Abstract
The use of helium-filled soap bubbles (HFSB) as flow tracers for particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to measure the properties of turbulent boundary layers is investigated in the velocity range from 30 to 50 m/s. The experiments correspond to momentum thickness-based Reynolds numbers of 3300 and 5100. A single bubble generator delivers nearly neutrally buoyant HFSB to seed the air flow developing over the flat plate. The HFSB motion analysis is performed by PTV using single-frame multi-exposure recordings. The measurements yield the local velocity and turbulence statistics. Planar two-component-PIV measurements with micron-sized droplets (DEHS) conducted under the same conditions provide reference data for the quantities of interest. In addition, the behavior of air-filled soap bubbles is studied where the effect of non-neutral buoyancy is more pronounced. The mean velocity profiles as well as the turbulent stresses obtained with HFSB are in good agreement with the flow statistics obtained with DEHS particles. The study illustrates that HFSB tracers can be used to determine the mean velocity and the turbulent fluctuations of turbulent boundary layers above a distance of approximately two bubble diameters from the wall. This work broadens the current range of application of HFSB from external aerodynamics of large-scale-PIV experiments towards wall-bounded turbulence.
Highlights
The use of helium-filled soap bubbles (HFSB) as tracer particles was initially intended for aerodynamic flow visualization, for instance to visualize the flow around a parachute (Pounder 1956)
The measurements with air-filled soap bubbles (AFSB) and HFSB are evaluated by particle tracking velocimetry (PTV) and require a large number of recordings in single-frame multi-exposure mode to compensate for the low particle concentration
Experiments conducted with planar particle image velocimetry (PIV) and using micron-sized droplets (DEHS) were performed under the same conditions to provide the reference distribution of mean velocity and the turbulence fluctuations statistics
Summary
The use of helium-filled soap bubbles (HFSB) as tracer particles was initially intended for aerodynamic flow visualization, for instance to visualize the flow around a parachute (Pounder 1956). Their use for quantitative measurements had been initially attempted with bubbles of several millimeters. The results discouraged follow-ups due to the difficulties in producing HFSB that could accurately follow the flow (Kerho and Bragg 1994) This issue has been resolved through the reduction of the bubble diameter to the sub-millimeter range (about 0.3 mm), and through refining the control of helium and bubble fluid solution (BFS) mass flows to match the bubble density to that of air (Scarano et al 2015). Studies dealing with HFSB and air-filled soap bubbles (AFSB) immersed
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