Fluid Mechanics Seminars
Abstracts 2002-2003
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479 EBU-II
Ann R. Karagozian
Department of Mechanical and Aerospace Engineering
“Experiments and Simulations of
Controlled Transverse Jets”
This talk will describe an experimental and computational study on the actively controlled gas jet injected transversely into crossflow, also known as the transverse jet. The transverse jet has widespread technological applications, ranging from dilution and fuel jet injection in gas turbine engines to thrust vector control systems for high speed aerospace vehicles. In our experimental studies, the jet actuation system consists of a loudspeaker placed within a plenum upstream of the jet nozzle. The dynamics of this actuator are characterized and modeled in the study, allowing a dynamic compensator or feed-forward controller to be developed which permits the jet to be forced in a more precisely prescribed manner. Ongoing studies on feedback control of jet excitation, in addition to the feed-forward control, allow for further improvement of the jet's temporal waveform. Use of the controllers allows for straightforward comparisons among different conditions for jet excitation. Clear identification can be made of specific excitation frequencies and characteristic temporal pulse widths which optimize transverse jet penetration and spread through the formation of distinct, deeply-penetrating vortex structures. Corresponding numerical simulations of the flowfield will also be described, wherein detailed examinations of the nature of vorticity generation are possible in addition to comparisons of simulation results with the experimental observations.
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479 EBU-II
Paul A. Libby
Professor (Emeritus) of
“THE INFLUENCE OF A THERMALLY
ACTIVE WALL ON PREMIXED TURBULENT COMBUSTION”
We consider combustion in a relatively simple flow, namely that arising from the impingement of a turbulent premixed reactant stream on a nearby wall whose temperature is controlled. We discuss first the virtues of this flow for fundamental studies and review some previous work related thereto. The influence of wall temperature on the combustion of reactants has not been investigated either theoretically or experimentally to date. To discuss this influence in a clear fashion we review first a variety of background issues; Favre averaging; the thermo chemistry and chemical kinetics of methane combustion involving element mass fractions, two scalars and a cross-over temperature; probability density functions and the k-epsilon model of turbulent transport. We then discuss in broad outline the set of transport equations describing the velocity and thermo chemical variables in the neighborhood of the axis, equations which involve two small parameters, one related to the relative intensity of the turbulence issuing from the jet and a second the reciprocal of the flow Reynolds number. We exploit the small magnitude of these parameters in applications and describe an asymptotic analysis in which both parameters are allowed to approach zero. There are identified three layers of flow with each layer described by a reduced set of equations. From representative solutions for each layer it is shown that the temperature of the wall can influence combustion only if the rate of strain, i.e., if the cross-over temperature, is sufficiently large as to cause the flame to be in the layer closest to the wall.
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479 EBU-II
“Fluid Dynamics in Combusting
Flows: Some New Results for Old Problems”
In the study of combustion certain fluid flows form an
integral part. For various reasons, prominent among them heat release and
gravity, these combusting flows are often considerably more complicated than
their pure isothermal-world fluid dynamics counterparts. Sometimes these
problems can be solved, more often they cannot. In certain cases, at least some
headway can be made, and simplifying approximations that are not possible can
be exploited to produce analytical solutions. I would like to discuss three
such problems.
The first is a recent solution technique that produces exact solutions of the Navier-Stokes equations in the limit as the crossflow Reynolds number vanishes. Many famous fluid mechanicians have studied problem of convective flow past a sphere. When the sphere is “blowing” the solution becomes somewhat simpler. I will discuss the theory of this flow.
The second problem comes from our recent work at Michigan State University (MSU) on simulated microgravity flame spread in a Hele-Shaw flow. Here we eliminate the influence of gravity by producing a mock “potential” flow between two parallel plates. The flow pattern has not completely been examined (in the combusting case) but preliminary measurements produce some very interesting results. Here I will mainly discuss experiments.
Finally, the third problem is an analytical/numerical problem involving the transition of a Rayleigh flow to a Blasius boundary layer flow. This problem was very popular in the 1950s and little work has been done on it since the early ‘70s, when numerical methods rendered such detailed theoretical pursuits “obsolete.” Nevertheless, this transition problem has many interesting features, and theoretical work, coupled with numerical analysis, is capable of answering important questions.
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479 EBU-II
“Pattern Formation in Diffusion
Flames Embedded in von Karman Swirling Flows”
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479 EBU-II
Stefan Llewellyn Smith
Department of Mechanical and Aerospace Engineering
“Tidal conversion”
Tidal conversion is the process by which energy is converted from the barotropic tide to internal gravity waves via flow over ocean bathymetry. These internal gravity waves are known as the "internal tide".
The internal tide propagates at a fixed angle to the vertical determined by the three fundamental frequencies: (1) the tidal frequency, (2) the Coriolis frequency and (3) the buoyancy frequency. The ratio of this slope to the topographic slope determines the strength of the conversion process. For shallow topographic slopes, simple estimates of the conversion rate are easily made using Fourier techniques. The conversion occurring over steeper topography is not yet fully understood.
We review research on tidal conversion from weak to strong topography, and examine the prospects for taking ocean measurements of bathymetry and using them to carry out forward tidal conversion calculations.
Questions of geophysical import include: How relevant is the weak topographic approximation to the real ocean? Is a spectral characterization of ocean bathymetry sufficient to compute conversion? How much resolution is enough? What are the upper and lower limits? Is spectral extrapolation a good strategy?
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479 EBU-II
Douglas MacMartin
Senior Research Fellow
California Institute of Technology
“Dynamics and Control of Shock Motion in a
Near-Isentropic Inlet”
Inlet pressure recovery of supersonic aircraft could be improved using a near-isentropic inlet with only a weak normal shock aft of the throat, however, such an inlet is highly susceptible to unstart. Small perturbations can move the shock ahead of the throat, where it is unstable. The dynamics of the inlet and shock are analyzed using a low order model that captures both the nonlinear shock motion and inlet acoustic propagation. This model allows parametric exploration of both the potential and limitations of using control to actively stabilize the shock, including actuator authority as a function of location, actuator authority and bandwidth requirements, and sensor requirements. A simple control law is shown to be sufficient to stabilize the shock motion. If time permits, I will briefly discuss recent (preliminary) results on feedback control of separation dynamics.
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479 EBU-II
“(Non)Faraday
Patterns on Vibrated Granular Layers”
A brief historical review is given of Faraday (1831) waves on vertically-vibrated liquid layers and similar patterned states found on the surface of shallow layers of non-cohesive granular media in vacuo. We then consider a simple continuum model of the type proposed by Bizon et al. (Phys. Rev. E, 85, 756, 1999). In this model, the granular layer is treated as a Newtonian liquid in flight, punctuated by solid-like rebound at the solid plate driving the layer.
A linear stability analysis indicates that the (Rayleigh-Taylor) mechanism underlying the Faraday instability in liquids is not operative in non-cohesive layers, which is contrary to the conclusions of previous work. The present analysis suggests that the patterns arise instead from non-linear collisional interactions between layer and plate.
Numerical solutions of an approximate "antiplane" solution to the problem for small viscosity, reveal localized "oscillon" structures that resemble those found experimentally. However, certain discrepancies among different types of numerical solution and experiment suggest that either the approximate solution or the model itself is in need of further refinement. Also, it is concluded that antiplane solutions cannot describe the so-called Faraday waves, which involve varicose "sloshing" modes.
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479 EBU-II
“Subgrid
Scale Dynamics and Simplified Two-Point Closure Modeling of Rayleigh-Taylor
Turbulence and Mixing”
In Part I of this presentation, the energy transfer process and the interaction of different scales in a flow induced by the variable-density Rayleigh-Taylor instability in miscible fluids is investigated using a three-dimensional DNS dataset with a resolution of 5122
2040. The method used to quantify the energy transfer
between the supergrid and subgrid
scales in the homogeneous planes, determined by partitioning the modes into
resolved and unresolved scales defined by a two-dimensional, artificial cutoff
wave number kc in Fourier space, is applied to
the kinetic energy evolution equation. Using a sharp Fourier cutoff filter, the
kinetic energy transfer is decomposed into the resolved part, a part
corresponding to the interaction between resolved and unresolved scales, and a
part corresponding to the interaction between unresolved scales. These z-dependent
spectra are computed for kc = 8, 16, and 32 to
investigate the dependence of the transfer process on the range of scales
contributing to the subgrid interactions. The kinetic
energy transfer is further decomposed into its positive and negative components
corresponding to the forward and backward cascades of energy, respectively. The
decomposition into resolved and unresolved scales is used to define an
effective eddy and backscatter viscosity. The principal conclusions are: (1)
the transfer spectra and eddy viscosities exhibit a strong dependence on kc; (2) the contributions from the interaction
between resolved and unresolved scales dominate the contribution to the total subgrid eddy viscosities and are responsible for the cusp
at large k/kc; (3) the contributions
from the interaction between unresolved scales dominates the contribution to
the total subgrid eddy viscosities at small k/kc and are responsible for the small, negative
contribution, and; (4) backscatter is strongest in the regions near the boundaries
of the mixing layer. The implications of these results for subgrid-scale
modeling in large-eddy simulation (LES) of Rayleigh-Taylor
instability-induced turbulence are discussed.
In Part II of this presentation, a simplified two-point statistical closure
model of turbulence is used to study the lowest order spectral dynamics of
incompressible Rayleigh-Taylor instability-induced
turbulence. In this single-fluid model, a prescribed source function given by
the linear instability growth rate is used to model the rate of energy input by
the instability and the energy removal by dissipation, and the eddydamped, quasi-normal Markovian
(EDQNM) model is used to model the transfer of kinetic energy due to nonlinear
interactions. Calculations are performed for a set of initial kinetic energy
and density variance spectra to investigate the evolution of turbulence spectra
and statistics from the linear regime through the weakly-nonlinear regime, and
finally, to a turbulent regime at late times. The spectral eddy viscosity and
backscatter are also studied. The application of this model for LES of Rayleigh-Taylor unstable flows with subgrid-scale
initial conditions is discussed. This
work was performed under the auspices of the U.S. Department of Energy by the
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479 EBU-II
Professor
Bud Homsy
“Novel Marangoni
Flows”
In this talk I will describe three recent studies of novel Marangoni flows, i.e. flows that are driven by tangential stresses that are produced by auxillary temperature, compositional, or electrical fields. The first two of these are flows driven or modified by the non-uniform in-situ production of surfactants by chemical reactions. Such surfactant gradients give rise to surface tension gradients which drive bulk flows.
We study experimentally the effect of such reactions on viscous fingering in the tip-splitting regime, finding that Marangoni stresses result in wider fingers and a suppression of the tip-splitting instability. We then describe an amazing phenomena of spontaneous, self-sustained chemically driven oscillations at the tip of a drop suspended from the tip of a needle and connect this phenomena to the well-known tip-streaming in extensional flow near drops. Finally, we describe theory and experiment on the manipulation of tangential electrical stresses to drive chaotic advection in translating drops of dielectric liquids.
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479 EBU-II
Professor Mohamed Gad-el-Hak
Virginia Commonwealth University
“Flow physics in Microdevices”
Interest in microelectromechanical systems (MEMS) has experienced explosive growth during the past few years. Such small devices typically have characteristic size ranging from 1 mm down to 1 micron, and may include sensors, actuators, motors, pumps, turbines, gears, ducts and valves. Microdevices often involve mass, momentum and energy transport. Modeling gas and liquid flows through MEMS may necessitate including slip, rarefaction, compressibility, intermolecular forces and other unconventional effects. In this presentation, I shall provide a methodical approach to flow modeling for a broad variety of microdevices. The continuum-based Navier-Stokes equations---with either the traditional no-slip or slip-flow boundary conditions---work only for a limited range of Knudsen numbers above which alternative models must be sought. These include molecular dynamics (MD), Boltzmann equation, Direct Simulation Monte Carlo (DSMC), and other deterministic/probabilistic molecular models. The present talk will broadly survey available methodologies to model and compute transport phenomena within microdevices.
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479 EBU-II
“Modeling of the Diffusion Process as
related to Stroke Imaging”
Important progress has been
achieved in several biomedical areas with essential contributions coming from
theory of porous media. Some of these features are reviewed in this talk. One
of these areas is related to Advanced Imaging, which has played an essential
role in clinical diagnosis such as Magnetic Resonance Imaging.
Diffusive-Weighted Magnetic Resonance Imaging has become a valuable clinical
imaging modality for the non-invasive detection and characterization of
cerebral ischemia compared to other conventional methods. Some of the
preliminary aspects in this area are discussed.
Also some preliminary aspects related to biochips with mechanical
detection systems are introduced and their main problems such as turbulence and
bimetallic effects are illustrated. Preliminary characterizations of these
problems as well as Methodologies for alleviating these features are discussed.
This will help in producing better measuring signals as well as in enhancing
detection capabilities.
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479 EBU-II
Professor Anutosh Moitra
Boeing Commercial Airplane
“Issues in 2-D High-Lift CFD Analysis: A Review”
The proposed talk will address lessons learned from a project recently completed at Boeing. The goal of the project was to develop an automated process for analyzing airplane high-lift systems using a Navier-Stokes CFD flow-solver on an unstructured grid system. High-lift flow-fields are characterized by very complex flow-physics. Consequently CFD analysis of high-lift configurations presents unique challenges in grid generation for multi-element airfoils and the ability of the flow solver to resolve complex multi-scale flow phenomena. The problem of high-lift analysis is rich in as yet unresolved issues and the associated CFD process is one of the most difficult of those being currently attempted by the CFD community. The mechanism through which the flow-fields for multi-element airfoils achieve high-lift is not completely understood. There are a number of competing and, in some instances, contradictory postulates that attempt to explain the high-lift phenomena arising from flow through slots between consecutive elements of the airfoil. Analysis of high-lift flows is further complicated by the need for automation of the CFD process for ease of routine use by high-lift design engineers. The aim of the proposed talk is to describe significant progress made in many of these areas as well as to identify issues that remain unresolved. The author hopes that these unresolved issues will be seen by the CFD community as an invitation for focused research and development, which will make routine, dependable, and accurate high-lift CFD analysis a reality in the near future.
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479 EBU-II
Professor Shihe Xin
LIMSI-CNRS, France
“Stability analysis of natural
convection flow in cavities”
Natural convection in
differentially heated cavities, since the pioneering work
by G. de Vahl
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479 EBU-II
Professor Paul Rightley
“The evolution of
shock-accelerated gaseous density interfaces and other fluid mechanical studies
at
I will present highly resolved
concentration and PIV measurements of two interacting Richtmyer--Meshkov-unstable gas cylinders. The heavy-gas (sulphur-hexafluoride) cylinders have an initial spanwise separation of S/D = 1.2 to 2.0, where D is the
cylinder diameter. The resulting flow morphologies are observed to be
highly sensitive to the separation, and vorticity
fields reveal that a principal interaction effect is the weakening of the inner
vortices of the system. We also observe a non-linear, threshold-type behavior
of inner-vortex formation around S/D = 1.5. A correlation-based
ensemble-averaging procedure is introduced, which permits decomposition of the
concentration fields into mean (deterministic) and fluctuating (stochastic)
components. This decomposition readily reveals a mixing transition in the
flow. These highly resolved data are used at
I will also present other examples of wide variety of experimentation being performed in the Dynamic Experimentation (DX) Division of Los Alamos to provide an experimental basis for our modeling efforts. DX Division is principally concerned with energetic materials research and development and understanding materials response to energetic materials.
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479 EBU-II
Alessandro
Gomez
Department of
“Gaseous and spray laminar diffusion flames interacting with toroidal vortices: inching towards turbulent combustion”
The interaction of a
laminar counter-flow diffusion flame with a toroidal
vortex injected in the mixing layer from either oxidizer or fuel side is a
canonical configuration, intermediate in complexity between steady laminar
flames and turbulent ones. As is well known to theorists, the controlling
parameter is not the easily measurable strain rate, but the scalar dissipation
rate, the two being proportional to each other only under steady-state
conditions, but not in time-dependent cases. The injection of a vortex
introduces an additional timescale in the problem, to be compared with the
other characteristic fluid timescales of the unperturbed flame and with the
chemical timescales. Under certain conditions, even edge flames phenomena, that are critical to flame stabilization, can be
observed in a natural way. In the case of spray flames, yet additional droplet
timescales appear, further contributing to the complexity of these flames. Key
findings of our group in these systems will be reviewed, with primary emphasis
on experiments, but including also some computational and theoretical results.
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479 EBU-II
Professor
Geert Schmid-Schonbein
UCSD Bioengineering
“Biomechanics of Microcirculation in the
Presence of Cell Activation: Application to Shock
and Multi-organ Failure”
In the past decades an increasing number of acute and chronic cardiovascular diseases (including coronary or cerebral ischemia and reperfusion, diabetes, hypertension, atherosclerosis) have been associated with enhanced levels of cardiovascular cell activation, a process that produces inflammation in the circulation. On one hand, inflammation is part of a protective mechanism in the circulation that is part of the immune response and wound healing. On the other hand, an excessive level of inflammation may have a direct influence on cellular properties and activities that impair the microcirculation and may lead to cell death and organ failure. Biomechanical analysis of inflammation opens the opportunity for development of new interventions against the deleterious effects of cell activation and for prevention. To achieve effective down regulation of the inflammation it is useful to know why cells are up regulated in the first place. Our analysis will be focused on the mechanisms that trigger cell activation.
In this presentation we
will focus on physiological shock, a condition with one of the highest
mortalities and with one of the most severe forms of cell activation.
Cell activation in this condition appears to be of non-genomic origin since it
is detectable within minutes after induction of shock. We will focus our
analysis on trigger mechanisms for inflammation and examine existing
hypotheses. We will trace the origin of inflammatory mediators in shock
and investigate the sequence of events leading to multiorgan
failure.
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479 EBU-II
David T. Leighton,
Jr.
Department of
“Hydrodynamic Dispersion in Micro-Fluidic
Geometries: Causes and Remedies”
Over the past decade microfabricated chip devices have emerged as powerful tools
for carrying out analytical scale separations. While these systems
significantly simplify analysis procedures, their separation efficiency is
largely limited due to analyte dispersion arising
from various sources. Many of these
arise from the hydrodynamics of the flow through the channels themselves,
including electrokinetic dispersion of solute
slugs in curved geometries, dispersion due to pressure driven shear flows, the
surprisingly large effect of side-walls in large aspect ratio channels, and the
effect of wall absorption in open channel chromatography. In this talk we
shall see how all of these phenomena may be understood as examples of Taylor-Aris dispersion. Further, detailed understanding of
these effects can show how appropriate microchannel
design can minimize such dispersion, reducing effective dispersivities
by an order of magnitude or more in many instances. Such reduction
directly translates into shorter required channel lengths and/or processing
times.
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479 EBU-II
Professor Kraig Winters
Scripps Institution of Oceanography
UCSD
“Stability of quasi-horizontal vortices in a
strongly-stratified fluid”
The dynamics of stably stratified turbulence governs the rate of vertical transport in naturally occurring stratified water bodies. Recent laboratory experiments have suggested that turbulent mixing events, unless provided with some external forcing mechanism, inevitably decay to a low Froude number regime in which stratification effects dominate. Recent experiments with collapsing turbulent wakes suggest that this "stratified-turbulence" regime is characterized by remarkably stable, quasi-horizontal vortex patches of alternating sign.
In this talk, I'll briefly
review some of the laboratory results, primarily to motivate a series of
"vortex-street" numerical experiments. Numerical results and scaling
arguments will be presented that suggest that the observed stability of
quasi-2d vortical motions in the laboratory may be a
low Reynolds number result.
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479 EBU-II
Professor
Allen Plotkin
Aerospace Engineering
San Diego State University
“Airfoil Ground
Effect Revisited”
The desire to predict aerodynamic forces during takeoff and landing gave rise to the classical set of problems in which a lifting surface translates steadily in close proximity to a solid boundary. The effect on the lifting surface forces due to the presence of the boundary is known in the literature as “ground effect”. There has been much recent interest in ground effect applications in automobile aerodynamics. Some major open questions relate to viscous effects for small values of ground height.
In this presentation, the incompressible potential flow past an airfoil in ground effect is studied both numerically and analytically. Discrete vortex and linear vortex panel methods are applied to a parabolic arc and symmetric Joukowski airfoil, respectively. Analytical solutions, valid either near or far from the ground, are compared with the numerical results. The numerical results are used to delineate the separate influences of angle of attack, camber and thickness.
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479 EBU-II
Swedish DRA (FOI)
“Optimal
control and estimation applied to three-dimensional boundary layers”
An overview of the activities in flow control at Mechanics, KTH and FOI is given with emphasis on application to three-dimensional boundary layers. First work is presented that extends previous research on linear controllers and estimators in temporal channel flow to spatially evolving boundary layer flow. Falkner--Skan--Cooke velocity profiles are used as the base flow in the Orr--Sommerfeld--Squire equations to compute the optimal feedback control through blowing and suction at the wall utilizing linear optimal control theory. Estimators using only wall measurements are computed in a similar manner and used in compensators applied to the same flow. The controllers and compensators are used in direct numerical simulations of the Navier-Stokes equations and it is shown that random cross-flow disturbances and wave packets can be stabilized. Second work is presented that describes the use of optimal control theory to extend the laminar region on aircraft wings by steady suction. An iteration procedure based on the boundary layer equations; parabolic stability equations and their adjoints are used to find the optimal suction distribution. Results for Fokker 100 and Airbus 310 wings are shown.
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479 EBU-II
Prof. Steven L. Ceccio
“Hydrofoil Trailing Edge Vortex Shedding
at High Reynolds Number”
An important hydroacoustic
noise source from a fully-submerged noncavitating
hydrofoil is the unsteady separated turbulent flow near its trailing edge. Here, hydroacoustic
noise may be produced by boundary layer turbulence scattered from the foil’s
trailing edge, and by the periodic formation of vortices in the near-wake. Such vortex shedding may generate an
energetic tone that rises above the broadband trailing-edge noise. This presentation describes results of an
experimental effort to measure the major flow features in the near-wake of a
high Reynolds number hydrofoil with trailing edge vortex shedding. The experiments were conducted at the US Navy's
William B. Morgan Large Cavitation Channel with a
two-dimensional hydrofoil (2.1 m chord, 3.0 m span) at chord-based Reynolds
numbers from 0.5 to 60 million. Two trailing edge shapes producing differing
strengths of vortex shedding were investigated.
Measurements include time-averaged and unsteady surface pressures, and
LDV and PIV derived velocity fields, and hydrofoil vibration. The dependence of the shedding strength on
changes in Reynolds number and trailing edge geometry were investigated. The results indicate a correlation between the
energy contained in the vortex shedding and the time-averaged shear of the
boundary layers approaching the trailing prior to separation at the trailing
edge bevel. These results have
implications for both the passive and active control of the shedding strength. Additionally, a brief discussion of our
upcoming high Reynolds number friction drag reduction experiment will be
briefly discussed where we will examine the use of micro-bubble gas injection
to reduce friction drag on submersed surfaces.
[Sponsored by Code 333 of the Office of Naval Research,
N00014-99-1-0341, N00014-99-1-0856, N00014-01-1-0880].
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479 EBU-II
Professor Donald Dabdub
“Mathematical Modeling of Air Pollution”
The focus of this talk is to describe the physical and chemical processes that affect the dynamics of pollutants in the atmosphere. Scientific studies have shown that human exposure to high ozone concentrations can impair lung functions in people with existing respiratory problems, and can cause chest pain and shortness of breath even in the healthy population. Furthermore, air pollution produces undesirable effects on animals, vegetation, and materials. The ozone problem raises serious concerns, as many urban areas around the world have recorded unhealthy levels of pollutants. Abatement planning is complex. Several key aspects such as emissions control are best addressed through the use of ambient air quality models. This talk will describe recent advances in our knowledge of atmospheric sciences, through the use of advanced computer systems and new developments of mathematical air quality models.
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479 EBU-II
Professor Mark Blyth
“Chaotic flow
in a pulsating pipe”
Unsteady
motion of a viscous incompressible fluid inside a cylindrical tube whose radius
is changing in a prescribed manner is examined. A class of exact solutions of
the Navier-Stokes equations is constructed in the
case when the vessel radius is a function of time alone so that the
cross-section is circular and uniform along the pipe axis. The flow is
categorized by two dimensionless parameters based on the forcing amplitude and
frequency respectively. At low frequencies, for a fixed
amplitude, the flow is synchronous with the wall motion, but as the frequency
increases a Hopf bifurcation is eventually
encountered beyond which periodic, quasiperiodic or
chaotic solutions occur. Most of the solutions presented are numerical,
although some analytical progress is possible in a small
amplitude, high frequency steady streaming limit. The axisymmetric
flow exhibits some marked differences from its two-dimensional counterpart in a
pulsating channel, previously studied by Hall \& Papageorgiou.
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479 EBU-II
Professor Carl
Gibson
Mechanical and
Aerospace Engineering
“Can the
Russians detect a submerged Hawaiian sewage outfall from a space satellite?”
We measured ocean microstructure as sea truth to compare with Russian claims that they could infer the location of submerged remnants of active turbulence far from the diffuser source using (a) optical properties of the sea surface inferred from satellite and helicopter images, and (b) a snapshot taken from the space station by one of Academician Bondur's students who happened to be overhead on September 6th while we were making our microstructure and surface wave spectral measurements.
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479 EBU-II
Professor Paul Linden
Mechanical and
Aerospace Engineering
"Gravity currents: the legacy of
von Karman"
In an influential article written
in 1940 on nonlinear dynamics, von Karman derives a
result for the speed of a gravity current. This
result, which comes from an application of Bernoulli’s equation, predicts that
in a deep environment, the dimensionless speed, the Froude
number F, is √2. In 1968, in
what has become a definitive study of gravity currents, Benjamin argues that
energy dissipation is an essential feature of gravity current dynamics. He
concludes that von Karman’s application of
Bernoulli’s equation is incorrect, and then provides an alternative argument
that gives the same theoretical value of F
= √2. Laboratory measurements give smaller values, typically F ≈ 1.2, and the slower speed of
the current giving this discrepancy with the theoretical value is usually
attributed to dissipation.
I will discuss a new theory of energy-conserving currents, applicable to flows generated by the release of dense fluid from a lock. I will show that Benjamin’s interpretation of energy dissipation is incorrect, and that his results should be discussed in terms of energy and momentum transfers by waves along the top of the current. So it seems that von Karman was right to apply Bernoulli’s equation after all. But he was not, and I will show that the correct value for the Froude number in a deep environment is F = 1. Finally I will present new experiments that confirm this theoretical value and I will explain why previous experiments have found larger values of F.
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For information: Bruno Etchepare at (858) 534-6029