Editing Particle image velocimetry (section)

==Pros and cons==

===Advantages===
The method is, to a large degree, nonintrusive. The added tracers (if they are properly chosen) generally cause negligible distortion of the fluid flow.<ref>
{{cite journal
 |last=Melling |first=A.
 |year=1997
 |title=Tracer particles and seeding for particle image velocimetry
 |journal=[[Measurement Science and Technology]]
 |volume=8 |issue=12 |pages=1406–1416
 |doi=10.1088/0957-0233/8/12/005
|bibcode = 1997MeScT...8.1406M |s2cid=250844330
 }}</ref>

Optical measurement avoids the need for [[Pitot tube]]s, hotwire [[anemometers]] or other intrusive [[Flow measurement]] probes. The method is capable of measuring an entire two-[[dimension]]al [[cross section (geometry)]] of the flow field simultaneously.

High speed [[data processing]] allows the generation of large numbers of image pairs which, on a [[personal computer]] may be analysed in [[Real-time computing|real time]] or at a later time, and a high quantity of near-continuous information may be gained.

Sub [[pixel]] displacement values allow a high degree of accuracy, since each vector is the statistical average for many particles within a particular tile. Displacement can typically be accurate down to 10% of one pixel on the image plane.

===Drawbacks===
In some cases the particles will, due to their higher density, not perfectly follow the motion of the fluid ([[gas]]/[[liquid]]). If experiments are done in water, for instance, it is easily possible to find very cheap particles (e.g. plastic powder with a diameter of ~60&nbsp;μm) with the same density as water. If the density still does not fit, the density of the fluid can be tuned by increasing/ decreasing its temperature. This leads to slight changes in the Reynolds number, so the fluid velocity or the size of the experimental object has to be changed to account for this.

Particle image velocimetry methods will in general not be able to measure components along the z-axis (towards to/away from the camera). These components might not only be missed, they might also introduce an interference in the data for the x/y-components caused by parallax. These problems do not exist in Stereoscopic PIV, which uses two cameras to measure all three velocity components.

Since the resulting velocity vectors are based on cross-correlating the intensity distributions over small areas of the flow, the resulting velocity field is a spatially averaged representation of the actual velocity field. This obviously has consequences for the accuracy of spatial derivatives of the velocity field, vorticity, and spatial [[correlation function]]s that are often derived from PIV velocity fields.

PIV systems used in research often use [[laser safety|class IV lasers]] and high-resolution, high-speed cameras, which bring cost and safety constraints.