Particle Image Velocimetry
Particle Image Velocimetry (PIV) is a non-intrusive, optical flow measurement technique through which velocity vectors can be obtained simultaneously at many points by applying statistical tools to particle images of uniformally seeded flow. PIV Measurments are typically made of 2 or 3 components of velocity in planar slices of a flow field.
Similar to LDV, for PIV tracer particles are added to the flow. The field of view (typically a plane) is illuminated twice by means of a laser that has been spread into a sheet by optics. The light scattered by the particles in the field of interest are captured on two separate frames by a digital camera. Once captured, these images are subdivided into small subareas called interrogation regions. Assuming that all the particles in one interrogation region have moved homogeneously between the two illuminations, the local displacement of each interrogation region is then determined using cross-correlation techniques. Taking into account the known time delay between the images and the magnification at imaging, an instantaneous velocity field for the entire area of interest is obtained. Development of high speed digital cameras over the years have allowed for the acquistion of images at rates >500Hz, permitting the determination of time-resolved velocity fields in a flow. Without a high speed camera, it is only possible to gather average statistics for the flow of interest.
The LaVision PIV system used by the cavitation and bubbly flows research group comprises of three CMOS 1024 x 1024 resolution sensor, 17 x 17 micron pixels, 1000 frames/second maximum frame rate, 10-bits dynamics range, Photron FASTCAM 1024 PCI cameras; one CMOS 1024 x 1024 pixel resolution sensor, 17 x 17 micron pixels, 3000 frames/second at full resolution (250,000 frames/second at reduced resoltion) windowing capability, 10-bits dynamic range, Photron FASTCAM APX-RS camera with an assortment of fixed focal length (28mm, 50mm, 60mm, 105mm) NIKKOR lenses; Camera mounting assembly including Scheimpflug camera lens adapters; camera lens filters (narrow band pass 523 nm and high pass, cut-off at 540 nm for LIF); high speed camera controller, DaVis Software with 2D PIV, Stereo PIV, SizingMaster (bubble, droplet, and non-spherical determination), and High-Speed image capture software packages; a single head, 30mJ/pulse (@527 nm) at 1 kHz, double-pulse option, up to 10kHz repitition rate, Nd:YLF single cavity diode pumped solid state, high repition laser; and adjustable light sheet optics.
Top: Steps to PIV analysis of two images.
Bottom: wake behind a 'bullet-shaped' model and resulting vector field determined from analysis of PIV images