bluuebunny/crossref_metadata_2025 huggingface dataset:
Distensibility of Small Pulmonary Blood Vessels
Authors: Mehrotra P. P. , Patel D. J. , Coleman B. R. , Tearney R. J. , Diggs J. A. , Cothran L. N. , Curry C. L.
Published: March 2008

Although vasomotor activity in small pulmonary vessels has been studied extensively in the past, using the concept of resistance to flow, information on the distensibility of these vessels is very sparse. In an attempt to reduce this deficit, we adapted a theoretical method developed for small systemic vessels, to estimate distensibility of pulmonary resistance vessels in experimental animals and man. Pressure-flow data from 11 dogs and 10 human subjects (5 control subjects and 5 patients with long-standing left heart failure) were used to calculate distensibility of small pulmonary vessels. The conductance, G, was calculated from these data as the ratio of blood flow to driving pressure. The slope of the relationship between the logarithm of G1/4 and the average distending pressure (ADP) provides a graphic picture of circumferential extensibility, E, defined as percent change in radius for an infinitesimal change in ADP. Results indicate that: (1) the value of E in dogs was 1.85 ± .40 mmHg−1 for the control state, which decreased to 1.45 ± .43 mmHg−1 during norepinephrine administration; however, the decrease in the value was not statistically significant (p = 0.53); (2) the value of E in control human subjects was 3.38 ± .47 mmHg−1 and the value of E in patients with left heart failure was −0.64 ± 0.39 mmHg−1; the difference was significant (P = .0001). Moreover, at a given ADP in the range of overlapping pressures, the “average” radius of small pulmonary vessels in patients with left heart failure was smaller than that in the control subjects; and (3) small pulmonary vessels were more distensible than both the small systemic vessels and the large pulmonary arteries.
The Steady Expiratory Pressure-Flow Relation in a Model Pulmonary Bifurcation
Authors: Collins J. M. , Shapiro A. H. , Kimmel E. , Kamm R. D.
Published: March 2008

Experiments were conducted over a range of Reynolds numbers from 50 to 8000 to study the pressure-flow relationship for a single bifurcation in a multi-generation model during steady expiratory flow. Using the energy equation, the measured static pressure drop was decomposed into separate components due to fluid acceleration and viscous energy dissipation. The frictional pressure drop was found to closely approximate that for an equivalent length of curved tube with the same curvature ratio as in the model bifurcation. The sensitivity of these results to changes in airway cross-sectional shape, non-planar configuration, and flow regime (laminar-turbulent) was investigated. In separate experiments using dye visualization and hot-wire anemometry, a transition to turbulent flow was observed at Reynolds numbers between 1000 and 1500. Transition had very little effect on the pressure-flow relation.
A Computer Simulation of the Blood Flow at the Aortic Bifurcation With Flexible Walls
Authors: Lou Zheng , Yang Wen-Jei
Published: March 2008

To understand the role of fluid dynamics in atherogenesis, especially the effect of the flexibility of arteries, a two-dimensional numerical model for blood flow at the aortic bifurcation with linear viscoelastic walls is developed. The arbitrary Lagrangian-Eulerian method is adopted to deal with the moving boundary problem. The wall expansion induces flow reversals or eddies during the decelerating systole while the wall contraction restricts them during the diastole. A flexible bifurcation experiences the shear stresses about 10 percent lower than those of a rigid one.
Numerical Studies of Three-Dimensional Arterial Flows in Reverse Curvature Geometry: Part I—Peak Flow
Authors: Banerjee R. K. , Cho Y. I. , Back L. H.
Published: March 2008

A three-dimensional flow simulation at Repeak = 192 and 580 was made in a smooth reverse curvature model that conformed to the gentle “S” shape from a human left femoral artery angiogram. The objective of this numerical investigation was to find the changes in pressure, shear stress, velocity profile, and particle path occurring in the double-curved arterial vessel. Due to the impingement of blood at the outer wall in the first bend region, the wall shear stress approached 40 dyne/cm2—a value over twice as large as in the straight upstream segment. Conversely, at the inner wall in the first bend, a low shear stress region was found where the value of the shear stress was consistently smaller than that in the straight section. The initiation of centrifugal effects caused by the first bend could clearly be seen at Repeak = 580, but due to the close proximity of the reverse curvature segment, the momentum effect due to the second bend overshadowed the centrifugal effect. Hence, only near the end of the second bend did the centrifugal effect due to the second bend result in a double-spiral-secondary motion. In addition, the numerically calculated pressure drop data were in agreement with prior experimental values.
Viscosity Sensing With Lamb-Wave Microsensor: Dimethylsulfoxide Solution Viscosity as a Function of Temperature
Authors: Eto T. K. , Costello B. J. , Wenzel S. W. , White R. M. , Rubinsky B.
Published: March 2008

Recently, a new microsensor employing low-velocity ultrasonic Lamb waves was developed and demonstrated to be capable of measuring the viscosity of solutions in small volumes. The microsensor, when attached to a temperature-controlled stage, can measure viscosity as a function of temperature. In this investigation, the ultrasonic Lamb-wave oscillator is employed to experimentally measure the viscosity of dimethylsulfoxide (Me2SO) solutions as a function of temperature. The microsensor and the experimental procedure are described and results for 1M, 3M, and 5M Me2SO aqueous solutions are presented. Dimethylsulfoxide is a compound commonly employed as a cryoprotectant in cryopreservation, the low-temperature preservation of biological materials. The temperature dependence of viscosity obtained through this study can be used in determining the probability for ice nucleation in biological materials, a parameter of importance during cryopreservation.
An Experiment on the Pulsatile Flow at Transitional Reynolds Numbers—The Fluid Dynamical Meaning of the Blood Flow Parameters in the Aorta
Authors: Nakamura Masahide , Sugiyama Wataru , Haruna Manabu
Published: March 2008

An experiment on the fully developed sinusoidal pulsatile flow at transitional Reynolds numbers was performed to evaluate the basic characteristics of the wall shear stress. In this experiment, the wall shear stress was calculated from the measured section averaged axial velocity and the pressure gradient by using the section averaged Navier-Stokes equation. The experimental results showed that the ratio of the amplitude of the wall shear stress to the amplitude of the pressure gradient had the maximum value when the time averaged Reynolds number was about 4000 and the Womersley number was about 10. As this condition is close to the blood flow condition in the human aorta, it is suggested that the parameter of the aorta has an effect to increase the amplitude of the wall shear stress acting on the arterial wall.
Steady Flow in Abdominal Aortic Aneurysm Models
Authors: Budwig R. , Elger D. , Hooper H. , Slippy J.
Published: March 2008

Steady flow in abdominal aortic aneurysm models has been examined for four aneurysm sizes over Reynolds numbers from 500 to 2600. The Reynolds number is based on entrance tube diameter, and the inlet condition is fully developed flow. Experimental and numerical methods have been used to determine: (i) the overall features of the flow, (ii) the stresses on the aneurysm walls in laminar flow, and (iii) the onset and characteristics of turbulent flow. The laminar flow field is characterized by a jet of fluid (passing directly through the aneurysm) surrounded by a recirculating vortex. The wall shear stress magnitude in the recirculation zone is about ten times less than in the entrance tube. Both wall shear stress and wall normal stress profiles exhibit large magnitude peaks near the reattachment point at the distal end of the aneurysm. The onset of turbulence in the model is intermittent for 2000 < Re < 2500. The results demonstrate that a slug of turbulence in the entrance tube grows much more rapidly in the aneurysm than in a corresponding length of uniform cross section pipe. When turbulence is present in the aneurysm the recirculation zone breaks down and the wall shear stress returns to a magnitude comparable to that in the entrance tube.
Bolus Contaminant Dispersion in Oscillatory Tube Flow With Conductive Walls
Authors: Jiang Yahong , Grotberg James B.
Published: March 2008

Dispersion of a bolus contaminant in a straight tube with oscillatory flows and conductive walls is solved by using a derivative-expansion method. Using asymptotic methods when small conductance exists, the axial dispersion, as measured by the time-averaged effective diffusivity, increases over the insulated case, as long as the dimensionless frequency (Womersley parameter), α, is smaller than a critical value. When α exceeds this value, axial dispersion is diminished by wall conductance. The functional dependence of this critical α on the system parameters is investigated. We examine the radial wall transport both for total mass and localized flux, which is found to be independent of velocity field, and compute the time-dependent total mass of wall transport and asymptotic Sherwood number for large times as a function of the wall conductance.
Mathematical Modeling of Ligaments and Tendons
Authors: Woo S. L.-Y. , Johnson G. A. , Smith B. A.
Published: March 2008

Ligaments and tendons serve a variety of important functions in maintaining the structure of the human body. Although abundant literature exists describing experimental investigations of these tissues, mathematical modeling of ligaments and tendons also contributes significantly to understanding their behavior. This paper presents a survey of developments in mathematical modeling of ligaments and tendons over the past 20 years. Mathematical descriptions of ligaments and tendons are identified as either elastic or viscoelastic, and are discussed in chronological order. Elastic models assume that ligaments and tendons do not display time dependent behavior and thus, they focus on describing the nonlinear aspects of their mechanical response. On the other hand, viscoelastic models incorporate time dependent effects into their mathematical description. In particular, two viscoelastic models are discussed in detail; quasi-linear viscoelasticity (QLV), which has been widely used in the past 20 years, and the recently proposed single integral finite strain (SIFS) model.
Nonhomogeneous Ventricular Wall Strain: Analysis of Errors and Accuracy
Authors: Waldman Lewis K. , McCulloch Andrew D.
Published: March 2008

Nonhomogeneous distributions of strains are simulated and utilized to determine two potential errors in the measurement of cardiac strains. First, the error associated with the use of single-plane imaging of myocardial markers is examined. We found that this error ranges from small to large values depending on the assumed variation in stretch. If variations in stretch are not accompanied by substantial regional changes in ventricular radius, the associated error tends to be quite small. However, if the nonuniform stretch field is a result of substantial variations in local curvature from their reference values, large errors in stretch and strain occur. For canine hearts with circumferential radii of 2 to 4 cm, these errors in stretch may be as great as 30 percent or more. Moreover, gradients in stretch may be over- or underestimated by as much as 100 percent. In the second part of this analysis, the influence of random measurement errors in the coordinate positions of markers on strains computed from them is studied. Arrays of markers covering about 16 cm2 of ventricular epicardium are assumed and nonuniform stretches imposed. The reference and deformed positions of the markers are perturbed with Gaussian noise with a standard deviation of 0.1 mm, and then strains are computed using either homogeneous strain theory or a nonhomogeneous finite element method. For the strain distributions prescribed, it is found that the finite element method reduces the error resulting from noise by about 50 percent over most of the region. Accurate measurements of cardiac strain distributions are needed for correlation with and validation of realistic three-dimensional stress analyses of the heart. Moreover, with the advent of increasingly effective noninvasive methods to measure cardiac deformation such as magnetic resonance imaging, the use of nonhomogeneous strain analysis to determine more accurate strain distributions has increasing clinical significance.
Hemodynamics and the Vascular Endothelium
Authors: Nerem Robert M.
Published: March 2008

The endothelium, once thought to be a passive, non-thrombogenic barrier, is now recognized as being a dynamic participant in vascular biology and pathobiology. Part of its dynamic nature is due to the influence of the mechanical environment imposed by the hemodynamics of the vascular system. Over the past two decades much has been learned about the influence of hemodynamics on the vascular endothelium. This has been in part through in vivo experiments; however, in the past 15 years a number of laboratories have turned to the application of in vitro cell culture systems to investigate the influence of flow and cyclic stretch on the biology of vascular endothelium. Taken together these studies demonstrate that flow and the associated shear stress modulate both endothelial cell structure and function. Cell culture studies employing cyclic stretch provide similar evidence. Furthermore, these effects of mechanical environment extend to the gene expression level, with there being a differential regulation of mRNA. A critical question is how does an endothelial cell recognize the mechanical environment in which it resides and, having done so, how is this transduced into the changes in structure and function observed? Although our knowledge of the recognition events remains limited, studies on signal transduction in response to a mechanical stimulus indicate that many of the second messengers known to be triggered by chemical agonists also are involved in transducing a mechanical signal. Over the past 20 years our understanding of the importance of the influence of the mechanical environment imposed by the hemodynamics of the system on vascular endothelial biology, both in the regulation of the normal biology of blood vessels and as a determinant of the distribution and development of atherosclerotic lesions, has grown immensely; however, there is still much to be learned.
The Artificial Heart: History and Current Status
Authors: Phillips Winfred M.
Published: March 2008

Twenty-years ago groups from California to Massachusetts were actively involved in the development of an artificial heart. From biomaterials development to biomedical power sources, the supporting industry and spin-off benefit was broad indeed. Young people were seeking careers in biomedical engineering and science. The National Institutes of Health was supporting artificial heart research at $10 to $12 million dollar levels. Groups at Andros, Inc. (now Baxter Novacor) and Stanford, Thoratec, Penn State and the Hershey Medical Center, Cleveland Clinic and the Division of Artificial Organs, the University of Utah, the Texas Heart Institute and the Baylor College of Medicine, Thermal Electron Corporation, and many more were the source of research and breakthrough development of pumps and systems for artificial hearts. We reported on performance criteria for an artificial heart pump at the First Biomechanics Symposium in 1973 [1]. By the beginning of the decade of the 90’s, thousands of presentations had been made and manuscripts written reporting significant progress in the development of artificial heart pumps and systems. The Heart, Lung and Blood Institute of the National Institutes of Health was supporting an artificial heart contract research and development program at a level of $6 million dollars in 1991 [2]. Broad basic research grant activity also continues. The National Institutes of Health’s artificial heart program received renewed support from the Institute of Medicine’s special review in 1991 [3]. In December of 1992, the 16th Annual Cardiovascular Science and Technology Conference attracted over 500 attendees. This annual conference has provided a continuing forum for an update on progress in artificial heart development.
The Role of Fluid Mechanics in the Localization and Detection of Atherosclerosis
Authors: Giddens D. P. , Zarins C. K. , Glagov S.
Published: March 2008

Fluid dynamics research over the past twenty years has contributed immensely to our knowledge of atherosclerosis. The ability to detect localized atherosclerotic plaques using noninvasive ultrasonic methods was advanced significantly by investigations into the nature and occurrence of velocity disturbances created by arterial stenoses, and diagnosis of carotid bifurcation disease using a combination of ultrasonic imaging and Doppler measurement of blood velocity is now quite routine. Since atherosclerotic plaques tend to be localized at sites of branching and artery curvature and since these locations would be expected to harbor complex flow patterns, investigators postulated that fluid dynamics might play an initiating role in atherogenesis. Several fluid dynamic variables were proposed as initiating factors. Investigations were undertaken during the 1980s in which fluid dynamic model experiments with physiologic geometries and flow conditions were employed to simulate arterial flows and in which morphometric mapping of intimal thickness was performed in human arteries. Correlations between fluid dynamic variables and intimal thickness revealed that atherosclerotic plaques tended to occur at sites of low and oscillating wall shear stress; and these observations were reinforced by studies in a monkey model of atherosclerosis. Concomitantly, it was realized that arteries adapt their diameters so as to maintain wall shear stress in a narrow range of values around 15 dynes/cm2, findings which were based both on observations of normal arteries and on animal studies in which flow rates were manipulated and arterial diameter adaptation was measured. Currently, a working hypothesis for the role of fluid dynamics in atherogenesis is that intimal thickening is a normal response to low wall shear stress, and this intimal thickening can develop into an early atherosclerotic plaque under certain circumstances such as excessive low density lipoprotein concentrations in blood.
Sinusoidal Variation of Wall Shear Stress in Daughter Tube Through 45 Deg Branch Model in Laminar Flow
Authors: Yamaguchi Ryuhei , Kohtoh Kenji
Published: March 2008

The present paper describes the experimental implemented work on the flow situation through a branch model having a daughter tube bifurcated from a parent tube at 45 deg. Experiments have been conducted utilizing an electrochemical method. The results show that, even in steady flow, the wall shear stress along the proximal wall in the daughter tube varies significantly with position in the form of a damped sine wave. For pulsating flow at the nondimensional pulsating frequency of α = 6.6, the above mentioned tendency appears to be severe, and the distribution of its amplitude in pulsating flow is similar to that of wall shear stress in steady flow.
In-Vivo Study of the Mechanical Properties of Epicardial Coronary Arteries
Authors: Manor Dan , Beyar Rafael , Shofti Rona , Sideman Samuel
Published: March 2008

This study proposes a method to examine the in-vivo pressure-volume (P-V) relationship of an epicardial coronary artery. With the proximal left anterior descending (LAD) artery in a dog occluded, the distal LAD flow oscillates around zero. Integration of the flow with respect to time yields the intraluminal arterial segment volume changes in the region between the site of occlusion and the flow probe. The distal LAD pressure is measured via a diagonal branch. The segmental P-V relationship exhibits a hysteresis loop. The area bound by the hysteresis loop corresponds to the energy loss on the arterial wall during one cycle (12.63±8.25 [erg·cm−1], n = 7), while the dynamic arterial compliance is calculated based on the ratio of the volume to pressure excursions (1.13±0.73 [m1·mmHg−1·cm−1·10−4]). These results represent first estimates of the in-vivo mechanical properties of the coronary arterial wall based on P-V measurements.
Shear Stress at a Compliant Model of the Human Carotid Bifurcation
Authors: Anayiotos A. S. , Jones S. A. , Giddens D. P. , Glagov S. , Zarins C. K.
Published: March 2008

To investigate the role of a compliant wall to the near wall hemodynamic flowfield, two models of the carotid bifurcation were constructed. Both were of identical internal geometries, however, one was made of compliant material which produced approximately the same degree of wall motion as that occurring in vivo while the other one was rigid. The inner geometries were formed from the same mold so that the configurations are directly comparable. Each model was placed in a pulsatile flow system that produced a physiologic flow waveform. Velocity was measured with a single component Laser system and wall shear rate was estimated from near wall data. Wall motion in the compliant model was measured by a wall motion transducer and the maximum diameter change varied between 4-7 percent in the model with the greatest change at the axis intersection. The mean shear stress in the compliant model was observed to be smaller by about 30 percent at most locations. The variation in peak shear stress was greater and occasionally reached as much as 100 percent with the compliant model consistently having smaller positive and negative peaks. The separation point was seen to move further upstream in the compliant cast. The modified flowfield in the presence of a compliant wall can then be important in the hemodynamic theory of atherogenesis.
Macromolecular Transport Through the Deformable Porous Media of an Artery Wall
Authors: Kim Woo-Sik , Tarbell John M.
Published: March 2008

To determine the macromolecular transport properties of the tunica media of an artery wall deformed inhomogeneously by the transmural pressure, we combine a simple mechano-hydraulic model based on a two parameter strain-dependent permeability function, which was developed by Klanchar and Tarbell (1987), with a fiber matrix theory. The combined theory allows us to calculate the spatial distributions of porosity, solute partition, fiber radius and macromolecular solute concentration in the media and their dependence on the transmural pressure. The predictions from the combined theory are in good agreement with experimental measurements of sucrose space, albumin space and albumin concentration profiles in the media of rabbit aortas at transmural pressures of 70 and 180 mmHg. The predictions indicate that albumin transport through the aortic media is dominated by convection rather than diffusion. It is further demonstrated that the transport properties of unstressed planar tissue samples, which are often used in in vitro experiments, may be quite different from those of intact vessels in their natural cylindrical configuration because of variation in tissue deformation.
Analysis of the Weinbaum-Jiji Model of Blood Flow in the Canine Kidney Cortex for Self-Heated Thermistors
Authors: Valvano Jonathan W. , Nho Sungwoo , Anderson Gary T.
Published: March 2008

The Weinbaum-Jiji equation can be applied to situations where: 1) the vascular anatomy is known; 2) the blood velocities are known; 3) the effective modeling volume includes many vessels; and 4) the vessel equilibration length is small compared to the actual length of the vessel. These criteria are satisfied in the situation where steady-state heated thermistors are placed in the kidney cortex. In this paper, the Weinbaum-Jiji bioheat equation is used to analyze the steady state response of four different sized self-heated thermistors in the canine kidney. This heat transfer model is developed based on actual physical measurements of the vasculature of the canine kidney cortex. In this model, parallel-structured interlobular arterioles and venules with a 60 μm diameter play the dominant role in the heat transfer due to blood flow. Continuous power is applied to the thermistor, and the instrument measures the resulting steady state temperature rise. If an accurate thermal model is available, perfusion can be calculated from these steady-state measurements. The finite element simulations correlate well in shape and amplitude with experimental results in the canine kidney. In addition, this paper shows that the Weinbaum-Jiji equation can not be used to model the transient response of the thermistor because the modeling volume does not include enough vessels and the vessel equilibration length is not small compared to the actual length of the vessel.
A Proposed Etiology for Tourniquet-Induced Neuropathies
Authors: Hodgson A. J.
Published: March 2008

Pneumatic tourniquets are used in limb surgery to provide a bloodless operating field, but they can also cause nerve injuries. These injuries are thought to be mechanically induced, although no previous work has quantified such a mechanism. This paper hypothesizes that axial compression of the nerve leads to damage at the nodes of Ranvier and presents simulation results based on this hypothesis which are in substantial agreement with clinical observations.
A Theoretical Model of Infant Incubator Dynamics
Authors: Simon Barry N. , Reddy Narender P. , Kantak Anand
Published: March 2008

A spatially lumped mathematical model was developed and used for a computer simulation of the neonate-incubator system for parametric analysis of the factors that influence neonatal thermo-regulation. The simulation examined the effects of the following parameters: (1) size of the infant; (2) respiratory rate; (3) metabolic rate; (4) heart rate; (5) thermal properties of the mattress; (6) specific heat capacity of the incubator wall; (7) air flow rate; (8) heater control mechanisms.
Mechanical and Dimensional Adaptation of Rat Aorta to Hypertension
Authors: Matsumoto T. , Hayashi K.
Published: March 2008

To investigate mechanisms of the mechanical adaptation of soft biological tissues to load, effects of hypertension on the mechanical properties and wall dimensions of thoracic aortas were studied in rats. Goldblatt hypertension was induced in male Wistar rats aged 8 to 9 weeks by constricting their left renal arteries. Two, 4, 8, or 16 weeks after the operation, thoracic aortas were excised and used to determine static pressure-diameter relations and wall dimensions. Wall thickness correlated significantly with the systolic blood pressure before sacrifice, Psys, at each period. The aortic hoop stress became almost constant at all Psys 2 weeks after the operation. On the other hand, the stress calculated for 100 and 200 mmHg correlated negatively with Psys. The incremental elastic modulus of the wall at Psys had a significant correlation with Psys having a positive slope at each period, although the correlation disappeared at 16 weeks after the operation. These results imply that: 1) thickness of the aortic wall increases very rapidly in response to hypertension; 2) wall stress developed by the in-situ blood pressure is kept constant at a normal level irrespective of hypertension; 3) elastic modulus of the wall of the hypertensive rats at the in-situ blood pressure becomes equal to the normal value after relatively long period of time; 4) in response to the alteration of the applied force, dimensional change appears much earlier than the change in the elastic properties.
The Effect of Wall Distensibility on Flow in a Two-Dimensional End-to-Side Anastomosis
Authors: Steinman D. A. , Ethier C. Ross
Published: March 2008

The development of intimal hyperplasia at the distal anastomosis is the major cause of long-term bypass graft failure. To evaluate the suspected role of hemodynamic factors in the pathogenesis of distal intimal hyperplasia, an understanding of anastomotic flow patterns is essential. Due to the complexity of arterial flow, model studies typically make simplifying assumptions, such as treating the artery and graft walls as rigid. In the present study this restriction is relaxed to consider the effects of vessel wall distensibility on anastomotic flow patterns. Flow was simulated in an idealized 2-D distensible end-to-side anastomosis model, using parameters appropriate for the distal circulation and assuming a purely elastic artery wall. A novel numerical approach was developed in which the wall velocities are solved simultaneously with the fluid and pressure fields, while the wall displacements are treated via an iterative update. Both the rigid and distensible cases indicated the presence of elevated temporal variations and low average magnitudes of wall shear stress at sites known to be susceptible to the development of intimal hyperplasia. At these same sites, large spatial gradients of wall shear stress were also noted. Comparison between distensible-walled and corresponding rigid-walled simulations showed moderate changes in wall shear stress at isolated locations, primarily the bed, toe and heel. For example, in the case of a distensible geometry and a physiologic pressure waveform, the heel experienced a 38 percent increase in cycle-averaged shear stress, with a corresponding 15 percent reduction in shear stress variability, both relative to the corresponding values in the rigid-walled case. However, other than at these isolated locations, only minor changes in overall wall shear stress patterns were observed. While the physiological implications of such changes in wall shear stress are not known, it is suspected that the effects of wall distensibility are less pronounced than those brought about by changes in arterial geometry and flow conditions.
A Computational Study of a Thin-Walled Three-Dimensional Left Ventricle During Early Systole
Authors: Yoganathan Ajit P. , Lemmon Jack D. , Kim Young H. , Walker Peter G. , Levine Robert A. , Vesier Carol C.
Published: March 2008

A numerical study was conducted to solve the three-dimensional Navier-Stokes equations for time-dependent flow in a compliant thin-walled, anatomically correct left ventricle during early systole. Model parameters were selected so that the simulation results could be compared to clinical data. The results produced endocardial wall motion which was consistent with human heart data, and velocity fields consistent with those occurring in a normally-contracting left ventricle. During isovolumetric contraction the posterior wall moved basally and posteriorly, while the septal wall moved apically and anteriorly. During ejection, the short axis of the left ventricle decreased 1.1 mm and the long axis increased 4.2 mm. At the end of the isovolumetric contraction, most of the flow field was moving form the apex toward the base with recirculation regions at the small pocket formed by the concave anterior leaflet, adjacent to the septal wall and near the left ventricular posterior wall. Fluid velocities in the outflow tract matched NMR data to within 10 percent. The results were also consistent with clinical measurements of mitral valve-papillary muscle apparatus displacement, and changes in the mitral valve annular area. The results of the present study show that the thin-walled, three-dimensional left ventricular model simulates observed normal heart phenomena. Validation of this model permits further studies to be performed which involve altered ventricular function due to a variety of cardiac diseases.
Estimation of the Thermal Effect of Blood Flow in a Branching Countercurrent Network Using a Three-Dimensional Vascular Model
Authors: Brinck Heinrich , Werner Ju¨rgen
Published: March 2008

A three-dimensional thermal model is presented in which convective heat transfer is defined in terms of the physical details of the vascular system. The convective heat exchange between prearteriole and postvenule vessels and the tissue across the vessel walls is calculated explicitly without using shape factors. Conduction in x-, y-, and z-direction is considered. This vascular model is applied to a human extremity. The spatial variations in the arterial, venous and tissue temperatures under basal conditions, hyperthermic conditions and in a cold environment are computed. Conclusions are drawn as to the validity of bioheat approaches.
Unsteady Entrance Flow Development in a Straight Tube
Authors: He Xiaoyi , Ku David N.
Published: March 2008

The entrance conditions for pulsatile flow are important in the understanding blood flow out of the heart and in developing regions at branches. The pulsatile entrance flow was solved using a spectral element simulation of the full unsteady Navier- Stokes equations. A mean Reynolds number of 200 and a range of Womersley parameters from 1.8 to 12.5 was used for a sinusoidal inlet flow waveform 1+sin (ωt). Variations in the entrance length were observed during the pulsatile cycle. The amplitude of the entrance length variation decreased with an increase in the Womersley parameter. The phase lag between the entrance length and the inlet flow waveform increased for Womersley parameter α up to 5.0 and decreased for α larger than 5.0. For low α, the maximum entrance length during pulsatile flow was approximately the same as the steady entrance length for the peak flow. For high α, the pulsatile entrance length was more uniform during the cycle and tended to the entrance length for the mean flow. The wall shear rate reached its far downstream value after only about half of the entrance length and also exhibited a dependence on α. The results quantify the entrance conditions typically encountered in studies of the arterial system.
Effects of Fit and Bonding Characteristics of Femoral Stems on Adaptive Bone Remodeling
Authors: Weinans H. , Huiskes R. , Grootenboer H. J.
Published: March 2008

Bone atrophy caused by stress-shielding may cause serious complications for the long-term fixation of hip stems. In particular, uncemented total hip arthroplasty is threatened by this problem, because the stems are usually larger and, as a consequence, stiffer than those of cemented implants. In the present study, the effects of fit and bonding characteristics of femoral hip stems were investigated, using the (nonlinear) finite element method in combination with adaptive bone remodeling theory to predict the bone density distribution in a bone or bone/implant configuration. Unknown parameters used in the theory, such as a reference equilibrium loading stimulus and a threshold (dead) zone of this stimulus, were established (triggered) by using the method to predict the density distributions in the natural femur and around fully coated uncemented implants. The computer simulation method can provide long term predictions of remodeling patterns around various implant configurations. Several cases were analyzed, whereby the coating conditions (fully, partly, or noncoated) and the fit characteristics (press fitted or overreamed) were varied. The computer predictions showed that partly coating can only significantly reduce bone atrophy relative to fully coated stems, when the coating is applied at a small region at the utmost proximal part of the stem. For smooth press-fit stems the predicted amount of bone loss (35 percent in the proximal medial region) was less than for a one-third proximally coated or a fully coated stem (50 to 54 percent predicted bone loss in the proximal medial region). The results showed that overreaming the femoral canal in the press fit case can have important effects. Distal overreaming gave reduced proximal atrophy. Proximal overreaming (or undersizing) resulted in a distal jam of the stem, a proximal “stress-bypass,” and dramatic proximal bone loss (up to 90 percent).
A Finite Element Analysis of the Human Temporomandibular Joint
Authors: Chen J. , Xu Liangfeng
Published: March 2008

A 2-D finite element model of the human temporomandibular joint (TMJ) has been developed to investigate the stresses and reaction forces within the joint during normal sagittal jaw closure. The mechanical parameters analyzed were maximum principal and von Mises stresses in the disk, the contact stresses on the condylar and temporal surfaces, and the condylar reactions. The model bypassed the complexity of estimating muscle forces by using measured joint motion as input. The model was evaluated by several tests. The results demonstrated that the resultant condylar reaction force was directed toward the posterior side of the eminence. The contact stresses along the condylar and temporal surfaces were not evenly distributed. Separations were found at both upper and lower boundaries. High tensile stresses were found at the upper boundaries. High tensile stresses were found at the upper boundary of the middle portion of the disk.
Analysis of Role of Bone Compliance on Mechanics of a Lumbar Motion Segment
Authors: Shirazi-Adl A.
Published: March 2008

A large deformation elasto-static finite element formulation is developed and used for the determination of the role of bone compliance in mechanics of a lumbar motion segment. This is done by simulating each vertebra as a deformable body with realistic material properties, as a deformable body with stiffer or softer mechanical properties, as a single rigid body, or finally as two rigid bodies attached by deformable beams. The single loadings of axial compression, flexion moment, extension moment, and axial torque are considered. The results indicate the marked effect of alteration in bone material properties on biomechanics of lumbar segments specially under larger loads. The biomechanical studies of the lumbar spine should, therefore, be performed and evaluated in the light of such dependency. A model for bony vertebrae is finally proposed that preserves both the accuracy and the cost-efficiency in nonlinear finite element analyses of spinal multi-motion segment systems.
A Biphasic Poroelastic Analysis of the Flow Dependent Subcutaneous Tissue Pressure and Compaction Due to Epidermal Loadings: Issues in Pressure Sore
Authors: Mak Arthur F. T. , Huang Lidu , Wang Qinque
Published: March 2008

A layer of skin and subcutaneous tissue on a bony substratum was modeled as a homogeneous layer of biphasic poroelastic material with uniform thickness. The epidermal surface and the bony interface were taken to be impervious. The soft tissue on the bony interface was assumed either fully adhered or completely free to slide on the bone. The cases for surface pressure loadings and displacement controlled indentations were simulated. The resultant biomechanical responses of the layer, including the transient tissue hydrostatic pressure and the tissue compaction, were presented. A new hypothesis is offered to interpret the threshold pressure-time curve for pressure sores in term of the time required for a particular area in the tissue layer to reach a critical compaction for a given level of applied pressure.
A Scaling Law for Wall Shear Rate Through an Arterial Stenosis
Authors: Siegel John M. , Markou Christos P. , Ku David N. , Hanson S. R.
Published: March 2008

Atherosclerosis of the human arterial system produces major clinical symptoms when the plaque advances to create a high-grade stenosis. The hemodynamic shear rates produced in high-grade stenoses are important in the understanding of atheromatous plaque rupture and thrombosis. This study was designed to quantify the physiologic stress levels experienced by endothelial cells and platelets in the region of vascular stenoses. The steady hemodynamic flow field was solved for stenoses with percent area reductions of 50, 75, and 90 percent over a range of physiologic Reynolds numbers (100–400). The maximum wall shear rate in the throat region can be shown to vary by the square root of the Reynolds number. The shear rate results can be generalized to apply to a range of stenosis lengths and flow rates. Using dimensions typical for a human carotid or coronary artery, wall shear rates were found to vary from a maximum of 20,000 s−1 upstream of the throat to a minimum of −630 s−1 in the recirculation zone for a 90 percent stenosis. An example is given which illustrates how these values can be used to understand the relationship between hemodynamic shear and platelet deposition.
Transient Behavior Analysis of a Mechanical Monoleaflet Heart Valve Prosthesis in the Closing Phase
Authors: Cheon Gill-Jeong , Chandran K. B.
Published: March 2008

In this paper, an analysis of the dynamics in the closing phase of the occluder of a mechanical monoleaflet heart valve prosthesis is presented. The dynamic analysis of the fluid in the vicinity of the occluder was based on the control volume approach. The backflow velocity of the fluid was computed by applying the continuity and momentum equations in the unsteady state. By considering the fluid pressure and gravity as external forces acting on the occluder, the moment equilibrium on the occluder was employed to analyze the motion of the occluder during closing and the force of impact between the occluder and the guiding struts. The computed magnitudes of the occluder tip velocities, as well as the backflow of the fluid during the closing phase using this model, were in agreement with previously reported experimental measurements. The maximum impact force between the occluder and guiding struts of 140–280 N was determined to occur during the initial impact for a duration of 35–45 μus. The results of such model studies may be extended for the analysis of the endurance limit of the valve prostheses as well as to determine the mechanical stresses on the formed elements and the incipience of cavitation bubbles during the closing phase of the valve function.
Steady Inspiratory Flow in a Model Symmetric Bifurcation
Authors: Zhao Yao , Lieber Baruch B.
Published: March 2008

Flow in a bifurcating tube system typifying a major bronchial bifurcation is studied experimentally with a two color, two velocity component laser Doppler anemometer. The flow loop is composed of a pumping station, flow stratifiers and a constant head pressure tank; it can accommodate steady, pulsatile or oscillatory flow. The test section is a symmetric bifurcation of constant cross sectional area and has a branching angle of 70 deg. The test section is a cast of clear silicon rubber in a plexiglass mold that was milled on a numerically controlled milling machine. The flow division ratio from the parent to daughter branches is about unity. Steady flow results that model the inspiratory phase at Reynolds numbers of 518, 1036 and 2089, corresponding to Dean numbers of 98, 196 and 395, show that in the bifurcation plane velocity profiles in the daughter branches are skewed toward the inner wall. In the transverse plane, “m” shaped velocity profiles are found with low velocity at the center. Secondary flow patterns, which are responsible for such phenomena, are first observed at the axial position where the flow begins to turn. Flow separation was not observed at any point in the bifurcation.
Modeling and Design Implications of Noncollocated Control in Flexible Systems
Authors: Spector V. A. , Flashner H.
Published: March 2008

In this paper we investigate generic properties of structural modeling pertinent to structural control, with emphasis on noncollocated systems. Analysis is performed on a representative example of a pinned-free Euler-Bernoulli beam with distributed sensors. Analysis in the wave number plane highlights the crucial qualitative characteristics common to all structural systems. High sensitivity of the transfer function zeros to errors in model parameters and sensor locations is demonstrated. The existence of finite right half plane zeros in noncollocated systems, along with this high sensitivity, further complicates noncollocated controls design. A numerical method for accurate computation of the transfer function zeros is proposed. Wiener-Hopf factorization is used to compute equivalent delay time, which is important in controls design.
Cross-Coupled Control of Biaxial Feed Drive Servomechanisms
Authors: Srinivasan K. , Kulkarni P. K.
Published: March 2008

A cross-coupled controller, designed to improve high-speed contouring accuracy independently of tracking accuracy in biaxial machine tool feed drive servomechanisms, is presented here. The controller parameters depend on the instantaneous slope of the desired contour and hence vary with time for curved contours, resulting in a time-varying controller. An approximate stability analysis of the controller is presented. The proposed controller is evaluated experimentally on a microcomputer controlled two-axis positioning table and compared to a more traditional uncoupled controller. Controller performance is evaluated for straight line, cornering and circular contours at feed rates varying from 2.25 m/min to 7.2 m/min. The experimental results show that the proposed controller reduces contouring error as compared to the uncoupled controller and leaves the tracking error practically unchanged. The cross-coupled controller is simple to implement and is practical.
On the Symmetrizability of Structural Control Systems With Noncolocated Sensors and Actuators
Authors: Piche´ R.
Published: March 2008

In problems of active vibration control of elastic structures, it is known that systems with symmetric feedback matrices (realized using colocated dual sensor/actuator pairs) possess many desirable properties, including real eigenvalues, infinite gain margin, no flutter instability, and the lower-bound character of Rayleigh estimation formulas. In this paper, we consider systems with nonsymmetric feedback matrices which may be transformed, via a similarity transformation, into symmetric systems. We propose constructive conditions to recognize a-priori which systems are symmetrizable. It is shown that lumped-mass structures are not symmetrizable. The results are applied in an example showing how to place sensors and actuators on a structure in such locations that the system remains symmetrizable.
Design and Analysis of a Transmission Hydraulic System for an Engine-Flywheel Hybrid-Vehicle
Authors: Karmel A. M.
Published: March 2008

Unlike hydraulic systems of conventional automotive transmissions, the hydraulic system of a hybrid vehicle must cope with large variations of pressure as well as flow. If supplied by traditional pump configurations, either fixed or variable displacement, the system will incur substantial efficiency losses. This paper presents a feasibility study of a dual-pump hydraulic supply system which consists of a low-volume fixed-displacement pump and a high-volume variable-displacement pump. The analysis addresses the dynamic characteristics of this system along with its sensitive design areas.
Fault Detection in Linear Discrete Dynamic Systems by a Pattern Recognition of a Generalized-Likelihood-Ratio
Authors: Tanaka S. , Mu¨ller P. C.
Published: March 2008

The detection of an abrupt change in the parameters of a linear discrete dynamical system is considered in the framework of the easily implemented generalized-likelihood-ratio (GLR) method. This paper proposes a robust detection method based on a pattern recognition of the maximum GLR provided by the conventional step-hypothesized GLR method. A numerical example demonstrates that the proposed method is highly superior to the conventional step-hypothesized GLR method and to the Chi-squared test in both detection rate and detection speed.
Order Reduction of Discrete-Time System Via Bilinear Routh Approximation
Authors: Hwang Chyi , Hsieh Ching-Shieh
Published: March 2008

In this paper, a method of combining the Routh approximation method with the bilinear transformation is presented for deriving stable reduced-order models of a strictly proper z-transfer function Gn(z). It is based on applying the bilinear transformation to the (z+1)Gn(z), and then deriving a new bilinear Routh γ − δ canonical expansion for Gn(z). The proposed bilinear Routh approximation method has all the advantages of the Routh approximation method [5] while without having initial-value problem caused by the bilinear transformation. A numerical example is included to illustrate the procedure.