Background Oriented Schlieren Method Research Articles

Super-resolution reconstruction of background-oriented Schlieren displacement field based on the convolution neural network with the gradient loss function

Abstract To refine the displacement field of the background-oriented Schlieren method, a novel super-resolution method based on deep learning has been proposed and compared with the bicubic interpolation in this study. The gradient loss functions were first introduced into the hybrid downsampled skip-connection/multi-scale model to improve the reconstruction effect. The reconstruction effects of the new loss functions were compared with that of the traditional mean square error (MSE) loss function. The results show that the Laplace operator with average pooling exhibits better performance than the origin loss function in all the indexes including peak signal-to-noise ratio, MSE, MSE of the gradient, and the maximum MSE. In these four indexes, the MSE of the gradient and the maximum MSE performed especially better than the others, where the MSE of the gradient was reduced from 3. 0× 10−05 to 3.30 × 10−05, and the maximum MSE was reduced from 0.392 to 0.360.

A non-axisymmetric temperature field reconstruction method based on the compressive sensing background oriented Schlieren method

Measurement of the combustion temperature field is an extremely important issue in industrial production. Temperature is one of the key parameters in combustion studies. With the temperature field distribution of the combustion field obtained, heat transfer, heat convection, and heat radiation can be calculated directly and efficiently. Traditional background oriented Schlieren (BOS) is an effective method for non-axisymmetric temperature field measurements, but it requires simultaneous Schlieren imaging at multiple angles for tomographic reconstruction, which will greatly limit its application. In this paper, the compressive sensing algorithm is introduced into the temperature field reconstruction, which establishes the system of equations between the deflection angle and the refractive index gradient. Then, the reconstruction of the non-axisymmetric temperature field is realized by solving the underdetermined system of equations by the method of solving the sparse solution through the compressive sensing. First, light offsets across the non-axisymmetric temperature field are measured by the under-angled BOS system and image processing method. Second, the spatial refractive index field is reconstructed by the compressive sensing BOS method proposed in this paper. Finally, the spatial temperature field is obtained. The experimental results show that by comparing the iRadon reconstruction algorithm and the compressive sensing reconstruction algorithm, the temperature field reconstructed by the compressive sensing under the condition of the under-angled sampling of projection data had a higher accuracy than that reconstructed by the tomographic reconstruction algorithm under the same condition. The average error of the temperature field was reduced from 34.6 to 29.7 K under the same measurement conditions; however, the accuracy is better maintained by using the compressive sensing algorithm under the condition of undersampling projection.

An Axisymmetric Temperature Field Reconstruction Method Based on the Fan-Shaped Light Path Background Oriented Schlieren Method

An Axisymmetric Temperature Field Reconstruction Method Based on the Fan-Shaped Light Path Background Oriented Schlieren Method

A non-axisymmetric temperature field reconstruction method based on the interferometric fringe schlieren method

The typical non-axisymmetric temperature field distribution in the tail nozzle outlet area of the combustion chamber of an aircraft engine can reflect the operating state of the engine. As an effective non-contact method for measuring a non-axisymmetric temperature field, the traditional background-oriented schlieren (BOS) method has the problem that the extraction accuracy of light deflection is not high. In this paper, interference fringes are introduced into the schlieren measurement, and the highly sensitive fringe offset replaces the traditional background speckle offset for measurement, which realizes accurate reconstruction of the non-axisymmetric temperature field. Firstly, through the interferometric fringe schlieren (IFS) method comprising multiple sets of double-slit interferometric systems and imaging sensors, the offset of the interference fringe after passing through the non-axisymmetric temperature field is measured by image processing. The spatial refractive index field is then reconstructed by the filtered back-projection algorithm under the sector model proposed in this paper and the spatial temperature field is obtained. The experimental results show that the temperature field reconstruction error is smaller for the IFS method than the BOS method, the measurement area is accurate from 0.5 mm for the BOS method to 0.2 mm for the IFS method and the temperature error range is reduced from 50 K for the BOS method to 25 K for the IFS method.

Boundary layer and near wake measurements of a two-dimensional airfoil with background-oriented schlieren method

Boundary layer and near wake measurements of a two-dimensional airfoil with background-oriented schlieren method

Fractal analysis of hydrogen jet mixing on the basis of BOS method

Structures formed as a result of mixing of hydrogen with air are analyzed by optical methods using detection of density non-uniformities. Methods for determination of fractal parameters for a random distribution of these non-uniformities are described, and information about the structure of gas mixing revealed is analyzed. The BOS (Background Oriented Schlieren) method is used to obtain the optical image of the formed structures and then treat by the correlation method that allows to obtain the quantitative information on the mixing. As a result, the possibility to link the characteristics of the injected gas source and the fractal parameters were demonstrated. The method can be used in the development of the non-intrusive technique for the evaluation of the gaseous system parameters based on the optical diagnostics and, potentially, for the obtaining more detailed information of the turbulence in gases.

Cavitation threshold pressure of focused ultrasound observed with sonochemiluminescence

Spatial distribution of sonochemiluminescence (SCL) from an argon-saturated luminol solution was measured in a focused sound field at 1 MHz in a standing-wave configuration. The SCL distribution was confined to pre-focal region at acoustic powers lower than 0.9 W, and was not located at the focus but at a few mm pre-focal side at a threshold for SCL inception. The threshold pressure amplitude for SCL inception was 3.6 atm at the focus, which value was obtained with a background-oriented schlieren method. The method is based on the broadening of multiple slits due to an optical deflection caused by ultrasound, and the broadening width measured provides an acoustic pressure amplitude. A qualitative image of the focused sound field was also obtained.

Features of the background oriented schlieren method for studying small axisymmetric plasma objects

Features of the background oriented schlieren method for studying small axisymmetric plasma objects

Spherical surface control by Background Oriented Schlieren method

The technique of automated quality spherical surfaces control using the background oriented schlieren method was presented. The results of theoretical refractive index gradient modelling for spherical lens were presented. The experimental results of spherical lenses quality diagnostics were shown.

Investigation of temperature field of weakly ionized gas jet by the background oriented schlieren method

The paper shows the possibility of using the background oriented schlieren technique to study the temperature field of a weakly ionized plasma jet created as a result of a spark discharge in argon at atmospheric pressure. The design of the spark source, the experimental setup, the experimental method and the processing of experimental data are presented. The temperature field of weakly ionized plasma jet and the temperature distribution around it were obtained.

Particle image velocimetry in refractive index fields of combustion flows

Optical measurements inside reacting flows are often disturbed by refractive index fields, e.g., due to the strong density gradients in flames. Although occurring measurement errors due to light refraction are a known problem for certain particle image velocimetry (PIV) applications, only a qualitative analysis of the resulting measurement uncertainty inside flame flows has been carried out to date. As an important step forward, a measurement approach is proposed, which enables a quantification of the resulting measurement uncertainties due to light refraction. As an example, the measurement approach is applied to a premixed propane flame. The uncertainty analysis is based on the determination of occurring particle position errors due to light refraction inside the flame. For three different measurement planes, the velocity field is measured with PIV and the particle position errors are experimentally measured and verified by ray-tracing simulation based on the measured refractive index field, which is determined by the background-oriented Schlieren method. In the examined flow, maximal position errors amount up to 14 μm and yield significant systematic velocity errors of up to 4% and random velocity errors of up to 6%. In contrast to the systematic velocity error, the random velocity error varies significantly for the analyzed measurement planes inside the flame flow.Graphic abstract

Video Recording of an Air Blast Wave Resulting from Initiation of a Light-Sensitive Explosive Composition

This paper presents experimental data on the parameters of an air blast wave resulting from initiation of VS-2 light-sensitive composition by laser diode radiation. The propagation of the air blast wave front was recorded by the background-oriented schlieren method. The air blast wave front was visualized using cross-correlation processing. Empirical dependences characterizing the propagation of the air blast wave front resulting from initiation of a light-sensitive composition of arbitrary weight. Experiments confirmed the possibility of initiating VS-2 light-sensitive composition using an EV-15 electric igniter.

Three-dimensional shock wave reconstruction using multiple high-speed digital cameras and background-oriented schlieren imaging

A methodology to perform three-dimensional reconstruction of an explosively driven shock wave’s position and shape as a function of time is developed here. A series of explosive tests are performed where the explosive process is imaged by multiple high-speed digital cameras spread over a wide area. The high-speed images are processed using the background-oriented schlieren method to visualize the shock wave. The data from the multiple camera views are then merged into a single three-dimensional point cloud representing the locations on the shock wave. The propagation of the shock wave is measured and fit to the Dewey equation. Analysis of the shock wave position and propagation allows identification of asymmetries on the shock front due to an asymmetrical explosion process. The techniques developed here are shown to be useful tools that can be implemented to augment the traditional point-wise instrumentation of current explosives research testing and provide an enhanced characterization of an explosion over traditional arena test methods.

Visualization of density variations produced by alternating-current dielectric-barrier-discharge plasma actuators using the background-oriented schlieren method

Gas density perturbations generated by an alternating-current dielectric-barrier-discharge plasma actuator (ac-DBDPA) are quantitatively visualised using the background-oriented schlieren (BOS) method. A method of setting the optimum boundary condition for solving the Poisson equation in the BOS method is studied, and an integration method for the boundary condition in the vicinity of the plasma where the density change is steep is proposed. The BOS method is applied in two cases with different voltage amplitudes, and the variation in the absolute value of the density is discussed with the discharge properties. The results show a decrease in density in the synthetic jet induced by the ac-DBDPA and a spatiotemporal variation indicating a step-wise gas-heating phenomenon due to plasma discharge.

Characterization of high-current pulsed arcs ranging from 100–250 kA peak

In this paper, we present the laboratory study on three experimental setups that produce a free arc channel subjected to the transient phase of a lightning current waveform. This work extends the high-current pulsed arc characterization performed in previous studies for peak levels up to 100 kA. Eleven high-current waveforms with peak value ranging from 100–250 kA with different growth rates and action integrals are studied, allowing the comparison of different test benches. These waveforms correspond to standard lightning ones used in aircraft certification processes. Hydrodynamic properties such as arc channel evolution and shock-wave propagation are determined by high-speed video imaging and the background-oriented Schlieren method. The arc diameter reaches around 90 mm at 50 μs for a current of 250 kA peak. Space- and time-resolved measurements of temperature, electron density and pressure are assessed by optical emission spectroscopy associated with the radiative transfer equation. It is solved across the arc column and takes into account the assumption of non-optically thin plasma at local thermodynamic equilibrium. For a 250 kA waveform, temperatures up to 43 000 K are found, with pressures in the order of 50 bar. The influence of current waveform parameters on the arc properties are analyzed and discussed.

Расчетно-экспериментальное исследование ударно-волнового нагружения оптически прозрачных тел

The results of computational and experimental studies of shock-wave loading of transparent bodies by two methods are presented: theoretical --- based numerical simulation on a regular three-dimensional grid using an explicit solver in a coherent Lagrangian-Eulerian formulation, experimental - - - using the method of shadow photography and background oriented schlieren method.

Practical Approach for Absolute Density Field Measurement Using Background-Oriented Schlieren

A practical approach for deriving the absolute density field based on the background-oriented schlieren method in a high-speed flowfield was implemented. The flowfield of interest was a two-dimensional compressible flowfield created by two supersonic streams to simulate a linear aerospike nozzle operated under a supersonic in-flight condition. The linear aerospike nozzle had a two-dimensional cell nozzle with a design Mach number of 3.5, followed by a spike nozzle. The external flow simulating the in-flight condition was 2.0. The wall density distribution used as the wall boundary condition for Poisson’s equation to solve the density field was derived by a simplified isentropic assumption based on the measured wall pressure distribution, and its validity was evaluated by comparing with that predicted by numerical simulation. Unknown coefficients in Poisson’s equation were determined by comparing the wall density distribution with that predicted by the model. By comparing the derived density field based on the background-oriented schlieren method to that predicted by the model and numerical simulation, the absolute density field was derived within an error of 10% on the wall distribution. This practical approach using a simplified isentropic assumption based on measured pressure distribution thus provided density distribution with sufficient accuracy.

Determination of Diffusion Coefficient in Hydrogel

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Observation of Supersonic Jet Using Small Volume High-Pressure Shock Tube

The unsteady supersonic jet and the shock wave injected by the small volume shock tube are experimentally studied in this paper. The experimental was performed by the background oriented schlieren method. The main parameters for the jet are the pressure ratio by the high pressure chamber/ a back pressure 10.9-53.0 and the length of high pressure chamber/diameter ratio 1 and 10. The velocity of the shock wave and supersonic jet were estimated by using the principle of the background oriented schlieren method. The results showed that the influence of the length of the high pressure chamber on the velocity of the jet.

Background Oriented Schlieren Method as an Optical Method to Study Shock Waves

Background oriented schlieren method is applied in diagnostics of shock waves in air. The method can be used for visualization of shock waves that are generated after explosion or due to motion at ultrasonic speeds. Experimental data make it possible to observe propagation of a shock wave in space, estimate the asymmetry of energy liberation in explosion, and determine parameters of shock wave.