A new type of tracer is making its entry in the scenario of wind-tunnel measurements: helium-filled soap bubbles (HFSB). The present work discusses the main fluid-dynamic and optical properties of HFSB to evaluate their use for quantitative measurements in aerodynamic experiments. In the past three decades, particle image velocimetry (PIV) has become a standard measuring technique in experimental fluid mechanics. Advances in both hardware components and software analysis have allowed achieving many milestones in flow diagnostics, mainly time-resolved and instantaneous volumetric measurements. In particular, the extension to the third dimension in space, i.e. tomographic PIV and 3D particle tracking velocimetry (PTV), has been used to provide quantitative visualizations of the coherent structures occurring in various turbulent flows and have provided insight in the spatial organization of the turbulent motions at different scales. The extension of the aforementioned techniques towards industrial practice in wind tunnel testing requires the development of a more efficient approach in terms of scaling and versatility. The present dissertation tackles the upscaling of PIV experiments towards industrial wind tunnels with the use of HFSB as tracing particles. The reasons and motivations behind this choice are addressed in the first chapter and followed by a description of the state-of-the-art of PIV. The second chapter aims at familiarising the reader with the working principles of PIV, which will be later recalled when presenting the advances towards large-scale experiments. Information on the mechanical behaviour of tracer particles and on the underlying physics are discussed in the third chapter, where also the case of HFSB is examined for use during quantitative measurements in the low-speed flow regime. The problem of seeding in wind tunnels is discussed in chapter 4, where a system for the injection of HFSB in a large wind tunnel is presented. Here, the relationship between HFSB production rate and the resulting spatial concentration and dynamic spatial range (DSR) are discussed. Specific experiments that examine the tracing fidelity of sub-millimetre HFSB tracers are presented in chapter 5. The behaviour of HFSB is compared to micro-size droplets, yielding a characteristic response time in the range of 10 μs. The latter milestone opens up to the applicability of HFSB tracers for quantitative velocimetry in wind tunnel flows. In chapter 6, a specific case of interest is presented whereby HFSB tracers are used to measure the flow velocity within steady vortices such as those released at the tip of wings. A dedicated experiment shows that the neutrally or slightly buoyant HFSB return a rather homogeneous spatial concentration within the core of vortices, solving the long-standing issue encountered for small heavy tracers, such as fog droplets, that are systematically ejected from highly vortical regions. An analysis of the light scattering by HFSB was conducted with theoretical and experimental approaches, as described in chapter 7. The light intensity scattered by the HFSB is characterised by two source points: the glare points. The overall scattered light appears to be 104-105 times more intense with respect to the oil-based micro-size droplets. This information is used to retrieve the maximum size of the measurement volume for a given light source. Chapter 8 closes this dissertation presenting a survey of all the experiments that have been conducted during this PhD research. The scale of experiments varies from the more academic case of a circular cylinder up to the one of a ship model installed in one of the large industrial wind tunnels operated at the German-Dutch Wind Tunnels laboratories (DNW), going through the visualization and quantification of large structures in the rotor region of a vertical axis wind turbine (VAWT).@en

Velocity measurements within the core of high-swirl vortices are often hampered by heavier-than-air particle tracers being centrifuged outside the vortex core region. The use of neutrally buoyant and lighter-than-air tracers is investigated to aim at homogeneous tracers concentration in air flow experiments dealing with high-swirl vortices using particle image velocimetry. Helium-filled soap bubbles (HFSB) of sub-millimeter diameter are employed as flow tracers. Their density is controlled varying the relative amount of helium and soap solution composing the bubbles. The dynamics of HFSB and micro-size droplets is modeled within a Lamb–Oseen vortex to retrieve the order of magnitude of the tracers slip velocity. A positive radial drift for heavier-than-air tracers leads to an empty vortex core. In contrast, the concentration at the vortex axis is expected to increase for lighter than air tracers. Experiments are conducted on a sharp-edged slender delta wing at 20° incidence. At chosen chord-based Reynolds numbers of 2 × 105 and 6 × 105, a stable laminar vortex is formed above the delta wing. Laser sheet visualization is used to inspect the spatial concentration of tracers. A comparison is made between micron-sized fog droplets and HFSB tracers in the nearly neutrally buoyant condition. Stereo-PIV measurements with fog droplets return a systematically underestimated axial velocity distribution within the vortex core due to drop-out of image cross-correlation signal. The nearly neutrally buoyant HFSB tracers appear to maintain a homogeneous spatial concentration and yield cross-correlation signal up to the vortex axis. The resulting velocity measurements are in good agreement with literature data.@en

The present work follows a previous study on the aerodynamic characterization of helium-filled soap bubbles (HFSBs) for large-scale PIV measurements. HFSBs were found to yield, on average, a time response of about 10s. However, the response of each individual tracer remained to be ascertained, which is the topic of the present study. The velocity of the bubbles in the stagnation region ahead of a circular cylinder is evaluated by the PTV technique. The results are compared with micro-size fog droplets taken as reference. The tracking error of individual trajectories is assessed by statistical analysis of the relative slip between the bubble and the airflow. The instantaneous particle relaxation time is retrieved from the ratio between slip velocity and local acceleration. Additional information on the bubble instantaneous properties is taken by inferring the diameter from the distance between the glare points. The results are discussed and related to the differences observed in the bubbling and jetting regimes for bubble production. Finally, the HFSBs relative density to the air is estimated using a modified Stokes drag law.@en

A new system for large-scale tomographic particle image velocimetry in low-speed wind tunnels is presented. The system relies upon the use of sub-millimetre helium-filled soap bubbles as flow tracers, which scatter light with intensity several orders of magnitude higher than micron-sized droplets. With respect to a single bubble generator, the system increases the rate of bubbles emission by means of transient accumulation and rapid release. The governing parameters of the system are identified and discussed, namely the bubbles production rate, the accumulation and release times, the size of the bubble injector and its location with respect to the wind tunnel contraction. The relations between the above parameters, the resulting spatial concentration of tracers and measurement of dynamic spatial range are obtained and discussed. Large-scale experiments are carried out in a large low-speed wind tunnel with 2.85 × 2.85 m2 test section, where a vertical axis wind turbine of 1 m diameter is operated. Time-resolved tomographic PIV measurements are taken over a measurement volume of 40 × 20 × 15 cm3, allowing the quantitative analysis of the tip-vortex structure and dynamical evolution.@en

The instantaneous volumetric pressure in the near-wake of a truncated cylinder is measured by use of tomographic particle tracking velocimetry (PTV) using helium-filled soap bubbles (HFSB) as tracers. The measurement volume is several orders of magnitude larger than that reported in tomographic experiments dealing with pressure from particle image velocimetry (PIV). The near-wake of a truncated cylinder installed on a flat plate (ReD = 3.5 × 104) features both wall-bounded turbulence and large-scale unsteady flow separation. The instantaneous pressure is calculated from the time-resolved 3D velocity distribution by invoking the momentum equation. The experiments are conducted simultaneously with surface pressure measurements intended for validation of the technique. The study shows that time-averaged pressure and root-mean-squared pressure fluctuations can be accurately measured both in the fluid domain and at the solid surface by large-scale tomographic PTV with HFSB as tracers, with significant reduction in manufacturing complexity for the wind-tunnel model and circumventing the need to install pressure taps or transducers. The measurement over a large volume eases the extension toward the free-stream regime, providing a reliable boundary condition for the solution of the Poisson equation for pressure. The work demonstrates, in the case of the flow past a truncated cylinder, the use of HFSB tracer particles for pressure measurement in air flows in a measurement volume that is two orders of magnitude larger than that of conventional tomographic PIV.@en

Aerodynamics plays a crucial role in cycling as in many other speed sports. Aerodynamic investigation is required primarily for two reasons: 1. Optimization of the athletes' posture and equipment geometry (e.g. bicycle frame, wheels, helmet) to minimize the aerodynamic drag, thus achieving higher velocity; 2. Enhancement of the directional stability to guarantee the cyclists' cornering capabilities and safety. To date, sport aerodynamics investigation has been conducted mainly via computational fluid dynamics (CFD) simulations or wind tunnel tests around scaled models. The effect cyclist's posture, helmet shape, wheel and frame design on bicycle aerodynamics has been investigated by several authors (Gibertini and Grassi, 2008; Alam et al, 2009; Kyle, 1990, among others). CFD simulations rely upon the introduction of turbulence models to predict the effect of turbulence. As a consequence, their accuracy is questionable when separated flows downstream of bluff bodies (as cyclists and their bike) are researched. Conversely, wind tunnel tests allow a direct measurement of the flow properties. However, measurements are typically conducted around a model in fixed position, giving little information on the actual aerodynamics occurring during races, when the cyclist is in motion while riding the bike. A technique for on-site aerodynamic measurements during the athletes' motion is currently missing. Particle image velocimetry (PIV) is an experimental technique that allows measuring the flow velocity in a two- or three-dimensional domain. Micrometric particles are added to the flow and carried by the fluid. The particles are illuminated twice by a light source, typically a laser, and imaged by a digital camera. The measurement of the particle images displacement within a short time interval Dt leads to the computation of the velocity field in the imaged region (Raffel el al, 2007). In tomographic PIV (shortly tomo-PIV), the particles are illuminated in a volume and imaged by several cameras at different viewing angles to determine the three-dimensional velocity field (Scarano, 2013). Up to date, small measurement volumes up to 50 cm3 have been achieved due to the low light scattering efficiency of conventional tracer particles. To achieve significantly larger measurement volumes (thousands of cubic centimeters), helium-filled soap bubbles (HFSB) have been proposed as flow tracers due to their large size (diameter of 300 mm) and scattering efficiency (Scarano et al, 2015). Tomo-PIV experiments with HFSB have been successfully conducted in the Aerospace Engineering department of Delft University of Technology. In particular, the technique has been used in the wind energy sector to assess the flow dynamics downstream of wind turbine blades. An example of application is illustrated in figure 1, where the flow induced by the blade tip of a vertical axis wind turbine (VAWT) is investigated over a measurement volume of 30,000 cm3. The use of large-scale tomographic PIV opens unprecedented possibilities for aerodynamics investigation in cycling. On-site aerodynamic measurements could be conducted during the cyclist motion, reproducing the same conditions as those encountered in actual competitions. The technique would allow aerodynamic measurements during cornering, which up to date are not possible neither via wind tunnel tests nor through numerical simulations. From the velocity measurements, pressure and forces can be calculated, which serve for optimization of the cyclist’s posture and equipment geometry. @en