A sediment bed shear by a pressure-driven flow. Source: Vowinckel, B., Biegert, E., Meiburg, E., Aussillous, P., & Guazzelli, É. (2021). Rheology of mobile sediment beds sheared by viscous, pressuredriven flows. Journal of Fluid Mechanics, 921.
Rio de la Plata estuary (Brazil). Source: https://en.wikipedia.org/wiki/Estuary#/ media/File:Rio_de_la_Plata_BA_2.JPG (accessed September 27, 2021).
Simulating sediment transport Researchers in the PRO-MUD project are working to simulating the transport of sediment in surface waters and coastal areas. Through these simulations researchers can track the position and velocity of each particle, as well as the forces acting upon them, and gain new insights into how nutrients and pollutants are dispersed, as Dr Bernhard Vowinckel explains. There is enormous scope for sediments in the world’s oceans to interact and form different kinds of interesting flocs and aggregates through thousands of metres of water column. This occurs in a process called flocculation through which silt and clay particles come together to form larger structures like flocs, building on the smallest possible entity, called the primary particle. “This is not an individual clay particle - in fact it can already be an assembly of particles. Those primary particles are platelets that are bonded together so strongly through van der Waals forces that the shear conditions, provided by the flow of the current, would not be enough to tear them apart,” outlines Dr Bernhard Vowinckel. The primary particles, as the building blocks of larger particles, have a fairly uniform size distribution. “These primary particles come together to form larger aggregates or flocs. These flocs of cohesive sediment remain cohesive even at larger sizes, but they may be torn apart more easily by shear forces because of this larger size,” continues Dr Vowinckel.
PRO-MUD project As the Principal Investigator of the PROMUD project, Dr Vowinckel is now working to simulate how these sediments are transported in shallow waters and coastal areas. This project has its roots in research Dr Vowinckel conducted earlier in his career
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Flocculation in isotropic turbulence. Source: Zhao, K., Pomes, F., Vowinckel, B., Hsu, T., Bai, B., & Meiburg, E. (2021). Flocculation of suspended cohesive particles in homogeneous isotropic turbulence. Journal of Fluid Mechanics, 921.
in the US, when he was part of a group which sent material to the international space station (ISS). “I proposed that we should look at fine-grained sediments, and look at how they form flocs together in micro-gravity,” he says. On earth, a suspension of fine-grain sediments typically falls to the ground due to gravity, but the environment on the ISS opened up new research opportunities. “The idea was that if we turned off gravity then we could look at flocculation over long periods of time within a tiny container,” explains Dr Vowinckel. “Containers were then flown to the ISS, and they were stirred to homogenous suspension. Then we observed flocs emerge over time with a camera.” The aim now in the project is to build on earlier findings to simulate sediment
transport, which is affected by gravitational, hydrodynamic and electrostatic forces. In this research, Dr Vowinckel and his colleagues in the project are using computational fluid dynamics to solve a partial differential equation called the Navier-Stokes equation numerically. “This equation is impossible to solve analytically, apart from a few exceptions with heavy simplifications,” he says. This equation is solved on what researchers call a computational grid. “There is a large domain that you want to simulate, and you sub-divide that into smaller and smaller pieces, in which you can then look to solve the Navier-Stokes equation numerically,” explains Dr Vowinckel. “With a partial differential equation there are gradients of a certain quantity as a function of space and time.” Researchers can then look at discrete timesteps, which is how a continuous fluid flow is discretized, essentially transferred into a form in which it can be analysed more easily. The tiny boxes that the domain is sub-divided in – the grid – must also reflect the Kolmogorov length scale for turbulent flow, the smallest scale over which an eddy can form. “We want to resolve this kind of motion. If we don’t have a grid capable of resolving this kind of motion then we won’t be able to capture that. This is how we compute the fluid motion” says Dr Vowinckel. It is possible to go to ever finer grids, although Dr Vowinckel says there are some practical limitations. “If
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