HFSB
An in-house seeding system was developed to generate neutrally buoyant helium-filled soap bubbles (HFSB) for large-scale 3D PIV/PTV measurements. The bubbles have a mean diameter around 0.5 mm. The HFSB system was shown in Fig 9. It consists of 4 modular ducts, each host 12 nozzles (a total of 48 nozzles). The supporting structure had a NACA 0012 profile and a thin wall thickness to minimize the disturbance to the flow field. The HFSB system was placed in the settling chamber, upstream of the contraction section. A photo of bubbles in production is presented in Fig 10.
The HFSB system
Bubbles in production
Alternative chamber configuration
In past studies, HFSB nozzles have not demonstrated sufficient bubble production rates and have not operated as consistently as required. By looking at the internal flow of the HFSB nozzle used previously, we look to come up with a new design that is more reliable and has a higher bubble production rate. Shadowgraphy will be used to characterize the internal flow of HFSB nozzles by imaging through a transparent outer nozzle cap, while simultaneously gathering images of the bubble stream outside the nozzle.
