An Improved Method for Estimating Turbulent Vortex Flow Properties from Stereoscopic DPIV Measurements

An improved method of analysis to correct for the effects of the inherent aperiodicity exhibited by rotor blade tip vortices was developed for flow measurements made using digital particle image velocimetry (DPIV). A correction technique for collocating the centers of the vortices based on helicity was shown to provide the most accurate and consistent results for all three flow components, and for both the mean and turbulent flow quantities. An analysis was performed to evaluate the minimum number of DPIV image pairs required to measure the vortex flow properties, which suggested at least 100 for the mean and about 800 for the turbulence. A statistical analysis performed using over 1,000 instantaneous velocity maps revealed a skewed distribution in the measurement data, suggesting the median over the mean as a better choice for estimating the core size of the vortices. The tip vortex was clearly non-axisymmetric, and showed a more elliptical shape at all wake ages. Furthermore, the measured turbulence characteristics were found to have an anisotropic distribution of eddy viscosity, suggesting that existing eddy viscosity models commonly used in CFD analyses are invalid for modeling vortex flows.