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Shaping covalent organic frameworks (COFs) into macroscopic objects with robust mechanical properties and hierarchically porous structure is of great significance for practical applications but remains formidable and challenging. Herein, a general and scalable protocol is reported to prepare ultralight and robust pure COF fiber aerogels (FAGs), based on the epitaxial growth synergistic assembly (EGSA) strategy. Specifically, intertwined COF nanofibers (100-200 nm) are grown in situ on electrospinning polyacrylonitrile (PAN) microfibers (≈1.7 µm) containing urea-based linkers, followed by PAN removal via solvent extraction to obtain the hollow COF microfibers. The resultant COF FAGs possess ultralow density (14.1-15.5 mg cm -3 ) and hierarchical porosity that features both micro-, meso-, and macropores. Significantly, the unique interconnected structure composed of nanofibers and hollow microfibers endows the COF FAGs with unprecedented mechanical properties, which can fully recover at 50% strain and be compressed for 20 cycles with less than 5% stress degradation. Moreover, the aerogels exhibit excellent capacity for organic solvent absorption (e.g., chloroform uptake of >90 g g -1 ). This study opens new avenues for the design and fabrication of macroscopic COFs with excellent properties., (© 2024 Wiley‐VCH GmbH.)

In this work, we prove the local well-posedness of strong solutions to the compressible flow of a chemically reacting gaseous mixture with the initial data containing vacuum. Moreover, we should emphasize that there is no assuming compatibility condition on the initial data. [ABSTRACT FROM AUTHOR]

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It is well known that people can derive the radiation MHD model from an MHD-P1 approximate model. As pointed out by F. Xie and C. Klingenberg (2018), the uniform regularity estimates play an important role in the convergence from an MHD-P1 approximate model to the radiation MHD model. The aim of this paper is to prove the uniform regularity of strong solutions to an isentropic compressible MHD-P1 approximate model arising in radiation hydrodynamics. Here we use the bilinear commutator and product estimates to obtain our result. [ABSTRACT FROM AUTHOR]

In this paper, long-wave anti-plane shear motion in a multilayered laminate composed of pre-stressed compressible elastic layers is investigated. The layers of the laminate are perfectly bonded, while a fixed-free boundary condition is prescribed on the outer faces of the laminate. The solution of the model is determined analytically via the propagator matrix and numerically through the asymptotic approach. Moreover, the numerical results featuring harmonic curves are presented graphically, together with an asymptotic long-wave analysis of the vibration modes. As a special case of materials, linear isotropic with one shear modulus is considered. A polynomial long-wave low-frequency approximation of the related dispersion relation is also studied. It governs dispersion curves including the lowest harmonic. It is revealed that a low-frequency mode exists in both the two- and three-layered laminates, which are taken as prototypical structures. Lastly, comparisons between the exact and approximate asymptotic results are presented, and excellent agreement is observed.

Here we develop general analytical results on the linear instability problem of inviscid compressible axial flows in the annular region between two coaxial cylinders to axisymmetric disturbances. For shear-free basic flows the non-existence of propagating subsonic modes is shown. An instability region for subsonic modes within which the complex wave velocity should lie is shown to be a semi-ellipse-type region that depends on the compressibility. For near-neutral subsonic modes some estimates for growth rates are derived and it is shown further that the growth rate tends to zero as the axial wave number tends to infinity. [ABSTRACT FROM AUTHOR]

With the development of mature Computational Fluid Dynamics (CFD) tools for fluids (air and liquid) and Finite Element Methods (FEM) for solids and structures, many approaches have been proposed to tackle the so-called Fluid-Structure Interaction or Fluid-Solid Interaction (FSI) problems. Traditional partitioned iterations are often used to link available FEM codes with CFD codes in the study of FSI systems. Although these procedures are convenient, fluid mesh adjustments according to the motion and finite deformation of immersed solids or structures can be challenging or even prohibitive. Moreover, complex dynamic behaviors of coupled FSI systems are often lost in these iterative processes. In this paper, the author would like to review the socalled monolithic approaches for the solution of coupled FSI systems as a whole in the context of the immersed boundary method. In particular, the focus is on the implicit monolithic algorithm for compressible solids immersed inside a compressible liquid. Notice here the main focus of this paper is on liquid or more precisely liquid phase of water as working fluid. Using the word liquid, the author would like to emphasize the consideration of the compressibility of the fluid and the assumption of constant density and temperature. It is a common practice to assume that the pressure variations are not strong enough to alter the liquid density in any significant fashion for acoustic fluid-solid interactions problems. Although the algorithm presented in this paper is not directly applicable to aerodynamics in which the density change is significant along with its relationship with the pressure and the temperature, the author did revisit his earlier work on merging immersed boundary method concepts with a fully-fledged compressible aerodynamic code based on high-order compact scheme and energy conservative form of governing equations. In the proposed algorithm, on top of a uniform background (ghost) mesh, a fully implicit immersed method is implemented with mixed finite element methods for compressible liquid as well as immersed compressible solids with a matrix-free Newton-Krylov iterative solution scheme. In this monolithic approach, with the simple modulo function, the immersed solid or structure points can be easily located and thus the displacement projections and force distributions stipulated in the immersed boundary method can be effectively and efficiently implemented. This feature coupled with the key concept of the immersed boundary method helps to avoid topologically challenging mesh adjustments and to incorporate parallel processing commands such as Message Passing Interface (MPI) and further vectorization of the numerical operation. Once these high-performance procedures are implemented coupled with the monolithic implicit matrix-free Newton-Krylov iterative scheme with immersed methods, effective and efficient reduced order modeling techniques can then be employed to explore phase and parametric spaces. The in-house developed programs are at the moment two-dimensional. Furthermore, based on the same approach implemented in one-dimensional test example with one continuum immersed in another continuum, such monolithic implicit matrix-free Newton-Krylov iterative approach can be extended for the study of composites with deformable aggregates and matrix. [ABSTRACT FROM AUTHOR]

In this manuscript, we consider the compressible Hall-MHD system, which is a mathematical model to describe magnetic reconnection in space plasmas, star formation, neutron stars, and geo-dynamics. In the system, there are two viscosity constants and a resistivity coefficient. Uniform regularity for the compressible isentropic Hall-MHD system is established in terms of these coefficients. [ABSTRACT FROM AUTHOR]

This paper presents a model for simulating a two-phase flow involving cavitation and condensation, focusing on its application to turbopump flows. The model is derived by extending Kapila's approach, which incorporates thermal and free energy relaxation terms, to compressible multiphase flow equations in a rotating reference frame. The numerical method employed is a finite volume approach based on the Godunov scheme, ensuring pressure equilibrium and total energy conservation through a relaxation step. The model utilizes separate Equations of State (EOS) for the liquid and vapor phases, with the experimental saturation curve, expressed as p sat (T) , being the only parameter needed for cavitation modeling. To broaden its applicability beyond axial pumps, a specific Riemann solver is developed to couple rotating and non-rotating regions within the flow domain. The thermodynamic phase change process is validated using simple cases, and the model is further utilized to simulate water flows in a turbopump inducer. The characteristic curve of the pump is obtained in the non-cavitating regime for several mass flow rates conditions, and the model is successfully assessed in severe cavitation conditions, demonstrating the performance breakdown caused by vapor pockets. The modeling also provides detailed flow descriptions, including analysis of vapor pockets and their impact on blade pressure loading. • Modeling multiphase flow in rotating reference frame. • A new Riemann problem to connect fixed and rotating flow regions. • Phase change modeling by a relaxation method from pure phase to mixture. • Cavitating flow in turbopumps without parameter for the growth of the cavitation pockets. [ABSTRACT FROM AUTHOR]

In compressed sensing the goal is to recover a signal from as few as possible noisy, linear measurements with the general assumption that the signal has only a few non-zero entries. The recovery can be performed by multiple different decoders, however most of them rely on some tuning. Given an estimate for the noise level a common convex approach to recover the signal is basis pursuit denoising. If the measurement matrix has the robust null space property with respect to the ℓ2-norm, basis pursuit denoising obeys stable and robust recovery guarantees. In the case of unknown noise levels, nonnegative least squares recovers non-negative signals if the measurement matrix fulfills an additional property (sometimes called the M+-criterion). However, if the measurement matrix is the biadjacency matrix of a random left regular bipartite graph it obeys with a high probability the null space property with respect to the ℓ1-norm with optimal parameters. Therefore, we discuss non-negative least absolute deviation (NNLAD), which is free of tuning parameters. For these measurement matrices, we prove a uniform, stable and robust recovery guarantee. Such guarantees are important, since binary expander matrices are sparse and thus allow for fast sketching and recovery. We will further present a method to solve the NNLAD numerically and show that this is comparable to state of the art methods. Lastly, we explain how the NNLAD can be used for viral detection in the recent COVID-19 crisis.

The accumulation of frozen liquid around a central passageway of melt as it flows through a freezing region can make calculations very challenging. To both illustrate and to quantify some of these challenges from freezing, a model equation is developed. It simplifies the solution of Holmes (2007, https://gfd.whoi.edu/wp-content/uploads/sites/18/2018/03/MHolmesGFDReport%5f30151.pdf) for low Reynolds number single component liquid flow through a long tube that has a wall kept at subfreezing temperature. This model equation is used in conjunction with three different upstream configurations, each with parameters expressing their behavior. Analytical and numerical results give the parameters that have criteria for: the freezing of a compressible upstream reservoir that includes oscillatory behavior; the freezing of flow fed through a constriction with a large upstream pressure, just like a dripping water faucet during winter; the evolution of flow in multiple tubes connected by an upstream manifold, where some tubes end up with full flow and others freeze shut. Numerical runs with 1,000 tubes give a formula for the spacing between actively flowing (non‐frozen) tubes over wide ranges of the two upstream parameters (flow rate and manifold resistance). Results have implications in various areas in earth science. Some are: oscillatory and freezing shut criteria for flow of magma from a compressible region, a criterion for wintertime ice accumulation at natural springs, and the spacing between volcanos. Plain Language Summary: The dynamics of liquid flow in an upstream region are considered in conjunction with flow through a freezing region. This is because when liquid flows into a freezing region, the resistance change that arises from the accumulation of solid modifies the upstream pressure and flow rate in both upstream and freezing regions. This study shows examples of three different upstream situations with dynamic interaction between upstream and freezing regions. The interaction leads to complicated results such as oscillations, intense flow channelization in subfreezing surroundings, and complete freezing shut in some portions of the downstream region. Through the use of three examples, the fundamental nature of the interaction between upstream and freezing flows helps to begin to explain the complicated nature of freezing flows in Earth Science. Key Points: A simple model is developed for liquid flow into a freezing tubeThree upstream conditions are considered: compressible, throttled, and multiple tubes fed by a manifoldTimes and locations of freezing are quantified [ABSTRACT FROM AUTHOR]

This paper presents an all-Mach method for two-phase inviscid flow in the presence of surface tension. A modified version of the Hartens–Lax–van Leer Contact (HLLC) solver is developed and combined for the first time with a widely used volume-of-fluid (VoF) method: the compressive interface capturing scheme for arbitrary meshes (CICSAM). This novel combination yields a scheme with both HLLC shock capturing as well as accurate liquid–gas interface tracking characteristics. It is achieved by reconstructing non-conservative (primitive) variables in a consistent manner to yield both robustness and accuracy. Liquid–gas interface curvature is computed via height functions and the convolution method. We emphasize the use of VoF in the interest of interface accuracy when modelling surface tension effects. The method is validated using a range of test-cases available in the literature. The results show flow features that are in sensible agreement with previous experimental and numerical work. In particular, the use of the HLLC-VoF combination leads to a sharp volume fraction and energy field with improved accuracy. [ABSTRACT FROM AUTHOR]

A new approach of the axisymmetric compressible lattice Boltzmann method (LBM) has been developed for the numerical simulation of supersonic inviscid flow using the finite volume method. The circular function idea has been used for capturing the compressibility effect in the supersonic flow field. In this study, the axisymmetric LBM equations based on the circular function have been derived for the first time and presented in detail and appropriate axisymmetric boundary conditions have been applied for the 2-dimensional, 13-velocity, 2-energy-level lattices. For validation of developed code, two supersonic axisymmetric flows have been simulated around the hemisphere nose and 60° blunt body. A comparison of the obtained results with some valid empirical data shows the accuracy of the developed numerical algorithm. Also, the results of 2D and axisymmetric simulations have been studied to evaluate the effects of the new derived axisymmetric formulation on flow macroscopic properties. [ABSTRACT FROM AUTHOR]

This work focuses on the performance and validation of compressible turbulence models for the pressure-strain correlation. Considering the Launder Reece and Rodi (LRR) incompressible model for the pressure-strain correlation, Adumitroaie et al., Huang et al., and Marzougui et al., used different modeling approaches to develop turbulence models, taking into account compressibility effects for this term. Two numerical coefficients are dependent on the turbulent Mach number, and all of the remaining coefficients conserve the same values as in the original LRR model. The models do not correctly predict the compressible turbulence at a high-speed shear flow. So, the revision of these models is the major aim of this study. In the present work, the compressible model for the pressure-strain correlation developed by Khlifi−Lili, involving the turbulent Mach number, the gradient, and the convective Mach numbers, is used to modify the linear mean shear strain and the slow terms of the previous models. The models are tested in two compressible turbulent flows: homogeneous shear flow and the newly developed plane mixing layers. The predicted results of the proposed modifications of the Adumitroaie et al., Huang et al., and Marzougui et al., models and of its universal versions are compared with direct numerical simulation (DNS) and experiment data. The results show that the important parameters of compressibility in homogeneous shear flow and in the mixing layers are well predicted by the proposal models.

The effects of Mach number on the skin friction and velocity fluctuations of the turbulent boundary layer are considered through a sonic eddy model. Originally proposed for free shear flows, the model assumes that the eddies responsible for momentum transfer have a rotation Mach number of unity, with the entrainment rate limited by acoustic signaling. Under this assumption, the model predicts that the skin friction coefficient should go as the inverse Mach number in a regime where the Mach number is larger than unity but smaller than the square root of the Reynolds number. The velocity fluctuations normalized by the friction velocity should be the inverse square root of the Mach number in the same regime. Turbulent transport is controlled by acoustic signaling. The density field adjusts itself such that the Reynolds stresses correspond to the momentum transport. In contrast, the conventional van Driest–Morkovin view is that the Mach number effects are due to density variations directly. A new experiment or simulation is proposed to test this model using different gases in an incompressible boundary layer, following the example of Brown and Roshko in the free shear layer.

This paper is concerned with a compressible shear thickening fluid-particle interaction model for the evolution of particles dispersed in a viscous non-Newtonian fluid. Taking the influence of non-Newtonian gravitational potential into consideration, the existence and uniqueness of strong solutions are established.

We study the Cauchy problem for a system of equations corresponding to a singular limit of radiative hydrodynamics, namely, the 3D radiative compressible Euler system coupled to an electromagnetic field. Assuming smallness hypotheses for the data, we prove that the problem admits a unique global smooth solution and study its asymptotics.

We study the Cauchy problem and the mixed initial boundary value problem of a fluid-particle interaction system in R 3 . A Serrin type criterion for the strong solution of the Cauchy problem is established in terms of ∥ ρ ∥ L t ∞ L x ∞ and ∥ u ∥ L t s L x r , where 2 / s + 3 / r ≤ 1 and 3 < r ≤ ∞ . In view of some useful integral inequalities, we prove the life span estimates of the regular solution. [ABSTRACT FROM AUTHOR]

We extend previous analyses of the origins of complex transport dynamics in compressible porous media to the case where the input transient signal at a boundary is generated by a multimodal spectrum. By adding harmonic and anharmonic modal frequencies as perturbations to a fundamental mode, we examine how such multimodal signals affect the Lagrangian complexity of flow in compressible porous media. While the results apply to all poroelastic media (industrial, biological and geophysical), for concreteness we couch the discussion in terms of unpumped coastal groundwater systems having a discharge boundary forced by tides. Particular local regions of the conductivity field generate saddles that hold up and braid (mix) trajectories, resulting in unexpected behaviours of groundwater residence time distributions and topological mixing manifolds near the tidal boundary. While increasing spectral complexity can reduce the occurrence of periodic points, especially for anharmonic spectra with long characteristic periods, other signatures of Lagrangian complexity persist. The action of natural multimodal tidal signals on confined groundwater flow in heterogeneous aquifers can induce exotic flow topologies and mixing effects that are profoundly different to conventional concepts of groundwater discharge processes. Taken together, our results imply that increasing spectral complexity results in more complex Lagrangian structure in flows through compressible porous media. [ABSTRACT FROM AUTHOR]

Lightweight polypyrrole/cellulose aerogel (PPy/CA) composite with three-dimensional (3D) conductive network was synthesized via in-situ polymerization and freeze-drying techniques. The PPy/CA composite shows good elasticity and compressibility owing to the three-dimensional (3D) porous structure. The microwave absorption properties of the PPy/CA composite were measured by wave guide methods. The results show that the complex permittivity of the PPy/CA composite can be effectively modulated through adjusting the compression ratios from 0 to 65%. The PPy/CA composite exhibits optimal microwave absorption performance at the compression ratio of 65%, which the minimum reflection loss (RL) value reaches − 12.24 dB at 8.53 GHz with the thickness of 5 mm. Besides, the effective bandwidth (RL less than −10 dB) of the PPy/CA composite can cover the whole X-band (8.2–12.4 GHz) by changing the thickness in the range of 4–5 mm. The controllable microwave absorption performance, green, lightweight and well microwave absorption may make it to be a promising microwave absorber in the future. Our research opens a new way to design microwave absorbers by a simple mechanical compression. [ABSTRACT FROM AUTHOR]

All-solid-state rechargeable Zn-air battery is deemed to be a promising class of power source for its attractive operational safety and mechanical flexibility. To achieve high performance of Zn-air battery, the density of active sites on the ORR/OER bifunctional electrocatalyst and the maximized access to them are of great importance. In this study, a nitrogen, sulfur and ferric co-doped three-dimensional bifunctional catalyst (FeN x /N,S–C) is developed via a facile and eco-friendly molten salt assisted synthesis. The homogeneously dispersed FeN x species, the surrounding S dopants and the 3D hierarchical architecture synergistically endow FeN x /N,S–C with dense active sites with high accessibility, abundant open pores/channels and efficient mass transfer. Compared with the commercial Pt/C–RuO 2 catalyst, the application of FeN x /N,S–C in all-solid-state rechargeable Zn-air batteries results in enhanced ORR/OER activities, lower charge-discharge voltage gaps and improved 52 h cycling durability, implying its good feasibility in practical application. The all-solid-state FeN x /N,S–C based battery shows no performance decay during charge-discharge cycles when its sodium polyacrylate hydrogel electrolyte is compressed up to 67% strain or stretched up to 400% length, implying the feasibility of the present FeN x /N,S–C catalyst in the usage of stable and flexible electronics. Image 1 [ABSTRACT FROM AUTHOR]

In this paper, we prove global well-posedness of strong solutions to a 1-D compressible non-isothermal model for nematic liquid crystals, provided that the initial datum satisfies a natural compatibility condition. The initial density may vanish in an open subset. [ABSTRACT FROM AUTHOR]

Portable and wearable electronics are undergoing rapid development, but these flexible devices may break and fail to work under high compressive strains. Here, presented is a new free‐standing compressible carbon nanotube array (CCNA) with a unique gradually crosslinking structure, which mimics the gradient structure of the beak of the giant squid. The CCNAs can tolerate various compressive strains and demonstrate high reversible compressibility up to 1 00 000 cycles with high electrical conductivities. On the basis of the CCNA, a novel all‐solid‐state compression‐sensing supercapacitor (CSS) that can store energy and tolerate and sense the external strain change is produced. It demonstrates a high capacitance of 93.2 mF cm−2 and can be maintained by 94% even after 3000 continuous compressing cycles at a strain of 60%. In addition, it also shows superior strain sensing capability and stability up to 1900 compressive cycles. These flexible CSSs promise a wide range of applications including electronic skins and advanced bioelectronic devices. [ABSTRACT FROM AUTHOR]

A double-distribution-function based lattice Boltzmann method (DDF-LBM) is proposed for the simulation of polyatomic gases in the supersonic regime. The model relies on a numerical equilibrium that has been extensively used by discrete velocity methods since the late 1990s. Here, it is extended to reproduce an arbitrary number of moments of the Maxwell--Boltzmann distribution. These extensions to the standard 5-constraint (mass, momentum and energy) approach lead to the correct simulation of thermal, compressible flows with only 39 discrete velocities in 3D. The stability of this BGK-LBM is reinforced by relying on Knudsen-number-dependent relaxation times that are computed analytically. Hence, high Reynolds-number, supersonic flows can be simulated in an efficient and elegant manner. While the 1D Riemann problem shows the ability of the proposed approach to handle discontinuities in the zero-viscosity limit, the simulation of the supersonic flow past a NACA0012 aerofoil confirms the excellent behaviour of this model in a lowviscosity and supersonic regime. The flow past a sphere is further simulated to investigate the 3D behaviour of our model in the low-viscosity supersonic regime. The proposed model is shown to be substantially more efficient than the previous 5-moment D3Q343 DDF-LBM for both CPU and GPU architectures. It then opens up a whole new [ABSTRACT FROM AUTHOR]

Turbulence models facilitated by Kolmogorov's theory play an important role for compressible flows. Typically the basis of these models is the power spectrum of the velocity u or of the density-weighted velocity w ≡ ρ 1 / 3 u . While for incompressible flow the quantity turbulent kinetic energy characterises turbulent motions, from the thermodynamic point of view, due to fluctuations of the density and the temperature other kinds of energies play a role at the different scales in compressible turbulence. We generalise the power spectrum of the velocity u from incompressible flows to compressible flows by introducing the exergy spectrum as an application of the exergy concept. Furthermore, we discuss the application of the concept of turbulent exergy to turbulence modelling and demonstrate this approach with a direct numerical simulation and a Large-Eddy-Simulation of homogeneous isotropic turbulence. The advantage of turbulence modelling based on turbulent exergy is shown on the example of the Approximate Deconvolution Model (ADM) where, at smallest scales for its newly introduced entropy formulation, more available energy is extracted from the flow, and this occurs in a more physical way than for the classical equation set of the model using the total energy. [ABSTRACT FROM AUTHOR]

With the development of technology and the improvement of living standards, wearable electronic devices have attracted more attention. Here, both stretchable and compressible hydrogel strain sensors based on carbon nanofiber powder (CFP) and polyvinyl alcohol (PVA) are prepared by freezing–thawing cycles. The PVA/CFP hydrogel exhibits excellent stretchable (366%) and compressible strains (70%). During 1000 loading–unloading cycles, the PVA/CFP hydrogel has a low plastic deformation (<10%, for both stretching and compressing), small energy loss efficiency (5.62% under stretching and 12.13% under compressing), and stable mechanical strength and excellent sensitivity, at conditions whether it is stretched to 100% or compressed to 50% strains. The stretchable and compressible PVA/CFP hydrogel can not only accurately detect multiple stretching behaviors of human activity, such as bending of joints, swallowing or breathing, but also detect the changes of pressure during walking. Besides, the PVA/CFP hydrogel can operate electronic screens due to its internal ions, with more potential application in soft robotics and electronic skin. [ABSTRACT FROM AUTHOR]

This study aimed to develop an explicit JKR-type model for adhesive contact of a rigid sphere (of radius R) on a compressible elastic thin layer (of thickness h, Young’s modulus E, and Poisson ratio ν ) bonded or sliding on a rigid substrate. Based on a simple Kerr model for a compressible elastic layer, an explicit expression for strain energy of the elastic layer is derived in terms of the two JKR-type variables δ , a , where a is the radius of contact zone and δ is the indentation depth of the rigid sphere. Thus the equilibrium values of δ , a can be determined as the stationary point of the potential energy. The explicit model is justified by detailed comparison of the predicted results (for Poisson ratio ν ≤ 0.45 ) with known data reported in recent literature. For example, the validity and accuracy of the present model are demonstrated for moderately soft elastic thin layers under the condition 2 1 - v 2 W R 2 / E h 3 ≥ 100 (where W is adhesion energy per unit contact area, for instance, for typical materials with R = 500 μ m E = 100 KPa and W = 100 mJ / m 2 , the condition requests h < 20 μ m ). As compared to existing methods that request more substantial numerical calculations, the present model achieves an explicit expression for strain energy of a compressible elastic layer and could offer a simpler analytical method for adhesion mechanics of a rigid sphere on a compressible thin elastic layer bonded or sliding on a rigid substrate. [ABSTRACT FROM AUTHOR]

The paper is concerned with the derivation of the hyperbolic-elliptic asymptotic model for surface wave in a pre-stressed, compressible, elastic half-space, within the framework of plane-strain assumption. The consideration extends the existing methodology of asymptotic theories for Rayleigh and Rayleigh-type waves induced by surface/edge loading, and oriented to extraction of the contribution of studied waves to the overall dynamic response. The methodology relies on the slow-time perturbation around the eigensolution, or, equivalently, accounting for the contribution of the poles of the studied wave. As a result, the vector problem of elasticity is reduced to a scalar one for the scaled Laplace equation in terms of the auxiliary function, with the boundary condition is formulated as a hyperbolic equation with the forcing terms. Moreover, hyperbolic equations for surface displacements are also presented. Scalar hyperbolic equations for surface displacements could potentially be beneficial for further development of methods of non-destructive evaluation. [ABSTRACT FROM AUTHOR]

With the development of mature Computational Fluid Dynamics (CFD) tools for fluids (air and liquid) and Finite Element Methods (FEM) for solids and structures, many approaches have been proposed to tackle the so-called Fluid–Structure Interaction or Fluid–Solid Interaction (FSI) problems. Traditional partitioned iterations are often used to link available FEM codes with CFD codes in the study of FSI systems. Although these procedures are convenient, fluid mesh adjustments according to the motion and finite deformation of immersed solids or structures can be challenging or even prohibitive. Moreover, complex dynamic behaviors of coupled FSI systems are often lost in these iterative processes. In this paper, the author would like to review the so-called monolithic approaches for the solution of coupled FSI systems as a whole in the context of the immersed boundary method. In particular, the focus is on the implicit monolithic algorithm for compressible solids immersed inside a compressible liquid. Notice here the main focus of this paper is on liquid or more precisely liquid phase of water as working fluid. Using the word liquid, the author would like to emphasize the consideration of the compressibility of the fluid and the assumption of constant density and temperature. It is a common practice to assume that the pressure variations are not strong enough to alter the liquid density in any significant fashion for acoustic fluid–solid interactions problems. Although the algorithm presented in this paper is not directly applicable to aerodynamics in which the density change is significant along with its relationship with the pressure and the temperature, the author did revisit his earlier work on merging immersed boundary method concepts with a fully-fledged compressible aerodynamic code based on high-order compact scheme and energy conservative form of governing equations. In the proposed algorithm, on top of a uniform background (ghost) mesh, a fully implicit immersed method is implemented with mixed finite element methods for compressible liquid as well as immersed compressible solids with a matrix-free Newton–Krylov iterative solution scheme. In this monolithic approach, with the simple modulo function, the immersed solid or structure points can be easily located and thus the displacement projections and force distributions stipulated in the immersed boundary method can be effectively and efficiently implemented. This feature coupled with the key concept of the immersed boundary method helps to avoid topologically challenging mesh adjustments and to incorporate parallel processing commands such as Message Passing Interface (MPI) and further vectorization of the numerical operation. Once these high-performance procedures are implemented coupled with the monolithic implicit matrix-free Newton–Krylov iterative scheme with immersed methods, effective and efficient reduced order modeling techniques can then be employed to explore phase and parametric spaces. The in-house developed programs are at the moment two-dimensional. Furthermore, based on the same approach implemented in one-dimensional test example with one continuum immersed in another continuum, such monolithic implicit matrix-free Newton–Krylov iterative approach can be extended for the study of composites with deformable aggregates and matrix.

In this article we prove the local well-posedness for an Ericksen-Leslie's parabolic-hyperbolic compressible non-isothermal model for nematic liquid crystals with positive initial density.

In this paper, we are investigating the interaction between different passive material models and the mechano-electrical feedback (MEF) in cardiac modeling. Various types of passive mechanical laws (nearly incompressible/compressible, polynomial/exponential-type, transversally isotropic/orthotropic material models) are integrated in a fully coupled electromechanical model in order to study their specific influence on the overall MEF behavior. Our computational model is based on a three-dimensional (3D) geometry of a healthy rat left ventricle reconstructed from magnetic resonance imaging (MRI). The electromechanically coupled problem is solved using a fully implicit finite element-based approach. The effects of different passive material models on the MEF are studied with the help of numerical examples. It turns out that there is a significant difference between the behavior of the MEF for compressible and incompressible material models. Numerical results for the incompressible models exhibit that a change in the electrophysiology can be observed such that the transmembrane potential (TP) is unable to reach the resting state in the repolarization phase, and this leads to non-zero relaxation deformations. The most significant and strongest effects of the MEF on the rat cardiac muscle response are observed for the exponential passive material law.

The National Taiwan University/Purdue (NTU/PU) nonhydrostatic model is used to study mountain waves, hydraulic jumps, and lee vortices. The NTU/PU model is based on a compressible and three-dimensional system of equations with a terrain-following vertical coordinate. A major advantage of the model is the use of a modified forward-backward integration scheme that retains internal gravity waves but suppresses unwanted sound waves. In flow around an isolated ideal elliptical mountain, the simulated flow from the NTU/PU model spreads broadly in the crosswind direction with a well-defined stagnation point at the windward side of the mountain. Two semi-idealized downslope wind cases were conducted using the real topography at White Sand Missile Range (WSMR), New Mexico, USA. Our simulated flows including hydraulic jumps and vortex shedding in the lee of the Organ Mountains agree very well with observations for the case on 25 January 2004, at 15 UTC, but the discrepancy is somewhat larger for the case on 19 January 2004, at 15 UTC. This paper shows that the model results are sensitive to the detailed numerical schemes, filters, and physics, etc. It also suggests that more observations and improvements are required for a model to realistically simulate the flow over complex terrain.

Hydrogen is the cleanest fuel available because its combustion product is water. The internal combustion engine can, in principle and without significant modifications, run on hydrogen to produce mechanical energy. Regarding the technological solution leading to compact engines, a question to ask is the following: Can combustion engine systems be lubricated with hydrogen? In general, since many applications such as in turbomachines, is it possible to use the surrounding gas as a lubricant? In this paper, journal bearings global parameters are calculated and compared for steady state and dynamic conditions for different gas constituents such as air, pentafluoropropane, helium and hydrogen. Such a bearing may be promising as an ecological alternative to liquid lubrication.

This paper presents an all-Mach method for two-phase inviscid flow in the presence of surface tension. A modified version of the Hartens–Lax–van Leer Contact (HLLC) solver is developed and combined for the first time with a widely used volume-of-fluid (VoF) method: the compressive interface capturing scheme for arbitrary meshes (CICSAM). This novel combination yields a scheme with both HLLC shock capturing as well as accurate liquid–gas interface tracking characteristics. It is achieved by reconstructing non-conservative (primitive) variables in a consistent manner to yield both robustness and accuracy. Liquid–gas interface curvature is computed via height functions and the convolution method. We emphasize the use of VoF in the interest of interface accuracy when modelling surface tension effects. The method is validated using a range of test-cases available in the literature. The results show flow features that are in sensible agreement with previous experimental and numerical work. In particular, the use of the HLLC-VoF combination leads to a sharp volume fraction and energy field with improved accuracy.

Lightweight and highly porous magnetic polymer foam (MPF) composites were synthesized using a simple, efficient, and environmentally friendly strategy. Their scaffold structure was well controlled using a surfactant and by varying the amount of Fe 3 O 4 nanoparticles (NPs) added to the emulsion solution used in their preparation. The three-dimensional (3D) sponge-like MPFs are microwave-absorbing materials, and their structural and microstructural characteristics were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The magnetic properties of the MPF composites were also characterized. The polymer matrix of the composites exhibited excellent thermal stability. The microwave-absorbing properties of the MPF composites with increasing amounts of Fe 3 O 4 NPs (5 wt%, 10 wt%, and 15 wt%) were systematically studied. The microwave absorption of the polymer foam was enhanced to as high as 82% with the addition of Fe 3 O 4 NPs. Our results demonstrate that the prepared MPF composites have the potential to be used as lightweight, microwave-absorbing materials. Image 1 • Magnetic polymer foam was fabricated by high internal phase emulsion method. • Fe 3 O 4 NPs provide magnetic properties without disturbing porous structure. • The foam exhibited magnetic properties and compressibility. • Microwave absorption was enhanced with macroporous polymer foam/Fe 3 O 4 composite. [ABSTRACT FROM AUTHOR]