Mobile visualization of density fields using smartphone background-oriented schlieren

The Background Oriented Schlieren (BOS) technique is one of the novel measurement techniques and its application range is very wide. The principle of BOS is similar to that of the conventional schlieren technique, it exploits the bending of light ray caused by a refractive-index change corresponding to the density change in the medium. The BOS technique allows the quantitative measurement of density with very simple experimental setup and proper image analysis. Only a background and a digital camera are required for the experiment, so that even the real scale experiments can be realized. In recent years, the development of the high-speed camera is remarkable and so many high-speed phenomena can now be captured. To realize the precise measurement with BOS technique using high-speed camera, higher resolution (larger number of pixels) is desirable. In this paper, with a technical support from Nobby Tech Ltd., a 4K high-speed camera (4096 × 2160 pixels) is applied to the BOS measurement of the lateral jet/cross flow interaction filed in the supersonic wind tunnel test as a trial of the quantitative density measurement with higher resolution. The measurement system consists of a 4K high-speed camera and a pulsed laser for background illumination. A telecentric optical system is also employed to improve the spatial resolution of the measurement. The measurement results of BOS technique up to 1000 fps with higher resolution are discussed.

The refractive index fields of several media, say, transparent solids, transparent liquids, and air are often crucial when using these media as optical elements, probing optical properties, and deriving other physical information related to the refractive index. For example, in an optical system using a high-power laser, the thermal lens effect of optical elements is one of great issues because the laser focal position will be changed by the effect [1]. Thus, a method to measure a gradient-index of the optical element should be useful to take a step to reduce the thermal lens effect. Basically, light rays passing through a medium with an inhomogeneous refractive index field will be deflected toward the area of higher refractive index. This way, it should be possible to obtain information about the refractive index field from measurement of such deflection. Consequently, there have been several methods for visualizing gradient-index fields, including schlieren photography [2] and shadowgraphy [3]. Although these visualization methods are powerful tools for qualitatively analyzing the gradient-index field, quantitative analysis requires additional methods. Rainbow schlieren photography is one candidate. In this method, the strength of deflection of light rays is obtained by using a rainbow aperture [4-6]. The quantitative phase-contrast imaging has also developed with incoherent-light [7, 8], coherent-light [9, 10], and asymmetric illumination [11]. There is also background-oriented schlieren (BOS) technique for measuring gradient-index field quantitatively [12-20]. In the BOS technique, displacements of dot-patterns that are printed on a background are caused by deflection of light rays passing through a gradient-index field. The optical flow algorithm can be used to obtain the displacements of the background-dot-patterns. However, obtaining depth information of the gradient-index field with the BOS technique is often difficult because the dot-pattern displacement results from the light ray deflected all along its path, requiring integration over the path. To address this, a method of reconstructing the depth information of a gradient-index field is proposed here on the basis of the Lagrangian optics with the BOS technique.

Background-Oriented Schlieren (BOS) is a non-intrusive and non-destructive technique for measuring density gradient in transparent media. Given the dependency of the refractive index and density of a medium, its static density field can be determined as a secondary measurand from BOS measurements. For an ideal gas, if the pressure variation is negligible, the static temperature field can be determined as a tertiary measurand. While the BOS technique for far-field measurements has been extensively researched, the implementation of this technique for near-field measurements still needs to be investigated. Near-field BOS is especially important for the measurement and visualization of internal flows, such as turbomachine flows, in which access to flow passage is limited and might be feasible using endoscopes (borescopes). This paper presents the features and challenges of the endoscopic BOS technique and presents methods and solutions for the application of this method in near-field measurements. The features of endoscopic BOS records as well as the comparison of endoscopic BOS and BOS measurements with camera are presented.

<div class="section abstract"><div class="htmlview paragraph">The measurement of spray penetration length is one of crucial tasks for understanding the characteristics of diesel spray and combustion. For this reason, many researchers have devised various measurement techniques, including Mie scattering, schlieren photography, and laser induced exciplex fluorescence (LIEF). However, the requirements of expensive lasers, complicated optics, delicate setups, and tracers that affect fuel characteristics have been disadvantages of previous techniques.</div><div class="htmlview paragraph">In this study, the background-oriented schlieren (BOS) technique is employed to measure the vapor penetration length of diesel spray for the first time. The BOS technique has a number of benefits over the previous techniques because of its quantitative, non-intrusive nature which does not require lasers, mirrors, optical filters, or fuel tracers. The objectives of this study are to establish the BOS technique as a viable method and then to use it to measure the vapor penetration of diesel spray as a function of ambient temperature, pressure, and orifice pressure drop by using BOS technique after verifying the possibility as a new useful method.</div><div class="htmlview paragraph">The experiments were performed on an optically accessible, constant-volume pre-combustion type chamber (CVPC) simulating the ambient conditions within a diesel engine. A single-hole solenoid injector and a common rail system were used for fuel injection. The structured background with randomly generated dots was taken by a high speed CCD camera, and the images were post-processed by analysis programs. The schlieren images were also taken under the same set of conditions to validate the results of the BOS technique. The BOS results show a good agreement with the schlieren results in terms of vapor penetration. It is worthwhile to develop a more convenient and quantitative diagnostic technique applicable for investigating the penetration characteristics of diesel spray and capable of providing fundamental information to the area of CFD study.</div></div>

Density measurements have been performed using Background Oriented Schlieren (BOS) and differential interferometry, in the case of supersonic flows around a cone-cylinder body, a hemisphere profile and two spike-tipped models for Mach numbers 2 and 3, and no angle of attack. Experiments have been driven in the blow-down wind tunnel of the FrenchGerman Research Institute of Saint-Louis (ISL). For BOS technique a specific Abel transform algorithm has been developed to rebuild density field from ray deflections. For differential interferometry an original and simple calibration and filtering process allows to obtain quantitative results in spite of experimental noise during the blow-down. Performances of these two very different techniques – accuracy, spatial resolution, limitations – have been evaluated and compared in the basic cases of cone and hemisphere. BOS measurements have been 2% accurate and spatial resolution has been 1 millimeter in a 100*100mm2 flowfield. In the same flowfield, differential interferometry has given 2% accurate measurements and spatial resolution has been 0.25 mm. Pretty good agreement has been found with RANS numerical simulations outside from boundary layers and bow shock regions. The latter is the main limitation to both of methods. In a bow shock region, BOS experimental set up is not “stigmatic”, while a “parasite” shadowgraph appears in differential interferometry. The lack of information leads to under-estimation of density in such a region. Having knowledge about performances and limitations of these two techniques, Background Oriented Schlieren and Differential Interferometry have been used to measure density in the case of a supersonic flow around a spike-tipped body: a disk-spike and a biconical-spike for Mach numbers 2 and 3 have been under study. Outside bow shock regions, a pretty good agreement has been found between measurements by BOS and by Differential Interferometry, which allow them to be used later for CFD validation.

Jets in a supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. In the present study, streamwise linear arrays of high-momentum microjets are used to generate either single or multiple oblique shocks in a supersonic crossflow. The shocks generated using microjets can be tailored in terms of their strength and be made either parallel or coalescing, depending on the application. Flow visualization using shadowgraph and density field measurements using background-oriented schlieren (BOS) technique were carried out for a range of microjet operating conditions. The results obtained using the two methods are consistent and complementary and show a linear variation of oblique shock angles with a microjet pressure ratio over the range of conditions tested. The density field obtained using BOS clearly shows the oblique shocks generated using these microjet arrays, the jump in density across the shock, the extent of the high-density field, the expansion fan, and the associated decrease in density. The results suggest that microjet arrays can be successfully used to develop techniques for sonic boom mitigation and high-performance supersonic inlets.

The background oriented schlieren (BOS) technique is one of the visualization techniques that enable the quantitative measurement of density information in the flow field with very simple experimental setup. BOS requires only a background and a digital still camera and it can realize the quantitative measurement of density. In this report we propose the colored grid background image for BOS technique (CGBOS). The experiments were carried out in the 0.6 m x 0.6 m test section of supersonic wind tunnel at JAXA-ISAS. The measurement setup consisted of metal halide ramps, a colored grid background image and a digital still camera. A colored grid pattern was used as background image and density gradient in vertical and horizontal direction was obtained. The measurement result and prospect of CGBOS technique are reported.

A method to reconstruct a localized gradient-index field using the background-oriented schlieren (BOS) technique is proposed here. The BOS technique can measure deflection angles of light rays passing through the gradient-index field. When a gradient-index field is localized in a limited area, a scalar potential that is the integral of the gradient-index field over the light ray path can be transformed in the form of the integral of the deflection angle over the localized area. The gradient-index field is then derived from the scalar potential assuming that the field is axisymmetric. When the gradient-index is anisotropic, a light ray path depends on its polarization. The localized reconstruction method is applicable not only to the isotropic gradient-index field but also to such anisotropic field. An anisotropic gradient-index field can be experimentally generated by applying stress to a glass block with a resin sphere that contacts the glass block at a point. The anisotropic gradient-index field can be reconstructed using the localized reconstruction method, and it agrees with that of theoretical calculation using structural analysis software.

An experimental study incorporating the use of the Background-Oriented Schlieren (BOS) technique was performed to measure the density field of a rectangular supersonic jet. This technique is easier to set up than conventional schlieren since the optical alignment involving the various mirrors, lenses and knife-edge is replaced by a background pattern and a single digital camera. The acquired images which contain information of density gradients in the flow are solved as a Poisson equation and further processed using deconvolution and tomographic algorithms to generate a 3D domain which contains information about the actual density. 2D slices can then be extracted to quantitatively visualise the density along any required planes. The results from supersonic axisymmetric jets are used for validation of the code; these show excellent agreement with pre-validated CFD data. The results for a rectangular supersonic jet are then obtained. These show good agreement with the CFD data, in terms of shock-cell spacing and overall structure of the jet. The technique has proved useful for investigating axis-switching, a phenomenon generally associated with non-axisymmetric jets.

An important aspect of 2D object recognition is matching. Based on a generalized distance transform method, Borgefors introduced the concept of the Chamfer scheme to 2D object matching. In Chamfer matching, a successful match is obtained when a scene in a reference image is correctly matched to the corresponding scene in the target image. The successful match corresponds to the lowest matching measure computed from the sum of Chamfer distances. However, if the target image has not been correctly corrected for distortions, there may not be an ideal best fit between the reference and target images. Instead, matching measures corresponding to local minima exist, for which it is difficult to choose the correct matching measure that corresponds to the best fit. The match fit corresponding to the global minimum measure may not necessarily be the best fit as there are many sub-optimal measures that could provide equally good fits between the target and reference images. This paper advocates that aside from the Chamfer matching measure, additional considerations need to be factored in to improve on finding the true global minimum measure which corresponds to the optimal fit. Int his paper, a new method has been designed to find the optimal fit between poorly-rectified target and reference images. The new approach uses simulated annealing to improve on the matching results. Simulated annealing is a powerful stochastic optimization technique for nonlinear problems. In our novel technique, the energy function for simulated annealing comprises two terms: a smoothness constraint term and the Chamfer matching measure term. The high computational burden of simulated annealing is reduced by using edge information for the matching process. Results are presented to illustrate the new matching technique.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Experimental measurements of shock wave propagation from explosions of C4 are presented. Each test is recorded with a high-speed digital video camera and the shock wave is visualized using background-oriented schlieren (BOS). Two different processing techniques for BOS analysis are presented: image subtraction and image correlation. The image subtraction technique is found to provide higher resolution for identifying the location of a shock wave propagating into still air. The image correlation technique is more appropriate for identifying shock reflections and multiple shock impacts in a region with complex flow patterns. The optical shock propagation measurements are used to predict the peak overpressure and overpressure duration at different locations and are compared to experimental pressure gage measurements. The overpressure predictions agree well with the pressure gage measurements and the overpressure duration prediction is within an order of magnitude of the experimental measurements. The BOS technique is shown to be an important tool for explosive research which can be simply incorporated into typical large-scale outdoor tests.

A new supersonic linear compressor cascade test section, designed to investigate supersonic aerodynamics of low-pressure compressor and fan blades of forthcoming aircraft engine architectures, is presented in this paper. This test section, installed in the short-duration VKI isentropic compression tube CT-2 facility, is composed of a 5-blade linear cascade and is designed to operate from subsonic up to supersonic (M1 = [0.5–1.4]) inflow conditions with independent Reynolds number variation up to engine-like conditions. A throttling mechanism allows the variation of the cascade pressure ratio for a fixed inlet Mach number, thereby aiming to cover a wide flow regime from unstarted to started “unique incidence” conditions. The first section of the paper outlines the design of the test section flow path in view of the challenges imposed for the assessment of the aerodynamic performance of supersonic compressor blade profiles at engine-like conditions in a linear cascade arrangement. The theoretical optical setup designed for the Background-Oriented Schlieren (BOS) technique, used for the visualization of the complex shock patterns, is also outlined. The second section of the paper provides a numerical validation of both the test section flow path design and the optical path of the BOS setup. Particular attention is given to the validation of the throttling mechanism that allows for various back pressures at fixed inlet conditions, while optimizing blade-to-blade periodicity. In the final section of the present paper, a selection of preliminary experimental results obtained at high inlet Mach number (M1 = 1.21) are briefly presented. The observed shortcomings of the current iteration of the test section are discussed and possible improvements are presented. Finally, the BOS technique proved to be an invaluable tool for the visualization of the complex shock patterns and for the qualification of the cascade inlet and outlet flow.

Methane presents a pervasive risk in most coal mines, its dilution being one of the primary objectives of the ventilation system. Although many different types of detectors and techniques exist for quantifying methane, little is known about the qualitative aspects of methane, such as visualising how this gas flows from coal. Background Oriented Schlieren (BOS) provides a potential method from qualitative analysis of methane emissions. BOS is an imaging technique designed to visualise normally invisible flows in transparent media. This paper is a preliminary investigation of the feasibility of using the BOS technique for imaging methane release from coal.

Background Oriented Schlieren (BOS) imaging of condensation from humid air on wettability-engineered surfaces

The flow field associated with supersonic free jets can be understood better by using visualization techniques like schlieren in the laboratory. Supersonic jets are used in the fields of aerospace, refrigeration, and precision manufacturing. The supersonic free jets are normally characterized by a compressible flow field with complex shock structures at off-design conditions. Supersonic free jets from nozzles with exotic shapes like Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle have three-dimensional flow fields that are difficult to completely understand using line-of-sight integrated techniques like schlieren. These nozzles present a challenge to the measurement of flow properties with novel flow diagnostic techniques. Density is one such flow property which is very informative yet difficult to obtain. The three-dimensional density fields can be obtained by the background-oriented schlieren (BOS) [1, 2]. Various nozzle configurations with non-axis symmetric profiles have been studied earlier [3] including the ESTS lobed nozzle [4, 5]. This is an attempt to capture the density flow field of an ESTS lobed nozzle free jet using BOS. In this study, a supersonic ESTS lobed nozzle with four lobes is operated to get a supersonic free jet. The density field of this jet is obtained at several azimuthal positions by turning the nozzle about its axis. The information from all the positions is used to reconstruct the three-dimensional flow field of the nozzle. From this density field, both the basic physics of the complex flow field can be understood as well as CFD modeling of the flow field can be validated.