Nonlinear waves in a fluid-filled thin viscoelastic tube

In the present paper the propagation property of nonlinear waves in a thin viscoelastic tube filled with incom- pressible inviscid fluid is studied. The tube is considered to be made of an incompressible isotropic viscoelastic material described by Kelvin-Voigt model. Using the mass conservation and the momentum theorem of the fluid and radial dynamic equilibrium of an element of the tube wall, a set of nonlinear partial differential equations governing the propagation of nonlinear pressure wave in the solid-liquid coupled system is obtained. In the long-wave approximation the nonlinear far-field equations can be derived employing the reductive perturbation technique （RPT）. Selecting the expo- nent c~ of the perturbation parameter in Gardner-Morikawa transformation according to the order of viscous coefficient 7, three kinds of evolution equations with soliton solution, i.e. Korteweg-de Vries （KdV）-Burgers, KdV and Burgers equations are deduced. By means of the method of traveling-wave solution and numerical calculation, the propagation properties of solitary waves corresponding with these evolution equations are analysed in detail. Finally, as a example of practical application, the propagation of pressure pulses in large blood vessels is discussed.

An Analysis of Structural-Acoustic Coupling Band Gaps in a Fluid-Filled Periodic Pipe

A periodic pipe system composed of steel pipes and rubber hoses with the same inner radius is designed based on the theory of phononic crystals. Using the transfer matrix method, the band structure of the periodic pipe is calculated considering the structural-acoustic coupling. The results show that longitudinal vibration band gaps and acoustic band gaps can coexist in the fluid-filled periodic pipe. The formation of the band gap mechanism is further analyzed. The band gaps are validated by the sound transmission loss and vibration-frequency response functions calculated using the finite element method. The effect of the damp on the band gap is analyzed by calculating the complex band structure. The periodic pipe system can be used not only in the field of vibration reduction but also for noise elimination.

Characteristics of Vibrational Wave Propagation and Attenuation in Submarine Fluid-Filled Pipelines

As an important part of lifeline engineering in the development and utilization of marine resources, the submarine fluid-filled pipeline is a complex coupling system which is subjected to both internal and external flow fields. By utilizing Kennard＇s shell equations and combining with Helmholtz equations of flow field, the coupling equations of submarine fluid-filled pipeline for n=0 axisymmetrical wave motion are set up. Analytical expressions of wave speed are obtained for both s=1 and s=2 waves, which correspond to a fluid-dominated wave and an axial shell wave, respectively. The numerical results for wave speed and wave attenuation are obtained and discussed subsequently. It shows that the frequency depends on phase velocity, and the attenuation of this mode depends strongly on material parameters of the pipe and the internal and the external fluid fields. The characteristics of PVC pipe are studied for a comparison. The effects of shell thickness/radius ratio and density of the contained fluid on the model are also discussed. The study provides a theoretical basis and helps to accurately predict the situation of submarine pipelines, which also has practical application prospect in the field of pipeline leakage detection.

Deep Learning Applied to Computational Mechanics:A Comprehensive Review,State of the Art,and the Classics

Three recent breakthroughs due to AI in arts and science serve as motivation:An award winning digital image,protein folding,fast matrix multiplication.Many recent developments in artificial neural networks,particularly deep learning(DL),applied and relevant to computational mechanics(solid,fluids,finite-element technology)are reviewed in detail.Both hybrid and pure machine learning(ML)methods are discussed.Hybrid methods combine traditional PDE discretizations with ML methods either(1)to help model complex nonlinear constitutive relations,(2)to nonlinearly reduce the model order for efficient simulation(turbulence),or(3)to accelerate the simulation by predicting certain components in the traditional integration methods.Here,methods(1)and(2)relied on Long-Short-Term Memory(LSTM)architecture,with method(3)relying on convolutional neural networks.Pure ML methods to solve(nonlinear)PDEs are represented by Physics-Informed Neural network(PINN)methods,which could be combined with attention mechanism to address discontinuous solutions.Both LSTM and attention architectures,together with modern and generalized classic optimizers to include stochasticity for DL networks,are extensively reviewed.Kernel machines,including Gaussian processes,are provided to sufficient depth for more advanced works such as shallow networks with infinite width.Not only addressing experts,readers are assumed familiar with computational mechanics,but not with DL,whose concepts and applications are built up from the basics,aiming at bringing first-time learners quickly to the forefront of research.History and limitations of AI are recounted and discussed,with particular attention at pointing out misstatements or misconceptions of the classics,even in well-known references.Positioning and pointing control of a large-deformable beam is given as an example.

NUMERICAL SOLUTION OF FLUID-STRUCTURE INTERACTION IN LIQUID-FILLED PIPES BY METHOD OF CHARACTERISTICS

Fluid-structure interaction （FSI） is essentially a dynamic phenomenon and always exists in fluid-filled pipe system. The four-equation model, which has been proved to be effective to describe and predict the phenomenon of FSI due to friction coupling and Poisson coupling being taken into account, is utilized to describe the FSI of fluid-filled pipe system. Terse compatibility equations are educed by the method of characteristics （MOC） to describe the fluid-filled pipe system. To shorten computing time needed to get the solutions under the condition of keeping accuracy requirement, two steps are adopted, firstly the time step Δt and divided number of the straight pipe are optimized, sec-ondly the mesh spacing Δz close to boundary is subdivided in several submeshes automatically ac-cording to the speed gradient of fluid. The mathematical model and arithmetic are validated by com-parisons between simulation solutions of two straight pipe systems and experiment known from lit-erature.

Six intragastric balloons:Which to choose?

Endoscopically placed intragastric balloons(IGBs)have played a significant role in obesity treatment over the last 30 years,successfully bridging the gap between lifestyle modification/pharmacotherapy and bariatric surgery.Since they provide a continuous sensation of satiety that helps the ingestion of smaller portions of food,facilitating maintenance of a low-calorie diet,they have generally been considered an effective and reversible,less invasive,non-surgical procedure for weight loss.However,some studies indicate that balloons have limited sustainable effectiveness for the vast majority attempting such therapy,resulting in a return to the previous weight after balloon removal.In this review we try to summarize the pros and cons of various balloon types,to guide decision making for both the physician and the obese individual looking for effective treatment.We analyzed the six most commonly used IGBs,namely the liquid-filled balloons Orbera,Spatz3,ReShape Duo and Elipse,and the gas-filled Heliosphere and Obalon-also including comments on the adjustable Spatz3,and the swallowable Obalon and Elipse-to optimize the choice for maximum efficacy and safety.

Efficient Solution of 3D Solids with Large Numbers of Fluid- Filled Pores Using Eigenstrain BIEs with Iteration Procedure

To deal with the problems encountered in the large scale numerical simulation of three dimensional(3D)elastic solids with fluid-filled pores,a novel computational model with the corresponding iterative solution procedure is developed,by introducing Eshelby’s idea of eigenstrain and equivalent inclusion into the boundary integral equations(BIE).Moreover,by partitioning all the fluid-filled pores in the computing domain into the near-and the far-field groups according to the distances to the current pore and constructing the local Eshelby matrix over the near-field group,the convergence of iterative procedure is guaranteed so that the problem can be solved effectively and efficiently in the numerical simulation of solids with large numbers of fluid-filled pores.The feasibility and correctness of the proposed computational model are verified in the numerical examples in comparison with the results of the analytical solution in the case of a single spherical fluid-filled pore under uniform pressure in full space and with the results of the subdomain BIE in a number of other cases.The overall mechanical properties of solids are simulated using a representative volume element(RVE)with a single or multiple fluid-filled pores,up to one thousand in number,with the proposed computational model,showing the feasibility and high efficiency of the model.The effect of random distribution of fluid-filled on overall properties is also discussed.Through some examples,it is observed that the effective elastic properties of solids with a large number of fluid-filled pores in random distributions could be studied to some extent by those of solids with regular distributions.

Influence of Shear Effects on the Characteristics of Axisymmetric Wave Propagation in a Buried Fluid‑Filled Pipe

The acoustic propagation characteristics of axisymmetric waves have been widely used in leak detection of fluid-filled pipes.The related acoustic methods and equipment are gradually coming to the market,but their theoretical research obviously lags behind the field practice,which seriously restricts the breakthrough and innovation of this technology.Based on the fully three-dimensional effect of the surrounding medium,a coupled motion equation of axisymmetric wave of buried liquid-filled pipes is derived in detail,a contact coefficient is used to express the coupling strength between surrounding medium and pipe,then,a general equation of motion was derived which contain the pipe soil lubrication contact,pipe soil compact contact and pipe in water and air.Finally,the corresponding numerical calculation model is established and solved used numerical method.The shear effects of the surrounding medium and the shear effects at the interface between surrounding medium and pipe are discussed in detail.The output indicates that the surrounding medium is to add mass to the pipe wall,but the shear effect is to add stiffness.With the consideration of the contact strength between the pipe and the medium,the additional mass and the pipe wall will resonate at a specific frequency,resulting in a significant increase in the radiation wave to the surrounding medium.The research contents have great guiding effect on the theory of acoustic wave propagation and the engineering application of leak detection technology in the buried pipe.

Micromechanics-Based Elastic Fields of Closed-Cell Porous Media

Fluid-filled closed-cell porous media could exhibit distinctive features which are influenced by initial fluid pressures inside the cavities.Based on the equivalent farfield method,micromechanics-based solutions for the local elastic fields of porous media saturated with pressurized fluid are formulated in this paper.In the present micromechanics model,three configurations are introduced to characterize the different state the closed-cell porous media.The fluid-filled cavity is assumed to be a compressible elastic solid with a zero shear modulus,and the pressures in closed pores are represented by eigenstrains introduced in fluid domains.With the assumption of spheroidal fluidfilled pores,the local stress and strain fields in solid matrix of porous media are derived by using the Exterior-Point Eshelby tensors,which are dependent of the Poisson’s ratio of solid matrix and the locations of the investigated material points outside the spheroidal fluid domain.The reliability and accuracy of the analytical elastic solutions are verified by a classical example.Moreover,for finite volume fraction of the fluid inclusions,the local elastic fields of the porous media subjected to the initial fluid pressure and external load are obtained.The results show that the present micromechanics model provides an effective approach to characterize the local elastic fields of the materials with closed-cell fluid-filled pores.

A novel heterozygous frameshift mutation in PKDl causing polycystic kidney disease

Objective Polycystic kidney disease(PKD),characterized by the presence of progressive fluid-filled cysts in renal mainly,is a lifethreatening genetic disorder which often develops into end-stage renal disease.Inherited pattern of PKD includes autosomal dominant and autosomal recessive.Autosomal dominant PKD is genetically heterozygous involving either of two genes,PKD1 or PKD2.The purpose of this study is to identify a novel frameshift mutation in PKD1 causing polycystic kidney disease.

VIBRATION OF FLUID-FILLED MULTI-WALLED CARBON NANOTUBES SEEN VIA NONLOCAL ELASTICITY THEORY

Vibration characteristics of fluid-filled multi-walled carbon nanotubes axe studied by using nonlocal elastic Fliigge shell model. Vibration governing equations of an N-layer carbon nanotube are formulated by considering the scale effect. In the numerical simulations, the effects of different theories, small-scale, variation of wavenumber, the innermost radius and length of double- walled and triple-walled carbon nanotubes are considered. Vibrational frequencies decrease with an increase of scale coefficient, the innermost radius, length of nanotube and effects of wall number are negligible. The results show that the cut-off frequencies can be influenced by the wall number of nanotubes.

Leaky modes and wave components of a fluid-filled borehole embedded in an elastic medium

Based on the analysis of the complex poles of the characteristic function of the borehole system on four major Riemann sheets corresponding to the leaky modes, the wave components, such as compressional head arrival, shear wave and Stoneley wave, are calculated. It is pointed out that the existing head wave theory which neglects the contribution of the leaky modes is not applicable to the wave component calculation of the formation of large Poisson ratio. An improved method is proposed to isolate and calculate the wave components effectively and accurately.

Wave Propagation in Fluid-Filled Single-Walled Carbon Nanotube Based on the Nonlocal Strain Gradient Theory

A dynamic Timoshenko beam model is established based on the new nonlocal strain gradient theory and slip boundary theory to study the wave propagation behaviors of fluid-filled carbon nanotubes （CNTs） at nanoscale. The nanoscale effects caused by the CNTs and the inner fluid are simulated by the nonlocal strain gradient effect and the slip boundary effect, respectively. The governing equations of motion are derived and resolved to investigate the wave characteristics in detail. The numerical solution shows that the strain gradient effect leads to the stiffness enhancement of CNTs when the nonlocal stress effect causes the decrease in stiffness. The dynamic properties of CNTs are affected by the coupling of these two scale effects. The flow velocity of fluid inside the CNT is increased due to the slip boundary effect, resulting in the promotion of wave propagation in the dynamic system.

Forced vibrational power flow input and transmission in a fluid-filled shell

The characteristics of vibrational power flow in an infinite elastic cylindrical shell filled with fluid are investigated. The simple harmonic motion of the shell and the pressure field in the contained fluid are described by the Fltigge shell equations and Helmholtz equation respectively. The vibrational equation of this system is obtained by using the coupling of shell and fluid. The dispersion curves are discussed for different circumferential orders. By using Fourier transform and its inverse transform, the input power into this coupled system excited by a simple harmonic linearly distributed driving force is studied. Along the shell, the transmission of the power flow carried by different shell internal forces and by the contained fluid are discussed

Propagation of flexural modal waves in a fluid-filled borehole

The propagation of multipole modal waves along the well-axis in a fluid-filled borehole surrounded by elastic and nonelastic, infinite and finite formation is analysed by using the wave equations. The phase velocity dispersion and the excitation curves are numerically calculated. The waveforms excited by attenuating bursts are also calculated. The measurements with long-spaced dipole transducers made of PZT thin disks vibrating in bending mode are carried out in a concrete model well and the experimental results are compared with the theoretical results.

THE ENVELOPE SOLITON OF STRESS WAVE ON THE WALL OF A FLUID-FILLED CIRCULAR TUBE

The reductive perturbation method of multiple-scales is used to investigate the weak nonlinear modulation of the stress wave on the wall of a fluid-filled elastic circular tube. In the case of a single mode, the nonlinear Schrodinger equation which the wave amplitude satisfies and its envelope soliton solution of stress wave are obtained.