A quantitative comparison of an over driven light emitting diode (LED) and a high intensity discharge lamp (HID) as illumination sources for high-speed schlieren imaging is presented. A custom pulser circuit utilizing a new and improved driver circuit was developed to overdrive the LED by a factor of ten while simultaneously reducing pulse widths to sub-microsecond durations. The LED system has been developed as a simple and inexpensive alternative light source to discharge lamps and pulsed laser systems, which are typical for high-speed schlieren imaging. Image quality of a decaying spherical shock wave, produced from the unsteady release of an under-expanded helium jet, is analyzed to assess comparative performance. The effects of framing rate, camera exposure time, and pulse duration on image quality were assessed and compared for the novel LED and a high intensity discharge lamp. Framing rates of 10,000 fps and 50,000 fps and exposure times of 1 µs and 10 µs were tested. Image quality was assessed qualitatively through side-by-side comparisons of fluid dynamic features such as the resolution of shock waves, compression waves, and shear layers. Quantitative analysis was performed through the comparison of the signal-to-noise ratio at the various conditions. LED performance was found to be superior when imaging fast events and inferior when imaging slower events. Results and potential system improvements indicate that the LED system is ideal for low-cost, high-speed flow imaging. Keywords schlieren imaging-light emitting diode (LED)-high intensity discharge (HID) lamp-shock propagation 1 Introduction Recent advances in digital camera technology have allowed for the visualization of high-speed, unsteady flows with high spatial and temporal resolution at a reasonable cost. This has enabled new experimental work to be performed on applications related to explosions and hypersonic aerodynamics using high-speed schlieren photography. For example, Mizukaki et al. (2013) performed background oriented schlieren (BOS) measurements of an open-air detonation of a C-4 explosive [1]. The optical displacement of natural obstacles in the background such as trees and grass were used to extract the shock wave overpressure during the explosion. In these experiments, a Phantom v10 (Vision Research Inc., NJ) camera, providing 800 x 600 pixels at 10,000 fps, was used. In an explosion chamber, Ciccarelli et al., (2013) used schlieren photography to study the flame acceleration process responsible for deflagration-to-detonation transition (DDT) [2]. A FASTCAM SA5 (Photron, CA) camera was used, which provides 1024 x 744 pixels at 10,000 fps. Saravanan et al., (2011) used a high-speed schlieren system in a shock tunnel to study the hypersonic flow around a missile-shaped body [3]. A Phantom v7.2 (Vision Research Inc., NJ) camera was used to provide 450 x 450 pixels at a framing rate of 10,000 fps.

Shock wave formation and acceleration in a high-aspect ratio cross section shock tube were studied experimentally and numerically. The relative importance of geometric effects and diaphragm opening time on shock formation are assessed. The diaphragm opening time was controlled through the use of slit-type (fast opening time) and petal-type (slow opening time) diaphragms. A novel method of fabricating the petal-type diaphragms, which results in a consistent burst pressure and symmetric opening without fragmentation, is presented. High-speed schlieren photography was used to visualize the unsteady propagation of the lead shock wave and trailing gas dynamic structures. Surfacemounted pressure sensors were used to capture the spatial and temporal development of the pressure field. Unsteady Reynolds-Averaged Navier-Stokes simulation predictions using the shear-stress-transport turbulence model are compared to the experimental data. Simulation results are used to explain the presence of high-frequency pressure oscillations observed experimentally in the driver section as well as the cause of the initial acceleration and subsequent rapid decay of shock velocity measured along the top and bottom channel surfaces. A one-dimensional theoretical model predicting the effect of the finite opening time of the diaphragm on the rate of driver depressurization and shock acceleration is proposed. The model removes the large amount of empiricism that accompanies existing models published in the literature. Model accuracy is assessed through comparCommunicated by isons with experiments and simulations. Limitations of and potential improvements in the model are discussed.

Simultaneous nitric-oxide and atomic-oxygen laser-induced-fluorescence experiments were performed in the Hypersonic Materials Environmental Test System facility at the NASA Langley Research Center. The data serve as an experimental database for validation for chemical and thermal nonequilibrium models used in hypersonic flows. Measurements were taken over a wide range of stagnation enthalpies (6.7−18.5 MJ∕kg) using an Earth atmosphere simulant with a composition of 75% nitrogen, 20% oxygen, and 5% argon (by volume). These are the first simultaneous measurements of nitric-oxide and atomic-oxygen laser-induced fluorescence to be reported in literature for the Hypersonic Materials Environmental Test System facility. The maximum atomic-oxygen laser-inducedfluorescence mean signal intensity was observed at a stagnation enthalpy of approximately 12 MJ∕kg, whereas the maximum nitric-oxide laser-induced-fluorescence mean signal intensity was observed at a stagnation enthalpy of 6.7 MJ∕kg. The experimental results were compared to a fluorescence model that assumes equilibrium conditions in the plenum and frozen chemistry in an isentropic nozzle expansion (Mach 5). The equilibrium calculations were performed using CANTERA v2.1.1 with 16 species. The fluorescence model captured the correlation in mean atomicoxygen laser-induced-fluorescence and nitric-oxide laser-induced-fluorescence signal intensities over a large range of stagnation enthalpies tested. The agreement between equilibrium calculations and the experimental laser-induced fluorescence signals was better for nitric oxide than atomic oxygen. Very weak correlations between single-shot atomic-oxygen laser-induced-fluorescence and nitric-oxide laser-induced-fluorescence intensities were observed in the experiments at all of the stagnation enthalpy conditions. It was found that the overall magnitude of nitric-oxide laser-induced-fluorescence fluctuations was much larger than atomic-oxygen laser-induced-fluorescence fluctuations.

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