Particle Image Velocimetry - PIV
The PIV method celebrated its 20th anniversary in 2004. The first articles were produced by Adrian and Pickering & Halliwell. The method has been generated from Laser Speckle Velocimetry, which is originally developed to investigate solid mechanics.
In the first applications the variation of illumination source was quite large from xenon flash lamps to continuous wave lasers, but eventually the when Nd:YAG lasers became available they started to dominate as the light source in PIV-experiments. Also the computational methods has been developed as the computational capacity has increased. First methods were based on particle tracking, but this led to low resolution results. Nowadays the calculations are based on statistical methods.
Particle Image Velocimetry (PIV) is an indirect way to measure flow fields in various applications. The method is based on imaging tracer particles that are placed into the flow stream. The particles are typically illuminated with a laser pulse and the scattered light is captured with a digital camera. (fig1 & 2)
Fig 1. A laser placed on a traversing system
Fig 2. The sheet optics transforms the laser beam into a light sheet
Fig 3. Light sheet in a stirred tank
In traditional PIV-system the laser is a double pulse Nd:YAG-laser, and the exact time of the taken images are known. The CCD-cameras that are designed for the PIV-system can take two images in a very short time period (with in few microseconds), therefore PIV-system can be applied also for gas flows and other relatively high speed flows. The recorded area with PIV can vary from approx. 5x5mm to square meters, depending on the used tracer particles and the power of the laser.
Fig 4. A typical image from PIV measurements
The vectors are calculated from the image by dividing the image into small interrogation areas and comparing the movement of the particles in each area. When a statistically matching pattern is found a vector is drawn at its place.
Fig 5 A vector field drawn from PIV-images
Possibilities of PIV
With the PIV system it is possible to measure flow fields in various applications. It is also possible to measure multiphase systems if the hold-up of the dispersed phase is low enough. PIV is most likely the best way to validate CFD-models, because the output is quite near the same. Also it is possible to analyze e.g. turbulence quantities from the PIV image.
Fig 6. An average vector field from 62 images
Fig 7. An instantaneous vector field
Fig 8. The vorticities from fig 7.
The PIV system in Chemical Engineering research group
Laser: DantecDynamics DualPower 250mJ-10Hz PIV-Laser (Nd:YAG)
Background illumination: A synchronized LED system and a continuous 2 kW HQI lamp for shadow imaging
Cameras: Flowsense 4M Mk2 (2pcs)
Optics: Several Nikon lenses
Software: Dantec Dynamics: Dynamic Studio 2.10
This PIV system has been designed for various multiphase applications and for investigating small scale phenomenon.