Determining the error levels in the calibration procedure when viewed through a transparent cylinder for engine flow diagnostics

Particle image velocimetry (PIV) has been widely used to investigate the flow fields in many areas. Images captured using PIV, however, are aberrated when viewed through a transparent cylinder such as in engines, and therefore, need to be compensated for distortions. The calibration procedure is an important step in flow diagnostics for the reconstruction of displacement vectors from image plane to physical space coordinates, also incorporating the distortion compensation. In engine flow diagnostics, the calibration procedure based on global pixel size, however, is commonly used; hence local pixel size variations are ignored, even with significant distortions. In the present work, an analysis is performed to quantify the error levels in the calibration procedure by acquiring the calibration images with and without the cylindrical liner at different measurement planes. Additionally, calibration is also performed utilizing the non-linear mapping functions to account for local pixel size variations, along with error determination. It is found that the error in the calibration procedure based on global pixel size is significant, hence highlights the importance of calibration based on mapping functions in engine flow diagnostics.