Air-filled soap bubbles for volumetric velocity measurements

The use of air-filled soap bubbles for volumetric velocimetry in air flows is demonstrated experimentally. The tracers are produced by a novel system that seeds high number density soap bubble streams for particle image velocimetry applications. Particle number density considerations, spatial resolution and response time scales are discussed in light of current seeding techniques for volumetric measurements. The micro-soap bubbles are employed to measure the 3D velocity field in intermediate-sized flow volumes of 75–500 \(\text {cm}^3\), which are difficult to measure with existing tracers such as liquid droplets or relatively large helium-filled soap bubbles, while resolving small-scale flow variations. The mean volumetric concentration of approximately 50 tracers per \(\text {cm}^3\) provides a potential for high-resolution measurements. Finally, the methodology is demonstrated for velocity measurements in the wake of a sphere immersed in a turbulent boundary layer. Both the wake structures and the boundary layer statistics are characterized successfully with a reasonable spatial resolution.