Apparent slip and drag reduction for the flow over superhydrophobic and lubricant-impregnated surfaces

2018 ◽  
Vol 3 (12) ◽  
Author(s):  
Edoardo Alinovi ◽  
Alessandro Bottaro
Keyword(s):  
Drag Reduction ◽  
Apparent Slip

scholarly journals SUPERHYDROPHOBIC SURFACES FOR DRAG REDUCTION

2014 ◽  
Author(s):  
Alessandro Bottaro
Keyword(s):  
Drag Reduction ◽  
Slip Length ◽  
Superhydrophobic Surfaces ◽  
Transition To Turbulence ◽  
Effective Coupling ◽  
Instability Waves ◽  
Low Surface Energy ◽  
Apparent Slip ◽  
Energy Material ◽  
Relevant Length Scale

Properties of superhydrophobic materials are examined in light of their possible use for drag reduction in naval applications. To achieve superhydrophobicity a low-surface-energy material must be structured so as to minimize the liquid-solid interactions. The crucial aspect is that of maintaining a layer of gas in between the (rough) wall and the liquid, and this can be achieved by hierarchical micro- and nano-structuring of the solid surface, to ensure a sufficiently large apparent slip of the fluid at the wall, thus reducing skin friction. The behavior of the liquid is quantified by a slip length; recent results have shown that this length can be as large as 400 μm. As far as transition to turbulence is concerned, we show that superhydrophobic surfaces are effective (i.e. they delay the onset of travelling instability waves) only in channels with characteristic dimensions of a few millimeters. Conversely, when the fluid flow has already attained a turbulent state, the gain in term of drag reduction can be very significant also in macroscopic configurations. This occurs because the relevant length scale of the boundary layer is now the thickness of the viscous sub-layer, which can be of magnitude comparable to the slip length, so that an effective coupling emerges. Finally, some procedures to produce superhydrophobic surfaces are examined, in light of the possible application of such innovative coatings on the hull of ships.

scholarly journals Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

2013 ◽  
Vol 727 ◽  
pp. 488-508 ◽  
Author(s):  
A. Busse ◽  
N. D. Sandham ◽  
G. McHale ◽  
M. I. Newton
Keyword(s):  
Drag Reduction ◽  
Mass Flow Rate ◽  
Layer Thickness ◽  
Mass Flow ◽  
Flow Rate ◽  
Superhydrophobic Surface ◽  
Slip Length ◽  
Apparent Slip ◽  
Viscosity Contrast ◽  
Gas Layer

AbstractAnalytic results are derived for the apparent slip length, the change in drag and the optimum air layer thickness of laminar channel and pipe flow over an idealised superhydrophobic surface, i.e. a gas layer of constant thickness retained on a wall. For a simple Couette flow the gas layer always has a drag reducing effect, and the apparent slip length is positive, assuming that there is a favourable viscosity contrast between liquid and gas. In pressure-driven pipe and channel flow blockage limits the drag reduction caused by the lubricating effects of the gas layer; thus an optimum gas layer thickness can be derived. The values for the change in drag and the apparent slip length are strongly affected by the assumptions made for the flow in the gas phase. The standard assumptions of a constant shear rate in the gas layer or an equal pressure gradient in the gas layer and liquid layer give considerably higher values for the drag reduction and the apparent slip length than an alternative assumption of a vanishing mass flow rate in the gas layer. Similarly, a minimum viscosity contrast of four must be exceeded to achieve drag reduction under the zero mass flow rate assumption whereas the drag can be reduced for a viscosity contrast greater than unity under the conventional assumptions. Thus, traditional formulae from lubrication theory lead to an overestimation of the optimum slip length and drag reduction when applied to superhydrophobic surfaces, where the gas is trapped.

Drag Reduction and Velocity Profiles Distribution of Crude Oil Flow in Spiral Pipes

2015 ◽  
Vol 9 (1) ◽  
pp. 1 ◽  
Author(s):  
Yanuar Yanuar ◽  
Kurniawan T. Waskito ◽  
Gunawan Gunawan ◽  
Budiarso Budiarso
Keyword(s):  
Drag Reduction ◽  
Crude Oil ◽  
Velocity Profiles ◽  
Oil Flow

Surfactant drag reduction

2017 ◽  
Author(s):  
A. Bismarck ◽  
L. Chen ◽  
J. M. Griffen ◽  
Geoffrey F. Hewitt ◽  
John Christos Vassilicos
Keyword(s):  
Drag Reduction

DYNAMICS OF COUNTER-FLOW INJECTION FOR WAVE DRAG REDUCTION

2018 ◽  
Author(s):  
Vishnu Prakash K ◽  
Siddesh Desai ◽  
Hrishikesh Gadgil ◽  
Vinayak Kulkarni
Keyword(s):  
Drag Reduction ◽  
Flow Injection ◽  
Wave Drag ◽  
Counter Flow ◽  
Wave Drag Reduction
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