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The SST turbulent model coupled with γ- Re θ was adopted in the present numerical research of the cooling performance of pulsating jet impingement onto dimpled/protruded surfaces. The turbulent model has been experimentally validated and it proves that the coupled turbulent model can well predict jet impingement. User Defined Function was used in the investigation. Grid independence validation has been performed and a set of structured grids was employed with y +<1. The results show the area-averaged Nu varies with the same pulsating frequency in different target morphology cases. The pulsating amplitude for the f = 80 Hz cases shows the largest value among all the cases. Both the area-averaged Nu and the area-averaged time-averaged Nu increase with the increasing pulsating velocity ratio and decrease with the increasing pulsating frequency. The heat transfer performance is clearly enhanced in the stagnation and secondary impingement regions in the protruded cases. Thus, the time-averaged Nu in all the protruded cases is higher than that in the flat cases. The secondary impingement also leads to a higher local Nu outside of the dimple edge compared with the flat case. The local stagnation Nu varies with the same frequency as the pulsating jet impingement. The pulsating amplitude of the local stagnation Nu shows the largest value in the protruded surface case. The largest value shows the smallest time in one cycle for the protruded surface cases. The local stagnation time-averaged Nu increases with the pulsating velocity ratio and frequency in both dimpled and flat cases, while it remains almost unchanged in the protruded case. The rate of increase in the dimpled cases is higher than that in the flat case. The case with lower pulsating frequency and higher pulsating velocity in the protruded target surface shows better heat transfer enhancement performance in the comparison.

We numerically investigate the morphology and disclination line dynamics of active nematic droplets in three dimensions. Although our model incorporates only the simplest possible form of achiral active stress, active nematic droplets display an unprecedented range of complex morphologies. For extensile activity, fingerlike protrusions grow at points where disclination lines intersect the droplet surface. For contractile activity, however, the activity field drives cup-shaped droplet invagination, run-and-tumble motion, or the formation of surface wrinkles. This diversity of behavior is explained in terms of an interplay between active anchoring, active flows, and the dynamics of the motile disclination lines. We discuss our findings in the light of biological processes such as morphogenesis, collective cancer invasion, and the shape control of biomembranes, suggesting that some biological systems may share the same underlying mechanisms as active nematic droplets.

PurposeThis study aims to identify the significant factors of the multi-dimpling process, determine the most influential parameters of multi-dimpling to increase the dimple sheet strength and make a low-cost model of the multi-dimpling for sheet metal industries. To create an empirical expression linking process performance to different input factors, the percentage contribution of these elements is also calculated.Design/methodology/approachTaguchi grey relational analysis is used to apply a new effective strategy to experimental data in order to optimize the dimpling process parameters while taking into account several performance factors and low-cost model. In addition, a statistical method called ANOVA is used to ensure that the results are adequate. The optimal process parameters that generate improved mechanical properties are determined via grey relational analysis (GRA). Every level of the process variables, a response table and a grey relational grade (GRG) has been established.FindingsThe factors created for experiment number 2 with 0.5 mm as the sheet thickness, 2 mm dimple diameter, 0.5 mm dimple depth, 8 mm dimples spacing and the material of SS 304 were allotted rank one, which belonged to the optimal parameter values giving the greatest value of GRG.Practical implicationsThe study demonstrates that the process parameters of any dimple sheet manufacturing industry can be optimized, and the effect of process parameters can be identified.Originality/valueThe proposed low-cost model is relatively economical and readily implementable to small- and large-scale industries using newly developed multi-dimpling multi-punch and die.

The effects on the mechanical properties of P22 and P91 steels were evaluated through uniaxial tensile (UT) and small-punch tensile (SPT) tests. UT fractures exhibited different distributions, while SPT fractures showed penetration from the lower surface. Numerous ductile dimples confirmed that both P22 and P91 exhibited ductile fracture at both room and elevated temperatures. Using a classical UT-SPT correlation equation, predictions were achieved for yield strength, ultimate strength, and elongation. This established a correlation model between UT test results and SPT outcomes for P22 and P91, demonstrating high prediction accuracy with a correlation coefficient reaching 0.9.

This study investigates the influence of vacuum level on the filling behavior of AlSi9Cu3 alloy in 3D-printed sand molds. Casting experiments were performed under gravity and varying vacuum conditions. At a vacuum of - 0.04 MPa, the microstructural evolution and strengthening mechanisms of low-temperature cast alloys were analyzed. The results show that an appropriate vacuum (- 0.04 MPa) markedly enhances the filling of thin-walled castings, allowing the pouring temperature to be reduced by about 30 ∼ 50 °C compared with gravity casting. With decreasing pouring temperature, the alloy microstructure transforms from columnar and rosette-like dendrites to predominantly equiaxed grains with refined size. Consequently, the mechanical properties improve, and the fracture mode changes to ductile dimples that are uniformly distributed and deeply formed.

Superplastic forming and diffusion bonding (SPF/DB) has been recognized as a viable manufacturing technology. However, the basic understanding of grain size and its effects on the quality of diffusion bonds is still limited. In this study, a certain type of SP700 alloy with different grain sizes is bonded at superplastic temperature. The experimental results indicate that the same materials, if coarse-grained, may not readily bond under identical conditions of pressure, temperature, and time. This type of bonding is possible because of the presence of many grain boundaries in fine-grained materials that act as short-circuit paths for diffusion. In addition, grain-boundary migration is also faster in fine-grained than in coarse-grained materials. Fractographic studies show that the dimples on the coarse-grained specimen have large dimensions compared with that in the fine-grained material, indicating that heterogeneous deformation develops in the coarse-grained specimen during tension.

Biological membrane fusion proceeds via an essential topological transition of the two membranes involved. Known players such as certain lipid species and fusion proteins are generally believed to alter the free energy and thus the rate of the fusion reaction. Quantifying these effects by theory poses a major challenge since the essential reaction intermediates are collective, diffusive and of a molecular length scale. We conducted molecular dynamics simulations in conjunction with a state-of-the-art string method to resolve the minimum free-energy path of the first fusion intermediate state, the so-called stalk. We demonstrate that the isolated transmembrane domains (TMDs) of fusion proteins such as SNARE molecules drastically lower the free energy of both the stalk barrier and metastable stalk, which is not trivially explained by molecular shape arguments. We relate this effect to the local thinning of the membrane (negative hydrophobic mismatch) imposed by the TMDs which favors the nearby presence of the highly bent stalk structure or prestalk dimple. The distance between the membranes is the most crucial determinant of the free energy of the stalk, whereas the free-energy barrier changes only slightly. Surprisingly, fusion enhancing lipids, i.e., lipids with a negative spontaneous curvature, such as PE lipids have little effect on the free energy of the stalk barrier, likely because of its single molecular nature. In contrast, the lipid shape plays a crucial role in overcoming the hydration repulsion between two membranes and thus rather lowers the total work required to form a stalk.

We study singular jets from the collapse of drop-impact craters, when the drop and pool are of different immiscible liquids. The fastest jets emerge from a dimple at the bottom of the rebounding crater, when no bubble is pinched off. The parameter space is considerably more complex than for identical liquids, revealing intricate compound-dimple shapes. In contrast to the universal capillary–inertial drop pinch-off regime, where the neck radius scales as $R\\sim t^{2/3}$, for a purely inertial air dimple the collapse has $R \\sim t^{1/2}$. The bottom dimple dynamics is not self-similar but possesses memory effects, being sensitive to initial and boundary conditions. Sequence of capillary waves can therefore mould the air dimple into different collapse shapes, such as bamboo-like and telescopic forms. The finest jets are only $12\\ \\mathrm {\\mu }\\textrm {m}$ in diameter and the normalized jetting speeds are up to one order of magnitude larger than for jets from bursting bubbles. We study the cross-over between the two power laws approaching the singularity. The singular jets show the earliest cross-over into the inertial regime. The fastest jets can pinch off a toroidal micro-bubble from the cusp at the base of the jet.

In this paper, the differences in macroscopic and microscopic fracture morphology of TC4 alloy blades damaged by soft/hard object impact were quantitatively studied. The results show that the crack length formed by a hard object impact is 2.83 mm, which is much smaller than that of a soft object impact. Meanwhile, a secondary crack perpendicular to the main crack tip is also formed by hard object impact. The scanning electron microscope (SEM) fracture morphology images of the cracks induced by hard object impact are mainly characterized by a large number of ductile dimples and a small amount of smooth areas. Instead, the fracture surface formed by soft object impact also exhibits fracture morphology characteristics of shear dimples and tear ridges. The quantitative characterization results of the dimple morphologies indicate that there is no significant difference in the mean diameter of dimples in different regions of cracks formed by hard object impact, while dimples formed by soft object impact exhibit inverse function distribution characteristics. The crack path is induced by hard object impact, which forms a crack bifurcation at the crack tip, causing a pronounced brittle fracture. In contrast, the crack propagation path resulting from soft object impact exhibits deflection at a shallower angle, leading to quasi-cleavage fracture.