Gravity Parameters

General

TopGridGravityBoundary (external)

A single integer which specified the type of gravitational boundary conditions for the top grid. Possible values are 0 for periodic and 1 for isolated (for all dimensions). The isolated boundary conditions have not been tested recently, so caveat emptor. Default: 0

SelfGravity (external)

This flag (1 - on, 0 - off) indicates if the baryons and particles undergo self-gravity.

SelfGravityGasOff (external)

This parameter is used in conjuction with SelfGravity so that only particles contribute to potential, not gas. Default = False (i.e. gas does contribute)

GravitationalConstant (external)

This is the gravitational constant to be used in code units. For cgs units it should be 4*pi*G. For cosmology, this value must be 1 for the standard units to hold. A more detailed decription can be found at Enzo Internal Unit System. Default: 4*pi.

GreensFunctionMaxNumber (external)

The Green’s functions for the gravitational potential depend on the grid size, so they are calculated on a as-needed basis. Since they are often re-used, they can be cached. This integer indicates the number that can be stored. They don’t take much memory (only the real part is stored), so a reasonable number is 100. [Ignored in current version]. Default: 1

GreensFunctionMaxSize

Reserved for future use.

S2ParticleSize (external)

This is the gravitational softening radius, in cell widths, in terms of the S2 particle described by Hockney and Eastwood in their book Computer Simulation Using Particles. A reasonable value is 3.0. [Ignored in current version]. Default: 3.0

GravityResolution (external)

This was a mis-guided attempt to provide the capability to increase the resolution of the gravitational mesh. In theory it still works, but has not been recently tested. Besides, it’s just not a good idea. The value (a float) indicates the ratio of the gravitational cell width to the baryon cell width. [Ignored in current version]. Default: 1

PotentialIterations (external)

Number of iterations to solve the potential on the subgrids. Values less than 4 sometimes will result in slight overdensities on grid boundaries. Default: 4.

MaximumGravityRefinementLevel (external)

This is the lowest (most refined) depth that a gravitational acceleration field is computed. More refined levels interpolate from this level, provided a mechanism for instituting a minimum gravitational smoothing length. Default: MaximumRefinementLevel (unless HydroMethod is ZEUS and radiative cooling is on, in which case it is MaximumRefinementLevel - 3).

MaximumParticleRefinementLevel (external)

This is the level at which the dark matter particle contribution to the gravity is smoothed. This works in an inefficient way (it actually smoothes the particle density onto the grid), and so is only intended for highly refined regions which are nearly completely baryon dominated. It is used to remove the discreteness effects of the few remaining dark matter particles. Not used if set to a value less than 0. Default: -1

BaryonSelfGravityApproximation (external)

This flag indicates if baryon density is derived in a strange, expensive but self-consistent way (0 - off), or by a completely reasonable and much faster approximation (1 - on). This is an experiment gone wrong; leave on. Well, actually, it’s important for very dense structures as when radiative cooling is turned on, so set to 0 if using many levels and radiative cooling is on [ignored in current version]. Default: 1

External Gravity Source

These parameters set up an external static background gravity source that is added to the acceleration field for the baryons and particles.

PointSourceGravity (external)

This parameter indicates that there is to be a (constant) gravitational field with a point source profile (PointSourceGravity = 1) or NFW profile (PointSourceGravity = 2). Default: 0

PointSourceGravityConstant (external)

If PointSourceGravity = 1, this is the magnitude of the point source acceleration at a distance of 1 length unit (i.e. GM in code units). If PointSourceGravity = 2, then it takes the mass of the dark matter halo in CGS units. ProblemType = 31 (galaxy disk simulation) automatically calculates values for PointSourceGravityConstant and PointSourceGravityCoreRadius. Default: 1

PointSourceGravityCoreRadius (external)

For PointSourceGravity = 1, this is the radius inside which the acceleration field is smoothed in code units. With PointSourceGravity = 2, it is the scale radius, rs, in CGS units (see Navarro, Frank & White, 1997). Default: 0

PointSourceGravityPosition (external)

If the PointSourceGravity flag is turned on, this parameter specifies the center of the point-source gravitational field in code units. Default: 0 0 0

ExternalGravity (external)

This fulfills the same purpose as PointSourceGravity but is more aptly named. ExternalGravity = 1 turns on an alternative implementation of the NFW profile with properties defined via the parameters HaloCentralDensity, HaloConcentration and HaloVirialRadius. Boxsize is assumed to be 1.0 in this case. ExternalGravity = 10 gives a gravitational field defined by the logarithmic potential in Binney & Tremaine, corresponding to a disk with constant circular velocity. Default: 0

ExternalGravityConstant (external)

If ExternalGravity = 10, this is the circular velocity of the disk in code units. Default: 0.0

ExternalGravityDensity

Reserved for future use.

ExternalGravityPosition (external)

If ExternalGravity = 10, this parameter specifies the center of the gravitational field in code units. Default: 0 0 0

ExternalGravityOrientation (external)

For ExternalGravity = 10, this is the unit vector of the disk’s angular momentum (e.g. a disk whose face-on view is oriented in the x-y plane would have ExternalGravityOrientation = 0 0 1). Default: 0 0 0

ExternalGravityRadius (external)

If ExternalGravity = 10, this marks the inner radius of the disk in code units within which the velocity drops to zero. Default: 0.0

UniformGravity (external)

This flag (1 - on, 0 - off) indicates if there is to be a uniform gravitational field. Default: 0

UniformGravityDirection (external)

This integer is the direction of the uniform gravitational field: 0 - along the x axis, 1 - y axis, 2 - z axis. Default: 0

UniformGravityConstant (external)

Magnitude (and sign) of the uniform gravitational acceleration. Default: 1