Single-pulse Multi-point Multi-component Interferometric Rayleigh Scattering Velocimeter

Abstract

SummaryRayleigh scattering has previously been used for multi-component velocity measurements 1 . However, past work has generally been limited to excitation with continuous wave lasers resulting in a longer time response. We are developing a diagnostic system for time-resolved simultaneous measurement of multiple components of velocity in gas flow using a l0-ns pulsed YAG laser. This system will be combined with Coherent anti-Stokes Raman spectroscopy (CARS) thermometry 2 to allow simultaneous temperature and velocity measurements in supersonic combustion flows. Since we will use the same green laser beam for CARS and Rayleigh we will ensure that the temperature and velocity are measured at the same time and place. Dedicated optics collect the Rayleigh scattered light from different angles. This light is mixed together with the unshifted laser light before it is passed through a Fabry-Perot interferometer 3 . At the output of the interferometer, a high-sensitivity CCD camera images the interference fringe pattern. This pattern contains both the Rayleigh scattered signal from multiple collecting angles and the reference frequency information. Image processing software is used to analyze interferograms for Doppler shift to solve for velocity. Figure 1 shows three examples of interferograms containing frequency information to solve for one velocity component. Experiments were performed in stagnant air and a 3-mm diameter underexpanded jet flow. The jet had a total temperature of 300 K and a total pressure of about 4 atm and issued into ambient air at atmospheric pressure. We used a pulsed Nd:YAG laser at 10 Hz and 9 nsec, at 90 mJ/pulse. Measurements to date have included up to 2 velocity components simultaneously and a third component can be measured using the same method.To improve light collection efficiency each part of the optical system was analyzed and optimized, including light collecting optics, light transmission through optical fibers, interferometer mirrors, and CCD camera. The use of lenses for collecting Rayleigh scattered light combined with an optical fiber for transmission, introduced photon loses about 15%. More importantly, the number of photons transmitted by the interferometer input mirror was found to be low: 60 to one (98% of the photons are rejected by the etalon). Pulse stretching of the investigation laser beam of up to 20 nsec allowed an increase in the number of photons collected of up to 1.85 times.The image processing methodology makes use of linearization of the concentric interference patterns from a planar Fabry-Perot interferometer and subsequent analysis 3 . The important variables are the center of the interferogram and the maximum fringe shift of the Doppler shifted signal. There are four steps of interferogram processing: 1. The image center is found. 2. The image is mapped by reading it in r-θ plane, where r is the radius from the center and θ is the angle, using the original (θ, r) coordinate as the new (x, y) coordinate. 3. The coordinates of the obtained data are rescaled...

Simultaneous CARS and Interferometric Rayleigh Scattering

“…In this way more than one component of velocity can be measured in one interferogram. 8 Reference 8 shows several different approaches for combining Rayleigh signals to determine two or three velocity components simultaneously at multiple closely-spaced points. The vertically polarized laser beam with the wave vector k 0 is focused in a small region at the measuring location.…”

“…A second approach that permits a larger dynamic range of measurable velocities, and works at low 6,7,8 as well as at high 5 temperatures, is the interferometric Rayleigh scattering method. This technique has previously been used for single-point velocity, density, and temperature measurements.…”

“…Planar Fabry-Perot and confocal etalons were used for Rayleigh signal analysis in conjunction with either cooled or intensified CCD cameras, or photo-multiplier arrays to image the interferogram. 5,6,7,8, , , , 9 10 11 12 The shape of the Rayleigh scattering spectrum is a function of the parameter y = α/Ka, where α = p/µ is the collision frequency (p is the gas pressure and µ is the shear viscosity) and Ka is the acoustic spatial frequency (K is the wave vector and a is the speed of sound). For low density gases and high temperatures, when the parameter y << 1, the Rayleigh spectrum can be approximated with a Gaussian shaped spectrum.…”

“…The peak of the spectrum is Doppler shifted by a frequency proportional to the bulk velocity, the spectral width is proportional to the square root of the gas temperature, and its amplitude is proportional to the gas density. To obtain these gas parameters, the Rayleigh spectrum is analyzed by applying a fitting technique to the experimental data using the laser spectrum as reference 8 or by correlation. 7 In this paper we present for the first time the combination of a dual-pump CARS system with an interferometric Rayleigh scattering system (CARS-IRS) to provide time-resolved simultaneous measurement of multiple flow properties including velocity.…”

Direct-View Multi-Point Two-Component Interferometric Rayleigh Scattering Velocimeter

“…We reported in Reference 4 a single-shot laser-based technique, consisting of the combination of an interferometric Rayleigh scattering (IRS) system 5 with a dual-pump CARS system 6 on a common platform. It provides, in gaseous media, spatially and temporally resolved simultaneous measurements of two non-orthogonal components of velocity using IRS, as well as of temperature, and species concentration (N 2 , O 2 , and H 2 ).…”