Enhanced upswing in immersed collisions of tethered spheres

Abstract

Experiments and simplified computations are performed to study the influence of liquid motions on the dynamics of immersed pendulums. The pendulums consist of tethered spheres, swinging alone or colliding with one another in liquid initially at rest. Surprisingly, oscillations initiated in identical fashion are found to be damped less rapidly when spheres collide. Using refractive-index-matched materials and laser-illuminated tracers to visualize the flow between and around the spheres, we link this effect to the response of wake vortices. Set in motion by impact, a sphere emerges nearly wake-free from the collision, leading to reduced drag and enhanced upswing.

Cross-sectional imaging of refractive-index-matched liquid-granular flows

“…The combination of PMMA and para-cymene to obtain refractive-index-matched solid-liquid mixtures was proposed by Haam et al (2000). It was also adopted in previous work by Chi (2007), Hsu and Capart (2007) and Huang et al (2008).…”

“…To overcome these restrictions, a number of researchers have sought to achieve full optical access by using transparent refractive-index-matched materials (Cui and Adrian 1997; Haam et al 2000;Hsu and Capart 2007;Mouilleron et al 2009;Sanvitale and Bowman 2012;Aussillous et al 2013;Houssais et al 2015). By using such materials, optical access to the flow interior is preserved in spite of the presence of solid grains, and optical measurement techniques can again be used including laser illumination, laser Doppler anemometry (Haam et al 2000), particle-tracking velocimetry (PTV, Hsu and Capart 2007) and particle image velocimetry (PIV, Aussillous et al 2013).…”

“…By using such materials, optical access to the flow interior is preserved in spite of the presence of solid grains, and optical measurement techniques can again be used including laser illumination, laser Doppler anemometry (Haam et al 2000), particle-tracking velocimetry (PTV, Hsu and Capart 2007) and particle image velocimetry (PIV, Aussillous et al 2013). In addition to optical techniques, other non-intrusive approaches that provide access to the interior of two-phase flows include magnetic resonance imaging (MRI, Gladden et al 2006;Ovarlez et al 2006) and acoustic methods (Revil-Baudard et al 2015).…”

“…Simultaneous velocity and concentration measurements have also been acquired by MRI and acoustic methods (Ovarlez et al 2006;RevilBaudard et al 2015). Whether acquired by optical or other techniques, the resolution of the velocity and concentration measurements has so far been limited to scales greater than the grain diameter, except for special circumstances like monolayers (Frey 2014) or systems composed of only a few large spheres (Hsu and Capart 2007).…”

“…For saturated granular media, this reduces to the determination of the void ratio or its complement, the solid fraction or granular concentration. To capture both liquid and granular velocities, various researchers have seeded the liquid with tracer particles and used PTV and/or PIV to capture both the tracer and grain velocities (Hsu and Capart 2007;Mouilleron et al 2009;Spinewine and Capart 2013;Aussillous et al 2013). Simultaneous measurements of the solid fraction, on the other hand, have been deduced from ratios of granular area to total area, for laser-illuminated cross sections or monolayers (Chi 2007;Frey 2014), or from distance-to-wall statistics acquired using a laser stripe, for near-wall flows of opaque grains (Spinewine et al 2011;Capart and Fraccarollo 2011).…”