Heterodyne Background-Oriented Schlieren for Measurement of Heat Release

This research is the result of Austrian-German joint research project, funded by the Austrian Science Fund FWF within grant FWF-I5392-N. This project is a Lead-Agency D-A-CH project in cooperation between Graz University of Technology, Austria and Technische Universität Dresden, Germany, named "FOUR-DIMENSIONAL MEASUREMENT OF THERMOACOUSTIC OSCILLATIONS".

The underlying hypothesis of this project claims that four-dimensional detection of local thermoacoustic oscillations, based on the combination of a (camera-based) laser interferometric vibrometer with multidirectional background-oriented schlieren method, density tagging velocimetry and deep neural networks as binding element, will reveal local and coupled information on combustion, acoustics and fluid dynamics. 

As of Nov 2025, this repository contains the Python routines and sample data from this project.

Data collected during the investigation on the capability of  HBOS to measure heat release.

1) Images acquired at different apertures and stored in the folder "Exposure.zip".

In the folder "Exposure.zip", there are three folders containing an example for acquisitions at an exposure time of 350 microseconds (the folders are named after the F-number used for the stored files). Inside this folder, two additional folders are present: one for the "Background" acquisition and one for the "Flame" acquisition. Inside each one of these two folders, 1000 .tiff images can be found. These are the images acquired and then elaborated with the Heterodyne Background Oriented Schlieren method. 

Exposure.zip contains folders:

2.4

4

5.6

Folders 2.4, 4 and 5.6 contain each:

350/Background/image.tiff   (1000 .tiff files)

350/Flame/image.tiff  (1000 .tiff files)

2) Spectra acquired during Laser Interferometric Vibrometry measurements are stored in the folder "LIV.7z".

In this folder, the different .txt files containing the raw data extracted with the Laser Interferometric Vibrometer at different radial positions are stored.

3) Python scripts provided are described here:

fft_from_txt.py is necessary to extract power spectra from .txt data provided by the vibrometer

Integral_Power_spectra.py is the code used to integrate the power spectral density over different frequency intervals and for all the radial positions scanned with the Laser Vibrometer

intergal_Power_spectra_Abel.py is a code used to approximate the Power Spectral Density integration with splines and then compute the corresponding Abel inversion to retrieve a 2D slice. 

HBOS_algorithm.py is a ready-to-run example which performs the HBOS modulation field extraction for both the Flame and the Background case. 

Additional material is available in the Reference section.