Seminars

Chris Hogan
Dept. of Mechanical Engineering
University of Minnesota

Abstract:

Aerosol nanoparticles are found ubiquitously in the earth’s atmosphere, though for most of human history, the inhalation of these particles has had a negligible influence on human mortality.  Recent technological advances, however, have led to (1) an increase in the number of nanoparticles an individual inhales over his/her lifetime, and (2) changes in the physical and chemical properties of inhaled particles.  There is therefore concern over the presence of aerosol nanoparticles in the environment, and there is a need to better understand aerosol particle growth rates and transport.  This talk focuses on recent developments in understanding the growth of aerosol nanoparticles by mass transfer, i.e. the rate of uptake of vapor by particles and the growth of particles via particle-particle collisions, momentum transfer from gas molecules to nanoparticles (drag), and the ionization of aerosol particles via collisions with atmospheric ions.  These three processes play critical roles in controlling particle concentrations in the environment, the design of collection and filtration systems to remove particles from air streams, and the design of instrumentation to detect and analyze aerosol nanoparticles. 

Our approach to examine mass and momentum transport to aerosol particles combines traditional engineering dimensional analysis with mean first passage time and direct simulation monte carlo (DSMC) calculations.  There are two main challenges in these analyses.  First, at atmospheric pressure, the hard-sphere mean free paths of gas molecules in air and the mean persistence distances of moving entities in an aerosol are on the order of tens to hundreds of nanometers, similar in size to atmospheric aerosol particles.  For this reason, neither classical continuum approaches nor free molecular mechanics can be used to evaluate mass and momentum transfer rates in aerosol systems; at atmospheric pressure aerosol nanoparticles exist in mass and momentum transfer transition regimes.  Second, aerosol particles produced in high temperature (combustion) processes are rarely spherical, and are more appropriately described as quasifractal agglomerates composed of primary spherules.  A complete theory describing mass and momentum transfer in aerosol systems must account for the morphological diversity of real particles.  Through dimensional analysis and mean first passage time calculations we are able to infer a mass transfer rate (collision rate) for vapor molecules to particles as well as between particles, for particles of both arbitrary size and shape.  We are further able to develop an expression for the low Reynolds number momentum transfer rate (friction factor) of arbitrarily sized and shaped aerosol nanoparticles.

George Lauder
Professor, Harvard University

Abstract:
There are over 28,000 species of fishes, and a key feature of this remarkable evolutionary diversity is a great variety of propulsive systems used by fishes for maneuvering in the aquatic environment. Fishes have numerous control surfaces (fins) which act to transfer momentum to the surrounding fluid. In this presentation I will discuss the results of recent experimental kinematic and hydrodynamic studies of fish fin function, and their implications for the construction of robotic models of fishes. Recent high-resolution video analyses of fish fin movements during locomotion show that fins undergo much greater deformations than previously suspected and fish fins possess an clever active surface control mechanism. Fish fin motion results in the formation of vortex rings of various conformations, and quantification of vortex rings shed into the wake by freelyswimming fishes has proven to be useful for understanding the mechanisms of propulsion. Experimental analyses of propulsion in freely-swimming fishes have led to the development of a variety of self-propelling robotic models: pectoral fin and caudal fin (tail) robotic devices, and a flapping foil model fish of locomotion. Data from these devices will be presented and discussed in terms of the utility of using robotic models for understanding fish locomotor dynamics.

Tina Katopodes-Chow
Associate Professor
Civil & Environmental Engineering
University of California, Berkeley

Abstract:
This presentation will describe challenges that arise when simulations of the atmospheric boundary layer are performed at higher and higher grid resolution. Improved parameterizations for turbulence allow representation of intermittent turbulence that occurs under moderate to strong stable stratification. Implications for boundary layer predictions important for wind energy applications are discussed. New techniques for representing complex topography such as steep mountains and buildings are also described, allowing mesoscale simulations to move from regional to urban scales.

Krishnan Mahesh
Aerospace Engineering & Mechanics
University of Minnesota

Abstract:
Jets in crossflow are central to a variety of applications; e.g. dilution jets in gas--turbine combustors, film--cooling, and fuel injection. This talk will discuss our work on the simulation and control of passive scalar mixing by turbulent jets in crossflow. We have developed an analytical scaling for jet trajectory that accounts for jet velocity profile and crossflow boundary layer thickness. Also direct numerical simulation has been performed under conditions corresponding to recent experiments. The simulation results will be used to propose physical mechanisms for entrainment and mixing. A simple model that explains jet deformation as a result of acceleration imposed by the crossflow will be discussed. The tak will then discuss the control of jets in crossflow using pulsing. The main idea pursued here is that pulsing generates vortex rings and the effect of pulsing can therefore be explained by studying the behavior of vortex rings in crossflow. A regime map is proposed that collapses optimal conditions from experiments.

Jeremy Venditti
Simon Fraser University
Burnaby, BC, Canada

Abstract:

The beds of alluvial river channels become finer grained moving downstream and often exhibit an abrupt transition from gravel to sand-bedded conditions. Most previous work documenting this phenomenon have focused on small upland streams where sediment supply to the channel is strongly connected to sediment delivery from hillslopes. Fewer studies have focused on the gravel-sand transition in large alluvial channels and none have documented the spatial variability through reaches where transitions occur. The downstream fining pattern observed in the Fraser River is widely cited as a classic example of an abrupt gravel-sand transition in a large alluvial channel. However, important questions regarding the exact current location of the transition, its morphology, and what controls its location remain unanswered.

Here, I present detailed observations bed material grain-size, river bed topography and fluid flow through the 15 km long reach where the transition is widely thought to occur in the Fraser River. Bed topography was measured using a multibeam echo-sounding system (Reson 8101 Seabat) at high flow (11,000 m3s-1) when all fractions of the bed material were mobile. Fluid flow and suspended sediment transport patterns were also mapped using an ADCP at 5 different flow stages during an annual snowmelt hydrograph. These observations indicate that there is a gravel front that occurs in the river at Yaalstrick Bar, the last bar along the river dominated by gravel (> 75% of the bar material > 2 mm). However, sorting patterns caused by the superior mobility of gravel over sand have lead to gravel patches on the upstream sides and surfaces of sand bars. There are also gravel patches along the thalweg through the apex of some river bends. Bedforms associated with sand-gravel mixtures appear on the river bed immediately downstream of Yaalstrick Bar in a sequence (sand ribbons, barchans, dunes) suggesting sand deposition from suspension. There is also a dramatic increase in bar amplitude downstream of Yaalstrick Bar, suggesting greater sand composition. Our fluid flow and sediment transport measurements do not indicate any significant downstream shear stress gradient at high flows, but there is more sand moving as bedload and suspended load in the sand-bedded part of the river. This can only happen if the sand supply from the gravel-bedded part of the river is intermittent. This implies that sediment dynamics in this transition are dominated by sand storage in the gravel-bedded reach at low flows and downstream release to the sand-bedded reach during large floods.

Roman Stocker
Associate Professor,
Massachusetts Institute of Technology

Abstract:

It is now widely recognized that microbial activities represent one of the main forces shaping biogeochemistry and productivity in the ocean. At the level of individual microbes, the ocean is a sea of gradients. Chemical gradients define heterogeneous resource landscapes, while flow gradients exert forces and torques on organisms. Our understanding of these interactions - both chemical and fluid mechanical - has been hampered by the difficulty of studying microbial behavior at appropriate spatiotemporal scales. Modern microfluidic and millifluidic tools afford unprecedented access to this microscale world. I will show how these approaches can help shed light on microbial behavior in both resource gradients (chemotaxis) and flow gradients (gyrotaxis).

Leonardo Chamorro
Research Associate,
Saint Anthony Falls Laboratory, U of MN

Abstract:

Wind power is one of the most abundant and easily accessible sources of clean and renewable energy on the planet. In line with wind power another promising renewable energy source that is gaining acclaim is the guaranteed flow of river, tidal, and ocean currents. These can be converted to energy via marine hydrokinetic devices and has an incredible potential to fill the ever increasing demand for energy. In spite of the valuable efforts to date, fundamental problems related to the flow and structure interaction, scale dynamics, power maximization, structural reliability, environmental assessments, among others, persist. Continued research geared towards properly addressing these issues is necessary if we are to efficiently expand and capitalize on these vast sources of energy.

In the first part of this presentation, I will present some of the most recent insights obtained from wind tunnel experiments carried out at St. Anthony Falls Laboratory. This research included uses of different sizes and numbers of model wind turbines. The focus of these tests was placed on understanding the complex mechanisms of the flow/structure interaction; tip vortices stability, drag reduction, scalability of the problem.

Secondly I will present preliminary research collected on hydrokinetic turbines. The focus of this research is placed on the conceptual similarities and differences of the hydrokinetic turbines and their wind counterparts, and the turbine’s unsteady response with energetic coherent turbulent structures.

In conclusion these studies have provided valuable information pertaining to the turbulent flow/structure interaction needed to improve the design of wind and hydro turbines. Also, this information is being used to test and guide the development of improved parameterizations of wind turbines in high-resolution numerical models, such as large-eddy simulations (LES). The applications of this research are far reaching and important in increasing the viability and production of wind and hydrokinetic turbines, an increase that is necessary if they are to be implemented on a large enough scale to significantly generate a portion of the energy needed.

Brad Gemmell, Ph.D.
Visiting researcher from Woods Hole
Oceanographic Institution, Boston
Presently working with Ellen Longmire, Dept. of AEM, U of M.

Abstract:
opepods are found in virtually all marine environments. They provide a key link in marine food webs between photosynthetic algae and higher trophic levels. As a result copepods have evolved a powerful escape behavior at all stages of development, in response to hydrodynamic stimuli created by an approaching predator. Young copepods are strongly influenced by viscous forces and may be at a disadvantage when exposed to larger predators at cold temperatures. Results show that the nauplius exhibits a compensatory mechanism to maximize escape performance across its thermal range. Some species have developed unique mechanisms to avoid predation such as breaking the water surface and making aerial escapes to avoid predators while in other cases, the predator has developed unique morphology in order to reduce the amount of hydrodynamic disturbance in the water which improves capture success of copepods.

Richard Christopher, St. Anthony Falls Laboratory Safety Officer, will present the Safety Seminar. 

Preceeding that will be introductions by and of all SAFL Faculty, Staff, and students.

Attendance at this seminar is mandatory for all who work and/or study at SAFL. If you cannot make it, you must notify both Richard Christopher and your Advisor. There will be a sign-in - sign-out sheet at the door. 

Raleigh L Martin
 PhD Student University of Pennsylvania, Department of Earth & Environmental Science
 

Abstract: Field and lab studies indicate that bedform geometries lag changes in flow through floods, producing hysteretic relationships between bed morphology, roughness, and water discharge.  Disequilibrium between bedform geometries and flow parameters complicates our ability to interpret stratigraphy for paleoenvironmental reconstruction.  This summer, I am conducting experiments in the SAFL Tilting Bed Flume to explore this bedform hysteresis.  In these experiments, repeat sonar scans are used to continuously track the response of sand bedform morphologies to abrupt changes in water discharge.   The timescale of bedform adjustment appears to be driven by three primary factors: 1. directionality of adjustment, 2. preexisting bedform geometry, and 3. sediment flux.  Directionality of adjustment (rising versus falling water discharge) determines whether bedforms grow quickly by irreversible merger (rising flows) or shrink slowly through secondary bedform cannibalization of relict larger bedforms (falling flows).  Preexisting bedform geometry (height and length) determines the amount of bed deformation required for adjustment to new equilibrium, and sediment flux determines the rate at which this change is effected.  These three factors all favor faster adjustment of bedforms to rising flows.  I will present preliminary results on bedform adjustment hysteresis for a variety of increasing and decreasing discharge changes.