Background oriented schlieren setup

In the next stage of the project the tablet and webcam will be mounted on a motorized gantry that rotates around the tank to acquire BOS images at multiple projection angles. During acquisitions, the transducer was continuously pulsed at 1. BOS acquisitions used background images comprising alternating white and black lines with varying positions, thicknesses and orientation angles.

A 2D simulation was performed in MATLAB to validate the principles underlying BOS tomography, by implementing the forward model relating a spatially-varying index of refraction pattern to acquired BOS projection images, and a conjugate gradient reconstruction to invert that model. A parallel beam geometry was assumed. Next we will translate our reconstruction to the true fan beam geometry and construct a motorized gantry to enable projections at multiple angles.

Thick widths can leave large gaps in the pattern, and thin widths can wash it out arrows. Different parts of the field pattern are emphasized at different angles. Elements of the displacement equation are embodied in the flow graph.

National Center for Biotechnology Information , U. Journal List J Ther Ultrasound v. J Ther Ultrasound. Published online Jun Author information Article notes Copyright and License information Disclaimer.

Corresponding author. Publication of this supplement was funded by the Focused Ultrasound Foundation. Open in a separate window. Figure 1. Acquisitions During acquisitions, the transducer was continuously pulsed at 1. Second, motion blur should be minimized by reduced exposure times. Retroreflective materials provide the most efficient return of light from speckled backgrounds, when on-axis lighting is used, because retroreflection does not follow the inverse-square law of lighting suffered by diffuse background.

Heineck et al. A recent technical improvement for BOS applications is the increasing availability of CMOS sensors with the active pixel sensor APS technology in which, in addition to the photodiode, a readout amplifier is incorporated into each pixel. This, together with highly parallel readout electronics storage devices, allows for the recording and handling of up to a few thousand frames per second at acceptable noise levels.

Several of the BOS experiments presented in the literature describe the use of these cameras. In many cases, they allowed for good temporal resolution of the flow phenomena under investigation.

Rouser et al. Kessler et al. Kushner et al. Mizukaki investigated transonic vortex rings discharged from the open end of a shock tube at 10 kHz frame rate and , together with co-workers, the wave propagation of an explosion at kHz Mizukaki et al. The very high framing-rate capability of the BOS technique was demonstrated in when Yamamoto et al. The pressure field was obtained by solving the Poisson equation. During the past decades, several velocimetry techniques have been proposed which do not require tracer particles to be added to the flow.

One of them utilizes BOS. Jonassen et al. Due to the need for naturally occurring density gradients, the applicability of their method is limited to turbulent compressible or thermal flows. Other authors used their BOS time-series data similarly to measure velocities of the density gradient generated by vortices, concentration or temperature gradients or shock waves e.

However, the velocities measured represent the velocities of the structures, for example the vortex convection speed or the speed of the compression shock, but not necessarily the local fluid velocity. Density tagging velocimetry DTV has been proposed to overcome this problem Raffel et al. The DTV method is an optical technique for pointwise velocity measurements based on the detection and tracking of a local refractive index or density variation which is intentionally induced in the flow by two subsequently recorded BOS images.

The density tagging method used was based on pulsed laser-induced ionization. The same tagging method can also be used in order to determine the speed of the resulting shock wave. Assuming that the laser pulse is strong enough to ionize the fluid, but weak enough to create only a weak shock wave, the changes in pressure and density can be assumed to occur isentropically.

In order to get an improved analysis of color images, the CBOS processing takes the fact into account that digital cameras have sensors for the colors red, green and blue. According to Leopold and Leopold et al. Due to the decomposition into the three colors, eight elementary dot patterns can be extracted from the image Fig. Extraction of the eight elementary dot patterns from the colored background image Leopold The assessment of the image distortion can be achieved by treating each of the eight elementary patterns separately.

An average intensity value for each location is determined by the value obtained at the location itself and by the results from interpolations between the opposite neighboring values.

A standard deviation criterion, which the intensities used for the averaging must satisfy, can be used in order to increase the accuracy of the procedure. Various publications report the advantages of the method and the determination of density distributions of axisymmetric flows Leopold et al. One of the advantages of the CBOS technique is its ability to treat regions of high refractive index gradients with the associated blurring of the background pattern see, e.

Details of this procedure are described by Leopold et al. The use of natural formation backgrounds for detecting refractive index gradients is intuitive since many effects, such as heat haze or mirage, can be seen in nature even without any instrumentation. The first question that needs to be answered while planning to visualize those effects with electronic equipment for measurements is the selection of the best-suited background.

Different quality indicators were introduced by Kindler et al. The earlier indicators are based on the signal-to-noise ratio of the autocorrelation of a background image. Plotted versus interrogation window size, it allows identifying suitable backgrounds and gives an indication of the obtainable spatial resolution.

The second method introduced by Bauknecht et al. This quality indicator is independent of residual misalignments of the two images, is not negatively affected by the autocorrelation of image noise and allows for a more direct evaluation of the backgrounds in a realistic BOS setup than the previously suggested methods. An analysis of natural backgrounds Fig.

Figure 11 contains plots of the quality indicator over interrogation window size for the corresponding background patterns in Fig. The graph indicates that larger interrogation window sizes decrease the correlation noise, at the cost of measurement resolution and accuracy. Example cutouts of the natural backgrounds used for the current study with increasing structure sizes from left to right.

Contrast enhanced for clarity Bauknecht et al. Quality indicator defined by Bauknecht et al. Despite this, obvious trend images have to be evaluated at small interrogation window sizes e.

For these backgrounds and magnifications, small-scale scree Fig. Smaller structures like grass Fig. This quality indicator is especially suited for the background selection for large-scale tests outside the laboratory. In addition to the flight tests described by Bauknecht et al. The image subtraction method and the conventional BOS correlation method have been found to be suited for this task Hargather The reference-free BOS method Raffel et al.

These parts can serve as a reference for a second measurement image, in which the density structures are located in front of another part of the background, rather than requiring an image without a phase object as a reference. In the stereoscopic configuration, two cameras record images at the same time but from different angles see Fig. If properly set up, the density variation is in front of different parts of the common background. Correlating the two images, the density variation appears twice in the resulting displacement field, but in different positions and with a different sign.

As the background is viewed from different angles, it must be reasonably planar. Because of the simultaneous acquisition of both images, the background may change over time without influencing the measurement. In principle, two images of the same background are recorded at consecutive points in time from one location. The evaluated displacement field of these two pictures features the density variation at both points in time, but with a different sign.

For this setup, either a single camera with a short interframing time monoscopic configuration or two paraxially mounted cameras triggered with a short time delay paraxial configuration are required. In the paraxial setup, the cameras have to be mounted very close together and the distance to the background has to be large enough for the alignment error to be negligible. Reference-free BOS setup Raffel et al.

A drawback of this method is that the integration of the density field from the displacements is not or not easily possible, as the reference pictures are not entirely free of density variations.

The major advantage over the stereoscopic setup, however, is the straightforward implementation for out-of-the-laboratory experiments. Because both cameras must be close to each other, they can be mounted on the same tripod or can easily be operated airborne Kindler et al.

It is then possible to move both cameras simultaneously while keeping the object of interest within the field of view. Another alternative application of BOS is the observation of free surface flows. Moisy et al. The liquid was contained in a tank with a transparent bottom allowing for an optimization of the background pattern distance Z D.

Tokgoz et al. Its strength lies in the simplicity of a basic setup, and this allows for its application in experimental environments which are unsuited for other measurement techniques. This advantage has further been developed to a state where BOS can be used for a large number of investigations in various fields. Due to new camera designs and the rapid development in the area of digital imaging and image processing, higher signal-to-noise ratios and increased resolution counteract the main disadvantage of the BOS, which is the limited resolution inherent with the statistical displacement computation.

It can be seen in Fig. More and more frequently the pure qualitative flow visualization is replaced by a more sophisticated analysis of the images allowing for three-dimensional density determination. A clear recent trend for laboratory investigations is the recording with high-framing-rate cameras frequently used together with LED illumination and retroreflective material. Coupling BOS with other optical flow techniques such as PIV, LIF and thermography has also enjoyed success and serves fluid mechanical research and development of machinery.

Table 1 depicts the number of BOS applications that have been performed together with another optical flow metrology. Within the past 15 years, the background-oriented schlieren techniques have developed from a robust and simple flow visualization to an increasingly accurate method for the measurement of complete density fields at very high framing rates. The combination of its application with other sophisticated flow metrologies further increases the completeness of the information obtained from fluid flow experiments.

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In: Proceedings of the 15th international symposium on flow visualisation, Minsk. Download references. The author would like to acknowledge the contributions of A. Bauknecht, F. Leopold, K. Mulleners, K. Kindler, E. Schairer, J. Heineck, A. Gardner and R. Sujith in the form of results, descriptions and annotations to the article.

You can also search for this author in PubMed Google Scholar. Correspondence to Markus Raffel. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author s and the source are credited. Reprints and Permissions. Raffel, M.

Background-oriented schlieren BOS techniques. Exp Fluids 56, 60