In the past decades, drag-reduction surfaces have attracted more and more attention due to their potentiality and wide applications in various fields such as traffic, energy transportation, agriculture, textile industry, and military.
OMT inhibited collagen synthesis, which might be associated with TGF-β/Smad signaling pathway. These findings suggest that OMT may be a promising candidate to prevent keloid and other fibrotic diseases.
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.
Inspired by the drag-reducing properties of the cone-like
spines
and elastic layer covering the pufferfish skin, important efforts
are underway to establish rational multiple drag-reducing strategies
for the development of new marine engineering materials. In the present
work, a new drag-reducing surface (CPES) covered by conical protrusions
(sparse “k-type” with rough height k
+ = 13–15) and an elastic layer are constructed
on copper substrate via a hybrid method, combining the sintering and
coating processes. The drag-reducing feature of the prepared CPES
biomimetic surface is achieved by rheometer and particle image velocimetry
(PIV) experiments. To comprehensively investigate its drag reduction
mechanism, the porous copper substrate (PCS), copper substrate (CS),
conical protrusion resin substrate (CPRS), and conical protrusion
porous copper substrate (CPPCS) were used for a comparative analysis.
In laminar flow, we discovered that the conical protrusion structure
and wettability of the elastic surface coupling affect the CPES sample’s
drag-reducing performance (7–8%) and that the interface produced
slip to reduce the viscous drag. In turbulent flow, the CPES biomimetic
surface exhibits an 11.5–17.5% drag-reducing performance. Such
behavior was enabled by two concurrent mechanisms: (i) The conical
protrusions as vortex generators enhance the number of vortices and
the wake effect, enabling faster movement of downstream strips, reducing
viscous drag; (ii) The conical protrusion elements break and lift
large-scale vortices to produce numerous small-scale vortices with
low energy, effectively weakening perturbations and momentum exchange.
Additionally, the elastic layer shows high adhesion and stability
on copper substrate after sandpaper abrasion and water-flow erosion
tests. The copper substrate surface formed by the sintering method
is also covered with dense porous structures, which gives the elastic
layer and conical protrusions excellent combined robustness. Our findings
not only shed new light on the design of robust drag-reducing surfaces
but also provide new avenues for underwater drag reduction in the
field of marine applications.
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