I looked through the previous threads on yt-users, and I don't see this particular problem described.  Hoping for some help.  So, the quantity dark_matter_density is a CIC-deposited particle quantity, which becomes a grid field.  I would like to do something like this: 

for every grid voxel, calculate the mean dm particle *velocity* inside that voxel

return as a grid field

Is that simple using CIC_deposit?  The only mean particle velocity I've seen used in yt/enzo before is the one calculated for halos, by using HOP or FOF identification. 

thanks!