Now showing 1 - 10 of 305
PublicationDesign of Body-Grounded Tactile Actuators for Playback of Human Physical Contact(2011-06-01) Kuchenbecker, Katherine J; Stanley, Andrew AWe present four wearable tactile actuators capable of recreating physical sensations commonly experienced in human interactions, including tapping on, dragging across, squeezing, and twisting an individual’s wrist. In seeking to create tactile signals that feel natural and are easy to understand, we developed movement control interfaces to play back each of these forms of actual human physical contact. Through iterative design, prototyping, programming, and testing, each of these servo-motor-based mechanisms produces a signal that is gradable in magnitude, can be played in a variety of temporal patterns, is localizable to a small area of skin, and, for three of the four actuators, has an associated direction. Additionally, we have tried to design toward many of the characteristics that have made high frequency vibration the most common form of wearable tactile feedback, including low cost, light weight, comfort, and small size. Bolstered by largely positive comments from naive users during an informal testing session, we plan to continue improving these devices for future use in tactile motion guidance. PublicationKelvin-Helmhotz Instability for Parallel Flow in Porous Media: A Linear Theory(1982) Bau, Haim H.Two fluid layers in fully-saturated porous media are considered. The lighter fluid is above the heavier one so that in the absence of motion the arrangement is stable and the interface is flat. It is shown that when the fluids are moving parallel to each other at different velocities, the interface may become unstable (the Kelvin-Helmholtz instability). The corresponding conditions for marginal stability are derived for Darcian and non-Darcian flows. In both cases, the velocities should exceed some critical values in order for the instability to manifest itself. In the case of Darcy's flow, however, an additional condition, involving the fluids' viscosity and density ratios, is required. PublicationBody-Biased Complementary Logic Implemented Using AIN Piezoelectric MEMS Switches(2009-01-01) Sinha, Nipun; Jones, Timothy S.; Guo, Zhijun; Piazza, GianlucaThis paper reports on the first implementation of low voltage complementary logic (< 1.5 V) by using body-biased aluminum nitride (AlN) piezoelectric MEMS switches. For the first time, by using opposite body biases the same mechanical switch has been made to operate as both an ntype and p-type (complementary) device. Body-biasing also gives the ability to precisely tune the threshold voltage of a switch. The AlN MEMS switches have shown extremely small subthreshold slopes and threshold voltages as low as 0.8 mV/dec and 30 mV, respectively. Furthermore, this work presents a fully mechanical body-biased inverter formed by two AlN MEMS switches operating at 100 Hz with a ± 1.5 V voltage swing. PublicationA Course in Micro- and Nanoscale Mechanics(2003-01-01) Carpick, Robert WAt small scales, mechanics enters a new regime where the role of surfaces, interfaces, defects, material property variations, and quantum effects play more dominant roles. A new course in nanoscale mechanics for engineering students was recently taught at the University of Wisconsin - Madison. This course provided an introduction to nanoscale engineering with a direct focus on the critical role that mechanics needs to play in this developing area. The limits of continuum mechanics were presented as well as newly developed mechanics theories and experiments tailored to study and describe micro- and nano-scale phenomena. Numerous demonstrations and experiments were used throughout the course, including synthesis and fabrication techniques for creating nanostructured materials, bubble raft models to demonstrate size scale effects in thin film structures, and a laboratory project to construct a nanofilter device. A primary focus of this paper is the laboratory content of this course, which includes an integrated series of laboratory modules utilizing atomic force microscopy, self-assembled monolayer deposition, and microfluidic technology. PublicationEvaluation of Some Thermal Power Cycles for Use in Space(2006-01-01) Tarlecki, Jason; Lior, Noam; Zhang, NaProduction of power in space for terrestrial use is of great interest in view of the rapidly rising power demand and its environmental impacts. Space also offers a very low temperature, making it a perfect heat sink for power plants, thus offering much higher efficiencies. This paper focuses on the evaluation and analysis of thermal Brayton, Ericsson and Rankine power cycles operating at space conditions on several appropriate working fluids. 1. Under the examined conditions, the thermal efficiency of Brayton cycles reaches 63%, Ericsson 74%, and Rankine 85%. These efficiencies are significantly higher than those for the computed or real terrestrial cycles: by up to 45% for the Brayton, and 17% for the Ericsson; remarkably 44% for the Rankine cycle even when compared with the best terrestrial combined cycles. From the considered working fluids, the diatomic gases (N2 and H2) produce somewhat better efficiencies than the monatomic ones in the Brayton and Rankine cycles, and somewhat lower efficiencies in the Ericsson cycle. The Rankine cycles require radiator areas that are larger by up to two orders of magnitude than those required for the Brayton and Ericsson cycles. The results of the analysis of the sensitivity of the cycle performance parameters to major parameters such as turbine inlet temperature and pressure ratio are presented, and the effects of the working fluid properties on cycle efficiency and on the power production per unit radiator area were explored to allow decisions on the optimal choice of working fluids. PublicationBrain-Tumor Interaction Biophysical Models for Medical Image Registration(2008-12-01) Hogea, Cosmina; Davatzikos, Christos; Biros, GeorgeState-of-the art algorithms for deformable image registration are based on the minimization of an image similarity functional that is regularized by adding a penalty term on the deformation map. The penalty function typically represents a smoothness regularization. In this article, we use a constrained optimization formulation in which the image similarity functional is coupled to a biophysical model. This formulation is pertinent when the data have been generated by imaging tissue that undergoes deformations due to an actual biophysical phenomenon. Such is the case of coregistering tumor-bearing brain images from the same individual. We present an approximate model that couples tumor growth with the mechanical deformations of the surrounding brain tissue. We consider primary brain tumors—in particular, gliomas. Glioma growth is modeled by a reaction-advection-diffusion PDE, with a two-way coupling with the underlying tissue elastic deformation. Tumor bulk, infiltration, and subsequent mass effects are not regarded separately but are captured by the model itself in the course of its evolution. Our formulation allows for updating the tumor diffusion coefficient following structural displacements caused by tumor growth/infiltration. Our forward problem implementation builds on the PETSc library of Argonne National Laboratory. Our reformulation results in a very small parameter space, and we use the derivative-free optimization library APPSPACK of Sandia National Laboratories. We test the forward model and the optimization framework by using landmark-based similarity functions and by applying it to brain tumor data from clinical and animal studies. State-of-the-art registration algorithms fail in such problems due to excessive deformations. We compare our results with previous work in our group, and we observed up to 50% improvement in landmark deformation prediction. We present preliminary validation results in which we were able to reconstruct deformation fields using four degrees of freedom. Our study demonstrates the validity of our formulation and points to the need for richer datasets and fast optimization algorithms. PublicationFlow Past a Liquid Drop with a Large Non-uniform Radial Velocity(1983-08-01) Sadhal, Satwindar S.; Ayyaswamy, Portonovo S.In this analysis, the translation of a liquid drop experiencing a strong non-uniform radial velocity has been investigated. The situation arises when a moving liquid drop experiences condensation, evaporation or material decomposition at the surface. By simultaneously treating the flow fields inside and outside the drop, we have obtained physical results relevant to the problem. The magnitude of the radial velocity is allowed to be very large, but the drop motion is restricted to slow translation. The solution to the problem has been developed by considering a uniform radial flow with the translatory motion introduced as a perturbation. The role played by the inertial terms due to the strong radial field has been clearly delineated. The study has revealed several interesting features. An inward normal velocity on a slowly moving drop increases the drag. An increasing outward normal velocity decreases the drag up to a minimum beyond which it increases. The total drag force not only consists of contributions from the viscous and the form drags but also from the momentum transport at the interface. Since the liquid drop admits a non-zero tangential velocity, the tangential momentum convected by the radial velocity forms a part of this drag force. The circulation inside the drop decreases (increases) with an outward (inward) normal velocity. A sufficiently large non-uniform outward velocity causes the circulation to reverse. In the limit of the internal viscosity becoming infinite, our analysis collapses to the simple case of a translating rigid sphere experiencing a large non-uniform radial velocity. By letting the radial velocity become vanishingly small the Stokes-flow solution is recovered. An important contribution of the present study is the identification of a new singularity in the flow description. It accounts for both the inertial and the viscous forces and displays Stokeslet-like characteristics at infinity. PublicationFinite state abstraction of a stochastic model of the lactose regulation system of Escherichia coli(2006-12-15) Julius, Agung; Halász, Ádám; Kumar, R. Vijay; Pappas, George JThis paper focuses on the lactose regulation system in Escherichia coli bacteria, one of the most extensively studied examples of positive feedback in a naturally occurring gene network. State-of-the-art nonlinear dynamical system models predict a bi-stability phenomenon that is confirmed in experiments. However, such deterministic models fail to explain experimental observations of spontaneous transition between the two stable states in the system and the simultaneous occurrence of both steady states in a population of cells. In this paper, we propose a stochastic model that explains this phenomenon. Furthermore, we also extract a coarser two-state continuous-time Markov chain as a higher level abstraction of this model, and show that macroscopic properties are retained in the abstraction. PublicationDirect Simulation of Initial Value Problems for the Motion of Solid Bodies in a Newtonian Fluid. Part 2. Couette adn Poiseuille Flows.(1994-05-11) Feng, James; Hu, Howard H.; Joseph, Daniel D.This paper reports the results of a two-dimensional finite element simulation of the motion of a circular particle in a Couette and a Poiseuille flow. The size of the particle and the Reynolds number are large enough to include fully nonlinear inertial effects and wall effects. Both neutrally buoyant and non-neutrally buoyant particles are studied, and the results are compared with pertinent experimental data and perturbation theories. A neutrally buoyant particle is shown to migrate to the centreline in a Couette flow, and exhibits the Segré-Silberberg effect in a Poiseuille flow. Non-neutrally buoyant particles have more complicated patterns of migration, depending upon the density difference between the fluid and the particle. The driving forces of the migration have been identified as a wall repulsion due to lubrication, an inertial lift related to shear slip, a lift due to particle rotation and, in the case of Poiseuille flow, a lift caused by the velocity profile curvature. These forces are analysed by examining the distributions of pressure and shear stress on the particle. The stagnation pressure on the particle surface are particularly important in determining the direction of migration. PublicationOn the Stability and Flow Reversal of an Asymmetrically Heated Open Convection Loop(1981) Bau, Haim H.; Torrance, Kenneth E.Experimental results are reported for a U-shaped, free convection loop. The top of the loop is open to an isothermal reservoir. The horizontal leg and one vertical leg are heated at rates Q1 and Q2, respectively. The loop is filled either with water or a watersaturated porous medium. Symmetric heating and asymmetric heating favouring the ascending leg of the loop both yield stable flows. Asymmetric heating favouring the descending leg leads to stable flows when the ratio Q1/Q2 is above a critical value. Below this critical value, the flow is observed to oscillate with increasing amplitude until the direction of flow in the loop undergoes a reversal. A steady flow follows the reversal. Analytical results include a stability analysis and time-dependent, one-dimensional numerical calculations, both of which compare favourably with experiment.