Robert L. McKenzie

Measurement capabilities of planar Doppler velocimetry using pulsed lasers

Analytical models of a spectral filter that contains iodine vapor and of the noise sources associated with charge-coupled-device (CCD) detector technology are combined with a planar Doppler velocimetry (PDV) signal analysis to evaluate the measurement capabilities of PDV for quantitative aerodynamic research and production wind-tunnel testing applications. The criteria for optimizing the filter cell and calibrating the frequency scale of its transmission function are described. The measurement uncertainty limits owing to scientific-grade CCD detector performance are then evaluated, and an analysis is developed of the scattering properties of aerosols suitable for aerodynamic flow seeding. The combined results predict that single-pulse PDV measurements with velocity measurement uncertainties as small as 2 m/s should be possible in aerodynamic test facilities for measurement distances of tens of meters.

Method for measuring temperatures and densities in hypersonic wind tunnel air flows using laser-induced O 2 fluorescence

Laser-induced fluorescence in oxygen, in combination with Raman scattering, is shown to be an accurate means by which temperature, density, and their fluctuations owing to turbulence can be measured in air flows associated with high speed wind tunnels. For temperatures above 60 K and densities above 0.01 amagat, the uncertainties in the temperature and density measurements can be <2%, if the signal uncertainties are dominated by photon statistical noise. The measurements are unaffected by collisional quenching and can be achieved with laser fluences for which nonlinear effects are insignificant. Temperature measurements using laser-induced fluorescence alone have been demonstrated at known densities in the range of low temperatures and densities which are expected in a hypersonic wind tunnel.

Measurements of temperature, density, pressure, and their fluctuations in supersonic turbulence using laser-induced fluorescence

A laser-induced fluorescence (LIF) method has been developed that provides simultaneous measurements of temperature, density, and their fluctuations owing to turbulence in unheated compressible flows. Pressure and its fluctuations are also deduced using the equation of state. Fluorescence is induced in nitric oxide that has been seeded into a nitrogen flow in concentrations of 100 ppm. Measurements are obtained from each laser pulse, with a spatial resolution of 1 mm and a temporal resolution of 125 ns. The method was applied to a supersonic, turbulent, boundary-layer flow with a free-stream Mach number of 2. For stream conditions in the range from 150–300 K and 0.3–1 atm, temperature is measured with an uncertainty of approximately 1% rms, while density and pressure uncertainties are approximately 2% rms.

Diatomic Gasdynamic Lasers

Predictions from a numerical model of the vibrational relaxation of anharmonic diatomic oscillators in supersonic expansions are used to show the extent to which the small anharmonicity of gases like CO can cause significant overpopulations of upper vibrational states. When mixtures of CO and N2 are considered, radiative gain on many of the vibration‐rotation transitions of CO is predicted. Experiments are described that qualitatively verify the predictions by demonstrating laser oscillation in CO‐N2 expansions. The resulting CO‐N2 gasdynamic laser displays performance characteristics that equal or exceed those of similar CO2 lasers.

Vibration-translation energy transfer in anharmonic diatomic molecules. I - A comparative evaluation of the semiclassical approximation

The semiclassical approximation (quantum oscillator, classical path) is applied to anharmonic diatomic oscillators in excited initial states. Multistate numerical solutions giving the vibrational transition probabilities for collinear collisions with an inert atom are compared with equivalent, exact qunatum‐mechanical calculations. Several symmetrization methods are shown to correlate accurately the predictions of both theories for all initial states, transitions, and molecular types tested, but only if coupling of the oscillator motion and the classical trajectory of the incident particle is considered. In anharmonic heteronuclear molecules, the customary semiclassical method of computing the classical trajectory independently leads to transition probabilities with anomalous low‐energy resonances. Proper accounting of the effects of oscillator compression and recoil on the incident particle trajectory removes the anomalies and restores the applicability of the semiclassical approximation.

Radiative processes in air excited by an ArF laser

The emission spectrum of air that is excited by an ArF laser has been investigated experimentally and theoretically to determine the conditions under which fluorescence from O2 can be used for the measurement of temperature in aerodynamic flows. In addition to the expected fluorescence from O2, the spectrum from excitation with an intense laser beam is shown to contain significant contributions from the near-resonant Raman fundamental and overtone bands, the four-photon fluorescence excitation of C produced from ambient CO2, and possibly the three-photon excitation of O2+. The nature of the radiative interactions contributing to these additional features is described.

The influence of molecular rotation on vibration–translation energy transfer

The role of molecular rotations in the exchange of vibrational and translational energy is investigated for collisions between anharmonic diatomic molecules and structureless atoms. A three‐dimensional, semiclassical, impact parameter description is applied with emphasis directed towards the influence of rotational coupling on the net rate of vibrational energy transfer summed over all final rotational states. These results are then related to the predictions of an equivalent collinear collision model, and their comparison allows an evaluation of the collinear approximation. The mechanisms of vibrational energy transfer including rotational transitions are shown to be separable into three classes, with the molecules belonging to each class identified first and foremost by their ratio of fundamental vibrational and rotational frequencies, ωe/Be, and second by the proximity of their initial state to a near‐resonant vibration–rotation transition with a small change in angular momentum. While the dynamics of ...

ERRATA: LASER POWER AT 5‐μm FROM THE SUPERSONIC EXPANSION OF CARBON MONOXIDE

Laser radiation at wavelengths between 4.78 and 5.4μm has been obtained from the supersonic expansion of CO in mixtures of N2 and Ar. The ratio of laser power to the total enthalpy flow rate supplied to the nozzle reservoir was equal to or greater than that obtained from similar CO2 gas‐dynamic lasers.

The two‐photon absorptivity of rotational transitions in the A 2Σ+(v′ = 0)–X 2Π(v′′ = 0) gamma band of nitric oxide

A predominantly single‐mode pulsed dye laser system giving a well‐characterized spatial and temporal output suitable for absolute two‐photon absorptivity measurements was used to study the NOγ(0,0)S11+R21(J\ = 7‐1/2) transition. Using a calibrated induced‐fluorescence technique, an absorptivity parameter of 2.8±1.4×10−51 cm6 was obtained. Relative strengths of other rotational transitions in the γ(0,0) band were also measured and shown to compare well with predicted values in all cases except the 012(J\ = 10‐1/2) transition.

Single-pulse measurements of density and temperature in a turbulent, supersonic flow using UV laser spectroscopy.

Nonintrusive measurements of density and temperature and their turbulent fluctuation levels have been obtained in the boundary layer of an unseeded, Mach 2 wind tunnel flow. The spectroscopic technique that was used to make the measurements is based on the combination of laser-induced oxygen fluorescence and Raman scattering by oxygen and nitrogen from the same laser pulse. Results from this demonstration experiment compare favorably with previous measurements obtained in the same facility from conventional probes and an earlier spectroscopic technique.