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.
A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy ͑CARS͒ 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 ͑NO͒ to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the ͑1,0͒ band of the A 2 ⌺ ϩ-X 2 ⌸ electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the ͑0,0͒ band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.
River channel erosion by plucking is poorly understood even though it is a dominant mechanism for bedrock river profile evolution. In an experimental flume with fractured slabs of plaster model bedrock, plucking from a bed lacking protrusion is produced by nonuniform flow, particularly in rapidly varied flow with hydraulic jumps and free-surface undulations. Model bedrock slides upstream toward water-surface depressions in regions lacking recirculation, groups of blocks bulge up when a trough of free-surface waves moves above, and bubbles and debris particles move in the bed crack network. The likelihood and completeness of plucking increases with average flow strength but relies on local flow properties for initiation of motion. Particle image velocimetry (PIV) analysis of flow during a plucking event suggests that flow structures smaller than the average size of the blocks may be important in the plucking process by generating velocity or pressure variations around the blocks. Because plucking typically occurred near free-surface undulations and we have observations consistent with crack network flow, we propose that the mechanism driving block lift starts in the static pressure gradients developed in the sub-bed crack network, which are locally and temporally enhanced by turbulent pressure fluctuations. Positive feedback occurs when plucked blocks alter flow character and allow other blocks to slide around the bed, promoting additional plucking. Negative feedback occurs where the deposition of plucked blocks downstream of nonuniform-flow reaches limits transport capacity by changing or damping the nonuniform flow upstream. Our experimental results are consistent with previous engineering studies of slab uplift under plunging jets and high-Froude-number hydraulic jumps in energy-dissipating spillways. Our results also point toward the ability of nonuniform flow in bedrock rivers with a low Froude number to generate lift of fractured bedrock below steps and constrictions, and suggest a need for further study of mechanisms that initiate block plucking in experimental and field settings.
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.
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