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PIV
Particle Image Velocimetry (PIV) has become one of the most important flow-field measurement techniques in experimental fluid dynamics since the 1980s. Its core principle is to record the displacement of tracer particles in a flow field over a short time interval and calculate the fluid velocity distribution using image-processing algorithms. Unlike traditional single-point measurement methods such as hot-wire anemometry and laser Doppler velocimetry, PIV enables non-invasive acquisition of instantaneous velocity data across the entire flow field. This method preserves flow structure integrity and provides high spatial and temporal resolution.
Laser and Particle Imaging in PIV System
As a revolutionary tool in modern flow-field diagnostics, PIV has expanded from laboratory research to widespread engineering applications, becoming an indispensable instrument in fluid mechanics studies.
1. PIV System Components and Workflow
A PIV measurement system mainly consists of four modules: a light source, a PIV synchronization system, an image acquisition system, and an image analysis system. The light source delivers stable and uniform illumination to guarantee imaging quality and measurement accuracy. The image acquisition system includes lenses and high-speed cameras that capture images formed by scattered light from tracer particles. The image analysis system uses dedicated software for real-time image capture, storage, and subsequent data processing and analysis.
PIV System
The workflow of a PIV system covers four core steps: tracer particle seeding, flow-field illumination, image acquisition and image processing.
Tracer particle selection is critical. Ideal particles feature excellent flow-following performance and favorable light-scattering characteristics. Popular options include hollow glass microspheres, fluorescent microspheres and oil mist droplets, with typical diameters ranging from 1 to 100 μm. Their density shall match the working fluid closely to reduce particle slip velocity.
The flow-field illumination unit generally adopts CW or pulsed lasers. Via Powell lenses or cylindrical lenses, the laser beam is shaped into a thin light sheet for 2D-PIV or volumetric light for 3D-PIV to illuminate the target measuring region inside the flow field.
High-sensitivity, high-frame-rate scientific CCD and CMOS cameras complete image capture. A minimum of two exposures is normally needed to record particle positional variations.
2. Technical Types of PIV Systems
PIV technology has evolved from basic two-dimensional planar measurements to true three-dimensional volumetric measurements. Conventional two-dimensional PIV (2D‑PIV) only captures two velocity components (u, v) in the measurement plane.
Stereo PIV (SPIV) uses two cameras from oblique viewing angles to extract two or all three velocity components from a 3D velocity field (often called 2D‑3C or 3D‑3C measurements). Essentially, it reconstructs the out‑of‑plane velocity component based on the stereoscopic principle.
The more advanced tomographic PIV (Tomo‑PIV) employs multiple cameras (typically 4–6) combined with tomographic reconstruction algorithms to realize full three‑dimensional, three‑component (3D‑3C) velocity field measurements. This marks PIV’s entry into a new era of 3D flow‑field diagnostics.
Comparison of main PIV technologies and their characteristics
Technology Type | Measuring Dimensions | Advantages | Limitations | Typical Applications |
2D PIV | 2D-2C | Simple and reliable, low cost | Missing vertical component, planar constraint | Conventional Flow Field Diagnosis |
3D PIV | 2D-3C | Obtain the three velocity components | Still limited to planar measurements | Wingtip vortices, boundary layer |
Tomo-PIV | 3D-3C | True 3D measurement, high concentration | The system is complex and computationally intensive. | Complex three-dimensional flow field |
Time Analysis PIV | 2D/3D | High time resolution | The spatial resolution is low. | Turbulent fluctuations, unsteady flow |
Microscale PIV | 2D-2C/3C | Micrometer-level resolution | Speed range is limited. | Microfluidic system |
Wide-Field PIV | 2D-2C/3C | Meter-level field of view | The system is large-scale and costly. | Wind tunnel and water tunnel experiments |
3. Typical Applications of the PIV System
Leveraging its non-intrusive nature, full-field measurement capability, and high precision, PIV technology has permeated various fields of fluid mechanics research, providing essential experimental means for addressing complex flow problems from fundamental science to industrial application development, across macro to micro scales.
Aerodynamics Research: In aerodynamics, PIV has become a standard tool in wind tunnel testing, used for detailed measurement of surface flow fields and wake vortex structures around aircraft and vehicles. Unlike traditional intrusive methods like pressure probes that disturb the flow, PIV acquires full-field information without interference. In airfoil studies, PIV has successfully revealed the evolution of complex vortex structures during processes like boundary layer transition, flow separation, and dynamic stall, providing key data for improving aerodynamic design.
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Aeroelastic PIV Measurements in Fluid-Structure Interaction (FSI ) | PIV System for Wake Flow Measurement in Automotive Wind Tunnels |
Wind Power Generation: In the wind energy sector, PIV technology is widely used to study wind turbine wake characteristics. The wake interference downstream of large wind turbines can significantly reduce the overall efficiency of a wind farm. Using large-field-of-view PIV measurements, researchers have quantified wake recovery length and turbulent mixing properties under different atmospheric stability conditions, offering a scientific basis for optimizing wind turbine layout.
Ship Hydrodynamics: Research in ship hydrodynamics has long benefited from PIV applications. In towing tank experiments, PIV systems clearly reveal the boundary layer structure and wake field characteristics around a ship's hull, providing visual evidence for evaluating hull resistance performance. In a case study optimizing the bulbous bow of a container ship, tomographic PIV technology completely recorded the interference process between the bow wave and the hull wave at different speeds, guiding the design of an optimized hull form that reduced wave resistance by 12%. Studies on underwater vehicles utilize matched refractive index techniques to eliminate optical distortion at solid-liquid interfaces, successfully measuring spatial scales of boundary layer transition and turbulent burst events.
PIV Measurement of 3D Propeller Flow Field
Cardiovascular flow research is a classic application of PIV technology in the biomedical field. By constructing transparent blood vessel models and circulatory systems with matched refractive indices, researchers use PIV to quantify the size of vortices downstream of arterial stenoses and the distribution of shear stress. These parameters are highly correlated with the formation sites of atherosclerotic plaques. Time-resolved PIV further captures the transient changes in flow field characteristics throughout the cardiac cycle, providing crucial validation data for the design of artificial heart valves. Recent studies have combined PIV with Optical Coherence Tomography (OCT), enabling depth-resolved measurement of red blood cell velocity fields within real blood vessels, achieving an axial resolution of 10 μm.
PIV Study of Aortic Valve Hemodynamics Under Varied Cardiac Output Conditions
4. Typical Lasers for PIV Systems
CNI provides both CW and pulsed lasers for flow field illumination systems. The laser beam is shaped via a Powell lens to form a thin light sheet (for 2D-PIV) or a volumetric light of specific thickness (for 3D-PIV), uniformly illuminating the target area of the flow field under measurement.
For more detailed information about PIV systems and lasers, please visit www.cnilaser.com/ www.cnilaser.net.
Changchun New Industries Optoelectronics Technology Co., Ltd.
Address: No. 888 Jinhu Road, High-Tech Zone, Changchun, 130103, P.R.China
International Tel:+86-431-85603799
Email: sales@cnilaser.com
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