Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

Abstract

Pufferfish is known for its extension of tiny spinecovered skin that appears to increase skin drag and may act as turbulisors, reducing overall drag while serving a protective function. Therefore, the present study addresses a neglected aspect of how spines affect the turbulent boundary layer (TBL) for drag reduction in the pufferfish skin. Particle image velocimetry (PIV) was utilized to investigate the TBL structure on the biomimetic spine-covered protrusion samples inspired by the back skin of the pufferfish. The comparison samples of two sparse "ktype" arrangements (hexagon and staggered) for three types of rough element sizes with roughness heights k + = 5.5−6.5 (nearly hydraulically smooth) and smooth case in bulk Reynolds numbers (Re b = 37,129 and 44,554) were tested. The results of turbulence statistics of these samples indicate that both the sample (type hexagon) for large rough density (λ = 0.0215) with small roughness elements and the sample (type staggered) for small rough density (λ = 0.0148) with large roughness elements have a drag reduction rate of 5−11%. These two kinds of bionic surfaces have a similar morphology to that seen in the distribution of pufferfish spines and probably serve a similar hydrodynamic function. Vortex identification shows that the spines in the front section for large density with small rough elements stabilize the TBL and generate many small-scale vortices and the dense spines with large rough elements at the back section have the effect of separating the vortices. The retrograde vortex generated by them is beneficial to increasing the driving force of the pufferfish. In addition, these two rough surfaces may effectively delay the separation of the TBL. These results will provide a preliminary research foundation for the development of a more practical prototype of the bionic drag-reducing surfaces and strengthen the theoretical investigation concerning drag reduction exploration.

Coupled Bionic Drag-Reducing Surface Covered by Conical Protrusions and Elastic Layer Inspired from Pufferfish Skin

“…However, the two-point correlation coefficient contours between the biomimetic surface CPES and the smooth surface CS are relatively significant. However, compared with the samples with the same protrusion CPRS is smaller, which is consistent with the previous work . This indicates that the drag reduction performance is not only positively related to the regular protrusion structure formation but also to the elastic layer coated on the surface.…”

“…The experimental test data are taken under the stable flow rate of the test system, and the experimental error can be guaranteed to be less than 2% under all sample experimental tests. Additionally, more details about the PIV experiment can be found in our previous work …”

“…The pufferfish is also known for its skin covered with scattered small spines, which are wrapped in an elastic outer skin, as shown in Figure S1. These spines are found to affect water flow and reduce drag when they are in a steady state. , Staggered arrangement of elastic layers wrapped around conical projections inspired by hard spines buried in flexible epidermis. In the present study, the biomimetic surface combined with hydrophobicity, an elastic layer, and protrusion microstructure has been successfully fabricated via a novel hybrid method.…”

“…Each curve's secondary peak shifts from y + = 40−50 to y + = 20−30 with the Reynolds number increasing, which is acquainted with previous research. 14 However, each curve peak presents at y + = 75−120 near the wall and then decreases rapidly to around η = 2 at y + = 250−300 and then flattens out, finally reaching the balance of η = 9, due to the generation, development, and fragmentation of hairpin vortices.…”

Bionic research on Paramisgurnus dabryanus scales for drag reduction

“… 12 The special spine-like structure on the body surface of puffer fish can effectively delay the separation of turbulent boundary layers. 13 Muthuramalingam et al studied the DR performance of European sea bass scales and found that the alternating high and low-speed stripes generated by scales in the near-wall region could reduce the drag. 14 Chen et al investigated the coupling DR function of tuna scales and flexible body surface, and the maximum DR rate reached 25.7% due to the vortexes effect of flexible epidermis.…”