7.1.10 Hydrodynamics control (MHD only)

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Subsections

7.1.10 Hydrodynamics control (MHD only)

The following parameters apply only to the MHD module (see also Sect.2.2 for details).

7.1.10.1 character hdcheckflux

With this parameter the CheckFlux routine of the HLLMHD solver can be (des)activated, as e.g. in

character hdcheckflux f=A80 b=80 n='Switch to activate checking of MHD fluxes' & c0='on/off' off

This parameter is only recognized by the HLLMHD scheme. Leaving it on might be safer but is definitely (slightly) slower.

7.1.10.2 integer n_orderconstrainedtransport

Order of reconstruction of the electric field in the constrained-transport step. The parameter can be set e.g. with

integer n_orderconstrainedtransport f=I4 b=4 & n='Number of hydrodynamics iterations' & c0='order of reconstruction of electric field in constrained-transport step' & c1='1: Simple arithmetic average (default), 2: Quadratic interpolation' 2

Possible values:

1: Simple arithmetic average of fluxes (default).
2: Quadratic interpolation of fluxes to the cell edges

This parameter is only recognized by the hdscheme=HLLMHD.

7.1.10.3 integer n_rescellsperchunk

The approximate number of grid cells per chunk in the 3D resistivity scheme can be specified e.g. with

integer n_rescellsperchunk f=I9 b=4 & n='Number of cells per chunk in resistivity routine' & c0='0 => one 2D slice at a time' & c1='1 => minimum chunk size (inefficient)' & c2='10000: typical value' 20000

The exact number is determined at run time to get (approximately) equal sizes of the individual chunks. The choice of this parameter does not affect the result of the computation but the memory usage and performance: Smaller (and more) chunks may result in an optimum cache usage and need the smallest amount of memory, but result in additional overhead due to frequent subroutine calls. Bigger (and less) chunks are to be preferred for vector machines and processors with large caches.

7.1.10.4 real c_rescourant

The tensor resistivity routines have their own time-step restriction. The recommended typical time step can be set e.g. with

real c_rescourant f=E15.8 b=4 n='resistivity Courant factor' u=1 & c0='range: 0.0 < C_resCourant, typically: 0.5' 0.5

7.1.10.5 real c_rescourantmax

The absolute upper limit for the resistivity time scale can be set with

real c_rescourantmax f=E15.8 b=4 n='maximum resistivity Courant factor' u=1 & c0='range: C_resCourant <= C_resCourantmax, typically 1.0' 0.95

Its value should be slightly above c_rescourant.

7.1.10.6 real c_resb

This parameter specifies the electric resistivity. Values between 0.0 and 1.0 may be reasonable. Higher values are possible but drastically reduce the time step. The default value is 0.0. Example:

real c_resb f=E15.8 b=4 & n='Parameter for numerical resistivity' u=1 0.0

The artificial electric resistivity of Stone & Pringle (2001) is used. It is given by $\eta_{B} = \verb\vert c_resB\vert \cdot \vert j\vert\cdot (\Delta x)^2 / \sqrt{\rho}$ , where $\eta_{B}$ is the magnetic diffusivity,

the current density, $\Delta x$ a mean width of the grid cell, and $\rho$ is the density. Values $\le 0$ disactivate this feature.

7.1.10.7 real c_resbconst

This parameter specifies a constant magnetic diffusivity $\eta_{\rm m}$ . The default value is 0.0. Example:

real c_resbconstant f=E15.8 b=4 & n='Parameter for constant magnetic diffusivity' u=cm^2/s 0.1

A reasonable value can be found by choosing a reasonable value for the magnetic Reynolds number ${\rm Re}_{\rm mag} = (\ell_0 v_0)/\eta_{\rm m}$ . Values $\le 0$ disactivate this feature.

7.1.10.8 real c_resepsilon

This parameter controls an additional numerical energy diffusion. Typical values are between 0.0 and 1.0. The default value is 0.0. Example:

real c_resepsilon f=E15.8 b=4 & n='Parameter for additional energy diffusion' u=1 0.5

The diffusion coefficient of the additional energy diffusion is given by $\eta_{E} = \verb\vert c_resepsilon\vert \cdot \vert j\vert\cdot (\Delta x)^2 / \sqrt{\rho}$ , analog to the magnetic diffusivity c_resB. Values $\le 0$ disactivate this feature.

7.1.10.9 real beta_inv

This parameter is used for the dual energy method. It determines which cells are updated with the thermal energy equation. Example:

real beta_inv f=E15.8 b=4 & n='1/beta dual energy parameter' u=1 -1.0

The criterion is as follows:
if $\beta < 1/\verb\vert beta_inv\vert$ the equation for the thermal energy is used.
if $\beta > 1/\verb\vert beta_inv\vert$ the equation for the total energy is used.
$\beta$ is the plasma $\beta$ , i.e., the ratio of the gas pressure to the magnetic pressure. If beta_inv is set to zero, the thermal energy equation is used for all cells. A reasonable value may be 100.0. To deactivate this feature, set beta_inv to a negative value. If not specified, this parameter is set to -1.0. This parameter works only with hdscheme=HLLMHD. When $\verb\vert va_max\vert > 0$ then a reasonable value is given by $\verb\vert beta_inv\vert \lessapprox v_{A {\rm max}}2/c_s2$ , where

is the speed of sound in the region where $\beta < 1/\verb\vert beta_inv\vert$ .

7.1.10.10 real va_max

This parameter limits the Alfvén speed to va_max by an arbitrary reduction of the Lorentz force. Example:

real va_max f=E15.8 b=4 & n='maximum Alfven speed' u=cm/s & -1.0

This parameter is only recognized in conjunction with hdscheme=HLLMHD. va_max=5.0E+06 might be a reasonable value for some specific applications to the solar photosphere. If not specified this parameter is set -1.0. Values $\le 0$ disactivate this feature. See also Eq. (2.19).

Next: 7.1.11 Tensor-viscosity control Up: 7.1 Parameter file: rhd.par Previous: 7.1.9 Hydrodynamics control (HD Contents Index