Effects Of Wall Flexibility Research Articles

Evaluation of hemodynamic parameters to study the variation of artery wall properties

Abstract The carotid artery is the utmost path of the blood to brain, scalp, and face. It is also a critical region of vascular diseases. Any change in hemodynamics of carotid artery brings impairment in the function of the whole cardiovascular system and may result in hypertension, kidney diseases, stroke, numbness, or paralysis on the side of the body opposite to the obstruction of the artery. In the current study, change in stiffness due to ageing and hypertensionis considered and investigated its effect on hemodynamics. Blood modeled as the non-Newtonian fluid flowing in pulsatile fashion which is not taken into account in many previous studies. In addition, viscoelastic properties of the arterial wall were incorporated to note the flow-induced deformation and effect of wall flexibility on hemodynamics by two-way fluid-structure interaction (FSI) algorithm.The variation in stiffness occurs in the range from healthy to disease arteryis 0.4 MPa to 4.5 MPa. Considering this fact, tenfold increase in wall stiffness caused a significant drop in the flow of blood

Amoeboid swimming in a compliant channel

Several prokaryotes and eukaryotic cells swim in the presence of deformable and rigid surfaces that form confinement. The most commonly observed examples from biological systems are motility of leukocytes and pathogens present within the blood suspension through a microvascular network, and locomotion of eukaryotic cells such as immune system cells and cancerous cells through interstices between soft interstitial cells and the extracellular matrix within the interstitial tissue. This motivated us to investigate numerically the flow dynamics of amoeboid swimming in a flexible channel. The effects of wall stiffness and channel confinement on the flow dynamics and swimmer motion are studied. The swimmer motion through the flexible channel is substantially decelerated compared to the rigid channel. The strong confinement in the amply flexible channel imprisons the swimmer by severely restricting its forward motion. The swimmer velocity in a stiff channel displays nonmonotonic variation with the confinement while it shows monotonic reduction in a highly flexible channel. The physical rationale behind such distinct velocity behaviour in flexible and rigid channels is illustrated using an instantaneous flow field and flow history displayed by the swimmer. This behavior follows from a subtle interplay between the shape changes exhibited by the swimmer and the wall compliance. This study may aid in understanding the influence of elasticity of the surrounding environment on cell motility in immunological surveillance and invasiveness of cancer cells.

Dynamic Behaviour and Seismic Response of Ground Supported Cylindrical Water Tanks

Liquid storage tank such as in water distribution systems, petroleum plants etc., constitute a vital component of life line systems. Reducing earthquake effects on liquid storage tanks, to minimize the environmental and economic impact of these effects, have always been an important engineering concern. In this paper, the dynamic behavior of cylindrical ground supported concrete water tanks with different aspect ratios is investigated using finite element software ANSYS. The natural frequencies and modal responses are obtained for impulsive and convective modes of vibration. The natural frequency of vibration of the tank is observed to be the lowest at maximum water depth. The fundamental impulsive frequency increases as water level reduces and for water level less than 1/3 of tank height, there is significantly no change in impulsive frequency. The effect of wall flexibility on dynamic behavior of the tank is investigated by performing the modal analysis of flexible and rigid tanks. For a partially filled tank, the results of the present study are of significant relevance. The response of the tank to the transient loading as horizontal ground motion of El Centro earthquake is studied for various water heights. As the height of water on the tank increases, the ultimate maximum seismic response parameters are also observed to be increased. The location of maximum hoop stress varies in accordance with the variations in input ground motion and water fill condition whereas shear and bending moment are maximum at the base.

Finite element analysis of stresses in a flexible bulk solid container

Current theories and design codes pertaining to storage structures for bulk solids have been developed in the context of rigid-walled silos and may not be applicable for smaller and highly flexible containers that are often used for industrial packaging and intermediate storage. The focus of this study is to investigate the effect of wall flexibility on the bulk stresses and wall pressures during storage using finite element analysis. The results show that when the wall stiffness is low, the computed bulk stresses in the vertical bin section are dominated by plasticity, while the stresses in the hopper section remain in the elastic state. In this situation, the wall pressure in the bin section is heavily influenced by the strength of the stored solid, which controls the extent of plastic flow. Overall, the normal wall pressure in the bin section is found to decrease with wall flexibility leading to a corresponding increase in vertical stress in the stored solid. As a consequence, the stresses in the hopper also increase leading to increasing loads on the hopper walls and potential exacerbation of handling issues for cohesive materials in highly flexible containers.

Seismic behavioral fragility curves of concrete cylindrical water tanks for sloshing, cracking, and wall bending

Seismic fragility curves of concrete cylindrical tanks are determined using the finite element method. Vulnerabilities including sloshing of contents, tensile cracking and compression failure of the tank wall due to bending are accounted for. Effects of wall flexibility, fixity at the base, and height-diameter ratio on the response are investigated. Tall, medium and squat tanks are considered. The dynamic analysis is implemented using the horizontal components of consistent earthquakes. The study shows that generally taller tanks are more vulnerable to all of the failure modes considered. Among the modes of failure, the bending capacity of wall was shown to be the critical design parameter.

Effect of Wall Flexibility on the Deformation during Flow in a Stenosed Coronary Artery

The effect of varying wall flexibility on the deformation of an artery during steady and pulsatile flow of blood is investigated. The artery geometry is recreated from patient-derived data for a stenosed left coronary artery. Blood flow in the artery is modeled using power-law fluid. The fluid-structure interaction of blood flow on artery wall is simulated using ANSYS 16.2, and the resulting wall deformation is documented. A comparison of wall deformation using flexibility models like Rigid, Linear Elastic, Neo-hookean, Mooney-Rivlin and Holzapfel are obtained for teady flow in the artery. The maximum wall deformation in coronary flow onditions predicted by the Holzapfel model is only around 50% that predicted by the Neo-Hookean model. The flow-induced deformations reported here for patient-derived stenosed coronary artery with physiologically accurate model are the first of its kind. These results help immensely in the planning of angioplasty.

Investigation of Soil Structure Interaction and Wall Flexibility Effects on Natural Sloshing Frequency of Vessels

The main purpose of this study is to establish the effects of vessel walls flexibility on its natural sloshing frequency considering soil-structure-fluid interaction theory. Furthermore, two new efficiently relations to find both of wall flexibility and soil-structure interaction effects on natural frequency are developed. Regarding the aim of current study three different conditions of elevated tanks are applied. Fixed base condition with an emphasis on recommendations of international code ACI-350, analytical FSSI regarding equivalent mass spring method, and the numerical direct method regarding theory of finite element are taken into consideration. Results indicate that there is no significant effect of walls flexibility on natural sloshing frequency regarding fixed base assumptions of vessels. On the contrary, significant effects of wall flexibility are achieved considering SSI theory. Results of international code ACI-350 show that, the international codes assumptions have imprecise estimations of natural sloshing frequency in the range of hard to very soft soil categories. On the other hand, it is observed that the wall flexibility has a more highlighted effect on natural frequency in soft soils rather than soil-structure interaction. The significance of wall flexibility effect on natural frequency is more than that of SSI considering soil softening.

Falling film on a flexible wall in the presence of insoluble surfactant

This work investigates the effect of an insoluble surfactant on the gravity-driven flow of a liquid film down a vertical flexible wall. The paper builds upon previous work [Matar et al., Phys Rev E 76(5):056301, 2007; Sisoev et al., Chem Eng Sci 65(2):950–961, 2010] to include the Marangoni effect attributable to the gradient of surfactant concentration on a free surface. Here we employ an integral method to derive a set of asymptotic evolution equations valid for a moderate flow rate, based on a long-wave approximation. A normal-mode approach is used to examine the linear stability of the system. Similar to the work presented by Matar et al., the results show that a flexible wall with weak damping acts to stabilize flow, while wall tension plays an unstable role. The insoluble surfactant, which acts to stabilize film flow, can reduce the effects of wall flexibility (wall damping and tension) on flow linear stability. The nonlinear evolution equations for the system are solved numerically for both a given initial perturbation wave packet and a periodic perturbation at the inlet boundary. The equations are mainly concerned with the evolution of the flow stability and wave interaction processes, during which solitary-like waveforms are observed. When wall damping is weak, it tends to deplete the ripples preceding the solitary-like humps. However, as wall damping increases in strength, the ripples intensify; a similar phenomenon is observed with an increase in wall tension. The surfactant, which reduces the amplitude and traveling speed of the solitary-like waveforms, acts to distinctly weaken the dispersion of the interfacial wave.

Effect of wall flexibility on the dynamic earth pressure for cantilevered retaining wall

Effect of wall flexibility on the dynamic earth pressure for cantilevered retaining wall

Numerical simulation of blood flow in a flexible stenosed abdominal real aorta

This study concerns an evaluation of pulsatile flow and arterial wall behavior in a real world model of a stenosed abdominal aorta and iliac arteries. Two different geometries of a healthy and severly stenosed abdominal aorta, extracted from CT scan images and simulations of fluid flow and tissue interaction (Fluid Solid Interaction), are carried out. The blood is taken as incompressible, non-Newtonian, and the arterial wall tissue is treated as isotropic, elastic material with uniform mechanical properties. The results of using two models with rigid and flexible walls are presented and compared. The computed pressure at the abdominal aorta for a flexible healthy wall is consistent with measured values in vivo conditions. Results show that the computed pressure is lower by 15% for the flexible model, as compared to the rigid and complaint models. Although results obtained from the FSI study of pulsatile healthy and stenosed models show a similar trend for Wall Shear Stress (WSS) patterns, considerable differences in magnitude exist. It is shown that for the cases presented here, the effects of wall flexibility and actual stenosed geometry on the flow performance of veins are noticeable.

Effect of Wall Flexibility on Dynamic Response of Concrete Rectangular Liquid Storage Tanks under Horizontal and Vertical Ground Motions

In this study, the finite-element method is used to investigate the seismic behavior of rectangular liquid tanks in two-dimensional space. This method is capable of considering both impulsive and convective responses of liquid-tank system. Two different finite-element models corresponding with shallow and tall tank configurations are studied under the effects of both horizontal and vertical ground motions using the scaled earthquake components of the 1940 El-Centro earthquake record. The containers are assumed fixed to the rigid ground. Fluid-structure interaction effects on the dynamic response of fluid containers are taken into account incorporating wall flexibility. The results show that the wall flexibility and fluid damping properties have a major effect on seismic behavior of liquid tanks and should be considered in design criteria of tanks. The effect of vertical acceleration on the dynamic response of the liquid tanks is found to be less significant when horizontal and vertical ground motions are considered together. The results in this study are verified and compared with those obtained by numerical methods and other available methods in the literature.

Liquid plug propagation in flexible microchannels: A small airway model.

In the present study, we investigate the effect of wall flexibility on the plug propagation and the resulting wall stresses in small airway models with experimental measurements and numerical simulations. Experimentally, a flexible microchannel was fabricated to mimic the flexible small airways using soft lithography. Liquid plugs were generated and propagated through the microchannels. The local wall deformation is observed instantaneously during plug propagation with the maximum increasing with plug speed. The pressure drop across the plug is measured and observed to increase with plug speed, and is slightly smaller in a flexible channel compared to that in a rigid channel. A computational model is then presented to model the steady plug propagation through a flexible channel corresponding to the middle plane in the experimental device. The results show qualitative agreements with experiments on wall shapes and pressure drops and the discrepancies bring up interesting questions on current field of modeling. The flexible wall deforms inward near the plug core region, the deformation and pressure drop across the plug increase with the plug speed. The wall deformation and resulting stresses vary with different longitudinal tensions, i.e., for large wall longitudinal tension, the wall deforms slightly, which causes decreased fluid stress and stress gradients on the flexible wall comparing to that on rigid walls; however, the wall stress gradients are found to be much larger on highly deformable walls with small longitudinal tensions. Therefore, in diseases such as emphysema, with more deformable airways, there is a high possibility of induced injuries on lining cells along the airways because of larger wall stresses and stress gradients.

Seismic response of concrete rectangular tanks for liquid containing structures

In this paper, a procedure for computing hydrodynamic pressures in rectangular tanks is proposed. The procedure, which is referred to as the sequential method, considers the effect of the flexibility of the tank wall in determining the hydrodynamic pressures. In this study, only the impulsive response of the tank is considered. Based on a two-dimensional model of the tank wall, dynamic time-history analysis is carried out to study the effect of wall flexibility on the response. In the analysis, both a tall tank and a shallow tank are considered. The results of analysis are compared with those obtained based on current design practice codes and standards. The well-known Housner's model, which assumes that the mass of liquid is lumped on the wall based on rigid wall boundary condition in the calculation of hydrodynamic pressure, is widely used in practice. A comparison shows that in most cases, the lumped mass approach overestimates the base shear. The effect of wall flexibility on wall displacements, base shears, and moments are also discussed.Key words: reinforced concrete, liquid containing, rectangular tank, seismic, dynamic analysis, tank flexibility.

Dynamics of Solid-Containing Tanks. II: Flexible Tanks

The analysis of the response to horizontal base shaking for solid-containing rigid tanks, described in a companion paper, is extended to flexible tanks. Simple, approximate expressions for the critical responses are formulated, and comprehensive numerical data are presented that elucidate the effects of wall flexibility over wide ranges of the parameters involved. The response quantities examined include the stresses induced on the wall, the maximum wall forces, and the forces transmitted to the foundation. In addition to long-period, effectively static excitations, harmonic motions of arbitrary frequency and an actual earthquake ground motion are considered. A brief account is also given of the interrelationship of the critical responses of solid-containing tanks and those induced in tanks storing a liquid of the same mass density.

Dynamic Response of Rectangular Flexible Fluid Containers

Analytic solution methods are developed that can be used to evaluate dynamic response characteristics of partially filled flexible rectangular fluid containers under horizontal or vertical ground excitation. Fluid-structure interaction effects on the dynamic responses of partially filled fluid containers are taken into account incorporating wall flexibility. The containers are assumed fixed to the rigid ground. Solutions based on three-dimensional modeling of the rectangular containers are obtained by applying the Rayleigh-Ritz method using assumed vibration modes of rectangular plate with suitable boundary conditions as admissible functions. Solutions by the present two-dimensional analysis method are compared well with those by a coupled boundary-element/finite-element method. Dynamic responses of twoand three-dimensional models are calculated by the proposed methods in time domain and the effects of three-dimensional geometry on the responses are investigated. Variation of dynamic response characteristics with respect to the length to height ratio of the flexible wall is studied. The effect of wall flexibility on the sloshing response is investigated.

Effects of Wall Flexibility on the Rotordynamic Coefficients of Turbulent Cryogenic Annular Seals

Abstract A theory for analyzing the effects of elastic deformations of the seal wall on the dynamic characteristics of high pressure cryogenic annular seals under concentric operation is presented. The bulk flow continuity, axial and circumferential momentum, and the energy transport equations are utilized to determine the pressure distribution in the seal. Thermophysical properties of the cryogenic fluid are assumed to be functions of the local pressure and temperature. The wall deformations are obtained using an iso-parametric, axi-symmetric Finite Element formulation of the seal wall. A perturbation analysis is employed to arrive at the first order solution which yields the rotordynamic coefficients. Results obtained for the case of Space Shuttle Main Engine Oxygen Turbopump (SSME-HPOTP) Preburner Seal show a significant impact of seal flexibility on the dynamic coefficients.

Finite Element Analysis Of An Axially Moving Beam, Part I: Time Integration

Axially moving materials arise in problems associated with spacecraft antennas, pipes conveying fluid and telescopic robotic manipulators. Axially moving beams are a special class of axially moving materials, in which the axially moving material is modelled as a slender beam and the mechanism of elastic deformation is transverse bending. The mass of this system is not constant and the general analytical solution to the equation of motion is not known. In this study, numerical solutions are obtained using finite element analysis. However, instead of following the obvious (but cumbersome) approach of using fixed-size elements and increasing their number, in a step-wise fashion, as mass elements enter the domain of interest, a more elegant approach is followed wherein the number of elements is fixed, while the sizes of the elements change with time. To this end, a variable-domain beam finite element the size of which is a prescribed function of time is formulated in Part I. The accuracy of this variable-domain beam element is demonstrated through the time-integration of equations of motion using various extrusion profiles. The effects of wall flexibility, tip mass, and high frequency axial motion perturbations to the transverse response of the flexible extendible beam are also examined.

A finite element scheme for acoustic propagation in flexible-walled ducts with bulk-reacting liners, and comparison with experiment

The effects of wall flexibility on the propagation of acoustical and structural waves in ducts lined with bulk-reacting sound-absorbing material is studied both theoretically and experimentally. The theoretical treatment involves the use of a coupled finite element model for the airway, liner and flexible wall. Coupled eigenmodes in the duct are determined and a composite solution is obtained, by the use of modal superposition, for the sound field in the vicinity of a transition from a rigid to a flexible-walled segment. Analogous experimental data are presented for the sound pressure level inside a simple test duct of rectangular cross-section with a flexible wall. The wall displcement is also measured, and good agreement is obtained between theoretical and measured results. Both indicate significant increases in sound absorption within the duct—compared with that in a rigid-walled duct—in the vicinity of “cut-on” frequencies for the axial structural modes in the duct wall. Some parametric studies of this effect are also presented.

Effects of wall flexibility on the dynamic response of liquid storage tanks

A knowledge of the free vibration characteristics of tank — liquid systems is essential in order to predict dynamic responses. It has been demonstrated that in a dynamic situation impulsive forces are significant because of the elastic compliance of the tank wall. In this paper, the effects of wall flexibility on the dynamic response of liquid storage tanks is considered. Variational principles are used to obtain a functional describing coupled oscillations between a linear elastic body and a liquid of small wave heights. A complementary Rayleigh's quotient has been introduced to obtain coupled natural frequencies and the dynamic pressure distribution in the axisymmetric natural modes of vibration. Both low frequencies (liquid sloshing mode) and high frequencies (tank bulging mode) have been included in this study.

Wall pressures in squat steel silos from simple finite element analysis

Many different procedures for predicting the pressures on circular silo walls have been developed. These include methods based on simple assumptions, plasticity theory, minimising the stored solid recoverable strain energy and some finite element procedures using complex non-linear material characterisation. The purpose of the present paper is to show that comparatively simple finite element techniques which include appropriate wall friction characteristics can accurately model the pressures exerted by stored solids on cylindrical silo walls, at least for the initial filling condition. The study is entirely heuristic in character, and it is not suggested that stored bulk solids are normally in an elastic state. The aim of this development is to study the pressure distributions in squat circular silos and circular silos with flexible walls. Almost all the above-mentioned existing procedures give a poor representation of pressures in squat silos, and only one of them considers silos with flexible walls. However, these two categories are becoming increasingly important as steel silos become cheaper than concrete silos, and as on-ground squat structures replace tower blocks. The finite element analysis presented in this paper is shown to compare well with existing predictions for initial filling pressures in deep stiff-walled silos. Some results are also given to show the effect of wall flexibility on the predicted pressures.