scholarly journals Numerical simulations of Hall MHD small-scale dynamos

2009 ◽  
Vol 5 (H15) ◽  
pp. 436-437
Author(s):  
Daniel O. Gómez ◽  
Pablo D. Mininni ◽  
Pablo Dmitruk
Keyword(s):  
Numerical Simulations ◽  
Hall Effect ◽  
Three Dimensional ◽  
Theoretical Framework ◽  
Mhd Equations ◽  
Small Scale ◽  
Hall Parameter ◽  
Hall Mhd ◽  
Three Dimensional Simulations

AbstractMuch of the progress in our understanding of dynamo mechanisms, has been made within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect eventually becomes non-negligible. We present results from three dimensional simulations of the Hall-MHD equations subjected to random non-helical forcing. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter, using a pseudospectral code to achieve exponentially fast convergence.

scholarly journals Three dimensional simulations of Hall magnetohydrodynamics

2010 ◽  
Vol 6 (S274) ◽  
pp. 433-436 ◽  
Author(s):  
Daniel O. Gómez
Keyword(s):  
Magnetic Fields ◽  
Hall Effect ◽  
Turbulent Flows ◽  
Three Dimensional ◽  
Mhd Equations ◽  
Hall Parameter ◽  
Astrophysical Objects ◽  
Hall Mhd ◽  
Three Dimensional Simulations

AbstractTurbulent flows take place in a large variety of astrophysical objects, and often times are the source of dynamo generated magnetic fields. Much of the progress in our understanding of dynamo mechanisms, has been made within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect eventually becomes non-negligible.We present results from simulations of the Hall-MHD equations. The simulations are performed with a pseudospectral code to achieve exponentially fast convergence. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter.

Vortex rings impinging on walls: axisymmetric and three-dimensional simulations

1993 ◽  
Vol 256 ◽  
pp. 615-646 ◽  
Author(s):  
Paolo Orlandi ◽  
Roberto Verzicco
Keyword(s):  
Numerical Simulations ◽  
Strain Tensor ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Vortex Rings ◽  
Compression Phase ◽  
Vortex Pairing ◽  
The Impact ◽  
Good Agreement ◽  
Three Dimensional Simulations

Accurate numerical simulations of vortex rings impinging on flat boundaries revealed the same features observed in experiments. The results for the impact with a free-slip wall compared very well with previous numerical simulations that used spectral methods, and were also in qualitative agreement with experiments. The present simulation is mainly devoted to studying the more realistic case of rings interacting with a no-slip wall, experimentally studied by Walker et al. (1987). All the Reynolds numbers studied showed a very good agreement between experiments and simulations, and, at Rev > 1000 the ejection of a new ring from the wall was seen. Axisymmetric simulations demonstrated that vortex pairing is the physical mechanism producing the ejection of the new ring. Three-dimensional simulations were also performed to investigate the effects of azimuthal instabilities. These simulations have confirmed that high-wavenumber instabilities originate in the compression phase of the secondary ring within the primary one. The large instability of the secondary ring has been explained by analysis of the rate-of-strain tensor and vorticity alignment. The differences between passive scalars and the vorticity field have been also investigated.

scholarly journals Simulations of Core Convection Dynamos in Rotating A-type Stars

2004 ◽  
Vol 215 ◽  
pp. 376-377
Author(s):  
Matthew Browning ◽  
Allan Sacha Brun ◽  
Juri Toomre
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Differential Rotation ◽  
Three Dimensional ◽  
Dynamo Action ◽  
The Core ◽  
Core Convection ◽  
Seed Field ◽  
Initial Seed ◽  
Three Dimensional Simulations

We have conducted preliminary numerical simulations of a core convection dynamo operating within an A-type star of two solar masses. Convection within the core clearly can admit magnetic dynamo action. Magnetic field strengths in our three-dimensional simulations grow by many orders of magnitude, from an initial seed field to kilo-Gauss levels. We discuss the differential rotation and magnetic field sustained in our simulations.

scholarly journals Dynamics of Orographically Triggered Banded Convection in Sheared Moist Orographic Flows

2007 ◽  
Vol 64 (10) ◽  
pp. 3542-3561 ◽  
Author(s):  
Oliver Fuhrer ◽  
Christoph Schär
Keyword(s):  
Small Amplitude ◽  
Three Dimensional ◽  
Horizontal Resolution ◽  
Small Scale ◽  
Mean Flow ◽  
Topographic Roughness ◽  
Wide Range ◽  
Orographic Convection ◽  
Wind Profiles

Abstract Shallow orographic convection embedded in an unstable cap cloud can organize into convective bands. Previous research has highlighted the important role of small-amplitude topographic variations in triggering and organizing banded convection. Here, the underlying dynamical mechanisms are systematically investigated by conducting three-dimensional simulations of moist flows past a two-dimensional mountain ridge using a cloud-resolving numerical model. Most simulations address a sheared environment to account for the observed wind profiles. Results confirm that small-amplitude topographic variations can enhance the development of embedded convection and anchor quasi-stationary convective bands to a fixed location in space. The resulting precipitation patterns exhibit tremendous spatial variability, since regions receiving heavy rainfall can be only kilometers away from regions receiving little or no rain. In addition, the presence of banded convection has important repercussions on the area-mean precipitation amounts. For the experimental setup here, the gravity wave response to small-amplitude topographic variations close to the upstream edge of the cap cloud (which is forced by the larger-scale topography) is found to be the dominant triggering mechanism. Small-scale variations in the underlying topography are found to force the location and spacing of convective bands over a wide range of scales. Further, a self-sufficient mode of unsteady banded convection is investigated that does not dependent on external perturbations and is able to propagate against the mean flow. Finally, the sensitivity of model simulations of banded convection with respect to horizontal computational resolution is investigated. Consistent with predictions from a linear stability analysis, convective bands of increasingly smaller scales are favored as the horizontal resolution is increased. However, small-amplitude topographic roughness is found to trigger banded convection and to control the spacing and location of the resulting bands. Thereby, the robustness of numerical simulations with respect to an increase in horizontal resolution is increased in the presence of topographic variations.

The importance of Hall effect on the magnetized thin accretion disc

2019 ◽  
Vol 492 (2) ◽  
pp. 1770-1777
Author(s):  
Maryam Ghasemnezhad
Keyword(s):  
Hall Effect ◽  
Equatorial Plane ◽  
Radial Direction ◽  
Vertical Structure ◽  
Accretion Rate ◽  
Accretion Disc ◽  
Mhd Equations ◽  
Self Similar ◽  
Similar Solutions

ABSTRACT To study the role of Hall effect on the structure of accretion disc, we have considered a toroidal magnetic field in our paper. To study the vertical structure of the disc, we have written a set of magnetohydrodynamic (MHD) equations in the spherical coordinates (r, θ, ϕ) based on the two assumptions of axisymmetric and steady state. Also, we employed the self-similar solutions in the radial direction to obtain the structure of the disc in the θ-direction. We have solved a set of ordinary differential equations in the θ-coordinate with symmetrical boundary conditions in the equatorial plane. In order to describe the behaviour of Hall effect, we introduced the ΛH parameter that was called the dimensionless Hall Elsasser number. The strength of the Hall effect is measured by the inverse of dimensionless Hall Elsasser number. We have shown that the strong Hall effect decreases the accretion rate or infall velocity and size of inflow part. It has also been found the Hall effect is maximum in the equatorial plane and gets the value close to zero near the boundary, and it has the antidiffusive nature. The results display that the strong Hall effect makes the standard accretion sub-Keplerian disc becomes thinner. Our solutions have shown the Hall effect leads to transport magnetic flux outward in the upper layer of the disc and it produces outflows in the surface of the disc.

scholarly journals Flow-accelerated platelet biogenesis is due to an elasto-hydrodynamic instability

2020 ◽  
Vol 117 (32) ◽  
pp. 18969-18976 ◽  
Author(s):  
Christian Bächer ◽  
Markus Bender ◽  
Stephan Gekle
Keyword(s):  
Hydrodynamic Instability ◽  
Blood Platelets ◽  
Three Dimensional ◽  
Immersed Boundary ◽  
Fusion Event ◽  
Specific Flow ◽  
Actomyosin Contractility ◽  
Biophysical Mechanism ◽  
Three Dimensional Simulations

Blood platelets are formed by fragmentation of long membrane extensions from bone marrow megakaryocytes in the blood flow. Using lattice-Boltzmann/immersed boundary simulations we propose a biological Rayleigh–Plateau instability as the biophysical mechanism behind this fragmentation process. This instability is akin to the surface tension-induced breakup of a liquid jet but is driven by active cortical processes including actomyosin contractility and microtubule sliding. Our fully three-dimensional simulations highlight the crucial role of actomyosin contractility, which is required to trigger the instability, and illustrate how the wavelength of the instability determines the size of the final platelets. The elasto-hydrodynamic origin of the fragmentation explains the strong acceleration of platelet biogenesis in the presence of an external flow, which we observe in agreement with experiments. Our simulations then allow us to disentangle the influence of specific flow conditions: While a homogeneous flow with uniform velocity leads to the strongest acceleration, a shear flow with a linear velocity gradient can cause fusion events of two developing platelet-sized swellings during fragmentation. A fusion event may lead to the release of larger structures which are observable as preplatelets in experiments. Together, our findings strongly indicate a mainly physical origin of fragmentation and regulation of platelet size in flow-accelerated platelet biogenesis.

scholarly journals The Fermi bubbles inflated by winds from the accretion flow in Sgr A*

2014 ◽  
Vol 10 (S312) ◽  
pp. 137-138
Author(s):  
Guobin Mou
Keyword(s):  
Numerical Simulations ◽  
Three Dimensional ◽  
Small Scale ◽  
Observational Constraints ◽  
Accretion Flow ◽  
Accretion Flows ◽  
The Past ◽  
Sgr A ◽  
Hydrodynamical Simulations

AbstractBy performing three-dimensional hydrodynamical simulations, we show that the Fermi bubbles could be inflated by winds launched from the “past” hot accretion flow in Sgr A*. The parameters of the accretion flow required in the model are consistent with those obtained independently from other observational constraints. The wind parameters are taken from small scale MHD numerical simulations of hot accretion flows.

Direct Numerical Simulations of Transitional Separation–Bubble Development in Swept–Blade Flow Conditions

2012 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Pressure Field ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Flow Conditions ◽  
Crossflow Instability

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a three-dimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical low-pressure gas-turbine blades with blade sweep angles of 0° and 45°. The three-dimensional pressure field in the swept configuration produces a crossflow-velocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and most amplified disturbance frequency is observed in the separated-flow region of the swept configuration, and is attributed to boundary-layer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to small-scale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the time-averaged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separation-bubble development by modifying the flow conditions at separation.

Sign in / Sign up

Export Citation Format

Share Document