Three-Dimensional Density Measurements of a Heated Jet Using Laser-Speckle Tomographic Background-Oriented Schlieren

Amjad, Shoaib; Soria, Julio; Atkinson, Callum

doi:10.2139/ssrn.4216611

Cited by 2 publications

(6 citation statements)

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“…Thus, the total number of views is restricted by the physical space available, and limits the overall measurement accuracy. Our previous studies [10,11] deliver simulated TBOS measurements with up to 10% RMS bias error in synthetic turbulence; experimental time-averaged measurements in heated jets [17] show even closer agreement with thermocouple measurements. These use appropriate modifications to combat artefacts and 14-22 views; using any more views gives greatly diminishing returns.…”

Section: Implementing Tomographic Bossupporting

confidence: 56%

“…Other modifications can be used within the ART iterations to improve the rate of convergence, or overall accuracy. For the current work, based on favourable results from our previous studies [10,11,17], the following modifications will be used: a sharp cut-off mask which sets gradients outside the mask to zero (the mask is defined by scanning the displacement fields and finding where the displacements return to less than 0.1 pixels while adding a 10% margin); gradual unmasking [19] updates the reconstructed gradients up to a certain threshold t 0 in each ART iteration in order to target artefact formation (with a total of N iterations),…”

Section: Implementing Tomographic Bosmentioning

confidence: 99%

“…The gradients of the refractive index are found with 6th-order central differences. [29] upon which this study's optical setup is modelled.…”

Section: Synthetic Bos Experimentsmentioning

confidence: 99%

“…A virtual optical setup is created around the DNS heated jet for raytracing. The virtual optical setup for this study is modelled after a real experimental setup [28,29], shown in figure 4, which conforms to the compromise between increasing background displacements and reducing geometric blurring. In both real and virtual setups, there are 15 cameras placed in a full circle in a y−z plane circumferentially around the jet axis; each camera has a corresponding background pattern on the opposite side of the jet.…”

Section: Synthetic Bos Experimentsmentioning

confidence: 99%

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Assessment of three-dimensional density measurements from tomographic background-oriented schlieren (BOS)

Amjad

Karami

Soria

et al. 2020

Meas. Sci. Technol.

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Tomographic background-oriented schlieren (BOS) is used to measure three-dimensional instantaneous density fields in turbulent flows. Density gradients are related to the refractive index through the Gladstone-Dale relation. Based on path-integrated apparent displacements in a background pattern from path variations in light rays travelling through the flow, a tomographic reconstruction of the three-dimensional refractive index gradients can be obtained. A Poisson equation of the reconstructed gradients is solved to obtain the refractive index field. We examine four sources of measurement error: (i) defocus blurring; (ii) spatial averaging in the finite difference scheme for solving the Poisson equation; (iii) limited-view tomographic reconstruction; and (iv) displacement field noise. Synthetic BOS displacements are generated from the instantaneous density fields of a heated jet at a Reynolds number of 10 000 and jet exit centreline-to-ambient density ratio of 0.83 computed via DNS. Synthetic background displacements are produced by raytracing through the refractive index field obtained using the Gladstone-Dale relation. The virtual BOS setup consists of 15 cameras placed circumferentially around the jet axis, based on a proposed experimental setup. We show that defocus blurring has the greatest impact on the measurement accuracy, using the RMS and peak errors between the measured and true DNS fields as metrics; its impact is greatest in the transitional region and decreases steadily downstream. Sources (ii) and (iv) have marginal impact (for noise 15% of mean displacements). Three reconstruction methods are tested: (i) filtered back-projection (FBP); (ii) an algebraic reconstruction technique (ART); and (iii) sequential FBP-ART. ART and FBP-ART are similarly accurate, producing under half the errors of FBP, which suffers from significant reconstruction artefacts. The reconstruction error is modest compared to the impact of defocus blurring.

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Section: Implementing Tomographic Bossupporting

confidence: 56%

Section: Implementing Tomographic Bosmentioning

confidence: 99%

“…The gradients of the refractive index are found with 6th-order central differences. [29] upon which this study's optical setup is modelled.…”

Section: Synthetic Bos Experimentsmentioning

confidence: 99%

Section: Synthetic Bos Experimentsmentioning

confidence: 99%

See 2 more Smart Citations

Assessment of three-dimensional density measurements from tomographic background-oriented schlieren (BOS)

Amjad

Karami

Soria

et al. 2020

Meas. Sci. Technol.

View full text Add to dashboard Cite

show abstract

“…This information can in turn be used to determine other flow field quantities such as temperature and density. Tomographic BOS algorithms take several forms including: a two-step method [1][2][3][4][5][6][7][8][9][10], a two-step wavefront distortion method [11], a direct reconstruction method [12][13][14][15][16][17], and most recently, a unified method [18]. b = 1 n 0 ˆ∇n ds.…”

Section: Introductionmentioning

confidence: 99%

Tomographic background oriented schlieren using plenoptic cameras

Davis¹,

Clifford²,

Kelly³

et al. 2021

Meas. Sci. Technol.

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The development of a tomographic BOS implementation system utilizing up to four plenoptic cameras is presented. A systematic set of experiments was performed using a pair of solid dimethylpolysiloxan (PDMS) cylinders immersed in a nearly refractive index matched gylcerol/water solution to represent discrete ﬂow features with known sizes, shapes, separation distances, and orientation. A study was conducted to assess the inﬂuence of these features on the accuracy of 3D reconstructions of the refractive index ﬁeld. It was determined that the limited angular information collected by a single plenoptic camera is insuﬃcient for single-camera 3D reconstructions. In multi-camera conﬁgurations, the additional views collected by a plenoptic camera were shown to improve the overall reconstruction accuracy compared to an equivalent single view per camera reconstruction, potentially reducing the number of overall cameras needed to achieve a desired accuracy. For the imaging of two cylinders, three or more cameras are generally needed to avoid signiﬁcant ghosting artifacts in the reconstruction. Quantitative results are presented that show that: (1) two separate cylinders will be individually resolved as long as measurements from one camera are able to observe separation between the cylinders; (2) the error in the reconstructed 3D refractive index ﬁeld increases as the size of the feature decreases; and (3) the use of volumetric masking within the reconstruction algorithm is critical in order to improve the accuracy of the solution.

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Three-Dimensional Density Measurements of a Heated Jet Using Laser-Speckle Tomographic Background-Oriented Schlieren

Cited by 2 publications

References 0 publications

Assessment of three-dimensional density measurements from tomographic background-oriented schlieren (BOS)

Assessment of three-dimensional density measurements from tomographic background-oriented schlieren (BOS)

Tomographic background oriented schlieren using plenoptic cameras

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