How to Set Up Field
Line Advection
- Load a Metadata file (*.vdf file) into VAPOR.
- In the Basic Flow Settings:
- Specify "Field Line Advection" as the flow type
- Specify three variables for the unsteady (e.g. velocity) field
- Specify three variables for the steady (e.g. magnetic) field
- Specify the "Unsteady field scale factor". (Normally with
WRF datasets this factor is 1.) This
depends on the space and time scales that were used when the data was
generated. It is the product:
(spatial-correction-factor)*(time-correction-factor), where:
- (time-correction-factor) is the number of
simulation time units (e.g. seconds) per Vapor (.vdf) time step. If the user times are correctly set in the metadata (vdf file)
then this factor should be 1.
- (spatial-correction-factor) is the ratio
of the full
domain size in VAPOR (see the VAPOR region panel) to the actual user
domain extent that was used in the simulation. If the data extents in the
metadata (vdf file)
are correctly set to the actual user coordinates then this factor should be 1.
- Set up flow seeding as follows:
- Decide whether to use a random rake, nonrandom
rake, or a
list of seeds, as the starting seed points for the field lines to be advected.
- If you are using a rake, the position
and size of
the rake can be established by using sliders, typing in numerical
values, or by using the rake tool in the visualizer window.
- To use a list of seeds, either type it in
(using the "edit seed list" button), or read the seeds from a text file, or
specify the seed points with the probe tool. Each
seed in the list has (x,y,z)-coordinates, as well as a time. If
the time is negative, that seed point is valid for any time step;
otherwise it will only be used to seed the flow at the specified
time step.
- Under the "Unsteady Flow Time Settings",
Specify the one time step when the seed points are to be
injected into the flow, using the first of the three values labeled
"Seed time start, end, increment". Only one time step is used
for seed injection.
- If the unsteady field data is not available at every
time step, you must specify the time steps that will be sampled, using
the “Unsteady Flow Time Settings." By default, every
time step is sampled from beginning to end, using values in “Time step
sample start, end, increment". Check the optional “Use time step
list" to specify an arbitrary set of time steps that will be sampled.
Optional Settings to Consider
Nonuniform random seed placement:
If you are using the “Random Rake" for seed
placement, the distribution of random seeds can be biased by a field
magnitude in the “Flow Seeding Settings.
" As you increase the bias (above 0), the
seeds are chosen at larger field magnitude values, approaching the
maximum field magnitude within the rake bounds. Bias values below
zero result in seed points having field magnitude nearer
zero. The three variables labeled “Seed Distrib. Field" identify
the vector field whose magnitude is used to determine the seed
distribution.
Seed Prioritization:
The field line advection algorithm
maximizes a field magnitude along each field line to determine
the points that will be advected to the next time step. The field
that is used for this prioritization is specified as the "Seed Priority Field".
Also, values labeled "Priority Field Min, Max" can be
specified. The search for the largest field value is
terminated when field magnitude exceeds the value of “Priority Field Max". If the
priority field magnitude at a point is less than its magnitude at the seed point
multiplied by the “Priority Field Min," then the search is terminated and
the field line is truncated at that point.
You can choose whether the seed prioritization occurs before or after the time-advection.
If you select "Prioritize before advection", then the seed for a field line at the next time
step is chosen by maximizing the priority field along the field line at the current time step.
If you select "Prioritize after advection", then several points (their number is specified as
the No. of samples) along the field line at the
current time step are all advected to the next time step, and the seed to be used at the next
time step is selected by maximizing the priority field among the points that were advected to
the next time step. The second approach, "Prioritize after advection" is useful when the seeds
from the first approach exit the current region, resulting in early termination of the
field line advection.
Color and opacity settings:
By default, path lines have a constant color and
opacity. Using the Color/Opacity mapping, you can make the
color and opacity depend on any of the following:
- Position along Flow (This is a value between 0
and 1 along
a steady field line indicating the integration position at that point
relative to the full integration)
- Field Magnitude (The magnitude of the steady
field at the
position along the path line)
- Seed Index. The seeds are numbered from 0
to N-1
where N is the number of different seed points. If this option is
selected, each field line has color or opacity determined by its seed
point.
- Variable. The color or opacity is a
mapping of
an arbitrary variable at the position of the point in the current VAPOR
data collection.
When one of these mappings is selected, be sure to
specify the color
(or opacity) map endpoints to include the range of values to be
mapped. Then use the color editor or the opacity editor to
specify the color or opacity at specific points along that range.