Turbulent transport of magnetic fields. III - Stochastic excitation of global magnetic modes

1988 ◽  
Vol 332 ◽  
pp. 857 ◽  
Author(s):  
P. Hoyng
Keyword(s):  
Magnetic Fields ◽  
Turbulent Transport ◽  
Stochastic Excitation

Electrostatic particle-in-cell simulation of heat flux mitigation using magnetic fields

2016 ◽  
Vol 82 (5) ◽  
Author(s):  
Karl Felix Lüskow ◽  
S. Kemnitz ◽  
G. Bandelow ◽  
J. Duras ◽  
D. Kahnfeld ◽  
...  
Keyword(s):  
Heat Flux ◽  
Magnetic Fields ◽  
Turbulent Transport ◽  
Optical Emission ◽  
Emission Region ◽  
Transport Channel ◽  
Particle In Cell ◽  
Heat Deposition ◽  
Cell Simulation ◽  
Pic Method

The particle-in-cell (PIC) method was used to simulate heat flux mitigation experiments with partially ionised argon. The experiments demonstrate the possibility of reducing heat flux towards a target using magnetic fields. Modelling using the PIC method is able to reproduce the heat flux mitigation qualitatively. This is driven by modified electron transport. Electrons are magnetised and react directly to the external magnetic field. In addition, an increase of radial turbulent transport is also needed to explain the experimental observations in the model. Close to the target an increase of electron density is created. Due to quasi-neutrality, ions follow the electrons. Charge exchange collisions couple the dynamics of the neutrals to the ions and reduce the flow velocity of neutrals by radial momentum transport and subsequent losses. By this, the dominant heat-transport channel by neutrals gets reduced and a reduction of the heat deposition, similar to the experiment, is observed. Using the simulation a diagnostic module for optical emission is developed and its results are compared with spectroscopic measurements and photos from the experiment. The results of this study are in good agreement with the experiment. Experimental observations such as a shrank bright emission region close to the nozzle exit, an additional emission in front of the target and an overall change in colour to red are reproduced by the simulation.

Nonlinear Magneto-Electric Response Characteristics of Giant Mag-Netostrictive-Piezoelectric Composite Sensors in Harmonic and Stochastic Excitation

2015 ◽  
Vol 764-765 ◽  
pp. 1339-1343
Author(s):  
Jia Xu ◽  
Chang Yi Hong ◽  
Zhi Wen Zhu
Keyword(s):  
Magnetic Fields ◽  
Piezoelectric Composite ◽  
Electric Response ◽  
Stochastic Excitation ◽  
Nonlinear Dynamic Model ◽  
Van Der Pol ◽  
Response Characteristics ◽  
Giant Magnetostrictive Material ◽  
Jump Phenomena ◽  
Stochastic Magnetic Fields

Nonlinear magneto-electric response characteristics of giant magnetostrictive-piezoelectric composite sensors in harmonic and stochastic magnetic fields were studied in this paper. Van der Pol nonlinear difference items were introduced to interpret the hysteresis phenomena of both giant magnetostrictive material and piezoelectric ceramics. The nonlinear dynamic model of giant magnetostrictive-piezoelectric composite sensors in harmonic and stochastic magnetic fields was developed. The expression of dynamic response of the system was obtained, and the bifurcation characteristics of the system were analyzed. The magneto-electric coefficient of the composite sensors was determined, and the effects of system parameters on the dynamic characteristics and magneto-electric coefficient of the system were analyzed. The simulation results show that there are stochastic Hopf bifurcation and jump phenomena of vibration amplitude in the system, and both of that can be avoided through adjusting parameters. The experimental results show that the system’s motion becomes chaotic when stochastic excitation is extremely large. The results of this paper are helpful for optimal design and improvement of giant magnetostrictive-piezoelectric composite sensors.

scholarly journals Magnetic and rotational quenching of the Λ effect

2019 ◽  
Vol 622 ◽  
pp. A195 ◽  
Author(s):  
P. J. Käpylä
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Magnetic Fields ◽  
Differential Rotation ◽  
Large Scale ◽  
Turbulent Transport ◽  
Vertical Direction ◽  
Slow Rotation ◽  
Rapid Rotation ◽  
Maxwell Stresses

Context. Differential rotation in stars is driven by the turbulent transport of angular momentum.Aims. Our aim is to measure and parameterize the non-diffusive contribution to the total (Reynolds plus Maxwell) turbulent stress, known as the Λ effect, and its quenching as a function of rotation and magnetic field.Methods. Simulations of homogeneous, anisotropically forced turbulence in fully periodic cubes are used to extract their associated turbulent Reynolds and Maxwell stresses. The forcing is set up such that the vertical velocity component dominates over the horizontal ones, as in turbulent stellar convection. This choice of the forcing defines the vertical direction. Additional preferred directions are introduced by the imposed rotation and magnetic field vectors. The angle between the rotation vector and the vertical direction is varied such that the latitude range from the north pole to the equator is covered. Magnetic fields are introduced by imposing a uniform large-scale field on the system. Turbulent transport coefficients pertaining to the Λ effect are obtained by fitting. The results are compared with analytic studies.Results. The numerical and analytic results agree qualitatively at slow rotation and low Reynolds numbers. This means that vertical (horizontal) transport is downward (equatorward). At rapid rotation the latitude dependence of the stress is more complex than predicted by theory. The existence of a significant meridional Λ effect is confirmed. Large-scale vorticity generation is found at rapid rotation when the Reynolds number exceeds a threshold value. The Λ effect is severely quenched by large-scale magnetic fields due to the tendency of the Reynolds and Maxwell stresses to cancel each other. Rotational (magnetic) quenching of Λ occurs at more rapid rotation (at lower field strength) in the simulations than in the analytic studies.Conclusions. The current results largely confirm the earlier theoretical results, and also offer new insights: the non-negligible meridional Λ effect possibly plays a role in the maintenance of meridional circulation in stars, and the appearance of large-scale vortices raises the question of their effect on the angular momentum transport in rapidly rotating stellar convective envelopes. The results regarding magnetic quenching are consistent with the strong decrease in differential rotation in recent semi-global simulations and highlight the importance of including magnetic effects in differential rotation models.

scholarly journals Ion heat and parallel momentum transport by stochastic magnetic fields and turbulence

2021 ◽  
Author(s):  
Chang-Chun Chen ◽  
Patrick Diamond ◽  
Steve Tobias
Keyword(s):  
Residual Stress ◽  
Magnetic Fields ◽  
Critical Parameter ◽  
Turbulent Transport ◽  
Momentum Transport ◽  
Viscous Stress ◽  
Scattering Rate ◽  
Stochastic Magnetic Fields ◽  
Parallel Momentum ◽  
Acoustic Dispersion

Abstract The theory of turbulent transport of parallel momentum and ion heat by the interaction of stochastic magnetic fields and turbulence is presented. Attention is focused on determining the kinetic stress and the compressive energy flux. A critical parameter is identified as the ratio of the turbulent scattering rate to the rate of parallel acoustic dispersion. For the parameter large, the kinetic stress takes the form of a viscous stress. For the parameter small, the quasilinear residual stress is recovered. In practice, the viscous stress is the relevant form, and the quasilinear limit is not observable. This is the principal prediction of this paper. A simple physical picture is developed and shown to recover the results of the detailed analysis.

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