A commonly used and very mature flow measurement method for a wide variety of flow phenomena in wind tunnel applications is the Particle Image Velocimetry (PIV). Applying such advanced non-intrusive flow visualization techniques to an in-flight environment is a challenging task which can provide invaluable data. The ability to capture complex flow structures under free atmospherical influences and in real flight conditions can potentially supply data for numerical technique validation and wind tunnel measurements. Integrating such a PIV system inside an aircraft will pose several problems which have to be resolved in order to receive the permission to flight and carry out the flight test itself as safe and efficient as possible. The main challenges of this particular flight test were the installation of the high energy laser system and the identification of natural aerosols / cloud droplets as seeding for PIV.
Within three flight tests conducted within the European project AIM and launched at the German airport Braunschweig-Wolfsburg, several flap settings, velocities and altitudes (i.e. cloud layers) were flown and PIV recordings have been acquired by means of the installed PIV system. The size of the observed flow field was approximately 68 x 90 mm² and located right behind the aircraft wing (aligned parallel to the fuselage of the aircraft). A highly disturbed flow field could be recorded which was mainly influenced by the propeller slipstream, the fuselage boundary layer and the flap downwash.
This feasibility study proved the applicability of this optical flow measurement tool for flight testing. In this regard, the main advantages of the PIV technique are its non-intrusive nature as well as the capability of providing information with high spatial resolution of a flow field. This ability ensures the affordable determination of valuable instantaneous velocity vector field data during flight tests, whereas on the other side external probes or intrusive sensors can only obtain discrete single point values of the desired parameters. Nevertheless, a number of significant challenges for the application of in-flight PIV to future flight tests still remain related to the aircraft operation and certification.