Robotic Volumetric Particle Tracking Velocimetry by Coaxial Imaging and Illumination

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. Aerospace Engineering | Aerodynamics and Wind Energ

The effect of hand posture on swimming efficiency

Abstract: Our quest is for the thumb and finger positions that maximize drag in front crawl swimming and thus maximize propulsion efficiency. We focus on drag in a stationary flow. Swimming is in water, but using Reynolds similarity the drag experiments are done in a wind tunnel. We measure the forces on real-life models of a forearm with hands, flexing the thumb and fingers in various positions. We study the influence on drag of cupping the hand and flexing the thumb. We find that cupping the hand is detrimental for drag. Swimming is most efficient with a flat hand. Flexing the thumb has a small effect on the drag, such that the drag is largest for the opened (abducted) thumb. Flow structures around the hand are visualized using robotic volumetric particle image velocimetry. From the time-averaged velocity fields we reconstruct the pressure distribution on the hand. These pressures are compared to the result of a direct measurement. The reached accuracy of ≈ 10% does not yet suffice to reproduce the small drag differences between the hand postures. Graphical Abstract: [Figure not available: see fulltext.].AerodynamicsFluid Mechanic