How to generate large number of random particles which are continually moving in to one direction for collision detection

Hi , I am using gazebo + Ros (melodic) + URDF for my robot. I have created a contact sensor as below. Now I need to generate huge number of random particles (like small bubbles) which are continuously moving towards my robot. (without effect of the gravity) and get the hit count for a given time Is there any way to generate these particles? Please help here