Seminar Series

Talk Abstract:

Stick-slip oscillations in frictional systems are a familiar source of unwanted vibration, noise, and cyclic stress. Though friction-induced oscillations can be partially controlled by choice of contacting surfaces, structural redesign, or the addition of passive (material) damping, friction vibration problems persist. This is particularly true in the field of brakes for automobiles, trucks, and aircraft, where high-frequency noise or “squeal” continues to be a problem. This talk will explain how the application of high frequency vibration, or “dither,” can be used to eliminate vibration in such systems. The method will first be applied to a single-degree-of freedom, frictional system. Using an averaging technique, it will be shown that dither essentially “smoothes” the friction nonlinearity, thus preventing the occurrence of stick-slip oscillations.  It will be shown that that effective dither-cancellation depends critically on the amplitude and frequency of the dither signals. Improper choice of dither signals can result in poor performance, and may even destabilize a system that is undergoing steady sliding. Extensions to continuous systems models will also be discussed.  

Al Ferri received his BS degree in Mechanical Engineering from Lehigh University in 1981, and a Ph.D. in Mechanical and Aerospace Engineering from Princeton University in 1985. He joined the School of Mechanical Engineering at Georgia Tech in 1985, and has served on the faculty since that time. His research interests include nonlinear dynamics, vibrations, control theory, and acoustics.  He is currently an Associate Editor of the ASME Journal of Computational and Nonlinear Dynamics, and a past Associate Editor of the ASME Journal of Applied Mechanics. He is a fellow of the ASME and past recipient of the Ralph Teetor Award from SAE.  His work has been supported by AFOSR, ONR, NASA, and NSF, as well as a number of industrial sources.

Talk Abstract:

To achieve energy security and independence, the U.S. needs to utilize all of its resources in an integrated and efficient manner. This will increase the demand for hydrogen which can be produced using the primary energy delivered from a very high temperature nuclear reactor (VHTR). Thermochemical cycles can be used to split water through a series of chemical reactions where the net result is the production of hydrogen and oxygen at much lower temperatures than direct thermal decomposition. All chemicals within the cycle are fully recycled and the heat to drive the reactions, which tend to be endothermic, must be provided by a primary energy source. When the primary energy driver is nuclear heat, hydrogen can be generated with a minimal green-house gas footprint.

The sulfur-based family of thermochemical cycles, including the Sulfur-Iodine cycle (S-I), and the Hybrid Sulfur cycle appear promising for producing hydrogen using nuclear heat. These cycles employ a high-temperature sulfuric acid decomposition reaction step. The reaction produces oxygen and generates SO2, which is used in other reaction steps of the cycles. The reaction takes place from 750 to 900°C, or higher, and is facilitated by heterogeneous catalysts. The S-I cycle produces hydrogen by the catalytic decomposition of HI. The various cycles all present requirements of high-temperatures and harsh chemical reaction conditions which present significantly challenging environments for catalytic materials. 

This presentation will focus on the catalyst needs of themochemical cycles that are candidates for being powered by nuclear reactors. Specific catalyst activity and stability testing results will be provided for the decomposition of sulfuric acid for the production of oxygen in the sulfur-based family of cycles and for the catalytic decomposition of hydroiodic acid for the production of hydrogen in the S-I process. Sulfuric acid decomposition results will focus primarily on platinum group metal catalyst while HI decomposition results will be presented mainly on activated carbon catalysts.

Talk Abstract:

Shock wave lithotripsy and high intensity focused ultrasound generate cavitation bubbles in confined spaces that interact in a variety of ways.  Densely packed bubble clusters are produced that exhibit strong collective interactions.  In lithotripsy, the bubble clusters themselves interact with the surface of a kidney stone.  Comminution of the stone generates an accumulation of small particles that are interspersed with, and dynamically coupled to, the cavitation bubbles.  Within tissues surrounding the stone, cavitation activity can occur in urinary ducts and blood vessels, constrained by the vessel walls.  This seminar describes a systematic approach to modeling coupled bubble dynamics in clusters, amidst particles, near surfaces, and in tubes in response to acoustic excitation.  The models are based on Lagrangian and Hamiltonian formulations of the dynamical equations for interacting bubbles and particles.  Via the method of images, the model is augmented to describe bubble dynamics in tubes having several cross-sectional geometries.  The model equations will be presented and discussed, and simulations will be shown that demonstrate different interactions taken into accoun

Talk Abstract:

The measurement of skin-friction is important for applied problems, such as improving the performance of transportation vehicles, and basic problems, such as characterizing surface flows.  To accurately measure time-resolved turbulent wall shear-stress, the sensor must possess sufficient spatial resolution to avoid spatial averaging.  In addition, the device must possess large enough measurement bandwidth to avoid excessive low-pass filtering of the data.  This seminar addresses the promise and progress of microelectromechanical systems (MEMS) scaling advantages to improve the spatial and temporal resolution and accuracy of shear stress measurement technology.  The advantages and limitations of existing devices are discussed.  Next, the development cycle of a micromachined sensor is presented.  Specifically, the design, fabrication and characterization are discussed.  Finally, unresolved technical issues are summarized for future sensor development.