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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.

A novel, to the best of our knowledge, method for multipoint hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering measurements is presented. The pump/Stokes and probe beams are each split into 16 discrete points with 90 and 24 µJ/pulse, respectively, using simple diffractive optical elements, which are used in combination with a focusing lens and narrowband spectral amplifier for 1 kHz excitation along a linear array of probe volumes. Single-shot and averaged temperature and O2/N2 profile measurements are demonstrated along a line with 1 mm spacing in room temperature and heated N2 flows. This enables measurements over varying spatial extents for 1D profiles and potentially 2D grids in a simple and compact optical arrangement.

We present a hybrid frequency- and time-domain solution, applicable to the case of picosecond coherent anti-Stokes Raman scattering (CARS), for gas-phase diagnostics. A solution has been derived based on both physical arguments and four-wave mixing equations for picosecond CARS, with pulse durations that are comparable to the dephasing time scale for gas-phase Raman coherence—a regime where commonly employed solutions for impulsive (femtosecond) or cw (nanosecond) pump/Stokes forcing are not strictly valid. We present the ps-CARS spectrum in the form of incoherent sums of CARS intensity spectra, calculated from the fundamental solution for impulsive pump/Stokes Raman preparation. The solution was examined for temperatures from 1000–3000 K, for four plausible experimental configurations, with laser pulse durations of 50–150 ps, and probe pulse delays from −20 to 240 ps. Approximations based on cw and impulsive pump/Stokes preparation to fit picosecond CARS spectra at atmospheric pressure were examined and the relative thermometric accuracy and computational cost of these approximations were quantified for the case of a zero nonresonant CARS contribution, and a nonresonant susceptibility equal to 10% of the Raman-resonant value at the N2 bandhead. The nanosecond CARS approximation can result in large fitting errors when the probe pulse time delay is less than the probe pulse duration. Errors as large as 10–20% are observed in the fit temperatures for a zero picosecond probe pulse delay, when the nonresonant background is neglected, largely due to an inability of the time-independent cw model to capture transient frequency spread dephasing effects at the Q-branch bandhead. The inclusion of a nonresonant background results in 40–60% thermometry errors with a nanosecond model at a zero-probe delay. Time-dependent impulsive calculations used for femtosecond CARS better approximate the structure of the N2 bandhead, reducing temperature fitting errors to 5–10% at a short probe pulse delay. The impulsive approximation results in errors up to 10% at intermediate probe pulse delays, where the coherence of the pump and probe pulses leads to multiple terms in the picosecond CARS solution. Both approximations improve as the probe pulse delay exceeds the probe duration. The nanosecond approximation results in a 2–3% error, while the impulsive model results in differences of less than 1% in some cases. Fits to experimental data obtained using short, ∼60ps pulses at a zero probe time delay and longer 100 ps pulses at a substantial 200 ps delay are presented with accuracies of 1–3% in the fit temperature.

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.

10 kHz-rate dual-plane stereoscopic particle-image velocimetry (DSPIV) was applied in a swirling jet flame measurement simultaneously with hydroxyl planar laser-induced fluorescence (OH-PLIF). A high-speed burst-mode laser system with an optical parametric oscillator were used in double-pulse laser generation of 532 nm for one plane and 607 nm for the second plane. Temporally-resolved particle image velocimetry (PIV) image sequences, nine-component velocity gradient tensor, and simultaneous OH-PLIF images are presented. Flow vorticities and strain rates are also obtained with the measured velocity gradients. Compared with previous polarization-based dual-plane PIV technique, two-color based DSPIV measurement has distinctive advantages in elimination of signal interference from different PIV plane due to polarization leakage and hence improves measurement accuracy.

The structure and dynamics of a hydrogen-air rotating detonation engine (RDE) are described based on 100-kHz laser absorption spectroscopy measurements of water temperature at four simultaneous locations within the detonation channel. The analysis focuses on the evolution of the flowfield over a 200 ms period for three separate air mass flow rate cases. Two-dimensional unwrapped visualizations of the temperatures show a flowfield structure containing regions with the detonation front, combustion products, oblique shock, and refilling reactants, qualitatively agreeing with previous simulations and experiments. A major conclusion is that water from the combustion products is measured throughout all space and time in the RDE, including near the injector, implying the presence of performance loss processes such as burning upstream of the detonation wave or the back recirculation of combustion products with fresh fuel–air. By analyzing the elevated temperatures of the reactants during the refill process, one estimation for the mass fraction of combustion products in the reactants is as high as 20–30% on average. This product mass fraction is found to be inversely proportional to the bulk air mass flow rate and decreases as time progresses. This indicates these non-ideal processes are more significant closer to RDE ignition for poorer performing operating conditions. For the largest air mass flow case, water temperatures near the nominally cold plenum conditions likely corroborate the presence of a recirculation region on the RDE inner body. Analysis of inter- and intra-cycle temperature dynamics further support non-ideal processes occurring behind the detonation wave and during the refill process. As a whole, the data indicates that the RDE performance is better as time progresses away from ignition or for higher air mass flow rates. These data are also important for comparison with numerical models.

This paper analyzes the flame dynamics of near LBO (lean blowoff) swirl stabilized flames, using simultaneous OH and CH2O (formaldehyde) PLIF (planar laser induced fluorescence) measurements. Prior studies have shown that recirculation stabilized flames approach blowoff through two distinct stages – “stage 1” characterized by local extinction, where the overall flame and flow field remain largely unchanged, and “stage 2”, characterized by a fundamental change in the flow field, accompanied by violent flame flapping and wake disruption. This paper quantifies extinction spots along the flame edge, and entrained reactants within the combustion product region to analyze these stages in greater detail. Extinction spots were quantified by the overlapping regions of OH and CH2O – numerous such spots were found near blowoff. The entraining of unburnt reactants into the recirculation zone was quantified by detecting low intensity OH pockets that were not surrounded by CH2O. As expected, the flame near blowoff displayed significantly more entrained reactant pockets relative to a stable flame. Unexpectedly however, the volume of these pockets is tiny compared to the products, even on the edge of blowoff. Once they enter the wake, they are short-lived, suggesting that they are diluted and/or quickly react. This was surprising given the non-trivial baseflow and flame position disruption at these conditions, suggesting a striking similarity between the average composition of the wake, to that of a stable flame.

The understanding and predictive modeling of explosive blasts require advanced experimental diagnostics that can provide information on local state variables with high spatiotemporal resolution. Current datasets are predominantly based on idealized spherically symmetric explosive charges and point-probe measurements, although practical charges typically involve multidimensional spatial structures and complex shock-flow interactions. This work introduces megahertz-rate background-oriented schlieren tomography to resolve transient, three-dimensional density fields, as found in an explosive blast, without symmetry assumptions. A numerical evaluation is used to quantify the sources of error and optimize the reconstruction parameters for shock fields. Average errors are ∼ 3 % in the synthetic environment, where the accuracy is limited by the deflection sensing algorithm. The approach was experimentally demonstrated on two different commercial blast charges (Mach ∼ 1.2 and ∼ 1.7 ) with both spherical and multi-shock structures. Overpressure measurements were conducted using shock-front tracking to provide a baseline for assessing the reconstructed densities. The experimental reconstructions of the primary blast fronts were within 9% of the expected peak values. The megahertz time resolution and quantitative reconstruction without symmetry assumptions were accomplished using a single high-speed camera and light source, enabling the visualization of multi-shock structures with a relatively simple arrangement. Future developments in illumination, imaging, and analysis to improve the accuracy in extreme environments are discussed.

Digital inline holography (DIH) is a three-dimensional (3D) measurement technique widely used in the characterizations of particles, droplets, and bubbly flows. When collimated coherent light passes an object field, the disturbed and undisturbed components will superimpose at the imaging plane and form an interference pattern (hologram) due to their phase variation. By analyzing the phase information encoded in the hologram, the shapes and locations of objects can be reconstructed. However, the reconstruction produces higher levels of uncertainty along the line of sight, which is the out-of-plane direction normal to the imaging plane. Additionally, the reconstructions algorithm cannot resolve structures blocked by other features along the recording path. To overcome these limitations, prior works have implemented DIH from two to three views on simple geometries. In this work, multiview digital inline holography is presented with (≥3) views to enable the reconstruction of 3D structures with complex surface topologies, including ligaments and droplets during the primary liquid breakup. The approach is similar to DIH but with a different postprocessing method that combines the information on 3D edge outlines extracted from different DIH viewing angles. Two reconstruction approaches, an outline-based method, and another cross section-based method, are developed and applied on holograms of a 3D-printed test model imitating droplet breakup. With only three views, both methods provide limited reconstruction results with various artifacts. The outline-based method uses more spatial information but, due to practical limitations, results in lower-fidelity reconstructions than the cross section-based method. In general, DIH reconstructions struggle with concave structures even with more than six views due to shadowing of obstructed structures. However, when the number of views increases to six, the cross section-based reconstruction method yields morphological details close to the test model.

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...

MHz-rate Nitric Oxide Planar Laser-Induced Fluorescence (NO PLIF) imaging and MHz-rate Focused Laser Differential Interferometry (FLDI) were performed in hypersonic boundary layer at CUBRC 48” Mach 10 shock tunnel.

High spectral purity of the laser source in Filtered Rayleigh Scattering (FRS) is necessary to remove stray scattering. In this study, the spectral purity of a burst-mode laser was improved to 0.9999987 using an etalon

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.

A burst-mode nitrogen (

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.