vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the 1 0 1 4 0 3 vibronic transition between the ground ( X ~ 1 A 1 ) and first excited ( A ~ 1 A 2 ) electronic states. This allows detection of C H 2 O at concentrations as low as 9 × 10 14 m o l e c u l e s / c m 3 (55 parts per million) in a calibration cell with C H 2 O and N 2 at 1 bar and 450 K with 3% uncertainty. The measurements show a quadratic dependence of the signal with C H 2 O number density. Pressure scaling experiments up to 11 bar in the calibration cell show an increase in signal up to 8 bar. We study pressure dependence up to 11 bar and further apply the technique to characterize the C H 2 O concentration in an atmospheric premixed dimethyl ether/air McKenna burner flame, with a maximum concentration uncertainty of 11%. This approach demonstrates the feasibility for spatially resolved measurements of minor species such as C H 2 O in reactive environments and shows promise for application in high-pressure combustors.

Two-photon, planar laser-induced fluorescence (PLIF) of carbon-monoxide (CO) and two-dimensional thermometry employing two-color, hydroxyl radical (OH) PLIF are used to characterize atmospheric-pressure inverse diffusion flames. These flames are important tools to aid the understanding of secondary reaction zones that may form in gas turbine engines when film-cooling air reacts with fuel-rich packets from the combustor. For the experiments performed in the present study, exhaust from a propane–air well-stirred reactor is channeled to a test section where three different film-cooling geometries are used to create inverse diffusion flames: (1) a single row of normal cooling holes, (2) a slot cut at an angle of 30° with respect to the wall, and (3) an 5 × 11 array of cooling holes. It is found that CO and H 2 concentrations of a few percent can lead to secondary reaction zones and that different cooling-hole geometries can produce dramatically different secondary reaction-zone shapes. These secondary reaction zone flames have Damkohler numbers greater than unity and are diffusion limited. The PLIF measurements show regions where CO is consumed, OH produced, and the temperature perturbed. For film-cooling flows that remain attached to the wall, the secondary reaction zone is also close to the wall and can cover a relatively long axial length. For film-cooling flows that separate from the wall, the secondary reaction zones protrude farther into the cross flow then quickly mix with the cross flow. By comparing the CO, OH, and temperature fields, three characteristic regions of flows with secondary reaction zones are identified: the injection region where cooling air displaces the vitiated cross flow, the secondary reaction zone region, and the mix-out region where all of the oxygen has been consumed and mixing with the vitiated cross flow controls the local composition and temperature.

Megahertz-rate hydroxyl radical planar laser-induced fluorescence (OH-PLIF) was demonstrated in a hydrogen/air rotating detonation combustor for the first time, to the best of our knowledge. A custom injection-seeded optical parametric oscillator (OPO) pumped by the 355 nm output of a high-energy burst-mode laser produced narrowband pulses near 284 nm for OH excitation. The system generated sequences of more than 150 ultraviolet pulses with 400 µJ/pulse at 1 MHz and 150 µJ/pulse at 2 MHz. The order of magnitude improvement in the repetition rate over prior OH-PLIF measurements and in the number of pulses over previous megahertz burst-mode OPOs enables spatiotemporal analysis of complex detonation combustion dynamics.

Optical diagnostics of highly dynamic supersonic and hypersonic flows requires laser sources with a combination of high pulse intensities and fast repetition rates. A burst-mode Nd:YAG laser system is presented for increasing the overall energy of 532 nm pulse trains by ∼ 100 × and the number of high-energy pulses by 30 × for extended duration megahertz-rate flow diagnostics. At a lower repetition rate of 100 kHz, unprecedented energies near 1 J/pulse are achieved at 532 nm over a 1.1 ms burst. The laser performance is characterized and demonstrated for megahertz-rate laser-induced breakdown spectroscopy in a Mach 2 turbulent jet.

A robust approach for acquiring background-free, multitransition absorption spectra under single-laser-shot conditions is demonstrated using broadband, ultrashort laser pulses. This technique—referred to as time-resolved optically gated absorption (TOGA)—exploits the inherent differences in timescales between broadband, femtosecond-duration light sources and the longer-duration responses of narrowband atomic or molecular absorption features. An optical temporal gate, based on frequency mixing via sum-frequency generation or difference-frequency generation, is used to isolate these long-lived time-domain absorption features from the ultrashort component associated with the broadband absorption light source. A proof-of-principle demonstration of TOGA is provided using atomic Rb as an absorbing medium. Application of this technique toward single-laser-shot simultaneous detection of hydroxyl radical concentration and the corresponding local temperature is also demonstrated in a reacting flow. These results indicate that TOGA can provide spectrally resolved, broadband, background-free absorption measurements at laser-source repetition rates.

Picosecond laser electronic-excitation tagging (PLEET) was demonstrated in a Mach-6 Ludwieg tube at a repetition rate of 100 kHz using a 1064 nm, 100 ps burst-mode laser. The system performance of high-speed velocimetry in unseeded air and nitrogen Mach-6 flows at a static pressure in the range of 5–20 torr were evaluated. Based on time-resolved freestream flow measurements and computational fluid dynamics (CFD) calculations, we concluded that the measurement uncertainty of 100 kHz PLEET measurement for Mach 6 freestream flow condition is ∼1%. The measured velocity profiles with a cone-model agreed well with the CFD computations upstream and downstream of the shockwave; downstream of the shockwave the discrepancy between the CFD and experimental measurement could be attributed to a slight nonzero angle of attack (AoA) or flow unsteadiness. Our results show the potential of utilizing 100 kHz PLEET velocimetry for understanding real-time dynamics of turbulent hypersonic flows and provide the capability of collecting sufficient data across fewer tests in large hypersonic ground test facilities.

Burst-mode femtosecond laser electronic excitation tagging (FLEET) of nitrogen is introduced for tracking the velocity field in high-speed flows at kilohertz–megahertz (kHz–MHz) repetition rates without the use of added tracers. A custom-built Nd:glass femtosecond laser is used to produce 500 pulses per burst with pulses having a temporal separation as short as 1 µs, an energy of 120 µJ, and a duration of 274 fs. This enables 2 orders of magnitude higher measurement bandwidth over conventional kHz-rate FLEET velocimetry. Characteristics of the optical system are described, along with a demonstration of time-resolved velocity measurements with ∼ 0.5 % precision in a supersonic slot jet.

Quantitative, kiloherz-rate measurement of carbon monoxide mole fractions by femtosecond two-photon, laser-induced fluorescence (TP-LIF) was demonstrated in high-pressure, luminous flames over a range of fuel-air ratios. Femtosecond excitation at 230.1 nm was used to pump CO two-photon rovibrational X1Σ+→B1Σ+ transitions in the Hopfield–Birge system and avoid photolytic interferences with excitation irradiance ∼1.7×1010 W/cm2. The effects of excitation wavelength, detection scheme, and potential sources of de-excitation were also assessed to optimize the signal-to-background and signal-to-noise ratios and achieve excellent agreement with theoretically predicted CO mole fractions at low and high pressure.