Particle Image Velocimetry is a state-of-the-art non-intrusive flow measurement technique. Small tracer particles advected by the flow are illuminated twice by very short pulses of a laser light sheet defining the measurement plane. Digital optical sensors capture the light scattered by the particles. Employing image analysis techniques instantaneous velocity vector fields are obtained in high spatial and temporal resolution.
The principle of the PIV measurement technique relies on the physical definition of velocity as a differential quotient. The trajectory of many tracer particles which follow the flow faithfully can be captured with a CCD- or CMOS-camera by illuminating a plane in the flow with two very short light pulses (few nanoseconds) within a time difference of a few microseconds. The two particle images captured at time t and t’ are stored on two frames of the CCD- or CMOS- sensor.
This allows to cross-correlate the two particle image distributions in small interrogation areas in order to determine their displacement on many positions of the observation field locally. Using the image magnification factor this method enables the measurement of typically more than 10,000 instantaneous velocity vectors for each double-image of the tracer particles inside the light sheet plane. Extending the PIV system to a stereoscopic camera set-up it is possible to determine all three components of the velocity vectors in the plane of the flow field instantaneously.
Nowadays PIV is used in microscale and in planes of more than 3 m² and in flows with velocities between a few mm/s and up to a km/sec. DLR has applied the PIV technique in industrial wind tunnels for e.g. high-lift applications, wake vortex studies, rotor/ propeller blades or in transonic flows on rolling delta wings.
The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transonic flows with shocks, rotors, test facilities with short run time, etc.) new experimental techniques, such as PIV are required which allow to capture the flow velocity of large flow fields instantaneously.
An important feature of PIV is that for the first time, a reliable basis of experimental flow field data is provided for direct comparison with numerical calculations and hence, for validation of computer codes. During the last years an increasing number of scientists have started to utilize the PIV technique to investigate the instantaneous structure of velocity fields in various areas of fluid mechanics.
See also: Tomographic Particle Image Velocimetry (Tomo-PIV)
Fields of Application