Publications

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We report 100-kHz burst-mode planar laser-induced fluorescence imaging of the nitricoxide molecule in the free-piston facility at Sandia National Laboratories. Cylinder wakestartup transients are visualized in high-temperature (T ~ 3000 K) post-shock flow with the facility in the shock-tube mode of operation. In the reflected shock-tunnel mode, NO PLIF visualization of a 4.6-MJ/kg, 3 km/s startup flow over a cylinder is presented, and free-stream molecular-tagging velocimetry exploiting the long fluorescence lifetime of free-stream NO is demonstrated.

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High-enthalpy hypersonic flight represents an application space of significant concern within the current national-security landscape. The hypersonic environment is characterized by high-speed compressible fluid mechanics and complex reacting flow physics, which may present both thermal and chemical nonequilibrium effects. We report on the results of a three-year LDRD effort, funded by the Engineering Sciences Research Foundation (ESRF) investment area, which has been focused on the development and deployment of new high-speed thermochemical diagnostics capabilities for measurements in the high-enthalpy hypersonic environment posed by Sandia's free-piston shock tunnel. The project has additionally sponsored model development efforts, which have added thermal nonequilibrium modeling capabilities to Sandia codes for subsequent design of many of our shock-tunnel experiments. We have cultivated high-speed, chemically specific, laser-diagnostic approaches that are uniquely co-located with Sandia's high-enthalpy hypersonic test facilities. These tools include picosecond and nanosecond coherent anti-Stokes Raman scattering at 100-kHz rates for time-resolved thermometry, including thermal nonequilibrium conditions, and 100-kHz planar laser-induced fluorescence of nitric oxide for chemically specific imaging and velocimetry. Key results from this LDRD project have been documented in a number of journal submissions and conference proceedings, which are cited here. The body of this report is, therefore, concise and summarizes the key results of the project. The reader is directed toward these reference materials and appendices for more detailed discussions of the project results and findings.

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This work presents a high-speed laser-absorption-spectroscopy diagnostic capable of measuring temperature, pressure, and nitric oxide (NO) mole fraction in shock-heated air at a measurement rate of 500 kHz. This diagnostic was demonstrated in the High-Temperature Shock Tube (HST) facility at Sandia National Laboratories. The diagnostic utilizes a quantum-cascade laser to measure the absorbance spectra of two rovibrational transitions near 5.06 µm in the fundamental vibration bands (v" = 0 and 1) of NO in its ground electronic state (X2 Π1/2 ). Gas properties were determined using scanned-wavelength direct absorption and a recently established fitting method that utilizes a modified form of the time-domain molecular free-induction-decay signal (m-FID). This diagnostic was applied to acquire measurements in shock-heated air in the HST at temperatures ranging from approximately 2500 to 5500 K and pressures of 3 to 12 atm behind both incident and reflected shocks. The measurements agree well with the temperature predicted by NASA CEA and the pressure measured simultaneously using PCB pressure sensors. The measurements presented demonstrate that this diagnostic is capable of resolving the formation of NO in shock-heated air and the associated temperature change at the conditions studied.

A new reflected shock tunnel has been commissioned at Sandia capable of generating hypersonic environments at realistic flight enthalpies. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25 cm core flow with 4.6 MJ/kg total enthalpy is achieved over an approximately 1 millisecond test time. Analysis shows that increasing piston mass should extend test time by a factor of 2-3.

Coherent anti-Stokes Raman scattering of the N2 molecule is performed at rates up to 100 kHz for thermometry in the Sandia free-piston, high-temperature shock-tube facility (HST) for reflected-shock conditions in excess of T = 4000 K at pressures up to P = 10 atm. A pulse-burst laser architecture delivers picosecond-duration pulses to provide both the CARS pump and probe photons, and to pump a solid-state optical parametric generator (OPG)/optical parametric amplifier (OPA) source, which provides frequency tunable Stokes pulses with a bandwidth of 100-120 cm-1 . Single-laser-shot and averaged CARS spectra obtained in both the incident (P = 1.1 atm, T = 2090 K) and reflected (P ~ 8-10.5 atm, T > 4000 K) shock regions of HST are presented. The results indicate that burst-mode CARS is capable of resolving impulsive, high-temperature events in HST.

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Measurements of bifurcated reflected shocks over a wide range of incident shock Mach numbers, 2.9 < Ms < 9.4, are carried out in Sandia’s high temperature shock tube. The size of the non-uniform flow region associated with the bifurcation is measured using high speed schlieren imaging. Measurements of the bifurcation height are compared to historical data from the literature. A correlation for the bifurcation height from Petersen et al. [1] is examined and found to over estimate the bifurcation height for Ms > 6. An improved correlation is introduced that can predict the bifurcation height over the range 2.15 < Ms < 9.4. The time required for the non-uniform flow region to pass over a stationary sensor is also examined. A non-dimensional time related to the induced velocity behind the shock and the distance to the endwall is introduced. This non-dimensional time collapses the data and yields a new correlation that predicts the temporal duration of the bifurcation.