TY  - JOUR
A1  - Feudel, Fred
A1  - Bergemann, Kay
A1  - Tuckerman, Laurette S.
A1  - Egbers, C.
A1  - Futterer, B.
A1  - Gellert, Marcus
A1  - Hollerbach, Rainer
T1  - Convection patterns in a spherical fluid shell
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - Symmetry-breaking bifurcations have been studied for convection in a nonrotating spherical shell whose outer radius is twice the inner radius, under the influence of an externally applied central force field with a radial dependence proportional to 1/r(5). This work is motivated by the GeoFlow experiment, which is performed under microgravity condition at the International Space Station where this particular central force can be generated. In order to predict the observable patterns, simulations together with path-following techniques and stability computations have been applied. Branches of axisymmetric, octahedral, and seven-cell solutions have been traced. The bifurcations producing them have been identified and their stability ranges determined. At higher Rayleigh numbers, time-periodic states with a complex spatiotemporal symmetry are found, which we call breathing patterns.
Y1  - 2011
U6  - https://doi.org/10.1103/PhysRevE.83.046304
SN  - 1539-3755
VL  - 83
IS  - 4
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Rüdiger, Günther
A1  - Schultz, Manfred
A1  - Hollerbach, Rainer
T1  - Destabilization of super-rotating Taylor-Couette flows by current-free helical magnetic fields
JF  - Journal of plasma physics
N2  - In an earlier paper we showed that the combination of azimuthal magnetic fields and super-rotation in Taylor-Couette flows of conducting fluids can be unstable against non-axisymmetric perturbations if the magnetic Prandtl number of the fluid is Pm not equal 1. Here we demonstrate that the addition of a weak axial field component allows axisymmetric perturbation patterns for Pm of order unity depending on the boundary conditions. The axisymmetric modes only occur for magnetic Mach numbers (of the azimuthal field) of order unity, while higher values are necessary for the non-axisymmetric modes. The typical growth time of the instability and the characteristic time scale of the axial migration of the axisymmetric mode are long compared with the rotation period, but short compared with the magnetic diffusion time. The modes travel in the positive or negative z direction along the rotation axis depending on the sign of B phi Bz. We also demonstrate that the azimuthal components of flow and field perturbations travel in phase if vertical bar B phi vertical bar >> vertical bar B-z vertical bar, independent of the form of the rotation law. Within a short-wave approximation for thin gaps it is also shown (in an appendix) that for ideal fluids the considered helical magnetorotational instability only exists for rotation laws with negative shear.
KW  - plasma instabilities
KW  - astrophysical plasmas
Y1  - 2021
U6  - https://doi.org/10.1017/S0022377821000295
SN  - 1469-7807
VL  - 87
IS  - 2
PB  - Cambridge University Press
CY  - London
ER  - 
TY  - JOUR
A1  - Feudel, Fred
A1  - Tuckerman, Laurette S.
A1  - Zaks, Michael
A1  - Hollerbach, Rainer
T1  - Hysteresis of dynamos in rotating spherical shell convection
JF  - Physical review fluids / American Physical Society
N2  - Bifurcations of dynamos in rotating and buoyancy-driven spherical Rayleigh-Benard convection in an electrically conducting fluid are investigated numerically. Both nonmagnetic and magnetic solution branches comprised of rotating waves are traced by path-following techniques, and their bifurcations and interconnections for different Ekman numbers are determined. In particular, the question of whether the dynamo branches bifurcate super- or sub-critically and whether a direct link to the primary pure convective states exists is answered.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevFluids.2.053902
SN  - 2469-990X
VL  - 2
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - BOOK
A1  - Rüdiger, Günther
A1  - Kitchatinov, Leonid L.
A1  - Hollerbach, Rainer
T1  - Magnetic processes in astrophysics : theory, simulations, experiments
Y1  - 2013
SN  - 978-3-527-41034-7
PB  - Wiley-VCH
CY  - Weinheim, Bergstr.
ER  -