Large-scale structures in the temperature field in turbulent Rayleigh-Bénard convection. - In: 80th Annual Meeting of the DPG and DPG-Frühjahrstagung (Spring Meeting) of the Condensed Matter Section (SKM), (2016), DY 57.11, insges. 1 S.
Numerical simulations of MHD flow transition in ducts with conducting Hartmann walls : Limtech Project A3 D4 (TUI). - Karlsruhe : KIT Scientific Publishing. - 1 Online-Ressource (X, 107 Seiten). - (KIT scientific reports ; 7713) ISBN 978-3-7315-0562-4
Literaturverzeichnis: Seite 23-24
http://nbn-resolving.de/urn:nbn:de:0072-577449
Hartmann duct flow at moderate magnetic Reynolds numbers. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 16 (2016), 1, S. 577-578
http://dx.doi.org/10.1002/pamm.201610277
Simulation and analysis of turbulent MHD channel flow with a streamwise magnetic field. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 16 (2016), 1, S. 631-632
http://dx.doi.org/10.1002/pamm.201610304
Heat and momentum transfer for magnetoconvection in a vertical external magnetic field. - In: Physical review, ISSN 2470-0053, Bd. 94 (2016), 4, 043108, insges. 7 S.
The scaling theory of Grossmann and Lohse [J. FluidMech. 407, 27 (2000)] for turbulent heat and momentum transfer is extended to the magnetoconvection case in the presence of a (strong) vertical magnetic field. A comparison with existing laboratory experiments and direct numerical simulations in the quasistatic limit allows us to restrict the parameter space to very low Prandtl and magnetic Prandtl numbers and thus to reduce the number of unknown parameters in the model. Also included is the Chandrasekhar limit, for which the outer magnetic induction field B is large enough such that convective motion is suppressed and heat is transported by diffusion. Our theory identifies four distinct regimes of magnetoconvection that are distinguished by the strength of the outer magnetic field and the level of turbulence in the flow, respectively.
https://doi.org/10.1103/PhysRevE.94.043108
Secondary optimal energy growth and magnetic damping of turbulence in Hartmann channel flow. - In: European journal of mechanics, ISSN 1873-7390, Bd. 60 (2016), S. 209-218
The transient amplification of secondary linear perturbations in Hartmann channel flow is investigated. Optimal linear growth on either antisymmetric or symmetric streaky base flow is calculated by iteratively solving the direct and adjoint governing equations. The result shows that there is still residual interaction between the top and bottom Hartmann layer at relatively large Hartmann number. Strong amplification of secondary perturbations due to inflectional instability of modulated Hartmann channel flow is observed when the primary perturbations have a sufficiently large amplitude. The characteristic streamwise wavelength of the secondary perturbation is finite and scales with the thickness of the Hartmann layer. For weak modulation of the basic flow by the streaks, the secondary perturbations are streamwise independent vortices that resemble the primary optimal perturbations. The influence of the magnetic field is examined by means of the perturbation energy budgets, and the Joule dissipation turns out to be weak compared with the viscous dissipation. The secondary instability is therefore similar to that of an asymptotic suction boundary layer. Only for the mean velocity profile the Lorentz force is decisive. The weak influence of the magnetic field on the dynamics within Hartmann layer is verified by additional direct numerical simulations where the Lorentz force is only taken into account in the mean streamwise momentum equation. The results are close to full simulations of turbulent Hartmann flow, and the differences reduce with growing Reynolds number RR based on the Hartmann layer thickness.
http://dx.doi.org/10.1016/j.euromechflu.2016.06.008
Extreme dissipation event due to plume collision in a turbulent convection cell. - In: Physical review, ISSN 2470-0053, Bd. 94 (2016), 4, 043104, insges. 8 S.
An extreme dissipation event in the bulk of a closed three-dimensional turbulent convection cell is found to be correlated with a strong reduction of the large-scale circulation flow in the system that happens at the same time as a plume emission event from the bottom plate. The reduction in the large-scale circulation opens the possibility for a nearly frontal collision of down- and upwelling plumes and the generation of a high-amplitude thermal dissipation layer in the bulk. This collision is locally connected to a subsequent high-amplitude energy dissipation event in the form of a strong shear layer. Our analysis illustrates the impact of transitions in the large-scale structures on extreme events at the smallest scales of the turbulence, a direct link that is observed in a flow with boundary layers. We also show that detection of extreme dissipation events which determine the far-tail statistics of the dissipation fields in the bulk requires long-time integrations of the equations of motion over at least a hundred convective time units.
https://doi.org/10.1103/PhysRevE.94.043104
Transition to turbulence in Hunt's flow in a moderate magnetic field. - In: epl, ISSN 1286-4854, Bd. 115 (2016), 4, S. 440012, insges. 6 S.
Pressure-driven magnetohydrodynamic duct flow in a transverse uniform magnetic field is studied by direct numerical simulation. The electric boundary conditions correspond to Hunt's flow with perfectly insulating walls parallel to the magnetic field (sidewalls) and perfectly conducting walls perpendicular to the magnetic field (Hartmann walls). The velocity distribution exhibits strong jets at the sidewalls, which are susceptible to instability even at low Reynolds numbers Re. We explore the onset of time-dependent flow and transition to states with evolved turbulence for a moderate Hartmann number Ha = 100. At low Re time-dependence appears in the form of elongated Ting-Walker vortices at the sidewalls of the duct, which, upon increasing Re, develop into more complex structures with higher energy and then the sidewall jets partially detach from the walls. At high values of Re jet detachments disappear and the flow consists of two turbulent jets and nearly laminar core. It is also demonstrated that, there is a range of Re, where Hunt's flow exhibits a pronounced hysteresis behavior, so that different unsteady states can be observed for the same flow parameters. In this range multiple states may develop and co-exist, depending on the initial conditions.
http://dx.doi.org/10.1209/0295-5075/115/44002
Global and local statistics in turbulent convection at low Prandtl numbers. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 802 (2016), S. 147-173
http://dx.doi.org/10.1017/jfm.2016.457
Lorentz force transient response at finite magnetic reynolds numbers. - In: IEEE transactions on magnetics, ISSN 1941-0069, Bd. 52 (2016), 8, S. 6201611, insges. 11 S.
In this paper, we investigate the transient response of Lorentz force at finite magnetic Reynolds numbers Rm on an electrically conducting rectangular bar that is strongly accelerated in the presence of a localized magnetic field. This is done through numerical simulations utilizing a coupled finite-difference boundary element approach. The results show good qualitative agreement with existing experiments with a circular cylinder. The Lorentz force rise time is seen to be a linear function of Rm. The linear dependence of Lorentz force on Rm is found to be valid only for low values of Rm, after which the slope decays leading to an apparent saturation in the Lorentz force at sufficiently large values of Rm. Our results provide important information for the development of Lorentz force flow meters for transient flow applications. Index Terms: Lorentz force velocimetry (LFV), magnetic Reynolds number, transient response.
http://dx.doi.org/10.1109/TMAG.2016.2546229