Particle Image Velocimetry (PIV)
The Particle Image Velocimetry (PIV) is a commonly used and mature method to determine instantaneous flow velocity fields for a wide range of wind tunnel applications. The main principle of the PIV technique is based on imaging tracer particles added to the flow of interest which are illuminated by a pair of co-planar pulsed laser light sheets. These laser light sheets are usually orientated normal to the imaging axis of the camera (Figure 1). The camera system captures the scattered light of the particles. The two particle images at times t and t’ are recorded on two different frames. The velocity information is derived from the displacement of the tracer images, the time delay between the two laser pulses and the magnification of the imaging system. The evaluation of the pair of PIV images at times t and t’ is based on a cross-correlation algorithm within small interrogation windows.
Extending the PIV system to a stereoscopic camera set-up makes it possible to determine all three components of the velocity vectors in the plane of the flow field instantaneously. This stereoscopic camera setup consists of at least two cameras observing the same area of the light sheet under different viewing angles. Generally, the experimental setup of a PIV system can be divided into five sub systems: particles (i.e. seeding), illumination, acquisition system, evaluation and post processing). One of the main advantages of PIV is the determination of the spatial information of any arbitrary flow field. In addition, PIV can acquire data within small time scales. This ability enables data acquisition of complex and unsteady 3D flow fields e.g. propeller slipstream, flap downwash.
The present PIV systems enable the detection of flow structures between the microscopic dimensions and up to field of views of a size of several square meters. Hence, PIV has been successfully used for a wide range of wind tunnel applications such as turbulent boundary layer, high lift configurations, wake vortex detections as well as the capturing of rotor blades and propeller flows. At the moment, several scientific attempts on the field of aerodynamics and flow technologies are devoting themselves to the investigation of unsteady and detached flow phenomenon which require a high temporal and spatial resolutions (e.g. transonic flows and shock detection, flows around pitching airfoils). Furthermore, the fast development of PIV systems allows data recordings up to ~ 4 kHz. In addition, further enhancements of the PIV algorithm are improving the tomographic reconstruction of complex flow field structures. Within AIM the transition between PIV for wind tunnel applications and flight tests has been successfully carried out for the first time worldwide. The in-flight PIV setup as installed on a DLR Do228-101 is shown in Figure 2. It was used to capture the instantaneous flow field behind the wing in the vicinity to the fuselage. As seeding natural aerosols like clouds had been used. Figure 3 shows one sample vector field of the aircraft fuselage boundary layer calculated out of the PIV images recorded during the flight through a cloud.
The detection and identification of instantaneous velocity vector fields of the aircraft boundary layers, shocks and vortices under free flight conditions are of high importance to assess and increase the aircraft performance. Nevertheless, the first in-flight PIV measurement campaign showed that further improvement is required to increase the quality and flexibility of the PIV system. On the one hand, the precise identification of the tracer particles (e.g. shape, size, scattering behaviour, number of particles within the field of view) will provide crucial information about quality of the seeding and therefore the quality and accuracy of the measurement itself. On the other hand, the integration of a high power laser system into the airplane was one of the most time consuming steps within the certification process of the PIV setup. Hence, the implementation of new light sources, such as LEDs, and camera systems could increase the safety and flexibility of the PIV system.
Within AIM² a reliable in-flight PIV setup and the respective test procedures is developed. Also further research in the field of particle detection and optimisation during flight test is performed, to enhance the performance of seeding based flow measurements. In addition, recent developments on the field of PIV hardware for in-flight application such as safer illumination sources and new CCD cameras are applied to reduce operational restrictions and increase the user friendliness of PIV. With in-flight PIV a powerful tool will exist to measure complete flow fields instantaneously and in a non-intrusive way.