Motion and wake structure of spherical particles
  This paper presents results from a flow visualization study of the wake structures behind solid spheres rising or falling freely in liquids under the action of gravity. These show remarkable differences to the wake structures observed behind spheres held fixed. The two parameters controlling the rise or fall velocity (i.e., the Reynolds number) are the density ratio between sphere and liquid and the Galileo number. 
Christian Veldhuis, Arie Biesheuvel, Leen van Wijngaarden and Detlef   Lohse