Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005. / Includes bibliographical references (leaves 102-104). / An experimental study of flow separation in an incompressible two-dimensional unsteady flow was undertaken with the aim of validating recently developed flow separation criteria, which are based on kinematic principles. Specifically, the so-called rotor-oscillator arrangement was utilized to perform a series of experiments in steady, unsteady periodic, and aperiodic two- dimensional viscous flows at low Reynolds number. Flow separation under these conditions was investigated by means of flow visualization, shear stress sensors, and numerical simulation. The existence of fixed and moving separation structures, as predicted by the recent criteria, was verified in the experiments. Fixed separation structures were encountered in periodic flows and random flows, while moving separation was observed in a slow periodic flow and a random flow with linear drift. It was determined that separation in the rotor-oscillator experiment is strongly correlated to two factors: flow unsteadiness and flow ejection from the wall. The balance of the characteristic length and time scales of unsteadiness and ejection determines whether the separating structure is moving or is fixed. / (cont.) The experimental and numerical results strengthen the notion that the instantaneous zero skin friction point alone does not denote flow separation in unsteady flow. Rather, flow separation in unsteady flow can be better understood from a Lagrangian perspective, in which case it can be treated in a robust and coherent manner. / by Raul Javier Coral Pinto. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007. / Includes bibliographical references (leaves 111-114). / In this thesis, a theoretical and numerical procedure for predicting the effects of viscosity on the hydrodynamic forces developed by a sailing yacht hull is presented. A simultaneous viscous/inviscid algorithm is developed by coupling a low order panel method with quasi three-dimensional integral boundary layer equations. A transom condition is used to prevent non-zero wave heights at the stern for a hull with overhangs. The influence of viscosity on the outer inviscid flow is modeled using a wall transpiration boundary condition and an edge velocity formula. The boundary layer edge velocity is expressed as a sum of the inviscid velocities and a correction dependent only on the boundary layer variables, determined by equations developed from the panel method calculation as a distribution of transpiration sources. These are superimposed on the body, including the lifting surfaces, as well as on the potential flow wakes. The boundary layer equations, with the global potential flow effects included via the transpiration source model, are solved by a full Newton's method. Numerical predictions for a sailing yacht hull are compared with experimental results obtained in a towing tank. / by Claudio Cairoli. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008. / In title on title page, "N" appears as italic. / Includes bibliographical references (leaves [181]-185). / A compact printable microfluidic valve composed of poly(N-isopropylacrylamide) has been designed, fabricated, and tested. The design of the valve consists of filling microwells with poly(NIPAAm) and bonding PDMS channels above them. This filling is achieved using thermal inkjet printing of a prepolymer solution and subsequent polymerization using UV irradiation. When the gel is swollen, it blocks flow from passing through the channel. Upon heating, the gel shrinks and allows flow in the channel. Poly(NIPAAm) is a thermosensitive hydrogel that exhibits an inverse temperature expansion behavior. When the temperature of the swollen gel is raised above a lower critical solution temperature (LCST) of approximately 32°C, the gel becomes hydrophobic. This change in hydrophobicity results in expulsion of the water molecules from within the hydrogel network, thus resulting in shrinking of the gel. By adding magnetic nanoparticles to the hydrogel and exposing it to an external magnetic field, volumetric change of the hydrogel can be locally and externally induced. External heating of the magnetic nanoparticles, however, is not included in this thesis. In order to ensure shrinkage that is predictable in favor of flow control, microanchor structures have been designed, modeled, and fabricated at the bottom of the microwells. These microanchors hold the poly(NIPAAm) at the bottom of the plug such that the shrinkage of the gel always acts to open the flow channel at the top yielding a minimum pressure drop. Design decisions were made using the principles of Axiomatic Design in order to minimize the response time and pressure drops in the valve. Modeling of the underlying mechanisms is described along with the application of these models to the final device. Results of fabrication suggest the feasibility while also eliciting possible improvements to the fabrication process. / (cont.) Profilometry measurements of the swollen and shrunken valves reveal flow control operation as intended. Additionally, design and modeling of magnetic heating using mixed-in nanoparticles is presented. A fabrication plan designed to include this mechanism is proposed. / by Nathan Edward Reticker-Flynn. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student submitted PDF version of thesis. / Includes bibliographical references (p. 179-197). / As mechanical devices move towards the nanoscale, smaller and more sensitive force and displacement sensors need to be developed. Currently, many biological, materials science, and nanomanufacturing applications could benefit from multi-axis micro- and nanoscale sensors with fine force and displacement resolutions. Unfortunately, such systems do not yet exist due to the limitations of traditional sensing techniques and fabrication procedures. Carbon nanotube-based (CNT) piezoresistive transducers offer the potential to overcome many of these limitations. Previous research has shown the potential for the use of CNTs in high resolution micro- and nanoscale sensing devices due to the high gauge factor and inherent size of CNTs. However, a better understanding of CNT-based piezoresistive sensors is needed in order to be able to design and engineer CNT-based sensor systems to take advantage of this potential. The purpose of this thesis is to take CNT-based strain sensors from the single element test structures that have been fabricated and turn them into precision sensor systems that can be used in micro- and nanoscale force and displacement transducers. In order to achieve this purpose and engineer high resolution CNT-based sensor systems, the design and manufacturing methods used to create CNT-based piezoresistive sensors were investigated. At the system level, a noise model was developed in order to be able to optimize the design of the sensor system. At the element level, a link was established between the structure of the CNT and its gauge factor using a theoretical model developed from quantum mechanics. This model was confirmed experimentally using CNT-based piezoresistive sensors integrated into a microfabricated test structure. At the device level, noise mitigation techniques including annealing and the use of a protective ceramic coating were investigated in order to reduce the noise in the sensor. From these investigations, best practices for the design and manufacturing of CNT-based piezoresistive sensors were established. Using these best practices, it is possible to increase the performance of CNT-based piezoresistive sensor systems by more than three orders of magnitude. These best practices were implemented in the design and fabrication of a multi-axis force sensor used to measure the adhesion force of an array of cells to the different material's surfaces for the development of biomedical implants. This force sensor is capable of measuring forces in the z-axis as well as torques in the [theta]x and [theta]y axis. The range and resolution of the force sensor were determined to be 84 [mu]N and 5.6 nN, respectively. This corresponds to a dynamic range of 83 dB, which closely matches the dynamic range predicted by the system noise model used to design the sensor. The accuracy of the force sensor is better than 1% over the device's full range. / by Michael A. Cullinan. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999. / Includes bibliographical references (leaves 98-101). / by Elmer C. Lee. / S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999. / Includes bibliographical references (p. 89-96). / by Richard Joseph Nelson. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000. / by Roger Shapley Cortesi. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 108-114). / The cytoskeletal network consisting mainly of actin and actin binding proteins is highly dynamic, provides structural integrity to cells, and plays a central role in a wide range of mechanical and biological functions such as migration and the sensation of external forces. Thus, knowledge of actin cytoskeleton is indispensable for understanding the mechanics and many biological processes of cells. Although various theoretical, computational, and experimental investigations have been conducted, the underlying bases for these critical mechanical properties are still poorly understood. This thesis examines the morphology and rheology of actin networks through the development of a 3-D computational model. First, the viscoelastic properties of actin networks irreversibly bound by actin crosslinking proteins (ACPs) were investigated. Relative contributions of the concentration and type of ACPs, the stiffnesses of actin filaments and ACPs, and thermal fluctuations were evaluated at various prestrain levels. These studies demonstrated for the first time that under typical biological conditions, extensional stiffnesses of both actin filaments and ACPs were surprisingly significant, but thermal fluctuations were relatively unimportant. At high tensions, only a small portion of networks supported a majority of the load. Second, the relative importance of two mechanisms of ACPs which control dynamic properties of actin networks, unbinding and subdomain unfolding, was evaluated. By analyzing the strain-stiffening, stress relaxation, and plastic deformation of the networks with unbinding and/or unfolding, it was found that despite the possibility of unfolding, ACP unbinding is the dominant mechanism governing actin rheology under typical experimental and physiological conditions. In addition, detailed processes by which unbinding plays such a role were investigated. Lastly, roles that molecular motors play in the morphology and rheology of actin networks were studied. Motors enhanced elasticity of actin networks and led to heterogeneous networks to a degree that was highly dependent on how easily the motors unbind from actin filaments. ACPs helped the motors to make networks elastic and prevented the networks from being heterogeneous. Also, morphology of actin-motor networks was significantly affected by boundary conditions. / by Taeyoon Kim. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000. / Includes bibliographical references (p. 37). / Residual gas tests were done on a 2.4 liter commercial engine to find a correlation for the effect on spark timing on residual gas fraction. The tests were done by sampling the charge mass during a non-firing cycle through a small hole into cylinder 4. The residual gas fraction was determined by measuring the CO2 content of the sample. The experiments were conducted for a variety of different spark timing and valve timing settings. Since the data was taken over a limited range, a physical basis for a correlation was developed. Following the approach used in a previous modeling effort, the residual gas fraction was modeled as the sum of two contributing factors: backflow from the exhaust port to the cylinder during valve overlap and gas trapped inside the cylinder at the time of intake valve open. Additionally, the phenomenon of choked backflow was introduced in the model. Based on the data, a correlation was developed that estimates the residual gas fraction as a function of: intake to exhaust pressure ratio (Pi/Pe), valve profile, engine speed, compression ratio, and spark timing. / by Rolf Rikard Waero. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004. / Includes bibliographical references (p. 166-167). / Understanding how fluids respond to various deformations is of great importance to a spectrum of disciplines ranging from bio-medical research on joint replacements to sealing technology in industrial machinery. Specifically, this work addresses the need for probing interfacial rheology to understand how lubricants fail as system scales are reduced from bulk dimensions to molecular length scales. In the pursuit of interfacial rheology, one needs a platform capable of the temporal and spatial range and resolution required to quantify the visco-elastic fluid properties in the interfacial regime. With the availability and versatility of AFMs and the mounting models and data related to the performance of SPM probes in a fluid environment, the AFM is an attractive platform to exploit. This thesis will discuss the use of thermal oscillations of an SPM probe to quantify the visco-elastic properties of fluids via spectral variations. There exist theoretical models for the Fluid-Structure Interactions (FSI) of vibrating bodies in incompressible viscous mediums that have been validated. This thesis will discuss how these models have been extended to develop a new visco-elastic FSI model. The analytical results of these models will be quantitatively compared to thermally driven SPM cantilevers to extract fluid properties. The new theory required for modeling the probe dynamics is outlined and the present limitations, for both the analytical and experimental techniques, are discussed. / by Ryan Edward Jones. / Ph.D.