Dot tracking methodology for background-oriented schlieren (BOS)

Abstract

We propose a dot-tracking methodology for processing Background Oriented Schlieren (BOS) images. The method significantly improves the accuracy, precision and spatial resolution compared to conventional cross-correlation algorithms. Our methodology utilizes the prior information about the dot pattern such as the location, size and number of dots to provide near 100% yield even for high dot densities (20 dots/32x32 pix.) and is robust to image noise. We also propose an improvement to the displacement estimation step in the tracking process, especially for noisy images, using a "correlation correction", whereby we combine the spatial resolution benefit of the tracking method and the smoothing property of the correlation method to increase the dynamic range of the overall measurement process. We evaluate the performance of the method with synthetic BOS images of buoyancy driven turbulence rendered using ray tracing simulations, and experimental images of flow in the exit plane of a converging-diverging nozzle.

Flow over an espresso cup: inferring 3-D velocity and pressure fields from tomographic background oriented Schlieren via physics-informed neural networks

“…The distortion is then evaluated by using digital image correlation methods (Pan 2018) for the reference image (without density gradients) and the distorted images. Alternatively, optical flow algorithms (Atcheson, Heidrich & Ihrke 2009) and dot tracking methods (Rajendran, Bane & Vlachos 2019) can also be applied to reconstruct the displacement field characterizing the distortion. In addition to the refractive index and the density, temperature field or other quantities can be further inferred by analysing the BOS images (Tokgoz et al 2012).…”

Hybrid Adaptive Wavelet-Based Optical Flow Algorithm for Background Oriented Schlieren (BOS) Experiments

“…In practical application, it is necessary to use simulation data to verify the accuracy of the algorithm. To assess the accuracy of the proposed displacement estimation algorithms in the paper, synthetic BOS images are required [3], which are simulated based on the ray tracing method [20,21]. e layout of the BOS experimental setup modeled in the simulation experiment is shown in Figure 2, and the parameters describing the placement of the elements are summarized in Table 1.…”

“…BOS is an optical flow diagnostic technique used to measure the refractive gradients in a flow field by tracking the distortion of a target dot pattern. Due to the uneven refractive index distribution in the transparent flow field, the light deflects after passing through the flow field; therefore, an object viewed through the transparent flow field will appear distorted due to the refraction of light rays traversing the flow field [3]. BOS relies on this principle to measure the transparent flow field.…”

Wavelet-Based Optical Flow Analysis for Background-Oriented Schlieren Image Processing

“…One recent alternative to PIV-or DIC-like processing that can produce higher resolution BOS images is dot tracking, similar to methods used for particle tracking velocimetry (PTV) [13]. Dot tracking methods restrict users to backgrounds that are made up of discrete particle-like images, however, and they have been shown to be more sensitive to imaging noise than cross-correlation, although still more accurate at high levels of noise.…”