Triadic resonances in non-linear simulations of a fluid flow in a precessing cylinder

We present results from three-dimensional non-linear hydrodynamic simulations of a precession driven flow in cylindrical geometry. The simulations are motivated by a dynamo experiment currently under development at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which the possibility of generating a magnetohydrodynamic dynamo will be investigated in a cylinder filled with liquid sodium and simultaneously rotating around two axes. In this study, we focus on the emergence of non-axisymmetric time-dependent flow structures in terms of inertial waves which - in cylindrical geometry - form so-called Kelvin modes. For a precession ratio ${\rm{Po}}=\Omega_p/\Omega_c=0.014$ the amplitude of the forced Kelvin mode reaches up to one fourth of the rotation velocity of the cylindrical container confirming that precession provides a rather efficient flow driving mechanism even at moderate values of ${\rm{Po}}$. More relevant for dynamo action might be free Kelvin modes with higher azimuthal wave number. These free Kelvin modes are triggered by non-linear interactions and may constitute a triadic resonance with the fundamental forced mode when the height of the container matches their axial wave lengths. Our simulations reveal triadic resonances at aspect ratios close to those predicted by the linear theory except around the primary resonance of the forced mode. In that regime we still identify various free Kelvin modes, however, all of them exhibit a retrograde drift around the symmetry axis of the cylinder and none of them can be assigned to a triadic resonance. The amplitudes of the free Kelvin modes always remain below the forced mode but may reach up to 6% of the of the container's angular velocity. The properties of the free Kelvin modes will be used in future simulations of the magnetic induction equation to investigate their ability to provide for dynamo action.Comment: 26 pages, 14 figures, submitted to New J. Phy

Towards a precession driven dynamo experiment

The most ambitious project within the DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is the set-up of a precession-driven dynamo experiment. After discussing the scientific background and some results of water pre-experiments and numerical predictions, we focus on the numerous structural and design problems of the machine. We also outline the progress of the building's construction, and the status of some other experiments that are planned in the framework of DRESDYN.Comment: 9 pages, 6 figures, submitted to Magnetohydrodynamic

Mode Conversion and Period Doubling at Plasma-β\beta Unity in an Alfv\'en-Wave Experiment with Liquid Rubidium

We report Alfv\'en-wave experiments with liquid rubidium at the Dresden High Magnetic Field Laboratory (HLD). Reaching up to 63 T, the pulsed magnetic field exceeds the critical value of 54 T at which the Alfv\'en speed becomes equal to the sound speed (plasma-$\beta$ unity). At this threshold we observe a period doubling of an applied 8 kHz CW excitation, a clear footprint for a parametric resonance between magnetosonic waves and Alfv\'en waves.Comment: 5 pages, 4 figure

Helical magnetorotational instability in a Taylor-Couette flow with strongly reduced Ekman pumping

The magnetorotational instability (MRI) is thought to play a key role in the formation of stars and black holes by sustaining the turbulence in hydrodynamically stable Keplerian accretion discs. In previous experiments the MRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds numbers by applying a helical magnetic field. The observation of this helical MRI (HMRI) was interfered with a significant Ekman pumping driven by solid end-caps that confined the instability only to a part of the Taylor-Couette cell. This paper describes the observation of the HMRI in an improved Taylor-Couette setup with the Ekman pumping significantly reduced by using split end-caps. The HMRI, which now spreads over the whole height of the cell, appears much sharper and in better agreement with numerical predictions. By analyzing various parameter dependencies we conclude that the observed HMRI represents a self-sustained global instability rather than a noise-sustained convective one.Comment: 30 pages, 22 figures, submitted to PR

Boost Camp’, a universal school-based transdiagnostic prevention program targeting adolescent emotion regulation; evaluating the effectiveness by a clustered RCT : a protocol paper

Abstract Background The transition from childhood into adolescence can be considered as a critical developmental period. Moreover, adolescence is associated with a decreased use of adaptive emotion regulation strategies and an increased use of maladaptive emotion regulation strategies increasing the risk of emotional problems. Targeting emotion regulation is therefore seen as an innovative prevention approach. The present study aims to evaluate the effectiveness of Boost camp, an innovative school-based prevention program targeting ER, on adolescents’ emotion regulation skills and emotional wellbeing. Also secondary outcomes and possible moderators will be included. Methods The aim is to reach 300 adolescents (16 class groups, 6 schools) in their first year of high school. A clustered Randomized Controlled Trial (RCT) with two conditions, intervention (n = 150) and control (n = 150), will be set up. Adolescents in the intervention condition will receive 14 lessons over the course of 2 days, followed by Booster sessions, and will be compared with adolescents in a non-intervention control group. The outcomes will be measured by self-report questionnaires at baseline, immediately after Boost camp, and at three and 6 months follow-up. Discussion Data-collection is planned to be completed in May 2018. Data-analyses will be finished the end of 2018. The presented paper describes the Boost camp program and the clustered RCT design to evaluate its effectiveness. It is expected that Boost camp will have beneficial effects. If found effective, Boost camp will have the potential to increase adolescent’s ER and well-being, and reduce the risk to become adults in need. The trials is registered on the 13th of June 2017 in ISRCTN registry [ISRCTN68235634]

FEDSM2005-77089 VELOCITY MEASUREMENTS IN METALLIC MELTS

ABSTRACT Various developments of velocity measuring techniques, their tests in different liquid metals, and applications in hot melts are reported. A Mechano-Optical Probe (MOP) performing local measurements up to temperatures of about 700°C has been developed and successfully tested. The Ultrasound Doppler Velocimetry (UDV) can be considered as another attractive technique to get velocity data from opaque flows. To extend the application range to higher temperatures and to abrasive liquids a new integrated ultrasonic sensor with an acoustic wave guide has been designed. First successful measurements in a CuSn melt of about 620°C and in liquid Al of about 750°C were carried out. A fully contactless investigation of the mean velocity field is possible by magnetic tomography. Local measurements of the induced magnetic field and the application of inverse reconstruction techniques allow an analysis of the flow structure. A first demonstration experiment showing the feasibility of this approach for the reconstruction of the three-dimensional mean velocity structure is presented