Thesis (Ph.D.)--Tufts University, 2001. / Adviser: Chris Rogers. Submitted to the Dept. of Mechanical Engineering. Includes bibliographical references (leaves 140-146). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Any natural surface is in essence non-smooth, consisting of more or less regular roughness and/or mobile structures of different scales. From a fluid mechanics point of view, these natural surfaces offer better aerodynamic performances when they cover moving bodies, in terms of drag reduction, lift enhancement or control of boundary layer separation; this has been shown for boundary layer or wake flows around thick bodies. The numerical simulation of microscopic flows around "natural" surfaces is still out of reach today. Therefore, the goal of this thesis is to study the modeling of the apparent flow slip occurring on this kind of surfaces, modeled as a porous medium, applying Whitaker's volume averaging theory. This mathematical model makes it possible to capture details of the microstructure while preserving a satisfactory description of the physical phenomena which occur. The first chapter of this manuscript provides an overview of previous efforts to model these surfaces, detailing the most important results from the literature. The second chapter presents the mathematical derivation of the volume-averaged Navier-Stokes equations (VANS) in a porous medium. In the third chapter the flow stability at the interface between a free fluid and a porous medium, formed by a series of rigid cylinders, is studied. The presence of this porous layer is treated by including a drag term in the fluid equations. It is shown that the presence of this term reduces the rates of amplification of the Kelvin-Helmholtz instability over the whole range of wavenumbers, thus leading to an increase of the wavelength of the most amplified mode. In this same context, the difference between the isotropic model and a tensorial approach for the drag term has been evaluated, to determine the most consistent approach to study these flow instabilities. This has led to the conclusion that the model that uses the apparent permeability tensor is the most relevant one. In the following chapter, based on this last result, the apparent permeability tensor, based on over one hundred direct numerical simulations carried out over microscopic unit cells, has been identified for a three-dimensional porous medium consisting of rigid cylinders. In these configurations the tensor varies according to four parameters: the Reynolds number, the porosity and the direction of the average pressure gradient, defined by two Euler angles. This parameterization makes it possible to capture local three-dimensional effects. This database has been set up to create, based on a kriging-type approach, a behavioral metamodel for estimating all the components of the apparent permeability tensor. In the fifth chapter, simulations of the VANS equations are carried out on a macroscopic scale after the implementation of the metamodel, to get reasonable computing times. The validation of the macroscopic approach is performed on a closed cavity flow covered with a porous layer and a comparison with the results of a very accurate DNS, homogenized a posteriori, has shown a very good agreement and has demonstrated the relevance of the approach. The next step has been the study of the passive control of the separation of the flow past a hump which is placed on a porous wall, by the same macroscopic VANS approach. Finally, general conclusions and possible directions of research in the field are presented in the last chapter.

Chainsaws have seen a constant improvement over the time of their production and are still an object of improvement by the bigger chainsaw companies. The chainsaw engine is highly developed and optimised for the assigned uses. This thesis project aims to further optimise the engine by implementing a system that allows for variable inertia on the rotational system. A high inertia is needed at higher velocity to reduce speed fluctuations but a high inertia also makes it harder for an object to accelerate, a low start inertia could help accelerate the system and then reach a higher inertia for stability at higher velocities.  The variable inertia system were developed for a Husqvarna 550 XP mk.II. Concepts were developed using the product development process and the evaluation is based on the interpreted needs of the product. The most promising concepts underwent dynamic system simulations to ensure the best performing concept is selected. The chosen concept were then further designed with an iterative process where each iteration was analysed with the finite element method to ensure structural rigidity. The developed product indicates a performance increase. The magnitude of this performance increase is mainly governed by the size constraints in the chainsaw.

The objective of this study was to determine the correlation of the drag coefficient of an accelerating sphere in the liquid medium over a range of Reynolds number from 0.1 to 100, 000. A magnetic accelerator was used to obtain an acceleration for the sphere, the data was recorded using a photograph and a digital computer was used to reduce the data.

No description available.

This thesis is separated into two parts: threshold graphs and inertia sets. First we present an algorithmic approach to finding the minimum rank of threshold graphs and then progress to counting the number of threshold graphs with a specific minimum rank. Second, we find an algorithmic and more automated way of determining the inertia set of graphs with seven or fewer vertices using theorems and lemmata found in previous papers. Inertia sets are a relaxation of the inverse eigenvalue problem. Instead of determining all the possible eigenvalues that can be obtained by matrices with a specific zero/nonzero pattern we restrict to counting the number of positive and negative eigenvalues.

No description available.

The purpose of this research was to study the human ability to perceive inertia and to investigate the relationship between individual sensitivity to mass discrimination and the accuracy of inertia judgment. The study involved two major experiments, a mass discrimination test to provide a measure of kinesthetic sensitivity and a push/aim task to measure inertia perception. The experimental apparatus allowed for the manipulation of an inertial load for both the mass discrimination test and the push/aim task. The mass discrimination test was based on the method of constant stimuli and involved a standard stimulus of 30 kg and seven comparison stimuli ranging from 24.3 kg to 35.7 kg. It determined Weber fraction values which were used as a measure of sensitivity. The push/aim task simulated a dynamic horizontal pushing task and was restricted to an initial exertion causing the inertial load to travel to a target under its own momentum. Performance was measured by the errors associated with the accuracy of the aimed pushes. These errors were separated into an absolute component representing an overshoot or undershoot, and a relative measure of error representing variability of repeated trials about the absolute error. Distance to the target (2 or 6 m), amount of load (25, 45, or 90 kg), and apparent size of the load (small or large) were manipulated in the dynamic pushing experiment. Hypothesis testing was used to determine whether performance exhibited a consistent overshooting or undershooting of the target. An ANCOVA was used to measure the effects of distance, amount of load, and apparent size on performance given a covariate of mass discrimination sensitivity. Since the ANCOVA showed no significant effect from the covariate, an ANOVA was used to analyze the effects of the independent variables on both measures of error. The hypothesis tests revealed that absolute error in the push/aim task was significant by less than zero for the combined data set and at all levels of independent variables, indicating a consistent undershoot of the target. The ANCOVA results il indicated no significant effect from the covariate, mass discrimination Correlation coefficients (R2) were calculated for the relation between sensitivity to mass discrimination and push/aim task performance with respect to absolute and relative error. These values were 0.004 for the absolute error and 0.008 for the relative error. The ANOVA results revealed that weight, distance, and the weight x distance interaction had significant effects on the absolute measure of error. Increasing the levels of either weight or distance resulted in significant increases in the amount of absolute error. Increasing the distance resulted in significant increases in the amount of relative error. The presence of a distance effect and the absence of a weight effect indicates the importance of rate of force over amount of force in the relative error associated with the push/aim task. Apparent size of the load did not have a significant effect on either absolute or relative error. / Master of Science

While human balance is known to be affected by altered sensory feedback, altered dynamics may also contribute to balance deficiencies in certain populations. The goal of this study was, therefore, to investigate the effects of altered dynamics, namely increased inertia and increased weight, on standing balance. Sixteen normal-weight male participants completed quiet standing in a custom-built backboard under four conditions: baseline, increased inertia only, increased weight only, and increased inertia and weight. Increased inertia did not affect body center of mass movement (COM) or center of pressure (COP) movement, suggesting that no additional ankle torque was necessary to control the increased inertial forces. Increased weight caused increased body COM movement (increased backboard angle range and angular speed) and greater acceleration of the COM (as evidenced by increased COP-COM), requiring an increased level of corrections needed to maintain upright posture (as evidenced by increased COP speed) and increased ankle torques (as evidenced by increased range of COP position). Increasing inertia and weight simultaneously had the same effects as increasing weight except that there was no increased COM movement when both inertia and weight were increased. This indicates that there may be a slight mediating effect of increasing inertia on the extreme changes in balance observed when only weight is increased. These results indicate that altered dynamics of the body have an effect on human standing balance, just as altered sensory function has an effect on balance. / Master of Science

The aim of this thesis is to develop a methodology for obtaining mass properties of a vehicle using specific test rig. Investigated mass properties are the mass, location of center of gravity and the inertia tensor. Accurate measurement of mass properties of vehicles is crucial for vehicle dynamics research. The test rig consists of a frame on which the vehicle is fixed and which is suspended from the ceiling of the laboratory using steel cables. Mass and location of center of gravity are measured using the data from the test rig in equilibrium position and basic static equations. Inertia tensor is measured using the data from dynamical response of the system. For this purpose an identification routine which employs prediction error method is developed using the built&ndash / in functions from the System Identification Toolbox of MATLAB&reg / . The experiment was also simulated using Simmechanics Toolbox of MATLAB&reg / . Identification code is verified using the results of the experiment simulations for various cases.