scholarly journals A flux-balanced fluid model for collisional plasma edge turbulence: Numerical simulations with different aspect ratios

2019 ◽  
Vol 26 (8) ◽  
pp. 082303 ◽  
Author(s):  
Di Qi ◽  
Andrew J. Majda ◽  
Antoine J. Cerfon
Keyword(s):  
Numerical Simulations ◽  
Fluid Model ◽  
Collisional Plasma ◽  
Plasma Edge ◽  
Aspect Ratios

scholarly journals Magnetic holes in plasmas close to the mirror instability

2007 ◽  
Vol 14 (4) ◽  
pp. 373-383 ◽  
Author(s):  
D. Borgogno ◽  
T. Passot ◽  
P. L. Sulem
Keyword(s):  
Numerical Simulations ◽  
Fluid Model ◽  
Instability Threshold ◽  
Landau Damping ◽  
Larmor Radius ◽  
Maxwell System ◽  
Unstable Regime ◽  
Finite Larmor Radius ◽  
Spatio Temporal ◽  
Hall Mhd

Abstract. Non-propagating magnetic hole solutions in anisotropic plasmas near the mirror instability threshold are investigated in numerical simulations of a fluid model that incorporates linear Landau damping and finite Larmor radius corrections calculated in the gyrokinetic approximation. This FLR-Landau fluid model reproduces the subcritical mirror bifurcation recently identified on the Vlasov-Maxwell system, both by theory and numerics. Stable magnetic hole solutions that display a polarization different from that of Hall-MHD solitons are indeed obtained slighlty below threshold, while magnetic patterns and spatio-temporal chaos emerge when the system is maintained in a mirror unstable regime.

scholarly journals Direct numerical simulations of dense suspensions: wave instabilities in liquid-fluidized beds

2007 ◽  
Vol 587 ◽  
pp. 303-336 ◽  
Author(s):  
J. J. DERKSEN ◽  
S. SUNDARESAN
Keyword(s):  
Numerical Simulations ◽  
Bulk Viscosity ◽  
Fluid Model ◽  
Travelling Waves ◽  
System Size ◽  
Direct Numerical Simulations ◽  
Fluid Particle ◽  
Spherical Particles ◽  
Particle Phase ◽  
Viscosity Term

We present results of direct numerical simulations of travelling waves in dense assemblies of monodisperse spherical particles fluidized by a liquid. The cases we study have been derived from the experimental work of others. In these simulations, the flow of interstitial fluid is solved by the lattice-Boltzmann method (LBM) and the particles move under the influence of gravity, hydrodynamic forces stemming from the LBM, subgrid-scale lubrication forces and hard-sphere collisions. We first show that the propagating inhomogeneous structures seen in the simulations are in agreement with those observed experimentally. We then use the detailed information contained in the simulation results to assess aspects of two-fluid model closures, namely, fluid–particle drag, and the various contributions to the effective stresses. We show that the rates of compaction and dilation of the particle phase in the travelling waves are comparable to the rate at which the microstructure relaxes, and that there is a pronounced effect of the rate of compaction on the average collisional normal stress. Although this effect can be expressed as an effective bulk viscosity term, this approach would require the use of a path-dependent bulk viscosity. We also find that the effective fluid–particle drag coefficient can be described well with the often-used closure motivated by the experiments of Richardson & Zaki (Trans. Inst. Chem. Engng vol. 32, 1954, p. 35). In this respect, the effect of the system size for determining the drag requires specific care. The shear viscosity of the particle phase manifests small, but clearly noticeable dependence on the rate of compaction/dilation of the particle phase. Our observations point to the need for higher-order closures that recognize the slow evolution of the microstructure in these flows and account for the effects of non-equilibrium microstructure on the stresses.

FOUR-FLUID MODEL AND NUMERICAL SIMULATIONS OF MAGNETIC STRUCTURES IN THE HELIOSHEATH

2009 ◽  
Vol 695 (1) ◽  
pp. 420-430 ◽  
Author(s):  
K. Avinash ◽  
Sean M. Cox ◽  
Dastgeer Shaikh ◽  
G. P. Zank
Keyword(s):  
Numerical Simulations ◽  
Fluid Model ◽  
Magnetic Structures

Shear localization during magma ascent: results from quasi-2D numerical simulations

2020 ◽  
Author(s):  
Oleg Melnik ◽  
Yulia Tsvetkova
Keyword(s):  
Velocity Profile ◽  
Numerical Simulations ◽  
Numerical Models ◽  
Discharge Rate ◽  
Shear Bands ◽  
Shear Zones ◽  
Shear Thinning ◽  
Magma Ascent ◽  
Variation Of Parameters ◽  
Aspect Ratios

<p>At large crystal contents magma exhibits non-Newtonian behavior, typically shear thinning due to crystal orientation along streamlines. 1D models widely used for extrusive eruption simulations cannot capture efficiently the complexity of cross-conduit variations of the properties of magmas as they assume parabolic velocity profile and averaged properties of magma. Large aspect ratios of volcanic conduits (length/diameter) makes use of fully 2D numerical models computationally expensive and not reliable because of extremely large cross-conduit variation of parameters.</p><p>Here we present results of numerical simulations of a quasi-2D model that accounts for magma crystallization with the release of the latent heat, shear thinning rheology, heat transfer and viscous dissipation. Simulated velocity profile is far from parabolic. Shear layers form initially near the wall of the conduit and migrate towards the interior as magma ascends. Shear heating results in significant increases in temperature of the magma in narrow shear bands. There is a drastic difference between the predictions of 1D and quasi-2D models in terms of pressure-discharge rate relations. Lava dome morphology can be strongly affected by the formation of shear zones inside volcanic conduits during magma ascent.</p>

scholarly journals Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence

2020 ◽  
Vol 86 (5) ◽  
Author(s):  
Plamen G. Ivanov ◽  
A. A. Schekochihin ◽  
W. Dorland ◽  
A. R. Field ◽  
F. I. Parra
Keyword(s):  
Temperature Gradient ◽  
Numerical Simulations ◽  
Blow Up ◽  
Fluid Model ◽  
Equilibrium Temperature ◽  
Plasma Phys ◽  
Turbulent Heat ◽  
Ion Temperature ◽  
Ion Temperature Gradient ◽  
Saturated State

The saturated state of turbulence driven by the ion-temperature-gradient instability is investigated using a two-dimensional long-wavelength fluid model that describes the perturbed electrostatic potential and perturbed ion temperature in a magnetic field with constant curvature (a $Z$ -pinch) and an equilibrium temperature gradient. Numerical simulations reveal a well-defined transition between a finite-amplitude saturated state dominated by strong zonal-flow and zonal temperature perturbations, and a blow-up state that fails to saturate on a box-independent scale. We argue that this transition is equivalent to the Dimits transition from a low-transport to a high-transport state seen in gyrokinetic numerical simulations (Dimits et al., Phys. Plasmas, vol. 7, 2000, 969). A quasi-static staircase-like structure of the temperature gradient intertwined with zonal flows, which have patch-wise constant shear, emerges near the Dimits threshold. The turbulent heat flux in the low-collisionality near-marginal state is dominated by turbulent bursts, triggered by coherent long-lived structures closely resembling those found in gyrokinetic simulations with imposed equilibrium flow shear (van Wyk et al., J. Plasma Phys., vol. 82, 2016, 905820609). The breakup of the low-transport Dimits regime is linked to a competition between the two different sources of poloidal momentum in the system – the Reynolds stress and the advection of the diamagnetic flow by the $\boldsymbol {E}\times \boldsymbol {B}$ flow. By analysing the linear ion-temperature-gradient modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality.

A Prediction of the Optimal Blank Contour of an Inner Elliptic Flange from Flanging

2012 ◽  
Vol 152-154 ◽  
pp. 931-934
Author(s):  
Chin Tarn Kwan ◽  
Jui Tsai Chang
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Numerical Simulations ◽  
Process Parameters ◽  
Material Flow ◽  
Flow Characteristics ◽  
Data Sets ◽  
The Finite Element Method ◽  
Aspect Ratios ◽  
Abductive Network

In this paper, the finite element method is employed in conjunction with the abductive network to predict the optimum blank contour of an inner elliptic flange with unevenness in the flanging process. Different flange heights combined with various aspect ratios of the inner elliptic flange are taken into account as the process parameters in this study. A finite element-based code is utilized to investigate the material flow characteristics under different process parameters, and the abductive network is then employed to synthesize the data sets obtained from numerical simulations, thus establishing a predictive model. From this model, an optimal blank contour for producing an elliptic inner flange with unevenness can be found.

scholarly journals Numerical Simulation of Size Effects on Countercurrent Flow Limitation in PWR Hot Leg Models

2012 ◽  
Vol 2012 ◽  
pp. 1-7
Author(s):  
I. Kinoshita ◽  
M. Murase ◽  
A. Tomiyama
Keyword(s):  
Numerical Simulation ◽  
Size Effect ◽  
Numerical Simulations ◽  
Size Effects ◽  
Fluid Model ◽  
Flow Channel ◽  
Countercurrent Flow ◽  
Flow Limitation ◽  
Two Fluid Model ◽  
Two Fluid

We have previously done numerical simulations using the two-fluid model implemented in the CFD software FLUENT6.3.26 to investigate effects of shape of a flow channel and its size on CCFL (countercurrent flow limitation) characteristics in PWR hot leg models. We confirmed that CCFL characteristics in the hot leg could be well correlated with the Wallis parameters in the diameter range of0.05 m≤D≤0.75 m. In the present study, we did numerical simulations using the two-fluid model for the air-water tests withD=0.0254 m to determine why CCFL characteristics forD=0.0254 m were severer compared with those in the range,0.05 m≤D≤0.75 m. The predicted CCFL characteristics agreed with the data forD=0.0254 m and indicated that the CCFL difference betweenD=0.0254 m and0.05 mm≤D≤0.75 mm was caused by the size effect and not by other factors.

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