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 ${\sim}{354.8}\;{\rm nm}$. The two pulse trains are colinear with 3 µs time interval between the pulse pairs. The injection-seeded OPO efficiently converts the burst-mode laser output to 285.62 and 285.67 nm to excite the ${Q}_2({12})$ and ${P}_1({8})$ OH transitions. PLIF images were collected from each of the two excitation transitions, and intensity ratios from the images were used to determine local temperatures. The development of fast, dual-wavelength switching, burst-mode OPO technology significantly reduces the experimental complexity of the high-speed TC-PLIF thermometry and simplifies its implementation in harsh combustion and flow test facilities.

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

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

100 kHz PLEET velocimetry in a Mach-6 Ludwieg tube.

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.

100 kHz krypton planar laser-induced fluorescence imaging.

Krypton planar laser-induced fluorescence (Kr PLIF) was demonstrated at a repetition rate of 100 kHz. To achieve this increased rate, a custom injection-seeded optical parametric oscillator was built to efficiently convert the 355 nm output of a high-energy, high-repetition-rate nanosecond burst-mode laser to 212.56 nm to excite Kr from the ground to the 5p[1/2]0 electronic state. Successful tracking of flow structures and mixture fraction was demonstrated using detection speeds 100 times greater than previously attained with a femtosecond laser source. The increase in repetition rate makes time-resolved Kr PLIF relevant for high-speed flows in particular.

Time-Resolved Measurements of Turbulent Mixing in Shock-Driven Variable-Density Flows.

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.

Development of a three-legged, high-speed, burst-mode laser system for simultaneous measurements of velocity and scalars in reacting flows.

We report the development of a three-legged, high-speed, high-energy, burst-mode laser system for the simultaneous measurement of velocity and key combustion species in turbulent reacting flows. The laser system is designed to simultaneously amplify a four-pulse sequence [including a doublet pulse for particle image velocimetry (PIV) measurements] with variable pulse separations at a repetition rate up to 500 kHz and a burst duration of 1-10 ms. With the three-legged, burst-mode laser system, we demonstrate simultaneous measurements of velocity using PIV and planar laser-induced fluorescence imaging of hydroxyl and formaldehyde in a turbulent jet flame.

20 kHz CH2O and OH PLIF with stereo PIV.

Planar laser-induced fluorescence (PLIF) of hydroxyl (OH) and formaldehyde (CH2O) radicals was performed alongside stereo particle image velocimetry (PIV) at a 20 kHz repetition rate in a highly turbulent Bunsen flame. A dual-pulse burst-mode laser generated envelopes of 532 nm pulse pairs for PIV as well as a pair of 355 nm pulses, the first of which was used for CH2O PLIF. A diode-pumped solid-state Nd:YAG/dye laser system produced the excitation beam for the OH PLIF. The combined diagnostics produced simultaneous, temporally resolved two-dimensional fields of OH and CH2O and two-dimensional, three-component velocity fields, facilitating the observation of the interaction of fluid dynamics with flame fronts and preheat layers. The high-fidelity data acquired surpass the previous state of the art and demonstrate dual-pulse burst-mode laser technology with the ability to provide pulse pairs at both 532 and 355 nm with sufficient energy for scattering and fluorescence measurement at 20 kHz.

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. The seed-laser linewidth permits delivery of a laser linewidth of < 150 MHz at 532 nm The technique is demonstrated in a high-speed jet, and high-repetition-rate image sequences are shown.

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging.

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.