Editing Particle image velocimetry (section)

===Tomographic PIV===
Tomographic PIV is based on the illumination, recording, and reconstruction of tracer particles within a 3-D measurement volume. The technique uses several cameras to record simultaneous views of the illuminated volume, which is then reconstructed to yield a discretized 3-D intensity field. A pair of intensity fields are analyzed using 3-D cross-correlation algorithms to calculate the 3-D, 3-C velocity field within the volume. The technique was originally developed<ref name=scarano_2013>
{{cite journal
 |last=Scarano |first=F.
 |year=2013
 |title=Tomographic PIV: principles and practice
 |journal=[[Measurement Science and Technology]]
 |volume=24 |issue=1 |pages=012001 |doi=10.1088/0957-0233/24/1/012001
 |bibcode = 2013MeScT..24a2001S |s2cid=119509301
 }}</ref>
by Elsinga et al.<ref name=elsinga_2006>{{cite journal | last1 = Elsinga | first1 = G. E. | last2 = Scarano | first2 = F. | last3 = Wieneke | first3 = B. | last4 = van Oudheusden | first4 = B. W. | year = 2006 | title = Tomographic particle image velocimetry | journal = Experiments in Fluids | volume = 41 | issue = 6| pages = 933–947 | doi=10.1007/s00348-006-0212-z| bibcode = 2006ExFl...41..933E | s2cid = 53701882 }}</ref> in 2006.

The reconstruction procedure is a complex under-determined inverse problem.{{citation needed|date=September 2014}} The primary complication is that a single set of views can result from a large number of 3-D volumes. Procedures to properly determine the unique volume from a set of views are the foundation for the field of tomography. In most Tomo-PIV experiments, the multiplicative algebraic reconstruction technique (MART) is used. The advantage of this pixel-by-pixel reconstruction technique is that it avoids the need to identify individual particles.{{citation needed|date=September 2014}} Reconstructing the discretized 3-D intensity field is computationally intensive and, beyond MART, several developments have sought to significantly reduce this computational expense, for example the multiple line-of-sight simultaneous multiplicative algebraic reconstruction technique (MLOS-SMART)<ref name=atkinson_2009>
{{cite journal
 |last1=Atkinson |first1=C. |last2=Soria |first2=J. 
 |year=2009
 |title=An efficient simultaneous reconstruction technique for tomographic particle image velocimetry
 |journal=[[Experiments in Fluids]]
 |volume=47 |issue=4–5 |pages=553–568 |doi=10.1007/s00348-009-0728-0
 |bibcode = 2009ExFl...47..553A |s2cid=120737581 }}</ref>
which takes advantage of the sparsity of the 3-D intensity field to reduce memory storage and calculation requirements.

As a rule of thumb, at least four cameras are needed for acceptable reconstruction accuracy, and best results are obtained when the cameras are placed at approximately 30 degrees normal to the measurement volume.<ref name=elsinga_2006/> Many additional factors are necessary to consider for a successful experiment.{{citation needed|date=September 2014}}

Tomo-PIV has been applied to a broad range of flows. Examples include the structure of a turbulent boundary layer/shock wave interaction,<ref>{{cite journal | last1 = Humble | first1 = R. A. | last2 = Elsinga | first2 = G. E. | last3 = Scarano | first3 = F. | last4 = van Oudheusden | first4 = B. W. | year = 2009 | title = Three-dimensional instantaneous structure of a shock wave/turbulent boundary layer interaction | url = http://resolver.tudelft.nl/uuid:1ea2ab47-a595-46f9-a162-039c860512c9| journal = Journal of Fluid Mechanics | volume = 622 | pages = 33–62 | doi=10.1017/s0022112008005090| bibcode = 2009JFM...622...33H | s2cid = 52556611 }}</ref> the vorticity of a cylinder wake<ref>{{cite journal | last1 = Scarano | first1 = F. | last2 = Poelma | first2 = C. | year = 2009 | title = Three-dimensional vorticity patterns of cylinder wakes | journal = Experiments in Fluids | volume = 47 | issue = 1| pages = 69–83 | doi=10.1007/s00348-009-0629-2| bibcode = 2009ExFl...47...69S | doi-access = free }}</ref> or pitching airfoil,<ref>
{{cite journal
 |last1=Buchner |first1=A-J. |last2=Buchmann |first2=N. A. |last3=Kilany |first3=K. |last4=Atkinson |first4=C. |last5=Soria |first5=J.
 |year=2012
 |title=Stereoscopic and tomographic PIV of a pitching plate
 |journal=[[Experiments in Fluids]]
 |volume=52 |issue=2 |pages=299–314 |doi=10.1007/s00348-011-1218-8
 |bibcode = 2012ExFl...52..299B |s2cid=121719586 }}</ref>
rod-airfoil aeroacoustic experiments,<ref>D. Violato, P. Moore, and F. Scarano, "Lagrangian and Eulerian pressure field evaluation of rod-airfoil flow from time-resolved tomographic PIV," Experiments in Fluids, 2010</ref> and to measure small-scale, micro flows.<ref>{{cite journal | last1 = Kim | first1 = S. Große S | last2 = Elsinga | first2 = G.E. | last3 = Westerweel | first3 = J. | year = 2011 | title = Full 3D-3C velocity measurement inside a liquid immersion droplet | journal = Experiments in Fluids | volume = 51 | issue = 2| pages = 395–405 | doi=10.1007/s00348-011-1053-y| bibcode = 2011ExFl...51..395K | doi-access = free }}</ref>  More recently, Tomo-PIV has been used together with 3-D particle tracking velocimetry to understand predator-prey interactions,<ref>{{cite journal | last1 = Adhikari | first1 = D. | last2 = Longmire | first2 = E. | year = 2013| title = Infrared tomographic PIV and 3D motion tracking system applied to aquatic predator–prey interaction | journal = Measurement Science and Technology | volume = 24 | issue = 2| page = 024011 | doi = 10.1088/0957-0233/24/2/024011 | bibcode = 2013MeScT..24b4011A | s2cid = 122840639 }}</ref><ref>{{cite journal | last1 = Adhikari | first1 = D. | last2 = Gemmell | first2 = B. | last3 = Hallberg | first3 = M. | last4 = Longmire | first4 = E. |last5 = Buskey | first5 = E. | year = 2015 | title = Simultaneous measurement of 3D zooplankton trajectories and surrounding fluid velocity field in complex flows | journal = Journal of Experimental Biology | volume = 218 | issue = 22| pages = 3534–3540| doi = 10.1242/jeb.121707 | pmid = 26486364 | doi-access = free }}</ref> and portable version of Tomo-PIV has been used to study unique swimming organisms in Antarctica.<ref>{{cite journal | last1 = Adhikari | first1 = D. | last2 = Webster | first2 = D. | last3 = Yen | first3 = J. |year =2016 | title = Portable tomographic PIV measurements of swimming shelled Antarctic pteropods | journal = Experiments in Fluids | volume = 57 | issue = 12| page = 180| bibcode = 2016ExFl...57..180A | doi = 10.1007/s00348-016-2269-7 | s2cid = 125624301 }}</ref>