Mechanical Engineering & Mechanics
Effect of Pitch Rate on Time Evolution of Surface Topology on a Delta Wing
A basic delta wing of moderate sweep angle, representative of Unmanned Combat Air Vehicles (UCAVs) and Micro Air Vehicles (MAVs), undergoes a pitching maneuver. Near-surface flow patterns are visualized by a technique of high-image-density particle image velocimetry for a wide range of pitch rates. Five different universal states are defined during the relaxation process following cessation of the pitching motion. These states involve distinct patterns that can be defined in terms of topological features such as negative (separation) and positive (reattachment) bifurcation lines, saddle points, foci, and nodes. Such universal states can be identified for all pitch rates, extending over an eightfold range. Irrespective of the severity of the flow distortion at the end of the pitching maneuver, the relaxation of the flow involves the same sequence of universal states. The time delay to occurrence of the first universal state is very sensitive to the pitch rate. The delay between subsequent states is, however, nearly independent of pitch rate. Due to the highly three-dimensional nature of the flow, the flow patterns and topological states will also be visualized by stereoscopic particle image velocimetry.
Using LIBS Measurements for Coal Quality Monitoring and Upgraded Power Plant Control
Laser-induced breakdown spectroscopy (LIBS) has been developed and applied to measure key inorganic components in coal ash such as Si, Al, Fe, Na, Ca, Mg, and K – , which contribute to the slagging and fouling behavior of pulverized coal. A coal inventory was assembled from fuels used at utility boilers with a range of slagging/fouling characteristics. These coals included Eastern US bituminous and sub-bituminous coals and some foreign fuels. These coals were tested in a custom-built LIBS analyzer for ash metal composition and major element concentration (i.e. O, S, N). Detection limits are on the order of 0.01 percent, with variations depending on the particular element and type of coal. Measurement repeatability and accuracy are typically within 10 percent (relative). The elemental analyses were used in concert with a neural network algorithm to calculate a slagging and fouling index for the prediction of deposition behavior. The values of the predicted indices are very similar to the resulting indices from standard coal analysis procedure. A future on-line version of the LIBS system will be installed at a 650 MW coal-fired unit and equipped with expert system-based software to demonstrate the real-time capabilities of this technology to monitor coal ash composition, slagging/fouling prediction and recommend actions to the operators for boiler operation modifications for slagging/fouling mitigation.
Fluid-Structure Analysis of Cellular Deformation and Detachment during Airway Reopening
Pathological conditions such as pneumonia and sepsis can lead to acute respiratory distress syndrome (ARDS), a condition characterized by fluid accumulation in the distal airways. Ventilation of ARDS patients produces microbubbles which reopen fluid-occluded airways and may deform, injure and/or detach epithelial cells (EpC) from the airway wall. Although in-vitro systems can mimic airway reopening conditions, current visualization techniques cannot quantify cell deformation and detachment during microbubble flows. To investigate these processes we developed 3D finite element models of EpC under airway reopening conditions. These models utilized in-vitro confocal microscopy to specify cellular morphology and optical tweezer measurements to specify the EpCs viscoelastic properties. Boundary element solutions were used to specify hydrodynamic loading on the EpC and adhesion properties were based on a steered molecular dynamic (SMD) simulation of the integrin-collagen complex. We also developed a hybrid boundary element/finite element method to investigate the effect of cell deformability on the hydrodynamic stresses generated by the air-liquid interface. Results indicate that both cytoskeletal and membrane mechanical properties can influence the risk of injury and detachment. These results have helped explain counter-intuitive experimental data and may lead to the development of improved treatments for ARDS
Hannah L. Dailey is a NSF Graduate Research Fellow and Samir N. Ghadiali is a Parker B. Francis Fellow in Pulmonary Research.
Finite Element Simulation of Electromigration Cracking
Electromigration is one of the major problems that limits the reliability of high density microelectronics. Since microelectronic packages are expected to function reliably for long periods of time, it is crucial that numerical simulations be developed to aid in comprehensive package design. Electromigration is the transport of atoms in a conducting material due to momentum transfer from flowing electrons which leads to a failure in metal structures with high current densities. The presence of crack like defects in the conduction path accelerates the time to failure. The purpose of this paper is to describe a finite element formulation of the coupled stress-diffusion behavior that is observed in electromigration phenomenon. Specialized enriched crack tip elements, which simplify the fracture and reliability analyses, are used in this finite element formulation. A unique aspect of this work is the incorporation of defects (cracks) in the finite element models, which permits the direct calculation of relevant fracture parameters, like strain energy release rate, stress intensity factors and crack opening displacements.