Department of
Mechanical and Aerospace Engineering
Abstracts 2002 - 2003
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Y-C. Fung Auditorium
Powell Fochts Bioengineering Hall
Dr. Mathias Fink
Laboratoire Ondes et Acoustique
ESPCI, University Denis Diderot, Paris
Time-reversal invariance is a very powerful concept in classical and
quantum mechanics. In the field of
acoustics, where time reversal invariance also occurs, time-reversal
experiments may be achieved simply with arrays of transmit-receive transducers,
allowing an incident acoustic field to be sampled, recorded, time-reversed and
re-emitted.
Time reversal mirrors may be used to study random media, inverse
scattering problems, dissipation effects and diffraction limits. They open the way to new signal processings.
Time reversal mirrors have also plenty of applications including
ultrasonic therapy and medical imaging, non destructive testing,
telecommunications, underwater acoustics and sound control. An overview of these fields will be presented.
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Thursday, March 20, 2003
1:30 p.m.
479 EBU-II
Dr. Michael Nicholl
Mining and Geological Engineering
The
“Exploring the Fundamental Processes
of Flow and Transport in Unsaturated, Fractured Rock”
Field and laboratory evidence
collected over the past decade suggest that two-phase flow in fractured rock
will not follow simplistic conceptual models extrapolated from the soil physics
literature. Application of volume
averaged models to thick sequences of unsaturated, fractured rock has led to
order of magnitude errors in the prediction of water migration and contaminant
transport. Such conceptual model
failures stem from our inadequate understanding of fluid flow in rock fractures
at scales ranging from the single fracture to three-dimensional fracture
networks. A simple laboratory experiment
in a two-dimensional fracture-matrix network shows complicated behavior at a
variety of temporal and spatial scales.
An experimental program to understand and explain this behavior is
presented. Uncertainty regarding flow in
fractured rock is broken down into three categories: single fracture processes,
processes at fracture intersections, and interaction between fractures and the
surrounding matrix. A set of simplistic
laboratory experiments exploring these processes are presented
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Thursday, March 13, 2003
3:30 p.m.
479 EBU-II
Dr. Andrew Altevogt
Department of Civil and Environmental
“Subsurface Transport of Carbon
Dioxide Associated with Geological Sequestration"
Geological sequestration is an emerging technology which holds promise for reducing carbon dioxide emission to the atmosphere from large point sources. Numerical modeling efforts are focusing on transport of supercritical CO2 in deep brine aquifers and potential escape pathways through the subsurface. Additionally, modeling of gas-phase transport of CO2 through unsaturated surface soils gives us the opportunity to assess possible ecological risks associated with leakage from sites of geological sequestration. The fluid mechanics and thermodynamics governing transport in varied subsurface settings are being carefully examined in order to accurately capture important physical and chemical phenomena. Accurate representations of the brine aquifer system require detailed knowledge of thermodynamics related to equilibrium partitioning and phase distributions. A detailed exploration of gas-phase fluid dynamics involving laboratory experiments and equation derivation indicates novel transport phenomena which may be important even in simple unsaturated soil systems.
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479 EBU-II
Dr. Markus Tuller
Departments of Plant, Soil & Entomological Sciences
and Biological & Agricultural
“Pore-Scale Structural Changes and Hydraulic
Properties of Clays:
A Fundamental Framework”
The shrink-swell behavior of clay minerals and associated changes in pore
space present a challenge to development of predictive models for hydraulic
properties of swelling porous media. The
present pore-scale study uses a comprehensive geometrical picture of the
arrangement and evolution of clay fabric mixed with other textural constituents
and linked to a physico-chemical model. Clay fabric is considered as an assembly of
colloidal-size stacks of lamellae (tactoids) whose
spatial organization is a function of clay hydration state. Silt and sand textural constituents are
represented as rigid spheres interspaced by clay fabric in two basic
configurations of "expansive" and "reductive" unit cells.
Bulk media properties such as porosity and surface area provide constraints for
the idealized geometry. Liquid retention
and liquid-vapor interfacial configurations within the pore space are
calculated. Saturated hydraulic conductivity functions are derived by solving Navier-Stokes equations for parallel plate and duct flow
regimes. Model predictions are compared
with experiments designed to determine shrink-swell behavior and hydraulic
properties of artificial mixtures.
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479 EBU-II
Dr. Mark W. Schmeeckle
Department of Geological Sciences
Florida State University
"Interaction of Turbulence and Sediment
Transport in Rivers"
Recent studies have detailed the presence and importance of large-scale, coherent turbulence structures in rivers. Failure to account for these structures often leads to poor prediction of the sediment transport field, because the sediment response to fluctuations in the near-bed fluid velocity is highly nonlinear. Previous models of sediment transport are based on time-averaged quantities of the turbulence (usually boundary shear stress), and, thus, cannot consider the coupling between turbulence structures and sediment transport. Results of several laboratory flume experiments will be presented in which velocity fields and the forces and motion of sediment particles were synchronously measured at turbulence resolving frequencies. Based on these experiments, several numerical models of bedload and suspended load transport, that directly incorporate the time-varying forces on grains by turbulence structures, will be presented. Because these models are based on the coupling between turbulence structures and sediment motion, they can be applied to turbulent flows in rivers which are generally complex and highly non-uniform.
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479 EBU-II
Dr. Steven G.
Buckley
Department of Mechanical Engineering
University of Maryland, College Park
"Combustion and Environmental Analysis Using Laser-Induced Breakdown Spectroscopy
(LIBS)"
The ability to conduct rapid
elemental analysis is useful in a variety of engineering settings, including
combustion / emissions analysis, materials processing, and hazard
analysis. Laser-Induced Breakdown
Spectroscopy (LIBS) is an emerging technique capable of sensitive elemental
analysis at relatively high data rates, up to 20 Hz with sensitive ICCD
detectors. In LIBS, a laser-induced
micro-plasma is formed, with peak temperatures on the order of 10,000 – 15,000
ºC. The plasma vaporizes solid material,
including complete vaporization of aerosol particles up to roughly 10 mm in diameter, and atomizes molecular
species. Atoms in the hot plasma may
become electrically excited, and relax as the plasma cools, emitting
characteristic atomic emission lines that may be used as a measure of elemental
concentrations.
This talk will focus on
combustion and environmental analysis applications of LIBS, specifically
discussing LIBS for real-time equivalence ratio measurements in engines and
real-time analysis of combustion generated and ambient particles. Experimental
configurations, calibration and quantification, and potential interferences
will be explored. Further examples from
materials science and the process industries will also be presented.
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479 EBU-II
Dr. Heinz Pitsch
Department of Mechanical Engineering
Stanford University
"Numerical Simulation of
Chemically Reacting Flows"
The numerical modeling of chemically reacting flows is of profound importance in the design of combustion engines, such as gas turbines or reciprocating engines, but also for the process control development of chemical reactors or the understanding of accidental fires. The most pertinent design parameters in technical processes are usually efficiency, stability, and emissions. Since these are very often counteracting, the challenge is the understanding and the simultaneous optimization of these parameters. Examples and modeling results will be given from various fields, such as the modeling of emissions from Diesel engines, development of reduced kinetic mechanisms for surrogate fuels describing auto-ignition in HCCI engines, modeling of combustion instabilities in stationary gas turbines, and the modeling of soot formation in turbulent flame and fires.
The second part of the presentation focuses on the development and validation of flamelet models for large-eddy simulation of turbulent combustion. Models for non-premixed, premixed, and partially premixed combustion will be presented and application to various test cases, such as the simulations of lifted turbulent diffusion flames and the simulation of a gas-turbine model combustor will be shown.
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479 EBU-II
Dr. Adonios N. Karpetis
Combustion Research Facility
Sandia National Laboratory
"Raman Spectroscopy in Flames: Two Extreme Examples"
Two experiments, centered around Raman spectroscopy, will be described. One of them was conducted at Yale University and examines the detailed structure of turbulent spray flames. The presence of the liquid drops makes this problem an “extreme” application of laser spectroscopy. By combining Raman thermometry with laser velocimetry, that experiment provides useful information on the possible similarity scalings that apply in burning, turbulent two-phase flows. The scalings of momentum, scalar and vapor source for the gas phase will be discussed, along with the effects of the second phase on the boundary-layer flow (usually referred to as two-way coupling). The second experiment is currently under way in Sandia National Labs, and involves the measurement (through Raman spectroscopy) of the scalar dissipation, as well as all major species mass fractions and temperature in gaseous turbulent flames. While the environment (gaseous flames) does not pose significant difficulties for laser spectroscopy, the experimental complexity that allows for measurement of scalar dissipation makes this case another “extreme” application of Raman spectroscopy. Doubly conditioned flamelets can be measured in turbulent flames using this technique. Some examples will be provided, showing the effect of local strain on the local state variables and the approach of a turbulent flame to extinction.
Adonios Karpetis graduated with a
Dipl. Ing. in Mechanical Engineering from Aristotle’s University in
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479 EBU-II
Joanna Austin
Graduate Aeronautical Laboratories
California Institute of Technology
"Cell Structure of Gaseous Detonation"
The propagation mechanism of detonation waves is not well understood. It has been known for some time that all unsupported detonation waves are unstable and unsteady. The leading shock front is perturbed by a system of transverse shock waves that periodically collide. This work is motivated by the lack of understanding of such fundamental issues as the collision process; the role of the transverse waves, if any, in sustaining the detonation; the propagation mechanism of detonation in different mixtures and the parameters that determine the behavior. Experimental visualizations of the wave front are combined with gas dynamic calculations and stability considerations. It is determined that the transverse waves are weak and play a minor role in mixtures with reasonably low heat release and activation energy. Detonation propagation in these mixtures occurs by shock-induced combustion behind the leading shock front. Other mixtures are found to exhibit a range of behavior including unstable shear layers, unreacted gas pockets, and turbulent-like structures. These features are indicative of more complex combustion mechanisms than are incorporated in traditional detonation models.
Joanna Austin is currently at the Graduate Aeronautical
Laboratories, Caltech, working with Prof. Joe Shepherd in the Detonation Physics
Laboratory. She expects to complete her
Ph.D. in March 2003. She received a M.S.
from Caltech in 1998, and concurrent B.E. (Mechanical and Space) and B.Sc. Mathematics) degrees in
1997 from the
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Information: (858) 534-0113