Calculating Velocity In Shells

Hi, I am trying to solve a problem with LAMMPS. I have a liquid nanoparticle that is moving through a box and wrapping around. It is surrounded by a cloud of gaseous atoms.

I am trying to track the velocity of the gaseous atoms by grouping them into “shells” based off the distance from the droplet and then aggregating the velocity of all atoms in the shell. Is this possible in LAMMPS? And how would I go about doing this. I am especially concerned about what happens when the droplet is in the process of wrapping around.

You can use a combination of fix recenter (which will maintain the dynamics, but retranslate the coordinates so that the fix group will remain at the same location; i.e. you can make the droplet position the frame of reference instead of the environment), and compute reduce/region (for summing the velocity components after defining suitable regions around the (centered) droplet).