Molecular Rayleigh scattering offers a means to measure gas flow
parameters including density, temperature, and velocity. No seeding
of the flow is necessary. The Rayleigh scattered power is proportional
to the gas density, the spectral width is related to the gas temperature,
and the shift in the frequency of the spectral peak is proportional
to one component of the fluid velocity. Velocity measurements
based on Rayleigh scattering are more suitable for high-speed
flow, where the bulk fluid velocity is on the order of, or larger
than, the molecular thermal velocities. Use of ultraviolet wavelengths
for Rayleigh scattering diagnostics is attractive for two reasons.
First, the Rayleigh scattering cross section is proportional to
the inverse 4th power of the wavelength. And second, the reflectivity
of metallic surfaces is generally less than it is at longer wavelengths.
This is of particular interest in confined flow situations, such
as in small wind tunnels and aircraft engine components, where
the stray laser light scattered from the windows and internal
surfaces in the test facility limits the application of Rayleigh
scattering diagnostics.
In this work at the NASA Lewis Research Center, molecular Rayleigh
scattering of the 266-nm fourth harmonic of a pulsed, injectionseeded
Nd:YAG (neodymium:yttrium-aluminum-garnet) laser was used to measure
velocity in a supersonic free air jet with a 9.3-mm exit diameter.
The frequency of the Rayleigh scattered light was analyzed with
a planar mirror Fabry-Perot interferometer used in a static
imaging mode, with the images recorded on a
cooled, high-quantum-efficiency charge-coupled discharge (CCD)
camera. In addition, some unshifted light from the same laser
pulse was imaged through the interferometer to generate a reference.
Data were obtained with single laser pulses at velocities up to
Mach 1.3. The measured velocities were in good agreement with
velocities calculated from isentropic flow relations.
Our conclusion from this study was that ultraviolet Rayleigh scattering
is preferable in confined flow situations because of the increase
in the ratio of Rayleigh scattering signal to stray laser light.
On the other hand, in open flows, such as free jets and larger
wind tunnels where stray laser light can be controlled, visible
Rayleigh scattering is preferable.
Seasholtz, R.G.: Single-Shot Spectrally Resolved UV Rayleigh
Scattering Measurements in High Speed Flow. NASA TM-107323, 1996.
Lewis contact: Dr. Richard G. Seasholtz, (216) 433-3754,
Richard.G.Seasholtz@grc.nasa.gov
Author: Dr. Richard G. Seasholtz
Headquarters program office: OA
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