434 research outputs found
Mauersegler weiter Wege. Mathias Enard: Kompass
No full textAnalysis of the peculiar scientific narrative in the novel of the Prix-Goncourt winning author Mathias Enard
Aspect-ratio dependence of the transition to the ultimate state of turbulent Rayleigh-Bénard convection
No full textWe report on measurements of the ultimate-state transition in turbulent Rayleigh-B\'enard convection obtained in a large facility known as the ``Uboot of G\"ottingen" and using pressurized sulfur hexafluoride as the convecting fluid. We found that the transition occurs over a range of which becomes more narrow as increases, ranging from which is at most weakly dependent on and close to to which varies from about for to about for
The Göttingen rotating turbulent Rayleigh-Bénard convection facility
No full textThermally driven turbulent convection under the influence of global rotation is ubiquitous in nature. Well known examples are the outer convective shell of our Sun and the outer liquid core of the Earth. Trying to understand the underlying dynamics of such flows is highly challenging, not only because of the enormous range in length- and time-scales that are involved with these geo/astrophysical cases and the complex interaction of hydrodynamics with electromagnetism, but also because direct measurements on these systems are most often impossible to carry out. We gain access to direct measurements by isolating part of the problem: We focus solely on the hydrodynamical aspects of turbulent convection by performing experiments in the lab and making comparisons with direct numerical simulations (DNS). The canonical system that we use to study such flows is Rayleigh-B\'enard convection (RBC), the flow between a warm bottom plate and cold top plate, in a fluid-filled upright cylindrical cell that is rotating around its geometrical axis. This presentation will focus on the newly constructed rotating RBC facility at the Max Planck Institute for Dynamics and Self-Organization (MPIDS) in G\"ottingen
Similarities between 2D and 3D convection for large Prandtl number
No full textUsing direct numerical simulations of Rayleigh-B\'enard convection (RBC), we perform a comparative study of the spectra and fluxes of energy and entropy for large and infinite Prandtl numbers in two (2D) and three (3D) dimensions. We observe close similarities between the 2D and 3D RBC, in particular the kinetic energy spectrum , and the entropy spectrum exhibits a dual branch with a dominant spectrum. We showed that the dominant Fourier modes in the 2D and 3D flows are very close
Stability and exact coherent structures of the asymptotic suction boundary layer with temperature gradient
No full textThe asymptotic suction boundary layer with a temperature gradient is a good point of entry to study the dynamics of thermal boundary layers by means of dynamical systems theory. The laminar flow without heating is parallel and its properties have been studied before. We add a temperature difference between the bottom plate and the free stream flow, and study the stability in dependence on Reynolds, Rayleigh and Prandtl number. In marked contrast to the usual Rayleigh-B\'enard problem, the onset of convection is subcritical. Tracking secondary bifurcations we identify time-periodic, spanwise, and doubly-localized exact coherent states for this flow
Direct Numerical Simulation of Turbulent Convection in a Rectangular Rayleigh-Benard Cell
No full textConvective Ripening and Rainfall
No full textThis paper discusses the evolution of the droplet size distribution for a liquid-in-gas aerosol contained in a Rayleigh-B\'enard cell. It introduces a non-collisional model for broadening the droplet size distribution, termed \lq convective ripening'. The paper also considers the initiation of rainfall from ice-free cumulus clouds. It is argued that while collisional mechanisms cannot explain the production of rain from clouds with water droplet diameters of , the non-collisional convective ripening mechanism gives a much faster route to increasing the size of the small fraction of droplets that grow into raindrops
Rayleigh-B\'enard convective motion of stratified fluids in the Earth's troposphere
No full textRecently, Kaladze and Misra [Phys. Scr. 99 (2024) 085013] showed that the
tropospheric stratified fluid flows may be unstable by the effects of the
negative temperature gradient and the temperature-dependent density
inhomogeneity arising from the thermal expansion. They also predicted that the
modification in the Brunt-V\"ais\"al\"a frequency by the density inhomogeneity
can lead to Rayleigh-B\'enard convective instability in the tropospheric
unbounded layers. The purpose of the present work is to revisit the
Rayleigh-B\'enard convective instability in more detail by considering both
unbounded and bounded tropospheric layers. We show that the conditions for
instability in these two cases significantly differ. The critical values of the
Raleigh numbers and the expressions for the instability growth rates of thermal
waves in the two cases are obtained and analyzed. In the case of the bounded
region, we also derive the necessary boundary conditions and note that the
vertical wave number is quantified, and the corresponding eigenvalue problem is
well-set.Comment: 10 pages, 5 figure
Dynamics of large-scale structures and heat transfer in turbulent mixed convection
No full textLow frequency oscillations have been observed in the heat transfer of mixed convection in a rectangular cavity with an aspect ratio of and .~Mixed convective flow at , , and has been studied to determine the nature of these oscillations. Therefore Particle Image Velocimetry (PIV) and temperature measurements have been performed under ambient and high pressure conditions. The PIV results have been analysed using Proper Orthogonal Decomposion (POD) to identify the characteristic frequencies of the coherent large-scale structures and their dynamics have been compared with the low frequency oscillations found in the heat transfer
Large-Scale Circulation Reversals in a 2D Rayleigh-Bénard cell
No full textWe consider the numerical simulation of a two-dimensional Rayleigh-B\'enard cell in the turbulent regime and . The flow is dominated by a large-scale inclined roll, the orientation of which switches intermittently in time. We use Proper Orthogonal Decomposition to identify the most energetic modes. We find that the first two modes respectively correspond to an antisymmetric and a symmetric mode. The next most energetic mode breaks the symmetry of the flow. During reversals, sharp variations in the temporal amplitudes of the modes are observed. We derive a low-dimensional model based on the first three most energetic modes which is able to reproduce the large-scale circulation reversals, in quantitative agreement with the simulation
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