bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave Excitation on a Flexible Surface in the Presence of a Uniform Mean Flow
Authors: Lucey Anthony D. , Sen Pradeep K. , Carpenter Peter W.
Published: June 2008

The excitation of neutrally stable waves in a system comprising uniform flow over a flexible wall is studied. Linear theory and numerical simulation are used. A feature of wave excitation in such systems is that distinct amplitude ratios exist when more than one wave is present. An amplitude-ratio equation is first derived for the two waves excited directly by oscillatory excitation applied to a spatially homogeneous plate-spring flexible wall. There after, a second amplitude-ratio equation is derived for waves on a wall with spatially varying foundation-spring stiffness coefficient. In this case, waves beyond a region of changing flexibility are continuously excited by waves propagating through the region of change. Both energyflux and WKB methods are used to derive the final amplitude-ratio equation. It is shown, inter alia, that the waves upstream and downstream of the region of slowly changing wall properties are well-described by the dispersion equation based upon local wall properties. Finally, the case of waves incident upon a region of rapidly changing wall properties is simulated and discussed.
Seismic Test and Analysis on a Neutron Reflector in APWR Reactor Internals: Fluid Damping and Added Mass Effect
Authors: Nakamura Tomomichi , Hirota Kazuo , Ichikawa Toshio , Yonemoto Satoshi , Murota Minoru , Tanaka Toshihiko , Kimura Masao
Published: June 2008

In the case of a structure surrounded by liquid in a narrow gap, the added fluid mass and the damping become large number, compared with the ones in air. A new method to estimate the added damping for this case has been introduced by some of the authors in 2001, which is based on a narrow passage flow theory. Following this theory, a model test has been conducted, where the structure modified the reactor component for a newly developed PWR power plant. The test model is a cylinder type structure with four narrow passages to modify the real component. It consists of four blocks connected with tie rods and immersed in water inside a rigid shell to simulate the surrounding core barrel. The cylinder is suspended in the space to move as a rigid body. In this paper, results of a model test are shown comparing the data with the calculated ones based on the above method. The natural frequency of the structure in air decreases almost to 1/5 in water, and it shows no great trend in relation to the amplitude. However, the damping increases with amplitude, and also shows the clear effect of the gap clearance. These trends agree reasonably well with the calculated trends.
Exact Solution of the Linear Hyperbolic Four-Equation System in Axial Liquid-Pipe Vibration
Authors: Tijsseling A. S.
Published: June 2008

The so-called “FSI four-equation model” describes the axial vibration of liquid-filled pipes. Two equations for the liquid are coupled to two equations for the pipe, through terms proportional to the Poisson contraction ratio, and through mutual boundary conditions. Skalak (1955/1956ab) defined this basic model, which disregards friction and damping effects. The four equations can be solved with the method of characteristics (MOC). The standard approach is to cover the distance-time plane with equidistantly spaced grid-points and to time-march from a given initial state. This approach introduces error, because either numerical interpolations or wave speed adjustments are necessary. This paper presents a method of exact calculation in terms of a simple recursion. The method is valid for transient events only, because the calculation time grows exponentially with the duration of the event. The calculation time is proportional to the temporal and spatial resolution. The exact solutions are used to investigate the error due to numerical interpolations and wave speed adjustments, with emphasis on the latter.
Stability of Empty and Fluid-Filled Circular Cylindrical Shells Subjected to Dynamic Axial Loads
Authors: Pellicano F. , Amabili M. , Pai¨doussis M. P.
Published: June 2008

In the present study the dynamic stability of simply supported, circular cylindrical shells subjected to dynamic axial loads is analyzed. Geometric nonlinearities due to finite-amplitude shell motion are considered by using the Donnell’s nonlinear shallow-shell theory. The effect of structural damping is taken into account. A discretization method based on a series expansion involving a large number of linear modes, including axisymmetric and asymmetric modes, and on the Galerkin procedure is developed. Both driven and companion modes are included allowing for travelling-wave response of the shell. Axisymmetric modes are included because they are essential in simulating the inward deflection of the mean oscillation with respect to the equilibrium position. The shell is simply supported and presents a finite length. Boundary conditions are considered in the model, which includes also the contribution of the external axial loads acting at the shell edges. The effect of a contained liquid is also considered. The linear dynamic stability and nonlinear response are analysed by using continuation techniques.
Formation Process of Taylor Cells of a Surfactant Solution
Authors: Watanabe Keizo , Takayama Tsukasa , Ogata Satoshi
Published: June 2008

The flow of surfactant solutions between two coaxial cylinders was investigated using the laser-induced-fluorescence flow visualization technique to clarify the effect of drag-reducing additives on the formation process of Taylor cells in Taylor-Couette flow. Test fluids were Ethoquad O/12 10, 50 and 100 ppm surfactant solutions. In the Taylor number range of, 1.2×105 ≤ Ta ≤ 7.1×105, tap water and 10 ppm surfactant solution flows consisted of Taylor vortices and much smaller Go¨rtler vortices at the rotating inner cylinder wall. However, in 50 and 100 ppm surfactant solutions, Taylor vortices are not apparent and Go¨rtler vortices are collapsed. Measurement of the wavelength of Go¨rtler vortices led to the conclusion that surfactant solutions have a stabilizing effect on Go¨rtler instabilities. This effect depends on the concentration of surfactant solutions, and becomes considerable with increasing acceleration of the rotating inner cylinder. By considering the experimental results for the surfactant solutions without counterions, in which Taylor cells were not formed, it was shown that the increase in the local field viscosity based on the shear-induced structure of the surfactant solutions has a stabilizing effect on Go¨rtler instability.
Heat Transfer in Steady Viscoelastic Flow in Pipes of Complex Shapes
Authors: Letelier Mario F. , Mardon˜ez Alejandro , Siginer Dennis A.
Published: June 2008

An analysis of heat transfer in pipe-flow is presented for the cases in which viscoelastic properties of the fluid induce secondary flows in no-circular tubes, thus enhancing the tranversal transport capacity of the flow. It is used the Phan-Thien-Tanner (PTT) viscoelastic model, and the flow is assumed steady and laminar. The velocity field is first determined through a technique previously developed by the authors, and the energy equation is solved afterward for the temperature field for conditions of constant wall temperature and constant longitudinal temperature gradient. Perturbation analysis is applied around the relaxation time as parameter, wherefrom all orders of velocity and temperature are analytically found up to third order. Plots of results for triangular and square tubes are presented, followed by a discussion of the relevant findings related to interactions among the main variables generated by viscoelastic fluid behaviour.
Numerical Model of Complex Porous Media
Authors: Chambarel Andre´ , Bolvin Herve´
Published: June 2008

In complex porous media we often notice a percolation phenomenon [KIR 71] [GRI 89]. Usually these media present discontinuous characteristics and a random space distribution [LET 00] [BIR 95]. There results that the classical models based on the resolution of a partial differential problem become inefficient because we have non-derivable function [MAU 01]. Statistical approaches based on the resolution of partial differential problems pose notably the questions concerning the continuity of the functions representing the physical properties of the medium. In this work we propose to study a numerical model of porous media based on a mixture of 2 components in a percolation context. In practice, the main difficulty is based on the complex physical properties. We present also a model of homogenization. Our numerical model is based on the Finite Element approach.
Automatic MRI Image Segmentation and Left Ventricle Surface Reconstruction for Characterizing Myocardial Muscle Function
Authors: Etebari Ali , Pierrakos Olga , Vlachos Pavlos P.
Published: June 2008

Normal left ventricular function is characterized by synchronous firing of myocardial muscle fibers. Healthy myocardial function is essential in maintaining cardiac output, and any disturbance to the contracting muscle cells can be detrimental to the health of the heart. A method for non-invasively monitoring left ventricle (LV) operation is necessary for documenting the progression of heart disease and malfunction, such that the point at which disease becomes irreversible can be determined.
Modeling and Simulation of Peristaltic Transport and Mixing in the Human Gastrointestinal Tract
Authors: Jeffrey Brian D. , Udaykumar H. S.
Published: June 2008

Despite active research, the mechanism by which food is transported, mixed and absorbed in the human gastro-intestinal tract remains poorly understood. At the University of Iowa Veterans Hospital, clinicians have been treating veterans for abnormal mechanical activity of the stomach and intestine and are pursuing research into such mechanisms, in order to better control and treat such ailments. The pathology experienced by these patients tends to be chronic and somewhat refractory to treatment particularly in the elderly, those with diabetes or after abdominal operations. Abnormal activity is responsible for gastro-esophageal reflux, gastroparesis, diarrhea, and abdominal pain syndromes. Current techniques of imaging and of image analysis should steadily improve ability to obtain information on mechanical activity of stomach and intestines in patients. Yet, for these imaging techniques to provide optimal diagnostic information, one needs to understand, in depth, how the visual parameters of contractions relate to digestion and absorption of nutrients.
Wing Geometry and Dynamic Similarity in Insect Flight
Authors: Bedoya Julian , Rincon Diana M.
Published: June 2008

The study of insect and bird flight has always been a curiosity, but it is yet to be described as plentifully as fixed wing aerodynamics. The United States military has expressed an interest in this topic, providing some institutions with funding. The main intention for this type of research is to develop small robots resembling insects or birds for use in exploration, surveillance and intelligence. While conceptually these applications could be accomplished with fixed-wing aircraft, there is a tremendous lack of stealth in these vehicles. The velocities associated with the required lift forces for small flapping-wing insect flights are significantly smaller than for insect-size fixed-wing aircraft. Therefore, it is more feasible and practical to aim for flapping wing flight.
Flow-Induced Wall Pressure Under Average Resting Hemodynamic Conditions for Patient-Specific Abdominal Aortic Aneurysms
Authors: Finol Ender A. , Hajiloo Shoreh , Keyhani Keyvan , Vorp David A. , Amon Cristina H.
Published: June 2008

Abdominal Aortic Aneurysms (AAAs) are characterized by a continuous dilation of the infrarenal segment of the abdominal aorta. Despite significant improvements in surgical procedures and imaging techniques, the mortality and morbidity rates associated with untreated ruptured AAAs are still outrageously high. AAA disease is a health risk of significant importance since this kind of aneurysm is mostly asymptomatic until its rupture, which is frequently a lethal event with an overall mortality rate in the 80% to 90% range. From a purely biomechanical viewpoint, aneurysm rupture is a phenomenon that occurs when the mechanical stress acting on the dilating inner wall exceeds its failure strength. Since the internal mechanical forces are maintained by the dynamic action of blood flowing in the aorta, the quantification of the hemodynamics of AAAs is essential for the characterization of their biomechanical environment.
Simulation of Gas Metal Arc Welding Short Circuiting Transfer Using a Front Tracking Method
Authors: Xu Guo , Kannatey-Asibu Elijah , Schultz William W. , Wang P. C.
Published: June 2008

A numerical model is developed to simulate the short-circuiting metal transfer process during gas metal arc welding (GMAW). To our best knowledge, for the first time, the energy equation is considered in analyzing the short-circuiting time, along with the continuity and momentum equations. A front tracking free surface method is implemented to explicitly track the profile of the liquid bridge. The electromagnetic field, distribution of velocity, pressure, and temperature are calculated using the developed model. Effects of welding current, Marangoni shear stress and initial drop volume on short-circuiting duration time are examined. The results show that both the electromagnetic force and Marangoni shear stress play significant roles in the short-circuiting transfer.
Method for Flexible Part Clamping Simulation and Optimization
Authors: Hsieh Ching , Liang Fei
Published: June 2008

EAVS is a GM in-house software package to simulate dimensional variations of manufacturing process in vehicle body assembly involving flexible parts. Methods used in EAVS for clamping simulation, variation analysis and sequence optimization are described in this paper. Two application examples, a reinforcement body assembly and a door ring assembly, are given to show the implementation and performance of the methods.
Contact Area Modeling and On-Line Estimation in Resistance Spot Welding
Authors: Li Wei
Published: June 2008

Contact areas at both electrode-to-sheet and sheet-to-sheet interfaces are important in the resistance spot welding process. Given electrode force and welding time, contact areas strongly affect the amount of electrical current needed to make a good weld. In production, process variation such as electrode wear and misalignment causes the contact areas to vary. This effect contributes largely to the quality variation of resistance spot welds. This paper proposes a model-based approach to contact area estimation in the resistance spot welding process. A finite element analysis procedure is used to characterize the contact area behaviors. Based on the understanding from the simulations, a lumped parameter model, together with its calibration and estimation procedures, is developed for on-line applications. The proposed method is demonstrated successful under various process conditions including electrode size, force, welding time and current. It provides important information for on-line monitoring and control of the resistance spot welding process.
Application of CFD Technology Analyzing the Three-Dimensional Aerodynamic Behavior of Dimpled Golf Balls
Authors: Ting L. L.
Published: June 2008

A computational modeling method using the CFD codes GAMBIT and FLUENT jointly has been developed for analyzing the three-dimensional aerodynamic behavior of surface dimpled golf balls. Drag and lift coefficient values and fluid field solutions have been obtained for the balls with three different types of dimple pattern distributions, which consist of the dimples with the same or different pocket sizes and depths. The baseline case golf ball carries 422 circular dimples with the dimple distribution pattern very similar to that of a Spalding TOP-FLITE PLUS II ball. The computed drag and lift coefficients of this baseline ball stationary or spinning have been made to compare with some published wind-tunnel experimental results, and the agreement was found to be good. This indicates that the modeling method developed is relevant and suitable for predicting the ball flight behavior for varying dimple geometry and dimple pattern distribution designs. From this, the necessary ball surface design parameter changes can be found either to enhance or to limit the ball’s aerodynamic performance, which controls and determines the ball flying distance and trajectory accuracy after initial impact.
Flow Characteristics of Low Concentration Non-Newtonian Fluid Through a Channel With an Obstruction at the Entry
Authors: Kabir M. A. , Khan M. M. K. , Rasul M. G.
Published: June 2008

The flow of Newtonian fluid (eg. water) in the test channel with an obstruction at the entrance placed in a wider channel was seen to be stagnant, forward or reverse depending on the position of the obstruction. This interesting flow phenomenon has potential benefit and can be employed in the control of energy and various flows in process engineering. This study was extended to non-Newtonian fluid for further investigation using flat plate as an obstruction. A low concentration polyacrylamide fluid solution (0.018%) showing non-Newtonian fluid behavior was used in this investigation. The parameters that affect the flow inside and around the test channel were the gap (g) between the obstruction geometry and the test channel, the Reynolds number and the length of the test channel. The maximum reverse flow inside the test channel observed was 20%–25% of the outside test channel velocity at g/w (gap to width) ratio of 1 for Reynolds number of 1000 to 3500. The results of the influence of the test channel length and the Reynolds number on the velocity ratio (Vi/Vo: inside velocity/outside velocity in the test channel) is also presented and discussed.
Influence of the Fixed Negative Charges on the Measured Poisson’s Ratio, Young’s Modulus and Electrical Response of Articular Cartilage
Authors: Likhitpanichkul Morakot , Sun Daniel D. , Guo X. Edward , Lai W. Michael , Mow Van C.
Published: June 2008

Under physiological conditions, the solid extracellular matrix (ECM) of articular cartilage derives negative charges from its proteoglycan content [1]. The load-deformation behavior of the tissue (i.e., its apparent mechanical property) comes from not only the intrinsic matrix properties, but also these charges that are attached to the matrix. Study shows that in 1D configurations (such as confined compression), at equilibrium, the osmotic pressure associated with these fixed charges may contribute as much as 50% of the apparent compressive modulus [1–3].
Mean and Oscillatory Flows Past Porous Plates
Authors: Wang J. , Downie M. J. , Graham J. M. R.
Published: June 2008

This paper reports on a study that has been carried out into the flow fields induced by mean and oscillatory flows incident on porous plates normal to the flow. The motivation for this work is the possibility of using porous plates to modify the damping and added mass of bodies floating in waves with a view to reducing their motion. A series of experiments including sea keeping experiments, flow visualization experiments and numerical simulations have been carried out for both solid plate and porous plates in the context of their effectiveness as bilge keels designed to damp the motions of floating structures. The porous plate with small size holes has been shown to be a more efficient damping device more often than the solid one. Both plates produce a pressure drag arising from flow separation that damps the motion, but the proportion of drag to total force is greater in the case of the porous plate.
Variation of Electrical Conductivity With Water Content in Agarose Gel
Authors: Vega Adriana L. , Yao Hai , Justiz Marc-Antoine , Gu Weiyong
Published: June 2008

Specific electrical conductivity, a material property of biological tissues, has been found to be greater in tumor tissue than in normal tissue on account of its higher water content [1]. Its value is related to water content, ion concentrations, and ion diffusivities within biological tissues [e.g., 1,2,3]. The variation in conductivity with water content is hypothesized to be related to the change in ion diffusivities [5,6]. The objective of this study is to investigate the relationship between conductivity and water content in hydrogels. The main goal is to develop a similar relationship for biological tissues and to understand deformation-dependent ion diffusivity in tissues under mechanical loading.
Symmetry Breaking and Mode-Interaction in Vortex-Structure Interaction
Authors: Mureithi Njuki W. , Kanki Hiroshi , Goda Syuki , Nakamura Tomomichi , Kashikura Tomoharu
Published: June 2008

This paper presents some results of experiments and numerical computations on various aspects of vortex-structure interaction. Experimental tests were conducted in a small wind tunnel to investigate the effect of mechanical cylinder oscillation in the flow direction on the wake vortex structure. Depending on the excitation to Karman shedding frequency ratio, mode locked states, in the form of spatio-temporally repeatable patterns, could be observed. Symmetrical excitation leads to breaking of the Karman mode spatial-temporal symmetry. Depending on the excitation to Karman frequency ratio, modes lacking the usual Karman symmetry is observed. The existence of a stable spatio-temporal structure appears to be strongly dependent on the interaction (lock-in) between the near wake symmetrical shedding and the ‘established’ far wake Karman pattern. Preliminary work based on symmetry (group) theory is presented to support the foregoing experimental observations. By considering two oscillators having the Dm (κ,2π / m) inline shedding symmetry and the Karman wake having the spatio-temporal symmetry Z2 (κ,π), the possible symmetries of subsequent flow perturbations resulting from the modal interaction are determined. A mode having the reduced symmetry, Z2 (I,π2) was theoretically predicted and confirmed in the experiments. Finally, experimental tests show strong subharmonic lockin for forcing at rational ratios the vortex shedding frequency in the range 1 < m/n < 2. This phenomenon was also predicted theoretically.
The Frictional Coefficient of Bovine Articular Cartilage Correlates With Interstitial Fluid Load Support in Creep
Authors: Krishnan Ramaswamy , Ateshian Gerard A.
Published: June 2008

Articular cartilage functions as the bearing material in joints and provides low friction and wear over a lifetime. The cartilage lubrication mechanism has not yet been fully characterized though several theories have been proposed. In previous studies [1–3] it was hypothesized that interstitial fluid load support contributes significantly to the reduction of the frictional coefficient due to load transfer from the solid to the fluid phase of the tissue. This study provides experimental verification for a theoretical model based on this hypothesis [1,4]. The specific aim of this study is to experimentally investigate the correlation between the frictional response of bovine articular cartilage, and its interstitial fluid load support during sliding against glass under a constant load.
A Self-Learning Model for Cylinder Wakes Using Neural Networks
Authors: Balasubramanian G. , Olinger D. J. , Demetriou M. A.
Published: June 2008

Coupled map lattice models (CML) that combine a series of low-dimensional circle maps with a diffusion model have predicted qualitative features of the wake behind vibrating flexible cables. However, there are always unmodelled dynamics if a quantitative comparison is made with wake patterns obtained from laboratory or simulated wake flows. To overcome this limitation, self-learning features can be incorporated into the simple CML model to capture the unmodelled dynamics. The self-learning CML uses radial basis function neural networks as online approximators of the unmodelled dynamics. The neural network weights are adaptively varied using a combination of a multivariable least squares algorithm and a projection algorithm. The adaptive estimation scheme, derived from a new convective diffusive CML, seeks to precisely estimate the neural network weights at each timestep by mimimizing the error between the simulated and measured wake patterns. Studies of this approach are conducted using wake patterns from spectral element based NEKTAR simulations of freely vibrating cable wakes at Re = 100. It is shown that the neural network based self-learning CML precisely estimates the simulated wake patterns within several shedding cycles. The self-learning CML is also shown to be computationally efficient.
A Numerical Simulation of Nonlinear Fluid-Rigid Structure Interaction Problems
Authors: Xing J. T. , Price W. G. , Chen Y. G.
Published: June 2008

A numerical method to simulate nonlinear fluid – rigid structure interaction problems is developed herein. The structure is assumed to undergo large rigid body motions and the fluid flow is governed by nonlinear, viscous or non-viscous, field equations with nonlinear boundary conditions applied to the free surface and fluid - solid interaction interfaces. An Arbitrary-Lagrangian-Eulerian (ALE) mesh system is used to construct the numerical model. A multi-block approach is adopted allowing relative motion between moving overset grids which are independent of one another. This provides a convenient method to overcome the difficulties of matching fluid meshes with large solid motions. Nonlinear numerical equations describing nonlinear fluid - solid interaction dynamics are derived through a numerical discretisation scheme of study. A coupling iteration process is used to solve these numerical equations. A numerical example is presented to demonstrate applications of the developed numerical model.
Structural Optimization of Buckling Load in Fluid-Structure Interaction Problems
Authors: Umeda Hiroki , Noguchi Hirohisa
Published: June 2008

This paper presents a procedure for the sensitivity analysis of buckling load in fluid-structure interaction problems. In the formulation, the buckling of thin structures subjected to the pressure of viscous flow is modeled, where the geometrically nonlinear equation and the Navier-Stokes equation should be considered. These equations are solved by the strong coupling formulation and the Newton-Raphson method. In order to confirm the validity of this procedure, the arch subjected to only the static pressure of fluid is analyzed. Finally, simple optimization considering fluid-structure interaction is performed using the calculated sensitivity along with the steepest descend method and the satisfactory result is obtained.
Multidisciplinary Design and Optimization for Fluid-Structure Interactions
Authors: Mastroddi Franco , Bonelli Claudia , Morino Luigi , Bernardini Giovanni
Published: June 2008

The paper presents an introductory overview of modeling techniques used by the authors for MDO–PD (MultiDisciplinary Optimization – Preliminary Design). The algorithms used by the authors in their MDO–PD code for modeling aerodynamics and aeroelasticity are reviewed. For the aerodynamic analysis, a boundary–element potential–flow method is used (for simplicity, only the incompressible–flow formulation is presented). The methodology is geared specifically towards MDO–PD for civilian aircraft. The numerical formulation is applied to a specific, highly–innovative aircraft configuration proposed by Frediani, which has, as a distinguishing feature, a low induced drag. A comparison with an MDO-PD of a standard wing configuration has been included.
Vitrification Solutions for the Cryopreservation of Tissue-Engineered Bone
Authors: Liu B. L. , McGrath J. J.
Published: June 2008

Osteoblast (OB)-seeded hydroxyapatite (HA) scaffold cortical bone substitutes are being developed at Michigan State University. Preservation methods need to be developed to preserve such living products to ensure a steady supply for transplantation. Theoretically vitrification is an attractive method for the cryopreservation of tissue-engineered bone because it can eliminate the destructive effect of ice formation [1]. However, relatively fast cooling and warming rates are required to avoid damage associated with ice crystallization and relatively high concentrations of cryoprotective agents (CPAs) are required to achieve a glassy (vitrified) state. These rapid rates of temperature change may not be possible as tissue-engineered structures become larger. In addition to cell damage, rapid rates may also cause destructive thermo mechanical damage to the scaffold itself. Slower rates can be used to achieve the vitrified state but this requires higher CPA concentrations, which are more toxic. As a means of studying the interactive determinants of an optimal vitrification process for osteoblasts, we have undertaken thermal analysis of a variety of vitrification solutions of interest using differential scanning calorimetry (DSC) to determine the critical cooling and warming rates. The toxicity dynamics and tendency for the scaffolds to be damaged mechanically by the vitrification process are also examined. Glycerol and dimethyl sulfoxide at a concentration of 40% were studied with and without an ice blocker. Two vitrification “cocktails” (VS55 and VEG) over a concentration range of 80% to 100% were studied with and without an ice blocker. On the basis of these studies 95% VEG with ice blocker was least toxic and yielded the highest recovery (∼90%) for OBs vitrified in liquid suspension. Vitrification does not seem to be detrimental to the bending strength of high density (low porosity) HA scaffolds, but lower density HA scaffolds break more easily after vitrification in some instances.
Use of Directional Solidification to Determine Characteristics of Hepatic Systems During Low Temperature Cooling
Authors: Zinchenko Yekaterina , Coger Robin N.
Published: June 2008

Cryopreservation using freeze-thaw methods is one way in which cells can be safely stored at low temperatures for indefinite periods of time. Unfortunately the process itself can damage the cells such that the number of viable cells is reduced after thawing. This problem is particularly important when dealing with non-proliferating cells, such as primary hepatocytes. In the current study directional solidification is used to help improve the successful cryopreservation of hepatocytes. Specifically, by analyzing the crystallization of cryoprotective solutions, cell suspensions, and cultured hepatocytes in the presence of DMSO, we determine how changes in the morphology and rate of crystal growth influence cell survival. The results demonstrate that the presence of extracellular matrix and alterations in DMSO concentration are two ways to affect cell viability. Next by modeling the directional solidification results as a Stefan Problem, the image analysis data is then used to quantify the thermal conductivities of each system.
Finite Element Modeling of the Cryopreservation Protocols for a Bio-Artificial Liver Assist
Authors: Balasubramanian Saravana Kumar , Coger Robin N.
Published: June 2008

Bio-Artificial Liver (BAL) devices have demonstrated potential as a temporary therapy for patients suffering from acute liver failure [1,2]. Of the variety of designs that exist [3], two BAL designs that have been tested clinically are the Hepatassist BAL [4] and the ELAD (Extra Corporeal Liver Assist Device) BAL [5]. To enable off the shelf availability of these devices, it would be helpful if they could be cryopreserved for safe low temperature storage. The aim of this study is to use finite element modeling to predict a suitable cryopreservation protocol for the Hepatassist BAL. Hepatassist incorporates a hollow fiber configuration in which nutrients (including oxygen) are supplied through the fibers to the hepatocytes surrounding the fibers [4]. In this study, we simulate the effects of different cryopreservation protocols in the presence of dimethyl sulphoxide (DMSO), a cryoprotective solution or cryoprotectant, on its integrity.
Effects of Womersely and Reynolds Numbers on a Symmetric Compliant Bifurcation
Authors: Yazdani Saami K. , Vlachos Pavlos P. , Telionis Demetri D. , Pierrakos Olga
Published: June 2008

Twelve million people worldwide die of diseases associated with atherosclerosis. It is estimated that atherosclerosis costs the US economy 100 billion dollars each year. From clinical studies, it is well known that atherosclerosis has preferred locations in the vascular system, primarily sited in the carotid arteries, coronary arteries, and in vessels supplying the lower extremities in the arterial system [1]. In the vicinity of a bifurcating, arterial flow tends to separate forming recirculation regions. In addition, due to the pulsatile character of blood flow during the deceleration part of the cycle, the flow becomes unstable and transition to turbulence may occur. The combination of the above-described phenomena generates abnormal shear stress levels and high shear stress gradients, especially at the reattachment point of the separated region.
A Modal Decomposition Approach to Noninvasive Cardiac Output Monitoring via Multi-Channel Blind System Identification
Authors: Zhang Yi , Asada H. Harry , Reisner Andrew T.
Published: June 2008

This paper presents a modal decomposition approach to continuous cardiac output (CO) monitoring via multi-channel blind system identification (BSI). The presented method estimates CO from multiple noninvasive sensors located at different branches of the systemic circulatory system.
Reduction of Flow Rate Into a Saccular Aneurysm After Stenting
Authors: Jou Liang-Der
Published: June 2008

Stents, wire-frame structures, are very effective devices in the treatment of vascular diseases, such as stenoses and aneurysms. One-third of patients who have stent placements develop restenosis over a six-month period, with the cause thought to be hemodynamic-related. The use of stent grafts to treat aneurysms often leads to exclusion of smaller vessels adjacent to the aneurysm from the circulation, and success of this procedure may therefore depend on the size of small vessels being occluded. An open stent is preferred to preserve the blood supply to neighboring vessels, but is considered to be less effective in aneurysm thrombosis and in reducing the pressure inside the aneurysm.
Multiscale Characterization of the Ultrastructure of Normal and Osteoporotic Human Trabecular Bone
Authors: Rubin Matthew A. , Jasiuk Iwona
Published: June 2008

In this paper we characterized hierarchical structure of healthy and osteoporotic human trabecular bone from microscale down to nanoscale. To characterize the hierarchical structure, trabecular bone was investigated at the microstructural level (i.e. trabecula, trabecular packets), sub-microstructural level (lamellar structure) and the nanostructural level (crystal-collagen collagen composite). There was an emphasis on evaluating the sub-microstructure and nanostructure of trabecular bone since detailed descriptions of the lamellar structure and of the crystal-collagen relationship in osteoporotic bone are relatively unknown. The ultrastructure of healthy and osteoporotic human trabecular bone was characterized experimentally by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The experiments also involved studying trabecular bone from C57BL/6J and C3H/HeJ mice. These mice have nearly the same size and weight, but have very different bone density. Thus, they were good candidates for a comparative study of healthy and osteoporotic human trabecular bone. TEM and SEM were used to characterize the hierarchical microstructure of trabecular bone in the inbred mice. The understanding of the hierarchical nature of healthy and osteoporotic bone microstructure is needed for a deeper understanding of the state of bone health and its mechanical properties.
Effects of Vertebral Body Changes on Cervical Spine Load Sharing
Authors: Zhang Jiangyue , Yoganandan Narayan , Pintar Frank A.
Published: June 2008

The objective of the study was to determine the effects of changes in the Young’s modulus of elasticity of the cancellous bone that occur due to the ageing process on the biomechanical behavior of the cervical spine. An anatomically accurate three-dimensional (3-D) nonlinear finite element model of the C4-C5-C6 cervical spinal unit was used. The inferior surface of the C6 vertebrae was fixed in all degrees of freedom, and external loads were applied to the top surface of the C4 vertebra. The model was exercised under an axial compressive force of 754 N. In addition, flexion and extension bending moments of 3.44 Nm were applied individually to the model. The effects of ageing on bone strength were simulated by decreasing the Young’s modulus of elasticity from 100 MPa in the healthy spine to 40 MPa in the degenerated spine. The degenerated spine was found to be more flexible than the healthy spine. In addition, the degenerated spine responded with increased forces in the outer anterior and posterior regions of the vertebral body. Furthermore, forces in the facet joints increased in the degenerated spine. In contrast, the middle region of the disc showed decreased forces. These increases in the forces leading to stress risers may explain the occurrence of osteophytes in the spine with age.
Genetic Effects on Skeletal Mechanosensitivity in Mice
Authors: Turner Charles H. , Robling Alexander G.
Published: June 2008

The accumulation of bone mass during growth can be enhanced by environmental factors such as mechanical loading (exercise) or calcium intake, but 60–70% of the variance in adult bone mineral density (BMD) is explained by heredity. Consequently, understanding the signaling pathways targeted by the genes governing bone accumulation holds perhaps the greatest potential in reducing fracture incidence later in life. Rodent models are particularly useful for studying the genetics of skeletal traits. Of the available inbred mouse strains, three in particular have been studied extensively in skeletal genetics: C57BL/6, DBA/2, and C3H/He. The C57BL/6 strain is characterized by low BMD and large total cross-sectional area (CSA) in the midshaft femur; the C3H/He strain exhibits very high femoral BMD and a smaller femoral CSA than the C57BL/6 mice; and DBA/2 mice have moderately high femoral BMD and a very small midshaft femur CSA. Mechanical loading of the skeleton during growth can substantially enhance periosteal bone apposition, and ultimately produce a diaphyseal cross section with enlarged area. Therefore we hypothesized that the mouse strain with greater femoral cross-sectional area (C57BL/6) might have a genetic predisposition for greater mechanosensitivity than mice with smaller cross sections (C3H/He and DBA/2).
Changes in Bone Mineral Density and Mechanical Properties Due to Alcohol and Ovariectomy in Trabecular Bone of Female Rats
Authors: Hogan Harry A. , Sampson H. Wayne
Published: June 2008

Alcohol consumption is a known risk factor exacerbating postmenopausal osteoporosis [1,2]. The current study was undertaken to investigate these effects using the rat animal model. Recent studies with rats have shown deleterious effects due to alcohol [3,4], but no significant effects when combined estrogen deficiency induced by ovariectomy (OVX) [5]. This last study did not report mechanical properties, however. The objective of the current study was to investigate the effects of alcohol and OVX on trabecular bone mechanical properties and bone mineral density (BMD) in adult female rats. An important issue in evaluating osteoporosis and related bone maladies clinically is the relation between bone mineral density (BMD) and fracture risk or incidence. Thus, an additional goal of the current study was to examine the relationships (correlations) between trabecular bone BMD and mechanical properties.
Finite Element Implementation of a Structural Constitutive Model for Heart Valve Biomaterials
Authors: Sun Wei , Sacks Michael S. , Scott Michael
Published: June 2008

Accurate computational modeling of the non-linear, anisotropic mechanical properties of heart valve biomaterials remains an important and challenging area. Unlike phenomenological models, structurally based constitutive models attempt to exploit the tissue composition and structure to avoid ambiguities in material characterization, and offer insight into the function, structure, and mechanics of tissue components. Current finite element (FE) simulations of heart valve biomaterials do not simulate the complete anisotropic mechanical response, limiting simulation realism. In this study, we implemented structural constitutive model developed in our lab that incorporates SALS-derived fiber orientation data. The FE model was validated by both analytical result and experimental data.
Extended “LMPHETS” Finite Element Models for Coupled Mechano-Electro-Chemical Transport in Soft Tissues
Authors: Simon B. R. , Radtke G. A. , Rigby P. H. , Williams S. K. , Liu Z. P.
Published: June 2008

Soft tissues are hydrated fibrous materials that exhibit nonlinear material response and undergo finite straining during in vivo loading. A continuum model of these structures (“LMPHETS” [1,2]) is a porous solid matrix (with charges fixed to the solid fibers) saturated by a mobile fluid (water) and multiple species (e.g., three mobile species designated by α, β = p, m, b where p = +, m = −, and b = ± charge) dissolved in the mobile fluid. A “mixed” LMPHETS theory and finite element models (FEMs) were presented [1] in which the “primary fields” are the displacements, ui = xi − Xi and the mechano-electro-chemical potentials, ν˜ξ* (ξ, η = f, e, m, b) that are continuous across material interfaces. “Secondary fields” (discontinuous at material boundaries) are mechanical fluid pressure, pf; electrical potential, μ˜e; and concentration or “molarity”, cα = dnα / dVf. Here an extended version of these models is described and numerical results are presented for representative test problems associated with transport in soft tissues.
Constitutive Characterization of the Nonlinear Viscoelastic Response of Vocal Fold Tissues
Authors: Chan Roger W. , Siegmund Thomas
Published: June 2008

Previous empirical studies have shown that vocal fold tissues exhibit nonlinear viscoelastic behaviors under different loading conditions. Hysteresis and strain rate-dependence of stress-strain curves have been observed for different layers of vocal fold tissues when subjected to cyclic tensile loading [1,2]. Nonlinear viscoelastic response has also been described for vocal fold tissues subjected to constant strain and constant stress tests under both tensile loading and large-strain shear deformation conditions [3,4]. These findings cannot be adequately described by many of the traditional constitutive formulations of linear and quasilinear viscoelasticity. For instance, models based on Y. C. Fung’s quasilinear viscoelastic theory typically apply two separate functions to describe the time dependence and the strain dependence of stress (e.g., the reduced relaxation function G(t) and the elastic response σe(ε), respectively), and combine the two functions by the Boltzmann superposition principle [5]. Such formulations assume that time dependence and strain dependence can be separated. However, recently obtained stress relaxation data of vocal fold tissues under various magnitudes of applied shear strain indicated that they are not separable, as relaxation became slower with increasing strain [4]. This paper attempts to characterize some nonlinear viscoelastic behaviors of vocal fold tissues under tensile and shear deformation conditions based on an implementation of the Bergstrom-Boyce model [6,7].
Coefficient of Start-Up Friction of Early Stage Osteoarthritic Cartilage Is Not Larger Than Normal Cartilage
Authors: Fujie Hiromichi , Suzuki Yoji , Ota Michi , Mabuchi Kiyoshi
Published: June 2008

It is well known that the disease of osteoarthritis (OA) deteriorates the lubrication properties of articular cartilage. Previous studies [1,2] have demonstrated that the coefficient of friction of rabbit knee cartilage increases significantly in OA models. The coefficient of start-up (static) friction in the normal canine knee joint has also been observed to increase with the duration of static loading [3], and further increases in the start-up friction of osteoarthritic cartilage were induced by surface abrasion and papain injection [4]. However, the change in the start-up friction due to OA disease induced by anterior cruciate ligament (ACL) transection (ACL transection model), has not been fully determined in previous studies, although such a model is considered to display symptoms similar to the clinical situation. Therefore, we investigated the effect of osteoarthritic deterioration on the start-up friction in the ACL transection OA model in the present study.
Development of a 3-D Non-Linear Finite Element Model of Human Knee Joint
Authors: Song Yuhua , Debski Richard E. , Gil Jorge , Woo Savio L.-Y.
Published: June 2008

A 3-D finite element (FE) model of the knee is needed to more accurately analyze the kinematics of a knee joint as well as the function of various soft tissues such as ligaments. The data obtained can provide a better understanding of mechanisms of injury and offer valuable information for ligament reconstruction and rehabilitation protocols. The objective of this study was to develop a 3-D non-linear FE model of a human knee and determine its kinematics and the force and stress distributions within the anterior cruciate ligament (ACL) in response to anterior tibial loads at full extension. This model was validated by comparing the computed results to data obtained experimentally by a Robotic/UFS testing system [1].
Analysis of Load Transmission in Intact and ACL-Deficient Knee Joints Under Drawer at Different Flexion Angles
Authors: Moglo K. E. , Shirazi-Adl A.
Published: June 2008

Adequate understanding of functional biomechanics of human knee joints is critical in assessment and improvement of current prevention, evaluation and treatment procedures of the joint disorders and injuries. The anterior cruciate ligament (ACL) normal function, injury, diagnostic and replacement have remained controversial despite the very high incidence of ACL rupture and replacement as well as considerable research efforts reported in this field. Similar situation holds true with regard to the other components of the joint such as menisci. Our earlier model studies have demonstrated coupling between the ACL and medial meniscus at full extension under horizontal forces suggesting a greater mechanical importance for the medial meniscus following an ACL transection [1]. The current model study, as a continuation of our earlier works [1,2], aimed to investigate the detailed passive response of the fully unconstrained intact and ACL-deficient tibiofemoral joints under up to 100N posterior femoral force at different flexion angles varying from 0° to 90°.
Effect of Tapered Geometry on the Load Transfer of an Idealized Cemented Hip Implant
Authors: Nun˜o N.
Published: June 2008

Implant looseining of cemented hip implants is one of the major causes of failure of the arthroplasty. In cemented hip implants, the polymethyl methacrylate (PMMA), also called bone cement, is used as grouting material between the stem and the surrounding bone. During polymerisation of the cement, residual stresses are generated in the bulk cement. The bone cement does not have a chemical bond with the stem nor the bone; however, it fills completely the space between the two and serves to distribute the load being transferred from the stem to the bone. Numerical analyses on the load transfer of cemented hip implants usually do not include the residual stresses due to cement curing at the stem-cement interface [1–2].