UCY-CompSci

We investigate the effects of the consistency of strong fringing decreasing magnetic fields on numerical simulations of classical experimental data. Studies about fringing magnetic fields have attracted the attention of the fusion community in relation to the design of the liquid-metal flow blankets for fusion nuclear reactors. One-dimensional fitting functions neglecting magnetic field consistency have been adopted in previous numerical studies. Thanks to complete three-dimensional numerical simulations, the effect of the physical consistency of the magnetic field on fluid flow can now be assessed. We present a technique for generating discretely consistent magnetic fields based on classical one-dimensional fittings. With this method, key magnetic field features, such as the bending of the magnetic lines, are accurately reproduced and, therefore, the validity of the technique is established. Consistent and inconsistent magnetic fields have been tested under very strong decreasing magnetic fields with insulating and conducting walls using direct numerical simulations. The results show a moderate, but systematic, improvement of the predictions with respect to the experiments. As an example, the repeated under-prediction of the peak transverse pressure gradient, observed in the results of asymptotic methods and of direct numerical simulations, is explained by the historically neglected consistency of the fringing magnetic field.

1] We perform three-dimensional (3-D) numerical simulations of an oceanic lithosphere cooling above a convective mantle to investigate mantle flow geometry and its associated lithospheric features. A constant horizontal velocity (half spreading velocity) of 2 (or 4) cm/yr is applied at the box surface to mimic plate motion. In all cases, because of the temperature-dependent viscosity, a rigid conductive lithosphere cools progressively beneath the two plates separated by the ridge. After 16-40 Ma, cold downgoing instabilities develop at the base of the lithosphere. Small-scale convection is then superimposed on the large-scale circulation. It produces, and then interacts with, a slowly evolving short-wavelength isotherms topography at the base of the lithosphere. The influence of the imposed ridge geometry on the mantle flow appears by comparison of simulations with case A, a linear ridge perpendicular to the plate motion; case B, a linear ridge strongly oblique to the plate motion; and case C, a ridge with transform faults along the spreading center with a mean orientation strongly oblique to the plate motion. All simulations reveal complex interaction between the isotherm topography within the base of the lithosphere, the small-scale flow generated at or just below the base of the lithosphere, and the large-scale flow dominating in the mantle below. The large-scale flow appears always controlled by the mean ridge axis direction and thus may be oblique to the imposed plate motion direction. It is enhanced by the instabilities development. We argue that the large-scale flow orientation results from the interaction with the small-scale velocity field which flows down the isotherm topography at the base of the lithosphere. The latter is either inherited (lithospheric cooling, transform faults) or develops as boundary layer instabilities. Citation: Morency, C., M.-P. Doin, and C. Dumoulin (2005), Three-dimensional numerical simulations of mantle flow beneath midocean ridges,

A physically consistent approach is considered for defining an external magnetic field as needed in computational fluid dynamics problems involving magnetohydrodynamics (MHD). The approach results in simple analytical formulae that can be used in numerical studies where an inhomogeneous magnetic field influences a liquid metal flow. The resulting magnetic field is divergence and curl-free, and contains two components and parameters to vary. As an illustration, the following examples are considered: peakwise, stepwise, shelfwise inhomogeneous magnetic fields, and the field induced by a solenoid. Finally, the impact of the streamwise magnetic field component is shown qualitatively to be significant for rapidly changing fields.

We present numerical simulations without modeling of an incompressible, laminar, unidirectional circular pipe flow of an electrically conducting fluid under the influence of a uniform transverse magnetic field. Our computations are performed using a finite-volume code that uses a charge-conserving formulation [called current-conservative formulation in references (Ni et al J Comput Phys 221(1): 174-204, 2007, Ni et al J Comput Phys 227(1):205-228, 2007]. Using high resolution unstructured meshes, we consider Hartmann numbers up to 3000 and various values of the wall conductance ratio c. In the limit c Ha −1 (insulating wall), our results are in excellent agreement with the so-called asymptotic solution (Shercliff J Fluid Mech 1: 1956). For higher values of the wall conductance ratio, a discrepancy with the asymptotic solution is observed and we exhibit regions of velocity overspeed in the Roberts layers. We characterise these overspeed regions as a function of the wall conductance ratio and the Hartmann number; a set of scaling laws is derived that is coherent with existing asymptotic analysis.

This paper presents direct numerical simulations of a liquid metal flow under a space-varying magnetic field in an insulating circular duct. This is a generalization of a magnetohydrodynamic (MHD hereafter) benchmark case (MHD benchmark, 2005) in the nuclear fusion field. The benchmark was proposed after the experiments of Reed et al. (1987) owing to its similarity to the typical parameters appearing in liquid metal blankets of nuclear fusion reactors (Molokov and Reed, 2003). The main task at hand is to accurately define the transitional effects (suppression, triggering, intermittency) related to fringing magnetic fields and turbulence. For that purpose, the present paper addresses two qualitatively different cases: the first one considers the case of a turbulent flow (Reb = 7972, Reτ ≈ 520) entering a strong magnetic field (case MHD1) while the later considers the MHD flow leaving a strong magnetic field (case MHD 2). For brevity, this paper presents only some illustrative results...

ABSRACT In this study, we perform Direct Numerical Simulations (DNS) of fully developed turbulent flows in eccentric annuli having different diameter ratios. Annuli of unit eccentricity, and diameter ratios of 0.2591 and 0.1395 have been investigated, for a Reynolds number of 14600 based on the bulk velocity and outer wall diameter. This study aims to investigate the different flow characteristics, including the effect of the diameter ratio on the pressure gradient and the friction factor. Furthermore, this work confirms the presence of mean secondary flow for the configurations considered, while its effect on the flow characteristics and vortex structures is discussed in relation to the diameter ratios investigated.