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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). |
