The standard approach in particle image velocimetry (PIV) data processing is to use fast Fourier transforms to obtain the cross correlation of two single-exposure subregions, where the location of the cross-correlation peak is representative of the most probable particle displacement across the subregion. This standard PIV processing technique is analogous to matched spatial filtering, a technique commonly used in optical correlators to perform the cross-correlation operation. Phase-only filtering is a well-known variation of matched spatial filtering which, when used to process PIV image data, yields correlation peaks that are narrower and up to an order of magnitude larger than those obtained using traditional PIV processing. Researchers at the NASA Glenn Research Center recently used this technique to improve PIV image processing.
PIV data from a chevron nozzle flow experiment where the stereo PIV system is configured for cross-stream measurements. The left and right camera views are overlapped to illustrate the flare light scattered off of the chevron tips.
In addition to possessing desirable correlation plane features, phase-only filters provide superior performance in the presence of direct-current (dc) noise (or flare light from surfaces) in the correlation subregion. When PIV image subregions contaminated with surface flare light or high background noise levels are processed using phase-only filters, the correlation peak pertaining only to the particle displacement is readily detected above any signal stemming from the dc objects. Many of the PIV data collected in the jet noise facilities at Glenn are contaminated with flare light from the model, see the photograph above, which leads to a loss of data in these regions (see the top plot). Phase-only filtering techniques developed at Glenn minimize the effect of the flare light and recover the complete velocity field (see the bottom plot).
Ensemble averaged, three-dimensional velocity field where only the out-of-plane, w, component of velocity is plotted here as a color contour plot. The model flare light contaminated images were used to compute the three-component velocity field, resulting in the spurious velocities at the bottom of the nozzle flow field.
Phase-only filtering processing of the same image data yielded the true flow field, and no evidence of the model flare light contamination can be observed in the final result.
Wernet, Mark P.: Symmetric Phase Only Filtering: A New Paradigm for DPIV Data Processing. Meas. Sci. Technol., vol. 16,no. 3, 2005, pp. 601-618.
Wernet, M.: Symmetric Phase Only Filtering for Improved DPIV Data Processing. AIAA-2006-0042, 2006.
Find out more about this research: http://www.grc.nasa.gov/WWW/OptInstr/planarvel.html
Glenn contact:
Dr. Mark P. Wernet, 216-433-3752, Mark.P.Wernet@nasa.gov
Author:
Dr. Mark P. Wernet
Headquarters program office:
Aeronautics Research Mission Directorate
Programs/projects:
Quiet Aircraft Technology, Constant Volume Combustion Cycle Engine
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