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

The practical aspects of an advanced schlieren technique, which has been pr esented by Meier (1999) and Richard et al (2000) and in a similar form by D alziel et al (2000), are described in this paper. The application of the te chnique is demonstrated by three experimental investigations on compressibl e vortices. These vortices playa 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 det ails of the vortex formation from the blade tips of two different helicopte rs in flight: a Eurocopter BK117 and a large US utility helicopter. In addi tion to this, simultaneous measurements of velocity and density fields were conducted in a transonic wind tunnel in order to characterize the structur e of compressible vortices. The background oriented schlieren technique has the potential of complement ing other optical techniques such as shadowgraphy or focusing schlieren met hods and yields additional quantitative information. Furthermore, in the ca se of helicopter aerodynamics, this technique allows the effect of Reynolds number on vortex development from blade tips to be studied in full-scale f light tests more easily than through the use of laser-based techniques.