Abstract
In this paper, the improved Background Oriented Schlieren technique called CBOS (Colored Background Oriented Schlieren) is described and used to reconstruct the density fields of three-dimensional flows. The Background Oriented Schlieren technique (BOS) allows the measurement of the light deflection caused by density gradients in a compressible flow. For this purpose the distortion of the image of a background pattern observed through the flow is used. In order to increase the performance of the conventional Background Oriented Schlieren technique, the monochromatic background is replaced by a colored dot pattern. The different colors are treated separately using suitable correlation algorithms. Therefore, the precision and the spatial resolution can be highly increased. Furthermore a special arrangement of the different colored dot patterns in the background allows astigmatism in the region with high density gradients to be overcome. For the first time an algebraic reconstruction technique (ART) is then used to reconstruct the density field of unsteady flows around a spike-tipped model from CBOS measurements. The obtained images reveal the interaction between the free-stream flow and the high-pressure region in front of the model, which leads to large-scale instabilities in the flow.
Highlights
For the investigation of compressible flows the determination of the density distribution is of prime importance
In order to increase the performance of the conventional Background Oriented Schlieren technique, the monochromatic background is replaced by a colored dot pattern
In order to measure the light deflection caused by density gradients in a compressible flow, the Background Oriented Schlieren technique (BOS) technique uses the distortion of a background image
Summary
For the investigation of compressible flows the determination of the density distribution is of prime importance For this purpose, the schlieren method, introduced by A. The schlieren technique transforms the phase variation of the light passing through a phase object into an intensity variation Other techniques such as the density speckle photography appeared in the seventies and allowed the direct measurement of the deflection of the light [2,3]. In order to measure the light deflection caused by density gradients in a compressible flow, the BOS technique uses the distortion of a background image. This paper describes how to improve the accuracy of the BOS technique and how to treat strong density gradients causing blurred images by using a background with colored dots and suitable correlation algorithms (CBOS technique). Supposing the flow to be axially symmetrical, an algebraic reconstruction technique (ART) is applied to reconstruct the density field around the spike
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