Relationship Between Skin Scales and the Main Flow Field Around the Shortfin Mako Shark Isurus oxyrinchus

Abstract

The aim of this study was to reveal potential relationship between the main flow field around a shortfin mako shark and the surface morphology of shark skin. Firstly, a numerical simulation using the large eddy simulation (LES) method was conducted to obtain the main flow field around a smooth shark model. Then, the surface morphology characteristics of a shark (Isurus oxyrinchus) at different positions were characterized by scanning electron microscope (SEM), which showed that the morphology, riblet size, and density of scales at different positions on the shark were significantly different. At positions where the surfaces face into the water flow direction (i.e., nose and leading edge of fins), the scales were flat and round, with a lower density, and the pressure or wall shear stress (WSS) was greater. Scales with three longitudinal riblets ending in three tips were found on the middle and trailing edges of the first dorsal fin and caudal fin, where water flow states progress from transitional to turbulent. The ranges of the ratio of riblet depth to spacing (RD/RS) in the anterior zone, middle zone and posterior zone of the shark were 0.05–0.17, 0.08–0.23, and 0.32–0.33, respectively. The riblet angle generally followed the flow direction, but it varied across different areas of the body. The turbulence intensity increased gradually across the first dorsal fin, pectoral fin, caudal fin, and the shark body overall. In summary, it was found that the microstructure riblets on the shark skin surface, generally thought to be drag reduction structures, were only located in transitional and turbulent regions at the middle and trailing edge of the shark body and fin surfaces, and there were almost no microstructural grooves in the laminar flow regions along the leading edge. These findings can provide design guidance for engineering applications of bionic riblet surfaces. Riblets placed in transitional and fully turbulent regions can be used to effectively reduce drag. The riblet direction should be consistent with the direction of flow.

Three-dimensional shape of natural riblets in the white shark: relationship between the denticle morphology and swimming speed of sharks

“…The ridge spacings of shortfin mako sharks are smaller than those of our white shark: the s 1 around L1, L2 and L3 of shortfin mako sharks reportedly range from 35 to 65 µm [13,15,27,28,33,35,36]. The s 1 for the pectoral, caudal fin and dorsal fins are smaller than those for the body, as observed for our white shark: from 20 to 45 µm [13,33,35,36]. The ridge heights of shortfin mako sharks are also smaller than those of our white shark: the ridge heights on the body range from 2 to 13 µm [15,27,28,33,35], and the ridge heights on the pectoral dorsal fins are 8 µm or less [33,35].…”

“…Among these studies, Raschi & Musick differentiated between high and low ridges in their measurements of the spacings and heights, while their body locations were not quantified [27,28]. Zhang et al used a digital microscope to measure both the spacings and heights at several locations on the body, except for the heights of the low side ridges [36]. In addition, Miyazaki et al separately measured the high and low ridges of a Galapagos shark (Carcharhinus galapagensis) by X-ray CT [37].…”

“…Zhang et al . used a digital microscope to measure both the spacings and heights at several locations on the body, except for the heights of the low side ridges [36]. In addition, Miyazaki et al .…”

Characterization of shark skin properties and biomimetic replication

“…Smaller dermal denticles were predominantly observed on the fins and in close proximity to the gills, while larger dermal denticles were mainly distributed on the body. According to Zhang et al (2022), who conducted research on the Shortfin Mako shark, the length of the denticles ranges between 105 ± 7 µm and 286 ± 25 µm and the width of the denticles between 87 ± 4 µm and 255 ± 22 µm. The length-to-width ratio ranged from 1.02 to 1.45.…”

Multi-fidelity modelling of shark skin denticle flows: insights into drag generation mechanisms

“…By contrast, methodology based on the Reynolds averaged Navier–Stokes (RANS) equations are not so limited by Reynolds number, but require validation against benchmark data. To date, only Zhang et al [ 20 ] have carried out RANS simulations on resolved shark scales, but it is unclear how well RANS methodology is able to predict the flow over geometrically resolved rough surfaces (i.e without using roughness wall functions, which have been used to simulate the flow over full shark bodies by Díez et al [ 21 ] and Zhang et al [ 22 ]). However, the method may provide valuable insight into the fluid dynamics of shark scales by simulating more realistic flow scenarios when compared to DNS.…”