High lift flow structures: In-flight PIV feasibility study

Within the scope of the European project AIM several optical measurement techniques including Particle Image Velocimetry (PIV) are applied to flight tests. A large amount of challenges have to be accomplished including the large observation area, the time constraints given by the utilisation of the aircraft, interaction of the measurement system and the rapidly changing environmental parameters during the flight. Also considered are accessibility inside the aircraft as well as high safety precautions for the laser operation. Nevertheless, the PIV technique is still an attractive tool in modern aerodynamics to capture velocity flow fields within a short time scale enabling the detection of a wide range of spatial features in the flow. Therefore, PIV is able to provide a significant amount of experimental data to validate numerical flow simulations. For this reason PIV is the method of choice to capture the complex flow structures around selected planes adjacent to an aircrafts surface. However, integrating a PIV system in an aircraft will cause several problems. On the one hand, a number of certification procedures have to be pursued. Such laser systems are hazardous in their operation and high security standards have to be defined to tackle the severe risk of such a high energy light source. This results in operational restrictions during the flight tests. The arrangement of this experimental procedure has been specified and certified for a Dornier DO228 aircraft by the German Aerospace Center (DLR) together with Cranfield University. Other critical factors include feasible tracer particles (i.e. seeding) for successful measurements. The PIV method requires tracer particles to follow the flow without any lag to determine the velocity vectors indirectly with the help of the particle trajectories. For PIV systems used for wind tunnel measurement campaigns, specialised aerosol generators are installed to inject the seeding (e.g. oil droplets, glass microballoons) precisely into the test section. During a flight test an additional installation of some kind of seeding generator outside the aircraft and upstream the region of interest would not only disturb the flow but also be very difficult to attach to the aircraft structure. Therefore another approach has to be pursued. The best way to provide feasible seeding is by flying through natural aerosols – such as those in clouds