Background Oriented Schlieren Technique Research Articles

Event-triggered background-oriented schlieren: high-frequency visualization of a heated jet flow.

This Letter reports a novel, to our knowledge, event-triggered background-oriented schlieren (EBOS) technique using a combination of an event-triggered camera and pulsed laser speckle projection. The BOS images are reconstructed using the event data generated by the pulsed laser speckle projection and then processed to obtain the density and temperature distribution of the flow. This technique enables continuous visualization and recording of flows at kFPS frame rates with a very low cost, breaking through the short operating times of existing high-frame-rate BOSs. To examine the event-triggered BOS technique, tests are conducted on a hot air gun. The measured temperature distribution coincides with the thermocouple data with an error of no more than 10.8%. Measurements taken during the start-up of the hot air gun demonstrate that the presented technique can measure the evolution of the jet temperature for at least 150 s, as well as capture the localized unsteady turbulent structure in the heated jet flow.

Optical Analysis of Ignition Sparks and Inflammation Using Background-Oriented Schlieren Technique

To determine the timing of inflammation in gas and gasoline combustion engines, the point of 10% mass fraction conversion of fuel (MFB10) is commonly used. The MFB10 can be determined from the heating curve, which in turn is calculated from the in-cylinder pressure curve. However, the cylinder pressure is an indirect parameter with regard to inflammation, as it is the result of the combustion that follows the inflammation. An attempt is made to derive a new, direct parameter of inflammation based on optical measurements in order to detect inflammation more rapidly and accurately. The background-oriented Schlieren technique (BOS) in combination with high-magnification optics and a high-speed camera is used to detect local density changes coming from the particle wave around the ignition kernel of a hydrogen combustion inside a combustion chamber. Via BOS and regular high-magnification high-speed imaging, the influence of ignition coil dwell time and in-cylinder pressure on the spark phases and the inflammation itself are evaluated. As a potential direct parameter for inflammation, the size of the particle wave resulting from the expanding ignition kernel is evaluated. It was found that a higher coil energy supports a faster propagation of the particle wave at ambient pressure. At higher pressures, general combustion effects override the effect of the influence of the coil energy on the propagation speed of the particle wave. In addition, the presence of successful inflammation was found to influence the spark phases. A directly measurable parameter for ignition could be found at a basic level, which will serve as a starting point for further detailed investigations.

Tomographic Reconstruction of Hypersonic Aircraft Flowfields in Large-Scale Wind Tunnel Experiments

Three-dimensional flowfield features around a hypersonic aircraft model were reconstructed using a computed tomography background-oriented schlieren technique. Datasets of projection images were acquired in hypersonic wind tunnel experiments conducted at the Japan Aerospace Exploration Agency’s 0.5m hypersonic wind tunnel facility, by rotating the test model with an increment of 2° azimuth. A telecentric optical configuration was applied to the background-oriented schlieren measurement at the large-scale hypersonic wind tunnel facility. A basic tomographic reconstruction method, such as filtered backprojection, was used to reconstruct the projection datasets with some combination of denoising calculations. Results showed that the reconstructed flowfield was first noisy without denoising. With the denoising considerations proposed in this study, the flow physics, such as oblique shock waves emanating from the vehicle nose tip and engine lip, were clearly visualized. Therefore, the telecentric optical setup and denoising processes proposed in this study can successfully reconstruct a complex three-dimensional flowfield obtained at a large-scale wind tunnel facility. This setup and denoising process can also apply to other applications in large-scale wind tunnel experiments.

Double-pass imaging background-oriented schlieren technique for focusing on measurement target

Double-pass imaging background-oriented schlieren technique for focusing on measurement target

Fuel/air mixing characteristics of a Micromix burner for hydrogen-rich gas turbine

As a promising hydrogen-rich gas turbine combustion technology, Micro-mixing combustion may produce a new flame stabilization mechanism due to the novel structure. To investigate the effects of nozzle structural parameters and inlet aerodynamic parameters on mixing characteristics, this paper firstly used the background oriented schlieren technique combined with the self-development program to obtain the mixing quality, and the program was based on the optical flow method and Gladstone-Dale equation. The fuel-air mixture process was predicted by three mass diffusion models, and the Kinetic Theory model matched the experimental results best compared with the Constant Dilute Approximation model and Dilute Approximation model. The results showed that the main mixing zone moved closer to the outer wall when the premixed channel diameter increased. And the vorticity decreased with the increased premixed length, leading to a decline in mixing efficiency. For inlet aerodynamic parameters, changes in equivalence ratios only affected the shear-layer vorticity. As the combustible component of syngas was in the range of 60%–40% (by vol.), the outer wall of the premixed channel could promote fuel-air mixture. Moreover, when the equivalence ratio ranged in 0.335–0.685, the heating value of fuel varied as 4.686–7.029 and 11.715 MJ/Nm3, fuel and air were premixed uniformly near the nozzle outlet.

Quantitative Investigation of Ballistics Flow Fields by Background Oriented Schlieren Technique

The ballistics field is known by the presence of several complex phenomena such as muzzles and flying projectiles flow fields. Consequently, numerical simulations are commonly used to model these complicated flows. However, the validation process of these codes has proven to be problematic due to the lack of experimental quantitative data. In this context, the present paper describes the application of the Background Oriented Schlieren technique (BOS) as a quantitative investigation tool in the ballistics field. We illustrate that BOS can accurately capture the main characteristics of the studied configurations: Firstly, we discuss the visualization and the density field reconstruction around a Bullet Simulated Projectile BSP flying at supersonic velocities and a sniper projectile flying at supersonic and transonic velocities. We demonstrate that these fields are in satisfactory agreement with the results of Taylor and Maccoll’s equation and numerical simulation. Then, the findings of the BOS visualization of the precursors and the propellant flow fields are presented. To this end, the salient features accurately captured by the BOS technique such as vortex rings, shock bottles, Mach, and blast wave are described both qualitatively and in terms of density profiles. Two improved approaches that are essential to the aforementioned analysis are proposed: the first is related to density field reconstruction based on Abel inversion and the second approach is a phase separation procedure.

Design rules for Background Oriented Schlieren experiments with least-squares based displacement calculation

Background Oriented Schlieren (BOS) is a non-intrusive optical method used to visualize density variations in fluids. A camera is used to measure the distortion of a background image displaying a random pattern due to the refraction of the light induced by the local gradients in the refractive index of the under study. In contrast to conventional schlieren, for which direct observation is possible, the images obtained with the BOS technique have to be processed. In this contribution, a computationally affordable approach based on linear least-squares minimization and adapted from optical flow techniques is presented in detail and tested in the context of BOS with the Matlab GUI application comBOS. A series of synthetic images are first analyzed to devise design rules for the definition of an optimal random Gaussian dot background pattern. Tests are performed for comBOS, the commercial BOS module included in DaVis 10, and the free PIV tool PIVlab. It is found that the diameter of the dot images on the camera sensor should be at least 3 pixels, but can be larger without affecting the accuracy of the measurement with comBOS and DaVis 10, while PIVlab severely degrades the quality of the results for dots larger than 5 pixels. The contrast and the density of dots must also be sufficient such that a minimum of 75 % of the surface of the background is shaded. If these conditions are met, the maximum systematic error in the measured distortions is 0.006 pixel for comBOS and 0.001 pixel with DaVis 10 when local gradients are small. Similar errors are found with PIVlab for dots smaller than 5 pixels in diameter, but they increase rapidly for larger dots. The random error is typically smaller than 0.004 pixel in optimal conditions, but it increases significantly for images affected by a random noise or local gradients of the distortion. Finally, it is concluded with a realistic test case that both comBOS and DaVis 10 are well suited to capture the main features of a complex compressible flow with Schlieren imaging. Thus, the main advantage of comBOS is its affordability and robustness for researchers and educators willing to implement BOS, for example in educational contexts.

Experimental characterisation and data-driven modelling of unsteady wall pressure fields induced by a supersonic jet over a tangential flat plate

This work deals with the investigation and modelling of wall pressure fluctuations induced by a supersonic jet over a tangential flat plate. The analysis is performed at several nozzle pressure ratios around the nozzle design Mach number, including slightly over-expanded and under-expanded conditions, and for different radial positions of the rigid plate. Pitot measurements and flow visualizations through the background oriented schlieren technique provided a general overview of the aerodynamic interactions between the jet flow and the plate at the different regimes and configurations. Wall pressure fluctuations were measured using a couple of piezoelectric pressure transducers flush mounted over the plate surface. The spectral analysis has been carried out to clarify the effect of the plate position on the single and multivariate wall pressure statistics, including the screech tone amplitude. The experimental dataset is used to assess and validate a surrogate model based on artificial neural networks. Sound pressure levels and coherence functions are modelled by means of a single fully connected network, built on the basis of a recently implemented fully deterministic topology optimization algorithm. The metamodel uncertainty is also quantified using the spatial correlation function. It is shown that the flow behaviour as well as the screech and broadband noise signatures are significantly influenced by the presence of the plate, and the effects on spectral quantities are correctly reproduced by the proposed data-driven model that provides predictions in agreement with the available data.

Flexibility and versatility enhancements due to a portable optical system for the Background-Oriented Schlieren techniques

Flexibility and versatility enhancements due to a portable optical system for the Background-Oriented Schlieren techniques

Simultaneous background-oriented schlieren and retroreflective shadowgraph visualization

This paper describes a large-scale visualization system that can simultaneously produce background-oriented schlieren (BOS) images and retroreflective shadowgrams (RS) of a compressible flow. The simultaneous measurements with BOS and RS in two optically equal systems allowed a direct comparison of the performance of the two methods and to quantify the relative differences, primarily with respect to identifying the broad range of length scales in a turbulent compressible flow. Further, direct comparisons are made between the schlieren image obtained from the BOS measurement, the displacement map obtained by subtracting a no-flow image from the unprocessed BOS record, the divergence of the BOS displacement field (the latter two are forms of a BOS-generated shadowgram), and the simultaneously obtained direct shadowgram from the RS setup. A qualitative comparison of the results indicated that all the three methods (BOS schlieren, direct difference, and divergence of the BOS-schlieren record) can capture the large-scale flow structures but that the smaller scales in the plume can only be resolved to a much lesser extent. These structures were, however, easily resolved in the RS images. The results show that, in the described system, the resolution of the BOS images is lower than that of the shadowgrams by a factor of about five. Further tests indicate that these differences in the spatial resolution of the BOS technique cannot be removed using a different design of the random-dot background pattern. These observations apply to the used large-scale system in which a minimum distance between the observed object and each of the two screens (background pattern/reflective) has to be maintained. This constraint also meant that neither system could be optimized with respect to spatial resolution.

Background-oriented schlieren technique with vector tomography for measurement of axisymmetric pressure fields of laser-induced underwater shock waves

This study aims to overcome the problems that existing background-oriented schlieren (BOS) techniques based on computed tomography (CT-BOS) face when measuring pressure fields of laser-induced underwater shock waves. To do this, it proposes a novel BOS technique based on vector tomography (VT-BOS) of an axisymmetric target. The remarkable feature of the proposed technique is the reconstruction of an axisymmetric vector field with nonzero divergence, such as the field of a laser-induced underwater shock wave. This approach is based on an approximate relation between the projection of the axisymmetric vector field and the reconstructed vector field. For comparison, the pressure fields of underwater shock waves are measured with VT-BOS, CT-BOS, and a needle hydrophone. It is found that VT-BOS is significantly better than CT-BOS in terms of better convergence, less dependence on the spatial resolution of the acquired images, and lower computational cost. The proposed technique can be applied not only to fluid dynamical fields, but also to other axisymmetric targets in other areas, such as electromagnetics and thermodynamics.Graphical abstract

High-speed photography and background oriented schlieren techniques for characterizing tulip flame

High-speed photography and background oriented schlieren (BOS) techniques are used to characterize the tulip flame formation in half-open and closed-ends type channels. Flame propagation for a premixed mixture of Liquefied Petroleum Gas (LPG) and air in the equivalence ratio range of 0.8–1.0 are tested. The classical tulip flame is observed only in the closed channel. Refractive index and flow density are calculated numerically using Poisson’s equation and Gladstone–Dale relation. Pseudo-schlieren images, shadowgram, and density contours are plotted using the BOS technique, and these reconstructed fields matched well with the direct observations. Instantaneous velocity fields are calculated using consecutive images as the reference image for the BOS correlation method. The results show that the velocity during the early stages of the flame expansion is close to the laminar burning velocity of the premixed LPG-air flame. As the flame started to inverse, the flame tip close to the wall accelerated to approximately 1.16 m/s. But later the fully-formed tulip flame propagates at 0.25–0.3 m/s till the end of the chamber. A reverse flow in the burned gas region is observed after the tulip flame formation. However, BOS could not capture any acoustic changes in the unburned gas except for a high gradient backflow just behind the flame front.

Background-Oriented Schlieren technique using Fast Checkerboard Demodulation for extending measurement range of pressure of underwater shock waves

We have successfully extended the measurement range by applying Fast Checkerboard Demodulation (FCD) to Background-Oriented Schlieren (BOS) technique, which is a contactless measurement technique. In the experiment, the apparent displacement field on the background image distorted by underwater shock waves were measured using FCD-BOS and contact measurement pressure (hydrophone). The results show that the measurement limitation of FCD is higher than the methods of previous studies which used Optical Flow (OF) and Digital Image Correlation (DIC) algorithms. Since the displacement is detected from the frequency change due to the distortion of the background image, FCD that combines fast Fourier transform (FFT) and a background image with a two-dimensional periodic pattern, can be applied to the large distortion which is difficult to be detected using OF and DIC. The numerical approach in our previous study(Shimazaki et al., 2022) showed that the measurement range of FCD-BOS is strongly influenced by the background pattern. Therefore, this study also performed numerical investigation on the use of synthetic background images with the frequency of the resultant wave of two sine waves of different wavelengths for FCD-BOS. The numerical results show that the measurement range using the synthetic background images with wavelengths of (60,60/12) μm is three times larger than the experimental measurement range using the lattice grid (width: 20 μm) background. In addition to the synthetic background of wavelengths of 60,60/12, the measurement accuracy, phase wrapping, overlap, and aliasing artifacts of those of (40,40/12) and (30,30/12) were investigated. Such results show the superiority of the background with the frequency of sine wave over that of square wave (lattice grid).

Generation And Evaluation Of Projected Moving Patterns For Reference-Free BOS With Increased Spatial Resolution

A method of taking background oriented schlieren technique (BOS) images using projected backgrounds is shown. The projected backgrounds can be scaled to the measurement requirements, providing more flexibility than printed backgrounds. The evaluation of the BOS image pairs using cross correlation or optical flow analysis was shown to produce equivalent results. The background can be dynamically moved using a rotating mirror, allowing evaluation using the reference-free shadowgraphy or reference-free BOS methods. Forward BOS is shown, where the projected speckle pattern shines through the density object onto a screen, and is recorded by a camera focused on the screen. In contrast to standard shadowgraphy, this technique is robust when the lighting of the field is uneven, and the point movement is related to the first derivative of density.

Background Oriented Schlieren Technique With Reconstruction Of Three-Dimensional Vector Fields For Pressure Measurement Of Laser Induced Underwater Shock Waves

We propose a novel reconstruction of the three-dimensional vector field (VR) of an axisymmetric target to overcome the problems that existing background-oriented schlieren (BOS) techniques based on reconstruction of three-dimensional scalar field (SR-BOS). This approach is based on an approximate relation between the projection of the axisymmetric vector field and the reconstructed vector field. The remarkable feature of the proposed technique is the reconstruction of an axisymmetric vector field with nonzero divergence, such as the field of a laser-induced underwater shock wave. Reconstruction of 3D vector fields, such as conventional vector tomography, is usually used in the field of electromagnetics with vector fields of zero divergence, and its application to fluid phenomena has been di fficult. We have successfully applied this technique to BOS (VR-BOS) and measured the pressure fields of underwater shock waves. Using the experimental data and previous research data, the superiority of VR-BOS was clarified by comparing the measurement results of SR-BOS and VR-BOS, for example, measurement accuracy, in better convergence, less dependence on the spatial resolution of the acquired images, and lower computational cost. In particular, the fact that the measurement accuracy of VR-BOS is almost independent of resolution is expected to expand the maximum pressure values that can be measured by the BOS technique. We also used synthetic data to investigate the effect of noise on VR-BOS. If the synthetic displacement contains noise, the noise is amplified in the vicinity of the axisymmetry axis in the reconstruction distribution calculated by VT. If the distance from the axisymmetry axis is more than 1/4 of the measurement range, the measurement accuracy of the reconstructed distribution can be sufficiently guaranteed. This proposed technique can be applied fluid dynamical fields, as well as other axisymmetric targets with vector reconstruction fields in other areas, such as electromagnetics and material mechanism.

Visualization of a Small Jet Synthetic Using a Particle Image Velocimetry and Background-Oriented Schlieren Techniques

This work reports the results of experimental investigations carried out on a submerged synthetic jet obtained through the use of a headset speaker piloted with an appropriate sinusoidal signal at a frequency of 380 Hz. The study begins with the construction of a device that, exploiting the well-known transport properties of impacting jets, was able to improve local ventilation and the removal of excess moisture, due to the natural transpiration of human skin in people forced to assume the same position for prolonged periods such as professional drivers; bedridden patients; etc… Subjects are substantially forced to have parts of their own body in contact with fabrics and coverings that hinder the normal conditions of skin transpiration. The experimental activity was first based on the study of the structure of the synthetic jets, then moving on to the creation of a sponge mat equipped with 80 individual jets. On this sponge mat, semi-empirical tests were carried out in order to remove moisture from a fabric soaked in distilled water. The experimental investigations were first carried out using the PIV technique, and, subsequently, the synthetic jet was visualized using the Background-Oriented Schlieren (BOS) technique which allowed to test the presence of the jets, installed directly on the mattress, in a relatively simple and fast way, requiring a very simplified set-up.

Experimental and LES investigation of ignition of a spinning combustion technology combustor under relevant operating conditions

SAFRAN Helicopter Engines has developed the spinning combustion technology in which the burnt gases from one injector travel tangentially along the combustor annulus towards the neighboring injectors. Compared to a conventional design, this arrangement modifies the ignition process, which is a critical phase for aeroengines. In order to understand the ignition process in this technology, experiments and Large-Eddy Simulation (LES) have been performed in a cylindrical combustion chamber where the flow is injected tangentially (named Radius chamber). Three cases are considered with different strain and turbulence levels representative of real combustor flows. Micro calorimetry and the Background-Oriented Schlieren technique allows for detailed temporal measurements of energy deposited in the flame kernel. Pressure measurement and Schlieren imaging are used to study the flame propagation. LES are performed with a 19-species and 184-reactions analytically-reduced chemistry together with the thickened flame approach allowing the description of the first instants of ignition in a quasi-DNS mode and ensuing flame propagation. Both a static and dynamic formulations of the wrinkling factor to describe sub-grid scale chemistry-turbulence interaction are used. Results show that LES is able to capture the flame kernel formation and trajectory as well as the time to reach maximum pressure within an error of 10% when using a dynamic formulation. On the other hand, the static formulation of the wrinkling factor predicts the time for maximum pressure within a maximum error of 20%.

A new approach for the reconstruction of axisymmetric refractive index fields from background-oriented schlieren measurements

Background-oriented schlieren (BOS) is an optical visualization technique that reconstructs a whole-field flow based on its density gradient. Thanks to the cost-effectiveness of its optical setup and its unlimited field of view, BOS has become an attractive technique for laboratory and large-scale experiments. In BOS, the reconstruction of the refractive index field involves various mathematical calculations, which depend on the flow geometry. In this context, the present study presents a new method for the reconstruction of three-dimensional axisymmetric refractive index fields. The proposed method takes advantage of the bi-sensitivity quality of BOS to gain more accuracy in the reconstructed fields. We demonstrate the method’s precision and robustness against experimental noise throughout a synthetic axisymmetric refraction index field. The application of this new approach is not restricted to the BOS technique but can be extended to several other measurement techniques such as moiré deflectometry, laser speckle photography, and rainbow schlieren.

Background oriented schlieren technique with fast Fourier demodulation for measuring large density-gradient fields of fluids

In order to measure the large density-gradient fields of fluids such as underwater shock waves, we employed a fast Fourier demodulation called Fast Checkerboard Demodulation (FCD) method in Background Oriented Schlieren (BOS) technique. BOS is a simple image-based measurement technique that detects the apparent displacement (local distortion) of a background image caused by the density gradient of the fluid in front of the background. The cross-correlation particle image velocimetry (PIV) method, which is commonly employed in the BOS technique for detecting the apparent displacement, uses a random-dot background, whereas FCD uses a periodic pattern as the background (e.g., checkerboard pattern, lattice grid pattern (grid scale)) to detect the apparent displacement as the phase change of the pattern in the Fourier space. In this study, we measured the apparent displacement, which is proportional to density gradient of fluid, of an underwater shock wave using FCD and PIV. The results showed that FCD can measure a displacement gradient of up to 2.5 times larger than that which PIV can measure. Furthermore, we systematically investigated the measurement limit of FCD-BOS by changing several parameters of the periodic patterns, such as the grid size. Also, we explored the related parameters in the Fourier space to understand the limitations. It is worth noting that FCD-BOS with lattice grid pattern (grid scale) can measure the apparent displacement as accurately as that with a custom-made pattern, indicating that large density-gradient fields of fluids can be measured using a simple setup with a commercially available (inexpensive) pattern.

Visualization of the Electrohydrodynamic and Thermal Effects of AC-DBD Plasma Actuators of Plate- and Wire-Exposed Electrodes

The dielectric barrier discharge plasma actuator is a promising flow control device that uses surface discharge. The actuator generates an electrohydrodynamic force and Joule heating that contribute to the flow control. Thus, it is important to investigate the electrohydrodynamic and thermal effects on the air flow. To this end, the flow velocity field, density field, and surface temperature distribution induced by an alternating current dielectric barrier discharge plasma actuator were experimentally examined, adopting particle image velocimetry, the background oriented schlieren technique, and an infrared camera. These experiments were conducted for plate- and wire-exposed electrode plasma actuators to investigate the effect of the shape of the exposed electrode. It was confirmed that the topology of the discharge is different between the two types of plasma actuators. This results in a difference in the spatial distributions of the velocity and density fields between the two actuators. In particular, we clarified that there is an obvious difference in the peak position of the density and temperature distribution between the two actuators. We also confirmed that the difference in the spatial distribution of the vertical velocity makes the above difference.