Ground Based Large Scale Vortex Investigations with BOS and PIV

Abstract

Understanding the nature of aircraft wake vortices is fundamental in aerospace science. The enhanced safety and efficiency given by better prediction and measurement of these complex vortex systems is significant. Within AIM several nonintrusive optical methods have been considered to visualise the wake vortex of an approaching aircraft. The main objective of the 5th workpackage of AIM was the application of the Particle Image Velocimetry (PIV) and the Background Oriented Schlieren (BOS) methods to the investigation of these vortex flows. The authors believe such measurements are unique and created a series of major challenges due to the large scale, outdoor experimental setups. The PIV method is a mature visualisation tool for small and medium scale wind tunnel applications. Capturing the dynamics of a full scale aircraft wake vortex outdoors causes complex problems. Standard PIV seeding systems are not designed to work under such severe conditions. The motions of the free atmosphere, the pressure field disturbances of the landing aircraft and the large field of view have a major impact on the seeding efficiency. On the other hand, restrictive laser safety requirements for the laser operation on an airport have also required definition by the German authorities. Due to these limitations the laser light has to propagate over a distance of about 385 m between the transmitting station inside a DLR building and the field of interest on the runway. Therefore, the laser and its light sheet stability under free atmospherical influences had been tested on a new laser testing facility of the DLR. When assessed, the BOS method was found to have fewer limitations than the PIV with a simple setup defined by one or two cameras and a suitable background pattern. Thus problems caused by sensitive seeding or high energy lasers can be avoided. The BOS measurement campaign has been carried out at a German airport. The setup was installed on a tower and close to the runway and the background defined by natural vegetation including grass, shrubs or woods. The following paper presents solutions for the PIV seeding problem and the outdoor laser operation. The measurement systems for the conducted tests for both the PIV and the BOS method will be outlined.