A modular, 3D-printed helium-filled soap bubble generator for large-scale volumetric flow measurements

Abstract

A modular helium-filled soap bubble (HFSB) generator consisting of 3D-printed nozzles and developed for use in a wind tunnel is characterized. A multi-syringe pump accurately feeds bubble film solution (BFS) to each nozzle while air and helium flow rates are regulated using mass flow controllers. The modular design of the system allows for the customization of the HFSB streamtube to each unique experiment. Modules can be arranged in various configurations to increase seeding density and manipulate the size of the streamtube. Shadowgraphy, particle image velocimetry, particle tracking velocimetry, and laser-based imaging are used to characterize the particle sizes, tracing fidelity, production rates, and seeding density of a system consisting of two modules (8 nozzles in total). It is shown that nozzle performance, particle time response, and production rates can be controlled by varying the flow rates of air, helium, and BFS into the system, respectively. The optimal operating case resulted in the production of approximately 70,000 bubbles/s from each nozzle. The bubbles were neutrally buoyant on average and had a mean diameter of 0.46 mm. The two modules resulted in a streamtube with an effective cross-section of 15 × 15 cm2. The streamtube was produced continuously, resulting in a seeding density of 1.6 bubbles/cm3 at free stream velocity of 10.3 m/s.