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

Abstract

The flow-tracing fidelity of sub-millimetre diameter helium filled soap bubbles (HFSB) for low speed aerodynamics is studied. The main interest of using HFSB in relation to micron-size droplets is the large amount of scattered light, enabling larger scale three-dimensional experiments by tomographic PIV. The assessment of aerodynamic behavior closely follows the method proposed in the early work of Kerho and Bragg (1994) who evaluated the tracers trajectories around the stagnation region at the leading edge of an airfoil. The conclusions of the latter investigation differ from the present work, which concludes sub-millimetre 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 speeds up to 30 m/s. The tracers are injected in the free stream and high-speed PIV and PTV are used to obtain the velocity field distribution. A qualitative assessment based on streamlines is followed by acceleration and slip velocity measurements using PIV experiments with fog droplets as a term of reference. The tracing fidelity is controlled by the flow rates of helium, liquid soap and air in HFSB production. A characteristic time response, defined as the ratio of slip velocity and the fluid acceleration, is obtained. The feasibility of performing time-resolved tomographic PIV measurements over large volumes in aerodynamic wind tunnels is also studied. The flow past a 5 cm diameter cylinder is measured over a volume of 20x20x12 cm³ at a rate of 2 kHz. The achieved seeding density of less than 0.01 ppp enables resolving the Kármán vortices, whereas turbulent sub-structures cannot be captured.