The Effects of Superhydrophobic Surfaces on Skin Friction Drag

2015 ◽  
pp. 357-365 ◽  
Author(s):  
Hyunwook Park ◽  
John Kim
Keyword(s):  
Skin Friction ◽  
Superhydrophobic Surfaces ◽  
Friction Drag

scholarly journals Sustained drag reduction in a turbulent flow using a low-temperature Leidenfrost surface

2016 ◽  
Vol 2 (10) ◽  
pp. e1600686 ◽  
Author(s):  
Dhananjai Saranadhi ◽  
Dayong Chen ◽  
Justin A. Kleingartner ◽  
Siddarth Srinivasan ◽  
Robert E. Cohen ◽  
...  
Keyword(s):  
Drag Reduction ◽  
Skin Friction ◽  
Superhydrophobic Surface ◽  
Pressure Fluctuations ◽  
Slip Boundary Condition ◽  
Superhydrophobic Surfaces ◽  
Friction Drag ◽  
Slip Boundary ◽  
Large Water ◽  
High Reynolds

Skin friction drag contributes a major portion of the total drag for small and large water vehicles at high Reynolds number (Re). One emerging approach to reducing drag is to use superhydrophobic surfaces to promote slip boundary conditions. However, the air layer or “plastron” trapped on submerged superhydrophobic surfaces often diminishes quickly under hydrostatic pressure and/or turbulent pressure fluctuations. We use active heating on a superhydrophobic surface to establish a stable vapor layer or “Leidenfrost” state at a relatively low superheat temperature. The continuous film of water vapor lubricates the interface, and the resulting slip boundary condition leads to skin friction drag reduction on the inner rotor of a custom Taylor-Couette apparatus. We find that skin friction can be reduced by 80 to 90% relative to an unheated superhydrophobic surface for Re in the range 26,100 ≤ Re ≤ 52,000. We derive a boundary layer and slip theory to describe the hydrodynamics in the system and show that the plastron thickness is h = 44 ± 11 μm, in agreement with expectations for a Leidenfrost surface.

scholarly journals Reduction of turbulent skin-friction drag by passively rotating discs

2021 ◽  
Vol 923 ◽  
Author(s):  
Paolo Olivucci ◽  
Daniel J. Wise ◽  
Pierre Ricco
Keyword(s):  
Skin Friction ◽  
Friction Drag ◽  
Rotating Discs

Abstract

Effects of the air layer of an idealized superhydrophobic surface on the slip length and skin-friction drag

2016 ◽  
Vol 790 ◽  
Author(s):  
Taeyong Jung ◽  
Haecheon Choi ◽  
John Kim
Keyword(s):  
Drag Reduction ◽  
Skin Friction ◽  
Superhydrophobic Surface ◽  
Slip Length ◽  
Joint Probability ◽  
Turbulent Channel Flow ◽  
Water Interface ◽  
Friction Drag ◽  
Recirculating Flow ◽  
Air Water Interface

The anisotropy of the slip length and its effect on the skin-friction drag are numerically investigated for a turbulent channel flow with an idealized superhydrophobic surface having an air layer, where the idealized air–water interface is flat and does not contain the surface-tension effect. Inside the air layer, both the shear-driven flow and recirculating flow with zero net mass flow rate are considered. With increasing air-layer thickness, the slip length, slip velocity and percentage of drag reduction increase. It is shown that the slip length is independent of the water flow and depends only on the air-layer geometry. The amount of drag reduction obtained is in between those by the empirical formulae from the streamwise slip only and isotropic slip, indicating that the present air–water interface generates an anisotropic slip, and the streamwise slip length ($b_{x}$) is larger than the spanwise one ($b_{z}$). From the joint probability density function of the slip velocities and velocity gradients at the interface, we confirm the anisotropy of the slip lengths and obtain their relative magnitude ($b_{x}/b_{z}=4$) for the present idealized superhydrophobic surface. It is also shown that the Navier slip model is valid only in the mean sense, and it is generally not applicable to fluctuating quantities.

scholarly journals Aviation Impacts on Fuel Efficiency of a Future More Viscous Atmosphere

2020 ◽  
Vol 101 (10) ◽  
pp. E1761-E1780
Author(s):  
Diandong Ren ◽  
Rong Fu ◽  
Robert E. Dickinson ◽  
Lance M. Leslie ◽  
Xingbao Wang
Keyword(s):  
Skin Friction ◽  
Climate Models ◽  
Climate Model ◽  
Fuel Efficiency ◽  
Aviation Fuel ◽  
Vertical Shear ◽  
Friction Drag ◽  
Dominant Term ◽  
Warming Climate ◽  
Additional Fuel

AbstractAircraft cruising near the tropopause currently benefit from the highest thermal efficiency and the least viscous (sticky) air, within the lowest 50 km of Earth’s atmosphere. Both advantages wane in a warming climate, because atmospheric dynamic viscosity increases with temperature, in synergy with the simultaneous engine efficiency reduction. Here, skin friction drag, the dominant term for extra aviation fuel consumption in a future warming climate, is quantified by 34 climate models under a strong emissions scenario. Since 1950, the viscosity increase at cruising altitudes (∼200 hPa) reaches ∼1.5% century‒1, corresponding to a total drag increment of ∼0.22% century‒1 for commercial aircraft. Meridional gradients and regional disparities exist, with low to midlatitudes experiencing greater increases in skin friction drag. The North Atlantic corridor (NAC) is moderately affected, but its high traffic volume generates additional fuel cost of ∼3.8 × 107 gallons annually by 2100, compared to 2010. Globally, a normal year after 2100 would consume an extra ∼4 × 106 barrels per year. Intermodel spread is <5% of the ensemble mean, due to high inter–climate model consensus for warming trends at cruising altitudes in the tropics and subtropics. Because temperature is a well-simulated parameter in the IPCC archive, with only a moderate intermodel spread, the conclusions drawn here are statistically robust. Notably, additional fuel costs are likely from the increased vertical shear and related turbulence at NAC cruising altitudes. Increased flight log availability is required to confirm this apparent increasing turbulence trend.

scholarly journals Experimental Study on the Friction Drag Reduction of Superhydrophobic Surfaces in Closed Channel Flow

2019 ◽  
Vol 12 (1) ◽  
pp. 69-76 ◽  
Author(s):  
M. Monfared ◽  
M. A. Alidoostan ◽  
B. Saranjam ◽  
◽  
◽  
...  
Keyword(s):  
Experimental Study ◽  
Drag Reduction ◽  
Channel Flow ◽  
Superhydrophobic Surfaces ◽  
Friction Drag ◽  
Closed Channel ◽  
Friction Drag Reduction

The Effect of Freshwater Biofilms on Skin Friction Drag

2009 ◽  
pp. 1940-1945
Author(s):  
Jessica Andrewartha ◽  
Jane Sargison ◽  
Alan Henderson ◽  
Kate Perkins ◽  
Greg Walker
Keyword(s):  
Skin Friction ◽  
Friction Drag ◽  
Freshwater Biofilms

Log Boom Drag Measurement at Sea

1994 ◽  
Vol 31 (02) ◽  
pp. 145-148
Author(s):  
Sheldon I. Green ◽  
John R. Garfitt ◽  
G. Glenn Young
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
Skin Friction ◽  
Wave Drag ◽  
Operating Conditions ◽  
Small Scale ◽  
Friction Drag ◽  
Drag Measurement

Measurements of the drag on a typical (52-section) log boom were made under calm conditions at sea. The drag coefficient based on planform area is essentially independent of Reynolds number over typical operating conditions, cD = 0.0083 + 0.0006. Engineering calculations suggest that the log boom drag is caused primarily by skin friction drag and that wake drag is slightly less important; wave drag is entirely insignificant. A small-scale (4-section) log boom was constructed to test the influence of log arrangement within the boom on boom drag. Neither aligning the front row of bundles transversely nor covering boom sections with an underwater shroud had a significant impact on the boom drag.

Decomposition of the mean skin-friction drag in compressible turbulent channel flows

2019 ◽  
Vol 875 ◽  
pp. 101-123 ◽  
Author(s):  
Weipeng Li ◽  
Yitong Fan ◽  
Davide Modesti ◽  
Cheng Cheng
Keyword(s):  
Heat Flux ◽  
Mach Number ◽  
Skin Friction ◽  
Wall Layer ◽  
Channel Flows ◽  
Wall Heat Flux ◽  
Generation Process ◽  
Friction Drag ◽  
Turbulent Channel Flows ◽  
The Mean

The mean skin-friction drag in a wall-bounded turbulent flow can be decomposed into different physics-informed contributions based on the mean and statistical turbulence quantities across the wall layer. Following Renard & Deck’s study (J. Fluid Mech., vol. 790, 2016, pp. 339–367) on the skin-friction drag decomposition of incompressible wall-bounded turbulence, we extend their method to a compressible form and use it to investigate the effect of density and viscosity variations on skin-friction drag generation, using direct numerical simulation data of compressible turbulent channel flows. We use this novel decomposition to study the skin-friction contributions associated with the molecular viscous dissipation and the turbulent kinetic energy production and we investigate their dependence on Reynolds and Mach number. We show that, upon application of the compressibility transformation of Trettel & Larsson (Phys. Fluids, vol. 28, 2016, 026102), the skin-friction drag contributions can be only partially transformed into the equivalent incompressible ones, as additional terms appear representing deviations from the incompressible counterpart. Nevertheless, these additional contributions are found to be negligible at sufficiently large equivalent Reynolds number and low Mach number. Moreover, we derive an exact relationship between the wall heat flux coefficient and the skin-friction drag coefficient, which allows us to relate the wall heat flux to the skin-friction generation process.

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