Particle Image Velocimetry

Traditionally, quantitative measurements of fluid velocity have been carried out using a pitot-static tube and hotwire anemometry. Both these techniques require insertion of a physical probe into the flow domain. This process is intrusive and can alter the flow field itself. In addition, measurements are averages over a small representative volume. The probe has to be physically displaced to various locations to scan the entire region of interest. The development of cost-effective lasers led to the development of Laser Doppler velocimeter (LDV) that uses a laser probe to enable non-intrusive velocity measurements. Velocity information by LDV however, is obtained point-wise similar to that of the pitot-static tube and the hotwire probe. Particle image velocimetry (PIV) is the state-of -the-art technique for velocity measurement in experimental fluid mechanics. Original contributions towards its development were made by Adrian (1991), Gharib (1991), Melling (1997), and Westerweel (1997). The most important advantage of PIV is that it is a non-intrusive technique and gives the spatial details of the flow field over a plane of interest. There is some flexibility in the choice of the measuring plane. The measurement process can be repeated in time to yield temporal evolution of the flow field. The ability to make global velocity measurements makes PIV a special tool in experimental fluid mechanics.With PIV, it is possible to acquire practically instantaneous velocity fields with high spatial resolution. The spatial resolution is limited by the thickness of the laser sheet and the choice of the interrogation spot during analysis. The latter is about 8 or 16 pixels. The smallest length scale that can be detected depends on the size of the pixel, and hence the spatial resolution of the camera. Depending on the camera speed, a time series of images can be recorded during experiments. The ensemble average of the instantaneous velocity vectors yields the time-averaged velocity field. This includes zones of reversed flow that cannot be dealt with by hotwire and pitot probes. Once the velocity field is obtained, other quantities such as vorticity, strain rates and momentum fluxes can be estimated. With developments in lasers, camera and high speed/low cost computers it is now possible to use PIV regularly for research and industrial applications.