Buoyancy driven spheres rising in quiescent flow at terminal velocity
Buoyancy driven bodies in a viscous flow experience complex fluid structure interactions. In the low Reynolds number regime associated with this motion, the unsteady vortex-shedding leads to transient loading around the body. The oscillatory motion of the body due to the unsteady forces have been studied extensively using numerical simulations, but experimental studies of the flow field around such bodies are sparse. Most of the existing experimental literature are concentrated on drag measurements around freely falling bodies, instead of the flow fields around it. Dynamics of free-falling bodies are vital in understanding many processes in nature and industrial applications. The spread of seeds falling from a tree, the rise of oil droplets accidentally discharged from an underwater well, the settling of solids in a thickening tank, the motion of agglomerated solids in chemical reactors etc. are some examples where flow fields and dynamics associated with free falling objects are relevant.
Typically, a combination of image tracking and Particle Image Velocimetry (PIV) are used record motion of the body and the associated flow around it, respectively. Several PIV techniques have been used in prior studies to investigate the flow around the bodies in a fluid, including two-dimensional, stereo, and tomographic PIV. However, these studies captured the whole three dimensional flow field around the body, and hence there are almost no direct measurements of the coupled fluid-structure interaction.
The current study aims to employ state-of-the-art Refractive Index Matched Time Resolve Tomographic Particle Image Velocimetry to capture the flow around freely rising spheres of different diameters using four high-speed cameras. The time-resolved three dimensional flow field around the body is then used to estimate the instantaneous pressure fields surrounding it using 3D omni-directional integration of the material acceleration. Using both the pressure and velocity fields, transient loading and the dynamics of the rising body can be investigated in great detail.