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A study of inner-outer interactions in turbulent channel flows by interactive POD

Hongping Wang, Qi Gao

Accepted Manuscript , doi: 10.1016/j.taml.2021.A003

[Abstract] (67) [FullText HTML] (27) [PDF 3929KB] (11)

Abstract:
The amplitude and frequency modulation of near-wall flow structures by the large-scale motions in outer regions is studied in turbulent channel flows. The proper orthogonal decomposition (POD) method is applied to investigate the interactions between the near-wall motions and the large-scale flow modes of the outer regions based on two datasets from direct numerical simulation of turbulent channel flows at Reynolds numbers of 550 and 1000. The fluctuations in the fields

\begin{document}$u^+$\end{document}

\begin{document}$v^+$\end{document}

\begin{document}$w^+$\end{document}

and Reynolds shear stress

\begin{document}$-(uv)^+$\end{document}

are studied to understand the mechanism of amplitude and frequency modulation of the near-wall structures by the outer large-scale motions. The amplitude modulation coefficient of the Reynolds shear stress is larger than that of the velocity components. The frequency modulation effect has an opposite influence in the spanwise direction compared to the streamwise direction. The streamwise characteristic frequency increases with increasing large-scale velocity. However, the spanwise characteristic frequency exhibits a decreasing trend with increasing large-scale velocity in the near-wall region.

A multi-scale algorithm for dislocation creep at elevated temperatures

Lichao Yuan, Yujie Wei

Accepted Manuscript , doi: 10.1016/j.taml.2021.A011

[Abstract] (26) [FullText HTML] (13) [PDF 2994KB] (3)

Abstract:
Dislocation creep at elevated temperatures plays an important role for plastic deformation in crystalline metals. When using traditional discrete dislocation dynamics (DDD) to capture this process, we often need to update the forces on N dislocations involving ~N² interactions. In this letter, we introduce a multi-scale algorithm to speed up the calculations by dividing a sample of interest into sub-domain grids: dislocations within a characteristic area interact following the conventional way, but their interaction with dislocations in other grids are simplified by lumping all dislocations in another grid as a super one. Such a multi-scale algorithm lowers the computational load to ~N^1.5. We employed this algorithm to model dislocation creep in Al-Mg alloy. The simulation leads to a power-law creep rate in consistent with experimental observations. The stress exponent of the power-law creep is a resultant of dislocations climb for ~5 and viscous dislocations glide for ~3.

Constrained large-eddy simulation of turbulent flow over inhomogeneous rough surfaces

Wen Zhang, Minping Wan, Zhenhua Xia, Jianchun Wang, Xiyun Lu, Shiyi Chen

Corrected proof , doi: 10.1016/j.taml.2021.A009

[Abstract] (31) [FullText HTML] (21) [PDF 3108KB] (1)

Abstract:
In this work we extend the method of the constrained large-eddy simulation (CLES) to simulate the turbulent flow over inhomogeneous rough walls. In the original concept of CLES, the subgrid-scale (SGS) stress is constrained so that the mean part and the fluctuation part of the SGS stress can be modelled separately to improve the accuracy of the simulation result. Here in the simulation of the rough-wall flows, we propose to interpret the extra stress terms in the CLES formulation as the roughness-induced stress so that the roughness inhomogeneity can be incorporated by modifying the formulation of the constrained SGS stress. This is examined with the simulations of the channel flow with the spanwise alternating high/low roughness strips. Then the CLES method is employed to investigate the temporal response of the turbulence to the change of the wall condition from rough to smooth. We demonstrate that the temporal development of the internal boundary layer is just similar to that in a spatial rough-to-smooth transition process, and the spanwise roughness inhomogeneity has little impact on the transition process.

Research on refined reconstruction method of airfoil pressure based on compressed sensing

Xuan Zhao, Lin Du, Xuhao Peng, Zichen Deng, Weiwei Zhang

Corrected proof , doi: 10.1016/j.taml.2021.A004

[Abstract] (92) [FullText HTML] (28) [PDF 3132KB] (5)

Abstract:
The placement of pressure taps on the surface of the wind tunnel test model is an important means to obtain the surface pressure distribution. However, limited by space location and experimental cost, it is difficult to arrange enough pressure measuring taps on the surface of complex models to obtain complete pressure distribution information, thus it is impossible to obtain accurate lift and moment characteristics through integration. The paper proposes a refined reconstruction method of airfoil surface pressure based on compressed sensing, which can reconstruct the pressure distribution with high precision with less pressure measurement data. Tests on typical airfoil subsonic flow around flow show that the accuracy of lift and moment after the pressure integration reconstructed by 4-8 measuring points can meet the requirements of the national military standard. The algorithm is robust to noise, and provides a new idea for obtaining accurate force data from sparse surface pressure tests in engineering.

Experimental investigation of sound absorption in a composite absorber

Nansha Gao, Hong Hou

Corrected proof , doi: 10.1016/j.taml.2021.A002

[Abstract] (35) [FullText HTML] (22) [PDF 2778KB] (1)

Abstract:
A composite absorber made of a polyurethane sponge and multi-layer micro-perforated plates is presented in this study. Results from an acoustic impedance tube test show that the polyurethane sponge can exhibits higher low-frequency sound absorption in front of the micro-perforated plate, while sound absorption at medium and high-frequencies remains low. The physical mechanism behind this is that the micro-perforated plate increases the denpth cavity. If the polyurethane sponge is placed behind the micro-perforated plate, the amplitude of the original absorption peak will remain constant, but the absorption peaks will shift to lower frequencies. The reason for this phenomenon is that porous materials with low flow resistance can be approximately equivalent to fluid, which not only does not affect the resonance absorption coefficient of micro-perforated plate, but also makes the peaks move to low frequency. This study has the potential applications in the sound absorption design of composite structure.

Rotational dynamics of bottom-heavy rods in turbulence from experiments and numerical simulations

Linfeng Jiang, Cheng Wang, Shuang Liu, Chao Sun, Enrico Calzavarini

Accepted Manuscript , doi: 10.1016/j.taml.2021.A007

[Abstract] (40) [FullText HTML] (24) [PDF 3005KB] (1)

Abstract:
We successfully perform the three-dimensional tracking in a turbulent fluid flow of small asymmetrical particles that are neutrally-buoyant and bottom-heavy, i.e., they have a non-homogeneous mass distribution along their symmetry axis. We experimentally show how a tiny mass inhomogeneity can affect the particle orientation along the preferred vertical direction and modify its tumbling rate. The experiment is complemented by a series of simulations based on realistic Navier-Stokes turbulence and on a point-like particle model that is capable to explore the full range of parameter space characterized by the gravitational torque stability number and by the particle aspect ratio. We propose a theoretical perturbative prediction valid in the high bottom-heaviness regime that agrees well with the observed preferential orientation and tumbling rate of the particles. We also show that the heavy-tail shape of the probability distribution function of the tumbling rate is weakly affected by the bottom-heaviness of the particles.

Bifurcation mechanism of interfacial electrohydrodynamic gravity-capillary waves near the minimum phase speed under a horizontal electric field

Gexing Xu, Zhi Lin

Corrected proof , doi: 10.1016/j.taml.2021.A005

[Abstract] (53) [FullText HTML] (27) [PDF 2926KB] (4)

Abstract:
We investigate the evolution of interfacial gravity-capillary waves propagating along the interface between two dielectric fluids under the action of a horizontal electric field. There is a uniform background flow in each layer, and the relative motion tends to induce Kelvin-Helmholtz (KH) instability. The combined effects of gravity, surface tension and electrically induced forces are all taken into account. Under the short-wave assumption, the expansion and truncation method of Dirichlet-Neumann (DN) operators is applied to derive a reduced dynamical model. When KH instability is suppressed linearly by a considerably large electric field, our numerical results reveal that in certain regions of parameter space, nonlinear symmetric traveling wave solutions can be found near the minimum phase speed. Additionally, the detailed bifurcation structures are presented together with typical wave profiles.

Stable heat jet approach for temperature control of Fermi-Pasta-Ulam beta chain

Baiyili Liu, Qian Zhang, Shaoqiang Tang

Accepted Manuscript , doi: 10.1016/j.taml.2021.A008

[Abstract] (29) [FullText HTML] (23) [PDF 2868KB] (0)

Abstract:
In this paper, we propose a stable heat jet approach for accurate temperature control of the nonlinear Fermi-Pasta-Ulam beta (FPU-

\begin{document}$ \bf{\beta}$\end{document}

) chain. First, we design a stable nonlinear boundary condition, with coefficients determined by a machine learning technique. Its stability can be proved rigorously. Based on this stable boundary condition, we derive a two-way boundary condition complying with phonon heat source, and construct stable heat jet approach. Numerical tests illustrate the stability of the boundary condition and the effectiveness in eliminating boundary reflections. Furthermore, we extend the boundary condition formulation with more atoms, and train the coefficient to eliminate extreme short waves by machine learning technique. Under this extended boundary condition, the heat jet approach is effective for high temperature, and may be adopted for multiscale computation of atomic motion at finite temperature.

Wall-resolved large-eddy simulation of turbulent channel flows with rough walls

Shilong Li, Xiaolei Yang, Guodong Jin, Guowei He

Corrected proof , doi: 10.1016/j.taml.2021.A006

[Abstract] (44) [FullText HTML] (26) [PDF 16305KB] (1)

Abstract:
Turbulent flows over rough surfaces widely exist in nature and industry. Investigating its mechanism is of theoretical and practical significance. In this work we simulate the turbulent channel flow with rough walls using large-eddy simulation with rough elements resolved using the curvilinear immersed boundary method and compare the results obtained in this work with those in the paper by Yuan and Piomelli (J. Fluid Mech. 2014), where the volume of fluid method was employed for modeling rough elements. The mean streamwise velocity profiles predicted by the two methods agree well with each other. Differences in Reynolds stresses and dispersive stresses are observed, which are attributed to the different approaches in dealing with the complex geometry of the rough surface.

A study on equivalence of nonlinear energy dissipation between first-order computational homogenization (FOCH) and reduced-order homogenization (ROH) methods

Jiajia Yue, Zifeng Yuan

Accepted Manuscript , doi: 10.1016/j.taml.2021.A010

[Abstract] (30) [FullText HTML] (20) [PDF 2879KB] (1)

Abstract:
Nowadays, studies on the mechanism of macro-scopic nonlinear behavior of materials by accumulation of micro-scopic degradation are attracting more attention from researchers. Among numerous approaches, multiscale methods have been proved as powerful and practical approaches in predicting macro-scopic material status by averaging and homogenizing physical information from associated micro-scopic material behavior. Usually in mechanical problem, the stress, consistent material modulus, and possible material state variables are quantities in interest through the upscaling process. However, the energy-related quantities are not studied much. Some initiative work has been done in the early year including but not limited to the Hill-Mandel condition in multiscale framework, which gives that the macro-scopic elastic strain energy density can be computed by volumetric averaging of that in the micro-scale. However, in the nonlinear analysis, the energy dissipation is an important quantity to measure the degradation status. In this manuscript, two typical multiscale methods, the first-order computational homogenization (FOCH) and reduced-order homogenization (ROH), are adopted to numerically analyze a fiber-reinforced composite material with capability in material nonlinearity. With numerical experiments, it can be shown that energy dissipation is the same for both approaches.

Physics-informed deep learning for digital materials

Zhizhou Zhang, Grace X. Gu

Corrected proof , doi: 10.1016/j.taml.2021.A001

[Abstract] (160) [FullText HTML] (57) [PDF 2871KB] (11)

Abstract:
In this work, a physics-informed neural network (PINN) designed specifically for analyzing digital materials is introduced. This proposed machine learning (ML) model can be trained free of ground truth data by adopting the minimum energy criteria as its loss function. Results show that our energy-based PINN reaches similar accuracy as supervised ML models. Adding a hinge loss on the Jacobian can constrain the model to avoid erroneous deformation gradient caused by the nonlinear logarithmic strain. Lastly, we discuss how the strain energy of each material element at each numerical integration point can be calculated parallelly on a GPU. The algorithm is tested on different mesh densities to evaluate its computational efficiency which scales linearly with respect to the number of nodes in the system. This work provides a foundation for encoding physical behaviors of digital materials directly into neural networks, enabling label-free learning for the design of next-generation composites.

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Letter

A modified Lin equation for the energy balance in isotropic turbulence

W.D. McComb

2020, 10(6): 377 -381. doi: 10.1016/j.taml.2020.01.055

[Abstract] (221) [FullText HTML] (99) [PDF 2541KB] (33)

Abstract:
At sufficiently large Reynolds numbers, turbulence is expected to exhibit scale-invariance in an intermediate ("inertial") range of wavenumbers, as shown by power law behavior of the energy spectrum and also by a constant rate of energy transfer through wavenumber. However, there is an apparent contradiction between the definition of the energy flux (i.e., the integral of the transfer spectrum) and the observed behavior of the transfer spectrum itself. This is because the transfer spectrum T(k) is invariably found to have a zero-crossing at a single point (at k = k_*), implying that the corresponding energy flux cannot have an extended plateau but must instead have a maximum value at k = k_*. This behavior was formulated as a paradox and resolved by the introduction of filtered/partitioned transfer spectra, which exploited the symmetries of the triadic interactions (J. Phys. A: Math. Theor., 2008). In this paper we consider the more general implications of that procedure for the spectral energy balance equation, also known as the Lin equation. It is argued that the resulting modified Lin equations (and their corresponding Navier–Stokes equations) offer a new starting point for both numerical and theoretical methods, which may lead to a better understanding of the underlying energy transfer processes in turbulence. In particular the filtered partitioned transfer spectra could provide a basis for a hybrid approach to the statistical closure problem, with the different spectra being tackled using different methods.

Influence of wing flexibility on the aerodynamic performance of a tethered flapping bumblebee

Hung Truong, Thomas Engels, Dmitry Kolomenskiy, Kai Schneider

2020, 10(6): 382 -389. doi: 10.1016/j.taml.2020.01.056

[Abstract] (292) [FullText HTML] (107) [PDF 18347KB] (18)

Abstract:
The sophisticated structures of flapping insect wings make it challenging to study the role of wing flexibility in insect flight. In this study, a mass-spring system is used to model wing structural dynamics as a thin, flexible membrane supported by a network of veins. The vein mechanical properties can be estimated based on their diameters and the Young's modulus of cuticle. In order to analyze the effect of wing flexibility, the Young's modulus is varied to make a comparison between two different wing models that we refer to as flexible and highly flexible. The wing models are coupled with a pseudo-spectral code solving the incompressible Navier–Stokes equations, allowing us to investigate the influence of wing deformation on the aerodynamic efficiency of a tethered flapping bumblebee. Compared to the bumblebee model with rigid wings, the one with flexible wings flies more efficiently, characterized by a larger lift-to-power ratio.

Efficient model for the elastic load of film-substrate system involving imperfect interface effects

Wenwang Wu, Huabin Yu, Rui Xue, Tian Zhao, Ran Tao, Haitao Liao, Zhongdong Ji

2020, 10(6): 390 -404. doi: 10.1016/j.taml.2020.01.048

[Abstract] (145) [FullText HTML] (88) [PDF 3834KB] (8)

Abstract:
In this paper, an efficient calculation method based on discrete Fourier transformation is developed for evaluating elastic load induced elastic deformation fields of film-substrate system. Making use of 2D discrete Fourier transformation, the elastic fields induced by Hertz load is harvested in frequency domain, and the displacement and stress fields across the interface are enforced to satisfy the elasticity conditions for each Fourier modes. Given arbitrary distributed stress field at free surface plane of the three types of film-substrate systems, unique resultant elastic field within the can be harvested. Hertz load of half space, elastic film on elastic substrate, elastic film on rigid substrate system and elastic film-substrate system with three types of imperfect interface models are investigated: (1) the spring-like imperfect interface model which can be described as:

\begin{document}${\left. {u_k^f} \right|_{{z^f} = - h}} - {\left. {u_k^s} \right|_{{z^s} = 0}} = {{{K}}_{T}} {\sigma _{kz}}$\end{document}

and

\begin{document}${\left. {u_z^f} \right|_{{z^f} = - h}} - {\left. {u_z^s} \right|_{{z^s} = 0}} = {{ {{K}}}_{N}} {\sigma _{zz}}$\end{document}

; (2) the dislocation-like interface model, where interface displacement and stress components relation can be described as:

\begin{document}${\left. {u_i^f} \right|_{{z^f} = 0}} = {{{{k}}}_{ij}^u} {\left. {u_i^s} \right|_{{z^s} = 0}}$\end{document}

and

\begin{document}${\left. {\sigma _{iz}^f} \right|_{{z^f} = 0}} = {\left. {\sigma _{iz}^s} \right|_{{z^s} = 0}}$\end{document}

; (3) the force-like interface model, where interface displacement and stress components relation can be described as:

\begin{document}${\left. {u_i^f} \right|_{{z^f} = 0}} = {\left. {u_i^s} \right|_{{z^s} = 0}}$\end{document}

and

\begin{document}${\left. {\sigma _{iz}^f} \right|_{{z^f} = 0}} = {{{{k}}}_{ij}^t} {\left. {\sigma _{iz}^s} \right|_{{z^s} = 0}}$\end{document}

respectively. Finally, several simulation examples are performed for verification of the reliability and efficiency of the proposed semi-analytical methods.

Investigation of Agave cantala-based composite fibers as prosthetic socket materials accounting for a variety of alkali and microcrystalline cellulose treatments

Sakuri Sakuri, Eko Surojo, Dody Ariawan, Aditya Rio Prabowo

2020, 10(6): 405 -411. doi: 10.1016/j.taml.2020.01.052

[Abstract] (172) [FullText HTML] (112) [PDF 3444KB] (18)

Abstract:
This study was aimed to determine the mechanical strength of composites made from Agave cantala with an unsaturated polyester matrix and microcrystalline cellulose. Cantala fiber (CF) was treated with 6% NaOH with immersion times of 0 h (UF), 3 h (AK3), 6 h (AK6), 9 h (AK9), and 12 h (AK12). Thermogravimetric analysis (TGA) analysis shows that treated CF has higher thermal stability than CF without treatment. Cantala fiber was tested by X-ray diffraction. After alkali treatment with a 6-h soaking, it had a crystallinity index of 73.65%. Scanning electron microscopy (SEM) showed that the fibers were cleaner after alkali treatment because hemicellulose, wax, and other impurities were removed. Examination of the contact angle and surface energy showed that treated CF has smaller contact angles and greater surface energy.

Interactions of human islet amyloid polypeptide with lipid structure of different curvatures

Le Mei, Wenhui Shen, Xuwei Wu, Jie Liu, Dechang Li, Baohua Ji

2020, 10(6): 412 -418. doi: 10.1016/j.taml.2020.01.053

[Abstract] (431) [FullText HTML] (146) [PDF 4519KB] (18)

Abstract:
Curvature is one of the most important features of lipid membranes in living cells, which significantly influences the structure of lipid membranes and their interaction with proteins. Taken the human islet amyloid polypeptide (hIAPP), an important protein related to the pathogenesis of type II diabetes, as an example, we performed molecular dynamics (MD) simulations to study the interaction between the protein and the lipid structures with varied curvatures. We found that the lipids in the high curvature membrane pack loosely with high mobility. The hIAPP initially forms H-bonds with the membrane surface that anchored the protein, and then inserts into the membrane through the hydrophobic interactions between the residues and the hydrophobic tails of the lipids. hIAPP can insert into the membrane more deeply with a larger curvature and with a stronger binding strength. Our result provided important insights into the mechanism of the membrane curvature-dependent property of proteins with molecular details.

Evolution of vortices in the wake of an ARJ21 airplane: Application of the lift-drag model

Jun-Duo Zhang, Qing-Hai Zuo, Meng-Da Lin, Wei-Xi Huang, Wei-Jun Pan, Gui-Xiang Cui

2020, 10(6): 419 -428. doi: 10.1016/j.taml.2020.01.054

[Abstract] (218) [FullText HTML] (90) [PDF 3566KB] (21)

Abstract:
Wake separation is crucial to aircraft landing safety and is an important factor in airport operational efficiency. The near-ground evolution characteristics of wake vortices form the foundation of the wake separation system design. In this study, we analysed the near-ground evolution of vortices in the wake of a domestic aircraft ARJ21 initialised by the lift-drag model using large eddy simulations based on an adaptive mesh. Evolution of wake vortices formed by the main wing, flap and horizontal tail was discussed in detail. The horizontal tail vortices are the weakest and dissipate rapidly, whereas the flap vortices are the strongest and induce the tip vortex to merge with them. The horizontal tail and flap of an ARJ21 do not significantly influence the circulation evolution, height change and movement trajectory of the wake vortices. The far-field evolution of wake vortices can therefore be analysed using the conventional wake vortex model.

Analytical and numerical studies for Seiches in a closed basin with bottom friction

I. Magdalena, H.Q. Rif'atin, A. Mauditra A. Matin

2020, 10(6): 429 -437. doi: 10.1016/j.taml.2020.01.057

[Abstract] (158) [FullText HTML] (101) [PDF 2992KB] (3)

Abstract:
A standing wave oscillation in a closed basin, known as a seiche, could cause destruction when its period matches the period of another wave generated by external forces such as wind, quakes, or abrupt changes in atmospheric pressure. It is due to the resonance phenomena that allow waves to have higher amplitude and greater energy, resulting in damages around the area. One condition that might restrict the resonance from occurring is when the bottom friction is present. Therefore, a modified mathematical model based on the shallow water equations will be used in this paper to investigate resonance phenomena in closed basins and to analyze the effects of bottom friction on the phenomena. The study will be conducted for several closed basin shapes. The model will be solved analytically and numerically in order to determine the natural resonant period of the basin, which is the period that can generate a resonance. The computational scheme proposed to solve the model is developed using the staggered grid finite volume method. The numerical scheme will be validated by comparing its results with the analytical solutions. As a result of the comparison, a rather excellent compatibility between the two results is achieved. Furthermore, the impacts that the friction coefficient has on the resonance phenomena are evaluated. It is observed that in the prevention of resonances, the bottom friction provides the best performance in the rectangular type while functioning the least efficient in the triangular basin. In addition, non-linearity effect as one of other factors that provide wave restriction is also considered and studied to compare its effect with the bottom friction effect on preventing resonance.

Ergodic sensitivity analysis of one-dimensional chaotic maps

Adam A. Śliwiak, Nisha Chandramoorthy, Qiqi Wang

2020, 10(6): 438 -447. doi: 10.1016/j.taml.2020.01.058

[Abstract] (137) [FullText HTML] (76) [PDF 3724KB] (5)

Abstract:
Sensitivity analysis in chaotic dynamical systems is a challenging task from a computational point of view. In this work, we present a numerical investigation of a novel approach, known as the space-split sensitivity or S3 algorithm. The S3 algorithm is an ergodic-averaging method to differentiate statistics in ergodic, chaotic systems, rigorously based on the theory of hyperbolic dynamics. We illustrate S3 on one-dimensional chaotic maps, revealing its computational advantage over naïve finite difference computations of the same statistical response. In addition, we provide an intuitive explanation of the key components of the S3 algorithm, including the density gradient function.

Vibration energy harvesting for low frequency using auto-tuning parametric rolling pendulum under exogenous multi-frequency excitations

Surat Punyakaew, Manukid Parnichkun

2020, 10(6): 448 -455. doi: 10.1016/j.taml.2020.01.059

[Abstract] (124) [FullText HTML] (55) [PDF 3289KB] (18)

Abstract:
An electromagnetic parametrically excited rolling pendulum energy harvester with self-tuning mechanisms subject to multi-frequency excitation is proposed and investigated in this paper. The system consists of two uncoupled rolling pendulum. The resonance frequency of each the rolling pendulum can be automatically tuned by adjusting its geometric parameters to access parametric resonance. This harvester can be used to harvest the energy at low frequency. A prototype is developed and evaluated. Its mathematical model is derived. A cam with rolling follower mechanism is employed to generate multi-frequency excitation. An experimental study is conducted to validate the proposed concept. The experimental results are confirmed by the numerical results. The harvester is successfully tuned when the angular velocity of the cam is changed from 1.149 to 1.236 Hz.