Particle Image Velocimetry (PIV)
Particle image velocimetry (PIV) is a velocity measurement technique that offers a high-resolution instantaneous measurement capable of capturing coherent turbulent structures across a field of view smaller than more widely used velocity measurement technologies in atmospheric science and wind energy applications, such as LiDAR, Sodar, and Scanning Radar.
PIV is based on measuring the displacement of tracer particles entrained in a flow field between image pairs over a known change in time, thus providing the velocity of the flow field (Tropea and Yarin 2007; Adrian and Westerweel 2011). A thin laser sheet is normally used to illuminate the tracer particles with a pulsed laser while a camera resolves the tracer particles in subsequent image frames/pulses to capture image pairs (Figure 10). Powerful LEDs or spotlights can also be used to form the light sheet (Hong et al. 2014). PIV uses a cross-correlation analysis to find the average displacement between small subdomains, or interrogation regions, in the image pairs (Figure 11), while particle tracking velocimetry uses computational methods to track individual particles within the image pairs (Nemes et al. 2017). Velocity is calculated from the displacement vectors between image pairs using the image magnification, measurement plane position, and time delay between light pulses.
Planar PIV measures the velocity components normal to the light-sheet plane using a single camera, whereas stereoscopic PIV uses two cameras to measure three components of the flow field within the planar field of the laser sheet. Tomographic PIV uses multiple cameras to measure three velocity components within an illuminated volume (Adrian and Westerweel 2011).
The ability to use PIV for wind energy or atmospheric science applications is driven by the particulate seeding of the flow field. As scales increase, a trade-off exists between resolving the particles across multiple image pixels and adequately capturing enough light to image each particle (Tropea and Yarin 2007; Adrian and Westerweel 2011). High-resolution cameras help to adequately resolve the particles in the imaging plane, whereas shorter working distances (or stand-off distances), high-powered lasers, large-diameter particles, and intensified cameras amplify the amount of scattered light acquired (Herges et al. 2015).
Presently, the largest PIV scales have been achieved by seeding large-diameter tracer particles into the flow because the amount of scattered light scales by the square of the particle diameter and only linearly with laser power (Bosbach et al. 2009; Pol and Balakumar 2013; Scarano et al. 2015). Successful PIV measurements of the atmospheric boundary layer (Toloui et al. 2014) and utility-scale wind-turbine tip vortices (Hong et al. 2014) have been achieved at scales not previously possible using snowflakes as the tracer particles. These measurements have provided insight into the unsteady flow structures, but they are limited by the conditions on which data can be acquired and the fidelity at which snowflakes can adequately track the flow (Herges et al. 2015).
These limitations are why PIV has been more suitable for wind tunnel applications. Outdoor applications require the use of extremely large tracer particles that need to be safe for outdoor dispersion. It is difficult to seed an atmospheric inflow with changing wind direction, and camera working distances become challenging for wind turbine investigations (cameras mounted on aerial lifts or towers).
Wind tunnel PIV has a TRL ranging between 6 to 8 depending on the size of the field of view, whereas a field test PIV system has a TRL of 5-6. The key area of research to enable field test use of PIV is particle seeding generation.