Conference in Les Houches (France), 20-25 January 2019
Organizers: Thierry Dauxois (CNRS & ENS de Lyon) and Thomas Peacock (MIT)

Environmental Fluid Dynamics: Confronting Grand Challenges



The school is open from Sunday 20th January until Friday 25th January in the afternoon. The conference will end just after Friday's lunch (around 2pm).



Afternoons will be free to enjoy Nature wilderness (Hiking, Walking or Skiing) and to stimulate scientific discussions.

Abstracts

Matthew Alford

Ocean turbulence: Importance, techniques and case studies.

Ocean turbulence is crucial to a variety of biology, chemistry and physics problems in the ocean including the existential issue of accurately modeling climate change in the coming decades. Techniques for measuring turbulence have advanced greatly recently, in part following reduced-cost digital electronics driven by the mobile phone industry, and now allow inexpensive and broadly distributed measurements. I'll review the fundamentals of ocean turbulence and how it is measured, its connection to climate and other problems of societal relevance, and present several case studies showing key mechanisms by which turbulence is generated in the ocean.

Alberto Aliseda

The Dynamics of Inertial Particles in Turbulence: Improved Microphysics for Environmental Flows in the Ocean and the Atmosphere and Recent Progress towards Better Parameterized Models.

The dynamics of inertial particles, liquid droplets or gas bubbles in turbulent flows represent a significant source of uncertainty in the modeling of environmental flows, including ocean/atmosphere coupling. The microphysics of individual particles, as well as of particle clusters, occurring at the smallest scales of the turbulent flow play an important role -sometimes dominant - in phenomena such as formation of rain drops in warm rain, condensation/evaporation of water vapour in hurricanes, and gas transfer in bubbles entrained by breaking waves in the ocean. I will present results from turbulent multiphase experiments created to provide conditions that are conducive to reproducing the physics in environmental flow problems, including condensation droplets in cumulus clouds and bubbles in the post-wave-breaking turbulence. Measurements of the evolution of small particles in homogeneous, isotropic turbulence are analyzed to understand the particle concentration field, and their velocities relative to both the carrier flow and the other particles. As an example of the role of microphysics in large scale geophysical flows, the hypothesis of turbulence-driven droplet collisions in clouds has the potential to close the problem of computing precipitation times and cloud lifetimes in warm rain environments for weather and climate, respectively, modeling purposes. The fundamental understanding of inertial particle interactions with turbulence strongly support this mechanism for rain formation, but quantifying it with experimental measurements under well-controlled conditions has proven a significant challenge. To shed light over the turbulence collision kernel for inertial particles, we analyze the preferential concentration and enhanced relative velocities resulting from their inertial interactions with the underlying turbulence. Similarly, the residence time, size and concentration of gas bubbles in the ocean is determined by their interaction with the residual turbulence induced by wave breaking. Laboratory experiments are necessary to gain insight into the microphysics so that the problem can be parameterized into models that focus on the large-scale phenomena that we associate with environmental challenges.

Michael Allshouse

Application of Lagrangian clustering to interpret coastal transport and assist search-and-rescue operations

Oil in the ocean, a man lost at sea, and the formation of plastic garbage islands all feature transport in a complex dynamic system. Lagrangian coherent structures provide a robust technique for partitioning the domain and understanding the underlying structures that shape transport. Through analysis of a predictive coastal ocean models, we analyze the transport near Martha's Vineyard using different coherent structure techniques to partition the local domain. These results provided a basis for the deployment of drifters in the area to observe how well both the model and the coherent structures predicted the types of transport present. Because the drifters would be released over the course of hours and the time scale of the observation was only six to twelve hours, it was necessary to develop a deployment strategy that would account for the currents to have the drifters reach the desired alignment at the initial time of analysis. In addition to CODE drifters, manikins were also released because search-and-rescue targets will also be exposed to wind, which can drastically modify the transport. We present the coherent structure analysis as well as the observational results.

Peter Bauer

The future of numerical weather prediction will be determined by computing

Progress in numerical weather prediction is intimately connected with progress in super-computing. Over the years, more computing power has enabled us to increase the skill and detail of our forecasts. This has brought huge value to society, not least through early warnings of severe weather. But as the forecasting system becomes more complex, it will soon be impossible to issue forecasts within schedule and at a reasonable cost with current computing architectures. A new generation of computing systems with exa-scale capabilities promise much greater energy efficiency - but they will rely on parallel processing at levels to which current prediction model codes are not adapted. Changes are needed throughout the entire processing chain if we are to exploit these new opportunities. This also affects choices in both modelling and data assimilation with regards to discretization, numerical methods, parameterised physics and observation operators, but also efficient workflows able to deal with massive amounts of data. Key to success will be portable code structures ensuring efficiency and code readability, and exploiting a range of expected future technologies. The need to redesign forecasting systems creates also new opportunities for emerging disciplines at the interface between applied and computational science. The talk will give an overview of the current state of affairs and present exciting new research projects aiming to change the way we do weather forecasting in the future.

Cynthia Bluteau

Quantifying turbulence processes in ocean flows for the development of mixing models

Turbulent mixing controls the distribution of contaminants, nutrients, temperature, and small organisms in the ocean, while strongly influencing the global ocean circulation. Obtaining direct mixing measurements at high Reynolds is challenging, hindering the assessment of turbulent parameterizations under field conditions. In this talk, I will summarise the most common ways of estimating diapycnal mixing and fluxes before detailing new methods developed to measure ocean mixing more directly and autonomously. The techniques were also adapted to relax the technical constraints involved in measuring fluxes of tracers near the bottom. The methods' theoretical framework will be presented first, followed by their applications to diverse and energetic field sites. The talk will conclude by discussing some methodological challenges that remain in certain environments, especially for measuring turbulence near the bottom of the ocean in complex flows.

Colm-cille Caulfield

Open questions in turbulent stratified mixing: do we even know what we don't know?

Understanding how turbulence leads to the enhanced irreversible transport of heat and other scalars (such as salt and pollutants) in density-stratified fluids is a fundamental and central problem in environmental fluid dynamics, with a wide range of highly important applications. Recently, due not least to the proliferation of data obtained through direct observation, numerical simulation and laboratory experimentation, there has been an explosion in research activity directed at improving the community's understanding, modelling and parameterization of the subtle interplay between energy conversion pathways, turbulence, and irreversible mixing in a wide range of environmentally relevant flow geometries. However, as I will discuss in this talk, there are still leading order open questions and areas of profound uncertainty concerning turbulent stratified mixing. Therefore, I will present a personal perspective on some future priorities for research into this hugely complex, interconnected and important fluid dynamical challenge.

Claudia Cenedese

Mixing and Entrainment at the Interface of Lock-Release Gravity Currents Flowing over a Smooth and Rough Bottom

We are still facing great challenges in understanding and quantifying mixing in stratified flows, relevant to transport processes in the ocean and atmosphere. Lock release gravity currents have been widely studied and provide a good set-up for advancing our understanding of mixing in stratified flows. In the ocean, dense overflows mix with surrounding waters along the descent down the continental slope. The amount of mixing and entrainment at the interface between the dense currents and the ambient fluid dictates the final properties of these overflows, and thus is of fundamental importance to the understanding of the formation of deep water masses. The results from laboratory experiments and Direct Numerical Simulations investigating the entrainment due to the shear-induced turbulent fluxes across the interface between the dense current and the surrounding fluid highlight how strong entrainment does not occur uniformly along the interface. Rather, the region of entrainment is located primarily behind the head of the current. When the dense current flows over a bottom roughness represented by an array of cylinders, laboratory experiments suggest that enhanced mixing can occur via two different mechanisms. For a 'dense' configuration the current propagates over the top of the cylinders and mixing is enhanced by the onset of convective instability between the dense current above the cylinders and the ambient lighter water between the cylinders. In this configuration, the region of top entrainment (E) due to the shear-induced turbulent fluxes across the interface is located primarily behind the head of the current as for the flat bottom configuration, but it becomes narrower for increasing values of the ratio λ = h/H between the cylinders' height h and the water depth H, while the average E is independent of λ and E ~ 0.08. In addition, bottom exchange within the roughness elements occurs over the entire current and it increases with increasing λ. For a 'sparse' configuration the current moves between the cylinders and the mixing is enhanced by the vortices generated in the wake of the cylindrical obstacles. In this configuration, both the top maximum entrainment and the bottom exchange are lower than for dense gravity currents propagating into a dense configuration, and they seem to be independent of λ. As expected, for small values of λ the dense current behavior approaches that of a current over a smooth bottom, while the largest deviations from the smooth bottom case are observed for large values of λ. Mixing processes like the one discussed above occur at scales which cannot be resolved in general circulation and climate models and a great challenge we are facing is to provide correct and physically driven parameterizations of these subgrid processes. The improved understanding of the mechanisms regulating entrainment in dense currents will lead to improved parameterizations of these processes in general circulation and climate models.

Amin Chabchoub

Rogue Waves- Modelling and Prediction

Rogue waves are extreme wave events that are known to occur in offshore as well as coastal areas. The highest wave measured in ocean has been about 35 m and as such, these waves can have catastrophic consequences on ships and offshore structures as well as occasionally on residential areas. Possible explanation for the formation of rogue waves in deep-water from the underlying wave dynamics only, is either the dispersive or nonlinear focusing. These are also referred to as linear interference and modulation instability, respectively. Even though significant progress in experimental research has been achieved lately, the relevance of each of both mechanisms in the ocean remains a subject of debate in the oceanographic community. Here, we will summarize the latest scientific progress and discuss the perspectives as well as challenges in predicting extreme waves in natural environments.

Francesca Chilla

Some issues on transport of heat and particle in turbulent thermal convection

The transport of heat or particles is a major challenge in environmental hydrodynamics. Turbulent convection is the main driver for many natural flows, such as the circulation over inhabited and industrial areas, or mixing in the ocean and lakes. Specifically Turbulent thermal convection is a rich multi-physics problem with fundamental outcomes regarding heat transfer issues but also important fluid mechanics features where the concomitant action of turbulence, coherent structures (plumes) and large scale circulation strongly influences the transport properties of the medium, not only of heat, but more generally of any substances or particles eventually dispersed in the fluid. Measurement exists on real systems, but the boundary condition are often not well controlled and son not easy to model. The Rayleigh Bénard Convection is a simple model in which properties of turbulent thermal transport can precisely be studied. In this talk we will do a point on some of the present measurements and knowledges on this system. Some point will presented as the statistics of heat transport, the lagrangian transport of particles, the influence of boundary condition on the global heat transfer.

Edwin A. Cowen

Remote Sensing of Surface Flows, from order Kolmogorov to Continental scales, as a tool for Hydraulic, Hydrologic, and Ecosystem Science, Engineering and Management

Stage and discharge are some of the oldest measurements in environmental fluid mechanics and are vital in forecasting water supply and flood safety. These measurements are traditionally manpower intensive, hence expensive, and dangerous under high flow conditions. Considering climate change and the planet's increasing population there is a critical need for better, more accurate, and frequent, in space and time, data for model and forecast guidance. This need spans monitoring small-scale turbulent processes to calibrating and nudging continental scale river dynamics models. Driven by applications from river gaging networks to fisheries management to flood and erosion forecasting, and more generally, the near-shore environment of lakes, estuaries and the coasts, remote sensing with quantitative imaging tools is a rapidly expanding field. Such tools can be deployed from fixed platforms, drones, planes and satellites with valuable information contained within the visible to infrared spectral bands. We will review the opportunities for societal impact in remote sensing of surface flows, present field validation of an infrared technique that accurately measures the instantaneous surface velocity field in natural flows over a broad range of scales, and discuss how such data can be leveraged to monitor stage, discharge and bed stress and be incorporated into hydraulic and ecosystem forecasting and management models.
Work performed with S. A. Schweitzer and Erika D. Johnson.

Luc Deike

Wave breaking in ocean-atmosphere interactions.

Breaking waves at the water surface is a striking example of turbulent mixing across a fluid interface. The impact of the jet generates turbulence, entrains air into the water and ejects droplets into the air. A fundamental understanding of the general multi-scale properties of the resulting air-water turbulent flow is necessary to develop more accurate gas transfer or spray generation parameterizations. I will discuss air entrainment, bubble statistics and the associated gas transfer by breaking waves in the ocean; as well as aerosol generation by bubble bursting. The bubbles and wave breaking scales are studied through laboratory experiments and direct numerical simulations and the results are then up-scaled to the ocean scale using measurements of the wave and wave breaking statistics; leading to semi-empirical formulation that can be implemented in coupled ocean-wave models.

Peter Diamessis

Spectral-element-based modeling of shoaling internal solitary waves and turbulence in long domains with gently varying bathymetry

Internal solitary waves (ISWs), highly nonlinear and dispersive waves of near-constant waveform, are key drivers of long-distance energy and particulate transport in the stratified interior of the ocean and lakes. The turbulence and mixing, generated in the interior of these waves by shear or convective instability, can strongly impact background temperature stratification structure and ecosystem equilibrium in natural water bodies. The high-accuracy/resolution simulation of ISW evolution and associated turbulence is faced with three outstanding challenges: preservation of the ISW waveform against spurious disintegration by numerical dissipation/dispersion over long propagation distances, ensuring maximum dynamic range in the wave-embedded turbulence and the computationally efficient treatment of non-hydrostatic effects. We will present the salient components of a spectral-element- method-based flow solver, custom-designed to address these challenges in the context of computational mathematics formulation and high-performance-computing implementation. Results will then be shown from an ongoing collaborative modeling/observational study on convective breaking and the formation of recirculating turbulent cores within shoaling ISWs in the South China Sea. Emphasis will be placed on the mechanism of recirculating core formation as a function of bathymetric slope, ISW amplitude and near-surface structure of the background current. Finally, ongoing work on elucidating the three-dimensional nature of the breaking process and the subsequent transition to turbulence will be discussed.

Michelle DiBenedetto

Microplastics transport in the nearshore environment

Plastic pollution in the environment and the oceans is a global reality. Small dispersed pieces of this anthropogenic waste impact ecosystems, fisheries, and water resources throughout the world. The transport and fate of microplastics in the nearshore environment, however, remains poorly understood. Of particular interest is how the nontrivial shape and inertia of these particles affects their dynamics, especially in wavy flows. Using numerical modeling, theory, and experiments, I will describe how consideration of the characteristics of these particles should inform how we model and predict transport of this persistent pollutant.
Work performed with N. T. Ouellette and J. R. Koseff.

Berengere Dubrulle

Can we predict abrupt transitions with present climate models?: The lesson of a laboratory experiment.

According to everyone's experience, predicting the weather reliably for more than 8 days seems an impossible task for our best weather agencies. At the same time, politicians and citizens are asking scientists for decades of climate predictions to help them make decisions, especially on CO2 emissions. To what extent is this request scientifically admissible? In this presentation, I will investigate this question, focusing on the topic of predictions of bifurcations of the atmospheric or oceanic circulations. In such case, the issue is whether present climate models, that have necessarily a finite resolution and a smaller number of degrees of freedom than the actual terrestrial systems, are able to reproduce spontaneous or forced bifurcations. For this, I will use recent results obtained by my group in a laboratory analog of such systems, so called von Karman flow, in which spontaneous bifurcations of the circulation take place. I will detail the analogy, and discuss what is the effect of reducing the number of degrees of freedom in such system.
Work performed with F. Daviaud, A. Chiffaudel, B. Saint-Michel, F. Ravelet, P. Cortet and E. Herbert.

Sofia Fellini

Effect of wall heating on street canyon ventilation

Understanding the dynamics of mass and heat exchange between a street canyon and the overlying atmosphere is crucial to predict air quality and microclimatic conditions within dense urban areas. Previous studies have demonstrated that the bulk transfer between the street and the overlying flow is entirely governed by the intensity of turbulent fluctuations within the street. The aim of this experimental study is to evaluate how the geometry of the street canyon and the solar radiation on building façades influence the turbulent velocity field within a two-dimensional street canyon and thus the global street canyon ventilation. The study was carried in a wind tunnel. The boundary conditions inside the canyon were modified by heating its windward and leeward walls and by changing the cavity aspect-ratio. The flow field in a cross-section of the street canyon was measured with particle image velocimetry. Temperatures were measured by means of thermocouples. The velocity and vorticity fields are analysed and discussed.

Philippe Gondret

Tsunami wave generation by a granular collapse : from laboratory experiments to Nature

Various past geological events have shown that landslides involving large volumes, up to several cubic kilometers, can happen near coastlines. These events may lead to tsunami waves of large amplitudes with important potential damages to infrastructure and populations. To better assess the associated hazards, one fundamental issue is to provide a deep understanding of the tsunami wave generation taking into account the granular nature of landslides. We investigate here the wave generation that followed the collapse of an initial vertical granular column in a small-scale laboratory experiment, with both water and granular interface tracking.

Catherine Gorle

Predictive Simulations of Urban Flow and Dispersion

Computational Fluid Dynamics is increasingly used to design buildings and cities for optimal pedestrian wind comfort, air quality, thermal comfort, energy efficiency, and resiliency to extreme wind events. However, the large natural variability and complex physics that are characteristic of these flow problems can compromise the accuracy of the simulations results, thereby hindering their use in the design process. In this talk I will show how uncertainty quantification and data assimilation can be leveraged to evaluate and improve the predictive capabilities of Reynolds-averaged Navier-Stokes simulations for urban flow and dispersion. I will focus on quantifying inflow and turbulence model form uncertainties for two different urban environments: Oklahoma City and Stanford’s campus. For both test cases, the predictive capabilities of the models will be evaluated by comparing the model results to field measurements.

George Haller

Nonlinear Dynamics of Lagrangian Transport

Describing transport of substances and scalar fields in unsteady fluid flows is a major challenge in applied science and engineering. Examples include the transport of salinity and temperature in the ocean and the re-distribution of aerosols in the atmosphere. In this talk, I give an overview of contemporary mathematical methods for uncovering the main flow structures governing unsteady transport phenomena in unsteady continua. The approach is based on the modern, geometric theory of nonlinear dynamical systems, which targets the most influential material surfaces (invariant manifolds) that create observed patterns in turbulent mixing. Beyond purely advective mixing, I also discuss techniques for identifying the main inhibitors and enhancers of diffusive and stochastic transport.

Isabel Houghton

Emergent hydrodynamics of aggregations of active particles

The state of the ocean depends on a variety of particle-fluid interactions which are difficult to predict and measure, particularly in a non-dilute regime. The complexity of multiphase dynamics makes relevant environmental flows computationally expensive to simulate or requires involved configurations to investigate in the field or the laboratory. Nevertheless, recent laboratory experiments with centimeter-scale organisms swimming in high concentrations demonstrate surprising emergent dynamics due to particle-fluid interactions, such as aggregation scale eddy formation in the animal swarms. If similar phenomena occur in the ocean, it would have far-reaching consequences for ocean mixing by marine zooplankton. Field efforts informed by recent laboratory results and augmented computational tools are necessary to further understand the uniquely important high concentration regime of multiphase flow and the accompanying broader implications for the ocean.

Greg Ivey

Length scales, mechanisms and mixing in density stratified flows

The talk will review the basic ideas in the evolution of our understanding of turbulent mixing in stratified fluids in the environment. The framework for this evolution is built on an understanding of the key length scales, and hence dimensionless numbers, necessary to describe mixing. We examine the two fundamental mechanisms that can drive turbulent mixing: shear instability and convective instability. Using results from laboratory, numerical and field observations, we examine the characteristics of both mechanisms to identify what is similar and what is different about the two, what are the rates of mixing, and importantly when each mechanism likely operates. We explore a general model of mixing which incorporates both mechanisms, and touch on the implications for future work.

Celia Laurent

Modeling marine particle transport with LTRANS v.Zlev: applications and sensitivity studies

A new version of the off-line Lagrangian ocean particle tracking model LTRANS was implemented to handle the Z-coordinate (constant depth layers) discretization of the hydrodynamic equations, instead of the sigma (terrain- following) coordinate system. The new LTRANS v.Zlev model also comprises the OILTRANS module for oil spill simulations and various new features including additional larval transport modules. We present different applications obtained by using LTRANS v.Zlev coupled off-line to the grids and hydrodynamic fields of the MITgcm general circulation model simulations. Model sensitivity is assessed by analyzing its response to various key factors among the Lagrangian parameters such as wind drag, number of particles released, and random displacement model representing sub-grid scale horizontal turbulent diffusion. We also tested the sensitivity to several characteristics of the geophysical model (i.e., modeling assumptions/parameterizations, grid resolution, atmospheric forcing) and to more process-related features such as temporal dependency for larval connectivity studies and weathering formulations for oil spill applications.
Work performed with D. M. Canu, S. Querin and C. Solidoro.

Greg Lawrence

The Evolution of a Lake Covering Oil Sands Tailings

The oil sands industry in Canada faces the challenge of dealing with over one billion cubic meters of tailings that form a gel-like mixture of clay, water, napthenic acids, metals and hydrocarbons. The cost of clean-up by conventional methods have been estimated at over $1011 Canadian. One remediation strategy that is being investigated is to backfill mine pits with tailings and then cover them with water. This presentation addresses the Grand Challenge of understanding the evolution of water quality (salinity, turbidity, dissolved oxygen and temperature) in the resulting brackish lake. Important, but poorly understood, processes include: mixing caused by the salt excluded during ice formation, mixing caused by gas ebullition from the tailings, convection driven by the warm tailings, expression of pore water as the tailings settle and the lake deepens, and the possibility of meromixis.

Michael Le Bars

Dynamics of mixed turbulent-stably-stratified fluids

The organization of fluid systems into a turbulent layer adjacent to a stably stratified one is common both in Nature (atmospheres, stellar interiors, ...) and in the industry (e.g. nuclear power plant in accidental configurations). Understanding the exchanges of momentum and the mixing at the interface is a fundamental "grant challenge" with applications in meteorology, stellar evolution, and nuclear safety, among others. It involves in particular considering the excitation of internal waves by turbulence, and studying the consequences of their non-linear interactions. I will present here the results of experimental investigations, complemented by numerical simulations, of two idealised configurations: a turbulent jet impinging a density interface with the associated mixing mechanism, and the convection in water around 4 degree Celsius with the associated generation of a large-scale reversing flow.

Paul Linden

The physics of natural ventilation of buildings and the connections to the urban flows.

The imperative to reduce energy consumption in cities drives a need to replace air conditioning with other forms of low-energy ventilation in buildings. Natural ventilation is an obvious candidate and, since it requires exchange between the interior of a building and the outside environment, the relative level of a contaminant inside and outside is very important in determining the quality of the internal environment. In this talk I will describe the basic fluid mechanics behind low-energy natural ventilation and the implications for contaminant movement within buildings. I will show that depending on the location of the pollution source the distribution within a building can differ significantly from one case to another. Using results from a recent field study in London I will also show that pollution levels in the city vary significantly in time and space and that these variations must be taken into account when operating a naturally ventilated building. Finally, I will discuss an ongoing project that aims to provide an integrated modelling system that will provide a tool to make accurate assessments of the potential to reduce energy consumption and excess heat emissions from buildings.

Ed Llewellin

Bubbles, burps, and bangs: multiphase fluid dynamics in volcanic eruptions

Predicting volcanic eruption behaviour is the grand challenge of volcanology. It depends upon accurate modelling of the fluid dynamic behaviour of magma, which is a complex multiphase material, comprising a newtonian silicate melt, suspending deformable bubbles and rigid crystals. The fractions of these phases varies dramatically during magma transport. I will present the results of scaled analogue experiments that reveal some of the phenomenological richness of the fluid dynamics of multiphase magma transport. I will focus on the ascent of the bubble-rich, low viscosity magma that drives basaltic fissure eruptions, which are the most common eruptions on Earth.

Detlef Lohse

Double diffusive convection.

In the talk we will discuss double diffusive convection. First, we will focus on the fingering regime, which is the relevant regime for double diffusion in the tropical ocean, with a destabilizing salt concentration field and a stabilizing temperature field. We will focus on the flow structure and on scaling laws of the transport quantities, the Reynolds number, and boundary layer and finger thicknesses. We will show that the unifying scaling theory developed for Rayleigh-Benard convection can predict the salinity flux and the Reynolds number in some regimes of double diffusive convection, too, without introducing any new parameters. Next, I will show how staircases form in this regime. Finally, we will discuss the scaling law in the two-scalar regime when both temperature gradient and salinity gradient drive the flow.
Work performed with Yantao Yang and Roberto Verzicco

Jim McElwaine

Particle-laden flows and how to avoid them

The talk will outline the underlying theory of particle-laden flows and how simplified theoretical models can be developed. These will be illustrated by comparison with laboratory experiments, DNS and field observations from Earth, Mars and other planetary bodies. The talk will finish with the latest advances in understanding snow avalanches and will hopefully prove useful in avoiding them in the afternoon slope sessions.

Eckart Meiburg

Future research perspectives in particle-laden flows

The talk will survey some current research directions in the field of particle-laden flows, such as double-diffusive sedimentation, grain-resolving simulations of cohesive and non-cohesive sediment transport, and salt crystal precipitation in hypersaline lakes. Open questions and future research directions will be outlined.

Tamay Ozgokmen

Do Submesoscales Exist in The Ocean and What Do They Do?

Submesoscale motions are long surmised to exist in the ocean on the basis of ocean modeling. Nevertheless, since these processes coincide with the mesh scale at the interface of hydrostatic/nonhydrostatic boundary, they have been challenging to capture using traditional OGCM/LES approaches. Similar challenges were also posed to observational techniques. Submesoscale processes became a central problem for a research team, Consortium of Advanced Research of Transport in the Environment (CARTHE,http://carthe.org/), which was formed in the aftermath of the Deepwater Horizon event, the largest accidental marine oil spill. CARTHE focussed on understanding the physical processes controlling the transport of material from a deep blowout all the way to the coast. A number of large coordinated experiments with new instruments designed to sample the surface of the ocean at unprecedented sampling densities were executed. Here, we summarize these efforts with special emphasis on the role of submesoscale processes in the northern Gulf of Mexico.

Thomas Peacock

The fluid mechanics of deep-sea mining

In the next decade, with pressing societal for mineral resources, it is anticipated that deep-sea mining activities will commence. Associated with such activities are numerous challenging fluid dynamics problems. A notable example is the behavior of sediment-laden plumes generated by collector vehicles operating on the sea floor at depths around 4000-5000m. Here we present an overview of the fluid dynamics associated with deep-sea mining activities and results of our studies incorporating classical and numerical modeling, and the results of recent field experiments.

Jimmy Philip

Growth of axisymmetric jets and forced plumes: experiments and a predictive entrainment model

The cornerstone model to predict turbulent jets and plumes is based on the Morton, Taylor and Turner (1956) entrainment assumption, which although successful, has been criticised over the years. Only recently has a consistent 'entrainment relation' been formulated by vanReeuwijk and Craske (2015) from the Navier-Stokes equations. However, this is not sufficient for predictions, but requires experimental data and an 'entrainment model'. Although several experiments have been performed previously, simultaneous velocity/density data is scarce. Here we conduct temporally and spatially well-resolved Particle Image Velocimetry (PIV)/Planar Laser Induced Fluorescence (PLIF) experiments. The use of refractive-index matched fluids for ambient and jet/plume ensures minimal optical distortion, and hence recovery of high-fidelity PIV/PLIF data for turbulent fluxes, especially in the near field region where the density mismatch is higher. Employing a new entrainment model obtained from our data, we predict the plume growth, and compare the present relation/model and other well-known ones against our measurements and those available in the literature.
Work performed with Himanshu Mishra.

Nadia Pinardi

Perspectives in ocean pollution modeling

Marine pollution, together with climate change produces one of the most threatening adverse effects on ocean health and the sustainable exploitation of marine resources. The first objective of the UN Sustainable Development Goal 14 is: “By 2025, to prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including marine debris and nutrient pollution”. Ocean pollution has been defined by the group of experts on the scientific aspects of Marine Pollution (GESAMP) as ” the introduction by man, directly or indirectly, of substances or energy into the marine environment (including estuaries) resulting in such deleterious effects as harm to living resources, hazards to human health, hindrance to marine activities including fishing, impairment of quality for use of sea water, and reduction of amenities.” There are several pollution forms, in this brief presentation we will concentrate on two different forms of organic pollutants: oil and biochemical pollutants. Modeling these ocean pollutants considers two basic processes: the advective / diffusive components of the transport and the changes due to biophysical-chemical reactions that change the constituent properties of the pollutant. Numerical simulations of these pollutants use different concepts of the reactions acting of the pollutant concentrations and we will try to show the fundamental differences. The assessment of the potential spreading patterns of pollution from sources requires advanced numerical methods applied to pollutants transport that in turn might depend on the way the pollutants are described in terms of biophysical and chemical reactions. A challenge for the future is to assess the uncertainties inherent to such models especially because the reactive part of the equations is normally empirical. It is then mandatory to use ensemble methods to estimate the prediction/simulation uncertainties. The challenge is here to couple the ensemble methods with machine learning algorithms capable to detect the most sensitive parameters and produce members of the ensemble that adequately sample the basic models errors. Further advances in the understanding of pollution modeling uncertainties require the statistical analysis of ensemble simulations of pollutant distributions, and the analysis of probability distribution functions. The main result is that oil and biogeochemical tracer concentration distributions are Weibull distributed and that a two-parameter PDF can appropriately represent the uncertainties. The presentation will end with a brief discussion of marine debris challenges for pollution modeling.
Work performed with Augusto Sepp-Neves.

Katherine M. Smith

Turbulence in Forced Stratified Exchange Flows

Continuously forced, stratified exchange flows occur in many geophysical systems, such as through channels between ocean basins, between coastal shelves and the deep ocean, and at the mouth of rivers and estuaries. These persistent exchange flows can be unstable to various instabilities that promote the growth of and even sustain the production of turbulence, which can in turn inhibit or enhance mixing between the two differing flows. The extent of mixing within these types of flows is an important factor for processes such as abyssal mixing, the set-up or destruction of hypoxic layers, injection of nutrients and buoyancy into the surface mixed layer, and many other important ocean processes. While these turbulent mixing processes are assumed to be important to global ocean budgets, they are unresolved within Earth system models and therefore must be fully understood in order to accurately include through subgrid scale parameterization.
A relatively well studied turbulence promoting instability is that of the shear broadening and overturning Kelvin-Helmholtz instability. This instability occurs at lower stratification and appears to behave super-diffusively through larger scale overturning billows. In contrast, the lesser known shear thinning and scouring Holmboe wave instability occurs at higher stratification and appears to behave anti-diffusively through a scouring process that promotes the formation of layers. While both of these instabilities have the ability to create and sustain turbulence in a persistent flow, an open question is whether the turbulent steady-state that arrises is characteristically different going from the overturning to the scouring regime. We hypothesize that given their differences in behavior, i.e. overturning vs scouring, these two regimes potentially have different steady-state turbulent properties and thus influence the flow and mixing around them differently. However, how these two regimes are separated, what parameters are they a function of, and how they differ from each other in mixing properties at a steady turbulent state are open questions that have yet to be answered.
In this talk, results from three-dimensional direct numerical simulations of stratified exchange flows that are continuously forced by weakly damping the buoyancy and streamwise velocity back to their initial mean profiles are presented. After an initial ‘spin-up’ period, two different turbulent steady states are observed, an overturning and a scouring state, that are dependent on several non-dimensional parameters. Both turbulence and mixing are characterized in each case and the implications for parameterization are discussed.
Work performed with Colm-cille Caulfield and John R. Taylor.

T. van den Bremer

The transport of plastic pollution by waves

Since the introduction of plastics in the 1950s, large concentrations of floating plastic debris have accumulated in oceans with harmful effects on marine wildlife and potentially to human health. In addition to currents and winds, waves play a role in their transport. Due to their inertia, plastics not follow the wave-induced Stokes drift. The large range of sizes and densities of plastic pollution can lead to different transport speeds. We solve the particle's equation of motion and derive asymptotic equations, validated using laboratory experiments, in order to give estimates of transport as a function of wave and particles characteristics.

Maarten van Reeuwijk

Wicked problems in urban fluid mechanics

I will discuss some of the challenges we face in understanding the physics of air flows in urban areas. These include the inherent complexity of the urban morphology, the importance of heat and moisture exchange with the atmosphere, the influence of vegetation as well as the influence of human activity. I will discuss how concepts from entrainment theory can be used to create idealised atmospheric conditions in non-neutral settings, the systematic study of city morphologies of intermediate complexity and strategies and how vegetation can be incorporated into Large-Eddy Simulation codes.

Valerie Vidal

Gas release at the seafloor, from laboratory experiments to risk assessment

Gas release at the ocean floor is a widespread phenomenon which can have drastic consequences on the environment or the industry. Striking examples include natural seep areas, possible links between giant methane release and climate change, gas blowout and subsequent bubble plumes during offshore drilling. This process was studied experimentally by injecting gas in water-saturated sands. We focused on the short and long-term dynamics of gas rising through the sediment layer, as well as bubble rise, sediment transport and particle suspension in the above liquid layer. Risk assessment for oil and gas offshore production or deep-sea mining will be discussed.

Cat Vreugdenhil

Transition to turbulence in the ocean boundary layer beneath ice shelves

Ocean-driven melting of ice shelves around Antarctica has implications for rising sea levels and changing climate. However, the picture of ocean-driven melting is incomplete. The mixing of warm, salty water towards the ice base drives melting, but the resulting meltwater freshens the water column and stabilizes the stratification, suppressing turbulence and mixing. Here we investigate ocean-driven melting using large-eddy simulations that resolve all but the smallest scales of turbulence. The transition from stratified to fully turbulent flow is examined and found to strongly influence the melt rate. The results will help to interpret real-world observations and improve parameterizations of ocean-driven melting.

Nathalie Vriend

Novel approaches and applications for granular and particle-laden flow

This talk will highlight new techniques in the field of granular and particle-laden flow, including photoelastic granular avalanches and sediment transport in a periodic domain. I will focus especially on new approaches that reveal information regarding multiphase flows that was previously hidden in experiments.

Andy Woods

Some Fundamental Models of Environmental Flows and their Application

In this talk, we describe some of the physical processes which arise in particle laden or bubble laden turbulent plumes. We explore the impact of the separation of the phases and of stratification of the ambient fluid on the flow evolution and the associated mixing. The relevance of the experiments and models in the context of volcanic plumes, deep sea mining and microbe transport in hospitals will be discussed. Finally, we will explore some complementary problems in particle laden gravity currents, with relevance for turbidite transport in the deep sea.

Tomohito J. Yamada

Specific energy in jet stream and formation process of blocking

Atmospheric blocking occurs with a strongly meandering jet pattern which remains in same place for several weeks, causing meteorological and hydrological extremes. Previous studies such as Rossby (1950) and Armi (1989) proposed a theoretical approach for atmospheric blocking utilizing analogies between open channel flow and jet stream. We extended this theory to realistic atmosphere and showed a relationship between typical blocking flow and specific energy in jet stream. We analyzed several blocking episodes in 1989 winter using specific energy theory and found energetic signals over Japan and its surroundings before Pacific blocking occurrences.
A series of laboratory experiments for baroclinic waves in a rotating cylindrical container with a differentially temperature gradient at the bottom without the inner wall have performed. The inner bottom is cooled and the outer bottom is head, respectively, and this experimental design can be classified as the dish-pan type. Baroclinic waves varied their amplitudes with time change. Under a certain experimental condition for the temperature gradient and the rotating speed, the dominant circumferential wavenumber varied 3 to 5, and the wavenumber becomes larger when the vorticity with rotation direction over the central area and the circumferential velocity at the area where the vortex is surrounded is larger.


Poster Session

Samuel O. Adesanya

Entropy generation analysis for a third grade fluid through an inclined channel with isothermal heating

In present poster focuses on the heat irreversibility in the third-grade fluid flow through a channel subjected to isothermal heating at the walls. Exact solution of the dimensionless nonlinear equations governing the momentum and energy equations are constructed and found to be valid for both small and large parameter values. Furthermore, results are presented and discussed for the heat transfer rate and the wall skin friction, velocity and temperature fields, Entropy generation and Bejan profiles.

Julie Albagnac

Dynamics of an inclined Vortex Ring interacting with a density stratification

Vortex Rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid (for instance, plumes can be considered as Vortex Rings). Such structures have the faculty to self-propagate along their revolution axis, conferring them capacities of transport and mixing that could be exploited. Among applications, one can mention nuclear safety and the need to mix fluids of different density to prevent explosion hazard. The scope of the present study is to identify and evaluate the mixing mechanisms associated with a Vortex Ring interacting with a density stratification, in particular, the reorganization of the flow and the generation of internal waves. The influence of the Vortex Ring propagation speed and propagation angle relative to the density gradient on its dynamics and mixing power are studied thanks to 2D and 3D time-resolved TOMO-PIV.

Nikolas O. Aksamit

Lagrangian coherent structures (LCSs) act as the skeleton of turbulent flows.

Due to the enormous complexity of high Reynolds number flows in the atmosphere, harnessing knowledge about transport from LCS can be exceedingly advantageous. Since as early as the 1960's, researchers in climate engineering have used climate models to investigate how the current rate of climate change may be slowed by the introduction of sulfate aerosols into the atmosphere, increasing reflectance of shortwave radiation. However, most simulations of climate engineering do not represent fine-scale structures of atmospheric mixing, which can pose transport barriers or accelerate mixing. The present research utilizes a recent, mathematically objective characterization of LCSs as barriers to diffusive and stochastic transport. This enables the identification of key material surfaces that enhance or inhibit transport of aerosols along 2-D isentropic surfaces. This is a novel application of exploiting LCS at synoptic and global scales to better harness the mathematical building blocks of atmospheric transport and mixing."

Peter G Baines

Stratified Flow Hydraulics - upstream disturbances in uniformly stratified flow over topography.

Hydrostatic layered flow (1, 2 or 3 layers) of finite depth over topographic features is governed by critical flow conditions (Froude number = unity) at the point of maximum obstacle height, which can determine the amplitude (in particular) of upstream motion generated by flow over the obstacle. In uniformly stratified flows, however, it is not possible for the flow to adjust to maintain the critical flow condition at this location, and critical flow occurs on the upstream and downstream sides of a single obstacle. Major questions are, what determines the upstream motion of stratified fluid over an obstacle under these conditions, and how does this relate to upstream effects in meteorological environments, in particular? This poster presents some results from steady-state flow solutions and some time-dependent flow solutions that explore these questions.
Work performed with Jimmy Philip.

Sridhar Balasubramanian

Energetics and mixing efficiency of gravity currents using simultaneous velocity-density measurements.

The energetics and mixing dynamics of a gravity current, released using a lock-exchange mechanism, is quantified experimentally using high-resolution simultaneous velocity (PIV) and density (PLIF) fields. The Reynolds number was varied from Re=485-12270. The terms in the turbulent kinetic energy budget equation, namely, the shear Production flux (P), buoyancy flux (B), and dissipation (ε) were measured to characterize the turbulent behaviour of the quasi-steady head region of the gravity current. Using the parameterization proposed by Osborn (1980), the mixing efficiency (or flux Richardson number), Ri_f , was calculated to understand the local mixing dynamics. It was found that P, B, and ε peak near the shear mixing layer. At low values of Re, the buoyancy flux was negative indicating counter- gradient fluxes. The mixing efficiency, Ri_f was found to attain a steady value of Ri_f = 0.22 at higher values of buoyancy Reynolds number, Re_b . The challenge that is being addressed here is the dynamics of episodes of mixing in weakly and strongly stratified environments (e.g. the Bay of Bengal, from an Indian monsoon context), where values of Ri f are highly debatable due to small-scale diapycnal mixing. The high-resolution experimental dataset will act as a platform for validating mixing models used for shear-stratified flow simulations.

Damien Bouffard

Convection in Lakes

Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries. Hence, convective processes are often the dominant drivers for shaping mixing and stratification structures in inland waters. Due to the diverse environments of lakes—defined by local morphological, geochemical, and meteorological conditions, among others—a fascinating variety of convective processes can develop with remarkably unique signatures. Whereas the classical cooling-induced and shear-induced convections are well-known phenomena due to their dominant roles in ocean basins, other convective processes are specific to lakes and often overlooked, for example, sidearm, under-ice, and double-diffusive convection or thermobaric instability and bioconvection. Additionally, the peculiar properties of the density function at low salinities/ temperatures leave distinctive traces. Here, we present these various processes and connect observations with theories and model results.

Samuel Boury

Axisymmetric Internal Waves

To date, axisymmetric internal wave fields, which have relevance to atmospheric internal wave fields generated by storm cells and oceanic near-inertial wave fields generated by surface storms, have been experimentally realized using an oscillating sphere or torus as the source. Here, we use a novel wave generator configuration capable of exciting axisymmetric internal wave fields of arbitrary radial form to generate axisymmetric internal wave modes. After establishing the theoretical background for axisymmetric mode propagation, taking into account lateral and vertical confinement, and also accounting for the effects of weak viscosity, we experimentally generate and study modes of different order. We characterize the efficiency of the wave generator through careful measurement of the wave amplitude based upon group velocity arguments. This established, we investigate the ability of vertical confinement to induce resonance, identifying a series of experimental resonant peaks that agree well with theoretical predictions. In the vicinity of resonance, the wave fields undergo a transition to non-linear behaviour that is initiated on the central axis of the domain and proceeds to erode the wave field throughout the domain.

Gisela D. Charò

Topological classification of Lagrangian geographies

Lagrangian geographies define regions in which fluid particles remain relatively isolated from the surrounding fluid. In this work, we shall consider whether particles within a relatively isolated region share the same type of dynamical behavior. In order to classify dynamical behaviors, we resort to concepts of algebraic topology, used to unveil and characterize the structure of chaotic flows from data. Understanding the relationship between the qualitative behavior of single fluid particles with the global organization of the fluid flow in terms of mixing, is an interesting issue that can be addressed from the perspective offered by Topology of Chaos.
Work performed with Guillermo Artana and Denisse Sciamarella.

Megan Davies Wykes

The effects of buoyancy on cross-ventilation.

Wind-driven cross-ventilation occurs when there are openings on opposite walls of a room, at equal heights. This poster will present the results of laboratory experiments using a water flume and a cross-ventilated model room at 1/10th scale. These experiments examine the effect of an initial indoor-outdoor temperature difference on the ventilation rate and the removal of contaminants from a cross-ventilated room. We present simple mathematical models that explain the observed ventilation rates, showing that buoyancy has little effect on the ventilation rate for wind-dominated ventilation and that wind enhances buoyancy-dominated ventilation. We discuss future directions for research on the natural ventilation of buildings and links to the understanding of outdoor air quality.

Morris Flynn

Modal decomposition of polychromatic internal wave fields in arbitrary stratifications

Internal waves e.g. those produced by tidal sloshing over bathymetry play a crucial role in the energetics of the oceanic overturning circulation. Understanding their spatial and temporal structure, which depend on both the details of the forcing topography and the forcing frequency, is relevant in predicting where mixing may occur, details of which remain poorly understood. Past work has largely focused on the case of a monochromatic wavefield; however, tides are composed of multiple frequency constituents. Here we present an approach by which the modal structure of a polychromatic internal wavefield may be computed from velocity data without any a priori knowledge of the details of the forcing topography.
Work performed with A. K. Kaminski .

Louis Gostiaux

Mixing efficiency in decaying stratified turbulence

We re-visit the historical 1850 Joule experiment in the context of stratified turbulence. We have considered by means of DNS a closed domain, linearly stratified, where turbulent kinetic energy is initially injected. In an homogeneous fluid, viscous dissipation would convert 100% of the mechanical energy into heat; in a stratified fluid, part of the energy is converted into an irreversible increase of the background potential energy; this ratio is directly related to the cumulative mixing efficiency computed over the turbulence decay. A closed energy budget is proposed for different values of bulk Richardson number Ri, in which all transfer terms are resolved in time and space and integrated. This configuration fundamentally differs from forced turbulence experiments, where stationary reversible and irreversible fluxes must balance : whether forced or decaying turbulence is the best model for deep oceanic mixing is the challenging question to be answered.
Work performed with A. Delache, E. Horne and A. Venaille.

Nicolas Grisouard

Extraction of Available Potential Energy from Geostrophic Fronts by Inertial-Symmetric Instabilities

Submesoscale oceanic density fronts are structures in geostrophic and hydrostatic balance, which are prone to inertial and/or symmetric instabilities. We argue that drainage of available potential energy from the geostrophic flow can be a leading-order source of their growth. We illustrate our point with two-dimensional Boussinesq numerical simulations of oceanic density fronts on the f-plane. A set of two-dimensional initial conditions covers the submesoscale portion of a three-dimensional parameter space consisting of the Richardson and Rossby numbers, and a measure of stratification or latitude. Because we let the lateral density gradient decay with depth, the parameter space map is non-trivial, excluding low-Rossby, low-Richardson combinations. Dissipation effectively selects the largest growing mode, and inertial-symmetric instability in a confined unstable region creates flow cells that recirculate outside the unstable region, disturbing isopycnal locations. As the ageostrophic flow grows in amplitude, the instability eventually displaces isopycnals significantly. In our experiments, a leading-order drainage of available potential energy from the geostrophic fronts to the ageostrophic perturbations ensues. Various constraints, some physical, some numerical, result in our experiments to behave like inertial rather than symmetric instabilities. Our results depend very weakly on the Richardson number, and more on the Rossby number and relative stratification. We cannot rule out the possible importance of diffusive effects, and will briefly discuss the possible implications for the energetics impacts of inertial-symmetric instabilities in the ocean.

Anaïs Guillemain

Plume intrusion in a two-layer stratification

Greenland's fjords play an important role in the transport and mixing of melt-water from glaciers to the ocean. Seasonal subglacial discharge at the base of the glacier forms a buoyant plume in the fjord waters, influencing the stratification of the fjord waters and the melting of the adjacent ice face. Systematic experiments have been carried out to model the behaviour of the plume in the stratified fjord waters using dye to visualise the flow and a conductivity probe to measure density profiles. Key results from the experiments include prediction of the maximum height of rise and the intrusion height of the plume, and its impact on the stratification of the water. We identify that three regimes can occur depending on the discharge rate and the ambient stratification: (i) plume intrusion at mid-depth in the fjord; (ii) plume rise to the surface, and subsequent intrusion at mid-depth and (iii) plume intrusion at the surface of the fjord, and we discuss the implications of these regimes for the overall circulation in the fjord.

Timothée Jamin

Nonlinear regime in internal waves attractors

Internal waves are ubiquituous in oceans and their nonlinear regime could lead to mixing and thus explain the upwelling part of meridional currents. We investigate experimentally and numerically this nonlinear regime of internal waves in stratified fluids. To do so, we tune the geometry of a water tank to focus energy on closed paths called attractors. It is well known that a primary internal wave can become unstable above a certain threshold and lead to the growth of two secondary waves through triadic resonant interaction. In attractors, energy density is large enough to observe serial triadic resonant interactions, thus populating the system with mutliple frequencies. By measuring velocity and density gradient fields (using PIV and synthetic Schlieren techniques), we analyse specific patterns observed in the spatiotemporal energy spectrum.

Sylvain Joubaud

Dynamics of single rising bubble in suspension

Suspensions (i.e. particle-laden fluids) are prevalent in a wide range of industrial and natural processes. Mixing and instabilities occurring during gas release in such multiphase flows may be crucial, for example, in oil recovery, gas sequestration, deep-sea mining. The present work focuses on studying the rise of a single bubble in a dense suspension. This study is based on a table-top experiment which consists of a non Brownian granular suspension inside a Hele-shaw Cell. The bubble and suspension dynamics are extracted and linked to the rheological properties of the suspension.

Vincent Legat

A new FEM-DEM Multiscale Model to Solve Immersed Granular Flows

An unresolved Computational Fluid Dynamic-Discrete Element Method (CFD-DEM) model for the simulation of flows mixing fluid and grains is described. The grains trajectories are solved at a fine scale using a discrete element method. It provides the velocities and the trajectories of the grains with an accuracy that is needed to describe microscopic phenomena like clogging in pipe happening in these flows. Solved at a coarse scale using the finite element method, the fluid motion is deduced from a mean continuous representation of the fluid phase giving computational performance and keeping variables evolutions that are of interest for a lot of simulation processes. The key point of this method lays in the coupling of the two different representation scales. An empirical drag formula for monodisperse granular media parametrises the transfer of momentum between the phases. Applying this model to the well-known problem of suspension drops provides validation and insight in this kind of methodology. Simulations in which inertia is non negligible are achieved to prove the generality and adaptability of the unresolved CFD-DEM model compared to other models.
Work performed with Matthieu Constant and Jonathan Lambrechts.

Matthieu Mercier

Vertical motions of particles in a linearly stratified fluid

Understanding the dynamics of settling/floating particles in a stratified fluid is of interest to model the global transport of dust in the atmosphere and of solid particles (dust, plankton, plastic) in the oceans. The modeling of the vertical dynamics of particles has a strong impact on the world carbon cycle, dispersion of pollutants or biological species, which are grand challenges for environmental fluid dynamics. I will present experimental and numerical results on single objects (sphere, disk) settling in a linearly stratified fluid over a large parameter space.

Nicola Mingotti

Transport of contaminants along a ventilated hospital corridor

We explore the transport of contaminants by the ventilation flows in healthcare buildings. We consider a corridor adjoining a number of public spaces such as wards and operating theatres and investigate the mechanisms for the transport and dispersal of microbes through the corridor. We present the results of new laboratory experiments, which illustrate how different schemes of ventilation and the motion of people along the corridor affect the contaminants distribution.

Eugene Morozov

Tidal Jet and Internal Waves in a Frozen Sea

We observed a tidal jet flowing from the Van Mijen Fjord to Lake Vallunden in Spitsbergen. Strong currents of the jet prevented ice freezing along a stripe in the lake continuing the flow in the narrow channel. The thickness of the ice cover around the jet was mapped. After the tidal current passes the channel, the tidal jet overflows a shallow bar and generates internal lee waves downslope. The generation of wave packets occurs every tidal cycle when the current flows into the lake, but no generation occurs during the ebb phase of the tide because the currents over the slope are low. Parameters of internal waves are estimated. Model simulations confirm generation of internal wave train by the tidal current descending down the slope.
Work performed with A.V. Marchenko.

Justin Pringle

Waves, Algae, Sunshine & Wastewater: A Recipe for Renewable Energy & Improved Water Quality on our Coasts?

Algal feedstock are a potential resource for new generation biodiesel production. This is because algae are highly efficient at converting solar energy (through photosynthesis) into biomass. However the space required to grow such algae will directly compete with agriculture. Recently the development of Offshore Membrane Enclosures for Growing Algae (OMEGA) provide an attractive solution to "space problem". OMEGA exploits the use of flexible bioreactors anchored offshore in coastal cities to grow algae. The system utilizes wastewater discharges as a nutrient source to cultivate algae. The algae treat the wastewater while sequestrating CO2. This study explores the use of ocean wave or wind energy resources to naturally power operational processes such as mixing, pumping, and harvesting. Results show the mean flow is a complicated function of the incident wave field. The unsteady effects of the wave pumping drive interesting features at a range of temporal and spatial scales.
Work performed with Katrin Tirok and Derek Stretch

Kristy Schlueter-Kuck

Model parameter estimation using coherent structure coloring

Lagrangian data assimilation is a complex problem in oceanic and atmospheric modeling. Tracking drifters in large-scale geophysical flows can involve uncertainty in drifter location, complex inertial effects, and other factors which make comparing them to simulated Lagrangian trajectories from numerical models extremely challenging. Temporal and spatial discretization, factors necessary in modeling large scale flows, also contribute to separation between real and simulated drifter trajectories. The chaotic advection inherent in these turbulent flows tends to separate even closely spaced tracer particles, making error metrics based solely on drifter displacements unsuitable for estimating model parameters. We propose to instead use error in the coherent structure coloring (CSC) field to assess model skill. The CSC field provides a spatial representation of the underlying coherent patterns in the flow, and we show that it is a more robust metric for assessing model accuracy. Through the use of two test cases, one considering spatial uncertainty in particle initialization, and one examining the influence of stochastic error along a trajectory and temporal discretization, we show that error in the coherent structure coloring field can be used to accurately determine single or multiple simultaneously unknown model parameters, whereas a conventional error metric based on error in drifter displacement fails. Because the CSC field enhances the difference in error between correct and incorrect model parameters, error minima in model parameter sweeps become more distinct. The effectiveness and robustness of this method for single and multi-parameter estimation in analytical flows suggests that Lagrangian data assimilation for real oceanic and atmospheric models would benefit from a similar approach.

Antonio Augusto Sepp Neves

Bridging ocean tracer concentration distribution and oil spill hazard mapping

The current lack of a standardized approach to computing the marine oil spill hazard has hindered an accurate estimate of its global distribution map, which is paramount to manage the associated risk. It has been observed that winds and surface ocean current fields are Weibull distributed and, for the specific case of the atmosphere, this characteristic was found to be transferred to tracer concentration distributions. Based on the outputs of an ensemble oil spill experiment with over 20,000 scenarios, we demonstrate that ocean tracer concentration distribution, limited to the oil spill case here, also fit a Weibull curve. Lastly, a statistically consistent method to quantify the oil spill hazard was proposed and applied to the Algarve, Portugal.

Sara Shamek

Do ocean warm anomalies favor the aggregation of deep convective clouds?

We investigate the role of ocean warm anomalies (hot spots) on the spontaneous aggregation of deep convective clouds in cloud-resolving simulations. We perform cloud-resolving simulations in radiative-convective equilibrium with circular SST anomalies of varying radius, keeping the domain mean SST constant between simulations. We find that the presence of a hot spot (or a cold spot) significantly accelerates the aggregation of convection. Additionally it can extend the range of SSTs for which aggregation occurs. Earlier studies with homogeneous SSTs find that radiative feedbacks are necessary for both the onset and maintenance of aggregation, at least at current tropical temperatures. With a hot spot, we find that aggregation can occur, even in the absence of radiative feedbacks (removed by homogenizing horizontally radiative cooling rates), if the hot spot is warm and/or large enough. The hot spot triggers the onset of self-aggregation by generating a temperature anomaly in the convective region, itself leading to a large scale circulation. Thus the large-scale circulation induced by the hot spot is key, in particular the compensating subsidence in dry regions. In reality, with planetary rotation, the scale of the large-scale circulation induced by SST anomalies is likely determined by the Rossby radius of deformation. Our results suggest that for large enough fractional area of SST anomalies compared to this large-scale circulation, self-aggregation feedbacks could play a role in organizing deep convection over SST anomalies.
Work performed with Caroline Muller, Jean-Philippe Duvel and Fabio D'Andrea

Dheeraj Varma

Internal wave resonant triads in finite-depth nonuniform stratifications.

Internal wave dynamics are now recognized to have a profound influence on the fundamental understanding of geophysical and astrophysical systems. In the ocean, internal wave instabilities and breaking represent a significant mechanism to cause deep ocean vertical mixing and dissipation. One of the important internal wave instability mechanisms occurs via Resonant Triad Interactions (RTI), a special case of which is the well-known Parametric Subharmonic Instability (PSI). In this poster, we focus on two important realistic features of the ocean: finite-depth and nonuniform stratification associated with the upper ocean, to highlight the enriching role of modal interactions resulting in superharmonic wave generation via RTI.

Karan Venayagamoorthy

On the inference of the state of turbulence and mixing in stably stratified geophysical flows

Accurate parameterization of the mixing coefficient Γ is essential to estimate mixing in geophysical flows. Direct measurement of Γ is not feasible and hence it must be parameterized. However, a universal parameterization of Γ remains elusive. Using dominate time scales that govern stratified turbulent flows, scaling arguments are used to provide functional relations between Γ and the turbulent Froude number Fr across a broad spectrum ranging from weak to strongly stratified turbulence. Validation with different direct numerical simulation data confirms the universality of the proposed parameterization. Considering that Fr is not measurable in the field, scaling analysis is used to infer Fr from two measurable length scales namely: the Thorpe Length scale L_T and the Ozmidov length scale L_O. It is shown that L_T/L_O is not only an indicator of age of turbulence but more importantly, also as an indicator of strength of stratification in a turbulent flow. These findings will be helpful in field oceanography to infer Fr using a microstructure profiler, and hence determine the dynamic state of ocean turbulence in order to use the most appropriate parameterization of Γ for estimating diapycnal (irreversible) mixing.

Cristina Vidali

Atmospheric dispersion of heavy gas and passive scalar emission from elevated source.

The atmospheric dispersion of heavy gases can induce relevant risks for the environment and human health, associated to the exceeding of toxic or flammability concentration threshold within the pollutant cloud. Predicting the probability of concentration levels exceeding these limits for dense gases releases remains a challenge for atmospheric dispersion modeling. The aim of this study is to investigate experimentally effects of buoyancy on the dispersion and mixing of a dense gas release and to enlighten its main differences compared to that of a passive scalar. To that purpose we simulate the emission of an elevated source placed within a turbulent boundary layer in the LMFA atmospheric wind tunnel for both heavy and passive scalar. In the experiments, we perform simultaneous velocity and concentration measurement for vertical and transversal profile at several distances downwind the source. A Flame Ionization Detector (FID) and the X-Probe Hot Wire Anemometry (HWA) are used respectively for concentration and velocity estimation. We obtain vertical and transversal profile at several distances downwind the source and we analyse the relative statistics up to fourth moments of the concentration and velocity Probability Density Functions (PDFs ). The information provided by the dataset, allow us to evaluate the high of the mass center (ZMC) and the vertical variance of concentration distribution (σz). Furthermore, we examine turbulent mass flux for vertical velocity and thus estimate turbulent dispersion coefficient (Dt).
Work performed with Massimo Marro, Louis Gostiaux, Horacio Correia, Simon Jaillais, Deborah Houssin, Elena Vyazminab and Pietro Salizzonia.

Andrew Wells

Convective flows impacting sea-ice growth and melt.

The seasonal freezing of the polar oceans, and summer sea-ice melting have significant implications for climate, ocean circulation, and economic activity in the Arctic. I will highlight key uncertainties regarding the growth and melting of sea ice, focussing on two case studies involving convective flows. Growing sea ice is a reactive porous medium of ice crystals and brine with evolving porosity. I will describe recent modelling work to predict the salt fluxes from ice into the ocean that result from convective flow through the mushy sea ice. Meanwhile, meltwater pools in ponds on the ice surface during Arctic summer, and provides a feedback on the heat fluxes driving melt. I will summarise recent field observations of melt-pond temperature, and corresponding modelling of turbulent convection arising from radiative heating of water in a shallow pond.

Hidekatsu Yamazaki

Long term observations of ocean mixing from a cabled observatory

Long term measurements of mixing in the ocean are extremely rare, yet very important if we are to quantify seasonal, annual and longer time frame mixing processes in the ocean. We have established a cabled observatory in the coastal area of Oshima Island, located south of Tokyo, Japan in 2014. The observatory was operated from 2014 to September 2018 thus providing a four-year record of ocean properties and ocean mixing at the one site. Despite some interruptions, we have collected a long time series of key physical properties using thermistor chains, ADV as well as ADCP. We use these data to calculate turbulent length scales and diffusivities observed from this long term observation site. A grand challenge is to compare these fine-scale mixing estimates against turbulent microstructure observation in a cohesive way.
Work performed with M. Tanaka, W. Aoyama and G. Ivey.