Hydrodynamic Drag Reduction Research Articles

Preliminary Analysis of Hydrodynamic Drag Reduction and Fouling Resistance of Surfaces Inspired by the Mollusk Shell, Dosinia juvenilis.

Many species of plants and animals show an ability to resist fouling with surface topographies tailored to their environments. The mollusk species Dosinia juvenilis has demonstrated the ability to resist the accumulation of fouling on its outer surface. Understanding the functional mechanism employed by nature represents a significant opportunity for the persistent challenges of many industrial and consumer applications. Using a biomimetic approach, this study investigates the underlying hydrodynamic mechanisms of fouling resistance through Large Eddy simulations of a turbulent boundary layer above a novel ribletted surface topography bio-inspired by the Dosinia juvenilis. The results indicate a maximum drag reduction of 6.8% relative to a flat surface. The flow statistics near the surface are analogous to those observed for other ribletted surfaces in that the appropriately sized riblets effectively reduce the spanwise and wall-normal velocity fluctuations near the surface. This study supports the understanding that nature employs ribletted surfaces toward multiple functionalities including the considered drag reduction and fouling resistance.

Enhancing hydrodynamic drag and lift reduction around two staggered obstacles via control plates

This study analyzes the enhancement of hydrodynamic forces reduction for flow around two identical square cylinders arranged in a staggered manner utilizing the control plates. Effects of attached flat plates on flow structures and fluid forces acting on cylinders are examined by considering plates length (l) = 0.1–10 times cylinder’s size. The gap distance between cylinders (g) is fixed at 2.5 times cylinders' size with a fixed Reynolds number of 200. Four different flow patterns revealed in this study: synchronized flow pattern for l = 0.1 to 0.6, quasi flip-flopping flow pattern for l = 0.7 to 1.9, modulated synchronized flow pattern for l = 2 to 4.1 and quasi-periodic flow pattern for l = 4.2 to 10 while non-synchronized flow pattern is observed for staggered cylinders without plates at g = 2.5. The flow patterns variation is found to be directly linked with variations in Strouhal number (St) of both cylinders. The fluid force parameters decreased significantly with increasing length of plates as compared to two staggered cylinders without plates. In particular, mean drag coefficient (CDmean) reduces maximum up to 91% at l = 0.8, root-mean-square value of drag coefficient (CDrms) reduces up to 82% at l = 0.1, root-mean-square value of lift coefficient (CLrms) reduces up to 94% at l = 0.3, St reduces up to 72% at l = 10, the amplitude of drag coefficient (CDamp) reduces up to 78% at l = 0.8 and amplitude of lift coefficient (CLamp) reduces up to 87% at l = 0.2. The plates are found more effective in controlling the flow around the second cylinder as compared to the first cylinder. This study also reveals that at some plate lengths, instead of reduction, an increase in hydrodynamic force values occurred compared to the uncontrolled case.

Hydrodynamic drag reduction in ribbed microchannel with infused non-Newtonian lubricants

Liquid-infused surfaces have recently gained prominence in engineering applications owing to their versatile characteristics such as self-cleaning, anti-fogging, drag reduction, and enhanced heat transfer. In this article, a numerical analysis of pressure-driven flow past a periodic array of rectangular transverse grooves infused with non-Newtonian immiscible lubricants is performed. The volume of fluid method is employed to capture the interface between primary and secondary fluids, and the power-law model is deployed to mimic the non-Newtonian lubricant. The drag reduction capability of the microchannel is examined for various parameters such as Reynolds number, liquid fraction, viscosity ratio, viscosity index, and contact angle. It is observed that the introduction of a non-Newtonian fluid (shear-thickening or shear-thinning) drastically modifies the interface velocity and hydrodynamic resistance. In particular, a shear-thinning lubricant enhances the slip length as the viscosity index (n) is reduced owing to the reduced viscosity at the interface. Note that, for a lubricant having n = 0.7, the percentage improvement in the slip length is 382% in comparison with a Newtonian counterpart having the same viscosity ratio, N = 0.1. Importantly, the introduction of a shear-thinning lubricant with a viscosity ratio N = 5, a liquid fraction of 0.8, and a behavior index n = 0.7 yielded a pressure drag reduction of 63.6% with respect to a classical no-slip channel and of 23% with reference to a microchannel with the Newtonian lubricant. Moreover, at high Reynolds numbers, Re→50, the drag mitigation is slightly lowered due to the primary vortex shift in the cavity. Furthermore, the effect of the interface contact angle (θc) is investigated, as θc drops from 90° (flat) to 45° (convex); the meniscus curvature is enhanced, and the effective slip length is reduced. These observations suggest that a shear-thinning lubricant-infused microchannel is a promising candidate for drag reduction in lab-on-chip applications.

Sustainability of the plastron on nano-grass-covered micro-trench superhydrophobic surfaces in high-speed flows of open water

This paper studies the sustainability of plastrons on superhydrophobic (SHPo) surfaces made of longitudinal micro-trenches covered by nano-grass with the main interest on hydrodynamic friction drag reduction in high-speed flows of open water, which represent the operating conditions of common watercraft. After revising the shear-driven drainage model to address the air diffusion for SHPo surfaces, the existing theories are combined to reveal the trends of how the immersion depth, air saturation level and shear stress affect the maximum attainable plastron length. Deviations from the theories by the dynamic effect at the two ends of the trench, the interfacial contaminations and turbulent fluctuation are also discussed. A combinatorial series of well-defined SHPo trench surfaces (4 cm × 7 cm in size with varying trench widths, depths, lengths and roughnesses) is microfabricated and attached underneath a 4 m long motorboat on seawater in turbulent flows up to 7.2 m s−1(shear rate ∼83 000 s−1and friction Reynolds number ∼5500). Because the plastron can provide a substantial slip only while its air–water interfaces are pinned (or only slightly depinned) at the trench top, two underwater cameras are employed to differentiate the pinned (and slightly depinned) interfaces from the depinned (and no) interfaces. In addition to achieving pinned plastrons on 6 cm long trenches aligned to high-speed flows in open water, the experimental results corroborate the theoretical estimations, supporting the design of SHPo surfaces for field applications.

Fabrication of Salvinia-inspired surfaces for hydrodynamic drag reduction by capillary-force-induced clustering

For decades, bioinspired functional materials have been attracting the interest of many researchers for their remarkable characteristics. In particular, some plant leaves are well known for their inherent superhydrophobic nature. Salvinia molesta, a free-floating aquatic fern, has egg-beater-shaped hierarchical trichomes on its surface of leaves. Due to the unique structure and complex wettability of the hairs, this plant has the ability to maintain a stable thick air layer upon the structure when it is submerged underwater. Often referred to as the “Salvinia Effect,” this property is expected to be suitable for use in hydrodynamic drag reduction. However, due to the complex shape of the trichome, currently applied fabrication methods are using a three-dimensional printing system, which is not applicable to mass production because of its severely limited productivity. In this work, artificial Salvinia leaf inspired by S. molesta was fabricated using a conventional soft lithography method assisted with capillary-force-induced clustering of micropillar array. The fabrication method suggested in this work proposes a promising strategy for the manufacturing of Salvinia-inspired hydrodynamic drag reduction surfaces.

Hydrodynamic drag reduction of shear-thinning liquids in superhydrophobic textured microchannels

Super-hydrophobic textured surfaces reduce hydrodynamic drag in pressure-driven laminar flows in micro-channels. However, despite the wide usage of non-Newtonian liquids in microfluidic devices, the flow behaviour of such liquids was rarely examined so far in the context of friction reduction in textured super-hydrophobic micro-channels. Thus, we have investigated the influence of topologically different rough surfaces on friction reduction of shear-thinning liquids in micro-channels. First, the friction factor ratio (a ratio of friction factor on a textured surface to a plain surface) on generic surface textures, such as posts, holes, longitudinal and transverse ribs, was estimated numerically over a range of Carreau number as a function of microchannel constriction ratio, gas fraction and power-law exponent. Resembling the flow behaviour of Newtonian liquids, the longitudinal ribs and posts have exhibited significantly less flow friction than the transverse ribs and holes while the friction factor ratios of all textures has exhibited non-monotonic variation with the Carreau number. While the minima of the friction factor ratio were noticed at a constant Carreau number irrespective of the microchannel constriction ratio, the minima have shifted to a higher Carreau number with an increase in the power-law index and gas fraction. Experiments were also conducted with aqueous Xanthan Gum liquids in micro-channels. The flow enhancement (the flow rate with super-hydrophobic textures with respect to a smooth surface) exhibited a non-monotonic behaviour and attenuated with an increase in power-law index tantamount to simulations. The results will serve as a guide to design frictionless micro-channels when employing non-Newtonian liquids.

Thermal behavior of wormlike micelles under turbulent and quiescent regimes

Under turbulent flow, wormlike micelles efficiently cause hydrodynamic drag reduction. However, it was demonstrated that beyond specific temperatures, the capacity of the micellar aggregates to produce drag reduction is abruptly lost. This can be associated with the decrease in size of long micellar chains, becoming shorter aggregates, in the sense that small aggregates are not capable to cause hydrodynamic drag reduction anymore. In this paper, we investigate the thermal effect on wormlike micelles under turbulent and quiescent regimes, in order to explore correlations between the size of the wormlike micelles and the hydrodynamic drag reduction phenomenon.

Wire mask assisted rolling as a cost-effective method for high-throughput surface micro-texturing

Micro-dimples are a specific class of surface textures that are ubiquitous in a wide range of applications where functionalities such as wetting, hydrodynamic drag reduction, and sliding friction reduction are needed. However, fabrication of the micro-dimples through the existing manufacturing techniques involves the use of complex equipment and requires relatively long cycle times. Consequently, we report here proof-of-concept of a method that enables in fabricating micro-dimples in a facile and fast manner. The industrial sieve wire masks were employed to deform metallic sheets at discrete locations by the cold rolling process. The geometry of projections on the wire masks allowed the formation of ellipsoid shaped micro-dimples in two distinct patterns on metallic sheets. The finite element modelling of process-material interactions was accurately predicted the formation of micro-dimples under different reduction ratios. We also demonstrate that the micro-dimples can be easily replicated on polymer surfaces using textured metallic moulds, which facilitates low-cost mass production in microfluidic applications. The presence of micro-dimples on polymer surfaces has led to an increase in water contact angles and resulted in a substantial drag reduction in microchannels. From the tribological viewpoint, the obtained patterns and surface area fractions of the micro-dimples are attractive. These findings provide a promising way to create textured surfaces in an economical manner, with functionalities suitable for microfluidic and tribological devices.

Superhydrophobic Drag Reduction for Turbulent Flows in Open Water

Trapping air under water, superhydrophobic surfaces on boats have long been coveted for hydrodynamic drag reduction without bubble injection. Despite many reports of drag reduction in lab tests, such surfaces have not been successful in real-world tests on open water, at high Reynolds numbers. The authors develop hydrophobic microtrenches that provide a large slip, with a subtle detail that maximizes air retention. Replacing a portion of a motorboat's hull with the developed surface, they obtain large drag reduction under high-speed flows on the open sea. This has significant implications for energy efficiency in oceanic shipping.

Superhydrophobic Surfaces Made from Naturally Derived Hydrophobic Materials

Functional coatings that can achieve stable superhydrophobicity have the potential to significantly enhance a plethora of industrial applications ranging from building environmental control, phase change heat transfer, thermoelectric power generation, and hydrodynamic drag reduction. In order to create superhydrophobic surfaces, scientists have utilized a variety of surface structuring methods in combination with organosilane based alkyl and perfluorinated synthetic chemical coatings. Unfortunately, organosilane based alkyl and perfluorinated chemicals tend to be toxic, flammable, corrosive, difficult to dispose of, and damaging to the environment. Here, we develop two new methods to achieve superhydrophobicity using liquid phase deposition of cinnamic acid or myristic acid, both organic compounds derived from natural sources. By varying the liquid phase solution concentration, we develop deposition methods on scalable copper oxide microstructured surfaces capable of achieving apparent advancing contact a...

Doubly Reentrant Cavities Prevent Catastrophic Wetting Transitions on Intrinsically Wetting Surfaces.

Omniphobic surfaces, that is, which repel all known liquids, have proven of value in applications ranging from membrane distillation to underwater drag reduction. A limitation of currently employed omniphobic surfaces is that they rely on perfluorinated coatings, increasing cost and environmental impact and preventing applications in harsh environments. Thus, there is a keen interest in rendering conventional materials, such as plastics, omniphobic by micro/nanotexturing rather than via chemical makeup, with notable success having been achieved for silica surfaces with doubly reentrant micropillars. However, we found a critical limitation of microtextures comprising pillars that they undergo catastrophic wetting transitions (apparent contact angles, θr → 0° from θr > 90°) in the presence of localized physical damages/defects or on immersion in wetting liquids. In response, a doubly reentrant cavity microtexture is introduced, which can prevent catastrophic wetting transitions in the presence of localized structural damage/defects or on immersion in wetting liquids. Remarkably, our silica surfaces with doubly reentrant cavities could exhibit apparent contact angles, θr ≈ 135° for mineral oil, where the intrinsic contact angle, θo ≈ 20°. Further, when immersed in mineral oil or water, doubly reentrant microtextures in silica (θo ≈ 40° for water) were not penetrated even after several days of investigation. Thus, microtextures comprising doubly reentrant cavities might enable applications of conventional materials without chemical modifications, especially in scenarios that are prone to localized damages or immersion in wetting liquids, for example, hydrodynamic drag reduction and membrane distillation.

Single bubble dynamics on superheated superhydrophobic surfaces

This work explores single bubble dynamics on superheated superhydrophobic surfaces and the corresponding heat transfer mechanism of water pool boiling. The superhydrophobic surfaces are prepared by coating a thin layer of polytetrafluoroethylene on top of silicon substrates with electroless etched silicon nanowires. It is observed that a vapor film covers the whole superhydrophobic surface when it is immersed in water. Once the surface is superheated, single bubble forms and departs from the vapor film. We find that the bubble growth rate shows little dependence on surface superheat and applied heat flux at large superheats due to the presence of the vapor layer, which seriously limits the heat transfer from the superheated superhydrophobic surface. The bubble departure diameter is mainly determined by the static force equilibrium, but local disturbance and surface superheat also play important roles. There is a plateau of bubble departure frequency at large superheats, which is a result of the constant departure velocity limit. Accompanying the continuous bubble formation and departure, a smooth boiling curve is observed. The corresponding heat transfer coefficient (HTC) first increases and then slowly decreases as the superheat increases, showing a peak at a superheat of ∼50K. Such HTC change is explained by the heat flux partitioning model, suggesting latent heat transport due to water vaporization and bulk liquid water convection after bubble departure contribute together to the heat removal on superhydrophobic surfaces. Insights gained from pool boiling on superhydrophobic surfaces might provide design guidelines for new surfaces excellent in heat removal or hydrodynamic drag reduction.

Tailoring of wormlike micelles as hydrodynamic drag reducers for gravel-pack in oil field operations

This paper describes a systematic investigation to verify the potential of wormlike micelles (WLMs) as additives to improve gravel-pack operations. This is an important step of the completion operations, in which a sand core along of an oil well is created. WLMs have high potential to reduce the hydrodynamic drag of sand suspensions during the gravel-pack operation. The capability of WLMs to survive in aqueous solution with high salt content and at the temperature of the reservoir was investigated. The screening of the best formulation was carried out in a rheometer under turbulent conditions. Based on the rheological results the best formulations were used to investigate the levels of hydrodynamic drag reduction (HDR) in circuits. The more appropriated WLMs were formed by the combination of commercial surfactants Ethoquad O/12 with Arquad 12–50 and sodium salicylate (NaSal). We also checked the effect of the presence of WLMs on the transportation of sand grains in conditions close to the ones used in gravel-pack operations.

Drag reducing surface fabrication with deformed sharkskin morphology

The superiority of sharkskin is the consequence of nature selection and self-evolution. The nano/microhierarchical structure covering over sharkskin can match its living surroundings perfectly, whereas the best drag reducing effect cannot be implemented at all circumstances. Therefore, it is necessary to adjust the size and shape of sharkskin morphology to accommodate more potential flowing conditions. In this paper, the stretching deformed fabrication process of sharkskin surface is explored and investigated, and the super-hydrophobic and hydrodynamic drag reduction effect is inspected. The experimental results in the water tunnel indicate that the stretched sharkskin can expand the speed scope of applications with satisfactory drag reduction effect. Additionally, the drag reduction mechanism is explained and derived comprehensively, which has important significance to apprehend sharkskin effect.

The thermal signature of wormlike micelles

The variations in enthalpy (ΔfHWLM) and critical concentrations associated with the formation of wormlike micelles (WLMs) from combinations of tetradecyltrimethylammonium bromide (C14TAB) and various aromatic co-solutes were determined using isothermal titration calorimetry (ITC). Three groups of aromatic molecules were investigated: neutral (phenol), benzoate derivatives and cinnamate derivatives. In addition, the thermal stabilities of the WLMs (of hexadecyltrimethylammonium bromide, C16TAB) and the aromatic co-solutes of the three groups were investigated by measuring the temperatures at which the WLMs break and lose their ability to produce hydrodynamic drag reduction. A comparison of the results was used to establish correlations between the spontaneity of WLMs formation, their thermal stability and the molecular structure of the aromatic co-solutes. A characteristic thermal pattern with four steps was observed when WLMs are formed, that depended on the co-solute structure. Micellar growth was found to be an exothermic process, related to the fusion of the end caps allied with the incorporation of more co-solutes. The co-solutes that had negative charge and were able to maintain planar configuration demonstrated stronger interactions and also showed higher thermal stability through drag reduction.

Experimental Investigations on Manufacturing Different-Shaped Bio-Inspired Drag-Reducing Morphologies and Hydrodynamic Testing

Fascinating sharkskin effect has drawn so many attentions in the recent several decades, and it been becoming more well-known since the Olympic Games in 2000, and there were many athletes wearing sharkskin swimming suit to obtain the good grades. Biological sharkskin is all covered by the tiny and stiff scales, and there are microgrooves sitting on them, which can protrude out of the viscous sublayer and reduce the wall friction. However, sharkskin effect is the imperative consequence of nature selection in a long time, which enable shark fit the living surroundings perfectly, and for expanding its application in more fields, it should be adjusted or deformed. In this research, experimental investigations on manufacturing different-shaped sharkskin morphologies are performed and the hydrodynamic drag reduction testing is conducted, the results show that deformed sharkskin morphologies can improve drag reduction effect, which has important potential applications in fluid engineering.

Improvement of water-repellent and hydrodynamic drag reduction properties on bio-inspired surface and exploring sharkskin effect mechanism

Bio-inspired/biomimetic surface technologies focusing on sharkskin, lotus leaf, gecko feet, and others have attracted so lots of attentions from all over the world; meanwhile, they have also brought great advantages and profits for mankind. Sharkskin drag-reducing/low-resistance surface is the imperative consequence of nature selection and self-evolution in the long history, which can enable itself accommodate the living environments perfectly. Generally speaking, sharkskin effect can become transparent only in some certain velocity scope. How to expand its application range and enhance the drag reduction function further has developed into the urgent issue. In this article, the water-repellent and hydrodynamic drag-reducing effects are improved by adjusting sharkskin texture. The experimental results show that contact angle of more than 150° is achieved, and drag-reducing property is improved to some extent. In addition, the drag-reducing mechanism is explored and generalized from different aspects adopting the numerical simulation, which has important significance to comprehend sharkskin effect.

Skin-friction drag reduction in the turbulent regime using random-textured hydrophobic surfaces

Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.

An experimental investigation into the hydrodynamic drag reduction of a flat plate using air-fed cavities

The Air-Cavity Ship (ACS) concept is currently being researched as one of the more promising concepts for economical forms of marine transportation. An Air-Cavity Ship uses an air injection system to isolate the bottom part of the ship's wetted surface from the water and thereby to reduce the frictional resistance. The air cavities are generated injecting air at the keel level of a vessel as such the cavities can extend along considerable portions of the hull bottom thus isolating it from the effect of the fluid. The main objective of an effective air cavity system is therefore to maximize the cavity length whilst minimising the air supply. Building on the lack of published experimental research, this paper presents a recent fundamental experimental investigation on air-cavities underneath a flat plane tested in the Emerson Cavitation Tunnel of Newcastle University. The experimental investigation uses this simplified model to understand the correlation of cavity length and volume with potential drag reduction due to the air cavity. The main objective of the research was to generate both fundamental and practical knowledge that furthered the understanding of the air-cavity phenomenon to help its potential application in practice. In addition an attempt was also made to correlate the findings from this experimental study with air-cavity theory and results from limited previous experimental studies performed in other facilities.

Bis-Urea-Based Supramolecular Polymer: The First Self-Assembled Drag Reducer for Hydrocarbon Solvents

The hydrodynamic drag reduction phenomenon, also termed the Toms effect, is an unusual case involving macromolecules in solution in which the resistance to flow is reduced comparatively to that of the pure solvent. Although the effect is relatively well characterized, it is still unclear from the molecular viewpoint. The presence of some amount of a polymer with high molecular weight can produce large levels of drag reduction in turbulent flow as a result of the interactions of the long structures with the small vortices developed during the flow. For this reason, the effect is very attractive in the pumping process because a significant amount of energy can be saved. In aqueous systems, giant micelles can be spontaneously formed, driven by the hydrophobic effect, and are effective drag reducers. Giant micelles are interesting in promoting drag reduction because the noncovalent and reversible aggregation of the surfactant molecules avoids mechanical degradation, which typically occurs with classical polymers, due to irreversible scission of the backbone. In this letter, we present the first hydrodynamic drag reducer for hydrocarbons based on a self-assembled polymer formed from the reversible aggregation of bis-urea monomers. This system forms two competitive polymeric structures--the tube (T) and the filament (F) forms--which are in equilibrium with each other. Our rheology results in octane and toluene are fully consistent with calorimetry data and show that only the longest form, T, is able to promote the drag reduction effect.