Are you doing anything special to the flow field to make the flight paths cluster around each other and the same area?

I tried out a flow field in processing the other day and the results seem to start out a little like this but eventually all the particles diverge and get further and further from each other.

Hey binary, I am doing something special. I have all the particles responding to the flow field, but they are also drawn towards the middle of the space with a slight gravitational pull. Well, not quite gravity. The further away the particle is from the center, the stronger the pull so they never can wander too far away. As for why they cluster around each other, not really sure. It was a pleasant side effect. I think it is because the perlin noise detail was set very low so you end up with smooth gradiations which make them behave similarly if they are close enough to each other.