PLIF AND IMAGE CHEMILUMINESCENCE
4.6 First-Pass Data Reduction
Image and data reduction is performed by an image processing program written specifically for this purpose. This program, called PLIF_II, is written entirely in GNU C (gcc) and runs under Linux. The program handles both the Andor .sif files and the data files from the AFC- 100 directly. From these it produces its own native files (.spif) with all the related data internally linked.
The PLIF_II program provides a semi-interactive interface allowing the user to run individual commands to perform specific data manipulation or reduction operations. However, the most powerful use of the program is through its interpreted scripting language. This allows the crafting of complicated data reduction sequences that may take quite some time to execute. These scripts can then be started and left to run unattended.
The scripts that are constructed can vary quite a bit in structure depending on the goal of the given script file. Nonetheless, the core procedures in the process are typically the same. The following sections describe the functions of the most common of these procedures.
4.6.1 Data Merging
The data merging process combines the image files collected from the Andor ICCD camera and the data files generated by the OPO software (from the data collected by the AFC-100). An .sif image file and the corresponding .dat data file are loaded and then combined into a native format record called SPIF (Standard PLIF Image File). The resulting group is then written out to a file with the .spif extension. It is these files that are used during all of the following data processing.
4.6.2 Background Image Generation
A background image is created in order to correct the data images for errors associated with intrinsic camera offsets such as CCD dark charge and CCD amplifier offset error, as well as for stray (yet relatively constant) light sources in the field of view such as laser sheet scattering and radiosity effects.
The background image is generated by loading images from selected data files and averaging them together. The images used are those where the laser sheet is present, but the burner is not operating. Images can be selected from multiple source files. Typically 400 images are used
in the average. The background frame is then stored in a special background frame register for later application.
4.6.3 Image Phase Averaging
With the collected data and given tools, it is now possible to exploit the periodic nature of the acoustic forcing and the corresponding periodic response of the flame through the data reduction process. The first step in this process is assigning the number of phase bins that will be used. If phase targeting was used in the data collection process, then the number of bins selected should produce the same phase spacing as was used in targeting. For the current experimental work, phase targeting with 15º spacing was used. This corresponds to the selection of 24 phase bins.
Next, each image of the specified dataset is loaded and the acoustic data for that image is processed by FFT. The phase information from the FFT is used to locate the rising-edge zero crossing of the fundamental mode. This is labelled as the zero-degree point. Then the phase location of the laser shot is identified within the dataset. The difference of these locations is taken (modulo 360º) and the result is used to choose the appropriate phase bin. This procedure is shown schematically in Figure 4-5 for the case of eight phase bins. Prior to averaging the image into the prescribed bin, corrections (if needed) are applied for variations in total laser energy and for the
“intensity and species concentration to fluorescence” function. These are handled on a case-by- case basis based on the species being fluoresced, and are discussed in the following sections. As mentioned earlier, due to the relative (ratiometric) use of the collected measurements, no attempt to correct for collisional quenching is made.
Depending on the processing steps used, the background image generated above may be applied on an image-by-image basis during the phase averaging process. Otherwise, once all the
Figure 4-5: Phase-averaging procedure for PLIF or chemiluminescence images. The phase location at the time of the laser shot is determined and the image is averaged into the
appropriate phase bin.
images from dataset files have been processed and averaged into their appropriate phase bins, the background frame is subtracted from each bin.
In the experiments presented a total of 9600 data images are taken per experiment for phase averaging. This corresponds to 400 images averaged per phase bin.
4.6.4 Report Generation
After the base reduction of the data is completed a detailed report can be generated. The report contains results from the phase-averaging process including spectral data for both the acoustic forcing, as well as for the response as seen in the varying fluorescence. The region of the
Image at Phase φ p´
φ
φ φ = 0 Phase reference
taken at rising edge zero
Record PLIF image from incident laser shot
Store image in associated phase bin
Acoustic Pressure at Flame
Phase Bins
image field over which the fluorescence is integrated to generate this response scalar is called the interrogation window. This is a rectangular region and is defined by the operator.
From this data, the global combustion response and global forced Rayleigh index are both computed. These are presented in their “raw” format (for the planar image, as is) as well as properly spatially integrated (over radius) to give the true global results for the given axisymmetric case.
Two lists of time-versus-magnitude ordered pairs are also output in the report file corresponding to the acoustic and fluorescence waveforms. Finally, certain statistical data is also recorded in the report file for validation of the reduction process.
4.6.5 Image Data Output
Another built-in command provides the ability to output frame data to image files for manipulation or plotting outside of the PLIF_II software. Images stored in frame registers (or any of the phase bins) can be output as individual .gif (graphics interchange format) files or as text data files in (x,y,z) triplet format for plotting purposes. If the .gif file option is chosen, the user has the ability to define the color map used.
Additionally, the phase bins can be output as a group into an animated .gif file. This allows the visualization of the unsteady response seen in the fluorescence. Again, the color map used is selected by the user.