Soap bubbles for large-scale PIV: Generation, control and tracing accuracy

Particle Image Velocimetry (PIV) relies upon the introduction of particle tracers that scatter sufficient light and follow the flow accurately. The use of submillimetre helium-filled soap bubbles (HFSB) as flow tracers for PIV is investigated for the purpose of enabling velocity measurements in large-scale industrial wind tunnels. That soap bubbles reflect more light than scattered by small liquid droplets or solid particles, allowing larger volumes to be illuminated, is a long known fact and has caught the attention of aerodynamicists since the 1930s. The difficulty encountered during initial efforts on using soap bubbles for accurate measurements revolves around the lack of control during the generation of these tracers, and the failure in presenting evidence that they could accurately follow the flow. Proof of concept that HFSB could be used for accurate flow measurements in wind tunnels was presented in the year that preceded the beginning of this work. In this thesis, the generation and control of HFSB and their tracing fidelity are studied through a series of experiments and simulations, bringing large-scale PIV using HFSB to the technology maturity level required for industrial measurements. High-speed shadowgraphy at the bubble generator exit revealed the main regimes of bubble generation. A regular, periodic and controlled generation bubbles of monodisperse size distribution, namely, the bubbling regime, was obtained by properly tuning of the input flow rates. The relation of the later with the bubble size and production rate was also obtained from these visualizations. Measurements of the HFSB velocity in the stagnation region ahead of a cylinder, obtained with Particle Tracking Velocimetry (PTV), relative to the flow velocity (slip velocity) were used to retrieve the HFSB time response and the ratio of helium to soap flow rates that satisfy the neutral buoyancy condition, in which the soap bubble density equals that of the surrounding air flow. Simulations of the particle motion in a rectilinear oscillatory flow was used to quantify the importance of the unsteady forces acting on a particle and to derive empirical relations for estimating the HFSB slip velocity in flows where the unsteady forces are relevant. In this case, the particle slip velocity is shown to depend on three parameters: the particle Reynolds number, the ratio of particle-to-fluid density and the flow time-scale. These cannot be combined into a single non-dimensional Stokes number. The validity of the empirical relations were extended for the analysis of the slip velocity of a particle travelling around an object. Based on the later, a method for deriving the density of a nearly-neutrally-buoyant particle that comprises the effects of unsteady forces and allows mismatch of acceleration between the particle and the flow was described. The tools developed for slip velocity analysis using the simulations were applied to assess experimental data from large-scale PIV measurements performed at the Low-Speed Tunnel (LST) of the German-Dutch Wind Tunnels (DNW). The experiments were realized in the flow around an airfoil of 70 cm chord at free stream velocity up to 70 m/s, reaching a chord-based Reynolds number of 3.2 million. PIV measurements using HFSB at this speed and Reynolds number were unprecedented. The results have indicated variations of the bubble density (20-30%) occurring post-generation. The tracing fidelity of HFSB in wall-bounded turbulence is investigated by comparing measurements in a turbulent-boundary layer of the mean velocity and Reynolds stress profiles, with those obtained with micrometre oil droplets (reference) and submillimetre air-filled soap bubbles (AFSB). The results have shown that the statistics of the first and second moments of velocity are well captured by all three investigated tracers, even by the heavier-than-air AFSB, which were shown to be poor tracers in the stagnation of a cylinder. Mechanisms of preferential concentration in turbulence were attributed as the cause of the better traceability observed. The thesis is concluded with a successful industrial application in the Large Low-Speed Facility (LLF) of DNW (9.5×9.5 m2 test section) around a tiltrotor aircraft in three flight modes, hover, transition and cruise, and tunnel speeds up to 60 m/s. The bubbles were introduced into the flow using a 3×3 m2 seeding rake, containing 400 bubble generators. The PIV measurements were performed in stereoscopic configuration in a field-of-view of 1.1×1.1 m2.