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Particle image velocimetry (PIV) is an optical method used to measure velocities and related properties in fluids. In Physics, velocity is defined as the rate of change of Position. FLUID ( F ast L ight '''U'''ser '''I'''nterface D esigner is a graphical editor that is used to produce FLTK Source code The fluid is seeded with particles which, for the purposes of PIV, are generally assumed to faithfully follow the flow dynamics. In fluid dynamics seeding a flow or a seeded flow means to introduce Particulates or other foreign substances into an otherwise transparent In marine and freshwater Ecology, a particle is a small object In physics the term dynamics customarily refers to the time evolution of physical processes It is the motion of these seeding particles that is used to calculate velocity information.

Contents

History

For a recent overview of the history of PIV technique see: Adrian, R. J. , "Twenty years of particle image velocimetry". Experiments in Fluids 39:159–169 (2005).

Technique

Typical PIV apparatus consists of a camera (normally a digital camera in modern systems), a high power laser, for example a double-pulsed Nd-YAG laser or a copper vapour laser, an optical arrangement to convert the laser output light to a light sheet (normally using a cylindrical lens), and the fluid/gas under investigation. A camera is a device used to capture images either as still Photographs or as sequences of moving images ( Movies or Videos. Many compact digital still cameras can record Sound and moving Video as well as still Photograph. A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission. NdYAG ( neodymium-doped yttrium aluminium garnet; NdY3Al5O12) is a Crystal that is used as a lasing medium A cylindrical lens is a lens which focuses light which passes through onto a line instead of onto a point as a spherical lens would A fibre optic cable often connects the laser to the cylindrical lens setup. An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length The laser acts as a photographic flash for the digital camera, and the particles in the fluid scatter the light. A flash is a device used in Photography that produces an instantaneous flash of artificial Light It is this scattered light that is detected by the camera.

In order to measure the velocity at least two exposures are needed. In Photography, exposure is the total amount of Light allowed to fall on the photographic medium ( Photographic film or Image sensor) during the They can be recorded on one or several frames. A film frame, or just frame, is one of the many single photographic images in a motion picture.

The frames are split in a large number of interrogation areas, often called tiles. It is then possible to calculate a displacement vector for each tile with help of signal processing (auto-correlation/cross-correlation). In Physics, displacement is the vector that specifies the position of a point or a particle in reference to a previous position or to the origin of the chosen Signal processing is the analysis interpretation and manipulation of signals Signals of interest include sound, images, biological signals such as Autocorrelation is a mathematical tool for finding repeating patterns such as the presence of a periodic signal which has been buried under noise or identifying the Missing fundamental In Signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them This is converted to a velocity using the time between image exposures.

Timing electronics allows one to control the spacing between image exposures. The electronics also permits image pairs to be acquired at various times along the flow. These digital delay and pulse generators provide several outputs that can be delayed and referenced to each other.

Advantages

The method is to a large degree nonintrusive. The added tracers (if they are properly chosen[1]) generally cause negligible distortion of the fluid flow.

Optical measurement avoids the need for Pitot tubes, hotwires or other intrusive Flow measurement probes. A Pitot (ˈpiːtoʊ tube is a Pressure measurement instrument used to measure Fluid flow Velocity. Flow measurement is the quantification of bulk Fluid movement Additionally the method is capable of measuring an entire two-dimensional cross section (geometry) of the flow field simultaneously. In mathematics the dimension of a Space is roughly defined as the minimum number of Coordinates needed to specify every point within it In Geometry, a cross section is the intersection of a body in 2-dimensional space with a line or of a body in 3-dimensional space with a plane etc

High speed data processing allows the generation of large numbers of image pairs which, on a modern personal computer may be analysed in real time or at a later time. A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated Thus 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. In Digital imaging, a pixel ( pict ure el ement is the smallest piece of information in an image Displacement can typically be accurate down to 10% of one pixel on the image plane.

Drawbacks

The particles will, due to their higher density, not exactly follow the motion of the fluid (gas/liquid). This page is about the physical properties of gas as a state of matter Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of

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.

The size of the recordable flow field is limited by the size of the tracer particles. The scattered light from each particle should be in the region of 2 to 4 pixels across on the image. If too large an area is recorded, particle image size drops and peak locking will occur with loss of sub pixel resolution. The typical maximum size of the recordable plane is in the region of 10cm to 50cm square, depending on the technology and complexity of the analysis algorithms used.

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 functions that are often derived from PIV velocity fields. Correlation functions contain information about the distribution of points or events or things across some space/time

Research grade PIV systems include a Class IV laser and high resolution/speed digital camera that make the systems potentially unsafe and very expensive. So they are prohibitevely expensive (around $100K) and potentially not safe to use in teaching.

Improvements to basic PIV

Each of the above limitations have been addressed by specialist techniques. For example, a similar velocimetry method known as molecular tagging velocimetry, or MTV, uses molecule sized tags, which are often already a part of the flow. Molecular Tagging Velocimetry (MTV is a specific form of Velocimetry invented by Daniel G Small molecules being much closer to the size and density of a flow minimize the error of particles not following the flow. One example used in humid air flows uses a laser to dissociate the water (H2O) in the flow into H + OH. The hydroxyl (OH) molecule serves as the tag. This method is known as hydroxyl tagging velocimetry (HTV). Hydroxyl tagging velocimetry (HTV is a Velocimetry method used in humid air flows

Stereoscopic PIV utilises two cameras with separate viewing angles to extract the z-axis velocity component. Stereoscopy, stereoscopic imaging or 3-D (three-dimensional imaging is any technique capable of recording three-dimensional visual In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called Holographic PIV similarly extracts the third component. Holography (from the Greek, ὅλος - hólos whole + γραφή - grafē writing drawing is a technique that allows the However Both these techniques drastically increase cost and complexity of the system.

Recent research has outlined the possibility of treating the flow images as a continuous system of flow structures, instead of a system of quasi-random points. Randomness is a lack of order Purpose, cause, or predictability This allows the imaging of a limitless size of flow field, provided appropriate seeding is ensured.

The molecules used as tracers in MTV are subject to Brownian motion. This limits the method to ultra-high speed flows. A typical example of the successful application of MTV is the investigation the intake flow in engines. Here the spray is very dense and flow speeds are high enough to allow accurate results for MTV.

The effect of the spatial averaging can be reduced by the use of more complicated algorithms based on deforming the interrogation areas or based on Particle tracking velocimetry using PIV as an initial estimate for the position where individual particles are advected. Particle tracking velocimetry (PTV is one of Velocimetry methods i

Because of safety concerns and very high price the potential benefit of using the highly visual nature of PIV in teaching fluid mechanics has not been realized. But recently Interactive Flow Studies introduced the Educational ePIV system which is safe and affordable designed for teaching fluid mechanics.

Applications

PIV has been applied to a wide range of flow problems, varying from the flow over an aircraft wing in a wind tunnel to vortex formation in prosthetic heart valves. Though less common, PIV can also be used for quantifying the deformation and motion of solid materials and tissues that have embedded markers or are in some other way visually heterogeneous (e. g. using fluorescent speckles or particle grains).

Rudimentary PIV algorithms based on cross-correlation can be implemented in a matter of hours, while more sophisticated algorithms may require a significant investment of time. Several open source implementations are available including URAPIV [2] and mpiv [3] (a Matlab Toolbox), PyPIV [4] (an implementation in Python), JPIV [5] (a Java implementation), OSIV [6] and Gpiv [7] (both implementations in C).

Recently Interactive Flow Studies [8] introduced an Educational PIV (e-PIV) system that is used in teaching fluid mechanics to students studying fluid mechanics in design, engineering and science including physiology. The development of the system is funded by the National Science Foundation (NSF). The National Science Foundation (NSF is a United States Government agency that supports fundamental Research and Education in all the non-medical The advantage of this system is that it is affordable, safe, easy to use and versatile.

See also

References

  1. ^ Melling, 1997 [1]

External links

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