Editing Particle image velocimetry (section)

==Equipment and apparatus==

===Seeding particles===
[[File:PIV through stagnation flame.jpg|thumb|Application of PIV in combustion]]
The [[Seeding (fluid dynamics)|seeding]] [[Particle (ecology)|particles]] are an inherently critical component of the PIV system.  Depending on the fluid under investigation, the particles must be able to match the fluid properties reasonably well.  Otherwise they will not follow the flow satisfactorily enough for the PIV analysis to be considered accurate.  Ideal particles will have the same density as the fluid system being used, and are spherical (these particles are called [[microspheres]]).  While the actual particle choice is dependent on the nature of the fluid, generally for macro PIV investigations they are [[glass]] beads, [[polystyrene]], [[polyethylene]], [[aluminum]] flakes or [[oil]] droplets (if the fluid under investigation is a [[gas]]).  Refractive index for the seeding particles should be different from the fluid which they are seeding, so that the laser sheet incident on the fluid flow will reflect off of the particles and be scattered towards the camera.

The particles are typically of a diameter in the order of 10 to 100 micrometers.  As for sizing, the particles should be small enough so that [[Latency (engineering)|response time]] of the particles to the motion of the fluid is reasonably short to accurately follow the flow, yet large enough to [[scattering|scatter]] a significant quantity of the incident laser light.  For some experiments involving combustion, seeding particle size may be smaller, in the order of 1 micrometer, to avoid the quenching effect that the inert particles may have on flames.  Due to the small size of the particles, the particles' motion is dominated by [[stokes' law|Stokes' drag]] and [[settling]] or rising effects.  In a model where particles are modeled as spherical ([[microspheres]]) at a very low [[Reynolds number]], the ability of the particles to follow the fluid's flow is inversely proportional to the difference in [[density]] between the particles and the fluid, and also inversely proportional to the square of their diameter.  The scattered light from the particles is dominated by [[Mie scattering]] and so is also proportional to the square of the particles' diameters.  Thus the particle size needs to be balanced to scatter enough light to accurately [[flow visualization|visualize]] all particles within the laser sheet plane, but small enough to accurately follow the flow.

The seeding mechanism needs to also be designed so as to seed the flow to a sufficient degree without overly disturbing the flow.

===Camera===
To perform PIV analysis on the flow, two [[Exposure (photography)|exposures]] of laser light are required upon the [[camera]] from the flow.  Originally, with the inability of cameras to capture multiple [[photograph|frames]] at high speeds, both exposures were captured on the same frame and this single frame was used to determine the flow.  A process called [[autocorrelation]] was used for this analysis.  However, as a result of autocorrelation the direction of the flow becomes unclear, as it is not clear which particle spots are from the first pulse and which are from the second pulse. Faster [[digital camera]]s using [[charge-coupled device|CCD]] or [[CMOS]] chips were developed since then that can capture two frames at high speed with a few hundred ns difference between the frames.  This has allowed each exposure to be isolated on its own frame for more accurate [[cross-correlation]] analysis.  The limitation of typical cameras is that this fast speed is limited to a pair of shots.  This is because each pair of shots must be transferred to the computer before another pair of shots can be taken.  Typical cameras can only take a pair of shots at a much slower speed.  High speed CCD or CMOS cameras are available but are much more expensive.

===Laser and optics===
For macro PIV setups, [[lasers]] are predominant due to their ability to produce high-power light beams with short pulse durations.  This yields short [[shutter speed|exposure times]] for each frame. [[Nd:YAG laser]]s, commonly used in PIV setups, emit primarily at 1064&nbsp;nm wavelength and its [[harmonics]] (532, 266, etc.)  For safety reasons, the laser emission is typically [[Band-pass filter|bandpass filtered]] to isolate the 532&nbsp;nm harmonics (this is green light, the only harmonic able to be seen by the naked eye).  A [[fiber-optic cable]] or liquid light guide might be used to direct the laser light to the experimental setup.

The optics consist of a [[spherical lens]] and [[cylindrical lens]] combination.  The cylindrical lens expands the laser into a plane while the spherical lens compresses the plane into a thin sheet.  This is critical as the PIV technique cannot generally measure motion normal to the laser sheet and so ideally this is eliminated by maintaining an entirely 2-dimensional laser sheet. The spherical lens cannot compress the laser sheet into an actual 2-dimensional plane.  The minimum thickness is on the order of the [[wavelength]] of the laser light and occurs at a finite distance from the optics setup (the focal point of the spherical lens).  This is the ideal location to place the analysis area of the experiment.

The correct lens for the camera should also be selected to properly focus on and visualize the particles within the investigation area.

===Synchronizer===
The synchronizer acts as an external trigger for both the camera(s) and the laser.  While analogue systems in the form of a [[Photoresistors|photosensor]], rotating [[aperture]] and a light source have been used in the past, most systems in use today are digital.  Controlled by a computer, the synchronizer can dictate the timing of each frame of the CCD camera's sequence in conjunction with the firing of the laser to within 1 ns precision.  Thus the time between each pulse of the laser and the placement of the laser shot in reference to the camera's timing can be accurately controlled.  Knowledge of this timing is critical as it is needed to determine the velocity of the fluid in the PIV analysis. Stand-alone electronic synchronizers, called [[digital delay generators]], offer variable resolution timing from as low as 250 ps to as high as several ms. With up to eight channels of synchronized timing, they offer the means to control several flash lamps and Q-switches as well as provide for multiple camera exposures.