Finite-sized rigid spheres in turbulent Taylor–Couette flow: effect on the overall drag

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 with 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 have a marginal effect on the drag, with smaller diameter particles showing only slightly lower drag. Increasing 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 with 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.

Statistics of rigid fibers in strongly sheared turbulence

“…We find a skewness of the angular velocity between [−0.14, 0.24]. The kurtosis of the angular velocity lies in the range [34,40], which is much larger than the kurtosis of spheres of similar size ratios [54][55][56]. This can be attributed to the fact that for elongated ellipsoids the rotational inertia is typically much lower than the rotational inertia of similar-sized spheres [22].…”

“…In such situations, no analytic expressions are available for the forces and torques acting on the particles. In general, it is considered that such finite-sized particles filter out the spatial and temporal flow fluctuations [27][28][29][30][31][32][33][34][35], and hence do not actively respond to the local gradients in the flow. Few experiments have explored this regime of finite sized rod-like fibers in sheared turbulence.…”

“…For comparison, a Gaussian distribution with the same mean and variance is added. The skewness and kurtosis are found to lie in the range [−0.14, 0.24] and [34,40], respectively. The inset shows the same data on a linear scale.…”

Towards boiling Taylor-Couette turbulence

“…Furthermore, it would be intriguing, in the spirit of Ref. [28], to study the influence of spanwise-varying regions of idealized high and low wall shear stress, without geometrical induced disturbances.…”

“…In multiphase TC flow, the presence of a dispersed phase can dramatically affect the wall shear stress and thus, the friction in the flow. Examples of dispersed phases are fibers, finite-sized particles, polymers, and bubbles [23][24][25][26][27][28]. The study of the interactions between the dispersed and carrier phase is of particular interest in the industry due to its potential applications.…”