bluuebunny/crossref_metadata_2025 huggingface dataset:
CO2 Emission Abatement in IGCC Power Plants by Semiclosed Cycles: Part B—With Air-Blown Combustion and CO2 Physical Absorption
Authors: Chiesa P. , Lozza G.
Published: February 2008

This paper analyzes the fundamentals of IGCC power plants with carbon dioxide removal systems, by a cycle configuration alternative to the one discussed in Part A (with oxygen-blown combustion). The idea behind this proposal is to overcome the major drawbacks of the previous solution (large oxygen consumption and re-design of the gas turbine unit), by means of a semiclosed cycle using air as the oxidizer. Consequently, combustion gases are largely diluted by nitrogen and cannot be simply compressed to produce liquefied CO2 for storage or disposal. However, CO2 concentration remains high enough to make separation possible by a physical absorption process. It requires a re-pressurization of the flow subtracted from the cycle, with relevant consequences on the plant energy balance. The configuration and the thermodynamic performance of this plant concept are extensively addressed in the paper. As in the first part, the influence of the pressure ratio is discussed, but values similar to the ones adopted in commercial heavy-duty machines provided here acceptable performance. Proper attention was paid to the impact of the absorption process on the energy consumption. The resulting net overall efficiency is again in the 38–39 percent range, with assumptions fully comparable to the ones of Part A. Finally, we demonstrated that the present scheme enables the use of unmodified machines, but large additional equipment is required for exhausts treatment and CO2 separation. A final comparison between the two semiclosed cycle concepts was therefore addressed.
An Iterative CFD and Mechanical Brush Seal Model and Comparison With Experimental Results
Authors: Chen L. H. , Wood P. E. , Jones T. V. , Chew J. W.
Published: February 2008

The position of the bristles within a brush seal is dictated by the pressure distribution within the seal, which is itself influenced by the position of the bristle matrix. In order to predict mass flows, pressure capabilities, bristle displacements, stresses, and contact loads at the rotor interface, a technique for iterating between a CFD and a mechanical model has been developed. The iterative technique is used to model the behavior of seals with an initial build clearance, where the application of pressure causes a change in the position of the bristle matrix. Frictional effects between neighboring bristles and at the backing ring influence the behavior of the bristles and these are accounted for within the mechanical part of the model. Results are presented and discussed for seals of both initial build clearance and interference. The mathematical predictions for flow, contact loads at the rotor interface, and the nature of the bristles displacements are compared with experimental results.
Correlations of the Convection Heat Transfer in Annular Channels With Rotating Inner Cylinder
Authors: Jakoby R. , Kim S. , Wittig S.
Published: February 2008

In the internal air system of gas turbine engines or generators, a large variety of different types of annular channels with rotating cylinders are found. Even though the geometry is very simple, the flow field in such channels can be completely three-dimensional and also unsteady. From the literature it is well-known that the basic two-dimensional flow field breaks up into a pattern of counter-rotating vortices as soon as the critical speed of the inner cylinder is exceeded. The presence of a superimposed axial flow leads to a helical shape of the vortex pairs that are moving through the channel. For the designer of cooling air systems there are several open questions. Does the formation of a Taylor-vortex flow field significantly affect the convective heat transfer behavior of the channel flow? Is there a stability problem even for high axial Reynolds-numbers and where is the location of the stability boundary? After all, the general influence of rotation on the heat transfer characteristics has to be known. By the results of flow field and heat transfer measurements, the impact of rotation and the additional influence of Taylor-vortex formation on the heat transfer characteristics in annular channels with axial throughflow will be discussed. The flow field was investigated by time-dependant LDA-measurements, which revealed detailed information about the flow conditions. By a spectral analysis of the measured data, the different flow regimes could be identified. Based on these results, the heat transfer from the hot gas to the rotating inner shaft was determined with a steady-state method. Thus, the influence of the different physical phenomena such as rotation with and without Taylor-vortex formation or the flow development could be separated and quantified. Finally, correlations of the measured results were derived for technical applications.
A Metallographic Technique for High Temperature Creep Damage Assessment in Single Crystal Alloys
Authors: Henderson P. , Komenda J.
Published: February 2008

The use of single crystal (SX) nickel-base superalloys will increase in the future with the introduction of SX blades into large gas turbines for base-load electricity production. Prolonged periods of use at high temperatures may cause creep deformation and the assessment of damage can give large financial savings. A number of techniques can be applied for life assessment, e.g., calculations based on operational data, nondestructive testing or material interrogation, but because of the uncertainties involved the techniques are often used in combination. This paper describes a material interrogation (metallographic) technique for creep strain assessment in SX alloys. Creep tests have been performed at 950°C on the SX alloy CMSX-4 and quantitative microstructural studies performed on specimens interrupted at various levels of strain. It was found that the strengthening γ′-particles, initially cuboidal in shape, coalesced to form large plates or rafts normal to the applied stress. The γ-matrix phase also formed plates. CMSX-4 contains ∼70 vol % γ-particles and after creep deformation the microstructure turned itself inside out, i.e., the gamma “matrix” became the isolated phase surrounded by the γ′-“particles.” This can cause problems for computerized image analysis, which in this case, were overcome with the choice of a suitable measurement parameter. The rafts reached their maximum length before 2 percent strain, but continued to thicken with increasing strain. Although of different dimensions, the aspect ratios (length/thickness ratio) of the gamma-prime rafts and the gamma plates were similar at similar levels of strain, increasing from ∼1 at zero strain to a maximum of ∼ 3 at about 1–2 percent strain. Analysis of microstructural measurements from rafting studies on SX alloys presented in the literature showed that the aspect ratios of the γ and γ′-phases were similar and that at a temperature of 950–1000°C a maximum length/thickness ratio of about 2.5–3.5 is reached at 1 to 2 percent creep strain. Measurement of gamma-prime raft or (or gamma plate) dimensions on longitudinal sections of blades is thus a suitable method for high temperature creep damage assessment of SX alloys. This gives a considerable advantage over conventional Ni-base superalloys whose microstructures are usually very stable with respect to increasing creep strain.
Influence of an Aluminide Coating on the TMF Life of a Single Crystal Nickel-Base Superalloy
Authors: Affeldt E. E.
Published: February 2008

TMF tests were conducted with bare and aluminide coated single crystal nickel-based superalloy specimens. Temperature cycling was between 400°C and 1100°C with a phase shift (135 deg) that is typical for damaged locations on turbine blades. Stress response is characterized by a constant range and the formation of a tensile mean stress as a result of relaxation in the high temperature part of the cycle which is in compression. Bare specimens showed crack initiation from typical oxide hillocks. Coated specimens showed life reduction with respect to the bare ones caused by brittle cracking of the coating in the low temperature part of the cycle. Isothermal bending tests of coated specimens confirmed the low ductility of the coating at temperatures below 600°C but quantitative correlation with the TMF test results failed.
Reduction of the Dynamic Load Capacity in a Squeeze Film Damper Operating With a Bubbly Lubricant
Authors: Diaz S. E. , San Andre´s L. A.
Published: February 2008

Squeeze film dampers (SFDs) are effective means to reduce vibrations and to suppress instabilities in rotor-bearing systems. However, at operating conditions while traversing critical speeds with large orbital whirl motions, ingestion and entrapment of air into the thin lands of SFDs generates a bubbly mixture (air in lubricant) that is known to reduce the dynamic film pressures and the overall damping capability. This pervasive phenomenon lacks proper physical understanding and sound analytical modeling. An experimental investigation to quantify the forced performance of a SFD operating with a controlled bubbly mixture is detailed. Tests are conducted in a constrained circular orbit SFD to measure the dynamic squeeze film pressures and journal motion at two whirl frequencies (8.33 and 16.67 Hz) as the air content in the mixture increases from 0 percent to 100 percent. The analysis of period-averaged film pressures reveals a zone of uniform low pressure of magnitude equal to the discharge pressure, independently of the mixture composition. The uniform pressure zone extends as the mixture void fraction increases. Radial and tangential film forces are estimated from the dynamic pressures at two axial locations of measurement. The tangential (damping) force decreases proportionally with the mixture volume fraction, while a radial hydrostatic force remains nearly invariant. The experimental results quantify effects previously known by qualitative description only, thus providing a benchmark towards the development of sound theoretical models.
Imbalance Response of a Rotor Supported on Open-Ends Integral Squeeze Film Dampers
Authors: de Santiago O. , San Andre´s L. , Oliveras J.
Published: February 2008

Rotor vibration attenuation and structural components isolation in jet engines are achieved with squeeze film dampers, many of them supported on long elastic squirrel cages. Integral squeeze film dampers (ISFDs) are comprised of arcuate pads and wire-EDM webs rendering a compact viscoelastic support. An experimental study is conducted to evaluate the effectiveness of ISFDs in attenuating the imbalance response of a massive test rotor. Measurements of the damper structural stiffness and rotor natural frequencies are detailed. Impact tests on the test rotor supported on its dampers reveal the supporting structure to be very flexible, thus requiring the experimental evaluation of an equivalent stiffness for the damper and supports system. System damping coefficients extracted from impact load excitations vary with the lubricant viscosity and include a significant structural damping from the bearing supports. Rotor coast-down tests demonstrate the ISFDs to damp well the rotor response with peak vibration amplitude proportional (linear) to the imbalance. Viscous damping coefficients estimated from the amplitude response at the critical speeds agree reasonably well with predictions from a full-film, finite element model.
Computational Evaluation of Low NOx Operating Conditions in Arch-Fired Boilers
Authors: Fueyo N. , Gambo´n V. , Dopazo C. , Gonza´lez J. F.
Published: February 2008

In the present paper, a computational model is used to simulate the aero-dynamic, thermal, and chemical conditions inside an arch-fired coal boiler. The model is based on the Eulerian-Eulerian concept, in which Eulerian conservation equations are solved both for the gas and the particulate phases. A NOx formation and destruction submodel is used to calculate the local concentration of NO. The model is used to simulate a range of operating conditions in an actual, 350 MW, arch-fired boiler, with the aim of reducing, using primary measures, the emissions of NOx. The model results shed some light on the relevant NOx-formation mechanisms under the several operating conditions. Furthermore, they correlate well quantitatively with the available field measurements at the plant, and reproduce satisfactorily the tendencies observed under the different operating modes.
Fatigue Strength Reliability Analysis of Diesel Connecting Rod Based on Stochastic Finite Element Method
Authors: Yi C. , Mingwu W. , Ling T.
Published: February 2008

In this paper, stress of a diesel connecting rod (CR) is analyzed by the perturbation stochastic finite element method (PSFEM). A fatigue failure criterion of a diesel CR is also put forward with Corten-Dolan fatigue cumulative damage theory. Based on fatigue failure criterion and the results of stochastic stress analysis, the advanced first order second moment (AFOSM) method is used for fatigue strength reliability analysis. It is shown that PSFEM is efficient and accurate in stochastic stress analysis by comparing with Monte-Carlo simulation. The analysis shows that the reliability of a certain type of diesel CR is 0.99917, which coincides with the statistical data from the factory. It is also found that operating parameters such as combustion peak pressure and engine rotary speed have the greatest influence on CR reliability because of the large variances and high stress response sensitivity.
Analysis of Heat, Mass, and Momentum Transfer in the Rain Zone of Counterflow Cooling Towers
Authors: de Villiers E. , Kro¨ger D. G.
Published: February 2008

The rate of heat, mass, and momentum transfer in the rain zone of three counterflow cooling tower geometries is analyzed using simplifying assumptions and numerical integration. The objective of the analysis is to generate equations for use in a one-dimensional mathematical cooling tower performance evaluations. Droplet deformation is taken into account and momentum transfer is calculated from the air flow’s mechanical energy loss, caused by air-droplet interaction. A comparison of dimensionless semi-empirical equations and experimental data demonstrates the method’s capability to predict the pressure drop in a counterflow rain zone.
Bifurcation and Chaos in Externally Excited Circular Cylindrical Shells
Authors: Chin Char-Ming , Nayfeh A. H.
Published: February 2008

The nonlinear response of an infinitely long cylindrical shell to a primary excitation of one of its two orthogonal flexural modes is investigated. The method of multiple scales is used to derive four ordinary differential equations describing the amplitudes and phases of the two orthogonal modes by (a) attacking a two-mode discretization of the governing partial differential equations and (b) directly attacking the partial differential equations. The two-mode discretization results in erroneous solutions because it does not account for the effects of the quadratic nonlinearities. The resulting two sets of modulation equations are used to study the equilibrium and dynamic solutions and their stability and hence show the different bifurcations. The response could be a single-mode solution or a two-mode solution. The equilibrium solutions of the two orthogonal third flexural modes undergo a Hopf bifurcation. A combination of a shooting technique and Floquet theory is used to calculate limit cycles and their stability. The numerical results indicate the existence of a sequence of period-doubling bifurcations that culminates in chaos, multiple attractors, explosive bifurcations, and crises.
A Higher-Order Theory for Vibration of Doubly Curved Shallow Shells
Authors: Liew K. M. , Lim C. W.
Published: February 2008

A higher-order shear deformation theory is presented for vibration analysis of thick, doubly curved shallow shells. An orthogonal curvilinear coordinate system is employed to arrive at the strain components. A third-order displacement field in transverse coordinate is adopted. Though no transverse normal stress is assumed, the theory accounts for cubic distribution of the transverse shear strains through the shell thickness in contrast with existing parabolic shear distribution. The unsymmetric shear distribution is a physical consequence of the presence of shell curvatures where the stress and strain of a point above the mid-surface are different from its counterpart below the mid-surface. Imposing the vanishing of transverse shear strains on top and bottom surfaces, the rotation field is reduced from a six-degree to a two-degree system. The discrepancy between the existing and the present theories is highlighted.
Freezing Range Effect on Shell Growth Instability During Alloy Solidification
Authors: Hector L. G. , Kim Woo-Seung , Richmond O.
Published: February 2008

During metal casting, nonuniform thermal strains due to small spatial perturbations in the cooling profile may lead to a condition of growth instability. This condition is one where the metal shell thickness becomes highly nonuniform due to the formation of microscopic air gaps along the mold surface. Documented experimental work with iron-carbon and aluminum alloy systems provides qualitative evidence that the freezing range width has an important influence on the macromorphology of the freezing front and hence the shell thickness. In this paper, we develop a thermomechanical model which points to a possible mechanism for growth instability during the solidification of a three-phase system where a small, spatially periodic cooling profile is superposed onto uniform cooling. This extends earlier work on two-phase (pure metal) systems. Temperature fields and shell growth are first calculated using perturbation theory, and then the associated thermal stresses and strains are determined from a hypoelastic constitutive law. The evolution of the shell/mold contact pressure beneath a shell thickness minimum is examined as a function of the freezing range width, and the onset of growth instability is assumed to occur as the contact pressure drops to zero indicating possible air gap nucleation. Under fixed cooling conditions, instability increases with freezing range width at small liquid metal pressures, decreases with freezing range width at somewhat larger liquid metal pressures, and disappears at still larger pressures.
Stability of Generalized Equilibria in Constrained Pseudodissipative Systems
Authors: Walker J. A.
Published: February 2008

We present improved stability criteria for the generalized equilibria of pseudodissipative systems. Compared to earlier results (Walker, 1988), the new criteria apply to many more kinematically constrained systems. We also obtain particularly simple stability criteria for holonomically constrained systems. Minimality is not assumed and linearization is not employed.
The Wrinkling of Rectangular Webs Under Nonlinearly Distributed Edge Loading
Authors: Lin C. C. , Mote C. D.
Published: February 2008

Nonlinearly distributed in-plane tension at the edges of a thin, flat, rectangular material with small flexural stiffness, called a web, can cause it to wrinkle. Airy stress functions are determined in closed form for webs under arbitrarily prescribed, in-plane edge loading. The loading at impending wrinkling, and the corresponding wrinkling shape in the web, are predicted by the solution of an eigenvalue problem. This analysis is illustrated through two examples: (I) wrinkling of a web induced by nearly uniform normal edge tension; (II) wrinkling of a web induced by uniform normal tension distributed over a section of the edge. Predictions of wrinkling using membrane theory are compared to those obtained with the current web theory. The contribution of small flexural stiffness to the prediction of the condition of wrinkling is shown to be substantial.
Electrorheological Dampers, Part II: Testing and Modeling
Authors: Gavin H. P. , Hanson R. D. , Filisko F. E.
Published: February 2008

Electrorheological (ER) materials develop yield stresses on the order of 5–10 kPa in the presence of strong electric fields. Viscoelastic and yielding material properties can be modulated within milli-seconds. An analysis of flowing ER materials in the limiting case of fully developed steady flow results in simple approximations for use in design. Small-scale experiments show that these design equations can be applied to designing devices in which the flow is unsteady. More exact models of ER device behavior can be determined using curve-fitting techniques in multiple dimensions. A previously known curve-fitting technique is extended to deal with variable electric fields. Experiments are described which illustrate the potential for ER devices in large-scale damping applications and the accuracy of the modeling technique.
Rotation of a Clamped Spherical Ball due to Linear Reciprocating Motion
Authors: Edelstein E. S. , Blech J. J.
Published: February 2008

A perfect spherical ball, symmetrically clamped between two identical surfaces, was found to rotate due to linear reciprocating excitation. A plausible theoretical explanation of this phenomenon follows. The rotation of the ball is a combination of the magnification of the dynamic response of the ball in the clamping system, near its natural frequencies, with frictional stick-slip at the contact areas. The ball rotates when the system is excited by a reciprocating motion not collinear with the preloading direction. The theoretical analysis of such rotation was corroborated by testing. This rotation may result in wear because of the slip motion involved. This work is focused on balls clamped between spherical (concave) surfaces. However, such rotation can also be developed in cylindrical hinges and rolling bearings, naturally loaded between a pair of surfaces. Furthermore, rotation of balls may develop in a nonrotating ball bearing under dynamic environmental conditions.
Distortion and Residual Stress in Quenched Aluminum Bars
Authors: Becker R. , Karabin M. E. , Liu J. C. , Smelser R. E.
Published: February 2008

Predictions of distortion and residual stress for rectangular bars, quenched on one surface, are compared with experimental measurements. The heat flow is unidirectional, and the dominant stress component acts along the length of the bars. Thermocouples measure the transient temperature distribution, and displacement transducers monitor the distortion of the bars into circular arcs. The thermal history inferred from an inverse heat transfer calculation drives the mechanical stress and deformation analysis. The numerical analyses demonstrate the sensitivity of the quench distortion to the thermal boundary conditions, the thermophysical properties and the constitutive model. Agreement between the predicted and measured distortion is only qualitative. The predicted residual stress field compares very well with the residual stress inferred from layer removal techniques.
Vertical Vibration of a Deep Bed of Granular Material in a Container
Authors: Wassgren C. R. , Brennen C. E. , Hunt M. L.
Published: February 2008

A deep bed of granular material (more than six layers of particles) was subjected to sinusoidal, vertical vibrations. Several phenomena were observed depending on the amplitude of excitation. These included heaping, surface waves, and arching; the transitions from one state to another involved various dynamic instabilities and bifurcations. The paper includes a description of these phenomena and the characteristic properties associated with each in addition to measurements of the transitions from one phenomena to another.
Simulation of Multivariate Gaussian Fields Conditioned by Realizations of the Fields and Their Derivatives
Authors: Ren Y. J. , Elishakoff I. , Shinozuka M.
Published: February 2008

This paper investigates conditional simulation technique of multivariate Gaussian random fields by stochastic interpolation technique. For the first time in the literature a situation is studied when the random fields are conditioned not only by a set of realizations of the fields, but also by a set of realizations of their derivatives. The kriging estimate of multivariate Gaussian field is proposed, which takes into account both the random field as well as its derivative. Special conditions are imposed on the kriging estimate to determine the kriging weights. Basic formulation for simulation of conditioned multivariate random fields is established. As a particular case of uncorrelated components of multivariate field without realizations of the derivative of the random field, the present formulation includes that of univariate field given by Hoshiya. Examples of a univariate field and a three component field are elucidated and some numerical results are discussed. It is concluded that the information on the derivatives may significantly alter the results of the conditional simulation.
Stepwise Loading of Half-Spaces in Elliptical Contact
Authors: Ja¨ger J.
Published: February 2008

A new analytical solution for Hertzian surfaces in contact under stepwise oblique loading is presented in this paper. It is written as a superposition of so-called Catta-neo-Mindlin functions, which represent the tangential stress distribution for constant normal and monotonically increasing tangential forces. The size of the stick zone is determined by cones in the force space, which are also known as yield cones. It has a simpler form than the differential force-displacement relations of plasticity theory and the numerical discretization of the influence integrals. A short computer algorithm is proposed and compared with a numerical solution of the discretized problem.
An Approximate Solution for the Compression of a Bonded Thin Annular Disk
Authors: Ling Yun
Published: February 2008

An approximate solution for the compression of a bonded thin annular disk is presented based on the so-called Perturbation-Ritz Method. The solution is essentially the outer expansion of the problem, with the unknown constants determined by the Ritz Method minimizing the potential energy. It is valid throughout the thin disk except in the boundary layers, which are confined to very narrow regions near the lateral surfaces. The solution asymptotically approaches the exact one as the disk thickness reduces towards zero. Both the incompressible and compressible cases are discussed. The relationships of the stiffness and stress distributions with the key parameters such as the shape factor, the radius ratio, and Poisson’s ratio are investigated. A better understanding of the compression of a bonded thin annular disk is achieved.
Diffusive Crack Growth at a Bimaterial Interface
Authors: Chuang Tze-jer , Chu June-Liang , Lee Sanboh
Published: February 2008

The high temperature microcrack growth behavior along a planar interface between two elastic dissimilar media is investigated with an aim at estimating service life of advanced ceramic composites under creep-rupture conditions. The crack is assumed to grow along the interface normal to a remote applied tensile stress via a coupled surface and grain-boundary diffusion under steady-state creep conditions. The crack-tip conditions were first derived from the asymmetric tip morphology developed by surface self-diffusion. The governing integro-differential equation containing the unknown tensile stress distribution along the interface ahead of the moving crack tip was derived and it was found that a new length parameter exists as a scaling factor for the interface for which the solution becomes identical to that of the single-phase media when plotted on the nondimensional physical plane. In contrast to the elastic stress solution which shows singularity at the tip and oscillatory character away from the tip, the creep stresses have a peak value away from the tip due to a wedging effect and interfacial sliding eliminates stress oscillation resulting in a decoupling between mode I and mode II stress fields. This stress solution ties the far-field loading parameter to the crack-tip conditions in terms of the unknown crack velocity to give a specific V-K functional relationship. It was shown that a stress exponent of 12 in the conventional power-law crack growth emerges at higher applied stress levels. An analysis on energy balance shows that the energy release during crack growth amounts to the J-integral which derives mostly from work done by “wedging,” not from strain energy loss. A constraint on interfacial diffusivities of the two species was found and its implications on possible microstructural developments were discussed.
An Energy-Based Approach to Computing Resonant Nonlinear Normal Modes
Authors: King M. E. , Vakakis A. F.
Published: February 2008

A formulation for computing resonant nonlinear normal modes (NNMs) is developed for discrete and continuous systems. In a canonical framework, internal resonance conditions are immediately recognized by identifying commensurable linearized natural frequencies of these systems. Additionally, a canonical formulation allows for a single (linearized modal) coordinate to parameterize all other coordinates during a resonant NNM response. Energy-based NNM methodologies are applied to a canonical set of equations and asymptotic solutions are sought. In order to account for the resonant modal interactions, it will be shown that high-order terms in the O(1) solutions must be considered (in the absence of internal resonances, a linear expansion at O(1) is sufficient). Two applications (‘3:1’ resonances in a two-degree-of-freedom system and ‘3:1’ resonance in a hinged-clamped beam) are then considered by which to demonstrate the resonant NNM methodology. It is shown that for some responses, nonlinear modal relations do not exist in the context of physical coordinates and thus a transformation to a canonical framework is necessary in order to appropriately define NNM relations.
On the Nonuniqueness of Solutions Obtained With Simplified Variational Principles
Authors: Mang H. , Helnwein P. , Gallagher R. H.
Published: February 2008

The attempt to satisfy subsidiary conditions in boundary value problems without additional independent unknowns in the form of Lagrange multipliers has led to the development of so-called “simplified variational principles.” They are based on using the Euler-Lagrange equations for the Lagrange multipliers to express the multipliers in terms of the original variables. It is shown that the conversion of a variational principle with subsidiary conditions to such a simplified variational principle may lead to the loss of uniqueness of the solution of a boundary value problem. A particularly simple form of the geometrically nonlinear theory of bending of beams is used as the vehicle for this proof. The development given in this paper is entirely analytical. Hence, the deficiencies of the investigated simplified variational principle are fundamental.
Feedback Control of Karman Vortex Shedding
Authors: Gunzburger M. D. , Lee H. C.
Published: February 2008

A computational study of the feedback control of the magnitude of the lift in flow around a cylinder is presented. The uncontrolled flow exhibits an unsymmetric Karman vortex street and a periodic lift coefficient. The size of the oscillations in the lift is reduced through an active feedback control system. The control used is the injection and suction of fluid through orifices on the cylinder; the amount of fluid injected or sucked is determined, through a simple feedback law, from pressure measurements at stations along the surface of the cylinder. The results of some computational experiments are given; these indicate that the simple feedback law used is effective in reducing the size of the oscillations in the lift.
Cavity-Opening Tensor Method and its Applications to Reflection and Transmission of Ultrasonic Wave by Defects
Authors: Xu Yonglin
Published: February 2008

The reflection and transmission of ultrasonic waves by an array of planar defects are investigated using the cavity-opening tensor (COT) method, whereby each defect or flaw can be simulated as a three-dimensional tensor in terms of the displacement distribution over the surface of the defect. The closed-form expressions of reflection and transmission coefficients represented by one set of COTs are derived, and the interaction factors of COTs are computed using the boundary integral equation with the approximate integral only on the single defect surface. Moreover, the analytical expressions of scattered fields at a distance from the defects are given by the representation of COT. As compared with the conventional numerical method, the COT method greatly simplifies the calculations, and also makes it possible to more efficiently characterize defects with more complicated distributions.
Micromechanics of Osteonal Cortical Bone Fracture
Authors: Guo X. E. , Liang L. C. , Goldstein S. A.
Published: February 2008

Microcracks have been associated with age-related bone tissue fragility and fractures. The objective of this study was to develop a simple osteonal cortical bone model and apply linear elastic fracture mechanics theory to understand the micromechanics of the fracture process in osteonal cortical bone and its dependence on material properties. The linear fracture mechanics of our composite model of conical bone, consisting of an osteon and interstitial bone tissue, was characterized in terms of a stress intensity factor (SIF) near the tip of a microcrack. The interaction between a microcrack and an osteon was studied for different types of osteons and various spacing between the crack and the osteon. The results of the analysis indicate that the fracture mechanics of osteonal cortical bone is dominated by the modulus ratio between the osteon and interstitial bone tissue: A soft osteon promotes microcrack propagation toward the osteon (and cement line) while a stiff one repels the microcrack from the osteon (and cement line). These findings suggest that newly formed, low-stiffness osteons may toughen cortical bone tissue by promoting crack propagation toward osteons. A relatively accurate empirical formula also was obtained to provide an easy estimation of the influence of osteons on the stress intensity factor.
Relation Between Diffuse Axonal Injury and Internal Head Structures on Blunt Impact
Authors: Nishimoto T. , Murakami S.
Published: February 2008

Diffuse axonal injury (DAI) is a severe head injury, which exhibits symptoms of consciousness disturbance and is thought to occur through rotational angular acceleration. This paper analyzes the occurrence of DAI when direct impacts with translational accelerations are applied to two-dimensional head models. We constructed a human model reproducing the human head structure, as well as modified human models with some internal head structures removed. Blunt direct impacts were applied from a lateral direction to the bottom of the third ventricle, considered to be the center of impact, using an impactor. The analysis was done by comparing the macroscopic manifestation of DAI with the shear stress as the engineering index. In the analytical data obtained from the human model, shear stresses were concentrated on the corpus callosum and the brain stem, in the deep area. This agrees with regions of the DAI indicated by small hemorrhages in the corpus callosum and the brain stem. The analytical data obtained by the modified human models show that the high shear stress on the corpus callosum is influenced by the falx cerebri, while the high shear stress on the brain stem is influenced by the tentorium cerebelli and the shape of the brain. These results indicate that DAI, generally considered to be influenced by angular acceleration, may also occur through direct impact with translational acceleration. We deduced that the injury mechanism of DAI is related to the concentration of shear stress on the core of the brain, since the internal head structures influence the impact stress concentration.
A Pedal Dynamometer for Off-Road Bicycling
Authors: Rowe T. , Hull M. L. , Wang E. L.
Published: February 2008

This paper describes the design and accuracy evaluation of a dynamometric pedal, which measures the two pedal force components in the plane of the bicycle. To realize a design that could be used during actual off-road cycling, a popular clipless pedal available commercially was modified so that both the form and the function of the original design were maintained. To measure the load components of interest, the pedal spindle was replaced with a spindle fixed to the pedal body and instrumented with eight strain gages connected into two Wheatstone bridge circuits. The new spindle is supported by bearings in the crank arm. Static calibration and a subsequent accuracy check revealed root mean square errors of less than 1 percent full scale (FS) when only the force components of interest were applied. Application of unmeasured load components created an error less than 2 percent FS. The natural frequency with half the weight of a 75 kgf person standing on the pedal was greater than 135 Hz. These performance capabilities make the dynamometer suitable for measuring either pedaling loads due to the rider’s muscular action or inertial loads due to surface-induced acceleration. To demonstrate this suitability, sample pedal load data are presented both for steady-state ergometer cycling and coasting over a rough surface while standing.
Cross-Sectional Deformation of the Aorta as Measured With Magnetic Resonance Imaging
Authors: Moreno M. R. , Moore J. E. , Meuli R.
Published: February 2008

Vessel geometry is commonly accepted as one of the primary factors influencing blood flow patterns. The vessels near the heart present a particular challenge because myocardial contraction creates dynamic changes in vessel geometry due to the movement created by the contraction of the myocardial muscle. The importance of vessel movement and deformation on blood flow patterns in the coronary arteries has been previously demonstrated. For larger vessels such as the aorta, the effects are less well understood, partially because no estimates of the dynamic variations in vessel cross section shape geometry have been reported. This study was undertaken to provide an estimate of the amount of dynamic variation in cross-sectional shape present in the aorta. Two young healthy male subjects were used, with measurements taken in the ascending aorta, aortic arch, and descending thoracic aorta using Magnetic Resonance Imaging (MRI). The magnitude of elliptical deformation was measured throughout the cardiac cycle by taking a discrete Fourier transform of the radius versus angle plot. Deformations of more than 7 percent of the mean vessel radius were noted. This level of deformation may be enough to influence flow patterns in the aorta significantly, and thus should be included in future flow studies.
A Method to Study Intracellular Ice Nucleation
Authors: Tatsutani K. , Rubinsky B.
Published: February 2008

The thermodynamics of intracellular ice nucleation are important in low-temperature biology for understanding and controlling the process of cell destruction by freezing. We have developed a new apparatus and technique for studying the physics of intracellular ice nucleation. Employing the principle of directional solidification in conjunction with light microscopy, we can generate information on the temperature at which ice nucleates intracellularly as a function of the thermal history the cells experience. The methodology is introduced, and results with primary prostatic cancer cells are described.
Numerical Solution of the Multidimensional Freezing Problem During Cryosurgery
Authors: Rabin Y. , Shitzer A.
Published: February 2008

A multidimensional, finite difference numerical scheme for the freezing process of biological tissues during cryosurgery is presented, which is a modification of an earlier numerical solution for inanimate materials. The tissues are treated as nonideal materials, freezing over a temperature range and possessing temperature-dependent thermophysical properties, blood perfusion, and metabolic heat generation. The numerical scheme is based on the application of an effective specific heat, substituting the intrinsic property, to include the latent heat effect within the phase transition temperature range. Results of the numerical solution were verified against an existing exact solution of a one-dimensional inverse Stefan problem in Cartesian coordinates. Results were further validated against experimental data available from the literature. The utility of the numerical solution for the design and application of cryodevices is demonstrated by parametric studies of the freezing processes around spherical and cylindrical cryoprobes. The parameters studied are the cryoprobe cooling power and the dimensions of the frozen region. Results are calculated for typical thermophysical properties of soft biological tissues, for angioma and for water.
A Microstructural Model for the Tensile Constitutive and Failure Behavior of Soft Skeletal Connective Tissues
Authors: Wren T. A. L. , Carter D. R.
Published: February 2008

We propose a microstructural model for the uniaxial tensile constitutive and failure behavior of soft skeletal connective tissues. The model characterizes the tissues as two-phase composites consisting of collagen fibers embedded in ground substance. Nonlinear toe region behavior in the stress versus strain curve results from the straightening of crimped fibers and from fiber reorientation. Subsequent linear behavior results from fiber stretching affected by fiber volume fraction, collagen type, crosslink density, and fiber orientation. Finally, the tissue fails when fibers successively rupture at their ultimate tensile strain. We apply the model to tendon, meniscus, and articular cartilage. The model provides a consistent approach to modeling the tensile behavior of a wide range of soft skeletal connective tissues.
A Model for Geometric and Mechanical Adaptation of Arteries to Sustained Hypertension
Authors: Rachev A. , Stergiopulos N. , Meister J.-J.
Published: February 2008

This study aimed to model phenomenologically the dynamics of arterial wall remodeling under hypertensive conditions. Sustained hypertension was simulated by a step increase in blood pressure. The arterial wall was considered to be a thick-walled tube made of nonlinear elastic incompressible material. Remodeling rate equations were postulated for the evolution of the geometric dimensions of the hypertensive artery at the zero-stress state, as well as for one of the material constants in the constitutive equations. The driving stimuli for the geometric adaptation are the normalized deviations of wall stresses from their values under normotensive conditions. The geometric dimensions are modulated by the evolution of the deformed inner radius, which serves to restore the level of the flow-induced shear stresses at the arterial endothelium. Mechanical adaptation is driven by the difference between the area compliance under hypertensive and normotensive conditions. The predicted time course of the geometry and mechanical properties of arterial wall are in good qualitative agreement with published experimental findings. The model predicts that the geometric adaptation maintains the stress distribution in arterial wall to its control level, while the mechanical adaptation restores the normal arterial function under induced hypertension.
The Effect of Dimethylsulfoxide on the Water Transport Response of Rat Hepatocytes During Freezing
Authors: Smith D. J. , Schulte M. , Bischof J. C.
Published: February 2008

Successful improvement of cryopreservation protocols for cells in suspension requires knowledge of how such cells respond to the biophysical stresses of freezing (intracellular ice formation, water transport) while in the presence of a cryoprotective agent (CPA). This work investigates the biophysical water transport response in a clinically important cell type—isolated hepatocytes—during freezing in the presence of dimethylsulfoxide (DMSO). Sprague-Dawley rat liver hepatocytes were frozen in Williams E media supplemented with 0, 1, and 2 M DMSO, at rates of 5, 10, and 50°C/min. The water transport was measured by cell volumetric changes as assessed by cryomicroscopy and image analysis. Assuming that water is the only species transported under these conditions, a water transport model of the form dV/dT = f(Lpg([CPA]), ELp([CPA]), T(t)) was curve-fit to the experimental data to obtain the biophysical parameters of water transport—the reference hydraulic permeability (Lpg) and activation energy of water transport (ELp)—for each DMSO concentration. These parameters were estimated two ways: (1) by curve-fitting the model to the average volume of the pooled cell data, and (2) by curve-fitting individual cell volume data and averaging the resulting parameters. The experimental data showed that less dehydration occurs during freezing at a given rate in the presence of DMSO at temperatures between 0 and −10°C. However, dehydration was able to continue at lower temperatures (<−10°C) in the presence of DMSO. The values of Lpg and ELp obtained using the individual cell volume data both decreased from their non-CPA values—4.33 × 10−13 m3/N-s (2.69 μm/min-atm) and 317 kJ/mol (75.9 kcal/mol), respectively—to 0.873 × 10−13 m3/N-s (0.542 μm/min-atm) and 137 kJ/mol (32.8 kcal/mol), respectively, in 1 M DMSO and 0.715 × 10−13 m3/N-s (0.444 μm/min-atm) and 107 kJ/mol (25.7 kcal/mol), respectively, in 2 M DMSO. The trends in the pooled volume values for Lpg and ELp were very similar, but the overall fit was considered worse than for the individual volume parameters. A unique way of presenting the curve-fitting results supports a clear trend of reduction of both biophysical parameters in the presence of DMSO, and no clear trend in cooling rate dependence of the biophysical parameters. In addition, these results suggest that close proximity of the experimental cell volume data to the equilibrium volume curve may significantly reduce the efficiency of the curve-fitting process.
Experiments on Steady and Oscillatory Flows at Moderate Reynolds Numbers in a Quasi-Two-Dimensional Channel With a Throat
Authors: Matsuzaki Y. , Ikeda T. , Matsumoto T. , Kitagawa T.
Published: February 2008

The study of steady and unsteady oscillatory static fluid pressures acting on the internal wall of a collapsible tube is essential for investigation of the complicated behavior observed when a flow is conveyed inside a tube. To examine the validity of two one-dimensional nonsteady theoretical flow models, this paper presents basic experimental observations of flow separation and reattachment and measured data on the static pressure distributions of the flow in a quasi-two-dimensional channel with a throat, together with information on the corresponding shape of the wall deflection and motion. For combinations of moderate Reynolds numbers and angles of the divergent segment of the channel, a smooth flow is separated from the wall downstream of the minimum cross section and reattached to the wall farther downstream. The measured data are compared with numerical results calculated by the two flow models.