Generation and control of helium-filled soap bubbles for PIV

The operating regimes of an orifice-type helium-filled soap bubbles (HFSB) generator are investigated for several combinations of air, helium and soap flow rates to establish the properties of the production process and the resulting tracers. The geometrical properties of the bubbles, the production regimes and the production rates are studied with high-speed shadowgraphy. The results show that the bubble volume is directly proportional to the ratio of helium and air volume flow rates, and that the bubble production rate varies approximately linearly with the air flow rate. The bubble slip velocity is measured along the stagnation streamline ahead of a cylinder via particle image velocimetry (PIV), yielding the particle time response from which the neutral buoyancy condition for HFSB is inferred. The HFSB tracing capability approaches that of an ideal tracer (i.e., minimum slip and 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, intended for PIV experiments. Graphical abstract: [Figure not available: see fulltext.]