VAPOR GUI supports the ability to animate field lines or streamlines under the influence of a velocity field. The algorithm in VAPOR GUI was based on the work of Aake Nordlund, who proposed it as a means of animating magnetic field lines in a velocity field. The algorithm generally works as follows:
The user specifies a set of seed points for the steady (magnetic) field at a starting time T0.
These seed points are integrated in the steady field, resulting in a set of field lines.
Along each field line, the steady field magnitude is maximized, finding a point with the strongest steady field along that line.
The maximizing points are then advected in the unsteady (velocity) field to the next time step T1, to be used as seeds in the steady flow.
Steps 2 to 4 are repeated for each subsequent time step Tn.
The following video illustrates the use of field line advection to show the motion of vortices in the eye-wall of a WRF-ARW simulation of a hurricane, based on the work of Yongsheng Chen:
Flow panel settings for field line advection are in many cases the same as in steady or unsteady flow. The button labeled "Field Line Advection Setup Help" provides guidance in setting up the Flow tab. The changes that are necessary are discussed below:
Basic Flow Parameters:
In this case there are two fields to specify, the unsteady flow field (usually velocity) and the steady flow field (defining the streamlines that are being advected in the unsteady field).
The unsteady field scale factor must be set as in unsteady flow, based on the time and space units that are used.
Press the button "Field Line Advection Setup Help" for a description of the settings needed
The Unsteady Integration Direction is always "As needed" because, with field line advection, the unsteady direction is either forward or backwards, depending on whether the current time step is before or after the initial seed time.
Flow Seeding Parameters:
For field line advection, the Flow Seeding includes the specification of a "Priority field". This field is used to determine which points at one time step are advected to the next time step. To select seeds, the priority field is maximized along the field line.
The Priority Field Min and Max are cutoff values. The process of finding the seed to advect is terminated when the Min or Max is attained. More details about this process are provided in the context-sensitive help for these values.
There is a selector for choosing "Prioritize Before Advection" and "Prioritize After Advection". This controls whether the point that is advected to the next time step is the single point that maximizes the priority field, or if many points are advected, and the maximum is chosen after the advection.
The value of "Points to advect" is only used with "Prioritize after Advection". It specifies the number of points that are advected to the next time step.
Unsteady Flow Time Settings: These are the same as the settings in the Unsteady Flow. However, the "Unsteady samples per time step" and "Timestep display interval min,max" are not used. The "Seed time start" is important, it is the (single) beginning time for the field line advection. There is no end or increment for seed times.
Appearance settings: These are identical to the steady flow appearance settings, and govern the appearance of the steady flow lines at each time step.