David F. James

Drilling in Bone: Modeling Heat Generation and Temperature Distribution

Thermo-mechanical equations were developed from machining theory to predict heat generation due to drilling and were coupled with a heat transfer FEM simulation to predict the temperature rise and thermal injury in bone during a drilling operation. The rotational speed, feed rate, drill geometry and bone material properties were varied in a parametric analysis to determine the importance of each on temperature rise and therefore on thermal damage. It was found that drill speed, feed rate and drill diameter had the most significant thermal impact while changes in drill helix angle, point angle and bone thermal properties had relatively little effect.

The laminar flow of dilute polymer solutions through porous media

Measurements of the pressure drop and flow rate were obtained for dilute solutions of polyethylene oxide flowing through beds of packed beads. When the velocity was sufficiently high, the pressure drop was above that for a Newtonian fluid of equal viscosity, often considerably above, and this viscoelastic effect was explored by varying the concentration and molecular weight of the polymer, by testing solutions over a wide range of flow rates, and by using several bead sizes. The non-Newtonian behaviour was most pronounced at moderate flow rates; at the highest velocities, the data were pseudo-Newtonian in character, i.e. the pressure drop still exceeded that for a Newtonian fluid, but was linearly related to the velocity. For some solutions, the large deviation from Newtonian values occurred over such a short range of flow rates that there was an interval in which the pressure drop decreased with velocity. It was not possible, therefore, to obtain steady-state measurements in this regime and a gap appears in the data curve of pressure vs. velocity. The pressure drop was monitored in steps along the test section, so that it was possible to detect molecular degradation of the solutions as they flowed through the porous media. In general, degradation was not extensive and the solutions became stably degraded by the midpoint of the test section. Degradation increased with velocity and, quite surprisingly, became more severe as the bead size increased. A visual examination of the flow field revealed that the streamline pattern for the polymer solutions was the same as that for water. The large non-Newtonian effects were therefore due to changes in the stress field, and in an effort to understand these effects, an analysis was carried out which examined how the stresses generated by each component of the deformation, i.e. by shear and pure strain, influence the pressure drop. This analysis, combined with a study of onset data, indicates that onset and the sudden large departures from Newtonian values are probably due to an interaction between extensional and shearing deformation, and that the reduced viscoelastic effect of higher flow rates may be due to the dominance of extensional stresses.

The laminar flow of dilute polymer solutions around circular cylinders

Experimental results are presented for heat transfer by free and forced convection at low velocities from small heated cylinders in dilute solutions of polyethylene oxide in water. The experiments were conducted for a range of velocities (less than 1. 0 ft/sec) and polymer concentrations, with several cylinder diameters, and for several polymer molecular weights. Experimental results are also presented for the drag of a small cylinder in similar liquids and for a comparable range of velocities. The heat transfer and drag results at low velocities were identical to those for a Newtonian liquid; at high velocities, the measured values departed considerably from Newtonian results. These departures result from the viscoelastic nature of the polymer solutions. Visualization studies of the flow around a cylinder and of a minute laminar jet were conducted to determine the gross magnitude of the viscoelastic effects. Due to the liquids elasticity, a significant enlargement of the flow field was observed for both configurations above a critical Reynolds number. An attempt is made to explain the heat transfer and drag results in light of these observations. Photographic materials on pp. 111 - 149 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered

Flow at the interface of a model fibrous porous medium

Planar flow in the interfacial region of an open porous medium is investigated by finding solutions for Stokes flow in a channel partially filled with an array of circular cylinders beside one wall. The cylinders are in a square array oriented across the flow and are widely spaced, so that the solid volume fraction ϕ is 0.1 or less. For this spacing, singularity methods are appropriate and so they are used to find solutions for both planar Couette flow and Poiseuille flow in the open portion of the channel. The solutions, accurate to O (ϕ), are used to calculate the apparent slip velocity at the interface, U s , and results obtained for U s are presented in terms of a dimensionless slip velocity. For shear-driven flow, this dimensionless quantity is found to depend only weakly on ϕ and to be independent of the height of the array relative to the height of the channel and independent of the cylinder size relative to the height of the channel. For pressure-driven flow, U s is found to be less than that under comparable shear-flow conditions, and dependent on cylinder size and filling fraction in this case. Calculations also show that the external flow penetrates the porous medium very little, even for sparse arrays, and that U s is about one quarter of the velocity predicted by the Brinkman model.

Measurement of thermal conductivity of bovine cortical bone

The thermal conductivity of cortical bone was characterized experimentally. Specimens were taken from the mid-diaphysis of bovine femora, and the rate of heat transfer was measured in three orthogonal directions. The conductivity was found to be 0.58+/-0.018 W/mK in the longitudinal direction, 0.53+/-0.030 W/mK in the circumferential direction, and 0.54+/-0.020 W/mK in the radial direction. Because the directional differences are small, it is concluded that bovine cortical bone can be treated as thermally isotropic.

An interlaboratory comparison of measurements from filament-stretching rheometers using common test fluids

Following development of a filament-stretching extensional rheometer at Monash University, similar rheometers have been designed and built in other laboratories. To help validate the basic technique, a collaborative program was undertaken to compare results from several instruments. First, three test fluids prepared at the University of California at Berkeley were characterized in steady and transient shear flows there and at the Massachusetts Institute of Technology (M.I.T.), and then tested in extensional rheometers at M.I.T., Monash and the University of Toronto. Each fluid is a constant-viscosity solution of narrow-molecular-weight-distribution polystyrene dissolved in oligomeric polystyrene. The solute molecular weights are 2.0, 6.5, and 20 million g/mol, and the polymer concentration in each fluid is 0.05 wt. %. From linear viscoelastic measurements, the Zimm relaxation times of the fluids are found to be 3.7, 31, and 150 s, respectively. The scaling of relaxation times with molecular weight indicat...

The meniscus on the outside of a small circular cylinder

The method of matched asymptotic expansions is used to solve the differential equation describing the shape of a meniscus on the outside of a circular cylinder. Since the perturbation quantity is proportional to the cylinder radius, the solution is valid basically for small oylinders. The predicted meniscus height is compared with numerical data to determine the accuracy of the two-term result; the third term is found but does not improve the estimate.

A general feature space for automatic verb classification

Lexical semantic classes of verbs play an important role in structuring complex predicate information in a lexicon, thereby avoiding redundancy and enabling generalizations across semantically similar verbs with respect to their usage. Such classes, however, require many person-years of expert effort to create manually, and methods are needed for automatically assigning verbs to appropriate classes. In this work, we develop and evaluate a feature space to support the automatic assignment of verbs into a well-known lexical semantic classification that is frequently used in natural language processing. The feature space is general – applicable to any class distinctions within the target classification; broad – tapping into a variety of semantic features of the classes; and inexpensive – requiring no more than a POS tagger and chunker. We perform experiments using support vector machines (SVMs) with the proposed feature space, demonstrating a reduction in error rate ranging from 48% to 88% over a chance baseline accuracy, across classification tasks of varying difficulty. In particular, we attain performance comparable to or better than that of feature sets manually selected for the particular tasks. Our results show that the approach is generally applicable, and reduces the need for resource-intensive linguistic analysis for each new classification task. We also perform a wide range of experiments to determine the most informative features in the feature space, finding that simple, easily extractable features suffice for good verb classification performance.

Extensional flow of dilute polymer solutions

The behaviour of dilute polymer solutions in sink flow, viz., radial flow toward a point, was investigated experimentally and theoretically. Solutions of polyethylene oxide, in the drag-reducing concentration range, were pushed through a 60° conical channel at Reynolds numbers of order 10 2 . Preliminary studies revealed a range of flow conditions in which the flow was free of secondary motion yet non-Newtonian effects were significant. Measurements of pressure differential between two radial positions yielded the non-Newtonian normal stress developed in the radial direction; the magnitude was the same order as the Newtonian stress, i.e., as the dynamic pressure ½ρ V 2 . Several fluid models were analysed to determine the stress generated by each in sink flow. It was found that a solution of Rouse–Zimm flexible macromolecules produces a stress which is three orders of magnitude below the observed level, and that macro-molecules with finite extension fall short by two orders. A suspension of elongated particles, of the type analysed by Batchelor, was also considered, but application to the present case was difficult because of the small scale required by the theory. Consequently, the theory was extended to include particles the size of the flow field, and an order-of-magnitude analysis revealed that for such particles to produce the desired stress, the aspect ratio must be

The effect of speech recognition accuracy rates on the usefulness and usability of webcast archives

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