How to create a particle path plot with the particles moving at a constant time step

If you use Display->Pathlines in the "Pulse" mode, you'll notice that the particles are updated using ***different*** time steps for
each particle stream! This is done for efficiency in integrating the particle acceleration equations. The ramification of this is that
particles in low velocity regions may in fact appear to move as fast as particles in high velocity regions!

So how can you update particles with a ***constant*** time step? A solution is provided below...

Here's how to update particles at a constant time step for a steady-state solution in Fluent 6...

Step 1: Read your steady-state case and data files into Fluent
Step 2: Turn on the unsteady solver in Define->Models->Solver (use First Order - this is actually arbitrary).
Step 3: Enable Unsteady Particle Tracking in Define->Models->DPM (use default settings).
Step 4: Set up one or more injections in the usual way using Define->Injections. If you want to mimic the pathline
plot, make sure to use small particles (e.g. 1 micron).
Step 5: To automatically plot the DPM particles and export the graphics files for animations, use Solve->Execute Command as follows:
A. Create a macro to display the DPM particle positions for all injections using Display->Particle Tracks
B. Create a second macro to save a tif file to disk (using filename such as particle%t.tif, where %t will be the current time step).
C. Set the "When" input to "Time Step" and "Every" input as desired (e.g. 2 = run the macros every two time steps).
Step 6: Access Solve->Controls->Solution and deselect (disable) ALL equations (e.g. flow, turbulence, etc.). This will allow you to
update the DPM model without changing your steady-state flowfield solution.
Step 7: In the Solve->Iterate panel, choose a time step which is suitable for updating the particles. A good choice is to use dt = dx / Vc,
where dx is the minimum cell size in the domain and Vc is a characteristic velocity. Also, set the number of subiterations to 0.
Step 8: Run the model for as many time steps as desired (e.g. the number of time steps it takes for particles to traverse the entire domain).
As the calculation proceeds, the Command macros will be executed, thereby saving graphics images of the particle positions to
tif files. When the calculation is complete, you can use xanimate, gif construction set, or similar software to animate the images.

Some notes on the foregoing method...

* For plotting the particles, use "Style" of Point, with a point size (under "Attributes") of about 0.2. Also, showing an outline or feature lines of
of the geometry is recommended, in order to provide a point of reference for the particles.
* Note that the unsteady DPM model is updated every time step when you run the model using Solve->Iterate. So when you access
Display->Particle Tracks, you will display the current positions of the particles at a given time.
* There are "Start Time" and "Stop Time" parameters for each injection that you can specify in Define->Injections. These control when
particles are released and removed from the domain. This can be useful for creating interesting displays (waves of particles initiated
at different times, for example).

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