Method to measure velocities in fluid
Particle image velocimetry (PIV) is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles which, for sufficiently small particles, are assumed to faithfully follow the flow dynamics (the degree to which the particles faithfully follow the flow is represented by the Stokes number). The fluid with entrained particles is illuminated so that particles are visible. The motion of the seeding particles is used to calculate speed and direction (the velocity field) of the flow being studied.
Other techniques used to measure flows are laser Doppler velocimetry and hot-wire anemometry. The main difference between PIV and those techniques is that PIV produces two-dimensional or even three-dimensional vector fields, while the other techniques measure the velocity at a point. During PIV, the particle concentration is such that it is possible to identify individual particles in an image, but not with certainty to track it between images. When the particle concentration is so low that it is possible to follow an individual particle it is called particle tracking velocimetry, while laser speckle velocimetry is used for cases where the particle concentration is so high that it is difficult to observe individual particles in an image.
Typical PIV apparatus consists of a camera (normally a digital camera with a charge-coupled device (CCD) chip in modern systems), a strobe or laser with an optical arrangement to limit the physical region illuminated (normally a cylindrical lens to convert a light beam to a line), a synchronizer to act as an external trigger for control of the camera and laser, the seeding particles and the fluid under investigation. A fiber-optic cable or liquid light guide may connect the laser to the lens setup. PIV software is used to post-process the optical images.
History.
Particle image velocimetry (PIV) is a non-intrusive optical flow measurement technique used to study fluid flow patterns and velocities. PIV has found widespread applications in various fields of science and engineering, including aerodynamics, combustion, oceanography, and biofluids. The development of PIV can be traced back to the early 20th century when researchers started exploring different methods to visualize and measure fluid flow.
The early days of PIV can be credited to the pioneering work of Ludwig Prandtl, a German physicist and engineer, who is often regarded as the father of modern aerodynamics. In the 1920s, Prandtl and his colleagues used shadowgraph and schlieren techniques to visualize and measure flow patterns in wind tunnels. These methods relied on the refractive index differences between the fluid regions of interest and the surrounding medium to generate contrast in the images. However, these methods were limited to qualitative observations and did not provide quantitative velocity measurements.
The early PIV setups were relatively simple and used photographic film as the image recording medium. A laser was used to illuminate particles, such as oil droplets or smoke, added to the flow, and the resulting particle motion was captured on film. The films were then developed and analyzed to obtain flow velocity information. These early PIV systems had limited spatial resolution and were labor-intensive, but they provided valuable insights into fluid flow behavior.
The advent of lasers in the 1960s revolutionized the field of flow visualization and measurement. Lasers provided a coherent and monochromatic light source that could be easily focused and directed, making them ideal for optical flow diagnostics. In the late 1960s and early 1970s, researchers such as Arthur L. Lavoie, Hervé L. J. H. Scohier, and Adrian Fouriaux independently proposed the concept of particle image velocimetry (PIV). PIV was initially used for studying air flows and measuring wind velocities, but its applications soon extended to other areas of fluid dynamics.
In the 1980s, the development of charge-coupled devices (CCDs) and digital image processing techniques revolutionized PIV. CCD cameras replaced photographic film as the image recording medium, providing higher spatial resolution, faster data acquisition, and real-time processing capabilities. Digital image processing techniques allowed for accurate and automated analysis of the PIV images, greatly reducing the time and effort required for data analysis.
The advent of digital imaging and computer processing capabilities in the 1980s and 1990s revolutionized PIV, leading to the development of advanced PIV techniques, such as multi-frame PIV, stereo-PIV, and time-resolved PIV. These techniques allowed for higher accuracy, higher spatial and temporal resolution, and three-dimensional measurements, expanding the capabilities of PIV and enabling its application in more complex flow systems.
In the following decades, PIV continued to evolve and advance in several key areas. One significant advancement was the use of dual or multiple exposures in PIV, which allowed for the measurement of both instantaneous and time-averaged velocity fields. Dual-exposure PIV (often referred to as "stereo PIV" or "stereo-PIV") uses two cameras to capture two consecutive images with a known time delay, allowing for the measurement of three-component velocity vectors in a plane. This provided a more complete picture of the flow field and enabled the study of complex flows, such as turbulence and vortices.
In the 2000s and beyond, PIV continued to evolve with the development of high-power lasers, high-speed cameras, and advanced image analysis algorithms. These advancements have enabled PIV to be used in extreme conditions, such as high-speed flows, combustion systems, and microscale flows, opening up new frontiers for PIV research. PIV has also been integrated with other measurement techniques, such as temperature and concentration measurements, and has been used in emerging fields, such as microscale and nanoscale flows, granular flows, and additive manufacturing.
The advancement of PIV has been driven by the development of new laser sources, cameras, and image analysis techniques. Advances in laser technology have led to the use of high-power lasers, such as s and diode lasers, which provide increased illumination intensity and allow for measurements in more challenging environments, such as high-speed flows and combustion systems. High-speed cameras with improved sensitivity and frame rates have also been developed, enabling the capture of transient flow phenomena with high temporal resolution. Furthermore, advanced image analysis techniques, such as correlation-based algorithms, phase-based methods, and machine learning algorithms, have been developed to enhance the accuracy and efficiency of PIV measurements.
Another major advancement in PIV was the development of digital correlation algorithms for image analysis. These algorithms allowed for more accurate and efficient processing of PIV images, enabling higher spatial resolution and faster data acquisition rates. Various correlation algorithms, such as cross-correlation, Fourier-transform-based correlation, and adaptive correlation, were developed and widely used in PIV research.
PIV has also benefited from the development of computational fluid dynamics (CFD) simulations, which have become powerful tools for predicting and analyzing fluid flow behavior. PIV data can be used to validate and calibrate CFD simulations, and in turn, CFD simulations can provide insights into the interpretation and analysis of PIV data. The combination of experimental PIV measurements and numerical simulations has enabled researchers to gain a deeper understanding of fluid flow phenomena and has led to new discoveries and advancements in various scientific and engineering fields.
In addition to the technical advancements, PIV has also been integrated with other measurement techniques, such as temperature and concentration measurements, to provide more comprehensive and multi-parameter flow measurements. For example, combining PIV with thermographic phosphors or laser-induced fluorescence allows for simultaneous measurement of velocity and temperature or concentration fields, providing valuable data for studying heat transfer, mixing, and chemical reactions in fluid flows.
Applications.
The historical development of PIV has been driven by the need for accurate and non-intrusive flow measurements in various fields of science and engineering. The early years of PIV were marked by the development of basic PIV techniques, such as two-frame PIV, and the application of PIV in fundamental fluid dynamics research, primarily in academic settings. As PIV gained popularity, researchers started using it in more practical applications, such as aerodynamics, combustion, and oceanography.
As PIV continues to advance and evolve, it is expected to find further applications in a wide range of fields, from fundamental research in fluid dynamics to practical applications in engineering, environmental science, and medicine. The continued development of PIV techniques, including advancements in lasers, cameras, image analysis algorithms, and integration with other measurement techniques, will further enhance its capabilities and broaden its applications.
In aerodynamics, PIV has been used to study the flow over aircraft wings, rotor blades, and other aerodynamic surfaces, providing insights into the flow behavior and aerodynamic performance of these systems.
As PIV gained popularity, it found applications in a wide range of fields beyond aerodynamics, including combustion, oceanography, biofluids, and microscale flows. In combustion research, PIV has been used to study the details of combustion processes, such as flame propagation, ignition, and fuel spray dynamics, providing valuable insights into the complex interactions between fuel and air in combustion systems. In oceanography, PIV has been used to study the motion of water currents, waves, and turbulence, aiding in the understanding of ocean circulation patterns and coastal erosion. In biofluids research, PIV has been applied to study blood flow in arteries and veins, respiratory flow, and the motion of cilia and flagella in microorganisms, providing important information for understanding physiological processes and disease mechanisms.
PIV has also been used in new and emerging fields, such as microscale and nanoscale flows, granular flows, and multiphase flows. Micro-PIV and nano-PIV have been used to study flows in microchannels, nanopores, and biological systems at the microscale and nanoscale, providing insights into the unique behaviors of fluids at these length scales. PIV has been applied to study the motion of particles in granular flows, such as avalanches and landslides, and to investigate multiphase flows, such as bubbly flows and oil-water flows, which are important in environmental and industrial processes. In microscale flows, conventional measurement techniques are challenging to apply due to the small length scales involved. Micro-PIV has been used to study flows in microfluidic devices, such as lab-on-a-chip systems, and to investigate phenomena such as droplet formation, mixing, and cell motion, with applications in drug delivery, biomedical diagnostics, and microscale engineering.
PIV has also found applications in advanced manufacturing processes, such as additive manufacturing, where understanding and optimizing fluid flow behavior is critical for achieving high-quality and high-precision products. PIV has been used to study the flow dynamics of gases, liquids, and powders in additive manufacturing processes, providing insights into the process parameters that affect the quality and properties of the manufactured products.
PIV has also been used in environmental science to study the dispersion of pollutants in air and water, sediment transport in rivers and coastal areas, and the behavior of pollutants in natural and engineered systems. In energy research, PIV has been used to study the flow behavior in wind turbines, hydroelectric power plants, and combustion processes in engines and turbines, aiding in the development of more efficient and environmentally friendly energy systems.
Equipment and apparatus.
Seeding particles.
The seeding 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 response time of the particles to the motion of the fluid is reasonably short to accurately follow the flow, yet large enough to 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' drag and settling or rising effects. In a model where particles are modeled as