Surfaces For Drag Reduction Research Articles

Effect of Hydrophilicity/Hydrophobicity of the Injector Wall on Atomization Performance

Hydrophilicity/hydrophobicity is a common physical property of the material. Water droplets roll on lotus leaf, and a lot of dust and dirt on the surface of the lotus leaf will be taken away, playing a certain cleaning role. The hydrophobic surface has drag reduction effect that would produce slip on the hydrophobic wall. There are some studies on hydrophilicity/hydrophobicity in channels, most of which focus on the effect of surface drag reduction and heat transfer on microchannels. However, few people pay attention to the effect of the hydrophilicity/hydrophobicity of the injector inner wall on the atomization performance. In this paper, three groups of the open-end swirl injector with different tangential channels were designed to study the effect of hydrophilicity/hydrophobicity on atomization performance. The hydrophobic coating was prepared and used on the inner wall of the injector, and the atomization experimental system was built. In the experiment, the liquid film thickness was measured using the conductance method. Details of the liquid film breakup and spray development were recorded with a high-speed camera. The average droplet diameter was measured by the Malvern particle size analyzer. The atomization performance of injectors with different tangential channels on the hydrophilicity/hydrophobicity was compared, and the effect of the velocity profile on the jet stability is discussed.

Simulation Study on the Screening of Hydrophobic Surface Materials for Pipeline Drag Reduction Based on Adsorption Properties.

Hydrophobic surface drag reduction techniques are effective in reducing the frictional resistance of fluids, the adsorption of liquid molecules on hydrophobic surfaces can reflect the resistance to fluid flow through such solid surfaces. Based on molecular simulation technology, we investigate the adsorption characteristics of water molecules on hydrophobic surfaces to achieve rapid screening of hydrophobic materials in fire-fighting water supply systems. The Monte Carlo method was used to simulate the adsorption process of polymers and to analyze the effects of temperature and fixed adsorption quantity. Contact angle tests were also done to verify polymer hydrophobicity. The isothermal adsorption heat, water molecule distribution, and energy distribution were studied by molecular mechanics and molecular dynamics methods. Then, adsorption localization simulations and electrostatic potential distributions were used to predict possible adsorption sites on hydrophobic surfaces and single-molecule chains. Finally, the interaction energy, diffusion coefficient, and free volume were investigated to explain the adsorption mechanism at the molecular level. Simulation results show that, overall, PTFE was more hydrophobic and PES was more hydrophilic and at 298 K, the number of adsorbed water molecules was ranked as follows: PTFE < PVDF < PVC < PMMA < PPS < CSM < BD-HDI < BD-MDI < BD-TDI < PES. Furthermore, PTFE, PVDF, PVC, PES, and PPS have more stable adsorption configurations on the (0 -1 0) surface. According to the findings, hydrogen bonding dominates the interaction between water molecules and hydrophilic polymers, whereas π-π interactions increase water molecules' diffusion resistance in polymers with benzene rings. In addition, PES contains many sulfone groups and ether bonds, which disorganize the chain arrangement to provide more free volume, whereas the water adsorption rate of PTFE is reduced because its molecular chains are less convoluted and more organized.

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.

Template-free scalable fabrication of linearly periodic microstructures by controlling ribbing defects phenomenon during forward roll coating

Linear periodic microstructures are of significant importance in various applications, including drag-reduction, biofouling, self-cleaning, and superhydrophobicity. However, practical applications of such surfaces require mass manufacturing techniques, which are highly limited. This study demonstrated a simple template-free scalable manufacturing technique to fabricate linearly periodic microstructure by controlling the ribbing defects in forward roll coating. A viscoelastic polymer nanocomposite with tailored properties was synthesized and utilized as the coating material. The ribbing instabilities were controlled with process parameters that resulted in variable periodicity of the linearly aligned microstructures with a spacing of 114 – 700 µm. The microstructure arrangement also had a linear to random transition as the instabilities increased. The manufactured surface has a high Wenzel roughness factor that ranges from 1.6 to 3.6, which results in water contact angles of 128o to 150o. The linear microstructure films can have critical applications in the mass manufacturing of drag reduction surfaces. The high aspect-ratio microstructure films can also have applications in superhydrophobic, self-cleaning, anti-icing, and anti-biofouling surfaces.

Coupled Bionic Drag-Reducing Surface Covered by Conical Protrusions and Elastic Layer Inspired from Pufferfish Skin

Inspired by the drag-reducing properties of the cone-like spines and elastic layer covering the pufferfish skin, important efforts are underway to establish rational multiple drag-reducing strategies for the development of new marine engineering materials. In the present work, a new drag-reducing surface (CPES) covered by conical protrusions (sparse "k-type" with rough height k+ = 13-15) and an elastic layer are constructed on copper substrate via a hybrid method, combining the sintering and coating processes. The drag-reducing feature of the prepared CPES biomimetic surface is achieved by rheometer and particle image velocimetry (PIV) experiments. To comprehensively investigate its drag reduction mechanism, the porous copper substrate (PCS), copper substrate (CS), conical protrusion resin substrate (CPRS), and conical protrusion porous copper substrate (CPPCS) were used for a comparative analysis. In laminar flow, we discovered that the conical protrusion structure and wettability of the elastic surface coupling affect the CPES sample's drag-reducing performance (7-8%) and that the interface produced slip to reduce the viscous drag. In turbulent flow, the CPES biomimetic surface exhibits an 11.5-17.5% drag-reducing performance. Such behavior was enabled by two concurrent mechanisms: (i) The conical protrusions as vortex generators enhance the number of vortices and the wake effect, enabling faster movement of downstream strips, reducing viscous drag; (ii) The conical protrusion elements break and lift large-scale vortices to produce numerous small-scale vortices with low energy, effectively weakening perturbations and momentum exchange. Additionally, the elastic layer shows high adhesion and stability on copper substrate after sandpaper abrasion and water-flow erosion tests. The copper substrate surface formed by the sintering method is also covered with dense porous structures, which gives the elastic layer and conical protrusions excellent combined robustness. Our findings not only shed new light on the design of robust drag-reducing surfaces but also provide new avenues for underwater drag reduction in the field of marine applications.

Bioinspired drag reduction surfaces via triple lithography method based on three-layer hybrid masks

Drag reduction is a significant challenge for many industries, such as ships, pipelines, aircraft, energy, and transportation. Multilayer hierarchical microstructures can inhibit the development of vortices near the wall, which is beneficial to drag reduction. However, existing methods have difficulty performing the controlled fabrication of complex multilayer hierarchical microstructure arrays. Here, a novel triple lithography method based on three-layer hybrid masks is proposed for the controlled fabrication of three-dimensional multilayer hierarchical microstructure surfaces. The capability of the proposed process is verified by the multilayer hierarchical microstructures. In the fabrication process, a special lithography sequence is designed based on the hybrid mask materials. The drag reduction ability of the multilayer hierarchical microstructures is investigated in a closed air channel measurement system. The experimental results demonstrate that the fabricated multilayer hierarchical microstructures exhibit significant drag reduction ability under certain conditions. Conceptual models based on the fluid-solid coupling interface interaction are proposed to explain the drag reduction mechanism of multilayer hierarchical microstructures. The proposed fabrication method provides a powerful means for practical engineering applications of various bioinspired functional surfaces, such as drag reduction, anti-icing, antifouling, self-cleaning, and superhydrophobic surfaces.

Structured porous surface for drag reduction and wake attenuation of cylinder flow

In the current study, a new passive flow control strategy characterized by 3D printing structured porous surface was introduced and experimentally investigated to manipulate the dynamic wake evolution and flow characteristics around a circular cylinder at a Reynolds number of 2.02×104. With the 3D printing structured porous surface, parts of the incoming flow could go through the cylinder and communicate with the wake flow spontaneously. As a result, the vortex shedding was suppressed and the drag was reduced. From POD analysis results, we observed shifted modes and decreased vorticity distribution, indicating that the periodic vortex shedding process was weakened. This was also verified by the analysis of dynamic wake evolution and by the power spectrum analysis. In addition, experimental results showed that the time-averaged turbulent kinetic energy (TKE) and the Reynolds shear stress (RSS) distributions were restrained in the wake of the porous cylinder. Moreover, the mean estimated drag coefficients witnessed a significant decrease up to 49.47%.

Improved Stable Drag Reduction of Controllable Laser-Patterned Superwetting Surfaces Containing Bioinspired Micro/Nanostructured Arrays.

Superwetting surfaces are widely used in many engineering fields for reducing energy and resistance loss. A facile and efficient method using laser etching has been used to fabricate and control superwettable drag reduction surfaces. Inspired by the self-cleaning theory of lotus leaves, we propose controllable patterned bionic superhydrophobic surfaces (BSSs) simulating the uneven micro/nanostructures of lotus leaves. The superhydrophobicity and drag reduction ratios at low velocities are highly improved using a laser ablation method on metal substrates. However, unstable air layers trapped on superhydrophobic surfaces are usually cut away by a high-velocity flow, which greatly reduces the drag reduction performance. The fabricated bionic superhydrophobic/hydrophilic surfaces (BSHSs) with alternated hydrophilic strips can build a large surface energy barrier to bind the three-phase contact line. It maintains the stable drag reduction by capturing the air bubbles attached to the hydrophilic strips at a high velocity. Three-dimensional simulation analysis and equipment to measure the weak friction of a self-assembled solid–liquid interface are used to explain the drag reduction mechanism and measure the drag reduction ratios at different flow speeds. BSSs achieve an improved drag reduction effect (maximum 52.76%) at a low velocity (maximum 1.5568 m/s). BSHSs maintain an improved and steady drag reduction effect at high speed. The drag reduction ratios can be maintained at about 30% at high speed, with a maximum value of 4.448 m/s. This research has broad application prospects in energy saving, liquid directional transportation, and shipping due to their robust superhydrophobic properties and stable drag reduction effect.

Bionic Nonsmooth Drag Reduction Mathematical Model Construction and Subsoiling Verification.

In this study, a bionic nonsmooth drag-reducing surface design method was proposed; a mathematical model was developed to obtain the relationship between the altitude of the nonsmooth drag-reducing surface bulges and the spacing of two bulges, as well as the speed of movement, based on which two subsoiler shovel tips were designed and verified on field experiments. The mechanism of nonsmooth surface drag reduction in soil was analyzed, inspired by the efficient digging patterns of antlions. The nonsmooth surface morphology of the antlion was acquired by scanning electron microscopy, and a movement model of the nonsmooth surface in soil was developed, deriving that the altitude of the nonsmooth drag-reducing surface bulge is proportional to the square of the distance between two bulges and inversely proportional to the square of the movement speed. A flat subsoiler shovel tip and a curved tip were designed by applying this model, and the smooth subsoiler shovel tips and the pangolin scale bionic tips were used as controls, respectively. The effect of the model-designed subsoilers on drag reduction was verified by subsoiling experiments in the field. The results showed that the resistance of the model-designed curved subsoiler was the lowest, the resistance of the pangolin scale bionic subsoiler was moderate, and the resistance of the smooth surface subsoiler was the highest; the resistance of the curved subsoiler was less than the flat subsoilers; the resistance reduction rate of the model-designed curved subsoiler was 24.6% to 33.7% at different depths. The nonsmooth drag reduction model established in this study can be applied not only to the design of subsoilers but also to the design of nonsmooth drag reduction surfaces of other soil contacting parts.

Towards Rapid Fabrication of Superhydrophobic Surfaces by Multi-Beam Nanostructuring with 40,401 Beams.

Superhydrophobic surfaces attract a lot of attention due to many potential applications including anti-icing, anti-corrosion, self-cleaning or drag-reduction surfaces. Despite a list of attractive applications of superhydrophobic surfaces and demonstrated capability of lasers to produce them, the speed of laser micro and nanostructuring is still low with respect to many industry standards. Up-to-now, most promising multi-beam solutions can improve processing speed a hundred to a thousand times. However, productive and efficient utilization of a new generation of kW-class ultrashort pulsed lasers for precise nanostructuring requires a much higher number of beams. In this work, we introduce a unique combination of high-energy pulsed ultrashort laser system delivering up to 20 mJ at 1030 nm in 1.7 ps and novel Diffractive Laser-Induced Texturing element (DLITe) capable of producing 201 × 201 sub-beams of 5 µm in diameter on a square area of 1 mm2. Simultaneous nanostructuring with 40,401 sub-beams resulted in a matrix of microcraters covered by nanogratings and ripples with periodicity below 470 nm and 720 nm, respectively. The processed area demonstrated hydrophobic to superhydrophobic properties with a maximum contact angle of 153°.

Focus on Bioinspired Textured Surfaces toward Fluid Drag Reduction: Recent Progresses and Challenges

After a long period of biological evolution, natural creatures will inevitably evolve body surfaces suitable for the living environment. The functions of natural biological surfaces are abundant and powerful, including antiadhesion, self‐cleaning, drag reduction, anti‐icing, wear resistance, and other characteristics. In the context of coping with the energy crisis, inspired by natural biological surface microstructures, researchers explore biomimetic surface microstructures that reduce fluid drag and investigate the mechanisms of drag reduction. Herein, mainly the current state of research on biomimetic textured surfaces that can be applied to fluid drag reduction is reviewed and the microstructures’ morphology, drag reduction mechanisms, and the fabrication techniques of textured surfaces are discussed in detail. In particular, the experimental methods for measuring the drag reduction effect of textured surfaces are introduced, which is extremely rare. The article concludes with a summary of existing problems and future directions in the research of biomimetic surface drag reduction technology. This Review can provide a reference for practical application and laboratory research of drag reduction in marine transportation, military weapons, aviation, and pipeline systems.

Multifunctional biopolymer coatings inspired by loach skin

Anti-biofouling surfaces are very important owing to their significant roles in microfluidic devices, biosensors and biomedical devices. However, traditional anti-biofouling surfaces could contaminate the environment. Thus, the development of environment-friendly coatings is an essential undertaking in efforts to resolve the problems associated with conventional anti-biofouling surfaces. Herein, a novel strategy inspired by the slippery surface of loach skin is proposed for the rational design of anti-biofouling surfaces. In this strategy, hydrophilic biopolymers, including chitosan, carboxymethyl cellulose, mPEG-amine (MW, 10,000 Da) and alginate, are grafted on a porous polycaprolactone (PCL) membrane. The porous PCL surface is a biomimetic surface inspired by the skin surface of a loach that secretes mucus. Previously developed antifouling surfaces were often toxic or the coating substances were easily released to the outside, and their drag reduction effects were not examined. The developed coating surface is not toxic and the coating material is not depleted to the outside. The resultant covalent biopolymer-coated surfaces (BCSs) exhibit excellent hydrophilic property and drag reduction effect in water. Especially, the additional coating of mPEG-amine on the alginate-coated surface exhibits the best drag reduction performance. In addition, the BCSs show superior anti-biofouling performance by resisting the adhesion of bacteria (Escherichia coli and Maribacter dokdonensis) and NIH3T3 fibroblasts. The proposed covalent biopolymer coatings could be potentially utilised as eco-friendly surfaces for drag reduction and anti-biofouling.

Which is the optimal surface for drag reduction?

Which is the optimal surface for drag reduction?

Model Design and Processing of Bionic Non-smooth Surface of Fish-scale Concavity

Based on the basic methods of engineering bionics research, this paper is inspired by the desert surface morphology on the basis of the existing drag reduction microstructures, and extracts the characteristics of ecological surface drag reduction information, thereby establishing a fish-scale pit model. This paper designs and builds a diamond stamping platform to stamp and stamp copper sheets, and proposes a method that can effectively process the non-smooth surface of the fish-scale pits of the required size.

Dual-composite drag-reduction surface based on the multilayered structure and mechanical properties of tuna skin.

Energy efficiency and friction reduction have attracted considerable research attention. To design low drag surfaces, researchers derived inspiration from nature on various types of drag reduction methods with exceptional functional surfaces, such as fish skin that possesses low friction. Fishes with high-performance swimming possess a range of physiological and mechanical adaptations that are of considerable interest to physiologists, ecologists, and engineers. Although tuna is a fast-swimming ocean-based predator, most people focus their attention on its nutritional value. In this study, the multilayered structures and mechanical properties of tuna skin are first analyzed, and then the drag-reduction effect of the bionic fish-scale and dual-composite surfaces are studied based on the computational fluid dynamics method. The results indicate that tuna skin is composed of five layers, with the fish scale covered by a flexible epidermis layer. According to the uniaxial tension results, the modulus and tensile strength of the epidermis are obtained as 1.17 and 20 MPa, respectively. The nanoindentation results show that the modulus and hardness of the outer surface of the fish scale are larger than that of the inner surface, while those of the dry state are larger than those of the hydrated state. The simulation results show that both the bionic fish-scale and dual-composite surfaces display drag reduction, with the maximum drag-reduction rate of 25.7% achieved by the bionic dual-composite surface. These findings can offer a reference for in-depth performance analysis of the hydrodynamics of tuna and provide new sources of inspiration for drag reduction and antifouling.

Investigation of the Turbulent Drag Reduction Mechanism of a Kind of Microstructure on Riblet Surface.

With the k-ε renormalization group turbulence model, the drag reduction mechanism of three- dimensional spherical crown microstructure of different protruding heights distributing on the groove surface was studied in this paper. These spherical crown microstructures were divided into two categories according to the positive and negative of protruding height. The positive spherical crown micro-structures can destroy a large number of vortexes on the groove surface, which increases relative friction between water flow and the groove surface. With decreasing the vertical height of the spherical crown microstructure, the number of rupture vortexes gradually decreases. Due to the still water area causes by the blocking effect of the spherical crown microstructure, it was found that the shear stress on the groove surface can be reduced, which can form the entire drag reduction state. In another case, the spherical crown microstructures protrude in the negative direction, vortexes can be generated inside the spherical crown, it was found that these vortexes can effectively reduce the resistance in terms of pressure and friction. In a small volume, it was shown that the surface drag reduction rate of spherical crown microstructures protrudes in negative directions can be the same as high as 24.8%.

One-step method using laser for large-scale preparation of bionic superhydrophobic & drag-reducing fish-scale surface

In this study, four bionic fish-scale models were extracted and produced based on the microscopic morphology and the arrangement of scales on the body of Sciaenops ocellatus. A simple, quick and low-cost method using laser was introduced for fabricating the bionic fish-scale surface on aluminum (Al) alloy. Under the action of laser beams, the fish-scale morphology with inclined surface was obtained by controlling the density of laser path and processing times. Biomimetic fish-scale surfaces with different sizes were produced through adjusting the four ratios of fish scale graph. Then wettability test was performed on different samples. Our results showed that all the samples were hydrophobic and the sample with 100% zoom ratio demonstrated excellent superhydrophobicity and low adhesion. Furthermore, we tested the drag reduction performance of the sample with zoom ratio of 100%. The experimental results indicated that under different flow conditions, the sample exhibited different performance values in drag reduction, and the drag reduction rate could reach 4.814% under laminar conditions. This study provided a simple method from the perspective of bionics to fabricate a bionic superhydrophobic surface with drag reduction performance, providing positive guidance for the fabrication of underwater drag-reducing surface.

Thriving artificial underwater drag-reduction materials inspired from aquatic animals: progresses and challenges.

In the past decades, drag-reduction surfaces have attracted more and more attention due to their potentiality and wide applications in various fields such as traffic, energy transportation, agriculture, textile industry, and military. However, there are still some drag-reduction materials that need to be deeply explored. Fortunately, natural creatures always have the best properties after long-term evolution; aquatic organisms have diversified surface microstructures and drag-reducing materials, which provide design templates for the development of thriving artificial underwater drag-reduction materials. Aquatic animals are tamed by the current while fighting against the water, and thus have excellent drag reduction that is unparalleled in water. Inspired by biological principles, using aquatic animals as a bionic object to develop and reduce frictional resistance in fluids has attracted more attention in the past few years. More and more aquatic animals bring new inspiration for drag-reduction surfaces and a tremendous amount of research effort has been put into the study of surface drag-reduction, with an aim to seek the surface structure with the best drag-reduction effect and explore the drag-reduction mechanism. This present paper reviews the research on drag-reduction surfaces inspired by aquatic animals, including sharks, dolphins, and other aquatic animals. Aquatic animals as bionic objects are described in detail, with a discussion on the drag-reduction mechanism and drag-reduction effect to understand the development of underwater drag-reduction fully. In bionic manufacturing, the effective combination of various preparation methods is summarized. Moreover, bionic surfaces are briefly explained in terms of traffic, energy sources, sports, and agriculture. In the end, both existing problems in bionic research and future research prospects are proposed. This paper may provide a better and more comprehensive understanding of the current research status of aquatic animals-inspired drag reduction.

Effects of surface morphological structure of a brown alga miyeok (Undaria pinnatifida) on sustainable drag reduction

Liquid-infused surfaces are a promising passive technique for frictional drag reduction. However, their practical application is limited by the easy depletion of the lubricant infused in the surface by external forces. Miyeok contains mucus on its surface, which induces a sustainable slippery surface under flow conditions. In this study, the effect of the surface morphological structures of miyeok on slippery characteristics was examined. The surface of miyeok has bump-like microstructures and tiny pores of mucilage gland cells with a re-entrant inlet configuration. A replica model of the miyeok surface was fabricated by adopting a molding process using polydimethylsiloxane. The miyeok replica model was dipped into a silicone oil solution to obtain a slippery surface property. The surface structures of fresh miyeok and the replica were checked by comparing their scanning electron microscopy images. A replica of a kelp surface was also fabricated by employing the same method. The heights of the surface microstructures of the miyeok replica were more uniform than those of the kelp replica. The sustainable drag reduction effect of the replica models’ surfaces was assessed by a sessile drop method. The liquid-infused miyeok replica surface (LIMRS) showed better sustainability than other surfaces. The slippery properties of the LIMRS were examined by measuring slip length and pressure drop. The drag reduction rates measured by the two methods were approximately 24% and 19%. The present results would be helpful for understanding the effects of the surface structure of natural miyeok on sustainable drag reduction and developing a biomimetic sustainable drag-reducing surface.

Airfoil profile surface drag reduction characteristics based on the structure of the mantis shrimp abdominal segment

To reduce the drag of fluid on a surface, the parameterized structure of the mantis shrimp abdominal segment was analyzed to design the airfoil profile, and a calculation model was established based on the biological characteristics of the mantis shrimp. By using a numerical simulation method, the change in the viscous sublayer and drag reduction of the biological surface under different flow velocities and with different numbers of airfoil profiles was analyzed, and the mechanism of the drag reduction of the biological airfoil profile was revealed. The results show that the biological surface with airfoil profiles exhibited a better drag reduction effect. With an increase in airfoil profile numbers, the drag reduction rate first decreased and then increased. When the flow velocity was 10 m/s and the number of airfoil profiles was 5, the drag reduction rate exhibited the largest value of 15.33%. This result showed that the biological airfoil profile could affect the structure of the wall turbulent boundary layer and diminish the velocity gradient of the boundary layer, which finally changed the interaction pattern between the fluid and wall. In addition, the vortex cushioning effect was created to change the friction between the fluid and the wall into a rolling friction with a lower friction coefficient, thus reducing the shear stress on the wall, which achieved drag reduction.