bluuebunny/crossref_metadata_2025 huggingface dataset:
Left Ventricular Geometric Remodeling and Residual Stress in the Rat Heart
Authors: Omens J. H. , Vaplon S. M. , Fazeli B. , McCulloch A. D.
Published: February 2008

Theoretical considerations and observations of residual stress suggest that geometric remodeling in the heart may also alter residual stress and strain. We investigated whether changes in left ventricular geometry during physiologic growth were associated with corresponding changes in myocardial residual strain. In anesthetized rats from eight age groups ranging from 2–25+ weeks, the heart was arrested and isolated, and equatorial slices were obtained. The geometry of the intact, unloaded state was recorded, as well as the “opening angle” of the stress-free configuration after radial resection of the tissue slice. The tissue was fixed and embedded for histological examination of collagen area fraction. Heart weight increased 10-fold with age and unloaded internal radius increased almost 4-fold. However, wall thickness increased only 66 percent, so that the ratio of wall thickness to internal radius decreased significantly from 2.22 ± 0.29 (mean ± SD) at 2 weeks to 0.81 ± 0.47 at 25 weeks. Opening angle of the stress-free slice decreased significantly from 87 ± 16 deg at 2 weeks to 51 ± 16 deg, and correlated linearly with wall thickness/radius ratio. Collagen area fraction increased with age. Hence physiologic ventricular remodeling in rats decreases myocardial residual strain in proportion to the relative reduction in wall thickness–radius ratio.
Thermal Stresses From Large Volumetric Expansion During Freezing of Biomaterials
Authors: Shi X. , Datta A. K. , Mukherjee Y.
Published: February 2008

Thermal stresses were studied in freezing of biomaterials containing significant amounts of water. An apparent specific heat formulation of the energy equation and a viscoelastic model for the mechanics problem were used to analyze the transient axi-symmetric freezing of a long cylinder. Viscoelastic properties were measured in an Instron machine. Results show that, before phase change occurs at any location, both radial and circumferential stresses are tensile and keep increasing until phase change begins. The maximum principal tensile stress during phase change increases with a decrease in boundary temperature (faster cooling). This is consistent with experimentally observed fractures at a lower boundary temperature. Large volumetric expansion during water to ice transformation was shown to be the primary contributor to large stress development. For very rapid freezing, relaxation may not be significant, and an elastic model may be sufficient.
Safe Touch Temperatures for Hot Plates
Authors: Subramanian B. , Chato J. C.
Published: February 2008

A finite difference heat transfer model has been developed to predict the Safe Touch Temperatures (STT) for plates made of different materials. SST can be defined as the highest temperature at which no pain is felt when the surface is touched for a long enough period to allow safe handling of the equipment. The criterion used to quantify damage is the “damage function” that was originally proposed by Henriques and Moritz. There are several uncertainties present in the physiological and thermal properties of the skin that give rise to a solution range rather than a single solution. Certain simplifying assumptions are made that tend to yield solutions for STT that are toward the lower or “safe” end of the solution range. The model developed is a two-dimensional axisymmetric model in cylindrical coordinates. A finite difference scheme that uses the Alternating Direction Implicit method is used to solve the problem. It is a second-order scheme in both space and time domains. A parametric analysis of the model is performed to isolate those factors that affect the STT to the greatest extent. Data are presented for a variety of cases, which cover commonly observed ranges in material and geometric properties. It is found that the material properties, namely thermal conductivity and volumetric heat capacity, and the plate thickness ratio are the three most important parameters. These three parameters account for a range of STT from 56°C–100°C with thick metals at the low end and thin metals and plastics in the high range. This method represents a significant improvement over existing standard practices.
Development of Wall Surface Tangent DPIV Measurement Techniques for Arterial Branch Models
Authors: Karn E. L. , Beale S. , Duitin˜o A. M. , Wei T. , Graham A. M. , Nackman G. B.
Published: February 2008

Experimental techniques for measuring unsteady flow in a glass arterial bifurcation model have been developed to aid in quantifying three-dimensional wall shear fluctuations associated with arterial disease. The unique feature of the current technique is the use of a “curved” laser sheet, which was everywhere tangent to the inner wall of a daughter tube in an arterial bifurcation model. Surface tangent velocity vector field measurements were made to demonstrate the potential of this technique. Ensemble-averaged data showing weak secondary flows as well as statistical distributions of flow angles are presented. Measurements of this type may be used to estimate mean and instantaneous wall shear magnitude and direction, data that are necessary for understanding the importance of circumferential motions on arterial disease.
Strain Distribution in the Layered Wall of the Esophagus
Authors: Gregersen H. , Lee T. C. , Chien S. , Skalak R. , Fung Y. C.
Published: February 2008

The function of the esophagus is to move food by peristaltic motion, which is the result of the interaction of the tissue forces in the esophageal wall and the hydrodynamic forces in the food bolus. To understand the tissue forces in the esophagus, it is necessary to know the zero-stress state of the esophagus, and the stress–strain relationships of the tissues. This article is addressed to the first topic: the representation of zero-stress state of the esophagus by the states of zero stress-resultant and zero bending moment of the mucosa–submucosa and the muscle layers. It is shown that at the states of zero stress-resultant and zero bending moment, these two layers are not tubes of smaller radii but are open sectors whose shapes are approximately cylindrical and more or less circular. When the sectors are approximated by circular sectors, we measured their radii, opening angles, and average thickness around the circumference. Data on the radii, thickness-to-radius ratios, and the opening angles of these sectors are presented. Knowing the zero-stress state of these two layers, we can compute the strain distribution in the wall at any in vivo state, as well as the residual strain in the esophageal wall at the no-load state. The results of the in vivo states are compared to those obtained by a conventional approach, which treats the esophageal wall as a homogeneous material, and to another popular simplification, which ignores the residual strains completely. It is shown that the errors caused by the homogeneous wall assumption are relatively minor, but those caused by ignoring the residual strains completely are severe.
A Poroelastic Continuum Model of the Cupula Partition and the Response Dynamics of the Vestibular Semicircular Canal
Authors: Damiano E. R.
Published: February 2008

Using mixture theory, an axisymmetric continuum model is presented describing the response dynamics of the vestibular semicircular canals to canal-centered head rotation in which the cupula partition is modeled as a poroelastic mixture of interpenetrating solid and fluid constituents. The solid matrix of the cupula is assumed to behave as a linear elastic material, whereas the fluid constituent is assumed to be Newtonian. A regular perturbation analysis of the fluid dynamics in the canal provides a dynamic boundary condition, which acts across the cupula partition. Numerical solution of the coupled system of momentum equations provides the spatio-temporal displacement fields for both the fluid and solid constituents of the cupula. Results indicate that at frequencies above 1 Hz, the fluid constituent is dynamically entrained by the solid matrix such that their motions are bound as if to exist as a single component. The resulting high-frequency response is consistent with the macromechanical response predicted by single-component viscoelastic models of the cupula. Below 1 Hz, the dynamic coupling between the fluid and solid constituents weakens and the transcupular differential pressure is sufficient to force fluid through the mixture with little deformation of the solid matrix. Results are sensitive to the precise value of the cupular permeability. One of the most important distinctions between the present analysis and previous impermeable models of the cupula arises at the micromechanical level in terms of the local fluid flow that is predicted to occur within the cupula and around the ciliary bundles and sensory hair cells. Another important result reveals that the permeation dynamics predicted below 1 Hz gives rise to the same low-frequency macromechanical response as would occur with an impermeable viscoelastic structure having a much greater stiffness. Current estimates of the mechanical stiffness of the cupula, based solely on afferent nerve data, may therefore overestimate the true value intrinsic to the solid matrix by as much as an order of magnitude.
Dynamic Contrast-Enhanced MRI and Fractal Characteristics of Percolation Clusters in Two-Dimensional Tumor Blood Perfusion
Authors: Craciunescu O. I. , Das S. K. , Clegg S. T.
Published: February 2008

Dynamic contrast-enhanced magnetic resonance imaging (DE-MRI) of the tumor blood pool is used to study tumor tissue perfusion. The results are then analyzed using percolation models. Percolation cluster geometry is depicted using the wash-in component of MRI contrast signal intensity. Fractal characteristics are determined for each two-dimensional cluster. The invasion percolation model is used to describe the evolution of the tumor perfusion front. Although tumor perfusion can be depicted rigorously only in three dimensions, two-dimensional cases are used to validate the methodology. It is concluded that the blood perfusion in a two-dimensional tumor vessel network has a fractal structure and that the evolution of the perfusion front can be characterized using invasion percolation. For all the cases studied, the front starts to grow from the periphery of the tumor (where the feeding vessel was assumed to lie) and continues to grow toward the center of the tumor, accounting for the well-documented perfused periphery and necrotic core of the tumor tissue.
Airway Closure: Occluding Liquid Bridges in Strongly Buckled Elastic Tubes
Authors: Heil M.
Published: February 2008

This paper is concerned with the airway closure problem and investigates the quasi-steady deformation characteristics of strongly collapsed (buckled) airways occluded by liquid bridges of high surface tension. The airway wall is modeled as a thin-walled elastic shell, which deforms in response to an external pressure and to the compression due to the surface tension of the liquid bridge. The governing equations are solved numerically using physiological parameter values. It is shown that axisymmetric configurations are statically unstable, as are buckled tubes whose opposite walls are not in contact. The quasi-steady deformation characteristics of strongly collapsed airways whose walls are in opposite wall contact show a pronounced hysteresis during the collapse/reopening cycle. Buckling is shown to occur over a short axial length with moderate circumferential wavenumbers. Finally, further implications of the results for the airway collapse/reopening problem are discussed.
A Perturbation Model for the Oscillatory Flow of a Bingham Plastic in Rigid and Periodically Displaced Tubes
Authors: De Chant L. J.
Published: February 2008

An approximate analytical model for the pulsatile flow of an ideal Bingham plastic fluid in both a rigid and a periodically displaced tube has been developed using regular perturbation methods. Relationships are derived for the velocity field and dimensionless flow rate. The solution compares adequately with available experimentally measured oscillatory non-Newtonian fluid flow data. These solutions provide useful analytical models supporting experimental and computation studies of arterial blood flow.
Conditions for Equivalency of Countercurrent Vessel Heat Transfer Formulations
Authors: Roemer R. B.
Published: February 2008

Previous models of countercurrent blood vessel heat transfer have used one of two, different, equally valid but previously unreconciled formulations, based either on: (1) the difference between the arterial and venous vessels’ average wall temperatures, or (2) the difference between those vessels’ blood bulk fluid temperatures. This paper shows that these two formulations are only equivalent when the four, previously undefined, “convective heat transfer coefficients” that are used in the bulk temperature difference formulation (two coefficients each for the artery and vein) have very specific, problem-dependent relationships to the standard convective heat transfer coefficients. (The average wall temperature formulation uses those standard coefficients correctly.) The correct values of these bulk temperature difference formulation “convective heat transfer coefficients” are shown to be either: (1) specific functions of (a) the tissue conduction resistances, (b) the standard convective heat transfer coefficients, and (c) the independently specified bulk arterial, bulk venous and tissue temperatures, or (2) arbitrary, user defined values. Thus, they are generally not equivalent to the standard convective heat transfer coefficients that are regularly used, and must change values depending on the blood and tissue temperatures. This dependence can significantly limit the convenience and usefulness of the bulk temperature difference formulations.
A Damage Model for Nonlinear Tensile Behavior of Cortical Bone
Authors: Fondrk M. T. , Bahniuk E. H. , Davy D. T.
Published: February 2008

To describe the time-dependent nonlinear tensile behavior observed in experimental studies of cortical bone, a damage model was developed using two internal state variables (ISV’s). One ISV is a damage parameter that represents the loss of stiffness. A rule for the evolution of this ISV was defined based on previously observed creep behavior. The second ISV represents the inelastic strain due to viscosity and internal friction. The model was tested by simulating experiments in tensile and bending loading. Using average values from previous creep studies for parameters in the damage evolution rule, the model tended to underestimate the maximum nonlinear strains and to overestimate the nonlinear strain accumulated after load reversal in the tensile test simulations. Varying the parameters for the individual tests produced excellent fits to the experimental data. Similarly, the model simulations of the bending tests could produce excellent fits to the experimental data. The results demonstrate that the 2-ISV model combining damage (stiffness loss) with slip and viscous behavior could capture the nonlinear tensile behavior of cortical bone in axial and bending loading.
Finite Element Analysis of Vertebral Body Mechanics With a Nonlinear Microstructural Model for the Trabecular Core
Authors: Overaker D. W. , Langrana N. A. , Cuitin˜o A. M.
Published: February 2008

In this study, a finite element model of a vertebral body was used to study the load-bearing role of the two components (shell and core) under compression. The model of the vertebral body has the characteristic kidney shape transverse cross section with concave lateral surfaces and flat superior and inferior surfaces. A nonlinear unit cell based foam model was used for the trabecular core, where nonlinearity was introduced as coupled elastoplastic beam behavior of individual trabeculae. The advantage of the foam model is that architecture and material properties are separated, thus facilitating studies of the effects of architecture on the apparent behavior. Age-related changes in the trabecular architecture were considered in order to address the effects of osteoporosis on the load-sharing behavior. Stiffness changes with age (architecture and porosity changes) for the trabecular bone model were shown to follow trends in published experimental results. Elastic analyses showed that the relative contribution of the shell to the load-bearing ability of the vertebra decreases with increasing age and lateral wall curvature. Elasto-plastic (nonlinear) analyses showed that failure regions were concentrated in the upper posterior region of the vertebra in both the shell and core components. The ultimate load of the vertebral body model varied from 2800 N to 5600 N, depending on age (architecture and porosity of the trabecular core) and shell thickness. The model predictions lie within the range of experimental results. The results provide an understanding of the relative role of the core and shell in vertebral body mechanics and shed light on the yield and post-yield behavior of the vertebral body.
Finite Element Analysis of the Lift on a Slightly Deformable and Freely Rotating and Translating Cylinder in Two-Dimensional Channel Flow
Authors: Zhao Y. , Sharp M. K.
Published: February 2008

Motivated by the lateral migration phenomena of fresh and glutaraldehyde-fixed red blood cells in a field flow fractionation (FFF) separation system, we studied the transverse hydrodynamic lift on a slightly flexible cylinder in a two-dimensional channel flow. The finite element method was used to analyze the flow field with the cylinder at different transverse locations in the channel. The shape of the cylinder was determined by the pressure on the surface of the cylinder from the flow field solution and by the internal elastic stress. The cylinder deformation and the flow field were solved simultaneously. The transverse lift exerted on the cylinder was then calculated. The axial and angular speed of the cylinder were iterated such that the drag and torque on the cylinder were nulled to represent a freely translating and rotating state. The results showed that the transverse lift on a deformable cylinder increased greatly and the equilibrium position moved closer to the center of the channel compared to a rigid cylinder. Also, with the same elastic modulus but a higher flow rate, a larger deformation and higher equilibrium location were found. The maximum deformation of the cylinder occurred when the cylinder was closest to the wall where a larger shear rate existed. The numerical results and experimental studies are discussed.
Numerical Analysis of the Deformation of an Adherent Drop Under Shear Flow
Authors: Kan H.-C. , Udaykumar H. S. , Shyy W. , Tran-Son-Tay R.
Published: February 2008

The adhesion of leukocytes to substrates is an important biomedical problem and has drawn extensive research. In this study, employing both single and compound drop models, we investigate how hydrodynamics interacts with an adherent liquid drop in a shear flow. These liquid drop models have recently been used to describe the rheological behavior of leukocytes. Numerical simulation confirms that the drop becomes more elongated when either capillary number or initial contact angle increases. Our results show that there exists a thin region between the drop and the wall as the drop undergoes large stretching, which allows high pressure to build up and provides a lift force. In the literature, existing models regard the leukocyte as a rigid body to calculate the force and torque acting on the drop in order to characterize the binding between cell receptors and endothelial ligands. The present study indicates that such a rigid body model is inadequate and the force magnitude obtained from it is less than half of that obtained using the deformable drop models. Furthermore, because of its much higher viscosity, the cell nucleus introduces a hydrodynamic time scale orders of magnitude slower than the cytoplasm. Hence the single and compound drops experience different dynamics during stretching, but exhibit very comparable steady-state shapes. The present work offers a framework to facilitate the development of a comprehensive dynamic model for blood cells.
Confocal Imaging of Flows in Artificial Venular Bifurcations
Authors: Hitt D. L. , Lowe M. L.
Published: February 2008

We describe a new experimental methodology for visualizing three-dimensional structures in microscopic tubes under flow conditions. Through the use of microfabrication techniques, artificial venular bifurcations are constructed from glass tubes with semicircular cross sections (radius = 50 μ). Aqueous fluorescent solutions are infused into the tubes at flow rates of about 1 μ1/min, a value comparable to blood flow in the microcirculation. The flow is imaged using a combination of confocal microscopy and three-dimensional image reconstruction software techniques. The quantitative accuracy of the experimental method is evaluated by measuring the “separation surface,” a formation resulting from converging flows at a bifurcation. Details of the fabrication process, fluidics, confocal microscopy, image reconstructions, optical effects, and computations are described. We show the first three-dimensional visualization of a microscopic flow structure using confocal microscopy, and within certain limitations, quantitative agreement between the measured and computed positions of the separation surface.
Study on Effect of Graded Facetectomy on Change in Lumbar Motion Segment Torsional Flexibility Using Three-Dimensional Continuum Contact Representation for Facet Joints
Authors: Natarajan R. N. , Andersson G. B. J. , Patwardhan A. G. , Andriacchi T. P.
Published: February 2008

Facet joints provide rigidity to the lumbar motion segment and thus protect the disk, particularly against torsional injury. A surgical procedure that fully or partially removes the facet joints (facetectomy) will decrease the mechanical stiffness of the motion segment, and potentially place the disk at risk of injury. Analytical models can be used to understand the effect of facet joints on motion segment stability. Using a facet joint model that represents the contact area as contact between two surfaces rather than as point contact, it was concluded that a substantial sudden change in rotational motion, due to applied torsion moment, was observed after 75 percent of any one of the facet joints was removed. Applied torsional moment loading produced coupled extension motion in the intact motion segment. This coupled motion also experienced a large change following complete unilateral facetectomy. Clinically, the present study showed that surgical intervention in the form of unilateral or bilateral total facetectomy might require fusion to reduce the primary torsion motion.
In Vitro Measurement of the Tracking Pattern of the Human Patella
Authors: Ahmed A. M. , Duncan N. A. , Tanzer M.
Published: February 2008

This study sought to determine whether a general pattern describing the three-dimensional tracking characteristics of the human patella could be established, and if not, then to determine the extent and nature of interspecimen variations in the characteristics in a normal population. Using 32 fresh-frozen knees subjected to extensor moment magnitudes similar to those in “static-lifting” and “leg-raising against resistance” maneuvers, patellar displacements were measured in the knee extension range 120 to 0 deg. For static-lifting, a constant foot-floor reaction of 334 N was applied. For leg-raising, a constant net quadriceps tension of 668 N was used throughout the extension range. Measurements were taken with a calibrated six-degree-of-freedom electromechanical goniometer and a displacement coordinate system referenced to the geometry of individual specinens. The three patellar displacements in the plane of knee extension/flexion (extension rotation, and anterior and proximal translations) consistently demonstrated the same pattern in the entire knee extension range with an average coefficient of variation of 13 percent. For knee angles greater than 45 deg, the three other displacements (medial lateral translation, and rotations about the anterior–posterior and proximal–distal axes) followed a general pattern. However, for knee angles less than 45 deg, these displacements differed considerably between specimens for each loading condition, both in terms of magnitude (average coefficient of variation: 70 percent), and direction.
Results From Demineralized Bone Creep Tests Suggest That Collagen Is Responsible for the Creep Behavior of Bone
Authors: Bowman S. M. , Gibson L. J. , Hayes W. C. , McMahon T. A.
Published: February 2008

Cortical and trabecular bone have similar creep behaviors that have been described by power-law relationships, with increases in temperature resulting in faster creep damage accumulation according to the usual Arrhenius (damage rate ~ exp (−Temp.−1)) relationship. In an attempt to determine the phase (collagen or hydroxyapatite) responsible for these similar creep behaviors, we investigated the creep behavior of demineralized cortical bone, recognizing that the organic (i.e., demineralized) matrix of both cortical and trabecular bone is composed primarily of type I collagen. We prepared waisted specimens of bovine cortical bone and demineralized them according to an established protocol. Creep tests were conducted on 18 specimens at various normalized stresses σ/E0 and temperatures using a noninvasive optical technique to measure strain. Denaturation tests were also conducted to investigate the effect of temperature on the structure of demineralized bone. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates at all applied normalized stresses and temperatures. Strong (r2 > 0.79) and significant (p < 0.01) power-law relationships were found between the damage accumulation parameters (steady-state creep rate dε/dt and time-to-failure tf) and the applied normalized stress σ/E0. The creep behavior was also a function of temperature, following an Arrhenius creep relationship with an activation energy Q = 113 kJ/mole, within the range of activation energies for cortical (44 kJ/ mole) and trabecular (136 kJ/mole) bone. The denaturation behavior was characterized by axial shrinkage at temperatures greater than approximately 56°C. Lastly, an analysis of covariance (ANCOVA) of our demineralized cortical bone regressions with those found in the literature for cortical and trabecular bone indicates that all three tissues creep with the same power-law exponents. These similar creep activation energies and exponents suggest that collagen is the phase responsible for creep in bone.
An Evaluation of Pseudoelastic Descriptors Used in Arterial Mechanics
Authors: Humphrey J. D.
Published: February 2008

Understanding how transmural distributions of stress relate to the mechanisms of vascular growth, remodeling, and disease necessitates computations that are based on a constitutive relation for the arterial wall. Although a number of candidate relations are in the literature, they have not been compared in detail. In this note, three commonly used descriptors of the passive behavior of common carotid arteries are compared using simple “thought experiments.” It is shown that two of the three relations are inherently limited in the degree of anisotropy they allow, that each predicts a different anisotropy, and that one yields physically unrealistic predictions given many of the published values of the material parameters. Based on this comparison, it appears that the relation proposed by Chuong and Fung is the best available, though there may be a need to search for an alternate form, particularly for muscular arteries. The methods presented herein are offered as a guide to help the experimentalist identify alternate forms of pseudostrain-energy functions for arteries.
Multichannel Optimal Linear Deconvolution Filters and Strip Thickness Estimation From Gauge Measurements
Authors: Grimble M. J.
Published: February 2008

A solution is presented to the H2 optimal deconvolution filtering, smoothing and prediction problems for multivariable, discrete, linear signal processing problems. A weighted H2 cost-function is minimized where the dynamic weighting function can be chosen for robustness improvement. The signal and noise sources can be correlated and signal channel dynamics can be included in the system model. The estimation of the thickness of steel strip given X-ray gauge measurements is then considered. The deconvolution problem arises because the thickness at the roll gap is required for control purposes whereas the measurement occurs some time later when the strip reaches the X-ray gauge.
Observer-Based Identification of Nonlinear System Parameters
Authors: Yi Kyongsu , Hedrick Karl
Published: February 2008

This paper deals with an observer-based nonlinear system parameter identification method utilizing repetitive excitation. Although methods for physical parameter identification of both linear and nonlinear systems are already available, they are not attractive from a practical point of view since the methods assume that all the system, x, and the system input are available. The proposed method is based on a “sliding observer” and a least-square method. A sufficient condition for the convergence of the parameter estimates is provided in the case of “Lipschitz” nonlinear second-order systems. The observer is used to estimate signals which are difficult or expensive to measure. Using the estimated states of the system with repetitive excitation, the parameter estimates are obtained. The observer based identification method has been tested on a half car simulation and used to identify the parameters of a half car suspension test rig. The estimates of nonlinear damping coefficients of a vehicle suspension, suspension stiffness, pitch moment inertia, equivalent sprung mass, and unsprung mass are obtained by the proposed method. Simulation and experimental results show that the identifier estimates the vehicle parameters accurately. The proposed identifier will be useful for parameter identification of actual vehicles since vehicle parameters can be identified only using vehicle excitation tests rather than component testing.
Nonlinear Diesel Engine Control and Cylinder Pressure Observation
Authors: Kao Minghui , Moskwa John J.
Published: February 2008

This paper presents novel approaches to nonlinear engine fuel controller and cylinder pressure observer designs based on mean torque production and cylinder-by-cylinder models. Sliding mode controllers and sliding mode observers have shown good robustness and high tracking performance. Accordingly, they are selected in this study to fulfill demanding engine electronic control and onboard diagnostics requirements.
On Relative Degrees and Zero Dynamics From Physical System Modeling
Authors: Wu S.-T. , Youcef-Toumi K.
Published: February 2008

Relative degrees and zero dynamics are intrinsic system properties associated with a given input-output pair. In the design of output tracking controllers, the relative degree and the stability of the zero dynamics of the control plant are usually assumed to be known in advance, and are determined by analyzing the system dynamic equations. With the help of bond graphs and through physical reasoning, a set of rules are proposed in this paper to determine the relative degree and the stability of the zero dynamics for a class of systems independent of the system dynamic equations. The rules establish a connection between these system properties and the physical structures, and are useful guidelines on the adjustment of relative degrees and zero dynamics for the purpose of control design.
A Hybrid Autonomous Control Approach
Authors: Clark Robert L.
Published: February 2008

A hybrid autonomous control approach for active damping and control of a persistent disturbance is presented. Direct velocity feedback control is coupled with an adaptive feedforward controller to minimize the transient response and that due to a persistent disturbance, respectively. A time-averaged gradient descent algorithm was selected to adapt the feedforward filter, rendering the controller model-insensitive and thus autonomous for the chosen application. The feedback control loop serves to increase the damping of the controlled structure, decreasing the convergence time associated with the adaptive feedforward filter. Thus the integration of the two control approaches affords increases in performance previously not possible with the adaptive feedforward controller alone.
Frequency Domain Criteria for Synthesizing Robustly Stabilizing Compensators for a Class of Interval Plants
Authors: Zhao Yongdong , Jayasuriya Suhada
Published: February 2008

Considered in this paper is the robust stabilization of a special family of single-input single-output, interval plants in which only the denominator polynomial or the plant poles are uncertain. Two frequency domain necessary and sufficient conditions are derived for the robust stability of the closed-loop system. The first stability criterion reduces to a question of whether or not a specially constructed polar plot intersects the box [−1, 1] × [−1, 1] in the complex plane and the second reduces to a question of whether or not the polar plot of the nominal loop transfer function intersects a specially constructed frequency dependent domain in the complex plane. Both criteria can be used for synthesizing controllers for the special class of interval plants considered. A loop shaping technique is proposed for the synthesis of a robustly stabilizing compensator. For the special class of interval plants considered, the polar plot of the nominal loop transfer function must not intersect a frequency dependent parallelogram. The four corners of the parallelogram can be explicitly computed at each frequency.
Closed-Type Solar Regenerator: Analysis and Simulation
Authors: Gandhidasan P.
Published: February 2008

Due to many potential problems associated with an open regeneration system in application to reconcentration with liquid desiccants, a closed-type solar regenerator has been simulated and analyzed in this paper. It consists of a flat, blackened, tilted surface with a transparent glazing as a covering. The weak desiccant to be concentrated flows as a thin film over the absorber and the water evaporating from the desiccant due to absorption of solar energy is condensed on the underside of the glass cover. A theoretical model, which includes the variation of rate of evaporation of water along the flow length of the regenerator, has been used to examine the thermal performance of the regenerator. Changes in thermal performance are reported in terms of insolation and the mass of water evaporated from the desiccant for different operating parameters.
A Unified Model for Slug Flow in Upward Inclined Pipes
Authors: Felizola H. , Shoham O.
Published: February 2008

The effect of pipe inclination on upward two-phase slug flow characteristics has been studied both experimentally and theoretically. Experimental data were acquired for the entire range of inclination angles, from horizontal to vertical. New correlations were developed for slug length and liquid holdup in the slug body as a function of inclination angle. A unified model has been developed for the prediction of slug flow behavior in upward inclined pipes. Reasonable agreement is observed between the pressure drop predicted by the model and the experimental data.
A Factorial Design Approach to Investigate the Effect of Geometry in Drill String Screw Connectors
Authors: Bahai H. , Esat I. I. , Rass L.
Published: February 2008

This paper describes a parametric study of drill string threaded connector design based on a “factorial design” method. The study is facilitated by a hybrid model which has been developed, validated, and reported previously, enabling efficient calculation of load and stress distribution along threaded connectors subjected to both axial and bending modes of loading. A parametric equation is produced where stress concentration factor is defined in terms of various geometrical variables. The equation is then utilized to carry out a constrained optimization within the feasible parameter space, and hence produce an “optimum” thread and connector design.
Creep Modeling of Rock Salts for Geoenvironmental Application
Authors: Hossain M. I. , Faruque M. O. , Zaman M.
Published: February 2008

The paper discusses a rate-dependent constitutive model to describe the creep deformation of rock salts. The model is based on the dislocation-related micromechanisms; the concept of effective stress is utilized by incorporating the back stress as an internal variable. The model is used to predict the creep potential of rock salt under general thermo-mechanical loading. The experimental test results from Avery Island dome salt are used in this work. The associated five model parameters are evaluated by using an optimization technique, called “box algorithm.” The parameters are then used with the model to predict unoptimized experimental data sets. The influence of temperature and confining pressure are investigated in detail for the selected rock salt. Overall, excellent correlations are observed.
An Experimental Study of Buckling and Post-Buckling of Laterally Constrained Rods
Authors: Suryanarayana P. V. R. , McCann R. C.
Published: February 2008

The effects of friction and curvature on buckling, post-buckling, and unbuckling behavior of rods laterally constrained within an enclosure are studied experimentally. The experimental apparatus, measurement procedures, and uncertainty analysis are described. Results indicate that friction significantly delays the onset of buckling, and causes noticeable hysteresis in the post-buckling behavior. As a result, the unbuckling loads are always less than the corresponding buckling loads. The drag-related loss, which eventually leads to lock-up, is also measured and reported in this work. Friction is also a cause of post-buckling snapping and reversals in the direction of wrap of the helix. As expected, the effects of friction become less significant as the inclination decreases. It is shown that predictions of current theory agree with experimentally measured unbuckling rather than buckling loads. When friction is significant, current theory underestimates the compressive loads at which buckling occurs in straight or curved wellbores. Ignoring friction or curvature limits the estimated weight on bit well below the safe load that can be used in many drilling and completion operations in extended reach or horizontal wells. Moreover, the hysteresis effect of friction means that once buckling has occurred, the compressive loads must be reduced to values much below the buckling initiation loads to fully straighten the buckled pipe.
Experimental and Theoretical Information on Solid Flow in a Circulating Fluidized Bed
Authors: Steward F. R. , Couturier M. F. , Poolpol S. , Wang S.
Published: February 2008

Data are presented on a laboratory-scale circulating fluidized bed for various column geometries, solid fluxes, and gas velocities. Particular attention is paid to the solid flux profiles under various operating conditions. A model based on the fundamental equations of motion has been developed to describe the flow patterns of the solids particles and gas within a circulating fluidized bed. The predictions of the model are compared with experimental results. The model predicts that instabilities at the base of the bed propagate up the column for significant distances before uniform flows of solids and gas are established.
Waste-Heat Recovery in Batch Processs Using Heat Storage
Authors: Stoltze S. , Mikkelsen J. , Lorentzen B. , Peterson P. M. , Qvale B.
Published: February 2008

The waste-heat recovery in batch processes has been studied using the pinch-point method. The aim of the work has been to investigate theoretical and practical approaches to the design of heat-exchanger networks, including heat storage, for waste-heat recovery in batch processes. The study is limited to the incorporation of energy-storage systems based on fixed-temperature variable-mass stores. The background for preferring this to the alternatives (variable-temperature fixed-mass and constant-mass constant-temperature (latent-heat) stores) is given. It is shown that the maximum energy-saving targets as calculated by the pinch-point method (time average model, TAM) can be achieved by locating energy stores at either end of each process stream. This theoretically large number of heat-storage tanks (twice the number of process streams) can be reduced to just a few tanks. A simple procedure for determining a number of heat-storage tanks sufficient to achieve the maximum energy-saving targets as calculated by the pinch-point method is described. This procedure relies on combinatorial considerations, and could therefore be labeled the “combinatorial method” for incorporation of heat storage in heat-exchanger networks. Qualitative arguments justifying the procedure are presented. For simple systems, waste-heat recovery systems with only three heat-storage temperatures (a hot storage, a cold storage, and a heat store at the pinch temperature) often can achieve the maximum energy-saving targets. Through case studies, six of which are presented, it is found that a theoretically large number of heat-storage tanks (twice the number of process streams) can be reduced to just a few tanks. The description of these six cases is intended to be sufficiently detailed to serve as benchmark cases for development of alternative methods.
A Stratified Flow Model for Adiabatic Losses in Regenerative Thermal Devices
Authors: Bauwens L.
Published: February 2008

A model has been developed for regenerative thermal devices such as Stirling engines and refrigerators, in which, while the heat exchangers are modeled as fully isothermal, the flow in the cylinders and dead spaces is modeled as adiabatic, one-dimensional, and stratified. Pressure is assumed to be spatially uniform, so that the two remaining conservation laws refer to mass and energy. It is shown that under the proposed assumptions, the conservation laws can be integrated in closed form with respect to the space variable, leaving only time integration to be performed numerically. As a result, the numerical integration is very cost-effective, sufficiently so that the resulting program can be used to chart large portions of the parameter space relatively quickly on widely available engineering workstations.
On the Simplified Modeling of NO Formation in Technical Combustion Processes
Authors: Kloeppner G. , Scholz R. , Jeschar R.
Published: February 2008

Measures reducing the NO formation by qualified firing (primary measures) are intensively investigated. Fundamental elements in this context are the staging of fuel and combustion air as well as the recirculation of exhaust gas, to control residence time and also temperature and concentration patterns. For prediction, but also for optimization and control of technical combustion processes, simplified models of NO formation are applicable and required. These models should be simplified in a way that allows on-line operation and is capable of reflecting sufficiently the process. The structure of such a model (kinetics, reactors) is described in the present paper. Introducing two model parameters (a characteristic process temperature and a fuel-NO parameter), the model has proved effective in predicting NO formation during combustion of natural gas and also visbreaker residue.
Differences Between Second Law Analysis and Pinch Technology
Authors: Sama D. A.
Published: February 2008

The use of second law analysis to design a heat exchanger network is compared with the pinch technology approach. Differences between the two methods are identified and discussed in the light of claims made by practitioners of pinch technology. Second law insights are used to easily identify and correct design errors in a heat exchanger network, and to design maximum energy recovery networks. More importantly, it is found that use of the second law provides an understanding of the process which is totally absent in the pinch technology approach. The claims that pinch technology can find global optimum solutions, that only pinch technology can find maximum energy recovery heat exchanger networks, and that pinch technology is a form of second law analysis, are considered, discussed, and shown to be invalid.
High-Power Multi-Stage Rankine Cycles
Authors: Ibrahim O. M. , Klein S. A.
Published: February 2008

This paper presents an analysis of the multi-stage Rankine cycle aiming at optimizing the power output from low-temperature heat sources such as geothermal or waste heat. A design methodology based on finite-time thermodynamics and the maximum power concept is used in which the shape and the power output of the maximum power cycle are identified and utilized to compare and evaluate different Rankine cycle configurations. The maximum power cycle provides the upper-limit power obtained from any thermodynamic cycle for specified boundary conditions and heat exchanger characteristics. It also provides a useful tool for studying power cycles and forms the basis for making design improvements.
Modeling of Roller Cone Bit Lift-Off Dynamics in Rotary Drilling
Authors: Spanos P. D. , Sengupta A. K. , Cunningham R. A. , Paslay P. R.
Published: February 2008

Rotary drilling with roller cone bit is often accompanied by wide fluctuations of the weight on bit (WOB). In certain cases, the WOB periodically reduces to null. It has been postulated that in these cases the drill bit lifts off from the formation surface. The present study models the interaction among the lift-off of the drill bit, the WOB variation, and the modulation of the amplitude of the lobes on the formation surface. The analytical model is used to obtain numerical results for a specific drillstring. From these results it is inferred that the rotary speeds corresponding to the axial resonant frequencies of the system, determined from the functions of the driving point mobility and impedance at the drill bit, are critical regarding the wide fluctuations of the WOB; they may be associated with the sustenance of the lobes with large amplitude.
Buckling and Lockup of Tubulars in Inclined Wellbores
Authors: Wu J. , Juvkam-Wold H. C.
Published: February 2008

This paper studies sinusoidal and helical buckling of tubulars in inclined wellbores and the “lockup” of tubulars due to buckling. The results show that tubular buckling starts from the tubular bottom in low-inclination wellbores, where the axial compressive load is largest due to tubular weight. In high-inclination wellbores it may start from the top portion of the tubular, where the axial compressive load is largest due to frictional drag. This clarifies the confusion on whether or not the tubular buckles at once on its entire length in inclined wellbores. New sinusoidal and helical buckling load equations are presented to better predict tubular buckling in inclined wellbores (0–90 deg). The lower the wellbore inclination angle, the smaller the axial compressive load required to initiate tubular buckling. However, a certain nonzero axial compressive load is still needed to buckle the tubulars in vertical wellbores. When tubulars buckle helically, a large wall contact force will be generated, and the “slack-off” weight at the surface will not be fully transmitted to the tubular bottom due to large resultant frictional drag. The “lockup” of tubulars may even occur, where the tubular bottom load cannot be increased by slacking-off weight at the surface.