Surfaces For Drag Reduction Research Articles

Analysis of Surface Drag Reduction Characteristics of Non-Smooth Jet Coupled Structures

To enhance the service life of shipping equipment and minimize surface wear, this study employs biomimetic principles, integrating fitted structures with jet dynamics to model three configurations: non-smooth structures, single jet structures, and non-smooth jet-coupled structures. We utilized the SST k-ω turbulence model for numerical simulations to investigate the drag reduction characteristics of these structural models. By varying the jet angle and speed, we analyzed the changes in viscous resistance, pressure differential resistance, and drag reduction rates at the wall surface. Furthermore, the mechanisms of compressive stress, velocity fields, vortex structures, and shear stress on drag-reducing surfaces were elucidated, revealing how these factors contribute to drag reduction in non-smooth jet-coupled structures. The results indicate that the non-smooth jet-coupled structure exhibits superior drag reduction performance at a main flow field velocity of 20 m/s. As the jet velocity increases, the viscous drag on the surface of the non-smooth jet-coupled structure decreases, while the pressure differential drag increases. Conversely, variations in the jet angle have a minimal effect on viscous drag but lead to a reduction in pressure differential drag. Specifically, when the jet velocity is set at 1 m/s, and the jet angle is 60°, the drag reduction achieved by the non-smooth jet-coupled structure peaks at 7.48%. Additionally, the non-smooth jet-coupled structure features a larger area characterized by low shear stress, along with an increased boundary layer thickness at the bottom; this configuration effectively reduces surface velocity and consequent viscous drag.

A strategy of hob-grinding the cylindrical riblet surface for drag reduction by the grinding wheel with ordered abrasive pattern

A strategy of hob-grinding the cylindrical riblet surface for drag reduction by the grinding wheel with ordered abrasive pattern

A laser-induced superhydrophobic surface with multiple microstructures for stable drag reduction

To address the issue of water resistance in ship navigation, we developed a drag-reducing surface based on the microstructure of shark skin and a superhydrophobic coating. First, grooves with various microstructures were fabricated on the substrate surface using laser processing. Then, a mixture of octadecyltrichlorosilane, water, and silica solution was uniformly sprayed onto the grooved substrate surface to achieve superhydrophobicity. To enhance and stabilize the drag reduction effect, we optimized the width and spacing of the grooves by adjusting the laser processing parameters, achieving a high drag reduction rate under various flow conditions. In the Reynolds number range of 2 × 105 to 5 × 105, the drag reduction rate of the superhydrophobic coated surface with grooves reached over 66.57 %, which is an improvement of 7 % to 26.51 % compared to the superhydrophobic coated surface without grooves. Additionally, the oxides accumulated on the ribs after laser ablation can slow down the wear of the superhydrophobic coating, significantly enhancing the surface's mechanical durability. This simple and cost-effective surface provides a new approach for laser preparation of drag-reducing surfaces.

Laser Ablating Biomimetic Periodic Array Fish Scale Surface for Drag Reduction.

Reducing resistance to surface friction is challenging in the field of engineering. Natural biological systems have evolved unique functional surfaces or special physiological functions to adapt to their complex environments over centuries. Among these biological wonders, fish, one of the oldest in the vertebrate group, have garnered attention due to their exceptional fluid dynamics capabilities. Fish skin has inspired innovation in reducing surface friction due to its unique structures and material properties. Herein, drawing inspiration from the unique properties of fish scales, a periodic array of fish scales was fabricated by laser ablation on a polished aluminum template. The morphology of the biomimetic fish scale surface was characterized using scanning electron microscopy and a white-light interfering profilometer. Drag reduction performance was measured in a closed circulating water tunnel. The maximum drag reduction was 10.26% at a Reynolds number of 39,532, and the drag reduction performance gradually decreased with an increase in the distance between fish scales. The mechanism of the biomimetic drag reduction surface was analyzed using computational fluid dynamics. Streamwise vortices were generated at the valley of the biomimetic fish scale, replacing sliding friction with rolling friction. These results are expected to provide a foundation for in-depth analysis of the hydrodynamic performance of fish and serve as new inspiration for drag reduction and antifouling.

An experimental investigation into the drag reduction performance of dimpled plates in a fully turbulent channel flow

Dimpled surfaces have gained increasing attention in recent years for their potential to reduce turbulent skin friction, a capability previously acknowledged for its beneficial implications on heat transfer. As a passive drag reduction method, dimpled surfaces offer significant advantages for marine applications due to their effectiveness and practical applicability. However, despite numerous studies, conflicting opinions and inconsistent drag reduction rates persist in the literature. This paper addresses these ambiguities and offers valuable insights into the effectiveness of dimpled surfaces for drag reduction in fully turbulent flows. We conducted an extensive experimental investigation involving various dimple configurations, including different depth-to-diameter ratios, diameters and orientations, utilising a specialised Fully Turbulent Flow Channel facility and a Particle Image Velocimetry system. Our findings demonstrated that circular dimple geometries, particularly those with low depth ratios, can achieve significant drag reduction of up to 27% as the Reynolds number increases. These results highlight the substantial potential of dimpled surfaces for improving energy efficiency in marine applications, where skin friction accounts for a significant portion of the total drag experienced by large vessels.

Preparation and performance study of highly durable silicone rubber superhydrophobic surfaces

Superhydrophobic coatings are widely applied in aerospace and other fields to address issues such as surface drag reduction, anti-icing, and corrosion resistance. Due to the poor durability and unstable performance encountered in the application of most superhydrophobic coatings, we have combined the template method with the spray method to prepare a superhydrophobic surface with excellent durability. Experimental results demonstrate that the contact angle and sliding angle of the prepared surface reached 160.9° and 2.5°, respectively. Importantly, even after undergoing tests such as high-load sandpaper abrasion, tape peeling, and high-speed steel ball impact, the surface maintained outstanding hydrophobic performance. Furthermore, the surface exhibited not only excellent chemical corrosion resistance and self-cleaning effects but also, after experiencing high-load mechanical wear, extended the ice formation delay time to 201 seconds compared to a silicone rubber surface. This study employed a simple and environmentally friendly preparation process, providing robust guidance for the theoretical analysis and practical operation of designing and preparing superhydrophobic surfaces with outstanding durability.

Optimization of Surface Drag Reduction Attribute of Non-Newtonian Nanofluids Flow Driven by Magnetic Dipole Enabled Curved Sheet

Optimization of Surface Drag Reduction Attribute of Non-Newtonian Nanofluids Flow Driven by Magnetic Dipole Enabled Curved Sheet

Taking inspiration from dolphins to develop drag-reducing surface structures

Understanding the effects of transverse ridges can lead to more efficient ships and underwater turbines.

Numerical simulation study on the drag reduction characteristics of grooves-microbubbles coupling surfaces

Surface drag reduction technologies can significantly reduce the resistance during ship navigation, enhancing speed, efficiency and adaptability under various operating conditions. This paper uses numerical simulation technology to analyze the drag reduction characteristics of grooved and grooves-microbubbles coupling surface, focusing on the effects of groove width, gas flow rate, and liquid flow velocity on the drag reduction performance. The research results indicate that the grooved surface is suitable for full surface drag reduction at velocity below 3 m/s with a maximum drag reduction rate of 4.02%. Microbubbles can greatly improve the drag reduction effect of the grooved surface, and the drag reduction effect of the coupling surface gradually increases with the gas flow rate increases. The maximum drag reduction rate can reach 89.86% at the gas inlet velocity of 1 m/s. The liquid flow velocity has a significant impact on the drag reduction. In both the groove model and the coupling model, the drag reduction rate initially rises and then declines with the liquid flow velocity increases.

Drag reduction using bionic groove surface for underwater vehicles.

Introduction: The reduction of drag is a crucial concern within the shipping industry as it directly influences energy consumption. This study addresses this issue by proposing a novel approach inspired by the unique ridge structure found on killer whale skin. The objective is to develop a non-smooth surface drag reduction method that can effectively decrease drag and improve energy efficiency for ships. Methods: The study introduces a technique involving the creation of transverse bionic groove surfaces modeled after the killer whale skin's ridge structure. These grooves are aligned perpendicular to the flow direction and are intended to modify the behavior of turbulent boundary layer flows that form around the ship's hull. Numerical simulations are employed using the Shear Stress Transport k-ω model to analyze the effects of the proposed groove surface across a wide range of flow conditions. The research investigates the impact of various parameters, such as the width-to-depth ratio (λ/A), groove depth, and inlet velocity, on the drag reduction performance of the bionic groove surface. Results: The study reveals several key findings. Optimal shape parameters for the bionic groove surface are determined, enabling the most effective drag reduction. The numerical simulations demonstrate that the proposed groove surface yields notable drag reduction benefits within the velocity range of 2∼12m/s. Specifically, the friction drag reduction ratio is measured at 26.91%, and the total drag reduction ratio at 9.63%. These reductions signify a substantial decrease in the forces opposing the ship's movement through water, leading to enhanced energy efficiency. Discussion: Comparative analysis is conducted between the performance of the bionic groove surface and that of a smooth surface. This investigation involves the examination of velocity gradient, streamwise mean velocity, and turbulent intensity. The results indicate that the bionic groove structure effectively mitigates viscous stress and Reynolds stress, which in turn reduces friction drag. This reduction in drag is attributed to the alteration in flow behavior induced by the non-smooth surface. Conclusion: The study proposes a novel approach for drag reduction in the shipping industry by emulating the ridge structure of killer whale skin. The transverse bionic groove surface, aligned perpendicular to flow direction, demonstrates promising drag reduction outcomes across diverse flow conditions. Through systematic numerical simulations and analysis of key parameters, the research provides insights into the drag reduction mechanism and identifies optimal design parameters for the groove surface. The potential for significant energy savings and improved fuel efficiency in maritime transportation underscores the practical significance of this research.

Revolutionary surfaces: boosting efficiency in aerospace and turbomachinery

Revolutionary surfaces: boosting efficiency in aerospace and turbomachinery ReSiSTant targets the optimisation of two industrial pilot lines by using micro and nanostructured surfaces for drag reduction. The objectives are to implement newly developed surfaces into aircraft turbofan engines and industrial compressors. The positive effects of using such surfaces could give benefits in terms of efficiency, CO2 and noise emission reduction, and a positive economic and ecological impact.

Study on surface fabrication and drag reduction performances of gradient hydrophobic aluminum alloy

An underwater superhydrophobic (SH) surface drag reduction is an effective drag reduction method. However, drag reduction failure is likely to occur on the SH surface at high water velocity. To solve this problem, we designed an SH and hydrophilic interphase (SH-H) and gradient SH-H (GSH-H) structure on the surface of aluminum alloy by laser etching and micro-electric discharge machining (EDM) methods. The laser scanning interval of the SH segment was 10 μm while that of the hydrophobic segment was 250 μm. The hydrophilic segment was cut by micro-EDM without laser processing. The SH and hydrophobic segments were treated with fluor silane to obtain surfaces with low surface energy. In this work, the flow field characteristics of the SH-H and GSH-H model with different Re were simulated by setting a sliding wall for numerical calculation. the drag reduction ability of samples for varying Reynold number (Re) were studied in pressure difference drag-reduction channel. The simulation results show that the SH-H structure can still seal a thin gas film at Re = 4000, and has a good drag reduction effect. The experiment results show that the drag reduction on the SH-H surface was up to about 45.45%. Moreover, when soaking in water for 30 min and Re = 3500, the sample surface still had the drag reduction effect, and the prepared SH surface had good mechanical stability. It is expected that this research can provide reference and ideas for solving the problem of gas film loss on the SH surface.

Antithrombotic and Flow Drag‐Reducing Material for Blood‐Contacting Medical Devices

AbstractBlood‐contacting medical devices are often associated with shear‐induced and contact activation thrombosis. Superhydrophobic materials have shown promise to reduce flow drag forces, but not contact activation. Here, a strategy of selectively grafting a potent anti‐thrombin compound on the tips of the surface microstructures of a superhydrophobic polytetrafluoroethylene foam, is presented, to concurrently achieve drag reduction and anti‐thrombosis. This work shows that two grafting approaches – Argon plasma or piranha solution treatment – followed by covalent cross‐linking can successfully graft the drug to the outer tips of the foam and provide anti‐thrombotic functionality. By avoiding grafting to the inner regions of the foam, the surface's drag reduction properties can be retained. The functional durability of the grafted surfaces is evaluated by strong water jetting, which demonstrates that the plasma approach can withstand substantial fluid shearing but not the piranha approach, although the plasma approach involves stronger compromise to the drag reduction capabilities. As the proposed selective grafting strategy is applied to a bulk foam, it can be complemented with a previously proposed strategy of supplying air pressure to the foam pores to bolster resistance to fluid impalement and plastron dissolution, allowing the material to be used in medical devices with high fluid pressures.

Wettability State Transition and Interfacial Slip Analysis of the Hydrophobic Nanostructure Surface Controlled by an Electric Field

Superhydrophobicity is widely used for reducing drag in pipeline transportation and preventing corrosion of metals. Interfacial slip is the leading factor of hydrophobic surface drag reduction. The presence of a gas–liquid interface is determined by the wetting state of a liquid on the surface. Therefore, investigating the influence of the wetting state on the interfacial slip is essential. In this study, an electric field was used to control the wetting state of a hydrophobic surface. Microchannel and micro-particle image velocimetry technologies were simultaneously used to investigate the effect of different wetting states on interfacial slip characteristics and to explore the mechanism of influence of these wetting states. As the performance changed from the Cassie contact state to the Wenzel contact state, the gas–liquid contact interface gradually moved downward and the interface slip weakened. The Wenzel contact state was achieved by filling the liquid to the bottom of the structure, and the gas–liquid interface and the interface slip disappeared. Consequently, the hydrophobic surface lost the drag reduction effect.

Drag reduction performance and mechanism of flexible conical microstructure film inspired by pufferfish epidermis

Flexible bionic drag reduction surfaces have attracted increasing attention in recent years owing to its potential to address the daunting energy crisis. Inspired by the skin of pufferfish, a flexible conical microstructure film with effective drag reduction is designed. The film has the advantages of cost-effective and tensile-bending resistance to meet commercial applications. The low price of raw materials and rapid solidification at room temperature are the core of cost reduction, and the manufactured film can withstand a maximum tensile force of 70 N and a strain of 484%. The practical application effect of the film has been confirmed by model ship tests, and it still has stable drag reduction ability after multiple stretching and bending. The water tunnel experiment results show that the drag-reducing efficiency of the flexible conical microstructure film is 10.65%–12.04% compared to the rigid plane, in which the conical topography suppresses turbulence burst, whereas the flexible surface has the effect of absorbing turbulent energy to buffer the near-wall fluid. The coupling effect enhances drag-reducing efficiency. Therefore, the flexible conical microstructure film in this work provides a practical and commercialized fluid drag reduction approach for ships, underwater vehicles, and pipeline transportation.

Antifouling activity of superhydrophobic PDMS/hydrophobic silica coating

ABSTRACT This paper assessed the antifouling activities of a superhydrophobic surface with a contact angle of 161.6° and a sliding angle of 4.1° which can be employed on large scales. The preparation of PDMS/silica nanoparticles composite was first described for the production of superhydrophobic surfaces with different contents of silica nanoparticles. Then, the surface roughness of the produced samples was investigated. Subsequently, the surface reaction against the algae was evaluated at different immersion times. The samples coated with PDMS-to-Silica ratio of 100% exhibited higher stability against algal growth for more than 10 h while samples with a coating ratio of 25% withstood algae for less than 2 h, indicating the influence of surface roughness. Although the withstanding time of this coating cannot compete with the commercial antifouling coatings, it becomes vital when it comes to the use of this coating as a drag-reducing surface in which fouling worsens the effectiveness.

Effect of anisotropic resistance characteristic on boundary-layer transitional flow

It is observed that the feather surface exhibits anisotropic resistances for the streamwise and spanwise flows. To obtain a qualitative understanding about the effect of this anisotropic resistance feature of surface on the boundary-layer transitional flow over a flat plate, a simple phenomenological model for the anisotropic resistance is established in this paper. By means of the large eddy simulation (LES) with high-order accurate finite difference method, the numerical investigations are conducted. The numerical results show that with the spanwise resistance hindering the formation of vortexes, the transition from laminar flow to turbulent flow can be delayed, and turbulence is weakened when the flow becomes fully turbulent, which leads to significant drag reduction for the plate. On the contrary, the streamwise resistance renders the flow less stable, which leads to the earlier transition and enhances turbulence in the turbulent region, causing a drag increase for the plate. Thus, it is indicated that a surface with large resistance for spanwise flow and small resistance for streamwise flow can achieve significant drag reduction. The present results highlight the anisotropic resistance characteristic near the feather surface for drag reduction, and shed a light on the study of bird’s efficient flight.

Hairy superhydrophobic surfaces with excellent mechanical robustness, underwater stability and drag-reduction property

Since superhydrophobic surfaces show extremely low friction with water, they have been tried for drag-reduction purpose for ships or pipe systems for liquid transportation in recent years. However, the superhydrophobic surfaces combining comprehensive performance, feasible fabrication method as well as low manufacturing cost are still big issues, limiting their real applications. Inspired by the fine hairs and cilia of creature, we prepare hairy surfaces by electrostatic flocking and subsequent surface modification. The optimal hairy surfaces exhibit excellent superhydrophobicity, satisfactory abrasion resistance that is superior to the superhydrophobic surfaces ever reported, long-term underwater stability with/without additional air pressure and good drag-reduction efficiency at low shearing rates. The correlations between microstructure of samples and key properties are analyzed. This work may shed new light on design and preparation of mechanical robust, underwater stable and drag-reduction surfaces that can be easily manufactured in large scale.

Recent progress in optimal design of superhydrophobic surfaces

Superhydrophobic surfaces have received tremendous attention due to their extraordinary physical characteristics and potential applications in various fields. However, the design of robust superhydrophobic surfaces showing both low adhesion and high antiwetting remains a great challenge. Here, we first introduce some important parameters for the characterization of the adhesion property of superhydrophobic surfaces and its physical meaning, including the apparent contact angle, rolling angle, and hysteresis angle. Then, we summarize the antiwetting property of superhydrophobic surfaces through thermodynamic and dynamic methods, and we point out the contradiction between low adhesion and high antiwetting of superhydrophobic surfaces. To achieve a trade-off between low adhesion and high antiwetting in superhydrophobic materials, we also review the recent progress made in regard to the optimal design of superhydrophobic surfaces by using chemical modification and physical design of microstructures. Finally, we review the application of superhydrophobic surfaces for drag reduction, with a few discussions and outlook. This Research Update is expected to provide useful guidance to rationally design superhydrophobic materials with low adhesion and high antiwetting properties and, thus, speed up the development of superhydrophobic materials or anti-icing materials.

Effect of the surface pattern on the drag property of the superhydrophobic surface

Superhydrophobic surfaces with a slip effect have been proven to be effective to achieve surface drag reduction. In this paper, we fabricated superhydrophobic disks via laser ablation and chemical modification, and their drag-reduction performance is tested by a rheometer. However, the superhydrophobic disk achieves a limited drag-reduction effect (33.5% in maximum) with poor consistency (decay rapidly as rotating speed increases). To enhance the drag-reduction performance of the superhydrophobic surfaces, surfaces with tunable patterns consisting of superhydrophobic and hydrophobic surfaces are proposed. The difference in wettability and slip length between different regions is employed to control the flow direction. The effect of different patterns is investigated both experimentally and numerically, and the results indicate that the pattern distribution performs a great impact on the drag-reduction effect. The pattern consistent with the primary flow can not only enhance the effect (60.3% in maximum) but also the consistency (maintained at a higher rotating speed) of drag-reduction. On the contrary, the pattern perpendicular to the flow direction is harmful to drag reduction and can even increase the drag.