The list also includes papers on related topics, e g molecular kinetic models and Mie scattering techniques. The articles are sorted chronologically with the last name of the first author used as a secondary sorting criterion for articles published the same year.

This paper discusses how the kinetic theory for monatomic gases can be extended to account for the internal degrees of freedom found in polyatomic gases.

This article shows how the Mandelstahm-Brillouin triplet is obtained from the BGK kinetic model under different conditions.

This article discusses constant-relaxation time kinetic modeling of a fluid as a means of finding its light scattering spectrum. The Morse (1964) model incorporates as well elastic as inelastic collisions modeled using separate collision frequencies or relaxation times. Inelastic collisions are the source of the energy transfer between internal and external energy modes in polyatomic molecules. The deviations from equilibrium are assumed to be small, allowing the equations to be linearized. The paper briefly notes that when fluctuations in temperature and molecular orientation are small, the scattered spectrum will be determined by (the double Fourier transform of) the time-dependent density correlation function G(r,t). The authors found that in the hydrodynamic (low wavelength to mean free path ratio), two temperatures (macroscopic and internal) gave a significantly better fit to experimental data than a single temperature. The experiments used in the comparison used a HeNe laser and CO₂ and CH₄ near ambient conditions and the deviations in the relative intensities of the Rayleigh and the Brillouin components between the model and the measurements are significant. One source of the differences may be inaccuracies in the applied values of elastic and inelastic collision frequencies.

Discussion of a 6-moment model for Rayleigh scattering broadening due to molecular movement and correlation in the kinetic range. This so-called Tenti S6 model is used frequently in later paper to determine the temperature based on an observed scattered light spectrum.

Historical discussion on the nomenclature of Rayleigh, Raman and Brillouin lines and on the development of a modern scattering theory primarily developed by Lord Rayleigh.

This article is the first to propose use of molecular vapor filters to remotely determine atmospheric temperature, pressure and aerosol content through studying the intensity and spectral composition of the back-scattered laser radiation. Molecular vapor filters offers higher rejection ratios and spectral resolution as well as better temperature stability than interference filters used previously. The article discusses separation of Mie and Rayleigh scattering based on their different bandwidths and supplies more references to papers on kinetic modeling of Rayleigh-Brillouin spectra of gas mixtures than later FRS papers. Based on earlier data, it is suggested that air can be treated as a single-species gas with a molecular weight of 28.8 g for kinetic modeling. It is discussed how a small temperature or pressure change affects the scattered spectrum and how the sign changes for this effect will determine a suitable range over which to integrate the light using tailored absorption filters. Several laser-filter systems are suggested based on a presentation of the factors necessary to consider in picking a suitable system. Since the article was written before the advent of the affordable Nd:YAG laser, alexandrite and dye lasers are suggested along with filters utilizing barium, rubidium, cesium and lead atomic absorption lines. A procedure to calculate atmospheric properties based on the measurements is presented and some sample calculations are carried out to demonstrate the potential performance of a dye laser/barium filter system. The authors also present some I₂ filter transmission curves obtained experimentally.

This article offers a good introduction with a brief discussion of Rayleigh-Brillouin scattering and how iodine cells can be used for background scatter suppression as well as the effects that the gas temperature and pressure has on the scattering spectrum. A few images of a M=2.5 boundary layer flow are used to illustrate the effect of varying the frequency of the incident laser light across the absorption range of the iodine cell are included. The authors favor using the 50% transmission point to determine the lines of constant velocity while minimizing any effects of non-uniform gas temperature and pressure.

Shear layer between M=2 and 3 layers in dry air

Step-by-step description on how to set up FRS experiment

Temperature and pressure dependency of I₂ filter profile

This Russian article offers an in-depth discussion of Rayleigh-Brillouin scattering in solids as well as in fluids.

A DGV system for use on the F-18 HARV aircraft is designed and its expected performance is discussed. Different combinations of lasers and molecular or atomic vapors are discussed and Nd:YAG+I₂ is found to be the best combination at present. Laboratory experiments for measuring the velocity profile of a subsonic jet with a 3x3 inch exit area and nominal exit velocities of 70, 100, 125 and 150 ms^-1 were carried out and DGV data was compared to Pitot data. The DGV system tested used a custom-built seeded Nd:YAG laser producing 20 mJ per pulse at 532 nm and being controlled by changing the seed laser frequency until minimum build-up time is achieved in the host cavity. The laser frequency was monitored using a separate unit with an absorption vapor filter and two photodiodes. Several different CCD cameras were evaluated for the image collection, but for the jet experiments simple RS-170 interlaced CCD cameras were used. One camera was used unfiltered, while the other camera observed the light passed through the iodine filter. LDA data acquired for the subsonic jet were discarded due to problems of achieving flow-tracking seeding. Since the seeding was introduced in the jet flow and not in the entrained ambient air, the data quality decreased towards the outer edge of the jet mixing layer. Near the center of the jet, DGV and Pitot measurements showed excellent agreement, even though the noise in the DGV data was significant (RMS 30 m/s at the mean velocity 150 m/s). It is unknown to what extent this large noise - to be compared to 1% in Pitot measurements, is an actual effect of turbulent flow rather than just a statistical scatter in the DGV measurements. The DGV mean velocity deviated at most by 4% from the Pitot data. In order to make the DGV data comparable to the lower-resolution Pitot measurements, spatial as well as temporal averaging was employed. The large noise caused by the low light intensity at the outer edge of the jet was removed from the ratio image between the two cameras through subtracting a smoothened ratio image from the raw one (essentially a high-frequency filter). This reduced the ratio image noise to 2.5%, which can be compared to the individual images that had RMS levels of over 3%. This shows that the images are correlated at high frequencies, i e that the overlap is good. Using 3x3 pixel spatial averaging it was reduced to 1.2%, while temporal averaging over 10 images gave 0.8% noise RMS. Combining the two averaging techniques brought the noise level down to 0.5%. The dominant noise source was found to be video noise of the cameras and the validated accuracy was stated to be 2%. To reduce image distortions, it was found that vapor cells with windows attached with optical contacting techniques are better than those with fused windows. Due to the Mie particle scattering efficiency dependency on polarization, it is important to consider the transmission ratio of the beamsplitter in the image acquisition system for different polarizations. The study also identified laser and ALF candidates that may be suitable for DGV in the future. Potential lasers include Nd:YAG/YLF/YAP, Ti:Sapphire and Alexandrite lasers that could be combined with I₂, Cs or Rb and Rb, respectively. Two basic laser/camera arrangements are introduced for 3D DGV, called R3 and L3. The R3 configuration uses a single laser sheet imaged from three different directions, while the L3 configuration uses three different laser sheets viewed by three cameras along a single optical axis. A potential arrangement of lasers and cameras for the F-18 testbed was simulated and the measurement errors estimated. The paper concludes with an estimate of the required scatterer density required to achieve a prescribed signal-to-noise ratio (and corresponding velocity uncertainty) for different ambient light conditions.

This paper offers an overview of Rayleigh and Raman scattering in flows as measurement techniques of the future, complementing PLIF, as well as of the principles of FRS rather than experimental data presentation. An FRS measurement on a Mach 5 overexpanded jet is quoted as having 4% accuracy in velocity and density and 18% accuracy for temperature measurements. The paper discusses how improvements in laser specifications, new narrowband absorption and transmission filters and improved intensified cameras with high linearity and frame rate can enable new Raman-based technologies and improve the quality of Rayleigh measurements through allowing the use of ultraviolet light.

Pulsed ArF laser used for Mach 6 density measurements in heated Mach 6 tunnel noting good accuracy (c f CFD) and no clusters

Linear photodiode array monitors laser sheet intensity profile during test

This paper focuses on the description of a new, more efficiently locked Ti:Sapphire laser that has been frequency tripled to 253.7 nm and is used with a mercury vapor filter on a Mach 2 free jet. The new locking method ensures single mode operation even during rapid changes of seed laser wavelength and checks the cavity status before a laser pulse is created rather than afterwards. This is made possible through replacing the conventional minimum build-up time technique with the novel "ramp and lock" technique, which essentially measures the host cavity length before the pump laser is fired. The linewidth of the seeded pulses was found to be 19.5 MHz - the transform limit of the FWHM 22.5 ns pulse length. This may be compared to the 140 MHz cavity mode spacing. The ability of the host cavity to track a seed scan at 300 MHz (UV) per second while retaining single-mode operation was demonstrated. The ability of the laser to handle a discrete step of 5.04 GHz (UV) between two pulses was also demonstrated. The paper also contains a brief discussion of experimental results where CO₂ seeding (in Rayleigh range) was used to compensate for the low (1 mJ) pulse energy. The Mach 2 nozzle had an exit diameter of 4.4 mm and was supplied a backpressure of 200 psi, leading to near-perfect expansion to ambient conditions. While the tuning capability of the laser and the selective absorption of the mercury vapor filter was demonstrated in a series of images, quantitative velocity information is not presented in this paper.

This paper presents a velocity measurement using the fourth harmonic of a Nd:YAG laser and a Fabry-Perot etalon to measure the velocity field in a Mach 1.3 free jet produced by a nozzle with an exit diameter of 9.3 mm. The paper contains a discussion of the relative merits of using the 266 and 532 nm Nd:YAG harmonics. The larger crossection of Rayleigh particles and the smaller reflectivity of windtunnel surfaces in the UV are offset by the lower power available from the laser and the lower efficiency of many lenses and cameras in the UV range. Rather than using an iodine vapor filter, the author uses a Fabry-Perot interferometer to filter the scattered light. A cooled, high-QE CCD camera is used to capture images of the interference fringes produced by the etalon, giving a scattered spectrum at a single point in the flow. The etalon can also be used as a conventional filter to determine the Doppler shift at different positions in the laser sheet. Through combining a reference beam and a Doppler shifted beam and pass them through the same Fabry-Perot etalon, the Doppler shift could be determined while compensating for laser drift. The velocity was measured at three different pressure ratios 15 mm downstream of the nozzle exit using two interference fringes. Velocity information was calculated from the fringe pattern using the Tenti S6 model (y=0.5) for the Rayleigh scattered light. The results show some variation both between the two points and when comparing to isentropic flow calculations. For the single shot measurements, the deviations are approximately 10 m/s +5% and the ten-shot averages show almost as large variations. The deviations seem mostly systematic and using the average of the two fringe measurements significantly improves the results with the mean values being within 2% of the isentropic calculations. The single shot averages show a standard deviation of 8% of the mean and the 10-shot averages 10% at the lowest mean velocity (176 m/s) and about 5% for the higher velocities. Two reasons put forward for the larger standard deviation found at the highest (M=1.3) flow speed is the larger air supply pressure variation and acoustic noise level found in this case. The author also investigated the reflectance of aluminum and stainless steel at 266 nm and 532 nm and found that while the reflectance for steel was reduced significantly (30-50%) through halving the wavelength, aluminum showed no significant change. One problem with working in the UV is the small selection of available imaging optics.

This paper is very similar to the 1995 paper by the same authors. It presents quantitative Rayleigh scattering measurements using a pulsed 193 nm, 120 mJ ArF laser in a Mach 6 flow around a 38.1 mm diameter cylinder. The facility allowed air to be heated up to a stagnation temperature of 700K, which led to the elimination of clusters observed earlier in hypersonic flows. Laser intensity variations were monitored using a linear photodiode array, which helped reduce the noise by almost 50% when used in normalization of the signal. Comparisons of the measured densities with CFD results in general showed good agreement, but the Rayleigh data showed a significantly more rapid decrease in the density moving away from the cylinder or the centerline upstream of the cylinder than did the CFD data. This difference is attributed to the poor spatial resolution of the CFD grid away from the cylinder not being able to accurately capture the strong bow shock upstream of the body.

Measuring velocity in sonic jet injected into M=1.98 crossflow using retro-reflected laser light and pressure-broadened iodine cell

Compared streamwise velocities and turbulence levels with circular and elliptical jets

A Mach 8 flat plate boundary layer is imaged using CO₂ seeding to increase the Rayleigh signal. It is mentioned that high Mach number flows are often studied at stagnation temperatures high enough to eliminate liquefaction of the air, but not condensation of water vapor. These droplets or crystals lead to Rayleigh signals frequently overwhelming any signal from the air molecules, resulting in over-estimates of the air density. The seeding is also temperature dependent, revealing the presence of warm boundary layers and heating shock waves, which is utilized in this paper. Using CO₂ rather than H₂0 for seeding was motivated by its smaller (one seventh) heat of condensation, leading to a smaller impact on the temperature of the air flow. CO2 would also condense at a lower temperature, leading to a later droplet formation and smaller particles more likely to be in the Rayleigh range. Smaller particles would also react more rapidly to changes in gas temperature than larger particles. Though no sizing is carried out, the particles are quoted as having a diameter of less than 100 nm based on a reference to a paper by Hill. The laser employed is an injection seeded Nd:YAG 532 nm laser with a pulse energy of 285 mJ and a linewidth of 150 MHz. Rayleigh measurements are carried out using a single intensified CID camera with a 210 mm/4.5 lens and an iodine vapor filter with a linewidth of ~1 GHz and a minimum transmission of ~10^-3. In the presented experiments, the seed/air mass ratio was 0.6%, but the authors suggest that with higher laser power and camera gain, a further reduction by a factor 5-10 is possible. Erosion of the ceramic material used for sealing the seed slot led to early transition of the flow along the centerline. The paper also describes PLIF in the same setup using sodium seeding. No quantitative velocity or density data is given.

The article describes how an intra-cavity etalon (FSR 35.5 GHz) can reduce stray light produced by the laser in non-seeded modes (FWHM 40 GHz) and how this improves the potential background suppression of the iodine filter. There was also a broadband component attributed to the flashlamp emission and fluorescence of the iodine, which could be reduced by a 10 nm FWHM interference filter. The spectrum of the laser was studied using a 111 GHz free spectral range Fabry-Perot interferometer after it was passed through the iodine vapor cell. A CCD camera was used to collect the concentric ring fringe pattern that was produced by the interferometer. The transmittance of the iodine cell (l=0.2 m, T_I2=30^oC, p_I2=0.46 torr) as a function of the laser wavenumber was plotted and compared to the Forkey model predictions. It was found that the minimum transmittance was as high as 3^.10^-3, while the predicted minimum transmittance for the line near 18788.44 cm^-1 was of the order of 5^.10^-6 under these conditions.

The problem with the intra-cavity etalon noted in the article is that the etalon pass wavelength tends to drift due to temperature fluctuations, requiring the etalon to be re-aligned in order to ensure proper stray light suppression.

The paper describes an attempt at making DGV less sensitive to environmental conditions in order to achieve better accuracy under windtunnel conditions. The work has been carried out in a number of very large wind tunnels under subsonic as well as supersonic conditions. The problems addressed included laser speckle noise, pixel-specific camera calibration, laser and iodine cell temperature drift and optical smoothing. The initial accuracy problems that were found as the DGV concept was moved from measuring the velocity of a rotating wheel to a flow over a delta wing were attributed mostly to the inability to perfectly align the reference and filtered images before calculating their ratio. This overlap was found to be required to be attained with subpixel accuracy in order for accurate measurements to be possible. Through the use of a 20x20 dotcard photographed by the camera pair in its data acquisition configuration and bi-linear dewarping of the images, this overlap was achieved. Furthermore, polarization-dependent Mie scattering was made more constant using circularized laser light and laser sheet intensity variation was reduced using a sweeping galvanometer rather than a cylindrical lens. These modifications significantly improved the quality of the collected data and were taken as the start of the development of even more refined measurement techniques. The problems that were addressed at this stage were: camera dark current, stray ambient room light, stray scattered laser light, pixel-to-pixel variations in CCD sensitivity, charge transfer noise, modulation transfer function of collection system, image distortions and field interlacing. In order to improve the signal quality, a pixel-to-pixel (but still linear) calibration for the CCD sensitivity is employed. Since interlaced video cameras were used for the image acquisition, linear interpolation was used to replace the missing lines in each image. A 5x5 top hat filtering kernel with a bandwidth slightly wider than the MTF of the cameras was used to reduce the charge transfer noise without reducing the actual resolution of the acquisition system. In order to monitor laser drift and mode hopping of the 5 W cw Ar⁺ laser that was used in the experiments, a second optical receiver system was constructed. In order to expand the measurement capabilities to simultaneous acquisition of 3D velocity data, a system was built using six RS-170 video cameras in three receiver pairs observing the target from different angles. To ensure that the mapping was sufficiently accurate to allow evaluation of the velocity field based on these images, a test was carried out with a rotating wheel. Applying the same setup in an open windtunnel demonstrated that misalignment of the images (up to 3 pixels in a single component) due to vibration reduced the data quality. In order to expand the capabilities to unsteady flows, the cw laser was replaced by a pulsed Nd:YAG laser. Furthermore, the iodine vapor cells were installed in insulated boxes to minimizle temperature fluctuations affecting their characteristics during wind tunnel testing. In addition, cursory testing revealed that the starved-cell design also reduced cell ambient temperature sensitivity. In a set of instantaneous windtunnel tests, the dominating issues were laser speckle noise, uneven seeding, iodine cell temperature fluctuation and laser frequency instability. In order to remove the severe laser speckle noise that was encountered when the Nd:YAG laser was used, a spatial median filter was used.

Error in mean and RMS velocities measured by DGV with an Ar⁺ laser and an iodine cell were measured using a rotating disk with a maximum speed of 29 ms^-1 and the experimental uncertainty were found to be 1-2 ms^-1

Point-wise measurement of temperature and velocity using a CW Ar⁺ laser, a Fabry-Perot interferometer and an unseeded flow

Measurement of scattered spectrum and application of kinetic gas theory

Good velocity accuracy, but large (30K) bias errors in temperature

A reference iodine cell calibrated using a FP etalon was used to monitor the laser frequency during the test using a small fraction of the laser energy. The reference iodine cell was kept at a low temperature in order to ensure high sensitivity of its absorption depending on the laser wavelength. The paper includes a discussion of laser speckle noise, including the importance of large-aperture collection optics and a large CCD chip. It also introduces the concept of a 'gain' file containing data on the sensitivity of each CCD pixel to a certain light level to account for e g vignetting in the collecting optics. No correction was made for the laser sheet spatial intensity variation, but it was mentioned that this effect was removed in the rationing of the filtered and reference images. Images of the scattered light both with and without iodine cell filtering were captured on a shared CCD chip simultaneously. Image alignment between the reference and filtered camera images was achieved through bi-linear warping of the Doppler image based on images taken of pixel-sized dust particles captured in both images simultaneously as the dust passed through the laser sheet. An inaccuracy in the mapping of no more than one-tenth of a pixel was required in order to obtain good quality velocity measurements. A comparison of the mean velocity profile obtained using PDV to Pitot measurements at about 6 diameters downstream of the nozzle exit show fair agreement with PDV overestimating the velocity by up to 50 m/s around 0.5-1 radius from the centerline. Due to the sharp velocity gradient in this region, this velocity difference corresponds to a radial offset of approximately 0.2 radii. In the far flank, 2 radii from the centerline, the PDV data shows a more gradual approach to zero than the Pitot data, suggesting that the Pitot data may not be more reliable than the PDV data.

Water vapor Rayleigh clusters and injected liquid Mie particles are studied downstream of Laval and convergent nozzles using a 100 mJ Nd:YAG laser. The study was purely qualitative and no filtering (except for a polarizer) was carried out of the scattered radiation. The convergent nozzle had 2.78 mm exit diameter and the Laval nozzle had a 3.12 mm throat and 3.86 mm exit diameter (A/A^*=1.53, M_design=1.88). For both nozzles, the stagnation pressures 168, 237 and 372 kPa were tested, the first pressure nominally giving M_e=0.89 and the higher pressures leading to under-expansion at the design Mach numbers.

Convergent nozzles were found to give better spread and atomization of droplets than a Laval nozzle

This paper describes how the signal-to-noise ratio of FRS can be improved using modulated light so that an inexpensive, rugged and compact low-power diode laser can be used rather than a more expensive high-power pulsed laser. The presented experiment utilized a NIR diode laser at 780 nm and a rubidium vapor filter. Using heterodyne detection with a lock-in amplifier, the small Rayleigh signal can be detected even in the presence of strong background noise. The modulation of the laser signal can also be used to sweep the laser wavelength rapidly. The laser frequency is monitored during the test through passing a small portion of the beam through a reference rubidium vapor cell. This signal was used to create a feedback loop for stabilizing the laser wavelength to the degree that the frequency fluctuations only amounted to 0.16 ms^-1 uncertainty in the measured velocity. Further improvement was deemed possible. The scattered light is split into two legs of which one is fed directly to a photo detector while the other first is passed through a rubidium vapor cell. This setup gave pointwise measurements in a 0.2 mm³ volume. Preliminary measurements were carried out in a supersonic jet seeded with CO₂ to increase the signal, but no quantitative data or global uncertainty estimates are given. The laser could also be scanned over several absorption bands in the spectra and then no reference sensor would be required provided that the mean density remained constant during the test.

This paper presents density measurement carried out along a line using a 80 mW CW Nd:YAG 532 nm laser. Images were acquired using an intensified CCD camera at a frame rate of 30 frames/s giving a peak Rayleigh signal of 28 counts and a background scatter level of 15 counts in a high-contrast run. A second standard video camera was also used to monitor potential clustering during the test. The 0.3 m transonic wind tunnel that was used in the test uses N₂ as flow medium. In one of the tests the static temperature was 255K, static pressure 0.43 MPa and Mach number 0.6, but the conditions were varied to obtain 6 different densities. The results 92 data points show a laser signal proportional to flow density and of the right magnitude, but large scatter (mean +100 to -20%). This large scatter is attributed to low signal-to-noise ratio, large laser power fluctuations in the windtunnel environment and digitization noise due to the low signal level. No clustering was found in the flow despite temperatures around 100K, but CO₂ condensed on windows at about 225K.

This paper discusses the application of FRS in the UV using a tripled Ti:Sapphire laser (253.7 nm) along with a photomultiplier tube fitted with a mercury vapor filter to measure the temperature at a point in a gas cell. The laser provides 10 mJ pulses with a linewidth of 200 MHz, which is an order of magnitude smaller than the thermally broadened scattered spectrum and filter linewidth. In order to monitor intensity and frequency fluctuations in the laser light, a second photomultiplier tube fitted with an iodine cell is collecting unscattered laser light and a photodiode collects the unfiltered laser light. An accuracy of about 3% is achieved near ambient conditions and measurements for a weakly ionized argon plasma showed a standard deviation of about 4% (at around 660K). The paper contains a discussion of the characteristics of a mercury vapor filter, a comparison to the iodine cell conventionally used as well as a discussion about how the temperature is determined from the scattered intensity. In a mercury cell, the vapor pressure goes from 0.001 to 20 torr as the temperature is increased from 20 to 200^oC. At 0.003 torr, absorption near 100%, a FWHM of 3 GHz and a cutoff frequency range of 1 GHz is achieved for one of the hyperfine lines near 253.5 nm. To obtain a temperature measurement, the laser wavelength is scanned through the range of one absorption line of the filter and the minimum transmission is measured and compared to a model value for the known filter profile and Rayleigh-Brilllouin spectrum under the given conditions. Single-point measurements are possible when the pressure of the gas is known and a normalization with the temperature measurement at the same frequency at a known temperature can be made. The comparison to conventional 532 nm FRS systems show that assuming similar detector efficiency, the systems have similar characteristics since the order of magnitude larger crossection in the UV is compensated for by the larger laser energies available in the visible region. The temperature sensitivity of the filters is determined by the wall steepness of the absorption wells and this is shown to be similar for the iodine and mercury vapor filters. The mercury background suppression is states as being significantly stronger than that of the iodine vapor, which is limited to 10⁵ due to continuum absorption.

This technical memorandum presents a crude measurement of soot concentration in the Mie or Rayleigh regime along four lines through a rocket nozzle exhaust plume. It also contains a discussion on measurement of the extinction coefficient and about particle size independence of total carbon concentration. The experiments employed an Ar⁺ ion laser with background correction using an optical chopper and filters in front of the photodiodes. The experiments were plagued by high noise levels, drift (temperature-induced?) and poor repeatability.

This paper presents Rayleigh scattering density measurements of a Mach 2.5 flow where an impinging shockwave interacts with a boundary layer on a flat plate. The incident light is produced by a Q-switched 532 nm Nd:YAG laser delivering 120 mJ per pulse in the form of a 10 mm wide, 0.2 mm thick light sheet. The light sheet was introduced 12 mm above the flat plate, perpendicular to the surface. The scattered light is collected by an intensified CCD camera. After the windtunnel tests were carried out, a laboratory model mimicking the windtunnel geometry was created to compare the signal levels in ambient condition nitrogen to those found in the windtunnel test. The detected signal was found to be 500 times stronger than anticipated due to unknown impurities condensing in the windtunnel. The scattering intensity was uniform down to the smallest resolved scales (0.3-0.5 mm) suggesting that these clusters must be small. A lengthy discussion is presented where different candidates for the impurity are presented and it is concluded that the most likely source of the extraneous scattering is oil clusters. Based on the expected light levels given the specifications of the light collection equipment, the particles are estimated to be Rayleigh scatterers with a diameter of no more than 86 nm, but it is not completely ruled out that they may be Mie scatterers with diameters of a few micron.

PLIF used in M=7 flow over 30^o half-angle cone with velocity determined by Doppler shift of tunable dye laser

PLIF chosen over Rayleigh because of large Mie signal

Thorough description of FRS theory and experimental setups for visualizations, thermometry and multi-property measurements

Presents simultaneous FRS measurement of density, velocity and temperature in underexpanded M=1.65 jet, but fairly large (~10%) errors

PLIF visualization of separated transitional boundary layers in a free-piston shock tunnel

This review article contains a thorough presentation of the theory behind Rayleigh and Raman scattering as well as a table over the scattering properties of standard air in the wavelength range 200-1000 nm. It also contains a discussion of the resulting scattered light lineshape under different conditions and the impact of larger (but still Rayleigh-sized) particles in the flow.

This paper presents a visualization of shock-boundary layer interactions in a M=2.5 flow subjected to 14^o and 24^o wedge compressions. The experiments are a continuation of those described in the 1997 paper by Lempert et al, but this time less emphasis is put on the pulse laser system and more on the flow being studied. 30 images were obtained at 0.5 MHz using a Nd:YAG laser giving 25 mJ per pulse (cf 1 mJ in 1997) and an iodine cell, but no quantitative velocity data is given.

This paper discusses how depolarized Rayleigh scattering may be an attractive alternative to using Raman scattering for determining fuel concentration in flames simultaneously with acquiring temperature measurements using conventional, polarized Rayleigh scattering. Depolarization is a species-dependent process typically yielding a signal an order of magnitude stronger than Raman scattering for depolarizing molecules. In the present paper methane, a very weak depolarizer, is used in a laminar combustion experiment where different Rayleigh components are acquired using polarizing filters. The fuel concentration is determined using the known efficiency of the polarizing filters and the depolarization ratios of the fuel, air and the products. Calibration measurements were carried out to determine the depolarizing ratio of the major species at the 532 nm Nd:YAG laser wavelength used in the experiment. The laser power was 260 mJ/pulse and the images were collected using a single CCD camera fitted with a 50 mm/1.4 lens, a sheet polarizer and a 10 nm FWHM interference filter. The temperature impact on the depolarizing ratio is estimated at only 2% per 1000K. The signal-to-noise ratio of the depolarized Rayleigh signal was ~10. The authors conclude that this is a promising technique, especially for fuels with a small depolarizing ratio diluted in noble gases as long as the effect of broadband fluorescence is kept under control.

Visualization of pulsed separation in M=0.2 flow

Unseeded and CO₂ seeded Rayleigh scattering compared to N₂ 0-1 vibrational Raman scattering showing that both techniques are feasible, but that unseeded flow gives low signal-to-noise ratio

This paper describes how a setup with two broadband pump Nd:YAG lasers and a third, injection-seeded probe Nd:YAG laser can be used to excite and probe density fluctuations in a scattering medium, providing information about the Rayleigh-Brillouin spectrum produced by the medium. This information can provide pointwise pressure and temperature data after the produced radiation has been analyzed using a Fabry-Perot interferometer. A kinetic theory that explains why the observed spectra are different from that of spontaneous Rayleigh-Brillouin scattering is presented and verified experimentally.

The use of a single camera in this paper suggests that the point-to-point variation in intensity in the laser beam may not be significant enough to warrant a correction on a pixel-by-pixel basis of the image data. Furthermore, the experiments were carried out while the laser shift was being monitored, but the fluctuations were found to be so small that in the data analysis this shift (smaller than the laser linewidth) was not considered. Images acquired while the laser Q-switch build-up time was found exceptionally large, i e when the seeding was considered not to be operating properly, were discarded. The authors do mention the issue of iodine cell saturation at high laser intensities and use beam expansion and neutral density filters to eliminate this problem.