SFB Colloquium Talk Abstract - 2017/2018 - Vercauteren

Nikki Vercauteren, Freie Universität Berlin

"What is… Turbulence closure models based on scale similarity principles?"

In the range of turbulent flow prediction tools, Large Eddy Simulations (LES) stand in the middle, between direct numerical simulations (DNS) where all the scales of motion are resolved, and Reynolds Averaged Navier-Stokes (RANS) methods where all the turbulent scales are modeled. In LES, all the large, energy containing scales that one can computationally afford to capture on a numerical grid (the resolved scales) are simulated, and the dynamics of the small turbulent eddies (subgrid scales) that cannot be captured and their effect on the larger scales are parameterized based on resolved, or filtered, quantities.
With scale similarity in the inertial range of turbulence, a subgrid-scale (SGS) model which respects the scale-similarity should in principle be applicable at different filter scales. This principle is exploited in the dynamic SGS model to determine numerical coefficients in the model. The most widely used version is the dynamic Smagorinsky model, in which the dynamic procedure is applied to determine the appropriate value of the Smagorinsky model coefficient. Usually the appropriate coefficient is determined as the one that most accurately represents energy transfer across scales and calculating it involves averaging over directions of statistical homogeneity of the flow (for example over flow trajectories). With a refined dynamic procedure, scale-dependent coefficients are used to mitigate the assumption of scale invariance; this has proven to be useful in the vicinity of the lower boundary where the subgrid scales account for a large portion of the flow and in stably stratified conditions.
Another, structural approach is to treat the turbulent flow as a set of flow structures moving in a Lagrangian frame and to track their interactions. This is used in coherent vortex simulation methods (CVS), in which a wavelet-filer decomposes the Navier-Stokes equations solutions Wavelets are then dynamically selected to track the flow evolution with a reduced number of modes. Quantifying self-similarity in such coherent vortex approaches would give a way to extrapolate ensemble of coherent flow structures from a coarse grid to generate unresolved fluctuations, thereby defining a new turbulence closure based on self-similarity principles. This is part of the aims of project B07.

SFB Colloquium Talk Abstract - 2017/2018 - von Larcher

Thomas von Larcher, Freie Universität Berlin

"What is... Scale similarity and self organisation in turbulent flows?"

Self organisation in turbulent flows leads to the emergence of coherent vortices at different scales. Such coherent structures have been highlighted by flow visualisation methods, for example by defining vortices as areas where the vorticity magnitude is greater than the rate of strain (so-called Q-criterion). To enable self-similar extrapolation of structures to small, unresolved scales, quantitative description of self-similar structures needs to be achieved. One challenging aspect is that the geometry of coherent structures can be variable; with increasing vorticity level, one typically sees an evolution from ribbon-like structures to elongated tubes. In order to be used in this context, pattern recognition techniques need to be able to detect structures despite being stretched or rotated. Furthermore, intense small-scale structures are not randomly distributed in space and time but rather form clusters of inertial-range extent, leading to an intermittent flow organization. With increasing Reynolds number, the intermittency becomes more pronounced and fluctuations in velocity gradients become more extreme, with longer tails in their probability distribution. Non-local scale interactions appear to also impact intermittency, to an extent that scales with the Reynolds number. Studying the organization in turbulent flows using data-driven methodologies will be part of project B07.

SFB Colloquium Guest Talk Abstract - 2017 - Settanni

Giovanni Settanni, Junior faculty Max-Planck Graduate Center, Physics Department, University of Mainz:

Nano-bio interfaces investigated using molecular dynamics simulations

The effectiveness of drugs and, more in general, therapeutic agents (therapeutic proteins, liposomes, nanoparticles, medical implants etc.) is strongly influenced by the interface they form with the surrounding biological fluids. For example, the protein shell (corona) forming around nanoparticles in contact with blood is a major factor in determining their circulation half-time and immuno- and thrombogenicity. The composition of the layer of adsorbed proteins, their structure and orientation all play a fundamental role in the reaction of the organism to the therapeutic agent. However, the molecular factors at the base of these interactions are not yet very well understood. Classical molecular dynamics simulations can be used to evaluate the interactions of proteins with (nano-) materials. Here, I will present how we used this technique to study the adsorption on model surfaces of fibrinogen, a protein involved in blood coagulation, and the adsorption of plasma proteins on nanoparticles coated with hydrophilic polymers. In both cases, the simulations provide a molecular-level picture of the adsorption process which can then be used to build more general models of protein adsorption and compared with experimental data.

SFB Colloquium Guest Talk Abstract - 2017/2018 - Schubotz

Wiebke Schubotz, Max Planck Institute for Meteorology, Hamburg

High definition clouds and precipitation - an overview of the HD(CP)² project

The project HD(CP)² (High Definition Clouds and Precipitation for advancing Climate Prediction) addresses the lack of understanding of cloud and precipitation (CP) processes, which is one of the foremost problems of climate simulations and climate predictions. In its first funding phase, the project leveraged rapid developments in simulation and measurement science (through its modeling and observation modules) and thus provided new insights to resolve the CP roadblock. This resulted in a significantly improved representation of clouds and precipitation in the ICON (Icosahedral non-hydrostatic) model that is used for hindcast simulations in HD(CP)². This model is currently utilized on a scale of 150m horizontal resolution over regions so diverse as central Europe, the Tropical and the Northern Atlantic. In its second funding phase, the work of the modeling and observation modules is utilized in several synthesis modules that investigate various topics such as the fast cloud adjustments to aerosols, convective organization of clouds or the influence of land surface heterogeneity. Data from observation campaigns is made available through the project own data base SAMD to the scientific community.

SFB Colloquium Talk Abstract - 2017 - Rosenau & Heida

Matthias Rosenau, Helmholtz Center, Potsdam  &  Martin Heida, WIAS Berlin

Matthias Rosenau: "What is ... Particle Image Velocimetry?"

Particle Image Velocimetry (PIV) is a visualization method for quantitative flow and deformation analysis. PIV is a key method used in the experiments of project B01 and, in this framework, supported by the DFG by means of a 80 k€ infrastrutural investment. PIV is based on digital image correlation and signal processing techniques. The output is basically a velocity field, in case of project B01, that of an experimentally simulated earth surface during earthquake cycles. I will give a quick overview of technical issues and applications.

Martin Heida: "What is ... GENERIC?"

We give a short introduction into the fundamental idea of the "General Equation for the NonEquilibrium Reversible-Irreversible Coupling", called GENERIC, and apply it to gain new insights into the well-established model of rate-and-state friction in geology.

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