Bubbly drag reduction using a hydrophobic inner cylinder in Taylor–Couette turbulence

Abstract

In this study we experimentally investigate bubbly drag reduction in a highly turbulent flow of water with dispersed air at 5.0 × 10 5 Re 1.7 × 10 6 over a non-wetting surface containing micro-scale roughness. To do so, the Taylor-Couette geometry is used, allowing for both accurate global drag and local flow measurements. The inner cylinder -coated with a rough, hydrophobic material -is rotating, whereas the smooth outer cylinder is kept stationary. The crucial control parameter is the air volume fraction α present in the working fluid. For small volume fractions (α < 4 %), we observe that the surface roughness from the coating increases the drag. For large volume fractions of air (α 4 %), the drag decreases compared to the case with both the inner and outer cylinders uncoated, i.e. smooth and hydrophilic, using the same volume fraction of air. This suggests that two competing mechanisms are at place: on the one hand the roughness invokes an extension of the log-layer -resulting in an increase in drag -and on the other hand there is a drag-reducing mechanism of the hydrophobic surface interacting with the bubbly liquid. The balance between these two effects determines whether there is overall drag reduction or drag enhancement. For further increased bubble concentration α = 6 % we find a saturation of the drag reduction effect. Our study gives guidelines for industrial applications of bubbly drag reduction in hydrophobic wall-bounded turbulent flows.

Superhydrophobic drag reduction in turbulent flows: a critical review

“…As a different approach to compensate for the degradation of a SHPo surface, hybrid methods of introducing gas (bubbles) into the bulk flow have been also explored. Bullee et al (2020) combined the bubbly flow and a random-roughness SHPo surface in a turbulent Taylor-Couette flow to investigate their complementary interaction for drag reduction. While the drag increased by about 14% on a smooth surface in a bubbly flow by the extension of the log layer in the velocity profile (roughness effect), the drag decreased by up to 5% on a SHPo surface in the same bubbly flow, overcoming the roughness effect.…”

“…Found capable of achieving 𝜆 ≳ O(10)𝜇m (Ou et al 2004;Truesdell et al 2006;Lee et al 2008), which is large enough to modify the dynamics of common bulk water flows considering their viscous sublayer thickness would typically range around O(10 − 100) m (Yeginbayeva and Atlar 2018), the SHPo surfaces have been widely investigated as a promising tool for flow control, especially hydrodynamic drag reduction. Drag reduction in large systems of turbulent flows is of particular interest, considering the significant practical benefits expected from the curtailment of energy consumption and pollutant emission in oceanic transportation, long-range liquid transport pipe systems, and others (van den Berg et al 2007;Ceccio 2010;Murai 2014;Park et al 2014;Gose et al 2018;Bullee et al 2020;Xu et al 2020b).…”

Sodium chloride inhibits effective bubbly drag reduction in turbulent bubbly Taylor–Couette flows

“…where ν is the kinematic viscosity of the working fluid and σ = (1 + η) 4 /(16η 2 ) can be seen as the quasi-Prandtl number [57], which is solely determined by the radius ratio η = r i /r o . For measurements where the working fluid contains bubbles, we correct the density and the viscosity using the Einstein equation [59], as was done previously [19,22,34,35,38]:…”

“…These power savings are, however, not nearly as large as the drag reductions measured in the laboratory. A possible explanation for the discrepancy between laboratory measurements and measurements on marine vessels could be size effects that come into play when scaling the flow from the laboratory to ships [7,31,32], surface details of the hull such as roughness [33][34][35][36][37], mineral/salt content of the water, particulate content or surfactants [22].…”

Performance of a Propeller Coated with Hydrophobic Material

“…Taghvaei et al [13] measured two superhydrophobic flat plates and found that the two hydrophobic plates had lower surface drag compared with that of a conventional material plate. Bullee et al [14] combined a hydrophobic surface with air film injection technique to reduce hydrodynamic drag on a rotating cylinder surface. Lyu et al [15] studied the friction reduction effect of a superhydrophilic and a superhydrophobic surfaces and found that, when Re < 200,000, the superhydrophobic plate had a significant friction reduction effect.…”