The trajectory of a particle moving in a fluid can be significantly influenced by its rotation, specially for large and/or heavy particles with high moments of inertia. In order to account for particle rotation, an ordinary differential equation for the angular momentum of the particle, Equation 2–1 , is solved along with the equation for the particle translational motion, Equation 2–2 .
One additional form of interaction between particle and fluid is the generation of an
angular moment, or a torque, over the particle when it moves in a fluid. In the Rocky DEM-CFD
coupling, the fluid generated torque is computed based on the torque coefficient
according to:
 | (3–76) |
where 
is the relative fluid-particle angular velocity, given by: 
The torque coefficient is usually given as a function of the Reynolds number based on
the relative angular velocity 
, calculated as
 | (3–77) |
Dennis et al. [21] have investigated the torque necessary to
keep a sphere rotating at an angular speed 
in an otherwise stagnant fluid. The torque coefficient is given by:
The range of validity for this expression is 
.