<<More

PIV

Particle Image Velocimetry (PIV)has evolved into one of the most crucial flow field measurement techniques in experimental fluid dynamics research since the 1980s. Its core principle involves recording the displacement of tracer particles within a flow field over a short time interval and calculating the fluid's velocity distribution using image processing algorithms. Unlike traditional single-point measurement techniques such as hot-wire anemometry or laser Doppler velocimetry, PIV enables non-invasive acquisition of instantaneous velocity information across the entire flow field. This approach preserves the integrity of the flow structures while offering high spatial and temporal resolution.

As a revolutionary tool in modern flow field diagnostics, PIV has expanded from laboratory research to broad engineering applications, becoming an indispensable asset in fluid mechanics studies.

1. PIV System Components and Workflow 

A PIV measurement system primarily comprises four components: light source, PIV synchronization system, image acquisition system, and image analysis system. The light source provides stable, uniform illumination to ensure imaging quality and measurement accuracy. The image acquisition system includes lenses and high-speed cameras responsible for capturing images formed by the scattered light from tracer particles. The image analysis system utilizes specialized software for real-time image acquisition, storage, and subsequent data processing and analysis.

The workflow of a PIV system involves four key stages: seeding tracer particles, flow field illumination, image acquisition, and image processing.

The selection of tracer particles is critical. Ideal particles should exhibit good flow-following capabilities and light-scattering properties. Commonly used materials include hollow glass microspheres, fluorescent microspheres, or oil fog droplets, with diameters typically ranging from 1 to 100um. Their density should be as close as possible to the fluid to minimize slip. 

The flow field illumination system often uses CW or pulsed lasers. The laser beam is shaped into a thin light sheet (for 2D-PIV) or a volume of light (for 3D-PIV) using Powell lenses or cylindrical lenses to illuminate the region of interest within the flow field.

Image acquisition is performed by high-sensitivity, high-frame-rate scientific CCD or CMOS cameras. At least two exposures are typically required to record the change in particle positions.

2. Technical Types of PIV Systems

PIV technology has advanced from initial two-dimensional planar measurements to genuine three-dimensional volumetric measurements. Conventional two-dimensional PIV (2D-PIV) can only obtain two velocity components (u, v) within the measurement plane.

Stereo PIV (SPIV), utilizing two cameras at oblique viewing angles, can extract either two or all three velocity components from a three-dimensional velocity field (often referred to as 2D-3C or 3D-3C measurements). Essentially, it reconstructs the out-of-plane component based on the stereoscopic principle.

More advanced Tomographic PIV (Tomo-PIV) employs multiple cameras (typically 4 to 6) combined with tomographic reconstruction algorithms to achieve full three-dimensional, three-component (3D-3C) velocity field measurement. This marks PIV's entry into a new era of three-dimensional flow field diagnostics.

Comparison table of main PIV technology types and their characteristics:

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.

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.

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.

The 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.