The iLA5150 Particle Image Velocimetry (PIV) system currently operating at CRIACIV is a planar optical measurement setup designed to provide detailed, quantitative characterization of fluid flow velocity fields. The system is configured as a 2D–2C measurement technique, meaning that two components of the velocity vector are measured over a two-dimensional illuminated plane within the flow domain. A schematic representation of the measurement principle is shown in the figure below, which reproduces the arrangement typically adopted in the CRIACIV wind tunnel facility.

The working principle of PIV is based on seeding the flow with micron-sized tracer particles, selected to ensure negligible inertia and therefore to faithfully follow the motion of the fluid. The seeded flow is illuminated by a thin laser sheet generated by a dual-pulse Nd:YAG Quantel Evergreen laser with a pulse energy of 200 mJ. The laser sheet defines the measurement plane and allows the visualization of particle motion within a precisely controlled region of the flow.

Particle images are acquired by means of a high-resolution sCMOS camera (pco.edge 26) equipped with a Sigma 85 mm f/1.4 lens, which guarantees excellent optical sensitivity and spatial resolution. The system captures pairs of particle images separated by a known and accurately controlled time interval. The displacement of particle groups between successive frames is evaluated using cross-correlation algorithms implemented in the PIVview processing software. From these displacements, the instantaneous velocity field is reconstructed over the entire illuminated area, providing simultaneous measurements of the two in-plane velocity components.

One of the major advantages of the PIV technique is its non-intrusive nature. Unlike intrusive probes, PIV does not alter the flow field and enables the measurement of spatially resolved velocity distributions over extended regions. The technique allows the evaluation of velocity gradients, vorticity fields, turbulent statistics, coherent flow structures, and flow separation mechanisms, offering a comprehensive insight into complex aerodynamic phenomena.

At CRIACIV, the PIV system represents a fundamental experimental tool for the investigation of flow behaviour around idealized geometries, particularly within the framework of bluff-body aerodynamics. The analysis of flows past bluff bodies such as circular cylinders, square and rectangular prisms, and other sharp-edged or non-streamlined sections plays a key role in wind engineering, as these geometries are representative of many structural elements found in civil and industrial applications.

In these configurations, the flow is typically characterized by large-scale separation, shear-layer instabilities, vortex shedding, and complex wake dynamics. PIV measurements allow detailed visualization and quantitative characterization of these phenomena, enabling the identification of vortex formation regions, recirculation zones, and turbulence production mechanisms. The technique is particularly effective in investigating the interaction between separated shear layers and wake structures, as well as in analysing the spatial evolution of coherent vortical patterns associated with periodic vortex shedding.

The availability of planar velocity fields also allows direct evaluation of key aerodynamic quantities such as Reynolds stresses, turbulence intensity distributions, and energy transfer mechanisms within the flow. These measurements are essential for the validation of theoretical models and numerical simulations, including CFD approaches, as well as for the development of aerodynamic mitigation strategies aimed at reducing wind-induced loads and vibrations.

Furthermore, PIV investigations on ideal bluff bodies provide valuable reference datasets for understanding the aerodynamic behaviour of more complex structural geometries. The experimental results obtained from simplified configurations often constitute the basis for interpreting flow mechanisms governing wind loading on buildings, bridge decks, towers, and other civil engineering structures.

The flexibility of the CRIACIV PIV setup allows adaptation of the optical arrangement, measurement window size, and acquisition frequency to match the specific requirements of each experimental campaign. This versatility enables the investigation of a wide range of flow regimes, from low-speed laminar flows to highly turbulent boundary layers and separated flows typical of atmospheric wind engineering applications.

The images clearly demonstrate how Particle Image Velocimetry (PIV) enables precise visualization and quantification of wind velocity fields around urban green elements. This technique allows the observation of flow distribution, areas of acceleration and deceleration, and vortices generated by trees and surrounding structures. The results not only highlight the complexity of aerodynamic phenomena in real urban environments but also provide quantitative information crucial for assessing vegetation vulnerability and supporting the design of safe and resilient green spaces.

The images below illustrate the PIV tests conducted within the framework of the Italian ReLUIS research project. These tests allowed detailed mapping of the wind flow around the roof and walls of the test building model. Comparison with pressure measurements on the same model shows very good agreement, highlighting one of the main strengths of PIV: its ability to provide accurate, validated data on complex aerodynamic phenomena. This demonstrates how PIV can serve as a powerful tool for both research and practical applications in wind engineering.