Maintenance of air layer and drag reduction on superhydrophobic surface

Du, Peng; Wen, Jun; Zhang, Zhaozhu; Song, Dong; Ouahsine, Abdellatif; Hu, Haibao

doi:10.1016/j.oceaneng.2016.11.028

Cited by 93 publications

(48 citation statements)

References 44 publications

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“…Du et al (2017) only found DR when air was being injected through their SH surface. The DR was the result of weakened near-wall vortices, pushed away from the SH surface, and smaller shear rates on top of the SH surface (Du et al 2017). A variety of flow geometries were studied by Fukuda et al (2000): rectangular pipe flow (5 × 10 4 Re 4 × 10 5 ), flat plate (3 × 10 5 Re L 1.7 × 10 7 ) and ship models in a towing tank (9 × 10 5 Re L 8 × 10 8 ).…”

Section: High Re Turbulencementioning

confidence: 97%

“…The amount of injected air was not enough to form an air bubbly flow, but was enough to maintain a plastron that was thick enough to prevent the surface roughness features from contacting the liquid. When the air injection was stopped, the air plastron became thinner, and roughness effects started to play a role (Du et al 2017). When the roughness elements are exposed to the flow, the Reynolds stresses become the main contributor to the wall shear stress, resulting in less DR (Ling et al 2016).…”

Section: The Air Plastronmentioning

confidence: 99%

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

et al. 2019

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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.

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Section: High Re Turbulencementioning

confidence: 97%

Section: The Air Plastronmentioning

confidence: 99%

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

et al. 2019

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“…The rapid attenuation of PSD demonstrates the low noise of our experimental system. Multiresolution analysis of OWT was used to extract the coherent structures in turbulence [9,25,26].…”

Section: Extraction Of Coherent Structuresmentioning

confidence: 99%

Extracting Coherent Structures in Near-Wall Turbulence Based on Wavelet Analysis

Hai-Bao¹,

Huang²

2020

Advances in Complex Analysis and Applications

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To analyze the properties of the coherent structures in near-wall turbulence, an extraction method based on wavelet transform (WT) and a verification procedure based on correlation analysis are proposed in this work. The flow field of the turbulent boundary layer is measured using the hot-film anemometer in a gravitational low-speed water tunnel. The obtained velocity profile and turbulence intensity are validated with traditional boundary layer theory. The fluctuating velocities at three testing positions are analyzed. Using the power spectrum density (PSD) and WT, coherent and incoherent parts of the near-wall turbulence are extracted and analyzed. The probability density functions (PDFs) of the extracted signals indicate that the incoherent structures of turbulence obey the Gaussian distribution, while the coherent structures deviate from it. The PDFs of coherent structures and original turbulence signals are similar, which means that coherent structures make the most contributions to the turbulence entrainment. A correlation parameter is defined at last to prove the validity of our extraction procedure.

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“…A slip velocity of 1.4 cm/s for Re = 11,000 was observed for water flowing on a super-hydrophobic hydrofoil (Gogte et al, 2005). Studies presenting apparent slip on super-hydrophobic walls have employed superhydrophobic coatings (Guan et al, 2015;Du et al, 2017), and super-hydrophobic coated surfaces with micro-sized features (Gogte et al, 2005;Daniello et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

“…However, the super-hydrophobicity and the contact angle tend to decrease with time under continuous immersion in water due to the loss of trapped air, causing the wetting state to transition from Cassie-Baxter to Wenzel. Super-hydrophobic surfaces that transition to the Wenzel state tend to have a higher drag than a smooth surface due to their high surface roughness (Du et al, 2017). This effect implies that the friction-reducing effect of superhydrophobic surfaces is temporary, which makes them unsuitable for engineering applications in which the material is continuously immersed in fluid.…”

Section: Introductionmentioning

confidence: 99%

Quantifying the Durability of a Friction-Reducing Surface with Recoverable Superhydrophobicity

et al. 2021

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The durability of super-hydrophobic surfaces in fully immersed conditions is a major obstacle to their application in engineering applications. We perform an experimental study to measure the friction factor f d as a function of time for a new super-hydrophobic surface that is capable of recovering the Cassie-Baxter wetting state. Values of f d were obtained by measuring the pressure drop and volume flux of a turbulent water flow in a 1.5 metre long duct containing one super-hydrophobic wall. The Reynolds number of the flow was approximately 4.5 × 10 4 for all experiments. Reductions in f d were 29-36 % relative to a hydraulically smooth surface. The Cassie-Baxter state could be recovered blowing air through the porous surface for 10 minutes. The durability of the drag-reduction, as quantified by the relaxation time T in which the surface loses its superhydrophobic characteristics, were measured to be between 10-60 minutes depending on the initial head. The relaxation time T was highly dependent on the pressure difference across the surface. In contrast to models based on Darcy flow through a porous medium, the study indicates that there seems to be a critical pressure difference beyond which the Cassie-Baxter state cannot be sustained for the material under consideration.

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Maintenance of air layer and drag reduction on superhydrophobic surface

Cited by 93 publications

References 44 publications

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

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

Extracting Coherent Structures in Near-Wall Turbulence Based on Wavelet Analysis

Quantifying the Durability of a Friction-Reducing Surface with Recoverable Superhydrophobicity

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