I am trying to simulate, using FLUENT 5.4's Discrete Phase Model, the terminal settling velocity of a single spherical particle. The fluid velocity is zero in all direction. I then compare the terminal settling velocity to that calculated using Stoke's equation : i.e. v_t = (rho_p - rho_f)*g*D_p^2 /(18*mu)

The simulated terminal settling velocity matches the velocity calculated from Stoke's equation for small particles (less or equal than 10 micron), but diverges when the particle size is larger than 100 micron. For example, the terminal settling velocity for a 2600 kg/m3, 100 micron diameter particle is -1.535e-01 m/s (from FLUENT) and -8.69e-03 m/s (from Stokes' equation). I have tried using smaller time step, but to no avail.

Am I missing something blantantly obvious ? Any advice /suggestion are most welcomed.

Many thanks in advance.

Sorry

"100 micron diameter particle is -1.535e-01 m/s (from FLUENT) and -8.69e-03 m/s (from Stokes' equation)"

should have read

"100 micron diameter particle is -8.08e-03 m/s (from FLUENT) and -8.69e-03 m/s (from Stokes' equation)"

Is what you said meaning that a particle is moving in static flow field and the particle move depending on its gravity? If yes, maybe, the reason is the stoke's equation cann't use again when the particle diameter is large.