Direct comparison of 2D PIV and stereoscopic PIV measurements

A stereoscopic particle image velocimetry (PIV) measurement system based on the translation configuration was developed and applied to the flow behind a forward-swept axial-fan with five blades in a water tank. The 3D calibration procedure was employed to compensate the distortion and refraction of particle images. The perspective error caused by the out-of-plane motion was estimated by direct comparison of the 2D PIV and stereoscopic PIV (SPIV) results. The SPIV and 2D PIV comparison was carried out for the particle images captured simultaneously. The difference of mean velocity data measured by 2D PIV and SPIV techniques is nearly proportional to the mean out-of-plane velocity component. The difference appears to be relatively large in the region near the fan blade having higher out-of-plane motion. The perspective error estimated from the calibration data and mean out-of-plane velocity data agrees with the mean velocity difference between the 2D PIV and SPIV results. The turbulence intensity measured by the 2D PIV method is overestimated in comparison to that of the SPIV method due to the projected velocity fluctuations of the out-of-plane velocity component. The turbulent fluctuations caused by the out-of-plane velocity component were estimated by multiplication of the incident angle obtained in the calibration procedure and out-of-plane velocity fluctuations. The estimated perspective fluctuation of the out-of-plane velocity component was compared with the difference measured by the two methods. In SPIV measurements of turbulent statistics, therefore, the ratio of out-of-plane to in-plane RMS error determined from the system set-up should be considered in order to obtain reliable results.