Tilgner, Andreas

[["dc.bibliographiccitation.artnumber","024606"],["dc.bibliographiccitation.issue","2"],["dc.bibliographiccitation.journal","Physical Review Fluids"],["dc.bibliographiccitation.volume","2"],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2020-12-10T18:25:37Z"],["dc.date.available","2020-12-10T18:25:37Z"],["dc.date.issued","2017"],["dc.description.abstract","Numerical simulations of the G.O. Roberts dynamo are presented. Dynamos both with and without a significant mean field are obtained. Exact bounds are derived for the total energy which conform with the Kolmogorov phenomenology of turbulence. Best fits to numerical data show the same functional dependences as the inequalities obtained from optimum theory."],["dc.identifier.doi","10.1103/PhysRevFluids.2.024606"],["dc.identifier.eissn","2469-990X"],["dc.identifier.isi","000396067700003"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/75769"],["dc.language.iso","en"],["dc.notes.intern","DOI Import GROB-354"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","PUB_WoS_Import"],["dc.publisher","Amer Physical Soc"],["dc.relation.issn","2469-990X"],["dc.title","Scaling laws and bounds for the turbulent G.O. Roberts dynamo"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","363"],["dc.bibliographiccitation.journal","Journal of Fluid Mechanics"],["dc.bibliographiccitation.lastpage","379"],["dc.bibliographiccitation.volume","492"],["dc.contributor.author","Lorenzani, S."],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T10:35:30Z"],["dc.date.available","2018-11-07T10:35:30Z"],["dc.date.issued","2003"],["dc.description.abstract","As a model for the precession-driven motion in the Earth's core, the flow of incompressible fluid inside a spheroidal shell with imposed rotation and precession is investigated by direct numerical simulation. In one set of simulations, free-slip boundary conditions are used in order to isolate inertial instabilities. These occur as triad resonances involving pairs of inertial modes which have the form of columnar vortices. The simulations reproduce the phenomenon of 'resonant collapses' in which the excited modes periodically grow and suddenly decay into turbulence. The experiments of Malkus (1968) are simulated using a hyperviscosity. A hysteretic transition towards developed turbulence observed in one of these experiments can be interpreted as a feature of the basic laminar flow rather than the instability itself. A similar transition can be excluded for Earth's parameters."],["dc.identifier.doi","10.1017/S002211200300572X"],["dc.identifier.isi","000186099500017"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/45114"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Cambridge Univ Press"],["dc.relation.issn","0022-1120"],["dc.title","Inertial instabilities of fluid flow in precessing spheroidal shells"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","101"],["dc.bibliographiccitation.issue","1-3"],["dc.bibliographiccitation.journal","Physics of The Earth and Planetary Interiors"],["dc.bibliographiccitation.lastpage","107"],["dc.bibliographiccitation.volume","153"],["dc.contributor.author","Simkanin, J."],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T10:54:03Z"],["dc.date.available","2018-11-07T10:54:03Z"],["dc.date.issued","2005"],["dc.description.abstract","An \"invisible dynamo\" is a dynamo which operates in an electrically conducting region surrounded by vacuum and which generates a magnetic field which is trapped in the electrically conducting region so that no magnetic field exists in the vacuum. Consequently, observations from outside the conducting region are unable to detect this dynamo. The search for an invisible kinematic dynamo is started in an infinite cylinder with periodic boundary conditions along the axis. The first results suggest that invisible magnetic decay modes exist in cylinders, but that no invisible growing field can be supported by the dynamo mechanism. (c) 2005 Elsevier B.V. All rights reserved."],["dc.identifier.doi","10.1016/j.pepi.2005.01.006"],["dc.identifier.isi","000233837100010"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/49487"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Elsevier Science Bv"],["dc.publisher.place","Amsterdam"],["dc.relation.conference","8th Symposium on the Study of the Earths Deep Interior (SEDI)"],["dc.relation.eventlocation","Garmisch Partenkirchen, GERMANY"],["dc.relation.issn","0031-9201"],["dc.title","Searching invisible helical dynamos in a cylinder"],["dc.type","conference_paper"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","015305"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","PHYSICAL REVIEW E"],["dc.bibliographiccitation.volume","80"],["dc.contributor.author","Schmitz, S."],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T08:28:31Z"],["dc.date.available","2018-11-07T08:28:31Z"],["dc.date.issued","2009"],["dc.description.abstract","Numerical simulation of rotating convection in plane layers with free slip boundaries show that the convective flows can be classified according to a quantity constructed from the Reynolds, Prandtl, and Ekman numbers. Three different flow regimes appear: laminar flow close to the onset of convection, turbulent flow in which the heat flow approaches the heat flow of nonrotating convection, and an intermediate regime in which the heat flow scales according to a power law independent of thermal diffusivity and kinematic viscosity."],["dc.identifier.doi","10.1103/PhysRevE.80.015305"],["dc.identifier.isi","000268616500008"],["dc.identifier.pmid","19658763"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/16440"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Amer Physical Soc"],["dc.relation.issn","1539-3755"],["dc.title","Heat transport in rotating convection without Ekman layers"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","169"],["dc.bibliographiccitation.issue","6988"],["dc.bibliographiccitation.journal","Nature"],["dc.bibliographiccitation.lastpage","171"],["dc.bibliographiccitation.volume","429"],["dc.contributor.author","Christensen, Ulrich R."],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T10:48:57Z"],["dc.date.available","2018-11-07T10:48:57Z"],["dc.date.issued","2004"],["dc.description.abstract","In the Earth's fluid outer core, a dynamo process converts thermal and gravitational energy into magnetic energy. The power needed to sustain the geomagnetic field is set by the ohmic losses (dissipation due to electrical resistance)(1). Recent estimates of ohmic losses cover a wide range, from 0.1 to 3.5 TW, or roughly 0.3 - 10% of the Earth's surface heat flow(1-4). The energy requirement of the dynamo puts constraints on the thermal budget and evolution of the core through Earth's history(1-5). Here we use a set of numerical dynamo models to derive scaling relations between the core's characteristic dissipation time and the core's magnetic and hydrodynamic Reynolds numbers dimensionless numbers that measure the ratio of advective transport to magnetic and viscous diffusion, respectively. The ohmic dissipation of the Karlsruhe dynamo experiment(6) supports a simple dependence on the magnetic Reynolds number alone, indicating that flow turbulence in the experiment and in the Earth's core has little influence on its characteristic dissipation time. We use these results to predict moderate ohmic dissipation in the range of 0.2-0.5 TW, which removes the need for strong radioactive heating in the core(7) and allows the age of the solid inner core to exceed 2.5 billion years."],["dc.identifier.doi","10.1038/nature02508"],["dc.identifier.isi","000221356300037"],["dc.identifier.pmid","15141208"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/48320"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Nature Publishing Group"],["dc.relation.issn","0028-0836"],["dc.title","Power requirement of the geodynamo from ohmic losses in numerical and laboratory dynamos"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","094103"],["dc.bibliographiccitation.issue","9"],["dc.bibliographiccitation.journal","Physics of Fluids"],["dc.bibliographiccitation.volume","26"],["dc.contributor.author","Kellner, M."],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T09:35:45Z"],["dc.date.available","2018-11-07T09:35:45Z"],["dc.date.issued","2014"],["dc.description.abstract","Finger convection is observed experimentally in an electrodeposition cell in which a destabilizing gradient of copper ions is maintained against a stabilizing temperature gradient. This double-diffusive system shows finger convection even if the total density stratification is unstable. Finger convection is replaced by an ordinary convection roll if convection is fast enough to prevent sufficient heat diffusion between neighboring fingers, or if the thermal buoyancy force is less than 1/30 of the compositional buoyancy force. At the transition, the ion transport is larger than without an opposing temperature gradient. (C) 2014 AIP Publishing LLC."],["dc.identifier.doi","10.1063/1.4895844"],["dc.identifier.isi","000342852400034"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/32459"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Amer Inst Physics"],["dc.relation.issn","1089-7666"],["dc.relation.issn","1070-6631"],["dc.title","Transition to finger convection in double-diffusive convection"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","30"],["dc.bibliographiccitation.journal","Physics of The Earth and Planetary Interiors"],["dc.bibliographiccitation.lastpage","38"],["dc.bibliographiccitation.volume","231"],["dc.contributor.author","Wei, Xing"],["dc.contributor.author","Arlt, Rainer"],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T09:39:28Z"],["dc.date.available","2018-11-07T09:39:28Z"],["dc.date.issued","2014"],["dc.description.abstract","We investigate numerically the self-sustained dynamo action in a spinning sphere whose sense of rotation reverses periodically. This system serves as a simple model of a dynamo in small bodies powered by frequent collisions. It is found that dynamo action is possible in some intervals of collision rates. At high Ekman numbers the laminar spin-up flow is helical in the boundary layers and the Ekman circulation together with the azimuthal shear powers the dynamo action. At low Ekman number a non-axisymmetric instability helps the dynamo action. The intermittency of magnetic field occurs at low Ekman number. (C) 2014 Elsevier B.V. All rights reserved."],["dc.description.sponsorship","program PlanetMag of Deutsche Forschungsgemeinschaft (DFG) [SPP1488]"],["dc.identifier.doi","10.1016/j.pepi.2014.03.004"],["dc.identifier.isi","000337660500003"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/33288"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Elsevier Science Bv"],["dc.relation.issn","1872-7395"],["dc.relation.issn","0031-9201"],["dc.title","A simplified model of collision-driven dynamo action in small bodies"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","469"],["dc.bibliographiccitation.issue","5-6"],["dc.bibliographiccitation.journal","Geophysical & Astrophysical Fluid Dynamics"],["dc.bibliographiccitation.lastpage","487"],["dc.bibliographiccitation.volume","101"],["dc.contributor.author","Kelley, Douglas H."],["dc.contributor.author","Triana, Santiago Andres"],["dc.contributor.author","Zimmerman, Daniel S."],["dc.contributor.author","Tilgner, Andreas"],["dc.contributor.author","Lathrop, Daniel P."],["dc.date.accessioned","2018-11-07T10:58:04Z"],["dc.date.available","2018-11-07T10:58:04Z"],["dc.date.issued","2007"],["dc.description.abstract","Dynamics occurring in the Earth's outer core involve convection, dynamo action, geomagnetic reversals, and the effects of rapid rotation, among other processes. Inertial waves are known to arise in rotating fluids, and their presence in the core has been previously argued using seismological data (Aldridge and Lumb 1987). They may also be involved in flows affecting the geodynamo. We report experimental observations of inertial wave modes in an Earth-like geometry: laboratory spherical Couette flow with an aspect ratio 0.33, using liquid sodium as the working fluid. Inertial modes are detected via magnetic induction and show good agreement with theoretical predictions in frequency, wavenumber, and magnetic induction structure. Our findings imply that linear wave behavior can dominate the dynamics even in turbulent flows with large Reynolds number Re, where nonlinear behaviors might be expected (here Re similar to 10(7)). We present evidence that strong differential rotation excites the modes via over-reflection. Earth's inner core may also super-rotate and thereby excite inertial modes in the same way. Zonal flows in the core, likely to have higher speeds than the super-rotation, may be a stronger source for exciting inertial modes in the Earth."],["dc.identifier.doi","10.1080/03091920701561907"],["dc.identifier.isi","000252668900009"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/50396"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Taylor & Francis Ltd"],["dc.publisher.place","Abingdon"],["dc.relation.conference","10th Symposium on the Study of the Earths Deep Interior (SEDI)"],["dc.relation.eventlocation","Prague, CZECH REPUBLIC"],["dc.relation.issn","0309-1929"],["dc.title","Inertial waves driven by differential rotation in a planetary geometry"],["dc.type","conference_paper"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","248501"],["dc.bibliographiccitation.issue","24"],["dc.bibliographiccitation.journal","Physical Review Letters"],["dc.bibliographiccitation.volume","109"],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T09:02:21Z"],["dc.date.available","2018-11-07T09:02:21Z"],["dc.date.issued","2012"],["dc.description.abstract","Numerical simulations of dynamos in rotating Rayleigh-Benard convection in plane layers are presented. Two different types of dynamos exist which obey different scaling laws for the amplitude of the magnetic field. The transition between the two occurs within a hydrodynamically uniform regime which can be classified as rapidly rotating convection. DOI: 10.1103/PhysRevLett.109.248501"],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft (DFG)"],["dc.identifier.doi","10.1103/PhysRevLett.109.248501"],["dc.identifier.isi","000312299400021"],["dc.identifier.pmid","23368398"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/24662"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Amer Physical Soc"],["dc.relation.issn","0031-9007"],["dc.title","Transitions in Rapidly Rotating Convection Driven Dynamos"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","42"],["dc.bibliographiccitation.journal","BioMedical Engineering OnLine"],["dc.bibliographiccitation.volume","5"],["dc.contributor.author","Bernhard, Stefan"],["dc.contributor.author","Moehlenkamp, Stefan"],["dc.contributor.author","Tilgner, Andreas"],["dc.date.accessioned","2018-11-07T09:40:38Z"],["dc.date.available","2018-11-07T09:40:38Z"],["dc.date.issued","2006"],["dc.description.abstract","Background: The pressure drop - flow relations in myocardial bridges and the assessment of vascular heart disease via fractional flow reserve (FFR) have motivated many researchers the last decades. The aim of this study is to simulate several clinical conditions present in myocardial bridges to determine the flow reserve and consequently the clinical relevance of the disease. From a fluid mechanical point of view the pathophysiological situation in myocardial bridges involves fluid flow in a time dependent flow geometry, caused by contracting cardiac muscles overlying an intramural segment of the coronary artery. These flows mostly involve flow separation and secondary motions, which are difficult to calculate and analyse. Methods: Because a three dimensional simulation of the haemodynamic conditions in myocardial bridges in a network of coronary arteries is time-consuming, we present a boundary layer model for the calculation of the pressure drop and flow separation. The approach is based on the assumption that the flow can be sufficiently well described by the interaction of an inviscid core and a viscous boundary layer. Under the assumption that the idealised flow through a constriction is given by near-equilibrium velocity profiles of the Falkner-Skan-Cooke (FSC) family, the evolution of the boundary layer is obtained by the simultaneous solution of the Falkner-Skan equation and the transient von-Karman integral momentum equation. Results: The model was used to investigate the relative importance of several physical parameters present in myocardial bridges. Results have been obtained for steady and unsteady flow through vessels with 0 - 85% diameter stenosis. We compare two clinical relevant cases of a myocardial bridge in the middle segment of the left anterior descending coronary artery ( LAD). The pressure derived FFR of fixed and dynamic lesions has shown that the flow is less affected in the dynamic case, because the distal pressure partially recovers during re-opening of the vessel in diastole. We have further calculated the wall shear stress (WSS) distributions in addition to the location and length of the flow reversal zones in dependence on the severity of the disease. Conclusion: The described boundary layer method can be used to simulate frictional forces and wall shear stresses in the entrance region of vessels. Earlier models are supplemented by the viscous effects in a quasi three-dimensional vessel geometry with a prescribed wall motion. The results indicate that the translesional pressure drop and the mean FFR compares favourably to clinical findings in the literature. We have further shown that the mean FFR under the assumption of Hagen-Poiseuille flow is overestimated in developing flow conditions."],["dc.identifier.doi","10.1186/1475-925X-5-42"],["dc.identifier.isi","000242393300001"],["dc.identifier.pmid","16790065"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/33546"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Biomed Central Ltd"],["dc.relation.issn","1475-925X"],["dc.title","Transient integral boundary layer method to calculate the translesional pressure drop and the fractional flow reserve in myocardial bridges"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]