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Scanning electron microscopy showing sections of skin at various key positions along the body for a spiny dogfish. Dermal denticles on the posterior-dorsal region present obvious prominent ridges, as seen in figure 1(E). The skin sample used in the present experiment was taken from (E).
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The effect of shark skin on the boundary-layer separation process under dynamic conditions (maneuvers) has been studied experimentally. We use a foil covered with biomimetic shark skin to explore how this type of surface impacts boundary-layer dynamics in both steady and accelerating conditions. The effect of denticles is assessed via particle imag...
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Context 1
... fin (BCF) undulation is the preferred mode of swimming for the majority of fish species. BCF swimming styles are classified into four modes Dermal denticles on the posterior-dorsal region present obvious prominent ridges, as seen in figure 1(E). The skin sample used in the present experiment was taken from (E). ...
Context 2
... the shape and size of the denticles vary depending on body location, as seen in figure 1. For instance, the denticles are comparatively flat on the nose tip (figure 1(A)) and middle region of pectoral/dorsal fin (figure 1(C)), while the denticles are significantly larger and with more prominent ridges on the lateral posterior portion of the tail ( figure 1(E)), corresponding to the point where the body can oscillate with the highest amplitude [3,18], where flow separation is prone to happen [1,3]. Therefore, these denticles may serve to expand the favorable pressure gradient region so as to eliminate flow separation and thereby reduce the cost of locomotion during free swimming and/or increase maneuverability during escape and predatory responses. ...
Citations
... Wen, Weaver & Lauder (2014) reported a reduction in energy consumption due to a formation of stronger leading-edge vortices. Guo et al. (2021) found that, for steady foils, the roughness elements resulted in a considerably thicker boundary layer when compared with the smooth foil, whereas for dynamic foils, the changes due to roughness in the wake characteristics were considerably smaller. Mostly, previous work conclude that shark-inspired surfaces can improve the performance of an unsteady body, but the potential benefit is strongly dependent on the shape and size of shark denticles, which often appear in highly complex geometries. ...
... Hence, we would expect that increasing the size of the roughness elements could have a major impact on the wake characteristics of the pitching foil. In contrast, in the static state, foils with 36 % and 70 % roughness have thicker shear layer in time-average compared with the smooth case, similar to the findings by Guo et al. (2021). The presence of thicker shear layers, which are caused by the additional pressure drag incurred by the roughness elements, can be responsible for the 76 % drag penalty shown in figure 6. ...
The hydrodynamic influence of surface texture on static surfaces ranges from large drag penalties (roughness) to potential performance benefits ( shark-like skin ). Although it is of wide-ranging research interest, the impact of roughness on flapping systems has received limited attention. In this work, we explore the effect of roughness on the unsteady performance of a harmonically pitching foil through experiments using foils with different surface roughness, at a fixed Strouhal number and within the Reynolds number ( $Re$ ) range of $17\,000\unicode{x2013}33\,000$ . The foils’ surface roughness is altered by changing the distribution of spherical-cap-shaped elements over the propulsor area. We find that the addition of surface roughness does not improve the performance compared with a smooth surface over the $Re$ range considered. The analysis of the flow fields shows near-identical wakes regardless of the foil's surface roughness. The performance reduction mainly occurs due to an increase in profile drag. However, we find that the drag penalty due to roughness is reduced from $76\,\%$ for a static foil to $16\,\%$ for a flapping foil at the same mean angle of attack, with the strongest decrease measured at the highest $Re$ . Our findings highlight that the effect of roughness on dynamic systems is very different than that on static systems; thereby, it cannot be estimated by only using information obtained from static cases. This also indicates that the performance of unsteady, flapping systems is more robust to the changes in surface roughness.
... Domel et al. (2018) used a rotary motor to drive a foil covered with bionic shark skin to perform a series of pitch motions (h) and frequency ( f ) to generate a fish-like swimming motion. Guo et al. (2021) further investigated the impact of shark skin on the boundary layer separation process under dynamic conditions through experimental research. It is shown that dynamic conditions and small-scale disturbances can mitigate boundary-layer separation through instantaneous modification of the local pressure-gradient distribution. ...
To reduce the drag of underwater vehicles during navigation, this paper proposes a skin imbricated with bionic placoid scale based on micro-Stewart mechanism. The skin is composed of bionic shark placoid scales and Stewart structure with multi-dimensional motion characteristics, which can well simulate the multi-dimensional oscillation motion of shark scales during swimming. A co-simulation platform of computational fluid dynamics and multi-body dynamics is established to investigate the impact of oscillating parameters (heave and pitch) on the drag reduction performance of the skin. The novel skin shows a remarkable drag reduction performance, with a relative drag reduction rate over 20% (up to 33%) in the range of Re = 105 ∼ 106. It is found that the oscillation motion generated by the placoid scales can cause the fluid inside the skin to spray upward, which can increase the thickness of the fluid boundary layer, revealing the drag reduction mechanism of the skin to some extent. Moreover, the pitching motion of the placoid scale is more effective in drag reduction than the heaving motion in the condition of Re = 105. It is expected that applying this skin to underwater vehicles can achieve satisfactory drag reduction effects.
... For all θ, acceleration causes the pressure-gradient field to be transformed from APG to FPG. Furthermore, the strength of the streamwise FPG is increased with increasing a * , which is in good agreement with the theoretical predictions based on potential theory for an accelerating sphere (Fernando et al. 2017) and for an accelerating flat plate at incidence (Guo et al. 2021), where the strength of the FPG was shown to be linearly dependent on the acceleration magnitude. In contrast, the pressure-gradient field is still covered by the streamwise APG for all decelerations and the strength of the APG increases also with increasing deceleration magnitude. ...
Time-varying flow separation on an accelerating prolate spheroid has been studied at various angles of incidence. Instantaneous pressure and scanning stereoscopic particle image velocimetry were used to shed light on the evolution of cross-flow structures for the Reynolds number ( $Re$ ) range of $1.0\times 10^6\leq Re \leq 1.5\times 10^6$ . The movement of separation lines is examined for various model accelerations to investigate on the interplay between acceleration and flow separation. The results demonstrate that for axial accelerations, the streamwise pressure distribution in the rear part of the prolate spheroid switches from an adverse to a favourable pressure gradient. At the same time, the circumferential adverse pressure gradient present during steady motion vanishes during said accelerations. In contrast, both streamwise and circumferential adverse pressure gradients strengthen when the model is axially decelerated. These dynamic pressure distributions influence the location of the separation line, which in turn moves closer to the model meridian during accelerations while moving outwards during decelerations. The streamwise vorticity distribution and the streamwise circulation both show how the separation-line position impacts the vortex formation. A high-vorticity region near the model surface is established during acceleration. In contrast, a decelerating model leads to transport of high-vorticity fluid into the outer area of the cross-flow separation. We further assess the memory effects following the near-impulsive velocity changes. The cross-flow retains the memory of moving separation lines shortly after the acceleration. However, the separation recovers quickly to a steady state.
... In many geophysical and cardiovascular flows, and in flows around flying or swimming animals, the turbulent boundary layer (TBL) formed on the surface of an object is subject to a time-varying, non-zero pressure gradient (Li et al. 2007;Momen & Bou-Zeid 2017;Guo et al. 2021). Changes in curvature or variation of the freestream pressure distribution might cause the boundary layer to thicken and detach from the surface under the effect of a strong enough deceleration (or adverse pressure gradient, APG). ...
The large-eddy simulation technique was used to investigate the dynamics of unsteady flow separation on a flat-plate turbulent boundary layer. The unsteadiness was generated by imposing an oscillating, wall-normal velocity profile at the top of the computational domain, and a range of reduced frequencies (k), from a very rapid flutter-like motion to a slow quasi-steady oscillation, was studied. Ambrogi et al. (J. Fluid Mech., vol. 945, 2022, A10) showed that the reduced frequency greatly affects the transient separation process, and at a frequency k = 1, the separation region became unstable and was advected periodically out of the domain. In this paper, we discuss the causes of the observed advection process and the effects of the unsteadiness on the second moments. The time evolution of turbulent kinetic energy, for instance, reveals that an advection-like phenomenon is also present at a very low reduced frequency, but its dynamic behaviour is completely different from that of the intermediate frequency (k = 1). At the intermediate frequency the entire recirculation region is advected downstream, keeping its shape. The advected structure is rotational in nature, and moves at constant speed. In contrast, in the low-frequency case the advected fluid originates at the reattachment point, and the structure is shear-dominated. Particle pathlines reflect the fact that the flow at the low frequency is quasi-steady-state, but show peculiar differences at the intermediate frequency, in which the flow response to the freestream forcing depends on the particle positions in the wall-normal direction.
... Greiner et al. [5] developed a biomimetic scale-like surface texture inspired by the scales on the skin of snakes and certain lizards and through tribological pin-on-disk testing, found that the surface texture reduced dry sliding-friction forces by more than 40%. In addition, inspired by sharkskin, Guo et al. [6] and Li et al. [7] studied the physical properties of biomimetic-textured sharkskin, showing that the sharkskin has a wear-reducing effect. Rong et al. [8] processed a micro-nano-structure array of biomimetic fish scales onto the surface of magnesium aluminum alloy, and resistance was reduced by about 50%. ...
... Similarly, the geometric relationship of the texture of sharkskin [6] can be expressed as ...
... Note that bar-shaped protrusions characterize the ridges on sharkskin, so it is necessary to carry out the secondary texture based on the original surface, which can be expressed as h = h + h p , |y − y c | + √ 3|x − x c | ≤ l and |y − y c | ≤ l and |y − y c | ≤ d h (6) where h and h p are the texture's final surface height and the height of the bar-shaped protrusions, respectively, and y c and d are the centerline and half-width of the bar-shaped protrusions, respectively. The simulation results of the texture are shown in Figure 1. ...
Rail transportation has dramatically improved travel convenience, but it has also led to environmental pollution and energy consumption issues. These challenges can be partially addressed by reducing friction loss in the mechanical transmission of rail systems. This paper examines the tribological properties of bionic-textured surfaces inspired by snake- and sharkskin. This study focuses on generating bionic textured surfaces with randomly distributed peaks through numerical simulation and connecting them to a transient Reynolds equation and friction fatigue model. The bionic surface wear lubrication model considers the lubricating film’s thickness and contact pressure obtained from the GT model. The results reveal that the existence of a bionic texture can reduce the friction coefficient and wear amount on the contact surface. The findings of this study not only offer a potential solution for reducing energy consumption and emissions in intelligent rail transit systems but also hold promise for providing further insights into the numerical simulation of bionic weaving and the investigation of tribological characteristics.
... Domel et al. 42 focused on how the arrangement of scales on the suction side of an airfoil passively alters fluid flow, and the experimental results showed that the scales lead to a large lift-to-drag ratio. Guo et al. 43 experimentally studied the flow separation of an airfoil covered with a biomimetic shark skin and found that scales can inhibit the flow separation. Du et al. 7 experimentally investigated the effect of tilted scales on reducing the flow separation of an inclined plate and found that it was better than that of aligned scales. ...
Flow separation control has a wide application prospect in drag reduction for industry. This paper numerically studies the effect of microstructures on flow separation and drag reduction. Simple morphological microstructures, derived from the tilted shark scales, are attached to the wing at an angle of attack. The spacing and height of microstructures are made dimensionless by using the microstructure width and half of the wing width, respectively, that is, d̃m=dm/dAB and h̃m=hm/(H/2). The angle of attack is set to 10°. It is found that microstructures can reduce the motion amplitude of shed vortices, thereby suppressing flow separation and reducing drag. Both the planar and curved microstructures have excellent drag reduction performance. The microstructure spacing d̃m and tilt angle θ should not be too large or too small; otherwise, it will weaken the drag reduction ability. Cases d̃m=1.51, θ=20°, and θ=30° exhibit excellent drag reduction performance. The microstructure has the characteristic for being small, yet it needs to reach a certain height h̃m to effectively reduce drag. The case h̃m=0.667 is the most superior choice. Based on the proposed microstructure shape and spacing, the drag reduction performance of microstructures can reach more than 28%. Meanwhile, the drag reduction performance of microstructures increases with the improvement of the attachment proportion pm, and case pm≥50% is suggested for significant drag reduction performance. Finally, we discuss the drag reduction performance of microstructures on the wing at different angles of attack and find that microstructures can achieve good drag reduction, provided that the pressure drag caused by the flow separation is a significant proportion of the total drag and the flow separation occurs within the controllable range of microstructures.
... The bristling is hypothesized to reduce pressure drag by inhibiting backflow in the boundary layer 143 . For an accelerating foil, a denticle-like texture was found to have delayed boundary layer separation compared to a foil with no texture 144,145 (Fig. 4d) 141,146 . Wen et al. 147 presented 3D-printed shark denticles that can be easily modified and applied to surfaces. ...
Flow control is the attempt to favorably modify a flow field’s characteristics compared to how the flow would have developed naturally along the surface. Natural flyers and swimmers exploit flow control to maintain maneuverability and efficiency under different flight and environmental conditions. Here, we review flow control strategies in birds, insects, and aquatic animals, as well as the engineered systems inspired by them. We focus mainly on passive and local flow control devices which have utility for application in small uncrewed aerial and aquatic vehicles (sUAVs) with benefits such as simplicity and reduced power consumption. We also identify research gaps related to the physics of the biological flow control and opportunities for device development and implementation on engineered vehicles.
... Additionally, it was clear that the riblet distribution Mawignon et al. impacted the drag reduction efficiency because the staggered distribution resulted in higher overall drag reduction than the aligned distribution ( Fig. 5(a) and (b)). Moreover, regardless of the inlet velocity, a number of the riblet surfaces investigated drag reduction, demonstrating the sharkskin effect (Guo et al., 2021;Lloyd et al., 2021). The calculated drag reduction efficiencies were lower than 12%. ...
The biomimetic sharkskin riblet has attracted more attention in engineering and research owing to the economic benefit of drag-reducing and antifouling properties. Although several works have optimized the parameters for a number of riblet designs, there is not yet a study dedicated to optimizing for rectangular cuboidal riblet parameters. Hence, this work proposes a new numerical optimization of riblet orientations and arrangements of a three-dimensional rectangular cuboidal riblet. The shear-stress transport k-omega was adopted as an appropriate turbulence model for the simulation. The results showed that the riblets efficiently improved drag reduction regardless of the flow state. Riblets oriented perpendicularly to the flow direction showed the greatest performance of 11.3% and 6% in the laminar and turbulent flow, respectively. Moreover, the analysis of the flow field characteristics near the wall revealed a significant improvement in terms of reduction in near-wall velocity and shear stress. This work offers new perspectives on the role of the intricate multifunctional architecture of shark-scale structures in improving swimming speed and opening new doors for marine and underwater applications.
... However, for real shark scales, the tilt angles are not uniform at different locations and are hard to measure, which makes it difficult to quantitatively analyze the effects of the tilt angles of the scales on flow separation. Thus, the effects of biomimetic shark scales on separated flow have been studied by researchers (Domel et al. 2018;Evans et al. 2018;Arunvinthan et al. 2021;Guo et al. 2021;Mao et al. 2021). Evans et al. (2018) simplified tilted shark scales as micro-pillars, and found that flow separation could be reduced because the micro-pillars could enhance the normal fluctuations in the boundary layer. ...
Sharks can swim with excellent hydrodynamic performance under a variety of flow conditions. The dermal scales of sharks, which can change from an aligned state to a tilted state, account for flow control in the attached and separated flow. However, the mechanism of using tilted biomimetic shark scales for flow separation control is still not clear. In the present work, the effects of biomimetic shark scales with fixed tilt angles on flow separation over an inclined plate are investigated experimentally using time-resolved particle image velocimetry. The Reynolds number based on the streamwise length of the plate and the freestream velocity is 2.0 × 10⁴. From the perspective of the time-averaged flow field, it is found that the tilted biomimetic shark scales decrease the reattachment length by 57% when the angle of attack of the plate is 10°. From the perspective of the instantaneous flow field, the shed vortices over the tilted biomimetic shark scales are closer to the wall than those over the flat plate. The fluid convection is strengthened in the separated shear layer by the tilted biomimetic shark scales, as the momentum transportation and the vorticity convection are enhanced. As a result, normal motions of fluid in the separated shear layer are improved, and energy is supplied to resist flow separation. A flow separation control strategy is proposed for different tilt angles of the biomimetic shark scales and angles of attack, which is significant for engineering applications involving flow control.
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