On the Use of Helium-filled Soap Bubbles for Large-scale Tomographic PIV Wind Tunnel Experiments

The use of particle image velocimetry in wind tunnels has evolved from experiments done at moderate scale (typically 10×10 cm2) up to large scale ones both for aeronautical as well as automotive applications (50×50 cm2). Considering three-dimensional techniques, tomographic PIV has been applied successfully in several wind tunnel experiments, however the typical size of the investigated volume does not exceed 50 cm3. The main factors limiting the upscale of particle image velocimetry to macroscopic dimensions are the limited pulse energy from the illumination source, the scattering efficiency of the tracers and the sensitivity and spatial resolution of the imagers. A possible solution to image the tracer particles in a large domain is to apply larger tracer particles instead of increasing the illumination intensity. However, the tracer particles should be light enough to ensure their fidelity to the flow motions. This requirement has suggested the helium filled soap bubbles (HFSB) as a viable solution due to their potential towards neutral buoyancy and their large diameter in the order of one millimetre. Kehro and Bragg (1994) investigated the aerodynamic performance of HFSB and found that, in their case, the trajectories of those deviated from the theoretical streamlines around a NACA 0012 airfoil because the neutral buoyancy condition was not satisfied. In the present work, sub-millimetre HSFB are considered, generated by a system developed by the German Aerospace Center DLR (Bosbach et al. 2009). Aim of the work is the aerodynamic characterization of the HFSB, varying the main parameters governing the bubbles production: helium relative to soap and relative to air flow rates. Furthermore,
the HFSB tracers are used for demonstrating the feasibility of large-scale tomographic PIV measurements in wind tunnels.

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