The present work reviews the recent developments in the domain of particle image velocimetry (PIV) with attention to its use for large-scale problems of interest for industrial aerodynamics. The article introduces the fundamental principles of flow seeding for large-scale experiments based on the helium-filled soap bubble (HFSB) technique. The measurement of flow tracers in a three-dimensional domain is presented, which is based on Lagrangian tracking methods. Furthermore, the concept of co-axial volumetric velocimetry is introduced and its combination with robotic arm manipulation is shown. The work presents two main examples of applications to aerodynamic problems: The first is the flow field around a propeller aircraft at wind tunnel free-stream velocity up to 50 m s-1, which to date is considered the boundary of large-scale PIV application with HFSB. The second example is the complex three-dimensional flow field around a cyclist at full scale. The work summarizes developments obtained in the last 3 years of research, demonstrating that the domain of PIV techniques is still undergoing vivid developments.

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Aerodynamics research and development often involves large-scale and turbulent flows; may this be a rotor flow, the wake of a car, or the flow passing a human athlete to name a few examples. Volumetric particle image velocimetry (PIV) is well suited for the study of such flows. The introduction of Helium Filled Soap Bubbles (HFSB) for wind tunnel measurements has pushed the achievable measurement domains using state-of-the-art tomographic PIV systems to the order of 50 liters. Despite the dramatic increase in the measurement volume size by the introduction of HFSB, the observation volumes in tomographic PIV are still relatively limited. Further, the system complexity is high, constraining optical access on complex geometries and prohibiting a rapid repositioning of the measurement domain.

To address the identified need of large-scale volumetric flow field measurements around complex geometries, a novel robotic PIV approach is presented. The proposed method relies on a coaxial volumetric velocimetry probe comprising four high speed cameras at low tomographic aperture, aligned with the illumination. The compact system greatly enhances optical access and it gives rise to a universal calibration, meaning that it can take full advantage of robotic manipulations. Therefore, large domains can rapidly be partitioned into a series of adjacent sub-volume measurements, controlling the velocimeter position by means of a robotic arm.

The feasibility of the proposed system is demonstrated in a study of the near flow field around a full-scale replica of a time-trialling cyclist at 14 m/s. 450 time-resolved volumetric PIV measurements are acquired to compound the time-averaged flow field on a 2 m³ domain. The analysis of the flow topology developing around the cyclist shows good agreement with available literature and it indicates the potential of the proposed robotic PIV approach in other engineering applications.