Standard Deviation ( Sort of ) Problem

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I have a bunch of particles spread over a 2D area. I plan to fit a a distribution to describe the spread.

To start off :

  1. I have about 100 particles arranged in a circular pattern in a "unit" consisting of 10 pieces ( like a pizza basically ). There are then 10 of these "units" arranged in a cylindrical shape all contained in a cylindrical shell.

  2. A charge ignites at the back of the cylinder, sending the projectiles forward.

My argument is that the front pieces will be pushed by the rear ones, and result in a higher spread for the first 2-3 rows, whereas the last few rows will see a tighter spread as there are no particles "pushing" them out of the way.

If I were to draw a scatter plot of the spread, it would be slightly concentrated in the middle and increase from there to a point and then drop off.

I estimate it to look something like this if I were to view a line from the centre to the edge:

Prob Plot

This distribution will then be rotated around the centre to from a kind of "doughnut" shape that is raised more in the centre than the sides.

Now the problem I have :

What do I call a distribution like this? What is the general formula for this type of distribution? It seems like a normal distribution with a twist.