How to Set Up Field Line Advection

  1. Load a Metadata file (*.vdf file) into VAPOR.
  2. In the Basic Flow Settings:
    1. Specify "Field Line Advection" as the flow type
    2. Specify three variables for the unsteady (e.g. velocity) field
    3. Specify three variables for the steady (e.g. magnetic) field
    4. 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.
  3. Set up flow seeding as follows:
    1. 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.
    2. 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:

    1. 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)
    2. Field Magnitude (The magnitude of the steady field at the position along the path line)
    3. 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.
    4.  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.