Editing Particle image velocimetry (section)

{{Short description|Method to measure velocities in fluid}}
'''Particle image velocimetry''' ('''PIV''') is an [[optical]] method of [[flow visualization]] used in education<ref name="Httpwwwinteractiveflowscomdownloads">[http://www.interactiveflows.com/downloads/ Interactive Flow Studies – Downloads<!-- Bot generated title -->]</ref> and research.<ref name="Httpwwwlavisionde">[http://www.lavision.de/ LaVision – We count on photons<!-- Bot generated title -->]</ref><ref name="Httpwwwtsicomenindexaspx">{{cite web |url=http://www.tsi.com/en-1033/index.aspx |title=TSI Incorporated |accessdate=2008-12-16 |url-status=dead |archive-url=https://web.archive.org/web/20081218163521/http://www.tsi.com/en-1033/index.aspx |archive-date=2008-12-18 }}</ref><ref name="Httpwwwdantecdynamicscom">[http://www.dantecdynamics.com/ Dantec Dynamics – laser optical measurement systems sensors<!-- Bot generated title -->]</ref><ref>{{Cite web|title=Microvec Pte Ltd|url=https://piv.com.sg/|access-date=2021-03-18|language=en-US}}</ref><ref name="Httpila5150de">[http://www.ila5150.de/ ILA_5150 - Particle Image Velocimetry (PIV) Solutions]</ref> It is used to obtain instantaneous [[velocity]] measurements and related properties in [[fluid]]s. The fluid is [[seeding (fluid dynamics)|seeded]] with tracer [[Particle (ecology)|particle]]s which, for sufficiently small particles, are assumed to faithfully follow the flow [[Dynamics (mechanics)|dynamics]] (the degree to which the particles faithfully follow the flow is represented by the [[Stokes number]]). The fluid with entrained particles is illuminated so that particles are visible. The motion of the seeding particles is used to calculate speed and direction (the [[vector field|velocity field]]) of the flow being studied.

Other techniques used to measure flows are [[laser Doppler velocimetry]] and [[hot-wire anemometry]]. The main difference between PIV and those techniques is that PIV produces two-dimensional or even three-dimensional [[vector field]]s, while the other techniques measure the velocity at a point. During PIV, the particle [[concentration]] is such that it is possible to identify individual particles in an image, but not with certainty to track it between images. When the particle concentration is so low that it is possible to follow an individual particle it is called [[particle tracking velocimetry]], while [[laser speckle]] velocimetry is used for cases where the particle concentration is so high that it is difficult to observe individual particles in an image.

Typical PIV apparatus consists of a [[camera]] (normally a [[digital camera]] with a [[Charge-coupled device|charge-coupled device (CCD) chip]] in modern systems), a strobe or [[laser]] with an optical arrangement to limit the physical region illuminated (normally a [[cylindrical lens]] to convert a light beam to a line), a [[Electronics|synchronizer]] to act as an external trigger for control of the camera and laser, the seeding particles and the fluid under investigation. A [[fiber-optic cable]] or liquid light guide may connect the laser to the lens setup. PIV software is used to post-process the optical images.<ref>{{cite web|title=MatPIV|url=http://folk.uio.no/jks/matpiv/}}</ref><ref>{{cite web|title=OpenPIV|url=http://www.openpiv.net}}</ref>