Generation and control of helium-filled soap bubbles for large-scale PIV

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The operating regimes of an orifice-type helium-filled soap bubbles (HFSB) generator are investigated to characterize the properties of the resulting tracers. The geometrical properties of the bubbles as well as the production rate are studied with high-speed shadowgraphy. The weight of the tracers relative to the mass of displaced air is determined by Particle Image Velocimetry (PIV) measurements along the stagnation streamline ahead of a cylinder. The latter data yield the particle time response for several combinations of air, helium and soap flow rates. The bubble production rate appears to increase linearly with the air flow rate, whereas the bubble volume is found to be directly proportional to the ratio of helium and air volume flow rates. The HFSB tracing capability approaches that of an ideal tracer (i.e. shortest response time), when the volume flow rate of helium is approximately one thousandfold the soap flow rate. This study provides guidelines for operating HFSB generation systems within large-scale PIV experiments.