Particle Image Velocimetry

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Ever wondered how engineers design aircraft, boats, submarines, rockets, jet engines, wind turbines or artificial hearts? These devices, though differing greatly in function, involve a fluid — air in aircraft, water in boats and blood in hearts. Thus, in order to properly design these devices, there is a need to analyze the motion of fluid itself – a field more formally called ‘fluid dynamics’.

Engineers cannot design a fluid system with just a pencil and paper as they are inherently complicated. Imagine a million birds flying in the air and you have to keep track of every single bird. There are more atoms in a glass of water than there are glasses of water in all the oceans of the world. There are so many particles under consideration that to keep track of every single one of them is impossible.

Particle image velocity (PIV) is a great aid in this regard. It is an experimental technique which yields the velocity field. The velocity field is a function of space and time which;  allows us to find the velocity at every location and at every time. A PIV measurement is like a snapshot of the flow, with the velocity measured at each pixel of the image. This knowledge of the velocity field is what engineers need to design all the above-mentioned devices (and countless more!).

PIV measurement of a bird’s flight
Source: Wikimedia

It is a laser-based photographic technique. Velocity is calculated using the measured displacements of the tracer particles injected into the fluid flow. Two laser pulses, sub-microseconds apart, are exposed to the flow, and the scattering of light by the tracer particles is recorded on the camera. After some post-processing on the two captured images, the velocity field is obtained. Recent advances in PIV technology have made it possible to obtain three-dimensional velocity profiles at the cross-section of flow by using two cameras.

Schematic of a PIV system
Source: Wikimedia

Nd: YAG laser (Nd: YAG Laser-Animation) is the most common source of illumination. The selection of tracer or seeding particles is very important. They must be able to follow fluid without disrupting the flow. Ideally, they should be of the same density as the fluid and should be very small in size. The refractive index of the seeding particles must be different from the fluid medium so that light reflects towards the camera. Common tracers used are silicon carbide particles, titanium dioxide particles, polystyrene particles, gas bubbles and sometimes atomized oil droplets.

As in any other field, fluid dynamics has its underlying fundamental equations, most notable of them being the Navier-Stokes Equations. Their solution gives the velocity field. But these equations are extremely difficult to solve for real-life flow configuration, the reason being the turbulence in fluids. People have spent whole lives trying to solve these equations. One of the seven-millennium problems is the Navier–Stokes existence and uniqueness problem. Unable to achieve the exact solution, we resort to numerical techniques. This is where the computer comes into play and the birth of CFD (computational fluid dynamics) takes place. Exponential growth in computational power has led to proportional developments in the field of fluid mechanics. But these algorithms can model the physical problem only to a certain degree, and further supercomputers required for these calculations are highly expensive and rare. Even supercomputers can’t handle all fluid flow problems. So, we need PIV to provide us with exact velocity field at a reasonable cost.

PIV provides the velocity field over the whole cross-section of the flow. So, it has an advantage over other experimental flow measurement techniques which measure velocity at a single location. PIV has led to great advances in research and industry. The accuracy, flexibility, and versatility of PIV systems make them extremely valuable tools in the study of supersonic flows, explosions, flame propagation, bubble growth and collapse, turbulence, and unsteady flow. However, the development of PIV systems for three-dimensional velocity measurement is still an active area of research. Furthermore, rapid advances in camera technologies will enable PIV to study the time evolution of a flow system. (a large number of flow snapshots microseconds apart, in sequence yield a video animation of the flow).

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