Print Email Facebook Twitter Helium-filled soap bubbles for vortex core velocimetry Title Helium-filled soap bubbles for vortex core velocimetry Author Caridi, Giuseppe Carlo Alp (TU Delft Aerodynamics) Sciacchitano, A. (TU Delft Aerodynamics) Scarano, F. (TU Delft Flow Physics and Technology) Department Flow Physics and Technology Date 2017-09-01 Abstract 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. To reference this document use: http://resolver.tudelft.nl/uuid:e3f36f5a-164e-4707-8a13-961e1f81f8f4 DOI https://doi.org/10.1007/s00348-017-2415-x ISSN 0723-4864 Source Experiments in Fluids: experimental methods and their applications to fluid flow, 58 (9) Part of collection Institutional Repository Document type journal article Rights © 2017 Giuseppe Carlo Alp Caridi, A. Sciacchitano, F. Scarano Files PDF 10.1007_s00348_017_2415_x.pdf 2.63 MB Close viewer /islandora/object/uuid:e3f36f5a-164e-4707-8a13-961e1f81f8f4/datastream/OBJ/view