Seminars

Katsumi Matsumoto, Professor, Department of Earth Sciences, University of Minnesota

Lake Superior is the largest lake in the world by surface area and provides essential resources to the surrounding cities. Yet there are a number of gaps in knowledge regarding its physical and biogeochemical properties. Here I use a ROMS-based numerical model of Lake Superior in an attempt to fill some of those gaps.

Christopher Zappa, Lamont Associate Research Professor, Lamont-Doherty Earth Observatory, Columbia University

Poor understanding of the complex physical controls of air-sea exchanges under high winds, in particular with respect to uptake and release of greenhouse gases, remains one of the major uncertainties in biogeochemical models and climate predictions. Adequate characterization of gas transfer across the air-sea interface is not only essential to quantify local and global sinks and sources of CO₂ but also to budget many other trace gases that influence Earth’s radiation.

At high wind speed, breaking waves become a key factor to take into account when estimating gas fluxes. Breaking results in additional upper ocean turbulence and generation of bubble clouds. Efforts have been made towards including the effect of bubble mediate transfer to reduce the uncertainties around gas transfer velocity K estimates at high wind speed. These parameterizations model the transfer velocity due to breaking as a function of fractional whitecap coverage (W) or windsea Reynolds numbers.

Limitations in these gas transfer models arise from the large scatter in W parameterizations that is not yet fully understood. Recent work has focused on linking W variability to wave field statistics in addition to wind speed. Other limitations arise from the lack of observations at high wind speed. While gas transfer coefficients and W have regularly been obtained under winds of up to ~15 m s^-1, few gas transfer measurements at wind speeds of 15 to 30 m s^-1 exist, and almost none with coincident wave physics observations.

The High Wind Gas exchange Study (HiWinGS) offers a diverse data set that presents the unique opportunity to gain new insights on the poorly understood aspects of air-sea interaction under high winds.  The HiWinGS cruise took place in the North Atlantic during October and November 2013. Wind speeds exceeded 15 m s^-1 25% of the time amounting to a total of 189 hours of wind speeds above 15 m s^-1 of which 48 hours wind speeds greater than 20 m s^-1.  On October 25^th, wind speeds exceeded 25 m s^-1 with gusts of 35 m s^-1 during the St Jude storm. Continuous measurements of turbulent fluxes of heat, momentum and gas were taken from the bow of the R/V Knorr. Visible imagery was acquired from the port and starboard side of the flying bridge during daylight hours at 20Hz and directional wave spectra were obtained when on station from a wave rider buoy. Additional wave field statistics were computed from a laser altimeter as well as from a Wavewatch III hindcast.

Taking advantage of the range of physical forcing and variable wave conditions sampled during HiWinGS we investigate how W and K vary with sea state, contrasting pure windseas to swell dominated periods. We distinguish between wind seas and swell based on a separation algorithm applied to directional wave spectra as described in [Hanson and Philips, 2001]. For mixed sea, system alignment is considered when interpreting results.  

The role of bubble-mediated transfer depends on gas solubility. The four gases (CO₂, DMS, acetone, and methanol) sampled during HiWinGS ranged from being mostly waterside controlled to almost entirely airside controlled. While bubble-mediated transfer appears to be small for moderately soluble gases like DMS, the importance of wave breaking turbulence transport has yet to be determined for all gases regardless of their solubility. This will be done by correlating measured gas transfer velocities to estimates of active whitecap fraction (W_A) and turbulent kinetic energy dissipation rate (ε). W_A and ε are estimated from moments of the breaking crest length distribution derived from the imagery, focusing on young seas, when it is likely that large-scale breaking waves (i.e., whitecapping) will dominate the TKE dissipation rate.

Trung Le, Postdoctoral Associate, St. Anthony Falls Laboratory, University of Minnesota

The mixing process of two branches in a river confluence is highly dynamic and can exhibit two flow modes depending on the momentum ratio: i) Kelvin-Helmholtz mode and ii) wake mode. In this work, we study the dynamics of a river confluence on Mississippi River branch in the city of Minneapolis, Minnesota, United States. Field measurements by Acoustic Doppler Current Profiler using on-board GPS tracking were carried out for five campaigns in the summer of 2014 and 2015 to collect both river bed elevation data and flow fields. High fidelity simulation (Large Eddy Simulation) is carried out to simulate the flow field with the total of 100 million grid points for the domain length of 3.2 km. The simulation results agree well with field measurements at measured cross-sections and reflect a complex interaction between two upstream branches at the confluence. The results show the existence of wake mode on the mixing interface of two branches near the upstream junction corner. The mutual interaction between the shear layers emanating from the river banks leads to the formation of large-scale energetic structures. The existence of these structures induce the emergence of  "switching mode", which enhances the lateral transfer of momentum and promote mixing. Our result here is a feasibility study for the use of eddy-resolving simulations in predicting complex flow dynamics in medium-size natural rivers.

Lorenz G. Straub Award Ceremony and Seminar: Dr. Stephen Monismith, Obayashi Professor in the School of Engineering and Senior Fellow at the Woods Institute for the Environment, Stanford University

2013 Lorenz G. Straub Award Recipient: Esther Eke

The problem of predicting how the salinity field in estuaries responds to freshwater inflows is one that draws attention from both physical oceanographers and hydraulic engineers since it has both scientific and practical dimensions. In Northern San Francisco Bay, examination of 20+ years of data spanning the estuary shows that the overall structure of the salt field can be described using a single parameter, X2, the distance in km measured from the Golden Gate Bridge along the channel thalweg to where the salinity on the bottom is 2. Analysis of long-term monitoring of biological data (e.g. fish abundance) shows that much of ecological functioning of the estuary depends on X2 and so regulations have been developed specifying X2 position depending on time of year and hydrologic conditions. Because these regulations can require substantial amounts of water, it is necessary to efficiently predict the behavior of X2 with some accuracy so as to help manage the competing demands for California’s limited water supply.

In this talk, using several data sets including one that goes back to ca. 1960, I will discuss the observed behavior of X2 and how it responds to flow, Q. In general, the tendency of freshwater flows to carry salt out  of the estuary is balanced by the tendency of dispersion to move salt upstream. A surprising aspect of the X2-Q relation in Northern San Francisco Bay is that it is much weaker than would be inferred from classical estuarine circulation theory, behavior that we attribute to the effects of stratification on the turbulent flows that support upstream salt flux. I will present a rigorously derived but simplified model of salinity dynamics that can be used to understand this behavior and that can be used to create a dynamically based (rather than purely empirical) model of unsteady salinity intrusion. Finally, examination of the relevant data also suggests that inability to accurately measure freshwater flows during relatively dry periods may be a bigger limitation on accurate predictions of low-flow behavior than is choice of model structure.

Alexander Densmore, Professor, Institute of Hazard, Risk, and Resilience and Department of Geography, Durham University

India, the largest agricultural user of groundwater in the world, has seen a revolutionary shift from large-scale surface water management to widespread groundwater abstraction in the last 40 years, particularly in the northwestern states of Punjab, Haryana and Rajasthan. As a result, these states are now a hotspot of groundwater depletion, with the largest rate of groundwater loss in any comparable-sized region on Earth. Despite this, there is no integrated view of the aquifer system in northwestern India, no detailed understanding of the decline in groundwater levels, and no regional-scale conceptual framework with which to understand these changes and forecast the evolution of the system.

In this talk, I describe our efforts to address these shortcomings, using the sedimentological and geomorphological framework of the aquifer system as a guide to its behavior. Groundwater in northwestern India is largely hosted within ancient, buried, sandy former river channels deposited by aggrading, avulsive fan systems. To understand the response of this aquifer system to future stresses, we must first understand its geology and geometry – the locations, sizes, and characteristics of the channels, their ages, and their three-dimensional pattern. The geomorphology of the fan systems provides a first-order framework for assembling and relating data on the aquifer system and its properties. In particular, the geomorphological framework determines key aquifer properties, such as the thickness, proportion, and stacking pattern of sandy channel beds. These properties vary in predictable ways across the study area, and this variation can be used to make testable hypotheses about aquifer characteristics in areas with no existing well data or subsurface information. The detailed pattern of water-level change is influenced by this geomorphological framework, in conjunction with district-level patterns of abstraction and recharge. Future large-scale assessments of aquifer characteristics and groundwater sustainability should adopt a similar framework, so that the widest possible range of surface and subsurface data can be integrated and understood.

WIlliam Anderson, Assistant Professor, Department of Mechanical Engineering, University of Texas at Dallas

High Reynolds number rough wall turbulent flows are ubiquitous in engineering and geophysical flows. Turbulent momentum transport influences the aero-/hydro-dynamic signature of bluff bodies and the performance of vapor power systems; in geophysical flows, turbulent mixing impacts urban dispersion, the hydrologic cycle, and sedimentary processes in fluvial/aeolian systems. Recently, it has been shown that spanwise topographic heterogeneity can induce a mean domain-scale (δ) circulation. We demonstrate that these circulations are Prandtl’s Secondary Flow of the Second Kind: sustained and driven by spanwise—wall-normal heterogeneity in the Reynolds stresses (all of which vanish in the absence of spanwise topographic heterogeneity). These findings are supported by large-eddy simulation (channel flow: Anderson et al., 2015: J. Fluid Mech.) and experimental measurement (boundary layer: Barros and Christensen, 2014: J. Fluid Mech.) Mejia-Alvarez and Christensen, 2013: Phys. Fluids termed the resulting heterogeneity in spanwise—wall-normal streamwise velocity low- and high-momentum pathways (in order to draw distinction against low- and high-momentum regions – LMR, HMR – which are a spatially meandering, transient feature of wall turbulence). This work has prompted closer inspection on how mean secondary flows alter the structural attributes of LMRs and HMRs. Results have demonstrated that the inclination angle of coherent structures is steepened within high-momentum pathways (i.e., the hairpin packet paradigm is preserved, but altered, due to turbulent secondary flows). We have also investigated how spanwise spacing, s, between topographic heterogeneities influences turbulent secondary flows, finding that s/δ > 2 is a necessary condition for formation of δ-scale mean circulations (i.e., δ-scale circulations can be attenuated by interaction with adjacent circulations). In other work, we have explored the presence of an “amplitude modulation” effect of the roughness sublayer by inertial layer LMRs and HMRs; we have shown that periods of momentum excess(deficit) in the inertial layer precede periods of elevated(depressed) streamwise—wall-normal Reynolds shearing stress in the roughness sublayer. This work is inspired by Marusic et al., 2010: Science, who showed that LMRs and HMRs in the logarithmic region of smooth wall turbulent boundary layers exhibit an amplitude modulation of the viscous wall region. A decoupling procedure presented by Mathis et al., 2009: J. Fluid Mech. is used to illustrate that an amplitude modulation effect is indeed present for rough wall flows. Finally, we present results from LES of neutrally stratified atmospheric boundary layer flow over a sparsely vegetated, arid landscape. On such landscapes, aeolian erosion is induced (and sustained) by kinetic energy fluxes in the aloft surface layer. Conceptual models typically indicate that sediment flux, q (via saltation or drift), scales with imposed aerodynamic stress raised to some exponent, n, where n > 1. Since aerodynamic stress (in fully rough, inertia-dominated flows) scales with incoming velocity squared, u², it follows that q ~ u²ⁿ (where u is some relevant component of the flow, u(x,t)). Thus, even small (turbulent) deviations of u from its time-averaged value may play an enormously important role in aeolian activity. In order to illustrate the importance of surface stress intermittency, we have used conditional averaging predicated on aerodynamic surface stress during LES (where threshold selection is guided by probability density functions of local surface stress). This averaging procedure provides an ensemble-mean visualization of flow structures responsible for erosion “events”. Preliminary evidence indicates that surface stress peaks are associated with the passage of inclined, high-momentum regions flanked by adjacent low-momentum regions.

Dr. Pavlos Vlachos, Professor, School of Mechnical Engineering and School of Biomedical Engineering, Purdue University

It is estimated that almost one third of the population above the age of forty-five can be suffering from heart diastolic dysfunction, yet appear to have no symptoms. Heart diastolic dysfunction is characterized by the impaired filling efficiency of the heart left ventricle. However, heart compensatory mechanisms and remodeling make diagnosis very difficult as well as introduces great challenges that limit our understanding and hinder the use of experimental methods or computational modeling for investigating the associated physics in-vitro. In this talk, we will explore the flow physics of the left ventricle filling using clinical and laboratory tools and by combining flow physics with medical imaging and clinical pathophysiology. Using Color M-mode Echocardiography (CMM) and phase-contrast Magnetic Resonance Imaging (pcMRI) velocimetry we characterize in-vivo the fluid dynamics in healthy and diseased patients. Based on such data we will show how the vortices forming in the heart evolve and how they play an active role in the presence of disease. We will show mechanisms associated with the filling of the heart that were previously unexplored or even not recognized and will demonstrate how this knowledge can be used not only to understand the processes but classify healthy from diseased patients and lead to new diagnostic tools with significantly improved classification ability and clinical utility. Finally, during the last part of the talk we will survey other research projects pursued in our group such as arterial flows, engineering of tumor micro-environments, flows in insect hearts, aerodynamics of flying snakes, and developments in instrumentation and measurement science.

Jiarong Hong, Benjamin Mayhugh Assistant Professor, Department of Mechanical Engineering and St. Anthony Falls Laboratory, College of Science and Engineering, University of Minnesota

Naturally-formed biological and geological surfaces provide a well of inspiration for fluid engineering applications. Our research work is focused on understanding the connection between turbulent flow and the formation and the function of various types of surface roughness. Specifically, we are investigating experimentally on how the variation of shark skin denticle morphology affects near-wall hydrodynamics and how the small-scale dynamics involved in turbulence-sediment interaction contributes to the formation of different bedforms. Our experiments use digital holographic microscopy, which is a 3D imaging technique suitable for quantifying fluid dynamics with high resolution at small scales. In this talk, I will present a general framework of this research, a description of our experimental setup and some preliminary data, followed by a discussion on challenges and future work.

Gordon Grant, Research Hydrologist, USDA Forest Service, Pacific Northwest Research Station and Courtesy Professor, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University

The western US is emerging as one of the more vulnerable regions to climate change, with changes in the magnitude, seasonality, and timing of streamflow among the most sensitive variables.  As the climate warms, diminishing snowpack and earlier snowmelt will cause reductions in summer streamflow, trends that are already apparent in the hydrologic record. Peak winter streamflows are also expected to change.  Most regional-scale assessments of climate change impacts on streamflow use downscaled temperature and precipitation projections from general circulation models (GCMs) coupled with large-scale hydrologic models.  But such “top down models can miss important dynamics due to the interaction between climate changes and the intrinsic geologically-mediated drainage efficiencies of diverse landscapes. To address this, we have developed an alternative strategy that uses simple hydrologic theory to derive landscape-level predictions of streamflow sensitivities (both peak and low flow) to climate warming.  We test these predictions against the historical record, and compare this “bottom up” method against the “top down” forecasts, revealing strengths and weaknesses of both approaches.  

Lorenz G. Straub Award Recipient: Edmund Tedford, Ph.D., Associate Specialist, Earth Research Institute, University of California, Santa Barbara, for his 2009 Ph.D. dissertation, "Laboratory, Field, and Numerical Investigations of Holmboe’s Instability" from the University of British Columbia. 

Lorenz G. Straub Award Ceremony Distinguished Speaker: S. “Bala” Balachandar, William F. Powers Professor, Department of Mechanical and Aerospace Engineering, University of Florida

NOTE: The event will be held in 210 Civil Engineering Building, in conjunction with the Warren Lecture Series. Visit the Warren Lecture Series webpage to access the link and instructions for the live stream of the Straub Ceremony and Seminar.

Penetration of one material into another is a fundamental fluid mechanical process that can be observed all around us in many industrial and environmental applications. Filling/emptying pipelines, coating flows, falling films and sedimentation fronts are some industrial applications. Tsunamis, volcanic plumes, lava and pyroclastic flows, dust storms, powder snow avalanches, submarine turbidity currents and supernovae offer fascinating examples of advancing material fronts. This talk will introduce the concept of gravity currents, where the density difference between the propagating and the ambient materials drives the flow. The examples mentioned above include both scalar and particulate gravity currents, where in the former the density difference is due to temperature or salinity, while in the later suspended particles contribute to density difference. Particular attention will be paid to the front velocity and simple theoretical models that attempts to predict it. The propagating fronts undergo  Rayleigh-Taylor, Lobe-and-cleft and Kelvin-Helmholtz instabilities, giving rise to fascinating pathways to turbulence. 

One particular example we will consider in greater detail is the sustained propagation of submarine turbidity currents, whose propagation depends on an interesting interplay between suspended particles and turbulence. The suspended particles drive the flow and are the source of turbulence in a turbidity current, while the flow turbulence enables resuspension of particles from the bed. If resuspension dominates over deposition the intensity of the current can increase, thereby further increasing resuspension and resulting in a runaway current. On the other hand, stable stratification due to suspended sediment concentration can damp and even kill turbulence. Then deposition dominates over resuspension and the current could laminarize resulting in massive deposits. 

In this talk we present results that indicate the existence of conditions for the total damping of the near-bed turbulence. Under these conditions, sediment in suspension rains out passively on the bed, even though the upper layer may remain turbulent. The above scenario provides a reasonable (but not unique) explanation for the formation of massive turbidities that have recently been reported from field observations.