When the flow Type in the Basic Flow Parameters is set to "Unsteady", the flow integration is performed in time as well as space. An example application of unsteady flow is the following video, based on WRF simulation by Ryan Maue and analysis by Mel Shapiro of a severe atlantic storm in 1989:
The above video illustrates the trajectories of particles emitted every few minutes at 500m where the temperature is warmest. The particles and ground plane are colored by equivalent potential temperature.
Most of the settings used for unsteady flow are the same as the steady flow parameters. The settings that are different are discussed here:
Basic Flow Parameters:
The STOP button can be pressed to terminate the unsteady flow integration at the next time step. This is useful if you made a mistake and do not want to wait for completion of a time-consuming integration. When this is clicked, the Auto Flow Refresh checkbox is un-set, so there will be no integration until the user either clicks the "Refresh Flow" button or checks the Auto Flow Refresh checkbox.
The unsteady flow direction is either forwards or backwards (in time). Bidirectional integration is not supported.
VAPOR flow integration assumes that the temporal and spatial units in the VDC are consistent with the units of the vector field; e.g. if the time is in seconds and the distances are in meters then the vector field components should be in meters/second. However these units may not be properly set during the creation of the VDC and it may be necessary for the user to compensate by setting the unsteady field scale factor. The unsteady field scale factor is multiplied by the field magnitude during flow integration. If the VDC was converted from WRF-ARW data, then this factor should be 1.0. The unsteady field scale factor should be set to the product (spatial-correction-factor)*(time-correction-factor) where:
(time-correction-factor) is the number of simulation time units per VDC time step. If the user times are correctly specified in the vdf file, then this factor should be 1.
(spatial-correction-factor) is the ratio of the full domain size in the VDC user coordinates to the actual user domain extents that were used in the simulation. If the coordinates in the VDC are correctly set to the actual user coordinates in the simulation, this factor should be 1.
Unsteady Flow Time settingsmust be set properly to visualize unsteady flow. These control for example what portions of a particle trace will be visible at a given time step.
Appearance settings for unsteady flow are similar to the settings for steady flow. The one difference is that, instead of allowing the position along a flow line to be used in the color/opacity mapping, the time step (associated with a position along the flow line) can be used in the color mapping.
