Visualization of vortical structures by density gradient detection

Abstract

Applications of an advanced schlieren technique, in the following referred to as Backround Oriented Schlieren (BOS) (Richard et al, 2000) or synthetic schlieren (Dalziel et al, 2000), are presented. Three experimental studies have been carried out in order to investigate compressible vortices. Beside other things, those vortices play a major role in the blade vortex interaction (BVI) phenomena, which leads to the typical impulsive noise at helicopters. Therefore, two experiments have been performed in order to investigate details of vortex formation from blade tips of two different helicopters in flight: an Eurocopter BK117 and a large US utility helicopter. Simultaneous measurements of the velocity and density fields have then been conducted to characterize the struture of compressible vortices. The background oriented technique appears to well complement other optical techniques like shadowgraphy or focussing schlieren methods and yield 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 studied in full-scale flight tests more easily than using laser-based techniques.