Principle and applications of the background oriented schlieren (BOS) method

The practical aspects of an advanced schlieren technique, which has been presented by Meier (1999) and Richard et al (2000) and in a similar form by Dalziel et al (2000), are described in this paper. The application of the technique is demonstrated by three experimental investigations on vortices. These vortices play a major role in the blade vortex interaction (BVI) phenomenon, which is responsible for the typical impulsive noise of helicopters. Two experiments were performed in order to investigate the details of the vortex formation from the blade tips of two helicopters in flight: a Eurocopter BK117 and a large US utility helicopter. In addition to this, simultaneous measurements of velocity and density fields were conducted in a transonic wind tunnel in order to characterize the structure of compressible vortices. The background oriented schlieren technique has the potential of complementing other optical techniques such as shadowgraphy or focusing methods and yields additional quantitative information. Furthermore, in the case of helicopter aerodynamics, this technique allows the effect of Reynolds number on vortex development from blade tips to be in full-scale flight tests more easily than through the use of laser-based techniques.