D. A. Marakasov

Structure of the spatiotemporal spectrum of a laser beam in the atmosphere under strong turbulence

In this paper, possibilities of spatiotemporal filtering of the correlation function of laser beam intensity in a turbulent atmosphere in conditions of strong fluctuations are analyzed. It is shown that despite significant distortions in the high-frequency band of the spatiotemporal spectrum in comparison with weak intensity fluctuations, an increase in the time delay between the frames allows separation of contributions of different optical path segments. The characteristic scales of spectral responses differ significantly for different path points. This allows extracting information about the wind velocity profile.

Study of Turbulent Supersonic Flow Based on the Optical and Acoustic Measurements

Aircraft and missiles moving in the atmosphere with supersonic velocities form air flows of a complicated spatial structure, in which compression shocks of different configurations and intensities arise. Since an air flow is strongly spatially inhomogeneous, the air density in the flow experiences random pulsations much exceeding turbulent pulsations of the air density in the atmosphere. Mean characteristics of supersonic flows are investigated rather well both theoretically and experimentally, but characteristic properties of turbulence in a supersonic flow are studied insufficiently. To study turbulence in supersonic flows, the real-time measurements of the value and spectral composition of pulsations are required. Currently used sensors distort the flow structure and often have the low response rate. The dynamics of fluid or gas flows can also be studied and the turbulent velocity field in fluid and gas flows can be visualized with the use of noncontact optical methods. Among these are speckle photography and speckle interferometry methods (Fomin, 1998), in which the source of information is represented by intensity fluctuations of the laser radiation passed through the flow and a diffuse plate, as well as Doppler methods (laser Doppler anemometers (LDA)) (Abbrecht et al, 2003) and Particle Image Velocimetry (PIV) methods (Raffel et al, 2007) based on the measurement of velocities of microparticles suspended in a flow. LDA and PIV devices are very expensive and difficult in use. They are successfully used mostly for the investigation of fluid flows and subsonic gas flows. In the supersonic gas dynamics, their application is limited owing to such factors as increased requirements to the instrumentation (laser pulse energy, operation rate and sensitivity of recording instruments) and still open problems of velocity relaxation of tracing particles. In the seeding process, the tracing particle sizes are not identical and their concentration is not always uniform in a flow and this inevitably leads to the loss in accuracy of measurements. The widely used methods of shadow visualization (Schlieren photography) do not allow the spatial spectrum of refractive index fluctuations to be determined from the obtained images. Shadow images are integral characteristics of the refractive index in the entire radiation propagation path, and the spatial distribution of the refractive index in different flow layers cannot be reconstructed from them. In this connection, it is interesting to study possibilities

Estimate of the change in the effective beam width by the streamline method for axisymmetric laser beams in a turbulent atmosphere

Results of studying how the initial distribution of the laser beam field affects the change in the effective width of the beam in the process of its propagation in a turbulent atmosphere are presented. The investigations are carried out using the method of streamlines of the average Poynting vector for axisymmetric light beams. The effective beam width in the receiving plane is studied depending on the shape of the initial intensity distribution and presence of the phase dislocation in the initial field. It is shown that parameters of ring and vortex beams can be chosen such that their effective width in the receiving plane will be less than for a Gaussian beam with the same initial effective width in the process of laser radiation propagation in a turbulent atmosphere.

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

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.

Comparative Assessments of the Crosswind Speed from Optical and Acoustic Measurements in the Surface Air Layer

A passive optical method for measurements of the average crosswind speed on the atmospheric path has been developed. The crosswind speed estimation is based on the correlation algorithm for measuring fluctuations of the energy centroids of images of topographic objects under natural daylight. Test results of a windspeed- meter prototype, constructed based on this principle, are described. The wind velocity assessments recorded by this passive optical meter and an acoustic weather station are compared. The optimal time of accumulation of the cross-correlation function is estimated, which ensures stable real-time wind measurements.

Verification of a passive correlation optical crosswind velocity meter in experiments with a Doppler wind lidar

The results of comparative experiments on measurements of the crosswind velocity by the passive correlation optical method and with a coherent Doppler wind lidar are analyzed. The passive correlation optical measurements were carried out with a crosswind velocity meter developed at the Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences (IAO SB RAS).

Investigations of an acoustic field, generated by a supersonic flooded jet

The paper presents the results of experimental studies of the acoustic field generated by a supersonic flooded jet on the wind tunnel of the Vertical Jet Unit (VJU) ITAM SB RAS. The submerged jet came out vertically upward into a large enough room. Measurements are made by 9 microphones, which are placed on a ring with a radius of 265 mm, which moved vertically. The microphone inputs are installed at a radius of 235 mm (through 36°) symmetrically with respect to the axis of the jet. The time spectra of the acoustic field and the shape of the phase front at npr = 5 at heights from -50 to 495 mm are considered. The Mach number in the output section was equal to unity.

Field tests of a passive optical meter of the structure characteristic of refractive index

Results of experimental studies carried out at the testing ground of the Basic Experimental Complex (BEC) of the V.E. Zuev Institute of Atmospheric Optics SB RAS in 2016 are reported. Statistical characteristics of jitter of video images of remote objects are analyzed. The possibility of estimating the path-integral structure characteristic of the refractive index at atmospheric paths by a passive optical method is demonstrated.

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

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.

Estimation of the path-averaged wind velocity by cross-correlation of the received power and the shift of laser beam centroid

We consider the problem of estimation of the average wind speed on atmospheric path from measurements of time series of average power of the laser radiation detected through the receiving aperture and the position of the centroid of the image of the laser beam. It is shown that the mutual correlation function of these series has a maximum, whose position characterizes the average speed of the cross wind on the path. The dependence of the coordinates and magnitude of the maximum of the correlation function from the size of the receiving aperture and the distribution of turbulence along the atmospheric path.