Hasil untuk "Mechanics of engineering. Applied mechanics"

Yuto Tanaka, Yoshiaki Ono, Yosuke Tomita

<b>Background/Objective:</b> Postural stability and motor coordination require precise regulation of agonist and antagonist muscle activities. Jaw clenching modulates neuromuscular control during static and reactive postural tasks. However, its effects on dynamic voluntary movement remain unclear. This pilot study aimed to investigate the effects of jaw clenching on muscle activity and kinematics during repetitive single-leg sit-to-stand task performance. <b>Methods:</b> Eleven healthy adults (age: 21.2 ± 0.4 years; 6 males and 5 females; height: 167.9 ± 9.6 cm; body weight: 59.7 ± 8.1 kg) performed repetitive single-leg sit-to-stand tasks for 30 s under jaw-clenching and control conditions. Electromyography (EMG) signals from eight muscles and kinematic data from 16 inertial measurement unit sensors were analyzed, focusing on the seat-off phase. <b>Results:</b> Jaw clenching resulted in a significantly lower success rate than the control condition (success rate: 0.96 ± 0.13 vs. 0.78 ± 0.29, <i>p</i> = 0.047). Under the jaw clenching condition, failed trials exhibited higher medial gastrocnemius and masseter EMG activity (<i>p</i> < 0.001), lower erector spinae longus EMG activity (<i>p</i> < 0.001), and altered kinematics, including increased trunk yaw and roll angles (<i>p</i> < 0.001). Jaw clenching increased the coactivation of the gastrocnemius and tibialis anterior muscles (<i>p</i> < 0.001), disrupting the reciprocal muscle patterns critical for task performance. <b>Conclusions:</b> These findings suggest that jaw clenching may reduce task performance by altering neuromuscular coordination during dynamic postural tasks.

Rahul Deshpande, Ricardo Vinuesa

The present study investigates streamwise ($\overline{u^2}$) energy-transfer mechanisms in the inner and outer regions of turbulent boundary layers (TBLs). Particular focus is placed on the $\overline{u^2}$-production, its inter-component and wall-normal transport as well as dissipation, all of which become statistically significant in the outer region with increasing friction Reynolds number ($Re_τ$). These properties are analyzed using published data sets of zero, weak and moderately strong adverse-pressure-gradient (APG) TBLs across a decade of $Re_τ$, revealing similarity in energy-transfer pathways for all these TBLs. It is found that both the inner and outer peaks of $\overline{u^2}$ are always associated with local maxima in the $\overline{u^2}$-production and its inter-component transport, and the regions below/above each of these peaks are always dominated by wall-ward/away-from-wall transport of $\overline{u^2}$, thereby classifying the $\overline{u^2}$-profiles into four distinct regimes. This classification reveals existence of phenomenologically similar energy-transfer mechanisms in the `inner' and `outer' regions of moderately strong APG TBLs, which meet at an intermediate location coinciding with the minimum in $\overline{u^2}$ profiles. Given that the wall-ward/away-from-wall transport of $\overline{u^2}$ is governed by the $\rm Q_4$(sweeps)/$\rm Q_2$(ejections) quadrants of the Reynolds shear stress, it is argued that the emergence of the $\overline{u^2}$ outer peak corresponds with the statistical dominance of $\rm Q_4$ events in the outer region. Besides unravelling the dynamical significance of $\rm Q_2$ and $\rm Q_4$ events in the outer region of turbulent boundary layers, the present analysis also proposes new phenomenological arguments for testing on canonical wall-turbulence data at very high $Re_τ$.

S. L. Gavrilyuk, H. Gouin

By dispersive models of fluid mechanics we are referring to the Euler-Lagrange equations for the constrained Hamilton action functional where the internal energy depends on high order derivatives of unknowns. The mass conservation law is considered as a constraint. The corresponding Euler-Lagrange equations include, in particular, the van der Waals--Korteweg model of capillary fluids, the model of fluids containing small gas bubbles and the model describing long free-surface gravity waves. We obtain new conservation laws generalizing the helicity conservation for classical barotropic fluids.

Jonas Heinzmann, Pietro Carrara, Chenyi Luo et al.

In the context of the Damage Mechanics Challenge, we adopt a phase-field model of brittle fracture to blindly predict the behavior up to failure of a notched three-point-bending specimen loaded under mixed-mode conditions. The beam is additively manufactured using a geo-architected gypsum based on the combination of bassanite and a water-based binder. The calibration of the material parameters involved in the model is based on a set of available independent experimental tests and on a two-stage procedure. In the first stage an estimate of most of the elastic parameters is obtained, whereas the remaining parameters are optimized in the second stage so as to minimize the discrepancy between the numerical predictions and a set of experimental results on notched three-point-bending beams. The good agreement between numerical predictions and experimental results in terms of load-displacement curves and crack paths demonstrates the predictive ability of the model and the reliability of the calibration procedure.

Heiko Gimperlein, Michael Grinfeld, Robin J. Knops et al.

This paper provides a new admissibility criterion for choosing physically relevant weak solutions of the equations of Lagrangian and continuum mechanics when non-uniqueness of solutions to the initial value problem occurs. The criterion is motivated by the classical least action principle but is now applied to initial value problems which exhibit non-unique solutions. Examples are provided to Lagrangian mechanics and the Euler equations of barotropic fluid mechanics. In particular, we show the least action admissibility principle prefers the classical two shock solution to the Riemann initial value problem to certain solutions generated by convex integration. On the other hand, Dafermos's entropy criterion prefers convex integration solutions to the two shock solutions. Furthermore, when the pressure is given by $p(ρ)=ρ^2$, we show that the two shock solution is always preferred whenever the convex integration solutions are defined for the same initial data.

Ivo Wandrol, Karel Frydrýšek, Daniel Čepica

The article focuses on the deformation and strain-stress analysis of the Earth’s crust under external thermal loading. More specifically, the influence of cyclic changes in the surface temperature field on the stress and displacement inside the crust over a two-year time span is investigated. The finite element program MSC.Marc Mentat was used to calculate the stresses and displacements. For practical analysis reasons, the Earth’s crust is simplified as a planar, piecewise homogeneous, isotropic model (plane strain), and time-varying temperature functions of illumination (thermal radiation) from the Sun are considered in the local isotropy sections of the model. Interaction between the Earth’s crust and mantle is defined by the Winkler elastic foundation. By applying a probabilistic approach (Monte Carlo Method), a new stochastic model of displacements and stresses and new information on crustal displacements relative to the Earth’s mantle were obtained. The results proved the heating influence of the Sun on the Earth’s crust and plate tectonics.

Carimalo Christian

The transmutation of central forces, or dual law, is a transformation linking potentials in power law relative to the distance, that is, those having a positive exponent to those having a negative exponent. A well known example is that of the Newtonian and Hookean potentials, which are also strongly linked by Bertrand’s famous theorem. This article shows how the use of dual law provides a better understanding of this theorem, and a new way to complete its demonstration

Alla Ali Ibrahim, Muhammet Kayfeci, Aleksandar G. Georgiev et al.

The current paper proposes a novel multi-generation system, integrated with compound parabolic collectors and a biomass combustor. In addition to analyzing the comprehensive system in a steady state, the feasibility of using nanofluids as heat transfer fluids in the solar cycle and their effect on the overall performance of the system was studied. The multi-generation system is generally designed for generating electricity, cooling, freshwater, drying, hot water, and hydrogen, with the help of six subsystems. These include a double stage refrigeration system, an organic Rankine cycle, a steam Rankine cycle, a dryer, a proton exchange membrane electrolyzer, and a multistage flash distillation system. Two types of nanoparticles (graphene, silver), which have various high-quality properties when used within ethylene glycol, were chosen as absorbing fluids in the solar cycle. The performance parameters of the base case thermodynamic analysis and some of the variable parameters were calculated, and their effect on system performance was determined. According to the results, a spike in solar irradiation, ambient temperature, output temperature of biomass combustor and nanofluids’ concentration positively affected the overall system performance. The results also clearly showed an improvement in system performance when using nanofluids as working fluids in solar collectors.

Younes Menni, Ali J. Chamkha, Houari Ameur et al.

The attachment of turbulators, such as baffles, fins, ribs, bars, and blocks, inside the thermal solar receiver ducts, is among the most effective mechanisms for important thermal exchange by creating the turbulence, extending the trajectory of the flow, increasing the surface of heat exchange, forcing recycling cells, and hence a high thermal exchange. The solar finned and baffled heat exchangers are employed in a wide application interval, and it is important to examine the design of a duct for this configuration of the flow field and its effect on the heat transport phenomenon. In This study, dynamic field simulations are reported in horizontal rectangular form ducts, using three obstacles with oil HTF (heat transfer fluid). Two various finned and baffled duct configurations are treated, i.e., case (A) with one fin and two baffles, and case (B) with two fins and one baffle. Different hydrodynamic fields, i.e., X-velocity and Y-speed, as well as various X-velocity profiles in many flow stations, related to Re value, are analyzed. A computational approach is applied in order to simulate the oil flow, using finite volume (FV) integration method, SIMPLE discretization algorithm, QUICK interpolation scheme, Standard k-epsilon turbulence model, and ANSYS FLUENT 12.0 software. Simulation results reported an unstable flow structure, with powerful recycling cells, on the backsides of each fin and baffle, as a result of fluid detachment at their upper front sharp edges, in both studied models (A and B). As expected, the first duct model, i.e., Case A, has better X- and Y-velocity values, due to its large recirculation regions. In This paper, many physical phenomena, such as the turbulence, instability, flow separation, and the appearance of reverse secondary currents, are reported. As its data confirmed by many previous numerical and experimental results, the suggested new models of finned and baffled heat exchangers filled with high thermal conductivity oil, allow an improvement in the dynamic thermal-energy behavior of many thermal devices such as flat plate solar collectors.

Yabin Jin, Shixuan Zeng, Zhihui Wen et al.

How to endow diverse functions for lightweight structures without increasing mass has always been a major challenge (e.g., to have simultaneous high static stiffness and extreme-low dynamic stiffness in one structure) from both fundamental science and engineering application. In this work, Archimedean spiral metastructures are proposed for low-frequency vibration isolation by combining a double-layer corrugated sandwich core and two spiral metamaterial plates, which the former provides lightweight and high static load-bearing properties, and the latter possesses deep-subwavelength bandgaps. Both of the two proposed metastructures made of resin and Aluminum show efficient vibration isolation below 150 Hz where the corresponding flexural wavelengths are about 40 times the lattice constant, being a highly compact design for deep-subwavelength vibration control. We further develop a machine learning method to realize on-demand inverse design of the metastructures with targeted bandgap properties. The experimental and numerical results of the bandgaps are highly consistent, and further validate the accuracy of the machine-learning prediction. The slits between the Archimedean spiral arms make the metastructures be possible to further integrate sound absorption and isolation functions in future. The proposed metastructures provide a promising way for potential functional integration devices for vibration and noise control in industries such as aerospace engineering.

Christoffer Fyllgraf Christensen, Fengwen Wang, Ole Sigmund

Much work has been done in topology optimization of multiscale structures for maximum stiffness or minimum compliance design. Such approaches date back to the original homogenization-based work by Bendsøe and Kikuchi from 1988, which lately has been revived due to advances in manufacturing methods like additive manufacturing. Orthotropic microstructures locally oriented in principal stress directions provide for highly efficient stiffness optimal designs, whereas for the pure stiffness objective, porous isotropic microstructures are sub-optimal and hence not useful. It has, however, been postulated and exemplified that isotropic microstructures (infill) may enhance structural buckling stability but this has yet to be directly proven and optimized. In this work, we optimize buckling stability of multiscale structures with isotropic porous infill. To do this, we establish local density dependent Willam-Warnke yield surfaces based on local buckling estimates from Bloch-Floquet-based cell analysis to predict local instability of the homogenized materials. These local buckling-based stress constraints are combined with a global buckling criterion to obtain topology optimized designs that take both local and global buckling stability into account. De-homogenized structures with small and large cell sizes confirm validity of the approach and demonstrate huge structural gains as well as time savings compared to standard singlescale approaches.

Sebastian Gajek, Matti Schneider, Thomas Böhlke

In this work, we propose a fully coupled multiscale strategy for components made from short fiber reinforced composites, where each Gauss point of the macroscopic finite element model is equipped with a deep material network (DMN) which covers the different fiber orientation states varying within the component. These DMNs need to be identified by linear elastic precomputations on representative volume elements, and serve as high-fidelity surrogates for full-field simulations on microstructures with inelastic constituents. We discuss how to extend direct DMNs to account for varying fiber orientation, and propose a simplified sampling strategy which significantly speeds up the training process. To enable concurrent multiscale simulations, evaluating the DMNs efficiently is crucial. We discuss dedicated techniques for exploiting sparsity and high-performance linear algebra modules, and demonstrate the power of the proposed approach on an industrial-scale three-dimensional component. Indeed, the DMN is capable of accelerating two-scale simulations significantly, providing possible speed-ups of several magnitudes.

Mahboobeh Azadi, Fahimeh Kamali

In this article, microstructural characteristics and mechanical properties of Al-Si-Cu/NCP composites were evaluated. Reinforced nanocomposites with 1 wt% nano-clay were fabricated by the method of the stir casting. Stirring times and temperatures were variable parameters to produce specimens. Consequently, the effect of a T6 heat treatment, which contained solutioning at 490 ºC for 5 hrs, quenching, and aging process at 200ºC for 2 hrs, on tribological behavior and compression properties of nanocomposites was inspected. The microstructural observation was conducted by optical microscopy (OM) and the field emission scanning electron microscopy (FESEM). The acquired results demonstrated that nano-clay particles were distributed in the aluminum matrix. The range of Vickers hardness values was 123 to 158 VHN for nanocomposites. The wear resistance of nanocomposites enhanced when the stirring time and temperature increased to 4 mins and 800 ºC, respectively.  The best compressive mechanical properties correlated to the nanocomposite fabricated at 750 ºC and stirred for 2 mins.  The higher stirring time and temperature resulted in the formation of AlSiFe intermetallic phase which reduced the ultimate compressive strength.