An approach to verification and validation of MHD codes for fusion applications. - In: Fusion engineering and design, ISSN 0920-3796, Bd. 100 (2015), S. 65-72
https://doi.org/10.1016/j.fusengdes.2014.04.049
Role of critical points of the skin friction field in formation of plumes in thermal convection. - In: Physical review, ISSN 1550-2376, Bd. 92 (2015), 4, S. 043006, insges. 10 S.
The dynamics in the thin boundary layers of temperature and velocity is the key to a deeper understanding of turbulent transport of heat and momentum in thermal convection. The velocity gradient at the hot and cold plates of a Rayleigh-Bénard convection cell forms the two-dimensional skin friction field and is related to the formation of thermal plumes in the respective boundary layers. Our analysis is based on a direct numerical simulation of Rayleigh-Bénard convection in a closed cylindrical cell of aspect ratio [Gamma]=1 and focused on the critical points of the skin friction field. We identify triplets of critical points, which are composed of two unstable nodes and a saddle between them, as the characteristic building block of the skin friction field. Isolated triplets as well as networks of triplets are detected. The majority of the ridges of linelike thermal plumes coincide with the unstable manifolds of the saddles. From a dynamical Lagrangian perspective, thermal plumes are formed together with an attractive hyperbolic Lagrangian coherent structure of the skin friction field. We also discuss the differences from the skin friction field in turbulent channel flows from the perspective of the Poincaré-Hopf index theorem for two-dimensional vector fields.
https://doi.org/10.1103/PhysRevE.92.043006
Electromagnetic interaction of a small magnet and liquid metal flow in a pipe with insulating or conducting walls. - In: Magnetohydrodynamics, ISSN 0024-998X, Bd. 51 (2015), 3, S. 579-588
We study the effects of electrically conducting walls on the interaction between a permanent magnet and a liquid-metal flow in a cylindrical pipe using experiments and numerical simulation. The problem is motivated by Lorentz force velocimetry, where the drag force on the magnet due to the induced eddy currents in the flow is used for flow measurement. Compared with insulating walls, the conducting walls lead to an increased drag force on the magnet. Except for low distances, the experimental results are satisfactorily reproduced in simulations using two different approximations of the magnetic field distribution.
Optimal linear perturbations in Hartmann channel flow: the influence of walls and magnetic damping. - In: Magnetohydrodynamics, ISSN 0024-998X, Bd. 51 (2015), 2, S. 225-235
The transient amplification of optimal linear perturbations in Hartmann channel flow is studied at low and moderate Hartmann numbers. These perturbations are streamwise-independent vortices. They develop into streaks, which are important for subcritical transition to turbulence. The influence of the opposite channel wall is examined by comparing antisymmetric and symmetric perturbations. Differences in energy amplification between these two types of perturbations decrease rapidly with the Hartmann number. Moreover, the energy amplification in Hartmann flow is close to that in an asymptotic suction boundary layer, i.e. the magnetic damping of perturbations has only a weak effect on transient growth.
Patterned turbulence as a feature of transitional regimes of magnetohydrodynamic duct flows. - In: Magnetohydrodynamics, ISSN 0024-998X, Bd. 51 (2015), 2, S. 237-247
We present results of a numerical analysis of transition to turbulence and laminarization processes in magnetohydrodynamic duct flows with a transverse magnetic field. The simulations continue our earlier work [1, 2], where flow regimes with localized turbulent spots near the side walls parallel to the imposed magnetic field have been discovered. The new results extend the analysis to the case of large Reynolds and Hartmann numbers and to ducts of various aspect ratios. The results show good agreement with experiments and confirm that the states with localized turbulent spots are a robust feature of transitional magnetohydrodynamic duct flows.
Enhanced enstrophy generation for turbulent convection in low-Prandtl-number fluids. - In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 1091-6490, Bd. 112 (2015), 31, S. 9530-9535
Turbulent convection is often present in liquids with a kinematic viscosity much smaller than the diffusivity of the temperature. Here we reveal why these convection flows obey a much stronger level of fluid turbulence than those in which kinematic viscosity and thermal diffusivity are the same; i.e., the Prandtl number Pr is unity. We compare turbulent convection in air at Pr=0.7 and in liquid mercury at Pr=0.021. In this comparison the Prandtl number at constant Grashof number Gr is varied, rather than at constant Rayleigh number Ra as usually done. Our simulations demonstrate that the turbulent Kolmogorov-like cascade is extended both at the large- and small-scale ends with decreasing Pr. The kinetic energy injection into the flow takes place over the whole cascade range. In contrast to convection in air, the kinetic energy injection rate is particularly enhanced for liquid mercury for all scales larger than the characteristic width of thermal plumes. As a consequence, mean values and fluctuations of the local strain rates are increased, which in turn results in significantly enhanced enstrophy production by vortex stretching. The normalized distributions of enstrophy production in the bulk and the ratio of the principal strain rates are found to agree for both Prs. Despite the different energy injection mechanisms, the principal strain rates also agree with those in homogeneous isotropic turbulence conducted at the same Reynolds numbers as for the convection flows. Our results have thus interesting implications for small-scale turbulence modeling of liquid metal convection in astrophysical and technological applications.
https://doi.org/10.1073/pnas.1505111112
Large-scale mean patterns in turbulent convection. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 776 (2015), S. 96-108
http://dx.doi.org/10.1017/jfm.2015.316
Simulation of flux expulsion and associated dynamics in a two-dimensional magnetohydrodynamic channel flow. - In: Theoretical and computational fluid dynamics, ISSN 1432-2250, Bd. 29 (2015), 4, S. 263-276
We consider a plane channel flow of an electrically conducting fluid which is driven by a mean pressure gradient in the presence of an applied magnetic field that is streamwise periodic with zero mean. Magnetic flux expulsion and the associated bifurcation in such a configuration are explored using direct numerical simulations (DNS). The structure of the flow and magnetic fields in the Hartmann regime (where the dominant balance is through Lorentz forces) and the Poiseuille regime (where viscous effects play a significant role) are studied, and detailed comparisons to the existing one-dimensional model of Kamkar and Moffatt (J Fluid Mech 90:107-122, 1982) are drawn to evaluate the validity of the model. Comparisons show good agreement of the model with DNS in the Hartmann regime, but significant differences arising in the Poiseuille regime when nonlinear effects become important. The effects of various parameters like the magnetic Reynolds number, imposed fieldwavenumber etc. on the bifurcation of the floware studied.Magnetic field line reconnections occurring during the dynamic runaway reveal a specific two-step pattern that leads to the gradual expulsion of flux in the core region.
http://dx.doi.org/10.1007/s00162-015-0352-y
Solutal Marangoni convection in a Hele-Shaw geometry: impact of orientation and gap width. - In: European physical journal special topics, ISSN 1951-6401, Bd. 224 (2015), 2, S. 261-271
We study Marangoni roll cell convection in a ternary mass transfer system composed of cyclohexanol, water and butanol (transferred species) placed in a Hele-Shaw cell. A detailed comparison of experimental and numerical results is carried out for four different cases including horizontal and vertical orientation as well as small and large gap width of the Hele-Shaw cell. The numerical simulations are based on a common gap-averaged model. For the small gap width, the numerical results qualitatively represent the experimental results. However, by comparison with the experiments, the time evolution in the simulation appears generally retarded. The results show that three-dimensional flow effects have to be expected in particular for a horizontal orientation and a large gap width.
http://dx.doi.org/10.1140/epjst/e2015-02358-2
Numerical simulation of solutal Rayleigh-Bénard-Marangoni convection in a layered two-phase system. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 14 (2014), 1, S. 643-644
We present a two-dimensional simulation of solutal Rayleigh-Bénard-Maragoni convection in a layered system. In the initial state, the solute concentration is homogeneous in each layer but not in partition equilibrium. Diffusive transfer of solute leads to convective instability. Marangoni convection dominates initially as it operates on a smaller length scale. Rayleigh convection appears later as an instability of the mixed unstably stratified fluid near the interface. Compared to pure Marangoni convection the dynamics is more disordered due to additional flow in the bulk.
http://dx.doi.org/10.1002/pamm.201410306