| 1. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Breakup of small aggregates in bounded and unbounded turbulent flows
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- Breakup of small tracer-like aggregates is studied by means of numerical simulations in four different flows, namely homogeneous isotropic turbulence, smooth stochastic flow, turbulent channel flow, and developing boundary layer flow. Aggregate breakup occurs when the local hydrodynamic stress σ ∼ ε1/2, where ε is the local energy dissipation, overcomes a given threshold value σcr [or equivalently εcr ∼ σcr2 ] characteristic for a given type of aggregates. Following the aggregate trajectory upon release and detecting the first occurrence of local energy dissipation exceeding the predefined threshold allows for estimating the breakup rate as a function of εcr. Results show that the breakup rate decreases with increasing threshold. For small values of the threshold, this decrease assumes consistent scaling among the different flows which is explained by universal small scale flow properties.
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| 2. |
- Bäbler, Matthäus, et al.
(författare)
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Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 766
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Tidskriftsartikel (refereegranskat)abstract
- Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.
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| 3. |
- Klose, Ann Kristin, et al.
(författare)
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Rate-induced tipping cascades arising from interactions between the Greenland Ice Sheet and the Atlantic Meridional Overturning Circulation
- 2024
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Ingår i: Earth System Dynamics. - 2190-4979 .- 2190-4987. ; 15:3, s. 635-652
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Tidskriftsartikel (refereegranskat)abstract
- The Greenland Ice Sheet (GIS) and Atlantic Meridional Overturning Circulation (AMOC) are considered tipping elements in the climate system, where global warming exceeding critical threshold levels in forcing can lead to large-scale and nonlinear reductions in ice volume and overturning strength, respectively. The positive–negative feedback loop governing their interaction with a destabilizing effect on the AMOC due to ice loss and subsequent freshwater flux into the North Atlantic as well as a stabilizing effect of a net cooling around Greenland with an AMOC weakening may determine the long-term stability of both tipping elements. Here we explore the potential dynamic regimes arising from this positive–negative tipping feedback loop in a physically motivated conceptual model. Under idealized forcing scenarios we identify conditions under which different kinds of tipping cascades can occur: herein, we distinguish between overshoot/bifurcation tipping cascades, leading to tipping of both GIS and AMOC, and rate-induced tipping cascades, where the AMOC, despite not having crossed its own intrinsic tipping point, tips nonetheless due to the fast rate of ice loss from Greenland. The occurrence of these different cascades is affected by the ice sheet disintegration time and thus eventually by the imposed forcing and its timescales. Our results suggest that it is necessary not only to avoid surpassing the respective critical levels of the environmental drivers for the Greenland Ice Sheet and Atlantic Meridional Overturning Circulation, but also to respect safe rates of environmental change to mitigate potential domino effects.
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| 4. |
- Merder, Julian, et al.
(författare)
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ICBM-OCEAN : Processing Ultrahigh-Resolution Mass Spectrometry Data of Complex Molecular Mixtures
- 2020
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Ingår i: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 92:10, s. 6832-6838
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Tidskriftsartikel (refereegranskat)abstract
- Untargeted molecular analyses of complex mixtures are relevant for many fields of research, including geochemistry, pharmacology, and medicine. Ultrahigh-resolution mass spectrometry is one of the most powerful tools in this context. The availability of open scripts and online tools for specific data processing steps such as noise removal or molecular formula assignment is growing, but an integrative tool where all crucial steps are reproducibly evaluated and documented is lacking. We developed a novel, server-based tool (ICBM-OCEAN, Institute for Chemistry and Biology of the Marine Environment, Oldenburg-complex molecular mixtures, evaluation & analysis) that integrates published and novel approaches for standardized processing of ultrahigh-resolution mass spectrometry data of complex molecular mixtures. Different from published approaches, we offer diagnostic and validation tools for all relevant steps. Among other features, we included objective and reproducible reduction of noise and systematic errors, spectra recalibration and alignment, and identification of likeliest molecular formulas. With 15 chemical elements, the tool offers high flexibility in formula attribution. Alignment of mass spectra among different samples prior to molecular formula assignment improves mass error and facilitates molecular formula confirmation with the help of isotopologues. The online tool and the detailed instruction manual are freely accessible at www.icbm.de/icbm-ocean.
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| 5. |
- Molkenthin, Nora, et al.
(författare)
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Edge anisotropy and the geometric perspective on flow networks
- 2017
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Ingår i: Chaos. - : AIP Publishing. - 1054-1500 .- 1089-7682. ; 27:3
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Tidskriftsartikel (refereegranskat)abstract
- Spatial networks have recently attracted great interest in various fields of research. While the traditional network-theoretic viewpoint is commonly restricted to their topological characteristics (often disregarding the existing spatial constraints), this work takes a geometric perspective, which considers vertices and edges as objects in a metric space and quantifies the corresponding spatial distribution and alignment. For this purpose, we introduce the concept of edge anisotropy and define a class of measures characterizing the spatial directedness of connections. Specifically, we demonstrate that the local anisotropy of edges incident to a given vertex provides useful information about the local geometry of geophysical flows based on networks constructed from spatiotemporal data, which is complementary to topological characteristics of the same flow networks. Taken both structural and geometric viewpoints together can thus assist the identification of underlying flow structures from observations of scalar variables.
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