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Vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (hybrid CARS) spectroscopy is evaluated for the simultaneous measurement of temperature and pressure through the use of a single probe pulse and multiple probe pulses at various time delays. The relevant temperature and pressure parameter space is explored with the goal of determining feasibility of simultaneous pressure and temperature measurements with simplified CARS probe schemes. The sensitivity of spectral features to pressure and temperature changes is evaluated quantitatively from 1-60 atm and 300-3000 K. This study establishes a framework for experimental design and testing under relevant engine conditions.

100-kHz particle image velocimetry is demonstrated using a double-pulsed burst-mode laser with record length of 100 ms. Pulse doublets with inter-pulse spacing of 1–5 μs and repetition rate of 100 kHz are generated using a fiber-based oscillator and amplified through an all-diode pumped burst-mode amplifier. Pre-compensation of oscillator pulse width and pulse intensity enable control of output pulse energy even for unequally spaced pulse sequences. Spatial and temporal resolution of the velocity measurements is quantified and demonstrated to be sufficient for investigating space–time correlations in highly turbulent diffusion jets with jet-diameter Reynold’s Numbers from 14,000–43,000. Quantitative assesments of standard integral time and length scale definitions are made along with highly resolved space–time correlations and compared with Taylor’s Frozen Turbulence Hypothesis for turbulent flows.

An all-diode-pumped quasi-continuous burst-mode laser with burst period of 100 ms and pulse energy up to 225 mJ at 1064 nm has been developed. The high-speed laser incorporates a fiber amplifier and six diode-pumped amplifiers in a transportable 2.5-foot × 4-foot system. The repetition rate can be varied from 10–100 kHz resulting in pulse sequences of 1,000–10,000 pulses, a nearly ten-fold increase in record length over current state-of-the-art systems. Individual-pulse shaping allows temporal tailoring of the burst envelope, reducing pulse-to-pulse standard deviation two-fold and producing nearly flat-top bursts. Finally, the generation of pulse pairs with 1-μs spacing is enabled by acousto-optic modulation, allowing straightforward implementation of particle-image velocimetry with a single laser.

A method for simultaneous vibrational and rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectroscopy is presented for multispecies detection and improved temperature sensitivity over a wide range of flame conditions. N 2 /CH 4 vibrational and N 2 /O 2 /H 2 rotational Raman coherences are excited simultaneously using fs pump/Stokes pulses at 675/799 nm and 799/799 nm, respectively, and a fourth narrowband ps pulse at 799 nm is used to probe all coherence states at a time delay that minimizes non-resonant background and the effects of collisions. The strength of these transitions is concentration dependent, while the distribution among observed transitions can be related to temperature through the Boltzmann distribution of energy states. The multiplexed signal covers the entire range of temperatures observed in combustion reactants and products (298‐2300 K) with one measurement and can detect four species (N 2 , O 2 , CH 4 , H 2 ) using broadband excitation. This approach is discussed in the context of the feasibility for simultaneous, high-speed, interference-free measurements of key combustion species and temperature in flames with widely varying conditions.

We discuss recent experiments and modeling for the chirped-probe-pulse generation of single-laser-shot femtosecond pure-rotational CARS/CSRS spectra from room-temperature gases. A pure-rotational Raman coherence is impulsively generated using near-transformlimited femtosecond pump/Stokes excitation, and the coherence is probed by stretching a nominally 100-fs near-transform-limited probe beam to approximately 1.7 ps via the refractive-index dispersion in a 30-cm long flint-glass rod. The linearly chirped probe spectrum and phase beat against the time-dependent Raman polarization to generate complex spectra. Chirped-probe-pulse rotational CARS/CSRS offers an interesting alternative to hybrid fs/ps rotational CARS, in which a band-limited pulse of limited energy is used, because all of the available probe pulse energy can be retained in a chirped-probepulse experiment. Our early chirped-probe spectra are presented and the details of our initial model calculations are provided. The temperature sensitivity of the chirped-probe results is illustrated using calculated spectra.

The primary goal of this work is to explore optical diagnostic strategies for studying the mechanisms that control the reaction of novel energetic materials. Thus far, predictive models of these mechanisms have been elusive, and advanced diagnostics can help close the gap in the understanding of energetic initiation, ignition, shock-wave formation, propagation, and energy release. The proposed effort focuses on the development of optical diagnostics for imaging the reactions induced via laser ignition, including high-speed schlieren, two-color thermometry, and micro-optical imaging. Measurements are used to draw comparisons between the combustion of microand nano-scale aluminum, the effects of different fuel-oxidizer mixtures, and the influence of different passivation methods.

A new measurement technique for obtaining time- and spatially-resolved image sequences in hypersonic flows is developed. Nitric-oxide planar laser-induced fluorescence (NO PLIF) has previously been used to investigate transition from laminar to turbulent flow in hypersonic boundary layers using both planar and volumetric imaging capabilities. Low flow rates of NO were typically seeded into the flow, minimally perturbing the flow. The volumetric imaging was performed at a measurement rate of 10 Hz using a thick planar laser sheet that excited NO fluorescence. The fluorescence was captured by a pair of cameras having slightly different views of the flow. Subsequent stereoscopic reconstruction of these images allowed the three-dimensional flow structures to be viewed. In the current paper, this approach has been extended to 50,000 times higher repetition rates. A laser operating at 500 kHz excites the seeded NO molecules, and a camera, synchronized with the laser and fitted with a beam-splitting assembly, acquires two separate images of the flow. The resulting stereoscopic images provide three-dimensional flow visualizations at 500 kHz for the first time. The 200 ns exposure time in each frame is fast enough to freeze the flow while the 500 kHz repetition rate is fast enough to time-resolve changes in the flow being studied. This method is applied to visualize the evolving hypersonic flow structures that propagate downstream of a discrete protuberance attached to a flat plate. The technique was demonstrated in the NASA Langley Research Center s 31-Inch Mach 10 Air Tunnel facility. Different tunnel Reynolds number conditions, NO flow rates and two different cylindrical protuberance heights were investigated. The location of the onset of flow unsteadiness, an indicator of transition, was observed to move downstream during the tunnel runs, coinciding with an increase in the model temperature.

This work expands on previous studies to utilize hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) in high-temperature flames for background-free thermometry. The goal of the current work is to quantify the precision and accuracy of the temperature measurements over an expanded temperature range from 1250 – 2400 K, while also quantifying the influence of nonresonant background on the measurements. Temporal suppression of the nonresonant background by three orders of magnitude while retaining up to 20% of the time-zero signal is demonstrated while the background-free single-shot spectra show a signal-to-noise ratio of ~ 200:1 at ~2400 K. The accuracy and precision of the singleshot temperature measurements are investigated over an expanded range of 1250 – 2400 K to address hybrid fs/ps CARS applicability to lower temperature flames. The resulting accuracy and precision of the measurements is better than 10% and 3.5%, respectively, at all conditions. Finally, the effect of the nonresonant background on the accuracy and precision of the temperature measurement is quantified at ~2400 K, and the need for nonresonant-free detection is discussed.

In two separate test entries, advanced laser-based instrumentation has been developed and applied to visualize the hypersonic flow over cylindrical protrusions on a flat plate. Upstream of these trips, trace quantities of nitric oxide (NO) were seeded into the boundary layer. The protuberances were sized to force laminar-to-turbulent boundary layer transition. In the first test, a 10-Hz nitric oxide planar laser-induced fluorescence (NO PLIF) flow visualization system was used to provide wide-field-of-view, high-resolution images of the flowfield. The images had sub-microsecond time resolution. However these images, obtained with a time separation of 0.1 sec, were uncorrelated with each other. Fluorescent oil-flow visualizations were also obtained during this test. In the second experiment, a laser and camera system capable of acquiring NO PLIF measurements at 1 million frames per second (1 MHz) was used. This system had lower spatial resolution, and a smaller field of view, but the images were time correlated so that the development of the flow structures could be observed in time.

This paper reports a series of wind tunnel tests simulating the near-field behavior of the Space Shuttle Orbiter Boundary Layer Transition Detailed Test Objective (BLT DTO) flight experiment. Hypersonic flow over a flat plate with an attached BLT DTO-shaped trip was tested in a Mach 10 wind tunnel. The sharp-leading-edge flat plate was oriented at an angle of 20 degrees with respect to the freestream flow, resulting in post-shock edge Mach number of approximately 4. The flowfield was visualized using nitric oxide (NO) planar laser-induced fluorescence (PLIF). Flow visualizations were performed at 10 Hz using a wide-field of view and high-resolution NO PLIF system. A lower spatial resolution and smaller field of view NO PLIF system visualized the flow at 500 kHz, which was fast enough to resolve unsteady flow features. At the lowest Reynolds number studied, the flow was observed to be laminar and mostly steady. At the highest Reynolds number, flow visualizations showed streak instabilities generated immediately downstream of the trip. These instabilities transitioned to unsteady periodic and spatially irregular structures downstream. Quantitative surface heating imagery was obtained using the Temperature Sensitive Paint (TSP) technique. Comparisons between the PLIF flow visualizations and TSP heating measurements show a strong correlation between flow patterns and surface heating trends.

We describe the further development and application of a dual-injection-seeded optical parametric oscillator and associated kHz-rate burst-mode pump laser for high-speed OH planar laser-induced fluorescence in unsteady flames. A detailed overview of the novel double-pulse configuration of the burst-mode laser is given, indicating the potential for use in systems where both high energy and high repetition rate are necessary. Dual-wavelength injection-seeded operation of the optical parametric oscillator is demonstrated and preliminary OH PLIF line switching in a well-characterized natural-gas/air laminar diffusion flame is presented. Applications to high-speed nearly-instantaneous background capture for the elimination of laser scatter and background contamination from polycyclic aromatic hydrocarbons in fuel-rich hydrocarbon flames are also discussed and preliminary data are shown. Lastly, the potential for high-speed two-line planar OH thermometry is described including advantages of utilizing the burst-mode laser and multi-wavelength OPO for simplifying high-speed experimental systems.

NO PLIF imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 inch Mach 10 hypersonic wind tunnel. Approximately two hundred time correlated image sequences, of between ten and twenty individual frames, were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The majority of the image sequences were obtained from the boundary layer of a 20 flat plate model, in which transition was induced using a variety of cylindrical and triangular shaped protuberances. The high speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified. A series of image sequences were also obtained from a 20 compression ramp at a 10 angle of attack in which the temporal dynamics of the characteristic separated flow was captured in a time correlated manner.

We report on a source for MHz-rate, pulse-burst, tunable high-energy UV radiation and its application to high-speed imaging and temperature measurements of combustion based on planar laser-induced fluorescence (PLIF) spectroscopy of the OH radical. The system, comprised of a high-speed injection-seeded optical-parametric oscillator (OPO) pumped by a frequency-doubled MHz-rate pulse-burst Nd:YAG laser, was characterized in a laminar hydrogen-air diffusion flame stabilized over a near-adiabatic burner. High-speed OH PLIF was demonstrated at rates up to 50 kHz in a sonic hydrogen-air diffusion flame, showing the capability to capture turbulent fluid-flame interactions. In addition to high-speed OH PLIF at a fixed excitation wavelength, methods for planar spectroscopy are discussed, including wavelength scanning during a pulse burst as well as dual-wavelength injection seeding. Excitation scans of the UV radiation during a pulse burst can be achieved using various cavity stabilization techniques, such as discrete linearization of the OPO cavity modes. Preliminary work on an alternative two-line approach is also presented, with wavelength switching times as low as 20 ns. The OPO in this case is pumped by a double-pulsed burstmode pump laser with inter-pulse separation as low as 25 ns and pulse energies of 30mJ at 532nm. Feasibility studies of high-speed planar OH scanning and two-line spectroscopy are planned in laminar and turbulent flames.