The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1 km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ∼10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.

Atmospheric turbulence effects measured along horizontal-path optical retro-reflector links

The results of the analysis of the backscattered radiation mean power enhancement in a turbulent atmosphere are presented. The above enhancement arises because of the correlation of the forward and backward waves in a random media. The analysis was performed for the strong optical scintillation regime in a turbulent atmosphere. It is shown that the laser return mean power in the turbulent atmosphere can two times exceed that in a homogeneous medium because of the correlation of forward and backscattered waves. With an increase in the optical turbulence strength, the enhancement effect decreases according to the power law.

Enhancement of the laser return mean power at the strong optical scintillation regime in a turbulent atmosphere

With the development of airborne imaging devices on the high-speed vehicle, aero-optical prediction of the turbulent boundary layer has attracted wide publicity. Based on the density distribution of supersonic (Mach 3.0) turbulent boundary layer obtained by nano-tracer-based planar laser scattering (NPLS) technique, we find that the time distribution characteristics of density fluctuation deviate from the Gaussian distribution mainly induced by the intermittency of a turbulent boundary layer. It may cause the prediction error of Sutton's linking equation to increase. The deflection-angle amplitude spectrum has a peak value at Stδ ≈1. With the increase of Stδ, the deflection-angle amplitude spectrum is more attenuated, which means that small-scale vortices make a relatively weak contribution to aero-optics, and which is mainly induced by large-scale vortices. With the assumption of locally homogeneous turbulence, the integral scale Λ is basically around 0.1δ. Without the assumption of locally homogeneous turbulence, the integral scale Λ is basically around 0.05δ. For the supersonic turbulent boundary layer with obvious inhomogeneity, based on the turbulence integral scale defined without the assumption of locally homogeneous turbulence, a fine aero-optical prediction result could also be obtained via the Sutton's linking equation. From the covariance distribution of density fluctuation, the prediction result based on the exponential distribution approximation is worse than that based on the Gaussian distribution approximation. The prediction effect is also shown through the change in coefficient. The prediction deviation of OPDrms based on the exponential distribution approximation is larger than that based on the Gaussian distribution approximation.

Research on aero-optical prediction of supersonic turbulent boundary layer based on aero-optical linking equation.

The absorption spectra of exhaust gases (H2O, CO, CO2, NO, NO2, and SO2) and aerosol (soot and Al2O3) particles were modeled at different temperatures for the first time and suitable spectral ranges were determined for conducting laser remote sensing of the combustion products of jet engines. The calculations were conducted on the basis of experimental concentrations of the substances and the sizes of the aerosol particles. The temperature and geometric parameters of jet engine exhausts were also taken from the literature. The absorption spectra were obtained via the line-by-line method, making use of the spectral line parameters from the authors’ own high-temperature databases (for NO2 and SO2 gases) and the HITEMP 2010 database, and taking into account atmospheric transmission. Finally, the theoretical absorption spectra of the exhaust gases were plotted at temperatures of 400, 700, and 1000 K, and the impact of aerosol particles on the total exhaust spectra was estimated in spectral ranges suitable for remote sensing applications.

Modeling absorption spectra for detection of the combustion products of jet engines by laser remote sensing.

Spectral remote sensing is a very popular method in atmospheric monitoring. The paper presents an approach that involves mid-infrared spectral measurements of combustion processes. The dominant feature in this spectral range is CO2 radiation, which is used to determine the maximum temperature of nonluminous flames. Efforts are also made to determine the temperature profile of hot CO2, but they are limited to the laboratory conditions. The paper presents an analysis of the radiation spectrum of a non-uniform-temperature gas environment using a radiative transfer equation. Particularly important are the presented experimental measurements of various stages of the combustion process. They allow for a qualitative description of the physical phenomena involved in the process and therefore permit diagnostics. The next step is determination of a non-uniform-temperature profile based on the spectral radiation intensity with the 8 m optical path length.

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Open-Path FTIR Spectral Radiation Intensity Of Hot Combustion Gases – Measurement And Interpretation

The research conducted on the thermodynamic parameters of a flame was based on its experimental transmission spectra in the 2.7 μm and 4.3 μm ranges. To produce the flame, alcohol was burned under atmospheric conditions. The hot gases resulting from the burning of ethanol (H2O, CO, and CO2) were accelerated and spun by the rotation of an impeller. The optical method developed by the authors was employed for predicting the temperature and partial pressure of the hot gases. The results demonstrate the practical significance of the method suggested. The spectroscopic database HITEMP 2010 was used for all line-by-line calculations presented in the article.

The optical method for determining the thermodynamic parameters of hot gases

Results of experimental investigations of liquid fuel combustion in the regime of a twisted jet (model of a fire tornado) are presented. To perform investigations, a stand equipped with measuring devices based on optical and thermal imaging methods has been developed and manufactured. Dependences of the geometrical characteristics, heat flux, combustion rate, and frequencies of maxima in the spectral function of the intensity fluctuations and in the centre of gravity of image of the laser beam that has passed through the tornado on the twist velocity are determined.

Some results of experimental investigations of fire tornadoes

We present the experimental results of studying the possibility of compensating the misalignment of the axes of a laser beam, focused in an aerosol volume, and a receiving telescope from the shift of an image formed by a laser beam backscattered by aerosol particles of 0.5–0.6 μm. It is shown that the compensation can be carried out accurate to a few microradians via control of the position of the intensity distribution centroid of the image of the beam formed by the scattering volume.

Laser Beam Pointing Based on the Backscattered Aerosol Beam. A Model Experiment

The results of experimental researches of intensity fluctuations of a laser beam propagating through a supersonic flooded jet are presented. The laser beam propagates across the jet at various distances from a Vitoshinskii nozzle. The experiment was carried out in an Eiffel chamber at pressure ratios from 1.7 to 7 at two modes of the outflow of the jet: with the use of chevrons on the outlet nozzle and without chevrons. Dependencies of the dispersion and fluctuation spectra on pressure and distances from the nozzle are determined. The inner scale of turbulence is estimated.

Intensity fluctuations of a laser beam propagating through a supersonic flooded jet

We consider the applicability of laser transillumination method for the investigation of the screech-tones generation by the supersonic jet. Results of the experiments on parallel registration of the acoustic noise and laser radiation passed through the jet are discussed. The presence of responses at screech frequencies are found in the radiation spectra. Strong dependence of spectral amplitudes at screech frequencies from the sensing path shows high sensibility of laser sensing method for the selection of jet areas producing screech.

V. M. Sazanovich

Papers

The optical method for determining the thermodynamic parameters of hot gases

Some results of experimental investigations of fire tornadoes

Laser Beam Pointing Based on the Backscattered Aerosol Beam. A Model Experiment

Intensity fluctuations of a laser beam propagating through a supersonic flooded jet

Investigation of the screech-tones excitation by the supersonic jet with laser transillumination method