Abstract
We report on the modification of drag by neutrally buoyant spherical finite-sized particles in highly turbulent Taylor–Couette (TC) flow. These particles are used to disentangle the effects of size, deformability and volume fraction on the drag, and are contrasted to the drag in bubbly TC flow. From global torque measurements, we find that rigid spheres hardly decrease or increase the torque needed to drive the system. The size of the particles under investigation has a marginal effect on the drag, with smaller diameter particles showing only slightly lower drag. Increase of the particle volume fraction shows a net drag increase. However, this increase is much smaller than can be explained by the increase in apparent viscosity due to the particles. The increase in drag for increasing particle volume fraction is corroborated by performing laser Doppler anemometry, where we find that the turbulent velocity fluctuations also increase with increasing volume fraction. In contrast to rigid spheres, for bubbles, the effective drag reduction also increases with increasing Reynolds number. Bubbles are also much more effective in reducing the overall drag.
Highlights
Flows in nature and industry are generally turbulent, and often these flows carry bubbles, drops or particles of various shapes, sizes and densities
We have conducted an experimental study on the drag response of a highly turbulent TC flow containing rigid neutrally buoyant spherical particles
Unlike the case of bubbles used in prior works, rigid particles barely reduce the drag on the system, even for cases where their size is comparable to that of the bubbles used in other studies
Summary
Flows in nature and industry are generally turbulent, and often these flows carry bubbles, drops or particles of various shapes, sizes and densities. Uhlmann (2008) conducted one of the first numerical simulations of finite-sized rigid spheres in a vertical particle-laden channel flow They observed a modification of the mean velocity profile and turbulence modulation due to the presence of particles. In the current Taylor number regime, it is known that Nuω ∝ Ta0.4 Because this response is well known, it can be exploited to study the influence of immersed bubbles and particles (van den Berg et al 2005, 2007; van Gils et al 2013; Maryami et al 2014; Verschoof et al 2016) on the drag needed to sustain constant rotational velocity of the inner cylinder. We study the effects of varying the particle size dp, the volume fraction α, the density ratio φ and the flow Reynolds number Re on the global torque (drag) of the TC flow. The findings are summarized and an outlook for future work is given in the last section
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