On the use of helium-filled soap bubbles for large-scale Tomographic PIV wind tunnel experiments

Abstract

Experiments are conducted with helium-filled soap bubbles (HFSB) in the sub-millimetre diameter range in the low speed flow regime to ascertain the aerodynamic behaviour of these tracers for PIV measurements in wind tunnels. The flow-tracing fidelity of HFSB relies on their neutral buoyancy and partly on their relative size to the flow time and length scales. This study follows closely the methodology proposed in an early work by Kerho and Bragg (1994) who investigated the capabilities of HFSB within a steady potential flow close to a stagnation point. The conclusions of the present investigation differ from those drawn in the latter work as it is concluded that HFSB do represent a valid alternative for quantitative velocimetry in wind tunnel aerodynamic experiments. The flow stagnating ahead of a circular cylinder of 25 mm diameter is considered at speed varying from 5 to 30 m/s. The HFSB are injected at the end of the wind tunnel nozzle contraction. Planar high-speed PIV and PTV techniques are used to obtain the velocity field distribution, velocity streamlines and to map the fluid flow acceleration along the trajectories. PIV experiments conducted in the same conditions with micron size fog droplets are taken as a term of comparison. The parameters governing the HFSB production are also varied: helium relative to soap and relative to air flow rates. The variation of the latter parameters affects the bubble size, characterized with the shadow technique, as well as the HFSB tracing fidelity. The latter is quantified by the velocity slip and with an estimate of the time response in analogy to the case of small heavy tracers.