Vector Field Grains: Conic Ellipse 1
next might have a variable you can modify.
I recommend Turbo speed for this.
The vector field is:
based on an Applet from MIT for vector fields (TEAL, for electromagnetism) at http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/vectorfields/licapplet/licapplet.htm (specifically, the "grass seeds" method of visualization)