Home – Home – Jornals – Avtometriya 2012 number 3

A new technology of visualization and diagnostics of transparent media with gradient inhomogeneities is described. This technology is called three-dimensional laser diffractometry; it is based on refraction of spatially structured laser radiation and digital recording and processing of twodimensional (in transmitted light) or three-dimensional (in scattered light) refractive patterns. Results of computer simulations of three-dimensional refractive patterns (3D refractograms) for spherically and plane layered media are presented. A setup for observation of 3D refractograms in scattered light is schematically described, the method of obtaining refractograms from experimental two-dimensional refractive patterns is presented, and examples of computer processing of experimental data are given.

The evolution of vortex structures induced by a pressure jump on the output hole of the chamber is experimentally studied by methods of the Hilbert diagnostics. Various types of optical filters that ensure the Hilbert and Foucault-Hilbert transforms are considered. Colored visualization of the fields of the phase optical density of the vortex structure in an air medium is performed by using a linear source of white light and a quadrature Hilbert filter.

A number of modern optical methods used for diagnostics of reactive flows are described. Various aspects of using advanced modifications of particle image velocimetry (Stereo-PIV, Highrepetition PIV, and Tomo-PIV) for measuring instantaneous velocity fields in reactive flows are discussed in detail. Capabilities of PIV and spectroscopy of flame radiation (CH_ radical) in obtaining data on the spatial flow structure and the flame are demonstrated by an example of studying a swirled turbulent propane-air flame and an isothermal jet.

A method of quasi-optimal processing of laser Doppler vibrometer (LDV) signals in the case of monoharmonic vibrations is considered. Under certain assumptions, the problem of quasi-optimal estimation of the information parameter of the LDV signal, related through a known constant multiplier to the relative amplitude of vibrations, is solved. A structure of the device for optimal estimation is proposed. Conditions of optimality of the method are given. Statistical characteristics of estimates of the relative vibration amplitude are investigated by a numerical modeling method. The error of these estimates is compared with the Rao-Kramer bounds.

A new type of stimulated scattering of light (stimulated low-frequency Raman scattering (SLFRS)) is found. SLFRS appears as a result of interaction of laser pulses with acoustic vibrations of structural elements of nanostructured materials. SLFRS is shown to arise in materials with different compositions and different degrees of morphology, both ordered and random. Frequency shifts of SLFRS components are measured. A number of practical applications of the effect are proposed, in particular, biharmonic pumping with the help of SLFRS.

This study is motivated by the advent of methods for measuring particle sizes in twophase flows based on the local interaction of light with the scattering particle. Localization is easily interpreted in terms of the ray optics of a particle, which, however, does not cover all interaction effects. This paper consider effects that extend beyond the traditional concept of the total effect of the mechanisms of geometrical optics and wave diffraction by a particle as an opaque disk, in particular, surface waves and complex rays.. The results of the mathematical modeling performed in this study clarifies the contributions of the effects to the scattering and the position of the localization points of the interaction with the particle.

A method for modeling refraction patterns of structured laser radiation propagating in optically inhomogeneous media was developed. Wave models of structured beams that simultaneously account for refraction and diffraction effects were used. Results of modeling refraction in gradient inhomogeneities using the Kirchhoff method and the spectral method are presented.

An combined optical and thermal imaging experimental system was designed to investigate Rayleigh-Benard convection of a fluid in a layer with two rigid isothermal boundaries and a free upper boundary under steady and unsteady thermal boundary conditions. The fluid surface structure was visualized using methods of reflected-light Hilbert optics. Noncontact control of the fluid layer thickness was performed using a specially designed remote meter based on an MBR-1 microscope with a smooth focusing unit based on the Meyer mechanism. The evolution of the dynamic structure of the surface and temperature field of the fluid being analyzed were studied experimentally, and the existence of flows in the form of two-dimensional rolls with axes of rotation parallel to the lateral boundaries (the walls of the cavity) was confirmed. It is shown that the highly viscous fluid flow has a thermal gravitational nature. Correspondence is found between the evolution of the thermograms and Hilbert schlieren patterns of surface structures in different modes of Rayleigh-Benard convection.

A new turbidimetric method for determining the parameters of submicron aerosols was developed and implemented in hardware using a wide range of probe radiation wavelengths and highspeed video recording. The particle size distribution function is found using an original algorithm for solving the inverse problem of the optics of aerosols based on a direct search computational procedure.

A prototype of a combined measurement system (MS) based on the fluorescent and smallangle methods of determining the parameters of a fuel-air spray using a pulsed laser as a light source and a color digital video camera to record spray sections was designed and tested. The tests of the MS showed that it has good performance and is suitable for determining the characteristics of advanced atomizers in a pressure chamber. Spatial concentration distributions of aerosols generated by a centrifugal atomizer were studied. Distributions of time-averaged Sauter diameters of droplets, their concentration, and the circumferential inhomogeneity of droplet concentration in the spray cross section were obtained. A study of fuel atomization from a plate was performed showing the possibility of using this device in power plants to improve the atomization performance compared to the atomization of a free jet in crossflow.

New algorithms for processing noisy specklegrams are described which allow quantitative diagnostics of the microstructure of shock-wave flows with subpixel accuracy with the use of statistical analysis of the speckle fields recorded numerically and perturbed by refraction in the studied flows. The developed software makes it possible to recover up to 10,000 vectors of deflection angles of the probing radiation in a two-dimensional region 20 × 30 mm in size in a speckle field image with a magnification M = 1.

The origin and development of buoyant jets over a sudden linear heat source in a highly viscous fluid with a Prandtl number equal to 2700 are experimentally and numerically studied. The time evolutions of the spatial flow form and temperature and velocity fields are investigated as functions of power supply.

The results of recording gas flow in a shock tube by the schlieren and background oriented schlieren (BOS) methods after initiating a pulsed (surface or volume) discharge are presented. Simultaneous recording of the flow field by the two methods allows a complete qualitative and quantitative analysis of the shock-wave processes resulting from the interaction of a pulse discharge with high-velocity flow. The vector displacement field of the BOS method was determined by the cross-correlation method. The density field was obtained by solving the Poisson equation with special boundary conditions. It was shown that the BOS method yields a good quality map of the flow structure that corresponds to the classical schlieren method and provides reliable quantitative results except in areas of high gradients. A modification of the BOS method was proposed and tested to measure the density jump at the shock-wave front. Recording was performed at an angle to the plane of the wave front. Various Schemes of processing of digital flow images were tested. The proposed method provides a resolution of large density gradients at the shock-wave front. The obtained quantitative results are consistent with the calculated values.

Flows in horizontal fluid layers with a free upper boundary with uniform heating from below are studied. With the use of thermal imaging scanning, the temperature fields on the free fluid surface in laminar and turbulent regimes of the Rayleigh-Benard convection are measured. Computer processing of thermal imaging movies makes it possible to obtain spatial-temporal characteristics of cellular and multiscale convective flows.

The possibility of simultaneous measurements of the distance and velocity of diffusely scattering objects by active laser interferometry with frequency modulation is theoretically shown and experimentally substantiated.