Kolja Kindler

Background-Oriented Schlieren Imaging for Full-Scale and In-Flight Testing

Background-oriented schlieren (BOS) methods suited for large-scale and in-flight testing are presented with special emphasis on the detection and tracing of blade tip vortices in situ. Retroreflective recording and photogrammetric epipolar analysis for the computation of the vortices’ spatial coordinates in the wind tunnel are described. Feasibility and fidelity of referencefree BOS in conjunction with natural formation backgrounds and related evaluation methods are discussed, additionally, illustrating their simplicity and robustness. Results of successful image acquisition from a chaser aircraft are presented allowing vortex wakes to be identified at a wide range of flight attitudes, including complex maneuvers.

Towards In-Flight Measurements of Helicopter Blade Tip Vortices

In the framework of the AIM project the near field of the blade tip vortex of a full-scale helicopter in simulated hover flight was investigated by combining three-component Particle Image Velocimetry and Background Oriented Schlieren measurements. The velocity field measurements in the range of wake ages of \({\uppsi _{\mathrm{{v}}}={1}^\circ }\) to \({30}^\circ \) in azimuth provided a reference for a quantitative analysis of the Schlieren results yielding vortex core density estimates. Ongoing vortex roll-up was observed at \({\uppsi _{\mathrm{{v}}}={1}^\circ }\) while considerable aperiodicity was persistent thereafter. The vortex parameters for \({\uppsi _{\mathrm{{v}}}>{1}^\circ }\) were consistent with the Scully vortex model. The particular challenges of full-scale, outdoor testing, especially the limited spatial resolution and aperiodicity effects, resulted in elevated measurement uncertainty as compared to sub-scale experiments.