Surfaces For Drag Reduction Research Articles

Controllable Adjustment of Bio-Replicated Shark Skin Drag Reduction Riblets

Although natural shark shin surface morphology has excellent drag reduction performance, it exhibits maximum drag reduction just within swimming speed of the shark. That is, drag reduction function of shark skin is unadjustable to surrounding environment. To expand applications of bio-replicated shark skin, two novel controllable adjustments of shark skin drag reduction riblets including one-direction elongation and 3D volume swelling amplification, were explored. The validity and efficiency of the two approaches to change the drag reduction riblets were verified by comparison between microstructure of adjusted and original shark skin. And the translation of drag reduction peak of natural surface function from living environment to various application environments was proved by experimental measurement. By comparison, the elongating method is efficient but low drag reduction, while the swelling way can get the same drag reduction as shark skin but cost more time. So they should be performed according to the situation. As the optimal application velocity range of the imitative shark shin morphology can be expanded by both of the two methods, the application field of the biomimetic drag reduction surface fabricated by the bio-replicated forming technology was extended.

Design on the Drag Reduction Surface of an Arc-Shaped Revolution Body with the Phyllotactic Pattern

In order to resolve the theorized design problems on the bionic non-smooth drag reduction surface of an arc-shaped revolution body, which mainly include the structure morphology and spatial arrangement of the non-smooth point, a kind of designing method on the bionic non-smooth drag reduction surface is presented based on biological phyllotactic theory, and the mathematical equation of the arrangement of the non-smooth point on the arc-shaped drag reduction surface has been established, the effects of the phyllotactic parameters on the arrangement form and density of the non-smooth point have been discussed, and the arrangement of the non-smooth point on an arc-shaped revolution body surface has been designed by using a 3D software. Research results have shown that the arrangement density of the non-smooth points increases with the decreasing of the phyllotactic coefficient, and the area ratio of the no-smooth point to the surface of revolution body increases with the geometric parameter of the non-smooth point.

Shark skin inspired riblet structures as aerodynamically optimized high temperaturecoatings for blades of aeroengines

This paper deals with different structuring methods for high temperature resistant nickelalloys. The ideal structured surface for a possible application on the blades of aeroenginescombines high oxidation resistance with low drag in a hot gas flow. The effect of dragreduction due to riblet structured surfaces was originally inspired by shark scales, whichhave a drag reducing riblet structure. The necessary riblet sizes for effective dragreduction depend on the temperature, pressure and velocity of the flowing medium(gas or liquid). These riblet sizes were calculated for the different sections in anaeroengine. The riblets were successfully produced on a NiCoCrAlY coating bypicosecond laser treatment. This method is suitable for larger structures within therange of some tens of micrometers. Furthermore, experiments were performed bydepositing different materials through polymer and metal masks via electrodepositionand physical vapor deposition. All fabricated structures were oxidized at 900–1000 °C for up to 100 h to simulate the temperature conditions in an aeroengine. The resultingshape of the riblets was characterized using scanning electron microscopy. The mostaccurate structures were obtained by using photolithography with a subsequentelectrodeposition of nickel. This method is suited for single digit micrometer structures.The reduction of the wall shear stress was measured in an oil channel. The riblet structuresprior to oxidation showed a reduction of the wall shear stress of up to 4.9% compared to anormal smooth surface. This proves that the fabricated riblet design can be used as a dragreducing surface.

Numerical simulation and experimental study of drag-reducing surface of a real shark skin

It is well known that shark skin surface can effectively inhabit the occurrence of turbulence and reduce the wall friction, but in order to understand the mechanism of drag reduction, one has to solve the problem of the turbulent flow on grooved-scale surface, and in that respect, the direct numerical simulation is an important tool. In this article, based on the real biological shark skin, the model of real shark skin is built through high-accurate scanning and data processing. The turbulent flow on a real shark skin is comprehensively simulated, and based on the simulation, the drag reduction mechanism is discussed. In addition, in order to validate the drag-reducing effect of shark skin surface, actual experiments were carried out in water tunnel, and the experimental results are approximately consistent with the numerical simulation.

Biomimetics inspired surfaces for drag reduction and oleophobicity/philicity

The emerging field of biomimetics allows one to mimic biology or nature to develop nanomaterials, nanodevices, and processes which provide desirable properties. Hierarchical structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in nature and possess properties of interest. There are a large number of objects including bacteria, plants, land and aquatic animals, and seashells with properties of commercial interest. Certain plant leaves, such as lotus (Nelumbo nucifera) leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical surface roughness and presence of a wax layer. In addition to a self-cleaning effect, these surfaces with a high contact angle and low contact angle hysteresis also exhibit low adhesion and drag reduction for fluid flow. An aquatic animal, such as a shark, is another model from nature for the reduction of drag in fluid flow. The artificial surfaces inspired from the shark skin and lotus leaf have been created, and in this article the influence of structure on drag reduction efficiency is reviewed. Biomimetic-inspired oleophobic surfaces can be used to prevent contamination of the underwater parts of ships by biological and organic contaminants, including oil. The article also reviews the wetting behavior of oil droplets on various superoleophobic surfaces created in the lab.

High-precision bio-replication of synthetic drag reduction shark skin

Nano-long chains were grafted over the replicated micro-grooves of shark skin in a novel attempt to replicate bio-synthetic drag reduction structure with high precision through synthetic bio-replication. Pre-treated shark skin was used as casting template to prepare a flexible female die of silicone rubber by soft die formation. A waterborne epoxy resin was then used to graft long-chains of drag reduction agent and prepare a synthetic drag reduction shark skin with nano-long chain drag reduction interface and lifelike micro-grooves. Replication precision analysis shows that this technology could replicate the complicated three-dimensional morphology of a biological drag reduction surface with high precision. Drag reduction experiments show that the material had an excellent synthetic drag reduction effect, with a maximal drag reduction rate of up to 24.6% over the velocities tested.

Underwater Restoration and Retention of Gases on Superhydrophobic Surfaces for Drag Reduction

Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surface structures can hold a lubricating gas film between the solid surface and the liquid in contact with it. However, the types of SHPo surfaces that would produce any meaningful amount of reduction get wet under liquid pressure or at surface defects, both of which are unavoidable in the real world. In this Letter, we solve the above problem by (1)discovering surface structures that allow the restoration of a gas blanket from a wetted state while fully immersed underwater and (2)devising a self-controlled gas-generation mechanism that maintains the SHPo condition under high liquid pressures (tested up to 7atm) as well as in the presence of surface defects, thus removing a fundamental barrier against the implementation of SHPo surfaces for drag reduction.

Study on the Micro-Replication Precision of Shark Skin

The biomimetic highly-efficient drag reduction surface can be directly replicated by shark skin as biological template, and the pre cured micro-embossing technology is put forward aiming at the thermosetting materials. In the process of bio-replicated forming, the replication precision is one of the most important factors in evaluating the forming effect, and the convincing and reliable assessing method has become an urgent problem to be solved. In this paper, the new evaluating way on the basis of statistical method is put into application, which provide the new idea and method to study the forming quality and effect, and the replication-precision of single scale and regional scales is in comprehensive and deep analysis. In addition, the factors causing forming errors are also illustrated and discussed, which lay the foundation for further research.

Self-organized nanostructuring of composite coatings at high temperatures for drag reduction and self-cleaning

Coated and nanostructured surfaces gain much importance for improving the efficiency of high temperature applications (e.g. in turbines). By embedding ceramic particles with a negative thermal expansion coefficient (NTEC) into a metallic matrix, a reversible thermal activation of a nanostructured surface can be established. At high temperatures a defined drag reducing surface microstructure (“shark-skin”) is formed in the coating surface, while at low temperatures a self-cleaning effect (in idle periods) is achieved by the reversal of the thermal deformation. A feedstock powder produced by high energy milling and consisting of nanocrystalline yttrium oxide and tungsten oxide particles embedded into a conventional MCrAlY alloy was used for the investigations. By using different thermal spray and cladding techniques the powder is deposited on steel- substrates. In the next step, the coating is implanted with yttrium or xenon in order to induce the formation of Y 2W 3O 12. The latter is a ceramic with a strong negative thermal expansion coefficient and is stable up to temperatures above 1373 K. The effects during annealing and the morphology changes are analyzed in detail. Results of the phase transformation, the surface micro-morphology and the microstructured properties of these high-temperature coatings are presented.

Bio-replicated forming of the biomimetic drag-reducing surfaces in large area based on shark skin

On the investigation of biomimetic drag-reducing surface, direct replication of the firm scarfskins on low-resistance creatures to form biomimetic drag-reducing surfaces with relatively vivid morphology relative to the living prototype is a new attempt of the bio-replicated forming technology. Taking shark skin as the bio-replication template, the hot embossing method was applied to the micro-replication of its outward morphology. Furthermore, the skins were jointed together to form the drag-reducing surface in large area. The results of the resistance measurements in a water tunnel according to the flat-plate sample pieces have shown that the biomimetic shark-skin coating fabricated by the bio-replicated forming method has significant drag reduction effect, and that the drag reduction efficiency reached 8.25% in the test conditions.

Dynamical Adjustment Theory for Boundary Layer Flow in Cold Surges

Abstract Theoretical responses of boundary layer flow for winter cold surges crossing a coastline are determined. Analytical solutions for linearized equations describing the mixed layer adjustments to impulsive heating and surface drag reduction are obtains Dynamical interpretation contrasts with nonturbulent adjustment processes and nonlinear simulations are made. The response depends upon various physical process length males, two reflecting dynamical adjustments and four reflecting turbulence influences. Upstream wind and inversion strength determine a Froude number Fr which strongly influences the flow, regimes resembling quasi-geostrophic and inertia-gravity behavior (modified by turbulence) are identified. These possess strong two-way coupling between wind and inversion, but primarily a one-way influence between temperature and wind. Surface pressure drops may commence over land (small Fr); their maxima (a few millibars) over the sea are smaller than simple boundary layer warming would imply. Examp...