BOS-based three-dimensional reconstruction of rotor blade tip vortices

Abstract

A research topic of the Institute of Aerodynamics and Flow Technology in Göttingen is the investigation of blade tip vortices created by helicopter rotors. These vortices are investigated both on free-flying helicopters in different flight situations and on a rotor test stand. A measurement technique that is used to visualize these vortices is the density based background-oriented schlieren (BOS) method. The subject of this work is the three-dimensional vortex reconstruction based on stereoscopic BOS measurements. Therefore, a reconstruction scheme based on epipolar geometry was developed and applied to vortex filaments that were recorded in two different experiments. First, a model helicopter in free flight was investigated and it was possible to reconstruct significant parts of the vortex system up to a maximum vortex age Ψv above 600° during an unsteady takeoff maneuver in ground effect. The developed evaluation scheme was subsequently adapted and applied to the vortex system generated by a rotor test stand with cyclic pitch control. Vortex filaments of the rotor in cyclic pitch conditions were reconstructed and the vortex convection was investigated. The evaluation of a larger sample number of 200 measurement images in quasi-steady conditions showed a significant scatter in the vortex locations, which is in agreement with previous findings. The developed reconstruction scheme allows an automated extraction of vortex positions for a multitude of test conditions at the rotor test stand for future investigations.