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The skin of a fast swimming shark reveals riblet structures that help reduce the shark’s skin friction drag, enhancing its efficiency and speed while moving in the water. Inspired by the structure of the shark skin denticles, our team has carried out a study as an effort in improving the hydrodynamic design of marine vessels through hull design modification which was inspired by this riblet structure of shark skin denticle. Our study covers on macroscaled design modification. This is an attempt to propose an alternative for a better economical and practical modification to obtain a more optimum cruising characteristics for marine vessels. The models used for this study are constructed using computer-aided design (CAD) software, and computational fluid dynamic (CFD) simulations are then carried out to predict the effectiveness of the hydrodynamic effects of the biomimetic shark skins on those models. Interestingly, the numerical calculated results obtained show that the presence of biomimetic shark skin implemented on the vessels give about 3.75% reduction of drag coefficient as well as reducing up to 3.89% in drag force experienced by the vessels. Theoretically, as force drag can be reduced, it can lead to a more efficient vessel with a better cruising speed. This will give better impact to shipping or marine industries around the world. However, it can be suggested that an experimental procedure is best to be conducted to verify the numerical result that has been obtained for further improvement on this research.

Shipping industry has been a thriving industry, since the start of the industrial revolution, and now, it is categorized as one of the large scale economies [

Our focus in this study is on hull form design modification through biomimetic riblet structure of shark skin denticle as an attempt of improving the hydrodynamic design of marine vessels. However, in this present paper, we described our approach by focusing on wall shear, velocity profile, and turbulence kinetic energy (TKE) produced by the modified and unmodified hull form through CFD activity. Macroscaled biomimetic riblet shark skin is applied on the frontal and rear vicinity of the container ship with the intention of improving the fluid flow around the ship which will provide better flow separation control especially at the ship hull surface area. Endless studies have been carried out recently as an effort upon improving performance of systems such as ship performance, duct or pump efficiency, and energy saving. However, these studies focus on the nanoscaled modifications, which require more financial resources in order to be implemented due to the complex structure of the shark skin denticles. The studies include the investigation on drag-reduction mechanism which are inspired by nature such as drag reduction through the unique structure of shark skin denticle, hydrophobic surface inspired by lotus leaves, and drag reduction by injections of micro bubbles on ships which sometimes might lead to cavitation problems [

Shark skin structures are rough and harsh, and they are made up of dermal tooth-like denticle which is called as miniscule placoid scales or denticles. These denticles have pulp and cavity covered with enamel or vitrodentine [

3D reconstructed micro-CT model of a single denticle of mako shark [

This is considered as a natural phenomenon occurred in living animals. Principally, the existence of these denticles helps to maintain the smooth flowing water in laminar pattern resulting in faster speed of fluid close to the surface of a shark body. This helps in reducing the velocity difference between near surface and away fluid flow from surface during locomotion in water. This velocity difference sustains the pattern of the fluid when it travels along the body of the shark before it splits of into turbulence flow. Thus, resulting in a lesser drag force and faster speed during swimming in a much lesser effort [

Fluid flow over bodies commonly occurs in real life and normally leads to physical phenomenon such as drag force which frequently happened in transportations, lift force in the wings of an airplane, and upward draft during raining or snowing and effects of high winds to dust particles. Pressure drag and friction drag are the two most basic type of fluid drag. Pressure drag can be envisioned by a cross-flow over an object. It requires power or energy needed to move the front object out from the fluid in contact and then return back behind the object. Streamlining an object can help in reducing pressure drag [

The relationship between drag and the velocity of the body is commonly described by the given equation: drag force, _{D} = 1/2_{d}^{2}. Where _{d} is the drag coefficient of the object relative to its shape, structure, orientation, and size. From the given equation of _{D}, the force acting on any object is proportional to the square of the fluid’s speed, the density of the fluid, projected areas of the fluid, and the drag coefficient, _{d}, of the object which encountered by the fluid. Since _{D} is proportional to drag coefficient, thus the decrease in _{d} value will reduce the amount of force that acts toward the body provided that all other parameters are constant. The aim of our study is to reduce drag force exerted on the body by doing modification on the hull form through riblets which mimic the structure of the shark skin denticle. With the help of powerful software like computational fluid dynamic (CFD), ANSYS will calculate the drag coefficient on both models, modified and unmodified hull forms. The amount of force acting on the body will be obtained through drag force equation. In this study, the graph of pressure along modified and unmodified ships is plotted in Section

Basically, there are four factors that affect drag force. These are velocity or speed, the size or length of the object or solid, the density of the fluid, and lastly, the viscosity of the fluid. However, the transition from laminar to turbulent flow also depends on other factors such as surface geometry, surface roughness, the upstream velocity, the type of fluid, and the temperature of the surface. The relationship of these four factors is used in order to determine the type of flow either laminar or turbulence which can be calculated using Reynolds number (Re) [^{3}),

Generally, drag force, _{D}, that acts on an object depended on the following given quantities: velocity (^{5} but still does not turn to a complete turbulent flow when Re reaches Re = 3 × 10^{6} [

Enhancement of functional properties of any mechanism today is made possible by altering or improving its original design. The act of modifying the design of any geometries will contribute significantly to the performance of the system either dynamically or statically. Ibrahim et al. describe on how geometrical optimization on the design of conventional in air-lubricated thrust bearing can improve significantly the dynamic stiffness of the air film on the bearing significantly [

Modelling is done using Solid Works software while computational fluid dynamic (CFD) calculations are conducted using Ansys Fluent. Simulations were done on two types of models; the first is on a rectangular plane model, and the other is on the surface of a container ship models. For clearer comparisons, simulations are conducted on both models: one with biomimetic shark skin and one without biomimetic shark skin.

Both container ship models have the same boundary conditions as the rectangular plane models. Generic models of planes and container ships are generated by using a 3-dimensional (3D) modelling software. Models used in this study were rectangular plane (RP) with thickness of 1.50 cm and container ship (CS) models, with both models with one without biomimetic riblet structure of shark skin and one with addition of biomimetic riblet shark skin. Figure _{1} = 1.0 cm and _{2} = 2.0 cm; riblet height, _{1} = 2.0 cm, _{2} = 4.0 cm, _{3} = 6.0 cm, and _{4} = 8.0 cm, respectively; and distance between riblets,

(a) Parameters of riblet structure of biomimetic shark skin. (b) 3D isometric and frontal view of biomimetic shark skin.

Meshing details for each models.

Models | Number of nodes | Number of elements | Maximum element quality |
---|---|---|---|

RP with biomimetic shark skin | 275975 | 1447984 | 0.99980 |

RP without biomimetic shark skin | 16927 | 84922 | 0.99984 |

CS with biomimetic shark skin | 171265 | 549536 | 0.99978 |

CS without biomimetic shark skin | 53862 | 224310 | 0.99980 |

The morphology of a real shark skin is complex 3D geometries and configurations. However, it is still feasible to simulate and study the drag reduction through these complex geometries upon using a powerful supercomputer. Meshing of complex models is a difficult process in CFD. It will consume a lot of time not only to generate those mesh but also to run the simulation itself. These technical assessments of users’ designs are still possible for CFD activity; however, these will require massive parallel computing supported by powerful hardware. Numerical solving of complex flows for complex geometries as an attempt of representing the real-like conditions will encompass vast amount of computational power in trying to solve such cases or such fluid’s dynamic problems which also reliant on the physical of the model itself. In this case study, it focuses on simplified version shark skin or known as riblet geometry in line to our target of proposing a much more economical and practical modifications for ship hull.

The geometry of the riblets in this paper is proposed through reference of several studies that focus on the study of riblet geometry which is also inspired by shark skin denticle. Riblets are considered as two-dimensional geometry. Real shark skins give complex 3D geometries which lead to cost constrain in fabrication and may not be feasible for mass application. However, this inspired researchers today to focus more on the optimization of this denticle morphology in developing a simpler structure (riblet) as an attempt to achieve the same effect as the real shark skin. There are several different riblet studies which have different riblet structures such as scalloped riblets, sawtooth riblets, blade riblets, L-shaped riblets, U-shaped riblets, and few other riblet geometry [

The parameters used are standardized according to the type of models. Inlet velocity for both rectangular plane models is 12.86 m/s which is equivalent to 25 knots, the standard speed of the ship during cruising while other parameters like pressure were set based on the normal room conditions. However, the inlet velocity for container ship models is 128.6 m/s as the models are scaled down to 0.1. Other parameters are summarized as shown in Table

Parameters set-up for flow simulation.

Models | Plain models | Biomimetic shark skin |
---|---|---|

Pressure (Pa) | 101325 | |

Viscosity (kg/m·s) | 1.09 × 10^{−3} | |

Density (kg/m^{3}) |
1025 | |

Gravitational acceleration (m/s^{2}) |
−9.81 | |

Type of fluid | Seawater | |

Type of flow | Turbulence |

This research covers an external flow field study on surfaces. To have an acceptable level of simulation accuracy, the selection of turbulence model is very important. In this paper, realizable k-epsilon (k-

The analysis of the results for every section is based on the flow trajectories or contour lines on the particular models. For all the models used in this research, the value of average, minimum, and maximum of any parameters used is calculated or determined by using calculator function which is provided in Ansys.

Figure

(a) Cross-section of velocity streamline of riblet RP. (b) Cross-section of velocity streamline of plain RP. (c) Cross-section of velocity curl of riblet RP. (d) Cross-section of velocity curl of plain RP.

From the results obtained, larger vortices formed above the plain RP model vicinity are shown in Figure

XY plot of velocity magnitude (m/s) to the direction vector (m) of both RP models.

The changes in the velocity distribution along the flow field lead to a reduction in wall shear stress. As velocity flow profile inside the valley of the riblet RP model is lower, it indirectly reduces the shear stresses experienced by the model. Since higher vortices are only experienced on the tip of the riblets, these cause only small amount of high shear stress interacted with the plane as shown in Figure

(a) Contour of wall shear stress on plain RP. (b) Contour of wall shear stress on biomimetic riblet RP.

Figure

(a) Contour of wall shear stress on container ship model without biomimetic shark skin. (b) Contour of wall shear stress on container ship model with biomimetic shark skin.

(a) Contour of wall shear stress on frontal ship model with biomimetic shark skin. (b) Contour of wall shear stress on frontal plain ship model. (c) Contour of wall shear stress on rear ship model with biomimetic shark skin. (d) Contour of wall shear stress on rear plain ship model.

Energy content of eddies in turbulent flow is known as turbulent kinetic energy (TKE). Energy content of eddies is proportional to the size of it. The greater the size, the more energy content of eddies. The high turbulent energy extraction from the mean flow leads to high region of turbulent kinetic energy [

(a) TKE on rear end model of container ship with biomimetic shark skin. (b) TKE on rear end model of plain ship. (c) TKE on front model of container ship with biomimetic shark skin. (d) TKE on front model of plain ship.

Figure

(a) Pressure, isoline on CS model with biomimetic shark skin. (b) Pressure, isoline on CS model without biomimetic shark skin.

The results in Table

Improvement percentage.

Container ship models | With biomimetic shark skin, |
Without biomimetic shark skin, |
Difference, |
Percentage |
---|---|---|---|---|

Drag coefficient, _{d} |
0.077 | 0.080 | 0.003 | 3.75 |

Drag force (N) | 29434 | 30581 | 1147 | 3.89 |

8-meter flow channel, Hydrology Lab of UNIMAS.

Design modification with biomimetic shark skin has numerically proven that the biomimetic shark skin could give improvement to both models: rectangular plane and container ship models in terms of reduction in wall shear stress, reduction in drag force, and improved fluid flow around the ship. The coefficient of drag reduced by 3.75% and gave the reduction of 3.89% in drag force experienced by container ship model. Since the numerical study gave some encouraging results, we can further improve this research by conducting experimental procedure for validation. This modification might give better impact to shipping or marine industries around the world.

The findings which are reported in this paper were presented at the 4th International Conference on Materials, Mechatronics, Manufacturing and Mechanical (ICMMMM), IPN Conference Kuching 2017.

The authors declare that there is no conflict of interest regarding the publication of this paper.

The present work was partially funded by the Fundamental Research Grant Scheme, Ministry of Higher Education, Malaysia, Grant nos. FRGS/TK01 (01)/1059/2013(05), and Universiti Malaysia Sarawak.