Joel P. Kuehner

Electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy of nitric oxide

A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.

High-resolution broadband N2 coherent anti-Stokes Raman spectroscopy: comparison of measurements for conventional and modeless broadband dye lasers.

We have performed high-resolution N2 coherent anti-Stokes Raman spectroscopy (CARS) measurements using a modeless dye laser (MDL) as the Stokes beam source to determine the effects of a reduction in mode noise on the accuracy and precision of the method. These results are compared with previous research that employed a conventional broadband dye laser (CBDL) as the Stokes beam source. A new spectral-fitting procedure was developed to avoid starting-point bias in the least-squares fitting results, which possibly had altered the previous measurements. Single-shot measurements of pressure were performed in a static-pressure vessel over the range of 0.1-4.0 atm to examine the pressure sensitivity of the technique. The precision of these measurements is a measure of the baseline noise level of the system, which sets the detection limit for flow-field pressure fluctuations. Centerline measurements of pressure and temperature in an underexpanded jet (Mj = 1.85) were also used to determine the performance of the technique in a compressible flow field. Our study represents the first known application, to our knowledge, of a MDL CARS system in a low-temperature, low-pressure supersonic environment. Improvements in accuracy for mean single-shot measurements and increased precision were found for pressure vessel conditions above 1.0 atm. For subatmospheric pressure vessel conditions (0.1-1.0 atm) and the underexpanded jet measurements, there was a decrease in accuracy and precision compared with the CBDL results. A comparison with the CBDL study is included, along with a discussion of the MDL system behavior.

Perturbative theory and modeling of electronic-resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy of nitric oxide.

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal. The frequency of the ultraviolet probe beam is selected to be in electronic resonance with rotational transitions in the A (2)Sigma(+)<--X (2)Pi (1,0) band of NO. This choice results in a resonance between the frequency of the ERE-CARS signal and transitions in the (0,0) band. The theoretical model for ERE-CARS NO spectra has been developed in the perturbative limit. Comparisons to experimental spectra are presented where either the probe laser was scanned with fixed Stokes frequency or the Stokes laser was scanned with fixed probe frequency. At atmospheric pressure and an NO concentration of 100 ppm, good agreement is found between theoretical and experimental spectral peak locations and relative intensities for both types of spectra. Factors relating to saturation in the experiments are discussed, including implications for the theoretical predictions.

Planar Fluorescence Imaging of a Supersonic Axisymmetric Base Flow with Mass Bleed

Acetone planar laser-induced fluorescence (PLIF) was used to study the supersonic flow over an axisymmetric base with mass bleed. Detailed side-view and global end-view images were obtained in the recirculation regions, shear layer, and trailing wake. For the three bleed rates studied, the instantaneous behavior of the flowfield deviated substantially from the expected mean flow, especially for bleed rates below the optimal case. Large pockets of unmixed bleed fluid dominated the flowfield for the low and intermediate bleed rates, even in the trailing wake, highlighting the unsteady nature of the flowfield. The power-on condition for the high-bleed-rate case was the most temporally stable flowfield, due to the high momentum of the bleed jet. A multiplateau stirring effect was prevalent for all cases, consistent with previous compressible shear layer findings, and the effect increased with increasing bleed rate and distance downstream. Probability density functions (PDFs) of PLIF intensity indicated a similar mixing character between the low and intermediate bleed rates in the near wake and for all bleed rate cases in the trailing wake. The multiplateau stirring effect and unmixed bleed fluid pockets were represented well in the mixing PDFs.

HIGH-RESOLUTION N2 CARS MEASUREMENTS OF PRESSURE, TEMPERATURE, AND DENSITY USING A MODELESS DYE LASER

In this study, high-resolution N2 CARS measurements have been made using a modeless dye laser as the Stokes beam source to reduce the presence of mode noise. Along with this modification to the CARS technique, a new spectra fitting procedure was developed to avoid a starting-point bias in the leastsquares fitting results. Time-averaged and single-shot measurements of pressure were made in a static pressure vessel over the range of 0. 1t o 4.0 atm to test the pressure sensitivity of the technique. In addition, the precision of single-shot measurements in the pressure vessel is indicative of the baseline pressurefluctuation detection limit of the system. The precision uncertainty of the measurements was studied to investigate the possibility of making property fluctuation measurements in high-speedflows. Centerline measurements of pressure and temperature in an underexpanded jet (M j =1 .85) were also used to determine the performance of the technique in a compressible flowfield. Improvements in accuracy for time-averaged and single-shot mean measurements and increased precision were found for pressure levels above 1.0 atm. For the subatmospheric pressure levels that are important in high-speed flows, the results indicated that the current method is incapable of making fluctuation measurements due to limited precision. Nevertheless, the increased precision above 1.0 atm indicates that fluctuation measurements may be possible with further modifications.

Pressure, temperature, and density measurements using high-resolution N2 CARS

* Graduate Research Assistant. Student Member AIAA. † Professor. Senior Member AIAA. ‡ W. Grafton and Lillian B. Wilkins Professor. Associate Fellow AIAA. ABSTRACT Mean pressure, temperature, and density measurements have been made in a gas cell and along the centerline of an underexpanded jet using highresolution N2 CARS. This method takes advantage of the line-broadening effects and population shifts of the rotational structure in the nitrogen ( υ = → 0 1) Qbranch which are pressureand temperature-sensitive. Experimental CARS spectra were acquired with high spectral resolution, ∆ω = − − 016 018 1 . . cm . Theoretical spectra, created by a N2 spectral modeling program, were fit to the experimental spectra in a least-squares manner. Temperature and pressure were determined as adjustable parameters in the least-squares fit and density was then determined through an equation-ofstate. For gas cell pressures between 1 and 5 atm, the pressure values extracted from the CARS spectra exhibited an approximately 0.15 atm bias above the transducer pressures, probably because of Raman saturation effects. However, in the underexpanded jet measurements, our Stokes laser power was lower and agreement between measured and predicted pressures and temperatures along the jet centerline was excellent. The potential of performing pressure and temperature measurements on single laser shots was also examined.

Characteristics of a Central Bleed Jet in Supersonic Axisymmetric Base Flow

The near-wake flow structure of a supersonic axisymmetric base flowwith base bleed is investigated in detail using acetone planar laser-induced fluorescence. Global side-view and end-view images are analyzed to detect unstirred bleed fluid, which has been found to persist from the bleed orifice through reattachment (i.e., rear stagnation point). The characteristic size, shape, and location of the bleed fluid pockets are studied. This enables a comparison of the bleed jet structure for three different cases of mass flow rate of bleed fluid. In addition, the average location of the forward stagnation point and the primary and secondary recirculation regions are linked to the distributions of several statistics. The similarities and differences between two suboptimal bleed rate cases identify possible optimal locations for the central bleed jet to be directed. A stabilizing effect caused by increasing bleed rate is documented in many of the statistical quantities. Agreement between side-view and end-view image analysis confirms the ability of the processing technique to distinguish the unstirred bleed fluid in all images.