Combination of Simultaneous Density and Velocity Measurements of Blade Tip Vortices under Cyclic Pitch Conditions

Abstract

Blade tip vortices are a dominant phenomenon in rotor wakes. The interaction of vortices with rotor blades can cause noise emissions, vibrations and even flow separation. Particle Image Velocimetry (PIV) has become a standard tool in vortex investigations. A disadvantage of this method is its dependence on the tracking accuracy of the particles used, which often leads to erroneous or missing data in the vortex core. An alternative approach is the Background-Oriented Schlieren (BOS) method. Unlike PIV, BOS acquires data related to the density gradient of a transparent medium. BOS has been widely used to visualize the vortex system of a rotor and quantitative results have been obtained with respect to vortex positions. In the present work, BOS and PIV are used to measure blade tip vortices simultaneously. Velocity data from PIV are linked to density-related displacements from BOS. Using data from a numerical simulation by Goerttler, the assumption of isothermal rather than isentropic flow in the vortex is made, resulting in excellent agreement between the two measurement techniques. The BOS data are further used to quantify the vortex sizes, positions, and dynamics.