Multiple-Point Mass Flux Measurement System Using Rayleigh Scattering

Abstract

[Abstract] A multiple-point Rayleigh scattering diagnostic is being developed to provide mass flux measurements in gas flows. Spectroscopic Rayleigh scattering is an established flow diagnostic that has the ability to provide simultaneous density, temperature, and velocity measurements. Rayleigh scattered light from a focused 18 Watt continuous-wave laser beam is directly imaged through a solid Fabry-Perot etalon onto a CCD detector which permits spectral analysis of the light. The spatial resolution of the measurements is governed by the locations of interference fringes, which can be changed by altering the etalon characteristics. A prototype system has been used to acquire data in a Mach 0.56 flow to demonstrate feasibility of using this system to provide mass flux measurements. Estimates of measurement uncertainty and recommendations for system improvements are presented.

Progress on a Rayleigh Scattering Mass Flux Measurement Technique

“…A detailed discussion of the theory of Fabry-Perot interferometry as applied to this type of Rayleigh scattering experiment can be found in Ref. 8. Figure 4 shows a typical fringe pattern that results when a planar single-frequency light source is imaged through a Fabry-Perot etalon.…”

“…In more recent work a rigid solid fused-silica FP etalon has been used to allow spectral analysis to take place in close proximity to the harsh facilities. A solid etalon was used to implement an approach to provide spatially-resolved Rayleigh mass flux measurements in a free jet by imaging scattered light from a laser beam in the flow field directly through the etalon 8 . A similar technique has also been applied in the Rayleigh velocimetry work of Bivolaru et al The original intent for the Rayleigh system developed in this work for use in the 15 cm x 15 cm Supersonic Wind Tunnel (SWT) at NASA GRC, was to install a direct-imaging system like the one developed in Reference 8.…”

Study of Fabry-Perot Etalon Stability and Tuning for Spectroscopic Rayleigh Scattering

“…It is often necessary in Rayleigh experiments to set the diameter of the innermost fringe to a specific location such that it falls within the illuminated image region at the detector or to set the fringe diameter to a value that minimizes the measurement uncertainty 9 . The tuning feature may also be used in multiple-point Rayleigh measurement systems to change the physical locations of the measurements in the flow field, which are set by the interference fringe locations in the image 7 . This feature could also be used to stabilize the etalon system in a non-thermally-controlled testing environment where controlling the etalon temperature cannot provide the required stability, as mentioned earlier.…”

“…However, in harsh conditions and environments it is advantageous to use a solid etalon, since vibrations can change the plate spacing of an air-spaced interferometer and corrupt the spectral measurements. Some recent studies concerning Rayleigh scattering have utilized the solid etalon in place of the air-spaced interferometer [5][6][7] . It is often desirable to adjust the positions of the interference fringes or to maintain a fixed radial position of the fringes for various reasons.…”

“…ayleigh scattering is an optical diagnostic technique that uses elastically scattered light from gas molecules to provide density, velocity, and temperature measurements [1][2][3][4][5][6][7][8] . One way to determine these properties from the scattered light is to analyze the signal strength, Doppler frequency shift, and spectral linewidth of the scattered light, which directly relate to density, velocity, and temperature, respectively.…”