1,208 research outputs found
Numerical study of laminar magneto-convection in a differentially heated square duct
Magnetohydrodynamic pressure drops are one of the main issues for liquid metal blanket in fusion reactors. Minimize the fluid velocity at few millimeters per second is one strategy that can be employed to address the problem. For such low velocities, buoyant forces can effectively contribute to drive the flow and therefore must be considered in the blanket design. In order to do so, a CFD code able to represent magneto-convective phenomena is required. This work aims to gauge the capability of ANSYS© CFX-15 to solve such cases. The laminar flow in a differentially heated duct was selected as validation benchmark. A horizontal and uniform magnetic field was imposed over a square duct with a linear and constant temperature gradient perpendicular to the field. The fully developed flow was analyzed for Gr = 10^5 and Hartmann number (M) ranging from 10^2 to 10^3. Both insulating and conducting duct walls were considered. Strong dampening of the flow in the center of the duct was observed, whereas high velocity jets appeared close to the walls parallel to the magnetic field. The numerical results were validated against theoretical and numerical results founding an excellent agreement
Magnetohydrodynamic flow and heat transfer around a heated cylinder of arbitrary conductivity
The interaction of the liquid metal with the plasma confinement magnetic field constitutes a challenge for the design of fusion reactor blankets, due to the arise of MHD effects: increased pressure drops, heat transfer suppression, etc. To overcome these issues, a dielectric fluid can be employed as coolant for the breeding zone. A typical configuration involves pipes transverse to the liquid metal flow direction. This numerical study is conducted to assess the influence of pipe conductivity on the MHD flow and heat transfer. The CFD code ANSYS CFX was employed for this purpose. The fluid is assumed to be bounded by rectangular walls with non-uniform thickness and subject to a skewed magnetic field with the main component aligned with the cylinder axis. The simulations were restricted to Re = (20, 40) and M = (10, 50). Three different scenarios for the obstacle were considered: perfectly insulating, finite conductivity and perfectly conducting. The electrical conductivity was found to affect the channel pressure penalty due to the obstacle insertion only for M = 10 and just for the two limiting cases. A general increment of the heat transfer with M was found due to the tendency of the magnetic field to equalize the flow rate between the sub-channels individuated by the pipe. The best results were obtained with the insulating pipe, due to the reduced electromagnetic drag. The generation of counter-rotating vortices close to the lateral duct walls was observed for M=50 and perfectly conducting pipe as a result of the modified currents distribution
On scaling laws in turbulent magnetohydrodynamic Rayleigh-Benard convection
We invoke the concepts of magnetic boundary layer and magnetic Rayleigh
number and use the magnetic energy dissipation rates in the bulk and the
boundary layers to derive some scaling laws expressing how Nusselt number
depends on magnetic Rayleigh number, Prandtl number and magnetic Prandtl number
for the simple case of turbulent magnetohydrodynamic Rayleigh-Benard convection
in the presence of uniform vertical magnetic field.Comment: This is typos-corrected version of the earlier version. It has some
minor changes. This brief work is not, in any sense, complete. Certain ideas
have been put forward whose applicability and validity have to be checked.
More work is on the way; constructive critisisms are most welcom
Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations
are reported for the interior of a rotating, perfectly-conducting, rigid
spherical shell that is insulator-coated on the inside. A previously-reported
spectral method is used which relies on a Galerkin expansion in
Chandrasekhar-Kendall vector eigenfunctions of the curl. The new ingredient in
this set of computations is the rigid rotation of the sphere. After a few
purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial
waves), attention is focused on selective decay and the MHD dynamo problem. In
dynamo runs, prescribed mechanical forcing excites a persistent velocity field,
usually turbulent at modest Reynolds numbers, which in turn amplifies a small
seed magnetic field that is introduced. A wide variety of dynamo activity is
observed, all at unit magnetic Prandtl number. The code lacks the resolution to
probe high Reynolds numbers, but nevertheless interesting dynamo regimes turn
out to be plentiful in those parts of parameter space in which the code is
accurate. The key control parameters seem to be mechanical and magnetic
Reynolds numbers, the Rossby and Ekman numbers (which in our computations are
varied mostly by varying the rate of rotation of the sphere) and the amount of
mechanical helicity injected. Magnetic energy levels and magnetic dipole
behavior are exhibited which fluctuate strongly on a time scale of a few eddy
turnover times. These seem to stabilize as the rotation rate is increased until
the limit of the code resolution is reached.Comment: 26 pages, 17 figures, submitted to New Journal of Physic
A GENERAL COMPUTATIONAL APPROACH FOR MAGNETOHYDRODYNAMIC FLOWS USING THE CFX CODE: BUOYANT FLOW THROUGH A VERTICAL SQUARE CHANNEL
The buoyancy-driven magnetoconvection in the cross
section of an infinitely long vertical square duct is investigated
numerically using the CFX code package. The
implementation of a magnetohydrodynamic (MHD) problem
in CFX is discussed, with particular reference to the
Lorentz forces and the electric potential boundary conditions
for arbitrary electrical conductivity of the walls.
The method proposed is general and applies to arbitrary
geometries with an arbitrary orientation of the magnetic
field. Results for fully developed flow under various thermal
boundary conditions are compared with asymptotic
analytical solutions. The comparison shows that the asymptotic
analysis is confirmed for highly conducting walls
as high velocity jets occur at the side walls. For weakly
conducting walls, the side layers become more conducting
than the side walls, and strong electric currents flow
within these layers parallel to the magnetic field. As a
consequence, the velocity jets are suppressed, and the core
solution is only corrected by the viscous forces near the
wall. The implementation of MHD in CFX is achieved
Toroidal Vortices in Resistive Magnetohydrodynamic Equilibria
Resistive steady states in toroidal magnetohydrodynamics (MHD), where Ohm's
law must be taken into account, differ considerably from ideal ones. Only for
special (and probably unphysical) resistivity profiles can the Lorentz force,
in the static force-balance equation, be expressed as the gradient of a scalar
and thus cancel the gradient of a scalar pressure. In general, the Lorentz
force has a curl directed so as to generate toroidal vorticity. Here, we
calculate, for a collisional, highly viscous magnetofluid, the flows that are
required for an axisymmetric toroidal steady state, assuming uniform scalar
resistivity and viscosity. The flows originate from paired toroidal vortices
(in what might be called a ``double smoke ring'' configuration), and are
thought likely to be ubiquitous in the interior of toroidally driven
magnetofluids of this type. The existence of such vortices is conjectured to
characterize magnetofluids beyond the high-viscosity limit in which they are
readily calculable.Comment: 17 pages, 4 figure
Mixed convection in a downward flow in a vertical duct with strong transverse magnetic field
The downward flow in a vertical duct with one heated and three thermally
insulated walls is analyzed numerically using the two-dimensional approximation
valid in the asymptotic limit of an imposed strong transverse magnetic field.
The work is motivated by the design of liquid metal blankets with poloidal
ducts for future nuclear fusion reactors, in which the main component of the
magnetic field is perpendicular to the flow direction and very strong heating
is applied at the wall facing the reaction chamber. The flow is found to be
steady-state or oscillating depending on the strengths of the heating and
magnetic field. A parametric study of the instability leading to the
oscillations is performed. It is found among other results that the flow is
unstable and develops high-amplitude temperature oscillations at the conditions
typical for a fusion reactor blanket
Thermal Radiation and Viscous Dissipation Effects on an Oscillatory Heat and Mass Transfer Flow of a Viscoelastic Fluid with Ohmic Heating
An anticipated outcome that is intended chapter is to investigate effects of magnetic field on an oscillatory flow of a viscoelastic fluid with thermal radiation, viscous dissipation with Ohmic heating which bounded by a vertical plane surface, have been studied. Analytical solutions for the quasi – linear hyperbolic partial differential equations are obtained by perturbation technique. Solutions for velocity and temperature distributions are discussed for various values of physical parameters involving in the problem. The effects of cooling and heating of a viscoelastic fluid compared to the Newtonian fluid have been discussed
Steady-State Magnetohydrodynamic Flow Around an Unmagnetized Conducting Sphere
The non-collisional interaction between conducting obstacles and magnetized
plasma winds can be found in different scenarios, from the interaction
occurring between regions inside galaxy clusters to the interaction between the
solar wind and Mars, Venus, active comets or even the interaction between Titan
and the Saturnian's magnetospheric flow. These objects generate, through
several current systems, perturbations in the streaming magnetic field leading
to its draping around the obstacle's effective conducting surface. Recent
observational results suggest that several properties associated with the
magnetic field draping, such as the location of the polarity reversal layer of
the induced magnetotail, are affected by variations in the conditions of the
streaming magnetic field. To improve our understanding of these phenomena, we
perform a characterization of several magnetic field draping signatures by
analytically solving an ideal problem in which a perfectly conducting
magnetized plasma (with frozen-in magnetic field conditions) flows around a
spherical body for various orientations of the streaming magnetic field. In
particular, we compute the shift of the inverse polarity reversal layer as the
orientation of the background magnetic field is changed.Comment: Preprint submitted to Astrophysical Journa
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