ISSN:0094-243X    

DOI:10.1063/1.1832131

出版商:AIP    

年代:1904

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.1832132

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Experiments in spherical Couette flow (flow between concentric rotating spheres) with an imposed magnetic field have yielded induced magnetic fields consistent with the magnetorotational instability. This might be expected due to the decreasing rotation rate profile in the base state. The observation is at odds though with existing theory, in that the base state has a significant turbulent component. We characterize the observed induced magnetic fields, as well as the velocity disturbance underlying the instability. The saturated state shows a variety of patterns and dynamics depending on applied magnetic field strength and rotation rate. The observed phase diagram is in qualitative agreement with linear stability theory. We also compare the observed stability diagram with that of MHD instabilities calculated by Hollerbach and Skinner. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832133

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

A concise review is given of an experimental project to study magnetorotational instability (MRI) in a short Couette geometry using liquid gallium. Motivated by the astrophysical importance and lack of direct observation of MRI in nature and in the laboratory, a theoretical stability analysis was performed to predict the required experimental parameters. Despite the long‐wavelength nature of MRI, local analysis agrees excellently with global eigenmode calculations when periodic boundary conditions are used in the axial direction. To explore the effects of rigidly rotating vertical boundaries (endcaps), a prototype water experiment was conducted using dimensions and rotation rates favored by the above analysis. Significant deviations from the expected Couette flow profiles were found. The cause of the discrepancy was investigated by nonlinear hydrodynamic simulations using realistic boundary conditions. It was found that Ekman circulation driven by the endcaps transports angular momentum and qualitatively modifies the azimuthal flow. Based on this new understanding, a new design was made to incorporate two independently driven rings at each endcap. Simulations were used to optimize the design by minimizing Ekman circulation while remaining within engineering capabilities. The new apparatus, which has been constructed and assembled, is currently being tested with water and will be ready for the MRI experiment with gallium soon. This development process illustrates the value of interplay between experiment, simulation, and analytic insight. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832134

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

It has been proposed to investigate the magnetorotational instability at a large scale liquid sodium facility. This sort of laboratory astrophysics is encouraged by the recent successful dynamo experiments. We report on our experiences with the Riga dynamo experiment where magnetic field self‐excitation is achieved in a cylindrical vessel filled with approximately 2 m3of liquid sodium which can reach flow velocities up to 20 m/s. The main experimental results on the kinematic and the saturation regime are compared with numerical modelling. Some focus is also laid on the spectra of the magnetic field and the pressure. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832135

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Convection in a spherical shell under the influence of a radial force field is an important problem in the geophysical and astrophysical framework. To turn out the earth gravity field which breaks the simulated central force field, such a experiment has to be performed in a microgravity environment. The radial force field is produced by applying a voltage difference between the inner and outer spheres: the dielectrophoretic force. This last field differs from the earth’s gravity one. This paper aims at simulating the bifurcated dynamics in this framework. In particular, motions with a 3D structure are presented. The most of simulation results are corroborated by the bifurcation analysis. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832136

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The magnetic field induced by the nonstationary screw flow of gallium in a toroidal channel is investigated experimentally. Some typical configurations of the imposed magnetic field are considered. The induced field is measured by sensors placed outside of the channel and by the moving probes in the shell. The induction effects observed are attributed to the mean screw flow. An upper bound to the action of the small‐scale flow perturbations is estimated. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832137

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The linear theory of MHD Taylor‐Couette flow (subject to an axial magnetic field and unbounded inz) is presented in order to prepare laboratory experiments to probe the MRI. Only stationary flow patterns are considered but also with nonaxisymmetry and for small magnetic Prandtl numbers. If the outer cylinder is at rest for a small interval of magnetic field amplitudes subcritical excitation is found but only for Pm > 1. For rotating outer cylinder beyond the Rayleigh line the situation is different. Characteristic minima are found for the Reynolds number of the inner cylinder for Hartmann numbers of order 10 for Pm = 1 and 1000 for Pm = 10−5. The minimalmagneticReynolds number in all these cases is of order 10 (see Fig. 7). For liquid sodium (Pm = 10−5) the characteristic values for a container withRin=Rout/2 = 10 cm are 20 Hz for the inner rotation frequency and 1700 Gauss for the magnetic field amplitude. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832138

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present calculations of instabilities in hydromagnetic Taylor‐Couette flow at finite and infinite aspect ratios. In the former case we are concerned with the existence of anomalous modes; in the latter case we describe the magneto‐rotational instability as well as kinematic and fully self‐consistent dynamos. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832139

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The magnetorotational instability (MRI) can destabilize hydrodynamically stable flows which are characterized by an angular momentum that is increasing with the radius and by an angular velocity that is decreasing with radius. Its astrophysical importance comes from the fact that the Kepler flow with &OHgr;(r) ∼r−3/2exactly such a behaviour. In order to investigate MRI in a laboratory experiment, the Taylor‐Couette flow with &OHgr;(r) =A + Br2withA> 0 has been proposed as a substitute for the Kepler flow. In this paper we consider the Taylor‐Dean flow as another example of a flow profile which can exhibit the necessary radial dependence. Taylor‐Dean flows are a combination of the traditional Taylor‐Couette flow with an additional flow that is produced by an azimuthal force. Special focus is laid on the case that the Taylor‐Couette part of the flow is a rigid body rotation and the magnitude of the Dean flow is adjusted in such a way that in the outer part of the flow the conditions for MRI are fulfilled. Based on the dispersion relation derived by Ji, Kageyama and Goodman, in combination with some preliminary global instability analysis, we give some first estimates for the physical parameters of a Taylor‐Dean MRI experiment with liquid sodium. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1832140

出版商:AIP    

年代:1904

数据来源: AIP