Richard B. Miles

Tunable pulse-burst laser system for high-speed imaging diagnostics

The pulse-burst NdrYAG laser has recently been developed and demonstrated as a useful tool for capturing very high-speed dynamic processes in fluid flows. Images have been taken at up to 1 million frames per second using this laser, together with a new MHz-rate CCD camera. The work reported here seeks to extend the pulse-burst laser technology both to make the laser tunable and to enable it to operate at higher power. Tunability is important over a narrow frequency range in order to utilize the laser for Filtered Rayleigh Scattering, and over a broad frequency range for applications such as laserinduced fluorescence. A first step along this path is to use a seeded diode laser as the initial master oscillator. The diode itself gives short pulses, which lead to higher power output and better frequency doubling. By seeding the diode laser, its frequency can be controlled and its output tuned for Filtered *Graduate Student, Mechanical & Aerospace Engineering. Current Address: Associate Professor, Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210, Phone: 614-2923163, Member AIAA. s Professor, Mechanical & Aerospace Engineering, Associate Fellow AIAA. Copyright

Flow field diagnostics by spectrally filtered Rayleigh scattering

We report here preliminary results for a new technique which uses spectrally Filtered Rayleigh Scattering (FRS) to determine flow field parameters including density, temperature, and velocity. In addition, it may be used to suppress background scattering from windows and walls while simultaneously giving two-dimensional images of Rayleigh scattering which show flow field structure. Narrow linewidth laser light scattered from a high-speed gas flow is frequency shifted by an amount which is directly proportional to the velocity of the flow due to the Doppler effect. The spectral width of the scattered light is related to the temperature of the flow due to thermal broadening, and the scattering intensity is proportional to the gas density. A cell containing a gaseous atomic or molecular species is placed in front of a camera which observes the Rayleigh scattered light from the flow. If the illuminating laser is tuned through an absorption line of the gas in the cell, then the recorded intensity versus frequency of each resolvable portion of the flow will be related to its density, velocity, and temperature. By using a broad bandwidth, sharp cutoff spectral feature of the gas in the cell, scattering from the windows and walls can be simultaneously eliminated. If the scattering is predominately from particles in the flow rather than from air molecules, then the velocity can be measured, but temperature and density cannot.We report here preliminary results for a new technique which uses spectrally Filtered Rayleigh Scattering (FRS) to determine flow field parameters including density, temperature, and velocity. In addition, it may be used to suppress background scattering from windows and walls while simultaneously giving two-dimensional images of Rayleigh scattering which show flow field structure. Narrow linewidth laser light scattered from a high-speed gas flow is frequency shifted by an amount which is directly proportional to the velocity of the flow due to the Doppler effect. The spectral width of the scattered light is related to the temperature of the flow due to thermal broadening, and the scattering intensity is proportional to the gas density. A cell containing a gaseous atomic or molecular species is placed in front of a camera which observes the Rayleigh scattered light from the flow. If the illuminating laser is tuned through an absorption line of the gas in the cell, then the recorded intensity versus freque...

Comparisons between flow tagging measurements and computations in a complex rotating flow

The phenomena of vortex breakdown has been the subject of much attention because of its practical importance in many different situations, as well as its attraction as a fundamental problem in fluid mechanics. Experiments by Vogel [15, 16] showed that the flow produced by a rotating end wall inside a closed cylindrical container can undergo vortex breakdown under certain conditions. Only two non dimensional parameters, the aspect ratio 6 = ^ and the Reynolds number Re = TM-, completely determine the flow in this geometry. Escudier [4] extended the work of Vogel and examined this system using laser sheet fluorescence dye visualizations over the range 1000 < Re < 4000 and 1.0 < 6 < 3.5 and found that in this parameter range, the flow could undergo up to three breakdowns as well as become unsteady. Because of its simple geometry and completely specified boundary conditions, this flow is an excellent system, both computationally and experimentally, in which to study fundamental issues related to vortex breakdown. Using the PHANTOMM flow tagging technique we obtain quantitative measurements of velocity as well as visualizations similar to those obtained with stan•Graduate Student. Member AIAA t Professor. Associate Fellow AIAA Senior Research Scientist. Member AIAA This paper is a declared a work of the U.S. Government and is not subject to copyright protection in the United States. dard laser sheet fluorescence techniques. In addition, the axisymmetric Navier-Stokes equations are solved using a vorticity-stream function formulation. The computed flow is compared to measurements in order to evaluate and validate the PHANTOMM technique so that one can be confident of measurements in flow regimes where accurate computations are not available.

PHANTOMM flow tagging measurements in complex 3D flows

Using the PHANTOMM flow tagging technique we obtain quantitative measurements of velocity, as well as visualizations similar to those obtained with standard laser sheet fluorescence techniques, of the flow in a cylinder driven by a rotating end wall. In addition, the axisymmetric Navier-Stokes equations are solved numerically using a vorticity stream function formulation. The computed flow is compared to measurements in order to evaluate and validate the PHANTOMM technique so that one can be confident of measurements in flow regimes where accurate computations are not available. (Author)

PHANTOMM flow tagging measurements in a swirling, turbulent flow

The development of turbulence in a lid driven cylinder is studied using the PHANTO&IM flow tagging technique. Ensembles of line displacement images are used to compute, average velocities, turbulence intensities, and spatial correlations. These measurements are obtained over a Reynolds number range of 0 5 Re 5 2 x 10’ for a cylinder of aspect ratio of two (a = 2.0). Practical, experimental considerations related to the use of this technique are also discussed.

Observation of Vibrational Relaxation Dynamics in X(sup 3)Sigma(sup -)(sub g) Oxygen Following Stimulated Raman Excitation to the v=1 Level: Implications for the RELIEF Flow Tagging Technique

The vibrational relaxation of ground-state molecular oxygen (O2, X(sup 3)Sigma(sup -)(sub g)) has been observed, following stimulated Raman excitation to the first excited vibrational level (v=1). Time delayed laser-induced fluorescence probing of the ro-vibrational population distribution was used to examine the temporal relaxation behavior. In the presence of water vapor, the relaxation process is rapid, and is dominated by near-resonant vibrational energy exchange between the v=1 level of O2 and the n2 bending mode of H2O. In the absence of H2O, reequilibration proceeds via homogeneous vibrational energy transfer, in which a collision between two v=1 O2 molecules leaves one molecule in the v=2 state and the other in the v=0 state. Subsequent collisions between molecules in v=1 and v>1 result in continued transfer of population up the vibrational ladder. The implications of these results for the RELIEF flow tagging technique are discussed.

A single laser apparatus for writing patterns into unseeded air

In this paper we report a single laser apparatus for vibrationally exciting oxygen molecules in air, which is the first step in the RELIEF flow tagging method. This apparatus is based on new results for stimulated Raman scattering in high pressure O2/He mixtures. Use of stimulated Raman scattering rather than a dye laser to generate the Stokes beam for RELIEF flow tagging results in pump and Stokes beams which are automatically overlapped, both spatially and temporally. In addition, the stimulated Raman cell is a simple, passive device and the Nd:YAG pump laser is operated broadband, eliminating the need for injection seeding. The resulting short coherence length enhances the Raman conversion by suppressing the gain for stimulated Brillouin backscatter. Proper choice of He and O2 mole fractions and the total pressure is determined by the He and O2 pressure narrowing and pressure shifting of the O2 Q-branch spectrum. CARS measurements enable us to properly choose this operating point.In this paper we report a single laser apparatus for vibrationally exciting oxygen molecules in air, which is the first step in the RELIEF flow tagging method. This apparatus is based on new results for stimulated Raman scattering in high pressure O2/He mixtures. Use of stimulated Raman scattering rather than a dye laser to generate the Stokes beam for RELIEF flow tagging results in pump and Stokes beams which are automatically overlapped, both spatially and temporally. In addition, the stimulated Raman cell is a simple, passive device and the Nd:YAG pump laser is operated broadband, eliminating the need for injection seeding. The resulting short coherence length enhances the Raman conversion by suppressing the gain for stimulated Brillouin backscatter. Proper choice of He and O2 mole fractions and the total pressure is determined by the He and O2 pressure narrowing and pressure shifting of the O2 Q-branch spectrum. CARS measurements enable us to properly choose this operating point.