Flow regimes of Rayleigh-Bénard convection in a vertical magnetic field. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 894 (2020), A21, S. A21-1-A21-21
The effects of a vertical static magnetic field on the flow structure and global transport properties of momentum and heat in liquid metal Rayleigh-Bénard convection are investigated. Experiments are conducted in a cylindrical convection cell of unity aspect ratio, filled with the alloy GaInSn at a low Prandtl number of Pr = 0.029. Changes of the large-scale velocity structure with increasing magnetic field strength are probed systematically using multiple ultrasound Doppler velocimetry sensors and thermocouples for a parameter range that is spanned by Rayleigh numbers of 10^6 ≤ Ra 6 × 10^7 and Hartmann numbers of Ha ≤ 1000. Our simultaneous multi-probe temperature and velocity measurements demonstrate how the large-scale circulation is affected by an increasing magnetic field strength (or Hartmann number). Lorentz forces induced in the liquid metal first suppress the oscillations of the large-scale circulation at low Ha, then transform the one-roll structure into a cellular large-scale pattern consisting of multiple up- and downwellings for intermediate Ha, before finally expelling any fluid motion out of the bulk at the highest accessible Ha leaving only a near-wall convective flow that persists even below Chandrasekhar’s linear instability threshold. Our study thus proves experimentally the existence of wall modes in confined magnetoconvection. The magnitude of the transferred heat remains nearly unaffected by the steady decrease of the fluid momentum over a large range of Hartmann numbers. We extend the experimental global transport analysis to momentum transfer and include the dependence of the Reynolds number on the Hartmann number.
https://doi.org/10.1017/jfm.2020.264
Classical 1/3 scaling of convection holds up to Ra = 10^15. - In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 1091-6490, Bd. 117 (2020), 14, S. 7594-7598
Im Titel ist "15" hochgestellt
https://doi.org/10.1073/pnas.1922794117
Fractal iso-level sets in high-Reynolds-number scalar turbulence. - In: Physical review fluids, ISSN 2469-990X, Bd. 5 (2020), 4, 044501, insges. 11 S.
We study the fractal scaling of iso-level sets of a passive scalar mixed by three-dimensional homogeneous and isotropic turbulence at high Reynolds numbers. The scalar field is maintained by a linear mean scalar gradient, and the Schmidt number is unity. A fractal box-counting dimension DF can be obtained for iso-levels below about three standard deviations of the scalar fluctuation on either side of its mean value. The dimension varies systematically with the iso-level, with a maximum of about 8/3 for the iso-level at the mean scalar value; this maximum dimension also follows as an upper bound from the geometric measure theory. We interpret this result to mean that mixing in turbulence is incomplete. A unique box-counting dimension for all iso-levels results when we consider the spatial support of the steep cliffs of the scalar conditioned on local strain rate; that unique dimension, independent of the iso-level set, is about 4/3.
https://doi.org/10.1103/PhysRevFluids.5.044501
Characterization and first results from LACIS-T: a moist-air wind tunnel to study aerosol-cloud-turbulence interactions. - In: Atmospheric measurement techniques, ISSN 1867-8548, Bd. 13 (2020), 4, S. 2015-2033
The interactions between turbulence and cloud microphysical processes have been investigated primarily through numerical simulation and field measurements over the last 10 years. However, only in the laboratory we can be confident in our knowledge of initial and boundary conditions and are able to measure under statistically stationary and repeatable conditions. In the scope of this paper, we present a unique turbulent moist-air wind tunnel, called the Turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T) which has been developed at TROPOS in order to study cloud physical processes in general and interactions between turbulence and cloud microphysical processes in particular. The investigations take place under well-defined and reproducible turbulent and thermodynamic conditions covering the temperature range of warm, mixed-phase and cold clouds (25 ˚C > T > 40 ˚C). The continuous-flow design of the facility allows for the investigation of processes occurring on small temporal (up to a few seconds) and spatial scales (micrometer to meter scale) and with a Lagrangian perspective. The here-presented experimental studies using LACIS-T are accompanied and complemented by computational fluid dynamics (CFD) simulations which help us to design experiments as well as to interpret experimental results. In this paper, we will present the fundamental operating principle of LACIS-T, the numerical model, and results concerning the thermodynamic and flow conditions prevailing inside the wind tunnel, combining both characterization measurements and numerical simulations. Finally, the first results are depicted from deliquescence and hygroscopic growth as well as droplet activation and growth experiments. We observe clear indications of the effect of turbulence on the investigated microphysical processes.
https://doi.org/10.5194/amt-13-2015-2020
Lagrangian perspectives on turbulent superstructures in Rayleigh-Bénard convection. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 19 (2019), 1, e201900201, insges. 2 S.
https://doi.org/10.1002/pamm.201900201
Decay of turbulence in a duct with transverse magnetic field. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 120-124
Decay of honeycomb-generated turbulence in a duct with a static transverse magnetic field is studied via high-resolution direct numerical simulations. The simulations follow the experimental study [1], in particular the paradoxical observation of high-amplitude velocity fluctuations, which exist in the downstream portion of the flow when the strong transverse magnetic field is imposed in the entire duct including the honeycomb exit, but not in other configurations. It is shown that the fluctuations are caused by the large-scale quasi-two-dimensional structures forming in the flow at the initial stages of the decay and surviving the magnetic suppression. The study demonstrates that turbulence decay in the presence of a magnetic field is a complex phenomenon critically depending on the state of the flow at the moment the field is introduced.
MHD Taylor-Couette flow with insulating walls at low magnetic Reynolds number. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 111-115
Instabilities in mixed convection at moderate and strong magnetic fields. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 82-86
Turbulent Rayleigh-Bénard convection in strong vertical magnetic field. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 27-31
Rayleigh-Bénard convection in a vertical magnetic field at low Prandtl number. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 2-7