Subsections
20.2 Externally Applied Fields
The FLASH distribution includes three externally applied gravitational
fields, along with a placeholder module for you to create your own. Each
provides the acceleration vector
directly, without using
the gravitational potential
(with the exception of
UserDefined, see below).
When building an application that uses an external, time-independent
Gravity implementation, no additional storage in unk
for holding gravitational potential or accelerations is needed
or defined.
This implementation creates
a spatially and temporally constant field parallel to one of the
coordinate axes. The magnitude and direction of the field can be set
at runtime. This unit is called Gravity/GravityMain/Constant.
This PlanePar version
implements a time-constant gravitational field that is parallel to
one of the coordinate axes and falls off with the square of the
distance from a fixed location. The field is assumed to be generated
by a point mass or by a spherically symmetric mass distribution. A
finite softening length may optionally be applied.
This type of gravitational field
is useful when the computational domain is large enough in the
direction radial to the field source that the field is not
approximately constant, but the domain's dimension perpendicular to
the radial direction is small compared to the distance to the source.
In this case the angular variation of the field direction may be
ignored. The PlanePar field is cheaper to compute than the
PointMass field described below,
since no fractional powers of the distance are
required. The acceleration vector is parallel to one of the coordinate
axes, and its magnitude drops off with distance along that axis as the
inverse distance squared. Its magnitude and direction are independent
of the other two coordinates.
This PointMass implementation
describes the gravitational field due to a point mass at a fixed
location. A finite softening length may optionally be applied. The
acceleration falls off with the square of the distance from a given
point. The acceleration vector is everywhere directed toward this point.
The UserDefined implementation is a placeholder module for the user to
create their own external gravitational field. All of the subroutines in this
module are stubs, and the user may copy these stubs to their setup directory to
write their own implementation, either by specifying the gravitational
acceleration directly or by specifying the gravitational potential and taking
its gradient. If your user-defined gravitational field is time-varying, you
may also want to set PPDEFINE FLASH_GRAVITY_TIMEDEP in your setup's
Config file.
