Background Oriented Stereoscopic Schlieren Applied to a Helicopter in Flight

Abstract

In this paper we present a novel measurement technique based on the refractive index variation in a compressible flow field. This concept is referred to as "Background Oriented Schlieren" (BOS) in the following. The differences between BOS and other optical techniques which it can best be compared with, will be discussed. Experimental studies have been carried out to investigate the blade tip vortices of a BK117 helicopter in hover flight. An extension will be presented which uses two cameras placed in stereoscopic arrangement. Both techniques allow very easily to visualize compressible structures like tip vortices and seem to well complete other optical techniques like shadowgraph or schlieren, and additionally they give some quantitative informations.