bluuebunny/crossref_metadata_2025 huggingface dataset:
Value‐Enhancing Capital Budgeting and Firm‐specific Stock Return Variation
Authors: Durnev Art , Morck Randall , Yeung Bernard
Published: January 2004

We document a robust cross‐sectional positive association across industries between a measure of the economic efficiency of corporate investment and the magnitude of firm‐specific variation in stock returns. This finding is interesting for two reasons, neither of which is a priori obvious. First, it adds further support to the view that firm‐specific return variation gauges the extent to which information about the firm is quickly and accurately reflected in share prices. Second, it can be interpreted as evidence that more informative stock prices facilitate more efficient corporate investment.
The Statistical and Economic Role of Jumps in Continuous‐Time Interest Rate Models
Authors: Johannes Michael
Published: January 2004

This paper analyzes the role of jumps in continuous‐time short rate models. I first develop a test to detect jump‐induced misspecification and, using Treasury bill rates, find evidence for the presence of jumps. Second, I specify and estimate a nonparametric jump‐diffusion model. Results indicate that jumps play an important statistical role. Estimates of jump times and sizes indicate that unexpected news about the macroeconomy generates the jumps. Finally, I investigate the pricing implications of jumps. Jumps generally have a minor impact on yields, but they are important for pricing interest rate options.
Appraisal of psychomotor skills of dental students at University Complutense of Madrid
Authors: De Andrés Alfonso García , Sánchez Esperanza , Hidalgo Juan J. , Díaz María J.
Published: January 2004

The aim of this study was to evaluate the psychomotor skills of the dental students at University Complutense of Madrid (UCM), using a battery of psychometric tests. The sample comprised 306 students of the 1st, 2nd, 3rd and 4th years of Odontology, who, during the last month of the academic term, took the test on rotation of solid shapes, the Embedded Figures test in its group form (GEFT), the O'Connor Tweezer test, the MacQuarrie test for mechanical ability and the indirect vision test. Before these tests began, some personal data of all students were collected. The results showed some statistically significant correlation between several personal variables (sex, manual dominance and previous skills) and performance in the psychometric tests, although no significant relation was found between these variables and the students' academic qualifications. When comparisons were made between the psychomotor tests and the academic results of the students, significant differences appeared in the tracing and dotting subtests of the MacQuarrie test, whereby those students with higher practical qualifications reached the best punctuation. In the O'Connor Tweezer test, the students who obtained the worst results were those with lower qualifications. These data could serve to detect at an early stage those students who need greater educational support. No relation was found between the GEFT and the indirect vision test.
Outcome assessment of the French internship graduate programme
Authors: Villat C. , Farge P. , Bois D.
Published: January 2004

In 1995, the first dental postgraduate residency programme was established in France. Selection of the applicants was performed by means of a selective national competitive examination among dental students in the final year of the undergraduate studies. The main objective of this 3‐year general practice internship was to provide a clinical training for future academic dentists. This study describes the characteristics and the professional outcome of the student population enrolled in this programme. The French internship programme represents a new and welcome modification on the training of future dental academicians and establishes similarities with other programmes in Europe.
Instructional multimedia program versus standard lecture: a comparison of two methods for teaching the undergraduate orthodontic curriculum
Authors: Aly M. , Elen J. , Willems G.
Published: January 2004

Aim:  To compare the effectiveness of an interactive multimedia courseware package versus standard lectures regarding knowledge, understanding, and transfer of content, as well as problem‐solving skills in orthodontics.

Methods:  Pre‐ and post‐test assessments of final‐year dental students (n = 26), who either used an interactive multimedia courseware package (n = 15) or attended standard lectures (n = 11) on equivalent material of the undergraduate orthodontic curriculum were carried out. Both groups were tested by written and multiple‐choice questions covering knowledge, understanding, and application areas in the curriculum. A one‐way anova was carried out in order to check statistical difference between the two groups. The P‐value was set at 0.05.

Results:  There was no difference in prior knowledge between the groups at baseline. Generally, no significant difference was seen between the two groups in relation to answers to questions about knowledge, understanding, and application in the orthodontic curriculum. However, both groups improved their scores after the course. In one question investigating the extent of understanding the instructional content of the multidisciplinary orthodontic treatment, the multimedia courseware package group scored significantly better.

Conclusion:  In this study, the instructional interactive multimedia program was found to be at least as effective as the standard lecture of the orthodontic curriculum for undergraduate training in orthodontics.
The effects of an interactive software application on the self‐assessment ability of dental students
Authors: Mattheos Nikos , Nattestad Anders , Christersson Cecilia , Jansson Henrik , Attström Rolf
Published: January 2004

A randomised controlled trial was designed to test an Internet‐based interactive application as a stand‐alone support for the development of self‐assessment skills among second‐year dental students at the Faculty of Dentistry, University of Malmö, Sweden. Fifty‐two students were randomised into an experimental (n = 26) and a control group (n = 26). Both groups went through four identical learning cases in elementary clinical Periodontology during a period of 1 month. The experimental group received the cases with the interactive software, while the control group received them through a static web page. After the end of the learning phase, both groups were assessed blindly by two assessors during the interactive examination methodology. There was a moderate agreement between the two assessors, but both failed to find any significant differences between the two groups with regard to self‐assessment skills and overall performance of the students. Students positively received the Internet support, but several attitude differences were observed between the two groups during the study. Time on task and competence with computers were positively correlated to assessment results for both groups. On the basis of these findings, it remains unclear if computer‐based applications with automated feedback can constitute an effective remedial support for the improvement of self‐assessment skills of students. In addition, for such applications to be successfully accepted, two factors are critical: students must have a clear perception of the benefits deriving from the use of the software and the use of the software should be integrated in the curriculum, replacing some of the existing workload.
Teaching received in caries prevention and perceived need for Best Practice Guidelines among recent graduates in Finland
Authors: Vehkalahti Miira M. , Widström Eeva
Published: January 2004

The present study evaluated teaching that recent graduates in Finland had received in caries prevention and their perceived need for updated Best Practice Guidelines. A two‐page questionnaire was mailed to all dentists in Finland who had graduated from 1995 to 1998 (n = 390). After one reminder, the response rate was 46%. The closed questions covered teaching in 14 caries‐preventive methods and its suggestions as to when and on whom to apply such methods. Each dentist's own opinion on the three most important methods for caries prevention in patients with various types of dentition was sought in open questions. The respondents reported that at dental school, on average 12.5 (SD = 2.4) of 14 aspects of caries prevention were covered in theoretical teaching, 5.5 (SD = 3.8) in demonstrations and 8.5 (SD = 3.0) in clinical training. The top four methods teaching suggested for every patient were toothbrushing (100%), use of fluoridated toothpaste (99%), interdental cleaning (98%), and use of xylitol (97%); followed by fluoride varnish (77%) and fissure sealants (54%). The three caries‐preventive methods the respondents felt to be most useful were the same for all types of dentitions, in order of importance: (i) toothbrushing and use of fluoridated toothpaste, (ii) supplementary use of fluorides at home and (iii) healthy dietary habits, in particular, avoidance of sucrose. As regards clinical work, such teaching had served the respondents' real‐life needs in patients' caries prevention either extremely (44%) or moderately (54%) well; 91% said, however, that they would benefit from nationwide Best Practice Guidelines. In conclusion, the recent graduates' emphasis on patient‐active methods in caries prevention may indicate a change in the current policy favouring patient‐passive methods.
Looking for caries…? Teachers evaluate a program to improve caries diagnosis from radiographs
Authors: Mileman P. A. , Van Den Hout W. B. , Sanderink G. C. H.
Published: January 2004

We developed a computer‐assisted learning (CAL) program to improve student caries diagnosis from bitewing radiographs. In order to decide on its appropriateness, dental teachers independently evaluated the program before it was introduced. Our objective was to describe an evaluation by the teachers of the CAL diagnostic module and to compare the diagnostic performance with the evaluation score of the program. Key dental teachers involved in teaching clinical caries diagnosis and epidemiology (n = 22) viewed the CAL program. They evaluated aspects of the program by scoring 10 criteria on a visual analogue scale. Measures of the teachers' initial diagnostic accuracy assessed during use of the program were: sensitivity and specificity for the presence of dentine caries. Using Pearson's correlation coefficients, these measures of the teachers' diagnostic accuracy were compared separately with their evaluation score of one aspect of the program: the overall functionality of the program for a 2nd‐year dental student. The teachers' mean sensitivity for dentine caries diagnosis was 76.5% (median 77.8%; SD 15.4%) and the mean specificity was 93.7% (median 94.2%; SD 3.9%). The teachers gave the program a mean overall functionality score of 68% (median 75.1%; SD 21%). Dentists with a lower sensitivity (P = 0.065) for dentine caries tended to give the CAL program a higher overall functionality score than the other teachers. The CAL program was valued as generally useful. It tended to be more highly valued by the dentists with initially lower scores for diagnostic accuracy.
The influence of providing a clinical practice guideline on dental students' decision making
Authors: Van Der Sanden Wil J. M. , Mettes Dirk G. , Plasschaert Alphons J. M. , Mulder Jan , Verdonschot Emiel H.
Published: January 2004

The aim of this study was to assess the effect of the provision of a clinical practice guideline (CPG) on dental students' decisions to remove asymptomatic, impacted lower third molars. All dental students, who in 2001 were in the 3rd, 4th or 5th (final) year of their study at the Nijmegen College of Dental Sciences, were invited to participate. A pre‐test–post‐test control group design was used. Given 36 patient cases, all dental students were asked to assess the need for removal of asymptomatic, impacted lower third molars. All pre‐test respondents were randomly allocated to the control or intervention group. After the provision of a CPG to the intervention group, both groups were asked to assess the same cases again. Frequencies of decisions to remove the third molars were calculated. Chi‐square tests and anova were used to test the influence of study year and gender on the drop‐out rate and on the effect of the provision of a CPG on students' treatment decisions. The decrease in indications to remove third molars by the intervention group was statistically significant (P < 0.05). In the control group, no significant decrease was observed. It was concluded that the provision of a CPG significantly influences dental students' decision making about treatment in a third‐molar decision task. Students who used the CPG showed more guideline‐conformed decision making.
Internal medicine education in dentistry: knowledge required varies according to dental specialty
Authors: Miyatake Yoshiko , Kazama Miyuki , Isoda Mari , Nejima Jun
Published: January 2004

Background: With recent increases in the number of elderly and medically compromised patients, dentists are required to have a more comprehensive understanding of medical problems. There is, however, little awareness of the importance of internal medicine at dental schools in Japan.

Objective: The aims of the present study were to examine whether dentists need medical knowledge, to examine how often they experience difficulties regarding internal medicine in dental practice, and to determine whether their specialty or career influences their needs or difficulties.

Design: A questionnaire based on the above objectives was designed and was sent to all the faculty members in the department of dentistry at Tsurumi University and all dentists who work at the university hospital.

Results: Of 408 surveys, 307 (75%) responded. Ninety‐nine per cent of respondents recognized the necessity of internal medicine. The dentists specializing in gerodontology, dental anesthesiology or oral and maxillofacial surgery (group S) felt the need of knowledge of internal medicine more frequently than those of other specialties (group O) (Odds ratio 17.5, 95% CI: 2.4 ∼ 129.9;P = 0.000). Seventy‐four per cent of the dentists experienced difficulties in day‐to‐day practice caused by a lack of medical knowledge. Those in group S, experienced difficulties more frequently than those in group O (Odds ratio 5.3, 95% CI: 2.1 ∼ 12.9;P = 0.000). While the more experienced dentists in group S encountered difficulties more often, the more experienced dentists in group O encountered difficulties less often.

Conclusions: The need for instruction in internal medicine varies according to dental specialty. Even very experienced dentists might fail to find solutions because of a lack of knowledge of internal medicine.
Solid variant of odontogenic keratocyst
Authors: Vered Marilena , Buchner Amos , Dayan Dan , Shteif Moshe , Laurian Adi
Published: January 2004

A case of an unusual lesion from the maxilla is presented. Macroscopically, the lesion was solid and histologically consisted of ‘multiple separate keratocysts’ of varying size that infiltrated into the surrounding bone and soft tissues. Panoramic image and CT scans showed a multilocular honeycomb ill‐defined radiolucency with infiltration into the maxillary sinus and floor of orbit. This lesion should be differentiated from similar odontogenic lesions, such as keratoameloblastoma and papilliferous keratoameloblastoma. As there was no evidence of follicles, islands of ameloblastoma, or papilliferous structures in the entire specimen, the lesion could not be diagnosed as either a keratoameloblastoma or a papilliferous keratoameloblastoma. The invasive and destructive growth behavior, the histopathological features, and the histochemical pattern of the collagen stroma imply that this solid lesion is a neoplasia. It is suggested that the proper term for this lesion is solid variant of odontogenic keratocyst.
Growth of malignant oral epithelial stem cells after seeding into organotypical cultures of normal mucosa
Authors: Mackenzie Ian C.
Published: January 2004

Background:  Oral squamous cell carcinoma (OSCC) is associated both with the local expansion of clones of malignant cells and with their further migration to regional and distant sites. The interactions that occur between normal and malignant cells during these events are not well modelled by standard culture conditions, but organotypical cultures, in which epithelial cells are grown on a matrix containing fibroblasts, provide a suitable environment for such investigations.

Methods:  Cells from five cell lines, each derived from OSCC and marked by retroviral transduction with alkaline phosphatase, were incorporated as small subpopulations (0.1–5%) in uniformly differentiating organotypical cultures constructed from normal oral mucosal cells. The patterns of growth of the malignant cells within the normal epithelium were examined for 3 weeks.

Results:  There was variation between the different cell lines in their rates and patterns of growth, but all cell lines produced clusters of malignant cells that had expanded within 3 weeks to replace the normal epithelium. The appearance and spacing of these clusters suggested that each was derived from a single progenitor cell. The number of malignant cells initially present within a given area of organotypical epithelium was much greater than the number of expanding cell clusters subsequently formed. Cluster‐forming cells thus represented only a subpopulation of the tumour cells.

Conclusions:  The organotypical model allows examination of interactions occurring between cells derived from OSCC and normal epithelia. The three‐dimensional nature of organotypical cultures, together with their more normal patterns of differentiation, provides an environment that more closely mimics the in vivo environment in which tumours develop. The finding that only a subpopulation of tumour cells forms expanding tumour colonies suggests a range of growth potentials within a tumour population and may provide preliminary evidence for some form of stem and amplifying cell pattern.
Orofacial granulomas after injection of cosmetic fillers. Histopathologic and clinical study of 11 cases
Authors: Lombardi T. , Samson J. , Plantier F. , Husson C. , Küffer R.
Published: January 2004

Background: Purposely, cosmetic injections in orofacial tissues of various resorbable, biodegradable, or permanent fillers may be followed by development of foreign‐body granulomas. The aim of this article is to contribute to the histologic identification of the filler material.

Methods: Histologic and clinical features of 11 cases of granulomas on orofacial fillers are described.

Results: Only 3/11 patients knew the nature of the injected product. Four histologic patterns were found: (i) Artecoll, (ii) Dermalive, and (iii) New‐Fill granuloma, all three of the classic giant cell granuloma type, differing in respect of foreign particles; and (iv) Liquid Silicone granuloma, which featured a cystic and macrophagic type. Information was often missing or misleading, patients or practitioners being reluctant to give the details.

Conclusion: Increasing demand for orofacial tissue augmentation makes pathologists face new, and sometimes, puzzling granuloma types. Identification of the foreign product might be required for therapeutic or medico‐legal reasons.
Efficient Modeling of Fretting of Blade/Disk Contacts Including Load History Effects
Authors: Murthy H. , Harish G. , Farris T. N.
Published: January 2004

Fretting is a frictional contact phenomenon that leads to damage at the contact region between two nominally-clamped surfaces subjected to oscillatory motion of small amplitudes. The region of contact between the blade and the disk at the dovetail joint is one of the critical locations for fretting damage. The nominally flat geometry of contacting surfaces in the dovetail causes high contact stress levels near the edges of contact. A numerical approach based on the solution to singular integral equations that characterize the two-dimensional plane strain elastic contact of two similar isotropic surfaces presents itself as an efficient technique to obtain the sharp near-surface stress gradients associated with the geometric transitions. Due to its ability to analyze contacts of any two arbitrary smooth surfaces and its computational efficiency, it can be used as a powerful design tool to analyze the effects of various factors like shape of the contact surface and load histories on fretting. The calculations made using the stresses obtained from the above technique are consistent with the results of the experiments conducted in the laboratory.
Experimental and Numerical Studies in a Centrifugal Pump With Two-Dimensional Curved Blades in Cavitating Condition
Authors: Coutier-Delgosha O. , Fortes-Patella R. , Reboud J. L. , Hofmann M. , Stoffel B.
Published: January 2004

In the presented study a special test pump with two-dimensional curvature blade geometry was investigated in cavitating and noncavitating conditions using different experimental techniques and a three-dimensional numerical model implemented to study cavitating flows. Experimental and numerical results concerning pump characteristics and performance breakdown were compared at different flow conditions. Appearing types of cavitation and the spatial distribution of vapor structures within the impeller were also analyzed. These results show the ability of the model to simulate the complex three-dimensional development of cavitation in a rotating machinery, and the associated effects on the performance.
High-Frequency Ultrasonic Atomization With Pulsed Excitation
Authors: Lozano A. , Amaveda H. , Barreras F. , Jorda` X. , Lozano M.
Published: January 2004

Ultrasonic atomization is a very convenient method to produce sprays of very small droplets. Resulting droplet size distributions are very narrow and the mean diameter is essentially only controlled by the excitation frequency. Generation of droplets in the micron range requires MHz waves, with voltages around 30 V, which translates into power requirements on the order of 10 W. Tunable wave generators with these characteristics are somewhat uncommon. To explore the capabilities of ultrasonic atomization for inhalation therapies, an excitation source, described in this paper, has been designed. The characteristics of the sprays obtained when driving with it a piezoceramic disk are analyzed.
A Finite Element Model for Spherical Debris Denting in Heavily Loaded Contacts
Authors: Kang Young Sup , Sadeghi Farshid , Hoeprich Mike R.
Published: January 2004

The objective of this study is to develop models to investigate the effects of contaminants (debris denting process) in heavily loaded rolling and sliding contacts. A dynamic time dependent finite element model (FEM) was developed to determine the elastic-plastic deformation and contact force generated between the mating surfaces and a spherical debris as debris passes through the contact region. The FEA model was used to obtain the effects of various parameters such as debris sizes, material properties, friction coefficients, applied loads, and surface speeds on the elastic-plastic deformation and contact force of the system. The FEM was used to predict debris and mating surfaces deformations as a function of debris size, material properties, friction coefficient, applied load, and surface speed. Using the FEM, a parametric study demonstrated that material properties (i.e., modulus of elasticity, yield strength, ultimate strength and Poisson’s ratio) and friction coefficients play significant roles on the height and width of dents on the mating surfaces. For lower friction coefficients μd<0.3 the debris and mating surfaces slip more easily relative to one another and therefore the debris has lower aspect ratio. As friction coefficient is increased the debris and mating surfaces stick to one another and therefore the debris deforms less and has higher aspect ratio. The results indicate that the pressure generated between the debris and mating surfaces is high enough to plastically deform the debris and mating surfaces and cause a permanent dent on the surfaces and cause residual stresses around the dent. Based on the FEM results, a dry contact model (DCM) was developed to allow similar analyses as the FEM, however, in significantly shorter computational time.
Lubricants With Non-Newtonian Rheology and Their Degradation in Line Contacts
Authors: Kudish Ilya I. , Airapetyan Ruben G.
Published: January 2004

A plane isothermal elastohydrodynamic problem for a line contact lubricated by a degrading fluid with non-Newtonian rheology is studied. The lubricant is represented by a base stock with a polymer additive which undergoes stress-induced degradation caused by scission of polymer molecules. The polymer molecules are considered to be of linear structure. The effective lubricant viscosity experiences reversible and irreversible losses. The reversible loss of the effective lubricant viscosity (shear thinning) is due to the non-Newtonian rheology of the fluid and variations in the fluid shear rate. The irreversible loss of the effective lubricant viscosity is caused by the degradation process of the polymer additive dissolved in the lubricant. The degradation process of the polymer additive while it passes through the contact is described by a kinetic equation. The kinetic equation is solved along the lubricant flow streamlines. The solution of the kinetic equation predicts the density of the probabilistic distribution of the polymer molecular weight versus polymer molecule chain length. The changes in the distribution of polymer molecular weight affect local lubricant properties. In particular, the lubricant viscosity experiences reversible and irreversible losses and, in general, is a discontinuous function. The changes in the lubricant viscosity alter virtually all parameters of the lubricated contact such as film thickness, friction stresses, pressure, and gap. The considered non-Newtonian rheology of the lubricant causes a small reversible loss of its viscosity. As a result of the polymer additive degradation the lubricant may experience a significant irreversible loss of its viscosity which, in turn, leads to a noticeable reduction in the lubrication film thickness in comparison with the case of a non-degrading lubricant with similar rheology. Some comparisons between the cases of lubricants with Newtonian and non-Newtonian rheologies with and without lubricant degradation are considered.
A Systems Approach to the Solid Lubrication of Foil Air Bearings for Oil-Free Turbomachinery
Authors: DellaCorte Christopher , Zaldana Antonio R. , Radil Kevin C.
Published: January 2004

Foil air bearings are self-acting hydrodynamic bearings which rely upon solid lubricants to reduce friction and minimize wear during sliding which occurs at start-up and shut-down when surface speeds are too low to allow the formation of a hydrodynamic air film. This solid lubrication is typically accomplished by coating the nonmoving foil surface with a thin, soft polymeric film. The following paper introduces a systems approach in which the solid lubrication is provided by a combination of self lubricating shaft coatings coupled with various wear resistant and lubricating foil coatings. The use of multiple materials, each providing different functions is modeled after oil-lubricated hydrodynamic sleeve bearing technology which utilizes various coatings and surface treatments in conjunction with oil lubricants to achieve optimum performance. In this study, room temperature load capacity tests are performed on journal foil air bearings operating at 14,000 rpm. Different shaft and foil coating technologies such as plasma sprayed composites, ceramic, polymer and inorganic lubricant coatings are evaluated as foil bearing lubricants. The results indicate that bearing performance is improved through the individual use of the lubricants and treatments tested. Further, combining several solid lubricants together yielded synergistically better results than any material alone.
Hydrodynamic Analysis of the Flow in a Rotary Lip Seal Using Flow Factors
Authors: Salant Richard F. , Rocke Ann H.
Published: January 2004

The flow field in the lubricating film of a rotary lip seal is analyzed numerically by solving the Reynolds equation with flow factors. The behavior of such a flow field is dominated by the asperities on the lip surface. Since previous analyses treated those asperities deterministically, they required very large computation times. The present approach is much less computationally intensive because the asperities are treated statistically. Since cavitation and asperity orientation play important roles, these are taken into account in the computation of the flow factors. Results of the analysis show how the operating parameters of the seal and the characteristics of the asperities affect such seal characteristics as the pressure distribution in the film, the pumping rate and the load support.
Mechanical and Thermomechanical Elastic-Plastic Contact Analysis of Layered Media With Patterned Surfaces
Authors: Gong Z.-Q. , Komvopoulos K.
Published: January 2004

An elastic-plastic finite element analysis of a sphere in normal and sliding contact with a layered medium possessing a patterned surface with regularly spaced rectangular pads was conducted in order to investigate the effect of pattern geometry on the contact pressure distribution and subsurface stress-strain field. Three-dimensional sliding simulations were performed for lateral displacement of the indenting sphere approximately equal to two times the pad spatial periodicity. Three complete loading cycles, involving indentation, sliding, and unloading of a rigid sphere, were simulated to assess the effect of repeated sliding on the stresses in the first (hard) layer and plastic deformation in the underlying (soft) layer. Thermomechanical sliding contact simulations of an elastic-plastic layered medium with a patterned surface and an elastic-plastic sphere with properties identical to those of the first layer were carried out to examine the effect of frictional heating on the deformation behavior of the medium. Results are presented for the temperature distribution and maximum temperature variation at the surface and the evolution of subsurface plasticity in terms of Peclet number. The likelihood of thermal cracking in the wake of microcontacts during sliding is interpreted in the context of the thermal tensile stress due to the temperature gradients in the layered medium.
A Static Friction Model for Elastic-Plastic Contacting Rough Surfaces
Authors: Kogut Lior , Etsion Izhak
Published: January 2004

A model that predicts the static friction for elastic-plastic contact of rough surfaces is presented. The model incorporates the results of accurate finite element analyses for the elastic-plastic contact, adhesion and sliding inception of a single asperity in a statistical representation of surface roughness. The model shows strong effect of the external force and nominal contact area on the static friction coefficient in contrast to the classical laws of friction. It also shows that the main dimensionless parameters affecting the static friction coefficient are the plasticity index and adhesion parameter. The effect of adhesion on the static friction is discussed and found to be negligible at plasticity index values larger than 2. It is shown that the classical laws of friction are a limiting case of the present more general solution and are adequate only for high plasticity index and negligible adhesion. Some potential limitations of the present model are also discussed pointing to possible improvements. A comparison of the present results with those obtained from an approximate CEB friction model shows substantial differences, with the latter severely underestimating the static friction coefficient.
Dynamic Indentation of an Elastic-Plastic Multi-Layered Medium by a Rigid Cylinder
Authors: Yang J. , Komvopoulos K.
Published: January 2004

A plane-strain analysis of dynamic indentation of an elastic-plastic multi-layered medium by a rigid cylinder was performed using the finite element method. Conversely to plane-strain static contact analysis, the solutions of a dynamic contact analysis within a subsurface domain adjacent to the contact region are independent of mesh size, provided the mesh dimensions are sufficiently large such that the propagating waves reflected from the artificial boundaries do not reach the domain of interest during the analysis. Simulation results for the normal force, contact pressure distribution, subsurface stresses, and evolution of plasticity in the multi-layered medium are presented in terms of the speed and radius of the rigid indenter. The likelihood of mechanical failure due to excessive plastic deformation and cracking is interpreted in terms of finite element results for the von Mises equivalent stress, first principal stress, and equivalent plastic strain obtained for different values of the indenter speed and radius of curvature.
Modeling of Film Thickness and Traction in a Variable Ratio Traction Drive Rig
Authors: Sharif K. J. , Evans H. P. , Snidle R. W. , Newall J. P.
Published: January 2004

The paper describes the application of a full non-Newtonian, thermal elastohydrodynamic lubrication (EHL) model for the prediction of film thickness and viscous traction force in a special high speed rolling traction rig. The primary objective of the work was to identify a suitable lubricant rheological model that would describe the behavior of practical EHL traction drive contacts over their operating range. Experiments were carried out on a special rolling contact rig at temperatures of 60, 90, and 120°C and contact loads giving maximum Hertzian pressures of 1, 2, and 3 GPa. Entrainment speeds of up to 18 m/s were investigated. Corresponding modeling work was carried out using lubricant physical properties obtained for Santotrac 50, the traction fluid used in the experiments. Viscosity data for this lubricant were available from the work of Bair and Winer, but a degree of extrapolation was required to this data to cover the range of the experiments. In view of the crucial importance of viscosity/pressure behavior in the prediction of traction attention was therefore focused upon the lower contact loads for which reliable viscosity/pressure data are available. A best-fit exercise was then carried out to establish an appropriate rheological model to account for shear thinning of the lubricant. Different non-Newtonian relationships were investigated including those of Johnson and Tevaarwerk, Bair and Winer, and a model which combined the features of both of these. The most encouraging agreement between experiment and theory over the range of temperatures and speeds considered was obtained with the Johnson and Tevaarwerk (Eyring) model.
A Rough Surface Contact Model for General Anisotropic Materials
Authors: Lin Yuan , Ovaert Timothy C.
Published: January 2004

A method for solving the two-dimensional (2-D) isothermal rough surface contact problem of general anisotropic materials with friction is presented. By using Stroh’s formalism, the surface displacements of an elastic half-space due to uniform distributions of traction over a strip are derived from the surface Green’s function. The surface displacement and subsurface stresses of the anisotropic half-space due to the distributed contact pressure may then be calculated by superposition. The real contact area and the contact pressure are determined via an iteration scheme using the conjugate gradient method.
Effect of Flow-Induced Radial Load on Brush Seal/Rotor Contact Mechanics
Authors: Zhao Haifang , Stango Robert J.
Published: January 2004

Brush seals comprised of special-alloy wire bristles are currently being used in lieu of traditional labyrinth seals for turbomachinery applications. This advancement in seal technology utilizes close-packed bristles that readily undergo lateral deformation arising from aerodynamic loads as well as loads imparted by the rotor surface. Thus, during operation, filament tips remain in contact with the rotor surface, which, in turn, inhibits leakage between successive stages of the turbine, and increases engine efficiency. However, contact forces generated at the interface of the rotor and fiber tips can lead to eventual bristle fatigue and wear of the seal/rotor system. Therefore, it is important that reliable modeling techniques be developed that can help identify complex relationships among brush seal design parameters, in-service loads, and contact forces that arise during the operation of turbomachinery. This paper is concerned with modeling and evaluating bristle deformation, bending stress, and bristle/rotor contact forces that are generated at the interface of the fiber and rotor surface due to radial fluid flow, and augments previous work reported by the author’s, which assessed filament tip forces that arise solely due to interference between the bristle/rotor. The current problem derives its importance from aerodynamic forces that are termed “blow-down,” that is, the inward radial flow of gas in close proximity to the face of the seal. Thus, bristle deformation, bristle tip reaction force, and bristle bending stress is computed on the basis of an in-plane, large-displacement mechanics analysis of a cantilever beam that is subjected to a uniform radial load. Solutions to the problem are obtained for which the filament tip is constrained to lie on the rotor surface, and includes the effect of Coulombic friction at the interface of the fiber tip and rotor. Contact forces are obtained for a range of brush seal design parameters including fiber lay angle, flexural rigidity, and length. In addition, the governing equation is cast in non-dimensional form, which extends the range of applicability of solutions to brush seals having a more general geometry and material composition.
Real Contact Area of Fractal-Regular Surfaces and Its Implications in the Law of Friction
Authors: Wang Shao
Published: January 2004

The concept of a fractal-regular surface, with a dual-section power spectrum, has been implemented in an elastic-plastic contact analysis. Under certain assumptions, the analysis of individual fractal domains can be decoupled from that of the macroscopic shape. Due to the increase in the number of contacting fractal domains associated with a macroscopic contact expansion, the contact area-load relationship for fractal-regular surfaces is nearly linear, with a load exponent of 1-1.11, in contrast to 1-1.33 for fractal surfaces. Thus, the Amontons law of friction can be reasonably explained with fractal-regular surfaces under the assumption of a linear friction-area relationship. The distribution of the local real-to-apparent contact ratio in a nominally Hertzian contact was found to vary with the fractal dimension. The plastic contact ratio tends to be more uniformly distributed as the fractal dimension approaches unity.
Phase-Averaged Velocity Field Measurements of Flow Around an Isolated Axial-Fan Model
Authors: Lee Sang-Joon , Choi Jayho , Yoon Jong-Hwan
Published: January 2004

The phase-averaged flow characteristics around a forward-swept axial fan model were investigated using a two-frame PTV method. The velocity fields in the sagittal and axial planes show clearly the periodic variations of flow structure and periodic shedding of the tip and trailing vortices according to the fan blade phase.
Rapid Transition to Turbulence in Pipe Flows Accelerated From Rest
Authors: Greenblatt David , Moss Edward A.
Published: January 2004

Rapid transition to turbulence in a pipe flow, initially at rest, was achieved by temporally accelerating the flow and then sharply decelerating it to its final Reynolds number. The acceleration phase was characterized by the growth of a laminar boundary layer close to the wall. The subsequent rapid deceleration resulted in inflectional velocity profiles near the wall, followed immediately by transition to turbulence. The time taken to transition was significantly less than the time to transition in a pipe flow monotonically accelerated to the same Reynolds number. Transition is intrinsically different to that observed in oscillatory pipe flows, but is qualitatively similar to pipe flows decelerated to rest.
Use of Large-Eddy Simulation to Characterize Roughness Effect of Turbulent Flow Over a Wavy Wall
Authors: Cui Jie , Lin Ching-Long , Patel Virenda C.
Published: January 2004

Large-eddy simulation is used to study turbulent flow over a sinusoidal wavy wall from the perspective of surface roughness effects. Simulation results are averaged in time and space to obtain the so-called roughness function (shift in the logarithmic law), the equivalent sand-grain roughness and the virtual origin, and the dependence of these quantities on the amplitude-to-wavelength ratio. The results demonstrate the usefulness of LES to quantify roughness effects that have hitherto fore been the purview of laboratory experiments.
Filling Process in an Open Tank
Authors: Lee S. L. , Sheu S. R.
Published: January 2004

A numerical simulation for a filling process in an open tank is performed in this paper. A single set of governing equations is employed for the entire physical domain covering both water and air regions. The great density jump and the surface tension existing at the free surface are properly handled with the extended weighting function scheme and the NAPPLE algorithm. There is no need to smear the free surface. Through the use of a properly defined boundary condition, the method of “extrapolated velocity” is seen to provide accurate migrating velocity for the free surface, especially when the water front hits a corner or a vertical wall. In the present numerical procedure, the unsteady term of the momentum equation is discretized with an implicit scheme. Large time-steps thus are allowed. The numerical results show that when the water impinges upon a corner, a strong pressure gradient forms in the vicinity of the stagnation point. This forces the water to move upward along the vertical wall. The water eventually falls down and generates a gravity wave. The resulting free surface evolution is seen to agree well with existing experimental data. Due to its accuracy and simplicity, the present numerical method is believed to have applicability for viscous free-surface flows in industrial and environmental problems such as die-casting, cutting with water jet, gravity wave on sea surface, and many others.
Analysis of the Flow Through a Vented Automotive Brake Rotor
Authors: Johnson David A. , Sperandei Bryan A. , Gilbert Ross
Published: January 2004

Particle image velocimetry (PIV) was used to measure air velocities through a high solidity radial flow fan utilized as an automotive vented brake rotor. A brake rotor is a somewhat unusual fan in that its sole purpose is not to pump air but to dissipate thermal energy, it has no conventional inlet or outlet housing and it has a continuously varying rotational speed. For three typical rotational speeds, the flow characteristics were captured at the inlet and exit of the rotor, as well as internally through the cooling passages. Inlet measurements showed a swirling entry flow condition with significant misalignment of flow onto the vanes. As a result large regions of flow separation were found in the internal vane-to-vane passages on the suction side surfaces, which would lead to poor heat transfer conditions. The main flow exiting the rotor consisted of a series of jets corresponding to the individual rotor passages and were found to be very unstable leading to a rapid decay in velocity.
The Stretching of a Viscoplastic Thread of Liquid
Authors: Al Khatib M. A. M.
Published: January 2004

The problem of stretching a viscoplastic (yield-stress) thread of a liquid hanging vertically is considered. The length of the thread at later times and the time at which it ruptures is determined. A Lagrangian coordinate system is used to analyze the extension of the thread as it sags under its own weight, with negligible inertial effects. The biviscosity model has been used to characterize viscoplastic fluids; the Newtonian and Bingham models can be recovered as limiting cases. The Bingham limit is of special interest.
Volumetric Gas Flow Standard With Uncertainty of 0.02% to 0.05%
Authors: Wright John D. , Moldover Michael R. , Johnson Aaron N. , Mizuno Akisato
Published: January 2004

A new pressure, volume, temperature, and time (PVTt) primary gas flow standard for calibrating flowmeters has an expanded uncertainty k=2 of between 0.02% and 0.05%. The standard diverts a steady flow into a collection tank of known volume during a measured time interval. The standard spans the flow range of 1 slm1 to 2000 slm using two collection tanks (34 L and 677 L) and two flow diversion systems. We describe the novel features of the standard and analyze its uncertainty. The thermostatted collection tank allows determination of the average gas temperature to 7 mK (0.0023%) within an equilibration time of 20 min. We developed a mass cancellation procedure that reduced the uncertainty contributions from the inventory volume to 0.017% at the highest flow rate. Flows were independently measured throughout the overlapping flow range of the two systems and they agreed within 0.015 %. The larger collection system was evaluated at high flows by comparing single and double diversions; the maximum difference was 0.0075%.
Fluid Flow Through Microscale Fractal-Like Branching Channel Networks
Authors: Alharbi Ali Y. , Pence Deborah V. , Cullion Rebecca N.
Published: January 2004

Flow through fractal-like branching networks is investigated using a three-dimensional computational fluid dynamics approach. Results are used to assess the validity of, and provide insight for improving, assumptions imposed in a previously developed one-dimensional model. Assumptions in the one-dimensional model include (1) reinitiating boundary layers following each bifurcation, (2) constant thermophysical fluid properties, and (3) negligible minor losses at the bifurcations. No changes to the redevelopment of hydrodynamic boundary layers following a bifurcation are recommended. It is concluded that temperature varying fluid properties should be incorporated in the one-dimensional model to improve its predictive capabilities, especially at higher imposed heat fluxes. Finally, a local pressure recovery at each bifurcation results from an increase in flow area. Ultimately, this results in a lower total pressure drop and should be incorporated in the one-dimensional model.
Modeling Free-Surface Flow in Part-Filled Rotating Vessels: Vertical and Horizontal Orientations
Authors: Sujatha K. S. , Webster M. F.
Published: January 2004

This paper reports on the numerical simulation of rotating flows with free surfaces, typically that arise in dough-kneading situations found within the food processing industry. Free-surface flow in a rotating cylinder is investigated when a fluid is stirred in a cylindrical-shaped vessel with a stirrer attached to its lid. The problem is posed in a three-dimensional cylindrical polar frame of reference. Numerical predictions are based on a Taylor-Galerkin/pressure-correction finite element formulation, with particle tracking to accommodate free-surface movement. Peeling and wetting conditions are incorporated to predict fluid-surface movement in contact with solid boundaries. Free-surface profiles are presented for different speeds of rotation and predictions compare closely to equivalent experimental results. The algorithmic implementation is validated against Newtonian analytical solutions. Typical results are presented to demonstrate the difference between Newtonian and inelastic model fluids.