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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.

Heat transfer enhancement in the interfin space of an air capacitor due to the use of a package of in-line oval-trench dimples inclined at an angle of 45° to the incoming flow at Re=103 is numerically simulated. When considering a narrow channel of 1 in height, 80 in length and 4 in width, the case of 31 single-row oval-trench dimples of 0.25 depth is the best. The growth of hydraulic losses does not exceed 46% at an almost 2-fold increase in heat transfer in comparison to the plane-parallel channel.

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.

Impinging gas jets can induce depressions in liquid surfaces, a phenomenon familiar to anyone who has observed the cavity produced by blowing air through a straw directly above a cup of juice. A dimple-like stable cavity on a liquid surface forms owing to the balance of forces among the gas jet impingement, gravity and surface tension 1 , 2 . With increasing gas jet speed, the cavity becomes unstable and shows oscillatory motion, bubbling (Rayleigh instability) and splashing (Kelvin–Helmholtz instability) 3 , 4 . However, despite its scientific and practical importance—particularly in regard to reducing cavity instability growth in certain gas-blown systems—little attention has been given to the hydrodynamic stability of a cavity in such gas–liquid systems so far. Here we demonstrate the stabilization of such instabilities by weakly ionized gas for the case of a gas jet impinging on water, based on shadowgraph experiments and computational two-phase fluid and plasma modelling. We focus on the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, which is induced by the momentum transfer from accelerated charged particles to neutral gas under an electric field. A weakly ionized gas jet consisting of periodic pulsed ionization waves 5 , called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional electrohydrodynamic gas flow and electric field parallel to the gas–water interface produced by plasma interacting ‘in the cavity’ render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering insights into physical processes and revealing an interdependence between weakly ionized gases and deformable dielectric matter, including plasma–liquid systems. A weakly ionized gas jet impinging on a water surface is shown to produce a more stable cavity than does a neutral gas jet, with implications for plasma–liquid interactions.

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.

The Super304H was studied after aging treatment at 650 °C. With the time extension of aging treatment, the austenitic grains have not changed significantly. The number of the precipitates in the microstructure of austenitic heat-resistant steel was increased gradually. The tensile strength was gradually increased for precipitation strengthening of carbide precipitates, Cr 23 C 6 , the tensile strength reached its maximum and then dropped down after 600 h of aging treatment. The aggregation and growth of precipitated phase M 23 C 6 and Nb(C, N) led to a decrease in the tensile strength. The fractography of the tensile specimens changed from ductile to brittle fracture, and the fracture morphology changed from dimples to relatively flat cleavage planes.

The Marangoni flow induced by an insoluble surfactant on a fluid–fluid interface is a fundamental problem investigated extensively due to its implications in colloid science, biology, the environment and industrial applications. Here, we study the limit of a deep liquid subphase with negligible inertia (low Reynolds number, $Re\\ll {1}$), where the two-dimensional problem has been shown to be described by the complex Burgers equation. We analyse the problem through a self-similar formulation, providing further insights into its structure and revealing its universal features. Six different similarity solutions are found. One of the solutions includes surfactant diffusion, whereas the other five, which are identified through a phase-plane formalism, hold only in the limit of negligible diffusion (high surface Péclet number $Pe_s\\gg {1}$). Surfactant ‘pulses’, with a locally higher concentration that spreads outward, lead to two similarity solutions of the first kind with a similarity exponent $\\beta =1/2$. On the other hand, distributions that are locally depleted and flow inwards lead to similarity of the second kind, with two different exponents that we obtain exactly using stability arguments. We distinguish between ‘dimple’ solutions, where the surfactant has a quadratic minimum and $\\beta =2$, from ‘hole’ solutions, where the concentration profile is flatter than quadratic and $\\beta =3/2$. Each of these two cases exhibits two similarity solutions, one valid prior to a critical time $t_*$ when the derivative of the concentration is singular, and another one valid after $t_*$. We obtain all six solutions in closed form, and discuss predictions that can be extracted from these results.

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.