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Volumetric density measurement in buoyant plumes using Tomographic Background Oriented Schlieren (TBOS)

Javed Mohd, Debopam Das

Abstract

Buoyant plumes are encountered in both natural and artificial scenarios, ranging from volcanic ash clouds and wildfires to smoke from chimneys and industrial pollutant discharge to rivers and lakes. These plumes are driven by the buoyancy forces arising from the density differences between the plume and the ambient fluids. Measurements of three dimensional density field is limited in literature and not available for buoyant plumes. Such data can lead to better understanding of the pollutant dispersion as well as fundamental nature of concentration dispersion in three-dimensional and turbulent flow. We present the three-dimensional density field measurement in buoyant plumes, particularly for forced and lazy plumes, using an in-house developed experimental rig and associated processing software. The density field reconstruction has been obtained using the tomographic background-oriented Schlieren (TBOS) technique. The experimental rig consists of eight cameras, facing random dot backgrounds placed diametrically opposite, mounted circumferentially around the buoyant plume. For the dot displacement calculation, we use a cross-correlation method. A finite difference-based Poisson solver is employed to calculate projected integrated density. The reconstruction has been performed using the Simultaneous Algebraic Reconstruction Technique (SART). The three-dimensional density field data, thus obtained is validated using existing theoretical models in the literature. We demonstrate that the 3D density field can serve as valuable data for validating and improving theoretical models of plume dynamics e.g. for the lazy plumes the measured density field is effectively used to show the puffing phenomenon.

Volumetric density measurement in buoyant plumes using Tomographic Background Oriented Schlieren (TBOS)

Abstract

Buoyant plumes are encountered in both natural and artificial scenarios, ranging from volcanic ash clouds and wildfires to smoke from chimneys and industrial pollutant discharge to rivers and lakes. These plumes are driven by the buoyancy forces arising from the density differences between the plume and the ambient fluids. Measurements of three dimensional density field is limited in literature and not available for buoyant plumes. Such data can lead to better understanding of the pollutant dispersion as well as fundamental nature of concentration dispersion in three-dimensional and turbulent flow. We present the three-dimensional density field measurement in buoyant plumes, particularly for forced and lazy plumes, using an in-house developed experimental rig and associated processing software. The density field reconstruction has been obtained using the tomographic background-oriented Schlieren (TBOS) technique. The experimental rig consists of eight cameras, facing random dot backgrounds placed diametrically opposite, mounted circumferentially around the buoyant plume. For the dot displacement calculation, we use a cross-correlation method. A finite difference-based Poisson solver is employed to calculate projected integrated density. The reconstruction has been performed using the Simultaneous Algebraic Reconstruction Technique (SART). The three-dimensional density field data, thus obtained is validated using existing theoretical models in the literature. We demonstrate that the 3D density field can serve as valuable data for validating and improving theoretical models of plume dynamics e.g. for the lazy plumes the measured density field is effectively used to show the puffing phenomenon.

Paper Structure

This paper contains 28 sections, 27 equations, 19 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Buoyant plume generation and characterization
  4. Identification of puffing and non-puffing cases
  5. BOS camera arrangement
  6. Data Processing work-flow
  7. Ray deflection estimation using cross-correlation
  8. Poisson solver to obtain line integrated density field
  9. Sinogram Interpolation
  10. Tomographic Reconstruction using SART
  11. Results
  12. Validation using plume theory
  13. Transverse Density Profile Characterization: Top-Hat vs. Gaussian
  14. Comparison with Similarity Solution and Simulation Data from the Literature
  15. Visualization of the Puffing Plume from the Reconstructed 3D Density Field
  16. ...and 13 more sections

Figures (19)

  • Figure 1: Schematic of experimental setup for generating buoyant plumes. All dimensions are in mm.
  • Figure 2: Classifying three plume releases into laminar or puffing regimes using the non-dimensional parametric space of Re-Fr. Solid curves taken from Bharadwaj_Das_2017 are the neutral stability curves of the primary mode obtained from the global instability analysis and separate the laminar (stable) and puffing (unstable) regions.
  • Figure 3: 8-camera laboratory setup for Tomographic BOS, centred around the plume: (a) schematic and (b) actual experimental setup. The symbols 'cam' and 'bkg' denote the camera and background, respectively.
  • Figure 4: Data processing chain of the developed TBOS system
  • Figure 5: Projected density gradients from eight cameras at background plane (scale: 0.0773 mm/px).
  • ...and 14 more figures